Applications of TiO₂ nanomaterials in photocatalysis, batteries, supercapacitors and solar cells, have seen widespread development in recent decades. Nowadays, black TiO₂ have won attention due to enhancing the solar light absorption by the formation of oxygen vacancies and Ti³⁺ defects, to promote the separation of photo-generated charge carriers leading to the improvement of the photocatalytic performance in H₂ production and pollutants degradation. The enhanced photocatalytic activity of black TiO₂ is also due to a lattice disorder on the surface and the presence of oxygen vacancies, Ti³⁺ ions, Ti-OH and Ti-H groups. Enhancing the optical absorption characteristics of TiO₂ and change of energy level and band-gap of materials have been successfully demonstrated to improve their photocatalytic activities, especially for black TiO₂ nanoparticles, which promote visible light absorption. The current review focuses on the investigation of the chemical reduction synthetic route for black TiO₂ nanomaterials, and their proposed association with green applications such as photodegradation of organic pollutants and photocatalytic water splitting. The synthesis methods of black TiO₂ involves the changes from Ti⁴⁺ to Ti³⁺ state, into different strategies: (1) The use of highly active hydrogen species such as H₂, H₂/Ar or H₂/N₂ gases, and metal hydrides (NaBH₄, CaH₂), (2) the reduction by active metals such as aluminum, magnesium and zinc, and (3) organic molecules such as imidazole and ascorbic acid.

Theoretical Consideration of Black TiO₂

Chen et al. first reported black TiO₂ with a narrowed band-gap of 1.5 eV to expand the full spectrum sunlight absorption and promote an increase in the photocatalytic activity, by introducing surface disorders in the TiO₂ (Chen et al., 2011). Hu et al. observed in 2012 a remarkable enhancement in the visible-light absorption and the photocatalysis of TiO₂ after hydrogen treatment, attributed to surface disorder and the formation of oxygen vacancies (Hu, 2012; Wang and Chou, 2016; Zhu et al., 2016).

In the past decade, a considerable effort has been committed to preparing black TiO₂ by introducing Ti³⁺ defects and oxygen vacancies into the titanium oxide lattice (Di Valentin et al., 2009; Su et al., 2014; Li et al., 2015; Tian et al., 2015; Xin et al., 2015). Oxygen vacancy and Ti³⁺ defects are more detectable in black TiO₂ compared with white TiO₂. Oxygen vacancies have been detected by a few techniques (Zhang and Park, 2017): electron paramagnetic resonance (EPR), electron spin resonance (ESR), and Raman spectroscopy. Ti³⁺ defects are not proved in white TiO₂, but they are detected in black TiO₂ based on X-ray photoelectron spectroscopy (XPS), by EPR (Jedsukontorn et al., 2017) or ESR spectroscopy (Tian et al., 2015). The yellow TiO₂ synthesis at a low temperature had more oxygen vacancies and Ti³⁺ defects compared with white TiO₂, which decreases the band-gap from 3.1 to 2.9 eV (Bi et al., 2020).

The formation defects in titanium oxide are given below (Jayashree and Ashokkumar, 2018):

Oxygen vacancy (Kröger-Vink notation $V_O^{··}$) formation at a low oxygen pressure

$\begin{array}{ll}{O}_O\leftrightarrow V_O^{··}+2e^{\prime }+1/2O_2& (1)\end{array}$

Titanium interstitials Ti³⁺ (Kröger-Vink notation $T{i}_i^{···}$):

$\begin{array}{ll}2O_O+T{i}_{Ti}\leftrightarrow T{i}_i^{···}+3e^{\prime }+O_2& (2)\end{array}$

Cui et al. (2014) described the generation of the Ti³⁺ and oxygen vacancies, by the equation of the defect (equation 3). They observed that the number of oxygen vacancies is half of the Ti³⁺ sites (equation 3). The absorption increases with the density of Ti³⁺ or O vacancies, as the density of Ti³⁺ increases with the Al reduction in temperature.

