Black Titanium Dioxide Nanomaterials in Photocatalysis

Titanium dioxide (TiO2) nanomaterials are widely considered to be state-of-the-art photocatalysts for environmental protection and energy conversion. However, the low photocatalytic efficiency caused by large bandgap and rapid recombination of photoexcited electrons and holes is a challenging issue that needs to be settled for their practical applications. Structure engineering has been demonstrated to be a highly promising approach to engineer the optical and electronic properties of the existing materials or even endow them with unexpected properties. Surface structure engineering has witnessed the breakthrough in increasing the photocatalytic efficiency of TiO2 nanomaterials by creating a defect-rich or amorphous surface layer with black color and extension of optical absorption to the whole visible spectrum, along with markedly enhanced photocatalytic activities. In this review, the recent progress in the development of black TiO2 nanomaterials is reviewed to gain a better understanding of the structure-property relationship with the consideration of preparation methods and to project new insights into the future development of black TiO2 nanomaterials in photocatalytic applications.


Introduction
Titanium dioxide (TiO 2 ) nanomaterials have been considered as the most promising semiconductor photocatalysts for pollutant removal and energy generation owing to their relatively good photocatalytic activity, low cost, nontoxicity, and high stability since the discovery of hydrogen evolution through the photoelectrochemical water splitting on TiO 2 electrode [1][2][3].Meanwhile, photocatalysis potentially can be an ideal solution to current environmental issues and energy crisis by only consuming solar energy.Over the past decades, nanotechnology has greatly contributed to the development of TiO 2 materials in photocatalysis across the globe [2][3][4][5][6][7].However, the large bandgaps (3.0-3.2 eV over different phases) of TiO 2 nanomaterials limit their optical absorption to ultraviolet (UV) light, along with the rapid recombination of photo-excited electrons and holes, resulting in low photocatalytic efficiency [1][2][3][4][5][6][7][8][9][10][11].Extending the utilization of solar energy to visible region has thus been the urgent need for practical applications of TiO 2 nanomaterials.
The optical and electronic properties of solid materials highly depend on the structure including the way the atoms are bonded and arranged, the phases and their distribution, and the defects [12][13][14][15][16]. Therefore, tuning these states in solid materials can potentially tailor the optical and electronic properties of the existing solid materials [17][18][19].Since the discovery of visible-light active nitrogen-doped TiO 2 [19], structural modification of TiO 2 nanomaterials has been at the research frontier to extend the utilization of solar light to visible region, while other methods, such as localized surface plasmon resonance of plasmonic nanostructures [18,20,21], have been exploited.In 2011, the electronic band structure of black TiO 2 nanocrystals was reported to be largely narrowed for massive visible light absorption and conversion to chemical energy, leading to markedly enhanced photocatalytic activity towards photocatalytic pollution removal and hydrogen generation from water [22].Since then black TiO 2 nanomaterials have attracted unprecedented interest in visible light utilization.Over the past decade, many methods, such as hydrogenation, aluminum reduction, and chemical reduction, have been developed to synthesize black TiO 2 nanomaterials.In this review, we focus on the recent research progress in black TiO 2 nanomaterials for photocatalytic applications, especially for photocatalytic hydrogen generation and pollutant removal.The basic properties of black TiO 2 nanomaterials are first discussed in brief, and then, typical examples are given for each preparation method.We aim to get a better understanding on the relationship between the structure and photocatalytic properties of black TiO 2 nanomaterials from different preparation methods, along with considering the effect of preparation methods on their structures.Finally, summary and perspectives on the development of black TiO 2 nanomaterials are addressed.

Basic Properties of Black TiO 2 Versus
White TiO 2 TiO 2 exists in different phases.Anatase, rutile, brookite, and TiO 2 (B) are the four main polymorphs of TiO 2 .It is widely acknowledged that rutile is thermodynamically the most stable phase, while anatase, brookite, and TiO 2 (B) are metastable phases.The metastable phases of TiO 2 will go through phase transformation at elevated temperatures.Usually, it is considered that anatase goes through phase transformation to rutile [23,24] and brookite and TiO 2 (B) to anatase then to rutile [25,26].However, exceptions have been reported.Direct transformation of brookite to rutile is observed, while in the case of the anatase-brookite mixture, anatase transforms firstly to brookite and then to rutile [27].It is suggested that the phase transformation process is affected by various parameters.Generally, anatase and rutile TiO 2 nanomaterials can be easily prepared using conventional preparation methods, while the preparation of brookite and TiO 2 (B) nanomaterials needs special attention.Anatase is relatively stable, and the phase transformation between anatase and rutile is dependent on a variety of factors, such as particle size and defects in anatase TiO 2 [28,29].So far, anatase and rutile TiO 2 nanomaterials are the most widely studied photocatalysts.
