Recent Advances in Black TiO2 Nanomaterials for Solar Energy Conversion

Titanium dioxide (TiO2) nanomaterials have been widely used in photocatalytic energy conversion and environmental remediation due to their advantages of low cost, chemical stability, and relatively high photo-activity. However, applications of TiO2 have been restricted in the ultraviolet range because of the wide band gap. Broadening the light absorption of TiO2 nanomaterials is an efficient way to improve the photocatalytic activity. Thus, black TiO2 with extended light response range in the visible light and even near infrared light has been extensively exploited as efficient photocatalysts in the last decade. This review represents an attempt to conclude the recent developments in black TiO2 nanomaterials synthesized by modified treatment, which presented different structure, morphological features, reduced band gap, and enhanced solar energy harvesting efficiency. Special emphasis has been given to the newly developed synthetic methods, porous black TiO2, and the approaches for further improving the photocatalytic activity of black TiO2. Various black TiO2, doped black TiO2, metal-loaded black TiO2 and black TiO2 heterojunction photocatalysts, and their photocatalytic applications and mechanisms in the field of energy and environment are summarized in this review, to provide useful insights and new ideas in the related field.


Introduction
With the rapid development of industry and human society, fossil fuels, including coal, natural gas, and petroleum, have been excessively consumed in the last decades, thereby creating energy crises and environmental pollutions. Searching for alternative energy resources and improving the living environment have become urgent issues all over the world. Photocatalytic technology, which can employ the inexhaustible solar energy to H 2 generation from water splitting [1][2][3], CO 2 reduction to small sustainable fuels [4][5][6], and environmental pollutant degradation [7][8][9], has been developed and attracted much attention due to a series of excellent physical and chemical characteristics, such as low energy consumption, simple operation, no secondary pollution, low cost, and sustainability [10]. Since 1972, TiO 2 has been used as a photocatalyst and developed rapidly in the field of energy conversion and environmental remediation [1]. Three main kinds of TiO 2 , including anatase, rutile, and brookite, can be distinguished according to their different crystal structures [11][12][13]. Anatase and rutile are the most frequently investigated TiO 2 photocatalysts because of their superior photocatalytic activity under UV irradiation than brookite. The photocatalytic performance and properties of TiO 2 are severely influenced by its preparation, morphology, and dimensions. Serga et al. reported an extraction-pyrolytic method for the synthesis of nanocrystalline TiO 2 powders using valeric acid as an extractant [14]. This method can be applied for the fabrication of anatase, rutile, or mixed anatase-rutile TiO 2 powders [14]. Poly (titanium dioxide) is found to have a significant influence on the component compatibility and relaxation behavior of
The synthesis of black TiO 2 with unique morphology, such as nanoflowers, tubes, and wires, usually necessitates specific synthetic procedure for TiO 2 nanomaterials, including hydrothermal, solvothermal treatment, anodization, etc. Lim et al. prepared partially reduced hollow TiO 2 nanowires (R-HTNWs) using the hydrothermal method and the subsequent treatment with NaBH 4 under the nitrogen atmosphere [47]. The local distribution of Ti 3+ species (oxygen vacancies) in reduced hollow TiO 2 nanowires was confirmed to be primarily present in the surface region compared to the core using electron energy loss spectroscopy (EELS) [47]. In addition, trace impurities including B, Na, N from NaBH 4 , and nitrogen were located mostly at the surface and the distorted rutile structure region of R-HTNWs [47]. The SEM, TEM image, and EELS Ti L 2,3 data are illustrated in Figure 2B [47]. Ti 3+ present on the surface of TiO 2 could be stabilized by the surface impurities [47]. Black TiO 2 materials generally possessed certain amounts of oxygen vacancies, which can be confirmed by X-ray photoelectron spectroscopy (XPS). The concentration of oxygen vacancies was normally controlled by the different thermal treatment time or temperature [41,45,51]. However, there is lack of precise, quantitative characterization techniques for oxygen vacancies present on the black TiO 2 surface. The band gap parameters of black TiO 2 were usually measured and calculated by XPS and UV-vis spectroscopy measurements. The decrease in Eg of black TiO 2 was assumed to be related to the surface disorder, including the presence of Ti 3+ and oxygen vacancies [47].
The mesoporous structure of TiO 2 can increase the surface area and phase stability. Zhou et al. synthesized mesoporous black TiO 2 hollow spheres (MBTHSs) via the combination of a template-free solvothermal method and amine molecules encircling strategy, and the subsequent atmospheric hydrogenation process [64]. The wall thickness and diameter of MBTHSs could be tuned by adjusting the solvothermal reaction time and the Ti precursor concentration, respectively [64]. Ti 3+ species were proved to be mainly present in the bulk but not on the surface of MBTHSs via XPS Nanomaterials 2023, 13, 468 4 of 27 measurements [64]. The light absorption of MBTHSs was effectively extended to the visible light range compared with the pristine TiO 2 [64]. The synthetic procedure, SEM image, and UV-vis absorption properties are present in Figure 2C [64]. The anatase phase remained unchanged after hydrogenation [64]. The band gap of mesoporous TiO 2 was largely reduced to 2.59 eV after hydrogenation [64]. The black TiO 2 consisted of mesoporous structure with cylindrical channels providing the relatively high surface area of~124 m 2 g -1 [64]. They also fabricated the heterojunctions of γ-Fe 2 O 3 nanosheets/mesoporous black TiO 2 hollow sphere to enhance the charge separation and photocatalytic tetracycline degradation efficiency [59]. Porous black TiO 2 photocatalysts tended to appear in three dimensional structures, such as foams, pillars, and hollow structures. Zhang et al. synthesized the 3D macro-mesoporous black TiO 2 foams via freeze-drying, cast molding technology, and high-temperature surface hydrogenation [65]. The large, closed pores were generated using polyacrylamide as the organic template, while plenty of open pores were formed in the frameworks and on the surface of the black TiO 2 thanks to the water evaporation in the freezedrying process [65]. This black TiO 2 material exhibited a self-floating amphiphilic property and an enhanced solar energy harvesting efficiency [65]. Zhou et al. prepared porous black TiO 2 pillars through an oil bath reaction and high-temperature hydrogenation reduction [66]. The porous structure and mesopores of black TiO 2 pillars were clearly observed by the Scanning electron microscope and transmission electron microscope [66]. The enhanced photocatalytic performance was attributed to more active surface sites offered by the porous pillar structure and the self-doped Ti 3+ [66]. The hollow structured black TiO 2 with plenty of pore channels and an exposed surface also showed an enhanced photocatalytic efficiency [57]. The pores of the porous black TiO 2, generally located in its whole frameworks with open pores connected with surface, providing abundant active sites and surface defects, thus promoting the photocatalytic performance. Ethylenediamine was often utilized to maintain the porous structure of black TiO 2 and to prevent its phase transformation from anatase to rutile.
In addition to oxygen vacancies, disordered structures and surface amorphization in black TiO 2 may have significant impacts on its photoresponsive properties. Kang et al. prepared black TiO 2 with amorphous domains through a glycol-assisted solvothermal method and subsequent calcination [67]. Oxygen vacancies were introduced in the amorphous domains of the black TiO 2 nanosheets [67]. Figure 2D shows the color and optical absorption property changes [67]. The color of the brown TiO 2 turned into black after 2 h of calcination at 350 • C under Ar [67]. The light absorption of black TiO 2 was significantly extended to the near-infrared region [67]. Oxygen vacancies were confirmed to be present in the subsurface of black TiO 2 by first-principle calculations [67]. Table 1 summarizes the properties of some black TiO 2 nanomaterials with varied morphology. The color of most reduced TiO 2 is black. The morphology of black TiO 2 is not determined by the reduction process. of calcination at 350 °C under Ar [67]. The light absorption of black TiO2 was significantly extended to the near-infrared region [67]. Oxygen vacancies were confirmed to be present in the subsurface of black TiO2 by first-principle calculations [67]. Table 1 summarizes the properties of some black TiO2 nanomaterials with varied morphology. The color of most reduced TiO2 is black. The morphology of black TiO2 is not determined by the reduction process.