$\begin{array}{ll}{2Ti}_{Ti}^x+O_O^x\to {2Ti}_{Ti}^{\prime }+1/2O_2+V_O^{··}& (3)\end{array}$

The colorful TiO₂ with better absorption properties and improved photocatalytic activities compared with white TiO₂ have been designed through (i) metal doping in which metal replaces Ti⁴⁺ ions in the TiO₂ lattice (Chen et al., 2015), (ii) non-metal doping to replace O²⁻ ions in the TiO₂ lattice (Hamilton et al., 2014), (iii) to replace partial Ti⁴⁺ and O₂ ions in the TiO₂ lattice. To replace Ti⁴⁺ in TiO₂ with any cation is more accessible than to substitute O²⁻ with anions (such as nitrogen, carbon, sulfur) due to the difference in the charge state and ionic radii (Lee et al., 2014).

The structural modifications in black titanium oxide, involving Ti³⁺ centers and oxygen vacancies, conduct significant changes in crystallinity, and optoelectronic as well as the surface properties, and the most marked effect is the color changing. Increasing the optical absorption properties and diminishing electron-hole recombination of TiO₂ are expected to be meaningful for excellent photoactivity.

The optical band gaps of white anatase and rutile TiO₂ are reported as 3.2 and 3.0 eV, respectively (Haider et al., 2019), that means TiO₂ can adsorb only the UV part of the solar spectrum. A significant reduction in the TiO₂ band-gap around 1.23 eV and optical absorption near 1,000 nm in the near-infrared region (Ullattil et al., 2018), has been reported by hydrogenation of anatase nanocrystals under pressure resulting in the black TiO₂ materials (Chen et al., 2011; Liu et al., 2013).

The color change of titanium oxide from gray, blue, brown or black color, reflects in turn the optical properties and structural changes (Yan et al., 2017). By exposure to visible light, a heterojunction type I (Isac et al., 2019) is formed between white TiO₂ and colored TiO₂, the band energy levels of colored TiO₂ are included in that of the white TiO₂, and both heterojunction semiconductors could be excited to produce electrons and holes (Figure 1).

FIGURE 1

Figure 1. The heterojunction between white TiO₂ and colored TiO₂.

The colored TiO₂ shows a light absorption around 2eV, by introducing oxygen vacancies ($V_O^{··}$) and Ti³⁺ formation into TiO₂ lattice (Naldoni et al., 2019) or introducing disordered layers in the surface of crystalline TiO₂ (Song et al., 2017) enhanced solar light adsorption and served to prove their photocatalytic performance. The oxygen vacancy can significantly affect the electric and optical properties of the materials, by forming a donor level below the conduction band, located at 1.8eV below conduction band of titania as shown in Figure 1. The Ti³⁺ defect is responsible for changes in the electronic conductivity and optical properties. The Ti³⁺ and $V_O^{··}$ defects can be created by the reduction of TiO₂, either electrochemically, or through gas annealing and exposure in a vacuum (Lee et al., 2014).

Hu emphasize the importance of the crystalline phase of titanium oxide in the synthesis of black TiO₂; the rutile phase is theoretically easier to obtain than anatase crystalline phase because the formation energy of an oxygen vacancy in the rutile surface (110) is lower (5.5eV) than in the anatase surface (001) (7.54 eV) (Hu, 2012). The most common phase of the black TiO₂ is rutile or anatase; the rutile phase is formed at a temperature below 500°C. The oxygen deficiency and amorphous surface of TiO_2−x were also reported by Tan et al. (2014). The photocatalytic behavior of TiO₂ was found to depend on the crystalline phase; the white TiO₂ anatase phase has been shown to have higher photocatalytic efficiency than rutile TiO₂. Contrary to the previous statement, black rutile TiO₂ has been reported to have the best photocatalytic performance.

The synthesis methods of black TiO₂ are significantly affected by the structural, morphological and optical properties; involving the changes from Ti⁴⁺ to Ti³⁺ state, into different strategies, (i) the use of highly active hydrogen species such as H₂, H₂/Ar or H₂/N₂ gases, and metal hydrides (NaBH₄, CaH₂), (ii) the reduction by active metals such aluminum, magnesium and zinc, (iii) organic molecules such as imidazole and ascorbic acid, have been confirmed to be capable of reducing white TiO₂ to black titania.