Different phases have different structures.Generally, all four types of polymorphs mentioned above comprise of TiO 6 octahedra, but differ in the distortion of the octahedron units and share edges and corners in different manners (Figure 1) [25].The differences in TiO 6 octahedra arrangement result in different physicochemical properties and thus  International Journal of Photoenergy different photocatalytic activities.To the best of our knowledge, black TiO 2 nanomaterials have been prepared from white anatase, rutile, brookite, and TiO 2 (B).It should be noted that phase transformation was observed in a very few cases when white TiO 2 nanomaterials converted into black ones during the modification process [23].
2.1.Basic Structure.Color change from white to black for TiO 2 nanomaterials reflects the change in optical properties and thus suggests the structural change after modification, at least the surface structural change.High-resolution transmission electron microscope (HRTEM) has revealed that a number of black TiO 2 nanomaterials have a crystalline/ amorphous core/shell structure, while white ones have clear lattice fringes throughout the crystals [22,23,[30][31][32][33][34][35][36][37][38][39][40][41][42][43][44].The amorphous or disordered surface layer has been considered the typical feature of black TiO 2 nanomaterials.Figure 2 shows the HRTEM images of black and white TiO 2 nanomaterials with Figure 2(a) showing the typical core/shell structure of the black TiO 2 [33].The combination of X-ray diffraction (XRD) and Raman measurements has also confirmed the surface structural differences between black and white TiO 2 nanomaterials.However, it should be noted that not all black TiO 2 nanomaterials have the crystalline/ disordered core/shell structure.
2.2.Surface Functional Group.Surface -OH groups exist in many TiO 2 nanomaterials depending on the preparation method.As hydrogen is used in hydrogen thermal treatment and hydrogen plasma treatment, it is very possible to generate -OH groups on the surface of hydrogenated black TiO 2 nanomaterials owing to the reduction effect of hydrogen.An increase in −OH groups has been detected in hydrogenated black TiO 2 nanomaterials by X-ray photoelectron spectroscope (XPS) [22,45].Fourier transform infrared (FTIR) spectroscopy is another important technique to reveal the change in surface −OH groups by comparing the magnitude of the intensity of the peak corresponding to the −OH vibrational band [33,34,46].In addition, 1 H nuclear magnetic resonance (NMR) technique has also been used to analyze the surface -OH groups [33,34].
The existence of -H groups on the surface of black TiO 2 nanomaterials is very debatable.They have only been detected in a few cases of hydrogenated TiO 2 nanomaterials [33,47].Wang et al. attributed the peak at 457.1 eV in the Ti 2p XPS spectrum of the hydrogenated black TiO 2 nanocrystals to surface Ti-H bonds [33].Zheng et al. found that hydrogenated TiO 2 nanowire microspheres exhibited one shoulder peak at the lower binding energy side of the broader Ti 2p peak in the XPS spectrum and attributed it to the surface Ti-H bonds formed under hydrogen atmosphere [47].Formation of surface Ti-H bonds was at the expense of surface Ti-OH groups [47].Such groups do not exist on the surface of white TiO 2 nanomaterials undoubtedly.
Oxygen vacancies and/or Ti 3+ impurities are believed to be highly related to the color change of the defectrich TiO 2 [14,[61][62][63][64][65][66].More importantly, oxygen vacancies and/or Ti 3+ defects extend the photoresponse of TiO 2 from UV to visible light region, which leads to high visible-light photocatalytic activity [14,59,[67][68][69][70][71][72].Note that high density of crystal defects, especially bulk defects, may accelerate electron-hole recombination as defects can  3 International Journal of Photoenergy act as charge annihilation centers [57,73,74].It is likely that surface oxygen vacancies are responsible for the enhanced photocatalytic activity [39,70,75], while bulk oxygen vacancies act as trap states and charge carrier recombination centers [76].Studies also showed that surface Ti 3+ defects could enhance hole trapping and thus facilitate the separation of photo-excited electrons and holes [57], while bulk Ti 3+ sites acted as charge annihilation centers, leading to enhanced nonradiative recombination and shorter lifetime of electrons and holes [57,60].In addition, Ti 3+ ions with oxygen vacancies can improve the electrical conductivity of TiO 2 [73,77], which may enhance the charge transport and charge-transfer reaction [73].
2.4.Electronic Band Structure.The introduction of defects and/or surface disorder could result in change in electronic and optical properties of black TiO 2 .Bandgap narrowing is a vivid demonstration of the change in band structure of black TiO 2 .The origin of the bandgap narrowing has been argued for years owing to the complexity of the surface defects/disorders and functional groups.One assumption is that Ti 3+ species in the bulk TiO 2 is responsible for the band gap narrowing [14,59,67,68], and the oxygen-vacancy midgap states further enhance the light absorption for photon energy below the direct bandgap by indirect electron transitions [14].Chen et al. thought that bandgap narrowing mainly originated from the disorder-induced midgap states rather than the Ti 3+ species, greatly upshifting the valence band (VB) edge of black TiO 2 , and that the possible conduction band (CB) tail states arising from the surface disorder could only slightly narrow the bandgap (Figure 3(a)) [22,34].Naldoni's group agreed with these analyses and further pointed out that oxygen vacancies could introduce localized states at 0.7-1.0eV below the CB minimum of black TiO 2 [23].As shown in the schematic illustration of the density of states (DOS) of black and white TiO 2 (Figure 3(b)), Wang et al. reported that the CB and VB tails slightly narrowed the bandgap by 0.8 eV and that the Ti-H bonds introduced the midgap electronic states at 0.92-1.37eV below the CB minimum of black TiO 2 [33].In summary, the change in band structure is mainly attributed to the tailing of VB and/or CB, and the midgap states are induced by oxygen vacancies or −H groups [22,23,33,39].