Synthesis of Black TiO 2
Currently, various methods were used to synthesize black TiO 2 , which can be divided into two main approaches: high temperature hydrogenation reduction and solid phase reduction [68]. The high temperature hydrogenation method often uses hydrogen or hydrogen-contained gas mixtures to treat samples at high temperatures [69,70]. The materials used in the solid phase reduction method are generally NaBH 4 [71], CaH 2 [72], Mg powder [72], or other reducibility materials [72]. The reduction method can be expressed in reaction Equation (1): In addition, researchers also use hot wire annealing [73], laser irradiation [74,75], anode reduction [76], and other methods to synthesize black TiO 2 [77,78].

High Temperature Hydrogenation
Hydrogen reduction involves the reduction of pure H 2 gas, H 2 /Ar, or H 2 /N 2 mixture at high or low pressures [69], which is a simple, effective, and straightforward method.
Zhou et al. successfully prepared the ordered mesoporous black TiO 2 material by hydrogenation at high temperature (500 • C) under atmospheric pressure ( Figure 3), which had a larger specific surface area and pore size compared with the pristine titanium dioxide [79]. As shown in Figure 3, after hydrogenation at high temperature, the regular hexagonal channel of the obtained black TiO 2 was completely maintained [79]. It can be seen from the XRD in Figure 3 that there was no phase change in black TiO 2 compared with the original materials, thus proving the high thermal stability of the sample prepared by this method [79]. Notably, it can also be clearly seen that its crystallinity decreased, proved by the XRD intensity, thereby indicating that the surface disorder of TiO 2 has been created after the hydrogenation process [79]. The color of the white TiO 2 turned into black after 3 h of hydrogenation [79].   [100] and [110] planes, HRTEM images of the ordered mesoporous black TiO2 materials, and X-ray diffraction patterns and the photos of the ordered mesoporous black TiO2 materials (a) and ordered mesoporous TiO2 materials (b). Reprinted with permission for ref. [79]. Copyright 2014, American Chemical Society.
Black TiO2 with different morphologies can be obtained via hydrogenation. Yang et al. prepared one-dimensional black TiO2 nanotubes by the hydrogenation method, with an inner diameter of 7 nm and a wall thickness of 6 nm, as presented in Figure 4A [80]. Spherical and lamellar structures have also received much attention due to their large specific surface areas. As shown in Figure 4B, after simple hydrogenation reduction, Li et al. successfully prepared black TiO2 nanospheres and observed the mesoporous structure in the TEM image [81]. Although the crystal surface structure of anatase became slightly  [100] and [110] planes, HRTEM images of the ordered mesoporous black TiO 2 materials, and X-ray diffraction patterns and the photos of the ordered mesoporous black TiO 2 materials (a) and ordered mesoporous TiO 2 materials (b). Reprinted with permission for ref. [79]. Copyright 2014, American Chemical Society.
Black TiO 2 with different morphologies can be obtained via hydrogenation. Yang et al. prepared one-dimensional black TiO 2 nanotubes by the hydrogenation method, with an inner diameter of 7 nm and a wall thickness of 6 nm, as presented in Figure 4A [80]. Spherical and lamellar structures have also received much attention due to their large specific surface areas. As shown in Figure 4B, after simple hydrogenation reduction, Li et al. successfully prepared black TiO 2 nanospheres and observed the mesoporous structure in the TEM image [81]. Although the crystal surface structure of anatase became slightly disordered after hydrogenation, its special lattice fringes (d = 0.35 nm) did not change [81]. Black TiO 2 nanotubes with the mesoporous nanosheet structure were successfully prepared by the hydrogen reduction method by Zhang et al. [82]. Ethylenediamine coating method was used before hydrogenation [82]. The original morphology of TiO 2 was completely retained [82]. Wu et al. also synthesized two dimensional ultrathin mesoporous black TiO 2 nanosheet materials using the similar ethylenediamine encircling strategy ( Figure 5) with 4 h hydrogenation reaction at 500 • C [83].   In addition to the method of hydrogen reduction at high temperature and atmospheric pressure, researchers also use the high pressure method. Wu et al. prepared black anatase TiO2 in a two-step process [84]. The sample was degassed at 200 °C firstly, then was heated to 400 °C, and hydrogenated under high pressure (5-bar) for 24 h. The black sample was finally obtained after cooling to room temperature. The black TiO2 was successfully prepared by Hamad et al. at a high pressure (8 bar) and relatively mild temper-   In addition to the method of hydrogen reduction at high temperature and atmospheric pressure, researchers also use the high pressure method. Wu et al. prepared black anatase TiO2 in a two-step process [84]. The sample was degassed at 200 °C firstly, then was heated to 400 °C, and hydrogenated under high pressure (5-bar) for 24 h. The black sample was finally obtained after cooling to room temperature. The black TiO2 was suc- In addition to the method of hydrogen reduction at high temperature and atmospheric pressure, researchers also use the high pressure method. Wu et al. prepared black anatase TiO 2 in a two-step process [84]. The sample was degassed at 200 • C firstly, then was heated to 400 • C, and hydrogenated under high pressure (5-bar) for 24 h. The black sample was finally obtained after cooling to room temperature. The black TiO 2 was successfully prepared by Hamad et al. at a high pressure (8 bar) and relatively mild temperature [85]. The hydrogenation time was longer than in other similar research (1-5 days) [85]. The synthesized samples were uniform and stable in size, and showed higher photocatalytic activity compared with the pristine white TiO 2 [85]. Mixed gases with H 2 were also used as reducing agents in the synthesis of black TiO 2 . Cai et al. successfully prepared black TiO 2 with the surface disorder structure using H 2 /N 2 mixed gas with 10% content of hydrogen [86].

Solid Phase Reduction
Compared with the high temperature hydrogenation, the solid phase reduction method has certain advantages. The high temperature hydrogenation process normally starts from the outside to the inside with a relatively moderate reaction rate, while the solid phase reduction method can provide a more complete and intense reaction and may produce a series of doping at the same time. The defect is a double-edged sword. Too many defects may be detrimental to the photocatalytic performance, so the proportion and dosage of reductants in solid phase reactions should be reasonably controlled.
Xiao et al. prepared the black TiO 2 by the solid-state chemical reduction strategy by mixing the sample with sodium borohydride in a certain proportion [87]. Then, the mixture was heated in a tubular furnace with N 2 atmosphere [87]. Finally, the resulting sample was washed with deionized water to remove the unreacted sodium borohydride [87]. As shown in Figure 6, the color of the sample was getting darker with the temperature [87]. The absorption of the visible light was much enhanced after the reduction of TiO 2 [87].
Zhu et al. showed that CaH 2 can also be used as a constant reducing agent to prepare black TiO 2 [88]. The reduction process was conducted at varied temperatures [88]. It was found that the obtained black TiO 2 after reduction treatment at 400 • C had the best absorption of sunlight (over 80%), which was 11 times that of the pristine TiO 2 [88]. This simple method provides an alternative for improving the absorption of visible light on the TiO 2 surface.
Sinhamahapatra et al. reported the reduction of TiO 2 particles to black TiO 2 by magnesium thermal reduction method, which was inspired by the Kroll process, for the first time [89]. The synthetic procedure of this method was approximately identical to the method of sodium borohydride reduction [89]. TiO 2 and magnesium powder were thoroughly mixed first, and then heated in a tube furnace at 650 • C with 5% H 2 /Ar for 5 h [89]. The obtained samples were placed in HCl solution for 24 h, and then washed with water to remove the acid, and finally dried at 80 • C [89].