Synthesis Approach of Black TiO₂ by Chemical Reduction

The synthesis methods explored through hydrogenation, plasma, chemical reduction, electrochemical reduction, laser ablation in liquid, and oxidation approaches were available in the literature over the last decade for black TiO₂ photocatalytic materials (Rajaraman et al., 2020).

The synthesis route influences the physicochemical properties and photocatalytic performance of black TiO₂. A significant number of studies highlight the formation of black TiO₂ by hydrogen thermal treatment when the samples had surface and bulk defects comparing with plasma treatment under Ar (95%)/H₂ (5%) atmosphere where the bulk defects were revealed (Wang and Chou, 2016). The color of the samples turned brown at 400°C, while the samples turned black at 500°C. The white TiO₂ Degussa powder was unchanged under hydrogenation, emphasized the role of precursors and synthesis route (Leshuk et al., 2013).

The reduction strategy can be generally explained in Equation 4, where Red represents the reductant:

$\begin{array}{ll}Ti{O}_2+Red\to Ti{O}_{2-x}+Red{O}_x& (4)\end{array}$

The noble gas atmosphere has been considered as reductant due to defective TiO_2−x formation in argon, nitrogen atmosphere, and the disordered layer forms only if crystallization is performed in an oxygen-free environment (Tian et al., 2015).

$\begin{array}{ll}Ti{O}_2\leftrightarrow Ti{O}_{2-x}+x/2O_2& (5)\end{array}$

The first synthesis of black titanium oxide consists of anatase nanoparticles through treating white TiO₂ nanoparticles (precursors, titanium tetra-isopropoxide, ethanol, hydrochloric acid, deionised water, and Pluronic F127 as an organic template) under a 20.0-bar H₂ atmosphere at about 200°C for 5 days (Chen et al., 2011).

NaBH₄ Reduction

By reducing hydrides, black titanium oxide can be obtained through three approaches: (i) physical mixing of white TiO₂ and hydride, followed by annealing in an inert atmosphere (Ar, N₂) at temperature of 300…400°C, (ii) hydrothermal synthesis (Ren et al., 2015) and (iii) sol-gel process (Fang et al., 2014).

The sodium borohydride (NaBH₄) is a commonly used reducing reagent (Table 1), due to its ability to reduce Ti⁴⁺ to Ti³⁺, as in Equations (6) and (7), and to produce in situ active H₂ at room temperature, that reduces the white TiO₂ into black TiO₂ (Equation 8). During the NaBH₄ reduction process, boron oxide species are produced due to their insolubility in the ethanol, and it can be easily washed out by HCl solution to remove the surface impurities and expose the color centers on the surface of the catalyst, significantly increasing the visible light absorption. The degradation efficiency increases 9 times after washing with HCl solution (Fang et al., 2014).

$\begin{array}{ll}NaB{H}_4+8O{H}^{-}\to NaB{O}_2+8e^{-}+6H_{2}O& (6)\end{array}$

$\begin{array}{ll}T{i}^{4+}+e^{-}\to T{i}^{3+}& (7)\end{array}$

$\begin{array}{ll}B{H}_4^{-}+2H_{2}O\to B{O}_2^{-}+4H_2& (8)\end{array}$

TABLE 1

Table 1. The chemical reduction synthesis methods, properties and photocatalytic applications of representative black TiO₂ materials.

The rate of the generation of H₂ is higher in acidic conditions at experiments performed at 25°C with 20 g water per gram of sodium borohydride, and a wt. ratio accelerators/NaBH₄, around 1. The representative acidic materials that act as accelerators are tartaric acid, citric acid, succinic acid, oxalic acid (85…98% hydrogen liberated after 3 min), ammonium carbonate, maleic acid, aluminum sulfate, sodium diacid phosphate (90….80% of H₂ after 10 min), maleic anhydride, ammonium chloride, benzoic acid (80….65% of H₂ after 10 min). The catalytic effect of metals salt (cobalt, aluminum) was also demonstrated (Schlesinger et al., 1953).

The treatment of the TiO₂ nanotube in NaBH₄ for a short time (20–40 min) reduced the surface of TiO₂ into Ti³⁺, and introduced an oxygen vacancy that creates localized states, producing a narrower band-gap of 2.46 eV, which extends its optical absorption to the visible region comparing with 3.09 eV for pristine TiO₂ nanotubes (Table 1) (Kang et al., 2013). The films show good stability and excellent reproducibility of the samples.