Fundamental Physicochemical Process in Photocatalysis
A simplified model representing the fundamental physicochemical process in photocatalysis is demonstrated in Figure 4.A typical process of photocatalysis involves three steps: light absorption, electron-hole separation, and surface reaction.Light with energy greater than the bandgap of TiO 2 nanocrystal excites an electron from the VB to the CB; meanwhile, a positive hole will be left in the VB.In the case of anatase TiO 2 with a bandgap of 3.2 eV, UV light with λ ≤ 387 nm is required for electron excitation.The electrons and holes that have been separated and transferred onto the surface of TiO 2 can trigger redox reactions (pathways 1 and 2).For example, the electrons scavenged by O 2 can yield superoxide radical anions, while the holes that react with H 2 O can produce hydroxyl radicals.These radicals can oxidize organic species, such as methylene blue [6], rhodamine B and salicylic acid [8], and methylene orange [9].Thus, photocatalysis can be applied in degradation of pollutants [6,8,9], reduction of CO 2 [10], and water splitting [1,6].However, electrons and holes may recombine on the surface (surface recombination, namely, pathway 3) or even in the bulk (volume recombination, namely, pathway 4) [74], which will compete with the desired redox reactions and thus greatly decrease the efficiency of the photocatalytic process.
To increase the utilization of solar energy, the bandgap of TiO 2 needs to be engineered to extend its optical absorption from UV to visible even to infrared light.On the other hand, the charge carrier separation must be exponentially enhanced in order to improve the photocatalytic efficiency and achieve the ultimate practical applications of TiO 2 in photocatalysis.[22].The raw TiO 2 nanocrystals were prepared with a precursor solution consisting of titanium tetraisopropoxide, ethanol, hydrochloric acid, deionized water, and Pluronic F127 as an organic template, followed by hydrolysis ay 40 °C for 24 h, solvent evaporation at 110 °C for 24 h, and calcination at 500 °C for 6 h in air [22].The disorder-engineered black TiO 2 nanomaterials contain two phases with a core/shell structure: a crystalline core and a disordered or amorphous shell (Figure 5).The crystalline phase of the black TiO 2 maintained the anatase structure of the white raw TiO 2 as evidenced by XRD analysis, whereas peak broadening and extra peaks besides the typical signals of anatase TiO 2 were observed in the Raman spectrum of the black TiO 2 owing to the disordered nature of the surface layer [22].When used as photocatalysts, the degradation rate of methylene blue on the black TiO 2 nanocrystals was found to be nearly 7 times that on the raw TiO 2 nanocrystals, and the photocatalytic H 2 production rate of the black TiO 2 using 1 : 1 watermethanol solution under sunlight reached as high as 10 mmol•h −1 g −1 [22].The high photocatalytic activity was ascribed to the substantially narrowed bandgap (experimentally ~1.54 eV) induced by the surface disorder, thus extending the optical absorption from UV to infrared region [22].First-principle density functional theory calculation showed that two midgap states (centered at ~1.8 and ~3.0 eV corresponding to the VB and CB band tails, resp.) were created in the black TiO 2 nanocrystals and thus well explained the origin of the change in the electronic and optical properties of black TiO 2 nanocrystals [22].Black rutile TiO 2 nanowires have been prepared by hydrogenation in a tube furnace filled with ultrahigh purity hydrogen gas at temperatures ≥ 450 °C for 30 min for photoelectrochemical water splitting [45].Pristine rutile TiO 2 nanowires were grown on a fluorine-doped tin oxide (FTO) glass substrate by hydrothermal method with titanium nbutoxide in aqueous hydrochloric acid solution at 150 °C for 5 h, followed by annealing in air at 550 °C for 3 h to increase the crystallinity of TiO 2 nanowires and improve their contact to the substrate [45].In this study, the authors found that the enhanced photoactivity of the black TiO 2 nanowires was mainly attributed to the facilitated charge transport in black TiO 2 and charge separation at the TiO 2 /electrolyte interface owing to the increased donor density or oxygen vacancies, while the visible light absorption only made a negligible contribution [45].Further study showed that the electronic band structure of the black TiO 2 nanowires was similar to that of the pristine TiO 2 nanowires, and the color change was owed to the impurity/defect states in the bandgap of the black TiO 2 nanowires caused by oxygen vacancies [45].Similar enhancement in photoactivity of the hydrogenated anatase TiO 2 nanotubes was observed in this study [45].Liu et al. fabricated black anatase TiO 2 nanotubes by anodic oxidation and hydrogenation [52].In a typical synthesis, clean titanium foils were anodized in a two-electrode configuration with platinum gauze as the counter electrode in the electrolyte containing ethylene glycol (less than 0.2 wt.