Hot-Wire Annealing Method
In addition to the high temperature hydrogenation reduction and solid-phase reduction, researchers have also explored some other methods to synthesize black TiO 2 , which has made the method of preparing black TiO 2 diversified. Wang et al. proposed a simple and direct hot-wire annealing (HWA) method [73]. The titanium dioxide nanorods were treated with highly active atomic hydrogen simply generated by hot wire [73]. The reduction mechanism was similar to that of the high temperature hydrogenation [73]. The resulted black TiO 2 nanorods had better stability and higher photocurrent density compared with the traditional hydrogenation method [73]. In addition, it had no damage to the photoelectric chemical devices [73].
Xiao et al. prepared the black TiO2 by the solid-state chemical reduction strategy by mixing the sample with sodium borohydride in a certain proportion [87]. Then, the mixture was heated in a tubular furnace with N2 atmosphere [87]. Finally, the resulting sample was washed with deionized water to remove the unreacted sodium borohydride [87]. As shown in Figure 6, the color of the sample was getting darker with the temperature [87]. The absorption of the visible light was much enhanced after the reduction of TiO2 [87].  , the corresponding selected-area electron diffraction pattern (d) of the hydrogenated rutile TiO 2 (300 • C) and the UV/vis absorption spectra (e) of the pristine rutile TiO 2 (i) and the hydrogenated rutile TiO 2 under 250 • C (ii), 300 • C (iii), and 350 • C (iv). Reprinted with permission for ref. [87]. Copyright 2018, Springer.

Anode Oxidation Method
The introduction of crystal defects to titanium dioxide can effectively extend the light absorption range to the visible light region without side effects. Anode oxidation is a simple and efficient method to synthesize defective black TiO 2 . Dong et al. successfully prepared black TiO 2 using a two-step anode oxidation method [76]. The first step was to anodize Ti foil in the ethylene glycol solution with a certain proportion of NH 4 F and distilled water, and the corresponding voltage was set at 60 V [76]. After 10 h of oxidation, an oxide layer was obtained [76]. Subsequently, the Ti foil was purified to remove organic impurities, and treated at high temperature (450 • C) for 1 h to form black TiO 2 [76].

Plasma Treatment
Zhu et al. prepared black TiO 2 nanoparticles via the one-step solution plasma method under mild conditions [27]. The structural disorder layer was assumed to be formed in TiO 2 after the solution plasma process [27]. The light absorption of TiO 2 in the visible and near infrared range was significantly enhanced after the plasma treatment, thus increasing its activity in the water evaporation under solar illumination [27]. Teng et al. prepared black TiO 2 using P25 as the precursor system, hydrogen plasma, and a hot filament chemical vapor deposition (HFCVD) device with H 2 as the reducing gas [77]. The visible and near-infrared light absorption of TiO 2 were much enhanced after the surface reduction [77]. Oxygen vacancies and Ti-H bonds were formed on the black TiO 2 surface, thereby improving the photocatalytic activity [77].

Gel Combustion
Ullattil et al. prepared black anatase TiO 2−x photocatalysts through a one-pot gel combustion process using titanium butoxide, diethylene glycol, and water as precursors [90]. Plenty of Ti 3+ and oxygen vacancies existed in the synthesized black anatase TiO 2 nanocrystals confirmed by XPS measurements [90]. The light absorption of TiO 2 was extended from UV to the near-infrared range [90]. Campbell et al. also synthesized black TiO 2 via the solgel combustion method using titanium tetraisopropoxide as the precursor [91]. The light absorption ability was significantly enhanced compared to commercial TiO 2 [91]. The obtained black TiO 2 with the high surface area demonstrated much improved photocatalytic degradation efficiency of the organic dye under the visible light irradiation [91].

Strategies for Promoting Photocatalytic Activity of Black TiO 2
Researchers have been trying to use metal and non-metal doping methods to prepare the modified TiO 2 with better light absorption ability and photocatalytic activity. The introduction of metal ions and non-metallic elements into the TiO 2 lattice can expand its absorption range to the visible light, thus enhancing the photocatalytic performance [92]. In recent years, doped black TiO 2 has also been widely explored to narrow its band gap, thereby improving its optical properties in the visible light region, and enhancing its photocatalytic activity in various reactions.

Metallic Doped Black TiO 2
It was found that by doping different metals in TiO 2 , Ti 4+ in TiO 2 lattice was replaced [93]. New impurity levels would be introduced in the band gap of TiO 2 [93]. The band gap would be narrowed by the doping process, thus improving the separation efficiency of the photoelectron-hole of TiO 2 , increasing the quantum yield, and expanding the light absorption to the visible light region [92]. Photocatalytic degradation, hydrogen production capacity, and light energy conversion can be significantly improved [92]. Previously, various metal elements, including Cu, Co, Mn, Fe, Mo, etc., had been used to produce the doped TiO 2 via different approaches [93]. Lately, some of the metal elements, such as Al, Ni, Na, etc., were also utilized to dope black TiO 2 for achieving the narrower band gap and better photocatalytic performance [47,94,95].
Yi et al. prepared the amorphous Al-Ti-O nanostructure in black TiO 2 via a scalable and low-cost strategy [94]. The commercial TiO 2 and Al powders were mixed and then grinded in an agate mortar at room temperature for 0-50 min [94]. The color of the light gray TiO 2 turned into gray after 2 min of milling [94]. Its color became much darker after the longer milling time [94]. Black Al-Ti-O oxide samples were obtained after milling for more than 5 min [94]. The color changes, UV-Vis-NIR diffuse reflectance spectra, and TEM image of the samples were shown in Figure 7 [94]. The crystalline Al and anatase TiO 2 were transformed into amorphous Al-black TiO 2 after the ball milling [94]. Al-black TiO 2 after 20 min milling exhibited the best light absorption in the visible light and near infrared region [94].
Zhang et al. prepared Ni 2+ -doped porous black TiO 2 photocatalysts through the combination of the sol-gel method and in situ solid-state chemical reduction process [95]. The reduction approach was performed by heating the mixture of Ni-doped TiO 2 and NaBH 4 at 350 • C under Ar atmosphere for 1 h [95]. The color of the white as-made TiO 2 became yellowish after Ni doping [95]. Black Ni-doped TiO 2 was obtained after the reduction with NaBH 4 [95]. Figure 8 shows the optical properties and the band gap of different materials [95]. The light absorption of TiO 2 was extended to the visible light range after Ni doping, and further enhanced after the chemical reduction, which was attributed to the generation of oxygen vacancies, Ti 3+ , and Ni 2+ [95]. The band gap of the black Ni-doped TiO 2 was only 1.96 eV [95]. Zhang et al. prepared Ni 2+ -doped porous black TiO2 photocatalysts through the combination of the sol-gel method and in situ solid-state chemical reduction process [95]. The reduction approach was performed by heating the mixture of Ni-doped TiO2 and NaBH4 at 350 °C under Ar atmosphere for 1 h [95]. The color of the white as-made TiO2 became yellowish after Ni doping [95]. Black Ni-doped TiO2 was obtained after the reduction with NaBH4 [95]. Figure 8 shows the optical properties and the band gap of different materials [95]. The light absorption of TiO2 was extended to the visible light range after Ni doping, and further enhanced after the chemical reduction, which was attributed to the generation of oxygen vacancies, Ti 3+ , and Ni 2+ [95]. The band gap of the black Ni-doped TiO2 was only 1.96 eV [95].   Zhang et al. prepared Ni 2+ -doped porous black TiO2 photocatalysts through the co bination of the sol-gel method and in situ solid-state chemical reduction process [95]. T reduction approach was performed by heating the mixture of Ni-doped TiO2 and NaB at 350 °C under Ar atmosphere for 1 h [95]. The color of the white as-made TiO2 beca yellowish after Ni doping [95]. Black Ni-doped TiO2 was obtained after the reduction w NaBH4 [95]. Figure 8 shows the optical properties and the band gap of different materi [95]. The light absorption of TiO2 was extended to the visible light range after Ni dopi and further enhanced after the chemical reduction, which was attributed to the generat of oxygen vacancies, Ti 3+ , and Ni 2+ [95]. The band gap of the black Ni-doped TiO2 was o 1.96 eV [95].  Zhang et al. reported Ti 3+ self-doped black TiO 2 nanotubes with mesoporous nanosheet structure via a two-step approach consisting of the solvothermal reaction and hydrogenation process [82]. The appearance of the white TiO 2 turned into black after hydrogenation at 600 • C for 2 h [82]. The optical absorption was significantly extended to the range of 400-800 nm after hydrogenation [82]. The band gap of pristine TiO 2 decreased from 3.2 eV to 2.87 eV after the surface hydrogenation [82].