Tan et al. report a solid-state chemical reduction of TiO₂ at mild temperatures (300–350°C), for different times up to 1 h, an approach for large-scale production for visible light photocatalysis and solar-driven H₂ production (Table 1). The preparation of black TiO₂ followed the procedure: 4 g of TiO₂ Degussa P25 powder was mixed at room temperature with 1.5 g of NaBH₄ (98%) and heated in a tubular furnace under Ar atmosphere, up to 300°C and held for 5–120 min. When the temperature increase to 350°C, the black titanium oxide was obtained in 60 min. The colored powders from light blue to black were washed with deionised water and ethanol several times to remove unreacted NaBH₄ and dried at 70°C (Tan et al., 2014).

Xu et al. obtained black TiO₂ powders in a dual-zone quart tube furnace using titanium oxide synthesized of TiCl₄ and ethylene glycol at 150°C for 6 h in a Teflon-lined stainless-steel autoclave with NaBH₄ as reductant agent (Table 1). The reduction was carried out in an argon atmosphere, between 200 and 500°C for 1 h. The presence of Ti³⁺ and oxygen vacancy defects significantly increased the intensity of the band absorption in the visible spectrum range (Xu et al., 2019).

Metal Reduction

In recent studies, active metals such as magnesium, lithium, aluminum and zinc were used for the synthesis of black TiO₂ with oxygen-deficient metal oxides (Zu et al., 2019).

Ou et al. developed a room-temperature lithium reduction strategy removing oxygen, and generating oxygen vacancies into the titanium dioxide nanoparticles lattice. Lithium metal with a high reductive capacity can reduce a significant number of metal oxides at room temperature (TiO₂, ZnO, SnO₂, CeO₂). TiO₂ Degussa P25 and lithium powders (0.5% wt%) were mixed with a dispersant (dimethyl carbonate), then washed with diluted hydrochloric acid to remove lithium oxide, centrifugate and washing. The dried powders appear in different colors ranging from blue to black, and shift with the increase in lithium content (Ou et al., 2018).

The aluminum reduction of titanium oxide produces black TiO₂ in two ways: (i) reduction approach in an evacuated two-zone vacuum furnace, low temperature (300–600°C) for TiO₂ and high temperature (800°C) for aluminum and (ii) thermal treatment of a mixture of TiO₂ and aluminum powder (Table 1) (Wang et al., 2013). The black TiO₂ nanotube arrays have been used as a photoanode of photoelectrochemical cells for water-splitting, which was about 5 times higher than that of pristine (Cui et al., 2014).

Song et al. Herein prepared the black TiO₂ nanoparticles through subsequent Al reduction, with hollow nanosphere morphology, high crystallinity, small grain size (~8 nm), and high surface area (168.8 m²·g⁻¹) for photocatalytic hydrogen generation (56.7 mmol h⁻¹·g⁻¹) 2.5 times higher than pristine TiO₂ nanostructures. The aluminum reaction was performed for 6 h in an evacuated two-zone furnace, pristine TiO₂ hollow nanospheres were placed in the low-temperature zone (400….600°C), and the aluminum powder was placed at 800°C (Table 1) (Song et al., 2017).

Sinhamahapatra et al. report in 2015 a magnesiothermic reduction under a 5% H₂/Ar atmosphere followed by acid treatment to synthesize reduced black TiO₂ nanoparticles with improved optical absorption in the visible and infrared region for enhanced photocatalytic hydrogen production in the methanol-water system in the presence of Pt as a co-catalyst (Sinhamahapatra et al., 2015).

Nanoparticles with different colors were synthesized by Ye et al. (2017) using Mg as a reductant (Table 1). Commercial P25 TiO₂ nanoparticles were mixed with Mg powder into wt. ratio 20:3, 10:3, 5:3, and 1:1, before being purged with argon for 15 min and calcined at 600°C under an Ar atmosphere for 4 h. The TiO_x (x <2) nanoparticles with different Ti/O ratios increased with the increasing addition of Mg in the reaction (Equation 9) and colors (turned gray, blue-gray, light black, and dark black, respectively). The nanoparticles are material for converting solar energy to the thermal energy for evaporation of water.