% H 2 O), deionized water (1 M), and NH 4 F (0.1 M) at 60 V for 15 min; black TiO 2 nanotubes were then achieved by atmospheric pressure hydrogenation in H 2 /Ar (5%) flow at 500 °C for 1 h [52].The authors compared the effect of a series of factors on the photoactivity of the hydrogenated TiO 2 .It was found that atmospheric pressure (H 2 /Ar) hydrogenated anatase TiO 2 nanotubes had more oxygen vacancies, while high-pressure (20.0 bar, 500 °C for 1 h) hydrogenated anatase TiO 2 nanotubes possessed more Ti 3+ species [52].Photocatalytic experiments demonstrated that high-pressure hydrogenated anatase TiO 2 nanotubes exhibited a high H 2 evolution rate of 7 μmol•h −1 cm −2 without any cocatalysts, while atmospheric pressure hydrogenated anatase TiO 2 nanotubes had negligible H 2 evolution [52].For reference, rutile nanorods hydrogenated in either atmospheric pressure or high-pressure hydrogen showed extremely small H 2 evolution rate [52].It is believed that different polymorphs using anatase and rutile as examples have different defect formation behaviors upon reductive treatments [66,81,82] and thus lead to different photoactivities of treated TiO 2 nanomaterials.Recently, ordered mesoporous black anatase TiO 2 was prepared through an evaporation-induced self-assembly method combined with an ethylenediamine encircling process, followed by atmospheric pressure hydrogenation at 500 °C for 3 h under H 2 flow [78].Figure 6(a) displays the schematic synthesis process.Interestingly, the ordered mesoporous TiO 2 prepared with ethylenediamine turned into a black color after hydrogenation (Figure 6(b)), whereas only gray TiO 2 was obtained with the porous TiO 2 synthesized without ethylenediamine [78].This suggested that the original surface functionalities and surface structural defects may be the very important factors determining the colorization or types of defects or electronic band structure of the hydrogenated TiO 2 nanomaterials.The VB XPS analysis found that the VB maximum energy for the black TiO 2 blue-shifted towards the vacuum level at ~1.6 eV owing to the possible Ti 3+ species, narrowing the bandgap to ~2.80 eV consistent with the experimental value of 2.82 eV [78].The resultant ordered mesoporous black TiO 2 showed an extended photoresponse from UV light to visible and infrared light regions and thus exhibited a high photocatalytic hydrogen evolution rate of 136.2 μmol•h −1 , which approached two times that (76.6 μmol•h −1 ) of the pristine ordered mesoporous TiO 2 [78].
4.2.Black TiO 2 Nanomaterials by Hydrogen Plasma.Hydrogen plasma technology has attracted increasing interest owing to its effectiveness in engineering surface-disordered TiO 2 nanomaterials with a typical crystalline/amorphous core/shell structure [33,39,57,60].Ti 3+ species and oxygen vacancies were reported to be the primary defects in some cases [57,58], while Ti-H groups and oxygen vacancies were believed to be the dominant defects in other cases [33,39].
H-doped black titania (TiO 2−x H x ) with high solar absorption (~83%) was converted from commercial TiO 2 (Degussa P25) in a thermal plasma furnace by hydrogen plasma for 4-8 h at 500 °C with the plasma input power of 200 W [33].The TiO 2−x H x presented a crystalline/amorphous core/shell structure, and the black coloration of TiO 2−x H x was possibly caused by the defects including H doping, oxygen vacancies and surface hydroxyl groups [33].As discussed above in Section 2.4, the tailing effect of VB and CB and the midgap states caused by H doping contributed to the Vis-NIR absorption of the black TiO 2−x H x and thus highly enhanced photocatalytic activity [33].The TiO 2−x H x showed much higher solar-to-electron efficiency in both photocatalytic water splitting (Figure 7(a)) and degradation of methyl orange over pristine TiO 2 (Figure 7(b)) and demonstrated a high cycling stability (Figure 7(c)) [33].