Non-Metallic Doped Black TiO 2
The doping mechanism of nonmetallic elements can be explained as follows: the doped elements act as overlapping impurity levels in the valence band inside the photocatalyst crystal, thereby reducing the band gap of semiconductors and promoting the migration of photogenerated electrons to the active site. The doping of non-metal elements in the crystal lattice of TiO 2 can slow down the electron-hole pair recombination rate, which is an effective modification way to improve the photocatalytic activity of TiO 2 .
The nitrogen atom, which has five outer shell electrons, has a similar radius to oxygen. The introduction of N into TiO 2 enhances its visible light photocatalytic activity, which is proved to be the most ideal non-metallic doping element in a large number of studies [96,97]. Since the 2p orbital of N has a similar energy level to that of the oxygen atom and is easy to hybridize, the researchers found that the doping of N can improve the defects of TiO 2 and broaden the response range of the absorption spectra [98]. The N-Ti-O bond generated by doping the crystal can change the energy level structure of TiO 2 and improve the quantum efficiency [99]. In addition, N can also replace O in the lattice with the formation of the Ti-N bond, which increases the absorption of the visible light by TiO 2 and improves the photocatalytic efficiency of TiO 2 [96].
Zhou et al. successfully synthesized the nitrogen-doped black titanium dioxide nanocatalyst by calcining white TiO 2 with or without urea at varied temperatures under the different atmosphere [31]. The N-doped TiO 2 using urea as N precursor has a better visible light absorption, narrower band gap and the most effective excitation charge separation, and higher photocatalytic activity [31]. Liu et al. prepared N-doped black TiO 2 spheres via a two-step process consisting of the solvothermal reaction and calcination in the nitrogen atmosphere [100]. The black N-TiO 2 photocatalysts were obtained after heat treatment at 500 • C in N 2 atmosphere for 3 h [100]. Ammonium chloride was used as the nitrogen source during the synthetic process [100]. The obtained black N-TiO 2 with a moderate mole ratio of ammonium chloride to TiO 2 (2:1) had the narrowest band gap and the highest photocatalytic pollutant degradation efficiency [100].
Gao et al. prepared black TiO 2 nanotube arrays with dual defects consisting of bulk N doping and surface oxygen vacancies [101]. Urea was utilized as the N precursor during the anodic oxidation process [101]. Black N-doped TiO 2 nanotube arrays were obtained after calcination at 600 • C in the Ar atmosphere using aluminum powder [101]. The doping of N generated a new energy level and shortened the carrier migration distance [101]. The synergistic effect of the two defects established an internal electric field, promoted the transfer of charge, and achieved the balance between kinetics and thermodynamics, thereby enhancing the photocatalytic hydrogen production efficiency [101].
Cao et al. successfully synthesized N and Ti 3+ co-doped mesoporous black TiO 2 hollow spheres (N-TiO 2−x ) by a step-by-step method [102]. The prepared three different kinds of TiO 2 had similar XRD peaks, indicating no impurities was formed during the synthetic process, as presented in Figure 9a [102]. The broader peaks of the black TiO 2 may be attributed to the lattice distortion caused by N doping [102]. Raman spectra (Figure 9b) showed that the main phase of the titanium dioxide hollow sphere was anatase [102]. The co-doping of Ti 3+ and N in N-TiO 2−x resulted in a certain amount of attenuation [102]. The absorption in the visible light was much enhanced after the co-doping of N and Ti 3+ in TiO 2 [102]. The color of the samples gradually became darker after the nitrogen and Ti 3+ doping [102]. In addition, the band gap of the black N-doped TiO 2 was much smaller than the pristine TiO 2 (Figure 9c,d) [102]. The charge separation and photocatalytic activity for photocatalytic pollutant removal and hydrogen generation were much improved after the co-doping of N and Ti 3+ in the lattice of TiO 2 (Figure 9e) [102].

Metal-Loaded Black TiO2
Metal nanoparticles, such as Ag, Cu, Pt, etc., can generate the surface plasmon resonance (SPR) effect, thus improving the UV-vis absorption ability of photocatalysts [103]. The introduction of metal nanoparticles or clusters to the black TiO2 photocatalyst surface could further expand its light absorption range and enhance the photo-induced charge separation and transfer efficiency, thus improving the photocatalytic performance [38,[104][105][106][107][108][109]. Silver nanoparticles or clusters have been extensively explored to construct Ag/black TiO2 photocatalysts due to the relatively lower cost of Ag than other noble metals and the SPR effect. Jiang et al. prepared the Ag-decorated 3D urchinlike N-TiO2-x via a facile photo-deposition method combined with a reduction process, as presented in Figure  10 [104]. The AgNO3 solution was used as the Ag precursor and deposited onto the N-TiO2 surface under UV illumination at the wavelength of 365 nm for 30 min. [104]. Notably, the unique 3D urchinlike structure was retained after the Ag deposition and NaBH4 reduction process [104]. The light absorption of photocatalysts in the visible light range was further enhanced after the Ag deposition, with a much smaller band gap (2.61 eV) [104]. The Ag/N-TiO2−x photocatalysts presented the most excellent photocatalytic H2 production rate (186.2 μmol h −1 g −1 ) [104].