$\begin{array}{ll}Ti{O}_2+(2-x)Mg\to Ti{O}_x+(2-x)MgO& (9)\end{array}$

Organic Molecules Reductant

Seok et al. synthesis Ti³⁺ self-doped TiO₂ using the sol-gel route: 5 g of TiOSO₄, 250 ml distilled water, 1.5 g urea as a dispersant, NaOH was added (pH=7), and precursors annealed under an oxidative atmosphere at 350°C for 6 h in the presence of 2-methylimidazole and HCl when the Ti⁴⁺ was reduced to Ti³⁺ which resulted in lower internal resistance and improved electronic conductivity with application in Li-ion batteries as anode materials with a capacity retention of 88% at 50°C (Seok et al., 2016).

A facile hydrothermal approach, described in Table 1, has been developed by Wajid et al. to prepare defective TiO_2−x high surface nanocore using ascorbic acid as a reductant, established theoxygen vacancy concentration and tunable band-gap by setting the amount of ascorbic acid (Wajid Shah et al., 2015).

The synthesis methods of black TiO₂ changed the phase and crystallinity, morphology, band-gap and BET surface area, essential elements in photocatalysis as described in Table 1. The experimental conditions (pollutants and catalysts concentration, light irradiation and intensity) influence the pollutant degradation efficiency and H₂ production rate by photocatalysis (Table 1).

The synthesis techniques to obtain black TiO₂ and defective TiO_2−x follow four strategies: introducing surface disorders, Ti³⁺ defects, oxygen vacancies, Ti-OH and Ti-H groups to narrowing the band-gap for photo-related applications (Liu et al., 2017; Yan et al., 2017).

Chemical reduction is associated with a change in the oxidation state of Ti⁴⁺ with the formation of Ti³⁺ species responsible for the electronic conductivity, essential for many applications of TiO₂, especially photocatalysis (Di Valentin et al., 2009). The surface Ti³⁺ species are unstable and can be quickly oxidized by oxygen in air or water, developing a method to synthesize black TiO₂ materials that with improved visible-light photocatalytic activity is a challenge (Zheng et al., 2013).

A Fundamental Process in Photocatalytic Activity of Black TiO₂

In the past years, black titanium oxide has attracted attention in different fields, such as photocatalytic pollutants degradation (Chen et al., 2011; Li et al., 2019; Plodinec et al., 2019), photocatalytic hydrogen production through water splitting (Wang et al., 2017; Pan et al., 2019), photocatalytic CO₂ reduction (Qingli et al., 2015; Zhao et al., 2016; Gao et al., 2020), solar–thermal material (Ye et al., 2017), supercapacitor (Zhi et al., 2016; Huang et al., 2018), photoanode in Dye-Sensitized Solar Cells (Ullattil et al., 2017), Lithium-ion batteries (Kim et al., 2017) (Yang et al., 2018), and medicine (Ni et al., 2017; Mazare et al., 2019).

The principle of the semiconductors photocatalysis consists of the following components: photon absorption, carriers separation, carrier diffusion simultaneously with carrier transport, catalytic efficiency and mass transfer of reactants and products (Takanabe, 2017).

Photon absorption: if the semiconductor has energy equal to or greater than E_g and consequent excitation of electrons (e⁻) to the CB leaving positively charged vacancies, holes (h⁺), in the VB (Figure 1)

Carriers separation: the heterojunction between nanoparticles can better band gap arrangement, to improve the separation of photo-generated charge carriers (Figure 1), which is advantageous of improving the photocatalytic performance. The oxygen vacancy defects and Ti³⁺ centers on the surface of TiO₂ favor the separation of charge carriers (electrons and holes) and can trap the hole.

Carrier diffusion simultaneously with carrier transport. The photo-generated electrons can initiate the reduction processes, including O₂ reduction to superoxides, H₂ generation, and CO₂ reduction to methane, methanol, or formaldehyde (Wen et al., 2015). The electron transfer is significant for the knowledge of the fundamental concepts of photocatalytic processes and to have an opinion about design and industrialization of the photocatalytic process (Mohamed and Bahnemann, 2012).