Black TiO 2 nanocrystals with the typical crystalline/ amorphous core/shell structure were prepared from hydrogen plasma treatment of commercial TiO 2 nanoparticles (Degussa P25) by Yan et al. [57].Surprisingly, the authors found that slightly hydrogenated TiO 2 nanocrystals without visible coloration exhibited enhanced photoactivity in both photocatalytic degradations of methylene blue and reduction of CO 2 with H 2 O, while gray and black TiO 2 showed worse photoactivity over pristine TiO 2 [57].It was proposed that improved photocatalytic performance of slightly hydrogenated TiO 2 could be attributed to the higher ratio of trapped holes (O − centers) and a lower recombination rate induced by the increase of surface defects, while the highly concentrated bulk defects in gray and black (overhydrogenated) TiO 2 acted as charge recombination centers, leading to worse photoactivity [57].Huang's group has reported on the synthesis of a series of black TiO 2 nanomaterials by aluminum reduction [9,32,37,38,41,53].The typical crystalline/amorphous core/shell structure was observed in almost all Al-reduced black TiO 2 [9,32,37,38,41], except for the black TiO 2 nanotubes [53].Also, Ti 3+ and oxygen vacancy defects were commonly detected in Al-reduced black TiO 2 [9,32,37,38,41,53].In a typical synthesis of the Alrecued black TiO 2 , aluminum and pristine TiO 2 (Degussa P25) were placed separately in a two-zone tube furnace (Figure 8(a)); the pressure in the tube was controlled at a base pressure below 0.5 Pa, and then in order to trigger the reduction reaction, aluminum was heated at 800 °C while TiO 2 was heated at 300-500 °C for 6 h [32].As shown in Figure 8(b), black TiO 2 nanoparticles can be produced on a large scale with aluminum reduction method.A unique crystalline/ amorphous core-shell structure was observed on all Alreduced TiO 2 prepared at different temperatures, and the thickness of the disordered outer layer increased with the Al-reduction temperature (Figures 8(c), 8(d), 8(e), and 8(f)) [32].The black TiO 2 absorbed ~65% of the total solar energy by improving visible and infrared absorption and thus exhibited markedly high photoactivity in both photocatalytic water splitting and degradation, superior to the pristine TiO 2 (~5% solar energy absorption) [32].[83,84]) by CaH 2 usually generates black Ti 2 O 3 rather than black TiO 2 [83,84].Recently, black TiO 2 was prepared from Degussa P25 by CaH 2 reduction at 400 °C [42].The black TiO 2 had a crystalline/amorphous core/shell structure with abundant oxygen vacancies, which led to a high solar absorption (~81% solar energy absorption) and significantly enhanced photocatalytic organic degradation and water-splitting performance [42].

Magnesium Reduction. Recently, Sinhamahapatra et al.
developed a new method to synthesize black TiO 2 with magnesium as the reductant [11].Typically, well-mixed reactant of commercial TiO 2 and magnesium powder was placed in a tube furnace and then heated at 650 °C for 5 h in the flow of 5% H 2 /Ar; the product was stirred for 24 h in 1.0 M HCl 8 International Journal of Photoenergy and then washed with sufficient amount of water to remove the acid and dried at 80 °C [11].A small amount of anatase was transformed into rutile during the reduction process [11].The maximum hydrogen production rates were 43 mmol•h −1 g −1 and 440 μmol•h −1 g −1 , along with remarkable stability under full solar wavelength light and visible light irradiation, respectively [11].This outstanding activity can be correlated with the extended absorption in visible light, perfect band position, the presence of an appropriate amount of Ti 3+ species and oxygen vacancies, and slower charge recombination [11].[86,87].Typically, 14 mg of metallic Li foils were dissolved in 20 mL of ethanediamine under dry conditions; then, 200 mg of TiO 2 nanocrystals (Degussa P25) was immersed into the ethanediamine solution for 6 h with continuous stirring; 1 M HCl was used to consume the excess Li or electrons when the reaction was complete; finally, the product was rinsed with deionized water several times and dried in vacuum oven at room temperature [86].Note that the Li-assisted reduction is phase selective: rutile phase is reduced into black TiO 2 while anatase phase is well-maintained [86].This offers us an opportunity to create abundant order/disorder junctions at the surface by controlling the phase composition in pristine TiO 2 for highly efficient photocatalytic hydrogen generation [86].The order/disorder/water junction was believed to efficiently internally drive the electron/hole separation through type-II bandgap alignment and to trigger a strong hydrogen evolution surface reaction [86].Furthermore, Zhang et al. found that the "crystal-deficient" layers on the surface of the rutile TiO 2 nanowires increased the conductivity by 50 times, which increased the electron diffusion length to ~20 μm and overcame the charge collection limitation at the solid/liquid interface for efficient conversion of solar energy to chemical energy [87].These studies highlight the importance of controlling the surface localization of defects and the solid/liquid interface towards enhanced photoactivity over TiO 2 photocatalysts [86][87][88].9 International Journal of Photoenergy [30].Typically, a yellowish gel was first obtained by reacting TiH 2 and H 2 O 2 for 12 h; then, the gel was diluted using ethanol, the pH of the mixture was adjusted to 9.0 by NaOH, and NaBH 4 as an antioxidant was added to the resulting mixture; after the solvothermal treatment at 180 °C for 24 h, the collected sample was washed with HCl, water, and ethanol; light blue TiO 2 nanocrystals (TiO 2−x ) were obtained after the precipitate was dried in vacuum for 12 h; finally, postannealing treatment was carried out at 300-700 °C for 3 h under a nitrogen flow [30].The annealing temperature was found to be a crucial factor affecting the color of the as-prepared TiO 2 nanocrystals [30].Light brown, brown, black, dark brown, and shallow dark brown TiO 2 nanocrystals were obtained at 300, 400, 500, 600, and 700 °C, respectively [30].It is found that the Ti 3+ species initially increased with annealing temperature up to 500 °C and then decreased with further temperature rise [30].The black TiO 2−x prepared at 500 °C had the highest Ti 3+ concentration and thus exhibited the highest photocatalytic activity [30].