Metal-Loaded Black TiO 2
Metal nanoparticles, such as Ag, Cu, Pt, etc., can generate the surface plasmon resonance (SPR) effect, thus improving the UV-vis absorption ability of photocatalysts [103]. The introduction of metal nanoparticles or clusters to the black TiO 2 photocatalyst surface could further expand its light absorption range and enhance the photo-induced charge separation and transfer efficiency, thus improving the photocatalytic performance [38,[104][105][106][107][108][109]. Silver nanoparticles or clusters have been extensively explored to construct Ag/black TiO 2 photocatalysts due to the relatively lower cost of Ag than other noble metals and the SPR effect. Jiang et al. prepared the Ag-decorated 3D urchinlike N-TiO 2-x via a facile photodeposition method combined with a reduction process, as presented in Figure 10 [104]. The AgNO 3 solution was used as the Ag precursor and deposited onto the N-TiO 2 surface under UV illumination at the wavelength of 365 nm for 30 min [104]. Notably, the unique 3D urchinlike structure was retained after the Ag deposition and NaBH 4 reduction process [104]. The light absorption of photocatalysts in the visible light range was further enhanced after the Ag deposition, with a much smaller band gap (2.61 eV) [104]. The Ag/N-TiO 2−x photocatalysts presented the most excellent photocatalytic H 2 production rate (186.2 µmol h −1 g −1 ) [104]. Li et al. constructed Ag nanoparticle-decorated black TiO2 foams through the wet impregnation and high temperature surface hydrogenation process [105]. Ag nanoparticles were formed in the open pores of black TiO2 foams after the hydrogen atmosphere reduction, thus decreasing its surface area [105]. The synthetic process and UV-vis absorption spectra of Ag-black TiO2 foams were presented in Figure 11 [105]. The Ag-black TiO2 foams with varied amounts of silver showed apparent absorption at around 500 nm thanks to the SPR effect of Ag nanoparticles [105]. The Ag-black TiO2 foams containing 3 wt.% Ag nanoparticles exhibit the highest photocatalytic efficiency for atrazine removal [105]. Excess amounts of Ag nanoparticles in black TiO2 foams would decrease its photocatalytic performance due to the aggregation of Ag nanoparticles [105]. Ag nanoparticles were also decorated onto black TiO2 nanorods [106,109] and nanotubes surface [38], to further improve its photocatalytic performance using the SPR effect. In addition, NiS and Pt nanoparticles were co-decorated onto the surface of black TiO2 nanotubes via the solvothermal and photo-deposition approach, respectively [109]. The SPR effect of Pt nanoparticles effectively improved the light absorption ability, thus enhancing the photocatalytic water splitting [109]. Wang et al. successfully deposited Pt single atoms onto the black TiO2−x/CuxO surface assisted by the presence of surface oxygen vacancies [110]. The deposition of Pt single atoms further improved the light absorption of black TiO2−x/CuxO in the entire visible region [110]. Cu, which was a much cheaper candidate than noble metal, was also used for the surface decoration of black TiO2 surface as a SPR effect metal, thereby improving its photothermal effect [108].  Li et al. constructed Ag nanoparticle-decorated black TiO 2 foams through the wet impregnation and high temperature surface hydrogenation process [105]. Ag nanoparticles were formed in the open pores of black TiO 2 foams after the hydrogen atmosphere reduction, thus decreasing its surface area [105]. The synthetic process and UV-vis absorption spectra of Ag-black TiO 2 foams were presented in Figure 11 [105]. The Ag-black TiO 2 foams with varied amounts of silver showed apparent absorption at around 500 nm thanks to the SPR effect of Ag nanoparticles [105]. The Ag-black TiO 2 foams containing 3 wt.% Ag nanoparticles exhibit the highest photocatalytic efficiency for atrazine removal [105]. Excess amounts of Ag nanoparticles in black TiO 2 foams would decrease its photocatalytic performance due to the aggregation of Ag nanoparticles [105]. Ag nanoparticles were also decorated onto black TiO 2 nanorods [106,109] and nanotubes surface [38], to further improve its photocatalytic performance using the SPR effect. In addition, NiS and Pt nanoparticles were co-decorated onto the surface of black TiO 2 nanotubes via the solvothermal and photo-deposition approach, respectively [109]. The SPR effect of Pt nanoparticles effectively improved the light absorption ability, thus enhancing the photocatalytic water splitting [109]. Wang et al. successfully deposited Pt single atoms onto the black TiO 2−x /Cu x O surface assisted by the presence of surface oxygen vacancies [110]. The deposition of Pt single atoms further improved the light absorption of black TiO 2−x /Cu x O in the entire visible region [110]. Cu, which was a much cheaper candidate than noble metal, was also used for the surface decoration of black TiO 2 surface as a SPR effect metal, thereby improving its photothermal effect [108].

Construction of Black TiO 2 Based Heterojunction Photocatalysts
Although the visible light absorption ability of TiO 2 has been much enhanced after the hydrogenation or reduction process, the charge separation and transfer efficiency of black TiO 2 is still far from satisfactory for photocatalytic applications. In addition to the surface modification with metal nanoparticles, the construction of black TiO 2 -based junctions is an efficient way to improve the photo-generated charge separation and migration efficiency. The types of black TiO 2 -based heterojunctions can be divided into three main categories, including type II heterojunctions [59,[111][112][113][114][115][116][117][118][119], Z-scheme heterojunctions [120], and tandem heterojunctions [29,121,122]. Tan et al. fabricated the Ti 3+ -TiO 2 /g-C 3 N 4 nanosheets heterojunctions through a facile calcinations-sonication assisted approach [118]. The photo absorption of Ti 3+ -TiO 2 in the visible light range had been evidently enhanced after the coupling with meso-g-C 3 N 4 [118]. The synthetic procedure of Ti 3+ -TiO 2 /g-C 3 N 4 nanosheets, the UV-vis absorption spectra, and the band gaps of different samples were shown in Figure 12 [118]. The separation and migration of the photo-induced charge carrier had been effectively improved due to the construction of heterojunctions [118]. Ti 3+ -TiO 2 /g-C 3 N 4 nanosheets exhibited the highest photocatalytic H 2 evolution rate and phenol degradation efficiency [118]. A type II heterojunction with enhanced charge separation and transfer efficiency had been proposed [118].
were also decorated onto black TiO2 nanorods [106,109] and nanotubes surface [38], to further improve its photocatalytic performance using the SPR effect. In addition, NiS and Pt nanoparticles were co-decorated onto the surface of black TiO2 nanotubes via the solvothermal and photo-deposition approach, respectively [109]. The SPR effect of Pt nanoparticles effectively improved the light absorption ability, thus enhancing the photocatalytic water splitting [109]. Wang et al. successfully deposited Pt single atoms onto the black TiO2−x/CuxO surface assisted by the presence of surface oxygen vacancies [110]. The deposition of Pt single atoms further improved the light absorption of black TiO2−x/CuxO in the entire visible region [110]. Cu, which was a much cheaper candidate than noble metal, was also used for the surface decoration of black TiO2 surface as a SPR effect metal, thereby improving its photothermal effect [108].