The transfer of electron/hole pairs to the interface initiates the redox reaction. The lifetime of the photo-generated charge carrier determines the efficiency of photocatalytic processes (Takanabe, 2017). Hence, increasing the efficiency of charge separation/transport in semiconductor nanoparticles is one of the major problems in photocatalysis to be addressed by the black TiO₂.

The presence of oxygen vacancies in TiO₂ can efficiently extend the visible light absorption range of titania because the localized oxygen vacancy states are located at 0.75 to 1.18 eV below the conduction band of TiO₂ (Asahi et al., 2001) (Figure 1). The hydroxyl radicals (HO∙) can be formed when hydroxyl anions (HO⁻), and adsorbed water trap the holes, which are capable of degrading the organic pollutants in wastewater.

Summary and Outlook

The current review focuses on the investigation of the chemical reduction synthetic route for black TiO₂ nanomaterials, and their applications related to the environmental application such as photodegradation of organic pollutants and photocatalytic water splitting.

Since 1972, when Fujishima and Honda (Fujishima and Honda, 1972) reported about the water-splitting process using a TiO₂ electrode under UV irradiation, photocatalysis has attracted attention. The solar-driven applications of TiO₂ have been limited due to its band-gap (around 3.2 eV). A remarkable step in solar-driven photocatalysis was presented in 2011 by Chen and co-authors when black TiO₂ enhanced the photocatalytic activity of TiO₂. In the last years, many studies have focused on the synthesis and explanation of different properties of black-TiO₂ to improve the activity of the photocatalyst under visible irradiation. An important drawback is the synthesis requirements such as long annealing treatments (a few days), and the high pressure of hydrogen atmosphere, up to 20 bar.

The colored TiO₂ can turn from white to yellow, blue, brown or black, due to the change in optical properties (modification of its band-gap), and defects in the surface layers that enhanced solar light adsorption and photocatalytic reactions. Among the colored forms, black TiO₂ has been one of the most investigated because it can get excellent optical, chemical and electronic properties due to at least one of these characteristics: the presence of Ti³⁺ ions, oxygen vacancies undetectable in white TiO₂ and usually present in black TiO₂, structural disorder/defects in the surface, Ti-OH groups, Ti-H groups, and modifications of the valence band edge. The colored TiO₂ has rich oxygen vacancies and Ti³⁺ defects, which conduct to better conductivity for electron transfer, increased visible absorption and higher conduction band potential. The oxygen vacancies and Ti³⁺ defects can act as traps for reducing the recombination of e⁻/h⁺ pairs and enhancing the photocatalytic activity.

The chemical reduction methods include the reduction of TiO₂ with active hydrogen species such as H₂, H₂/Ar or H₂/N₂ gases, using high temperatures with active metals such aluminum, magnesium and zinc powders, or the reduction of TiO₂ in solution with NaBH₄ and organic molecules such as imidazole and ascorbic acid can effectively lead to the color change of TiO₂ into black color. The color change of TiO₂ depends on the synthesis conditions, such as pressure, temperature, time, and the reducing agent, featuring different structural (lattice changes or disordering), chemical (formation of Ti³⁺, oxygen vacancies, Ti-H, Ti-OH), physical properties (such as optical properties), and photocatalytic activities in both hydrogen generation and organic pollutant removal. The thermal treatment changes the color of the samples between yellow at 300–350°C, brown at 400°C and black above 450°C, the crystal structure has no major changes due to hydrogenation.

Black titanium oxide is a versatile photocatalyst with an extended absorption spectrum into the vis light range of the solar spectrum. From both a material and a chemical reaction perspective, this may provide new opportunities in efficiently utilizing the visible-light region of the spectrum to finally improve the efficiency of black TiO₂ nanomaterials for practical photocatalytic applications.

Author Contributions

LA planned the content and wrote the manuscript. AE contributed to the photocatalysis chapter. All authors contributed to the article and approved the submitted version.

Funding

This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CCCDI-UEFISCDI, Project number 114/2019 ERANET-M.-TESTIMONIES, within PNCDI III.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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