The reported black TiO 2 nanomaterials with a nanotube morphology and an anatase phase are prepared by electrochemical reduction in ethylene glycol electrolytes [56,90,91].However, it should be noted that electrochemically reduced black TiO 2 in ethylene glycol electrolytes was not stable [56,91], because glycerol has a higher viscosity making it difficult for the protons to insert into TiO 2 [91].It is worth noting that the electrochemically reduced black TiO 2 nanotubes were recently found unstable in air [17].
Xu et al. reported on the electrochemically hydrogenated black TiO 2 nanotubes [90].The pristine anodic TiO 2 nanotubes were prepared at 150 V for 1 h in an ethylene glycol electrolyte containing 0.3 wt.% NH 4 F and 10 vol.%H 2 O with carbon rod and Ti foil as the cathode and anode, respectively [90].After the pristine TiO 2 nanotubes were annealed in air at 150 °C for 3 h and then 450 °C for 5 h, the electrochemical reduction was performed at 5 V for 5 to 40 s in 0.5 M Na 2 SO 4 aqueous solution at room temperature with the TiO 2 nanotubes as the cathode and Pt as the anode to achieve black TiO 2 nanotubes [90].The surface oxygen vacancies were considered to contribute to the substantially enhanced electrical conductivity and photoactivity [90].
Similarly, Li et al. synthesized black anatase TiO 2 nanotubes by electrochemical reduction [91].In their study, a so-called "activation" step was adopted in order to obtain stable black TiO 2 nanotubes, where anodization was carried out in an ethylene glycol solution of 0.2 M HF and 0.12 M H 2 O 2 at 60 V for 30 s before the electrochemical reduction synthesis of black TiO 2 nanotubes at a cathodic voltage of −40 V for 680 s in an ethylene glycol solution of 0.27 wt.% NH 4 F [91].The authors proposed the doping mechanism as follows:  reported on the water-plasma-assisted synthesis of black titania spheres (H-TiO 2−x ) with efficient visible-light photocatalytic activity [44].The H-TiO 2−x was composed of a mixture of rutile, anatase, and oxygen-deficient phases (e.g., Ti 10 O 19 , Ti 5 O 9 , and Ti 3 O 5 ).The abundant oxygen vacancies and Ti 3+ species with the presence of Ti 2+ species resulted in a narrowed bandgap of 2.18 eV [44].Under visible-light irradiation for 180 min, the methylene blue is almost completely degraded in the presence of H-TiO 2−x (90%), whereas it is degraded only by 18% in the presence of P25 [44].
4.6.2.Nitrogen Doping.Wei's group focused on the synthesis of crystalline/disordered core-shell black anatase TiO 2 (TiO 2 @TiO 2−x ) by a one-step calcination in N 2 [43,92].Typically, two mixture solutions were prepared: one containing tetrabutyl titanate, urea, and ethanol absolute and another containing hydrochloric acid, deionized water, and ethanol absolute; the latter was added dropwise to the former solution and stirred until white colloid was formed; the mixture was placed in a water bath at 35 °C for 30 min and then stirred magnetically for 2 h; the TiO 2 @TiO 2−x was obtained by annealing at 550 °C for 3 h in a nitrogen atmosphere [43].
Oxygen vacancies and nitrogen species were detected in TiO 2 @TiO 2−x which explained its narrowed bandgap and high visible light photocatalytic degradation performance on methyl orange [43].The authors also investigated the effect of urea concentration on the structure and photocatalytic activity of the black TiO 2 and found that a lower urea concentration triggered the largest amount of oxygen vacancies [43].