Construction of Black TiO2 Based Heterojunction Photocatalysts
Although the visible light absorption ability of TiO2 has been much enhanced after the hydrogenation or reduction process, the charge separation and transfer efficiency of black TiO2 is still far from satisfactory for photocatalytic applications. In addition to the surface modification with metal nanoparticles, the construction of black TiO2-based junctions is an efficient way to improve the photo-generated charge separation and migration efficiency. The types of black TiO2-based heterojunctions can be divided into three main categories, including type II heterojunctions [59,[111][112][113][114][115][116][117][118][119], Z-scheme heterojunctions [120], and tandem heterojunctions [29,121,122]. Tan et al. fabricated the Ti 3+ -TiO2/g-C3N4 nanosheets heterojunctions through a facile calcinations-sonication assisted approach [118]. The photo absorption of Ti 3+ -TiO2 in the visible light range had been evidently enhanced after the coupling with meso-g-C3N4 [118]. The synthetic procedure of Ti 3+ -TiO2/g-C3N4 nanosheets, the UV-vis absorption spectra, and the band gaps of different samples were shown in Figure 12 [118]. The separation and migration of the photo-induced charge carrier had been effectively improved due to the construction of heterojunctions [118]. Ti 3+ -TiO2/g-C3N4 nanosheets exhibited the highest photocatalytic H2 evolution rate and phenol degradation efficiency [118]. A type II heterojunction with enhanced charge separation and transfer efficiency had been proposed [118]. Ren et al. prepared magnetic γ-Fe2O3/black TiO2 heterojunctions via the metal-ion intervened hydrothermal method and high temperature hydrogenation process [59]. In addition, α-Fe2O3 nanosheets were transformed to the surface defected γ-Fe2O3 after the hydrogenation process [59]. The light utilization of black TiO2 in visible light even near the infrared region has been improved after the combination with γ-Fe2O3 [59]. Figure 13 presented the UV-vis absorption spectra and the proposed band structure and charge transfer mechanism of γ-Fe2O3/black TiO2 heterojunctions [59]. The fabrication of the type II heterojunctions efficiently enhanced the photo-generated charge separation and transfer process [59]. The photocatalytic degradation of tetracycline on the heterojunction photocatalysts surface had been much improved, compared with the pristine TiO2 [59]. In addition, Bi2MoO6 [111], CdS [115], CeO2 [117], SrTiO3 [119], etc. had also been used for the construction of type II heterojunctions with black TiO2 with much improved photocatalytic efficiency. Ren et al. prepared magnetic γ-Fe 2 O 3 /black TiO 2 heterojunctions via the metal-ion intervened hydrothermal method and high temperature hydrogenation process [59]. In addition, α-Fe 2 O 3 nanosheets were transformed to the surface defected γ-Fe 2 O 3 after the hydrogenation process [59]. The light utilization of black TiO 2 in visible light even near the infrared region has been improved after the combination with γ-Fe 2 O 3 [59]. Figure 13 presented the UV-vis absorption spectra and the proposed band structure and charge transfer mechanism of γ-Fe 2 O 3 /black TiO 2 heterojunctions [59]. The fabrication of the type II heterojunctions efficiently enhanced the photo-generated charge separation and transfer process [59]. The photocatalytic degradation of tetracycline on the heterojunction photocatalysts surface had been much improved, compared with the pristine TiO 2 [59]. In addition, Bi 2 MoO 6 [111], CdS [115], CeO 2 [117], SrTiO 3 [119], etc. had also been used for the construction of type II heterojunctions with black TiO 2 with much improved photocatalytic efficiency. Sun et al. synthesized the CdS quantum dots/defective ZnO1−x-TiO2−x Z scheme heterojunction via the combination of hydrothermal synthesis, chemical reduction, and electroless planting process [120]. The visible light absorption was enhanced by the formation of ZnO-TiO2 heterojunction, and further improved after combining with CdS [120]. The formation process of the heterojunction, UV-vis spectra, and proposed charge transfer mechanism are shown in Figure 14 [120]. The charge separation efficiency and photocatalytic organic pollutant removal rate were much improved upon the formation of CdS QDs/defective ZnO1−x-TiO2−x Z scheme heterojunction [120]. To further promote the visible light utilization and photo-induced charge separation and transfer efficiency, black TiO2-based tandem heterojunction photocatalysts have been proposed by researchers for photocatalytic hydrogen production. Sun et al. prepared a hierarchical hollow black TiO2/MoS2/CdS tandem heterojunction photocatalyst through the combination of the solvothermal method and high-temperature hydrogenation treatment [122]. The black TiO2/MoS2 heterojunction effectively enhanced the photon absorption in visible light and the near infrared region [122]. The tandem system further promoted visible light utilization compared to other combinations [122]. The schematic view of the construction of black TiO2/MoS2/CdS, UV-vis absorption spectra, and photocatalytic hydrogen evolution rate are illustrated in Figure 15. The charge separation and migration efficiency were much promoted by the formation of black TiO2/MoS2/CdS tandem heterojunction [122]. The tandem heterojunction exhibited the highest photocatalytic hydrogen production rate under AM 1.5 illumination [122]. In addition, a sandwich-like mesoporous black TiO2/MoS2/black TiO2 nanosheet photocatalyst was proposed for visible light photocatalytic hydrogen generation with a much-promoted photo-generated charge transfer efficiency [121]. The mesoporous TiO2 and Cu2S were also combined with MoS2 to synthesize the hierarchical tandem heterojunctions [29]. The near-infrared energy Sun et al. synthesized the CdS quantum dots/defective ZnO 1−x -TiO 2−x Z scheme heterojunction via the combination of hydrothermal synthesis, chemical reduction, and electroless planting process [120]. The visible light absorption was enhanced by the formation of ZnO-TiO 2 heterojunction, and further improved after combining with CdS [120]. The formation process of the heterojunction, UV-vis spectra, and proposed charge transfer mechanism are shown in Figure 14 [120]. The charge separation efficiency and photocatalytic organic pollutant removal rate were much improved upon the formation of CdS QDs/defective ZnO 1−x -TiO 2−x Z scheme heterojunction [120]. Sun et al. synthesized the CdS quantum dots/defective ZnO1−x-TiO2−x Z scheme heterojunction via the combination of hydrothermal synthesis, chemical reduction, and electroless planting process [120]. The visible light absorption was enhanced by the formation of ZnO-TiO2 heterojunction, and further improved after combining with CdS [120]. The formation process of the heterojunction, UV-vis spectra, and proposed charge transfer mechanism are shown in Figure 14 [120]. The charge separation efficiency and photocatalytic organic pollutant removal rate were much improved upon the formation of CdS QDs/defective ZnO1−x-TiO2−x Z scheme heterojunction [120]. To further promote the visible light utilization and photo-induced charge separation and transfer efficiency, black TiO2-based tandem heterojunction photocatalysts have been proposed by researchers for photocatalytic hydrogen production. Sun et al. prepared a hierarchical hollow black TiO2/MoS2/CdS tandem heterojunction photocatalyst through the combination of the solvothermal method and high-temperature hydrogenation treatment [122]. The black TiO2/MoS2 heterojunction effectively enhanced the photon absorption in visible light and the near infrared region [122]. The tandem system further promoted visible light utilization compared to other combinations [122]. The schematic view of the construction of black TiO2/MoS2/CdS, UV-vis absorption spectra, and photocatalytic hydrogen evolution rate are illustrated in Figure 15. The charge separation and migration efficiency were much promoted by the formation of black TiO2/MoS2/CdS tandem heterojunction [122]. The tandem heterojunction exhibited the highest photocatalytic hydrogen production rate under AM 1.5 illumination [122]. In addition, a sandwich-like mesoporous black TiO2/MoS2/black TiO2 nanosheet photocatalyst was proposed for visible light photocatalytic hydrogen generation with a much-promoted photo-generated charge transfer efficiency [121]. The mesoporous TiO2 and Cu2S were also combined with MoS2 to synthesize the hierarchical tandem heterojunctions [29]. The near-infrared energy To further promote the visible light utilization and photo-induced charge separation and transfer efficiency, black TiO 2 -based tandem heterojunction photocatalysts have been proposed by researchers for photocatalytic hydrogen production. Sun et al. prepared a hierarchical hollow black TiO 2 /MoS 2 /CdS tandem heterojunction photocatalyst through the combination of the solvothermal method and high-temperature hydrogenation treatment [122]. The black TiO 2 /MoS 2 heterojunction effectively enhanced the photon absorption in visible light and the near infrared region [122]. The tandem system further promoted visible light utilization compared to other combinations [122]. The schematic view of the construction of black TiO 2 /MoS 2 /CdS, UV-vis absorption spectra, and photocatalytic hydrogen evolution rate are illustrated in Figure 15. The charge separation and migration efficiency were much promoted by the formation of black TiO 2 /MoS 2 /CdS tandem heterojunction [122]. The tandem heterojunction exhibited the highest photocatalytic hydrogen production rate under AM 1.5 illumination [122]. In addition, a sandwich-like mesoporous black TiO 2 /MoS 2 /black TiO 2 nanosheet photocatalyst was proposed for visible light photocatalytic hydrogen generation with a much-promoted photo-generated charge transfer efficiency [121]. The mesoporous TiO 2 and Cu 2 S were also combined with MoS 2 to synthesize the hierarchical tandem heterojunctions [29]. The near-infrared energy utilization was enhanced by the tandem system, thereby promoting the photothermal effect [29]. The visible light photocatalytic H 2 generation rate was significantly improved, achieving 3376.7 µmol h −1 g −1 , which was approximately 16 times that of black TiO 2 [29]. Nanomaterials 2023, 13, 468 19 of 28 utilization was enhanced by the tandem system, thereby promoting the photothermal effect [29]. The visible light photocatalytic H2 generation rate was significantly improved, achieving 3376.7 μmol h −1 g −1 , which was approximately 16 times that of black TiO2 [29].

Applications of Black TiO2
The absorption of light can be extended from the ultraviolet light to visible light, and even to near-infrared light by changing the white TiO2 into black TiO2 via various methods. This strategy can be utilized for enhancing the photocatalytic activity in the visible light range. Changing the color of TiO2 from white to black is one of most efficient ways for improving its photocatalytic efficiency in various fields, such as photocatalytic water splitting, photocatalytic pollutant degradation, etc.

Photocatalytic Water Splitting
Black TiO2 has a modified band structure, thereby improving the charge separation and migration efficiency. Its enhanced photocatalytic performance has been extensively investigated in water splitting. Black mesoporous TiO2 synthesized by Zhou et al. has excellent hydrogen production performance [79]. As shown in Figure 16A, black TiO2 had a higher rate of hydrogen production than original TiO2 under the condition of AM 1.5G and had excellent hydrogen production ability in visible light [79]. In addition, almost no attenuation was detected during photocatalytic measurements after 10 cycles [79]. As presented in Figure 16B, its apparent quantum efficiency at each single wavelength was much higher than that of the original sample [79]. The mesoporous black TiO2 photocatalysts showed remarkable photocatalytic stability in 10 cycling hydrogen evolution measurements within 30 h (each cycling test was conducted in the presence of fresh 1 mL methanol) [79]. The black TiO2 nanotubes prepared by Yang et al. showed an excellent photocatalytic hydrogen production performance (9.8 mmol h −1 g −1 ) through high temperature hydrogenation [80]. The enhanced photocatalytic activity could be attributed to two aspects: (1) the special one-dimensional hollow tube structure improved the charge separation efficiency; (2) the high temperature hydrogenation strategy improved its ability for

Applications of Black TiO 2
The absorption of light can be extended from the ultraviolet light to visible light, and even to near-infrared light by changing the white TiO 2 into black TiO 2 via various methods. This strategy can be utilized for enhancing the photocatalytic activity in the visible light range. Changing the color of TiO 2 from white to black is one of most efficient ways for improving its photocatalytic efficiency in various fields, such as photocatalytic water splitting, photocatalytic pollutant degradation, etc.

Photocatalytic Water Splitting
Black TiO 2 has a modified band structure, thereby improving the charge separation and migration efficiency. Its enhanced photocatalytic performance has been extensively investigated in water splitting. Black mesoporous TiO 2 synthesized by Zhou et al. has excellent hydrogen production performance [79]. As shown in Figure 16A, black TiO 2 had a higher rate of hydrogen production than original TiO 2 under the condition of AM 1.5G and had excellent hydrogen production ability in visible light [79]. In addition, almost no attenuation was detected during photocatalytic measurements after 10 cycles [79]. As presented in Figure 16B, its apparent quantum efficiency at each single wavelength was much higher than that of the original sample [79]. The mesoporous black TiO 2 photocatalysts showed remarkable photocatalytic stability in 10 cycling hydrogen evolution measurements within 30 h (each cycling test was conducted in the presence of fresh 1 mL methanol) [79]. The black TiO 2 nanotubes prepared by Yang et al. showed an excellent photocatalytic hydrogen production performance (9.8 mmol h −1 g −1 ) through high temperature hydrogenation [80]. The enhanced photocatalytic activity could be attributed to two aspects: (1) the special onedimensional hollow tube structure improved the charge separation efficiency; (2) the high temperature hydrogenation strategy improved its ability for sunlight utilization [80]. The photocatalytic activity of the black TiO 2 nanotubes for H 2 production remained stable for 5 cycles in 15 h using H 2 PtCl 6 as the co-catalyst and methanol as the sacrificing agent [80]. sunlight utilization [80]. The photocatalytic activity of the black TiO2 nanotubes for H2 production remained stable for 5 cycles in 15 h using H2PtCl6 as the co-catalyst and methanol as the sacrificing agent [80]. Two-dimensional lamellar structures with plenty of active sites are often used in TiO2 photocatalysis because of their large specific surface area. The black TiO2 nanosheets prepared by Zhang et al. shortened the band gap to 2.85 eV, thereby broadening the light response to the visible light region [123]. The hydrogen production rate was up to 165 μmol h −1 0.05 g −1 , which was twice as much as that of the original sample [123]. The chemical stability, light corrosion resistance, and photocatalytic activity for H2 generation were confirmed by 5 cycling tests in 25 h, using H2PtCl6 and methanol as the co-catalyst and sacrificing agent, respectively [123]. Similarly, Wu et al. synthesized another black TiO2 nanosheets, which had a high hydrogen production rate of 3.73 mmol h −1 g −1 [83]. This photocatalyst exhibited an unchanged photocatalytic H2 evolution rate in 5 cycling measurements with 15 h [83]. Li et al. designed and synthesized black TiO2 nanospheres by the self-assembly solvothermal method combined with the hydrogenation strategy [81]. The charge separation efficiency had been effectively improved after the hydrogenation process, confirmed by experimental results and DFT calculations [81]. The photocatalytic performance for H2 formation was also repeated 5 times in 15 h and remained stable in the 5 cycles [81].
The black rutile TiO2 prepared by Xiao et al. by the solid-phase reduction method showed much-enhanced hydrogen production performance, stability, and high apparent quantum efficiency, which was about 1.5 times that of the original sample ( Figure 17) [87]. In addition, the defects in TiO2 could be regulated by varying the hydrogenation temperature, and the optimal hydrogenation temperature was proved to be 300 °C [87]. The stability of photocatalytic activity was verified by 5 cycling hydrogen formation measurements in 12 h under AM 1.5 illumination [87]. No obvious decrease in the activity was observed in the cycling tests [87]. The maximum hydrogen production rate of black TiO2 synthesized by Sinhamahapatra et al. using the controllable magnesium thermal reduction method was 43 mmol h -1 g -1 in the full solar wavelength range with excellent stability, which was better than the black TiO2 material previously reported [89]. The black TiO2 nanoparticles presented great stability in the photocatalytic hydrogen evolution confirmed by 10 cycling measurements, which were conducted for 10 consecutive days using the same solution [89]. The aging of the black TiO2 materials was generally not mentioned in the reported publications. The photocatalytic stability of the hydrogen generation was  [123]. The hydrogen production rate was up to 165 µmol h −1 0.05 g −1 , which was twice as much as that of the original sample [123]. The chemical stability, light corrosion resistance, and photocatalytic activity for H 2 generation were confirmed by 5 cycling tests in 25 h, using H 2 PtCl 6 and methanol as the co-catalyst and sacrificing agent, respectively [123]. Similarly, Wu et al. synthesized another black TiO 2 nanosheets, which had a high hydrogen production rate of 3.73 mmol h −1 g −1 [83]. This photocatalyst exhibited an unchanged photocatalytic H 2 evolution rate in 5 cycling measurements with 15 h [83]. Li et al. designed and synthesized black TiO 2 nanospheres by the self-assembly solvothermal method combined with the hydrogenation strategy [81]. The charge separation efficiency had been effectively improved after the hydrogenation process, confirmed by experimental results and DFT calculations [81]. The photocatalytic performance for H 2 formation was also repeated 5 times in 15 h and remained stable in the 5 cycles [81].
The black rutile TiO 2 prepared by Xiao et al. by the solid-phase reduction method showed much-enhanced hydrogen production performance, stability, and high apparent quantum efficiency, which was about 1.5 times that of the original sample ( Figure 17) [87]. In addition, the defects in TiO 2 could be regulated by varying the hydrogenation temperature, and the optimal hydrogenation temperature was proved to be 300 • C [87]. The stability of photocatalytic activity was verified by 5 cycling hydrogen formation measurements in 12 h under AM 1.5 illumination [87]. No obvious decrease in the activity was observed in the cycling tests [87]. The maximum hydrogen production rate of black TiO 2 synthesized by Sinhamahapatra et al. using the controllable magnesium thermal reduction method was 43 mmol h -1 g -1 in the full solar wavelength range with excellent stability, which was better than the black TiO 2 material previously reported [89]. The black TiO 2 nanoparticles presented great stability in the photocatalytic hydrogen evolution confirmed by 10 cycling measurements, which were conducted for 10 consecutive days using the same solution [89]. The aging of the black TiO 2 materials was generally not mentioned in the reported publications. The photocatalytic stability of the hydrogen generation was mostly measured in 5 cycling tests within 15 h using the same solution. Some photocatalysts were tested in 10 repeated photocatalytic hydrogen formation measurements for more than 20 h. Black TiO 2 photocatalysts usually showed good stability and light corrosion resistance in the photocatalytic H 2 evolution reaction, providing the possibility of long-term usage of black TiO 2 photocatalysts for H 2 production. Nanomaterials 2023, 13, 468 21 of 28 mostly measured in 5 cycling tests within 15 h using the same solution. Some photocatalysts were tested in 10 repeated photocatalytic hydrogen formation measurements for more than 20 h. Black TiO2 photocatalysts usually showed good stability and light corrosion resistance in the photocatalytic H2 evolution reaction, providing the possibility of long-term usage of black TiO2 photocatalysts for H2 production.