4.6.3.Electrochemical Oxidation.Defective black anatase TiO 2 nanotubes were synthesized via two-step anodization on Ti foil in ethylene glycol containing 0.25 wt.% NH 4 F and 2 vol.% distilled water at 60 V for 10 h, followed by calcination in the air (Figure 12) [54].The black TiO 2 with controllable level of defects exhibited a high photocatalytic activity under visible light [54].Mechanistic analysis and characterization results indicated that oxygen vacancies were formed in an oxygen-deficient environment during the anodization Figure 10: (a) UV-vis diffuse reflectance spectra and (b) CO 2 reduction rate of the TiO 2−x , T300, T500, and T700.TiO 2−x is the as-prepared TiO 2 by hydrothermal reaction.T300, T500, and T700 are the products after postannealing treatment at 300, 500, and 700 °C, respectively.Reprinted with permission from [89].Copyright (2016) Nature Publishing Group.4.6.4.Ionothermal Synthesis.Black Ti 3+ -doped anatase TiO 2 was synthesized by treating metal Ti in an N-N-dimethylformamide solution containing 1-methyl-imidazolium tetrafluoroborate (ionic liquid), lithium acetate, and acetic acid in an autoclave at 200 °C for 24 h [55].The ionic liquid enriched with fluorine and acetic acid play key roles in dissolving and oxidizing the Ti foil, respectively [55].The in situ generated H 2 from the reaction between Ti foil and acetic acid could be strongly adsorbed onto the surface of TiO 2 and dissociated into H atom to form H-TiO 2-x disordered layer [55].The Ti 3+ -rich black TiO 2 exhibited high activity in photocatalytic degradation of organic pollutants under visible light (λ > 420 nm) and also a high hydrogen evolution rate of 0.26 mmol•h −1 m −2 in water splitting under simulated solar light [55].4.6.5.Laser Irradiation.Black TiO 2 nanospheres were fabricated by laser irradiation on suspended solution containing 20 mg anatase TiO 2 nanospheres and 1 mL distilled water [93].Phase transformation from anatase to rutile was observed when laser irradiation was performed more than 15 min, and black TiO 2 was obtained after 120 min laser irradiation [93].The degradation ratio of rhodamine B with black TiO 2 nanospheres could reach up to 33% under green light for 5 h, while the pristine and P25 showed no degradation ability on rhodamine B [93].The high photoactivity was attributed to the Ti 3+ defects and disordered surface layer which resulted in a narrowed bandgap of 2.2 eV [93].Recently, black amorphous TiO 2 film was achieved by pulsed laser deposition at 100 °C for 10 min under vacuum condition using a commercial TiO 2 target and a KrF excimer laser at a repetition rate of 2 Hz with a laser fluence of 2 J•cm −2 [94].This black amorphous TiO 2 film was deposited on a predeposited crystalline TiO 2 film to construct a bilayer structure similar to the crystalline/amorphous core/shell structure of black TiO 2 nanoparticles in order to create a simpler model to elucidate the working mechanism of black TiO 2 nanomaterials in many applications [94].Metallic conduction was achieved at the crystalline/amorphous homointerface via electronic interface reconstruction [94].This points to a research direction that may partly eluciate the high performance of black TiO 2 in many applications.4.6.6.Proton Implantation.Liu's group applied proton implantation method to the synthesis of black TiO 2 nanotubes [95].Proton implantation was carried out at an energy of 30 keV and a nominal dose of 10 16 ions•cm −2 using a Varian 350 D ion implanter [95].While the ion implantation on a (001) surface plane of an anatase crystal led to a low H 2 12 International Journal of Photoenergy production efficiency, implantation of TiO 2 nanotubes markedly enhanced hydrogen evolution due to the length effect [95].That is, a synergistic interaction between the implanted upper part of the TiO 2 nanotube, acting as light absorber, and the intact lower part, acting as catalytically active center, was proposed [95].

Summary and Prospective
TiO 2 photocatalyst as an ideal model for the investigation of photocatalysis in a variety of areas has attracted enormous attention over the past decades, especially on the water splitting for hydrogen production, pollutant removal for environmental protection and CO 2 reduction for solar fuels.These aspects are significant to the sustainable development of the economy and human society.However, the solar energy conversion efficiency is quite low owing to the limited optical absorption of TiO 2 only to UV spectrum and the rapid electron-hole recombination.Structural engineering that mainly leads to surface structure change of TiO 2 with color change from whiteness to blackness is a promising strategy.This strategy has the potential to take care of both sides, that is, black TiO 2 nanomaterials often exhibit a large absorption of visible light and highly enhanced charge separation.Many synthetic approaches have been developed to synthesize surface-structure-engineered black TiO 2 nanomaterials since the report of the hydrogenated black TiO 2 crystals in 2011.Despite these progress, two main challenges are present in terms of synthesis and applications of black TiO 2 .(i) Optical absorption of visible light, at present, does not mean their successful conversion to solar fuels; increasing the effective utilization of visible light is still a challenge for black TiO 2 .
(ii) Scalable synthesis of black TiO 2 with highly controllable quality is of vital importance for its practical applications.On the other hand, there are still quite a few open questions: why black TiO 2 nanomaterials have much higher photocatalytic activity over normal white TiO 2 nanomaterials (however, note that not all black TiO 2 nanomaterials exhibited enhanced photoactivity); what on earth primarily triggers the optical absorption of black TiO 2 nanomaterials to visible light; what are the individual roles of different types of defects in black TiO 2 nanomaterials; how the defect-rich surface region in black TiO 2 nanomaterials affects the charge transfer and photoactivity; etc.The answers to these fundamental questions vary with preparation method, properties of pristine TiO 2 , et al.This may be owing to the high sensitivity of the optical and electronic properties of TiO 2 to its surface structure.The huge differences in the surface structures (including surface groups, types and quantities of defects, and heteroatom contamination) among different black TiO 2 make the fundamental understanding even harder.Therefore, novel strategies along with new technologies that can precisely control and probe the surface structural evolution during the preparation process are highly desired.TiO 2 with quantitatively controlled surface defects may be a good model to reveal the underlying working principle of black TiO 2 nanomaterials in photocatalysis.The interface between the crystalline core and the disordered surface layer may also be an important consideration for understanding the basic physicochemical properties of black TiO 2 , but less attention has been paid to that.On the other hand, the interaction between the reactant compounds/ions and the surface of black TiO 2 nanomaterials, especially the effects of their surface states on the adsorption and activation of the targeted reactants, also calls for more attention as different photocatalytic processes need slightly different reaction environments which are highly related to the surface structures.We hope this review can inspire more work to advance the understanding and development of the black TiO 2 in photocatalysis.

Figure 3 :
Figure 3: (a) Schematic illustration of the DOS of black and white TiO 2 nanocrystals.Reprinted with permission from [22].Copyright (2011) American Association for the Advancement of Science.(b) Schematic illustration of the DOS of TiO 2 (Degussa P25) and black TiO 2 nanocrystals.Reprinted with permission from [33].Copyright (2013) Wiley-VCH.

Figure 5 :
Figure 5: (a) Schematic illustration of the structure and electronic DOS of a disorder-engineered semiconductor nanocrystal with dopant incorporation.Dopants are depicted as black dots, and disorder is represented in the outer layer of the nanocrystal.E C and E V represent the conduction and valence levels of a bulk semiconductor, respectively.The intrinsic bands of the nanocrystal are shown at the left.The disorder-induced CB and VB tails extending into the otherwise forbidden levels are shown at the right.(b) Digital images of the white and black TiO 2 nanocrystals.(c and d) HRTEM images of the white and black TiO 2 nanocrystals.Reprinted with permission from [22].Copyright (2011) American Association for the Advancement of Science.

Figure 6 :
Figure 6: (a) Schematic synthesis process for the ordered mesoporous black TiO 2 materials and (b) the ordered mesoporous TiO 2 before and after hydrogenation.Reprinted with permission from [78].Copyright (2014) American Chemical Society.

Figure 8 :
Figure 8: (a) Schematic low-temperature reduction of TiO 2 in a two-zone furnace.(b) Digital images of white and black TiO 2 .(c-f) HRTEM images of TiO 2 nanocrystals before (c) and after (d-f) the Al reduction at different temperatures for 6 h.Reprinted with permission from [32].Copyright (2013) The Royal Society of Chemistry.

+
≡Ti IV − OH + e − → ≡Ti III − OH 1 H + ions were likely driven inside the TiO 2 under the cathodic field [91].As shown in Figure 11, black TiO 2 in a ti o n (3 5 0 °C) w it h N a B H4 in A r B a l l m i l l i n g MT/MoS 2 /MT sandwich-like nanosheet MBT/MoS 2 /MBT sandwich-like nanosheet

Figure 11 :
Figure11: Ratio of the actual and initial concentrations (C/C 0 ) of rhodamine B as a function of the UV illumination time for the asanodized sample (anodized at 80 V for 7200 s), sample A0 (anodized at 80 V for 7200 s, following by annealing at 450 °C for 4 h), and sample A2 (black TiO 2 nanotubes converted from sample A0 via the activation step and subsequent cathodic treatment).The blank experiment was carried out in the absence of any photocatalyst ("no photocatalyst").Reprinted with permission from[91].Copyright (2014) The Royal Society of Chemistry.

Figure 12 :
Figure 12: Experimental process and optical images of the stripped TiO 2 layer.Reprinted with permission from [54].Copyright (2014) American Chemical Society.

4. Black TiO 2 Nanomaterials as Visible Light-Active Photocatalysts
[22,23,36,78]covery of black TiO 2 by Chen et al. in 2011[22], hydrogenation or hydrogen reduction has become a powerful tool to synthesize black TiO 2 nanomaterials.A variety of parameters, such as source of raw TiO 2 , hydrogenation time and temperature, H 2 pressure, exposed crystal facet of TiO 2 , and even reactor materials, will affect the colorization, surface structure and groups, and photocatalytic performance of hydrogenated TiO 2 nanomaterials[22, 23, 36, 40, 45-47,  52, 73, 76-80].Note that the crystalline/disordered core/shell structure is only observed in a few hydrogenated black TiO 2 nanomaterials[22,23,36,78].Chen et al. synthesized black TiO Black TiO 2 nanomaterials have been synthesized by various methods including hydrogen thermal treatment, hydrogen plasma treatment, chemical reduction, chemical oxidation, and electrochemical reduction.Although those black TiO 2 nanomaterials have similar appearance, their microstructures may differ owing to the differences in preparation methods and reaction parameters, and thus, black TiO 2 nanomaterials from different research groups worldwide possess different physicochemical properties.Herein, we will give a few typical examples for each preparation method.4.1.Black TiO 2 Nanomaterials by Hydrogenation. 2 nanocrystals by high-pressure hydrogenation in a 20.0-bar H 2 atmosphere at about 200 °C for 5 days