Photocatalytic Degradation of Pollutants
In addition to the hydrogen production, pollutant degradation is also one of the main applications of photocatalysis. Titanium dioxide was often used in the photocatalytic degradation of organic dye and pesticides [124][125][126]. Black TiO2 with an enhanced light absorption ability would have much improved the photocatalytic pollutant removal efficiency. The black TiO2 obtained by CaH2 reduction not only presented an enhanced hydrogen generation rate, which was 1.7 times that of the original sample, but also achieved a huge improvement in the degradation of pollutants with the complete removal of methyl orange within 8 min [72]. Hamad et al. synthesized black TiO2 using a new method of controlled hydrolysis [85]. The oxygen vacancy concentration was significantly increased with a much-reduced band gap, thereby showing an excellent organic pollutant degradation rate under visible light irradiation [85].
The oxygen vacancy plays an important role in photocatalysis. Black TiO2 prepared by Teng et al. via vapor deposition had a high photocatalytic oxidation activity for organic pollutants in the water, due to the formation of Ti-H bonds and a large number of oxygen vacancies [77]. All pollutants (rhodamine B) could be completely degraded within 50 min detected by the UV-vis spectrophotometer [77]. The defective TiO2−x prepared by the anodic oxidation method was characterized by the electron paramagnetic resonance spectroscopy, confirming the existence of oxygen vacancies and the extension of the absorption from the ultraviolet to visible light region [76]. This black TiO2 material showed excellent photocatalytic degradation activity for rhodamine B under 400-500 nm light irradiation [76].
The black TiO2-based heterojunction could significantly improve its photocatalytic efficiency in pollutant remediation thanks to the enhanced charge separation and transfer efficiency. Jiang et al. prepared black TiO2/Cu2O/Cu composites via in-situ photodeposition and the solid reduction method [127]. The light energy harvesting in the visible and infrared range was much enhanced after the formation of the composites [127]. The photocatalytic efficiency of the composites for Rhodamine B degradation was improved compared with the commercial P25, due to the enhanced charge separation efficiency [127]. Qiang et al. synthesized the RuTe2/black TiO2 photocatalyst through gel calcination and the microwave-assisted process [128]. The light absorption range of the as-made composites was enlarged compared to the pristine TiO2. The photocatalytic efficiency of the diclofenac degradation was 1.2 times higher than the pure black TiO2 [128]. The stability of RuTe2/black TiO2 for the photocatalytic diclofenac degradation was confirmed via 5 repeated experiments [128]. Reprinted with permission for ref. [87]. Copyright 2018, Springer.

Photocatalytic Degradation of Pollutants
In addition to the hydrogen production, pollutant degradation is also one of the main applications of photocatalysis. Titanium dioxide was often used in the photocatalytic degradation of organic dye and pesticides [124][125][126]. Black TiO 2 with an enhanced light absorption ability would have much improved the photocatalytic pollutant removal efficiency. The black TiO 2 obtained by CaH 2 reduction not only presented an enhanced hydrogen generation rate, which was 1.7 times that of the original sample, but also achieved a huge improvement in the degradation of pollutants with the complete removal of methyl orange within 8 min [72]. Hamad et al. synthesized black TiO 2 using a new method of controlled hydrolysis [85]. The oxygen vacancy concentration was significantly increased with a much-reduced band gap, thereby showing an excellent organic pollutant degradation rate under visible light irradiation [85].
The oxygen vacancy plays an important role in photocatalysis. Black TiO 2 prepared by Teng et al. via vapor deposition had a high photocatalytic oxidation activity for organic pollutants in the water, due to the formation of Ti-H bonds and a large number of oxygen vacancies [77]. All pollutants (rhodamine B) could be completely degraded within 50 min detected by the UV-vis spectrophotometer [77]. The defective TiO 2−x prepared by the anodic oxidation method was characterized by the electron paramagnetic resonance spectroscopy, confirming the existence of oxygen vacancies and the extension of the absorption from the ultraviolet to visible light region [76]. This black TiO 2 material showed excellent photocatalytic degradation activity for rhodamine B under 400-500 nm light irradiation [76].
The black TiO 2 -based heterojunction could significantly improve its photocatalytic efficiency in pollutant remediation thanks to the enhanced charge separation and transfer efficiency. Jiang et al. prepared black TiO 2 /Cu 2 O/Cu composites via in-situ photodeposition and the solid reduction method [127]. The light energy harvesting in the visible and infrared range was much enhanced after the formation of the composites [127]. The photocatalytic efficiency of the composites for Rhodamine B degradation was improved compared with the commercial P25, due to the enhanced charge separation efficiency [127]. Qiang et al. synthesized the RuTe 2 /black TiO 2 photocatalyst through gel calcination and the microwave-assisted process [128]. The light absorption range of the as-made composites was enlarged compared to the pristine TiO 2 . The photocatalytic efficiency of the diclofenac degradation was 1.2 times higher than the pure black TiO 2 [128]. The stability of RuTe 2 /black TiO 2 for the photocatalytic diclofenac degradation was confirmed via 5 repeated experiments [128].
Tetracycline is a toxic antibiotic which is difficult to remove. Li et al. synthesized black TiO 2 modified with Ag/La presented an improved visible light photocatalytic performance for the tetracycline degradation [26]. The photocatalytic stability and reusability of black TiO 2 -based photocatalysts were studied via 5 cycling tests without apparent deactivation [26]. Wu et al. reported that the synthesized black anatase TiO 2 exhibited impressive photocatalytic degradation of tetracycline [84]. Its degradation efficiency of tetracycline was 66.2% under the visible light illumination, which was higher than that of the white titanium dioxide and doped titanium dioxide [84]. In addition, ·O 2− and h + were found to play important roles in the degradation process, which was different from the original TiO 2 , providing new insights for environmental protection [84]. The stability of the photocatalytic tetracycline degradation was measured in four repeated experiments within 960 min without apparent deactivation after four cycles [84]. Table 2 summarizes the applications and photocatalytic stability of the black TiO 2 nanomaterials. The long-term photocatalytic stability of pollutant removal is often overlooked and unverified in most reported research. Therefore, researchers should pay more attention to aging and photocatalytic stability in pollutant degradation in the future. Anatase H 2 Diclofenac degradation 5 [128] Notes: N/S: not studied; : yes.

Summary and Outlooks
The utilization and conversion of sunlight in a more efficient way has gained much interest due to the energy crisis and global warming effect. The construction of black TiO 2 -based materials is proved to be an effective approach for promoting visible light utilization. Black TiO 2 with various morphologies, such as nanospheres, nanotubes, nanowires, etc., have been rationally designed. The properties of black TiO 2 , such as the surface area, are easily affected by its morphology. Although the photon absorption of TiO 2 in the visible light region can be effectively increased after the surface reduction process, its photocatalytic performance still needs to be improved for practical applications. The photocatalytic activity of black TiO 2 photocatalysts can be further enhanced by three main methods: element doping, decoration with metal nanoparticles, and fabrication of heterojunctions. The introduction of metal or nonmetal elements into the black TiO 2 lattice can create new impurities, thus narrowing its band gap. The SPR effect caused by metal decoration on the black TiO 2 surface can efficiently improve its visible light utilization. The fabrication of black TiO 2 heterojunctions, including type II, Z scheme, and tandem heterojunctions, can significantly enhance the photo-induced charge separation and transfer efficiency, thereby promoting the photocatalytic performance. The photocatalytic activity of black TiO 2 -based materials are mainly evaluated in the photocatalytic hydrogen production and pollutant removal. Although these black TiO 2 -based nanomaterials exhibit excellent photocatalytic activity in the visible light region, technologies for enhancing light harvesting in nearinfrared should be developed. In addition, the enhancement of the photo-generated charge separation and transfer should be further reinforced to meet the standard for practical application. Applications of black TiO 2 -based materials in industrial and outdoor fields, such as self-cleaning surfaces, should also be investigated in future.