Perspective Chapter: Black Titania – From Synthesis to Applications

Abstract

Titanium dioxide (TiO2) nanomaterials are very important for generating hydrogen through photoelectrochemical water splitting and remediation of environmental pollution. It has remained the focus of many researchers during last few decades due to their wide applications. Optical absorption properties of TiO2 can lead to increase their photocatalytic activities. However, its overall solar performance is very restricted because of its large bandgap. The emergence of black titania nanomaterials has recently triggered worldwide research interest due to its significantly improved solar absorption and enhanced photocatalytic performances. In this chapter, various synthetic approaches employed to obtain black titania are outlined, and the structural features of the black titania nanomaterials are described in detail, along with their photocatalytic performances towards various applications.

1. Introduction

By virtue of their large-scale applications in the removal of environmental pollutants and photocatalytic water splitting, TiO₂-based nanomaterials have fascinated massive interest [1, 2, 3]. TiO₂ exists in three main crystal phases that is brookite, anatase, and rutile. Electronic bandgaps of all these phases are above 3.0 eV, which is considered to be high bandgap. This limits their optical absorption in the ultraviolet (UV) region of the solar spectrum, which is below 5% of overall solar energy. If TiO₂ utilized this UV light very efficiently, its solar activity is still not better. It is the number of electrons and holes of the photocatalysts that determine its photocatalytic activity [4].

Upon appropriate light absorption, TiO₂ produces excited electrons in conduction band and excited holes in the valence band. For performing photocatalytic reactions, these excited charges, apart from each other, travel towards the surface. From all the excited charges, few of them are combined and vanished during the charge separation and migration processes. This absorption process leads to the generation of excited charges on the surface. If TiO₂ absorbs more light, then more excited charges come on the surface. The process is schematically illustrated in Figure 1. Therefore, if we improve the optical absorption properties of TiO_2, its whole activity can be increased [5, 6].

Figure 1.

Mechanistic illustration of the photocatalysis.

During last decades, doping techniques have been extensively employed to make TiO₂ colourful for desirable optical absorption [7]. For instance, in the early 1990s, a number of metallic species were employed to substitute the Ti⁴⁺ ions in the TiO₂ lattice [8]. More efforts lead to the doping of several nonmetals till 2001. Currently, many metal and nonmetal elements have been used to substitute partial Ti⁴⁺ and O₂ ions in the TiO₂ lattice. All the aforementioned efforts lead to improved light absorption by TiO₂ and consequent photocatalytic performance.

Recently, black TiO₂ has become the focus of the research community because of much-enhanced photocatalytic performance. Materials appear black in colour if it absorbs 100% light from the overall visible light region. From the entire visible light region, if TiO₂ absorbs certain percentage evenly, then it will become partially black or grey. If no light absorbs from the overall visible light regions, then it will show white colour. In the other case in which light is not absorbed appropriately, different colours (e.g., green, yellow and brown) will be observed. The focus of the research about titania is to tune their colour from lighter to darker. The properties and performance of the TiO₂ nanomaterials are affected by the apparent colour. In the proceeding sections, different synthetic strategies to black titania as well as characteristic features and their properties related applications will be discussed.

2. Synthetic strategies used to obtain black Titania nanostructures

Black titania nanostructures have attracted extensive interest, and various reductive and oxidative approaches have been established to successfully fabricate the black or coloured titania [9]. A variety of structural and chemical modifications are in practice to impart unique features to black titania, like surface amorphousity, oxygen vacancy/Ti³⁺, hydroxyl groups and Ti–H bonds. Various strategies have been briefly explained below.

2.1 Hydrogen treatment

To reduce TiO₂ nanocrystals using heat and hydrogen is an easy approach to obtaining black titania. Thermal hydrogen treatment changes TiO₂ (Ti⁴⁺) into other chemical species, such as Ti³⁺ or any other reduction states. Consequently, their lattice structure and chemical/physical properties also change by changing reduction states. The chemistry involved in this reaction is illustrated in the following scheme.

$$Ti{O}_2+\times {\text{H}}_2\underrightarrow{\Delta }Ti{O}_2,Ti{O}_{2-\times },Ti{O}_{2-\times }{\text{H}}_{2\times }$$E1

The above scheme illustrates that the chemical properties and concentration of starting TiO₂ nanomaterials, reaction temperature and time, pressure and concentration of hydrogen gas are the various factors that determine the final product of the reaction. When TiO₂ nanomaterials are treated with hydrogen, their final properties and the pathway through which the reaction proceeds will be different. This is because the final properties and the direction of reaction depend upon the conditions of hydrogen treatment. Many other factors that affect the chemical properties of nanomaterials are like morphology, shape, size, crystal facets and vacancies contents [10]. Reaction will be more complicated due to these variables. Preparation of these black TiO₂ nanomaterials is achieved normally by different research groups using different synthetic strategies. These alterations in structures lead to variations in their properties and functionalities. These structural and functional developments enable us to tune the structural features of a material and then consequent performances.

Hydrogenated environment used to obtain black titania may vary and it includes simply hydrogen thermal treatment, high-pressure pure hydrogen treatment, ambient or low-pressure pure hydrogen treatment, ambient hydrogen-argon treatment, ambient argon treatment and hydrogen plasma treatment.

Chen et al. synthesized black titania NPs via treatment of pure white titania NPs with 20.0-bar pure H₂ at 200°C for several days [11]. The precursor white titania NPs were synthesized following a solution-based route using titanium tetraisopropoxide as precursor, Pluronic F127 surfactant, hydrochloric acid, deionized water, and ethanol solvents. Figure 2 illustrates the schematic of the formation of black titania NPs (Figure 2a), along with digital photographs (Figure 2b) and electron microscopy images (Figure 2c and d) of white and black titania NPs. The obtained black titania NPs contained a well crystalline lattice core fenced by a disordered lattice shell from the hydrogen treatment. The amorphous boundary was expected to host the external hydrogen dopant and impart black colour to the hydrogenated titania NPs. The black titania NPs had broadband absorption as compared to corresponding untreated white titania as indicated in the UV-visible absorption spectrum (Figure 2e).

Figure 2.

(a) Schematic of the formation of black titania. (b) Digital photographs of white and black titania nanomaterials. High-resolution transmission electron microscopy images of (c) white and (d) black titania NPs, (e) UV-vis spectra of white and black titania NPs. Reprinted with permission from reference [11]. Copyright 2011 AAAS.

2.2 Chemical reduction

Chemical reduction is another route to obtain black titania where the Ti⁺⁴ species from the corresponding white titania precursors are reduced into low valent Ti species. The reducing agents involved in chemical reduction may include various natural products obtained from plants such as flavonoids, vitamins, phenolic acids, reducing sugars, polysaccharides, triterpenoids, tannins, and polysaccharides as all these are electron rich in nature. The resulting structure of the obtained materials can be tuned by controlling various reaction conditions. Besides these green reductants, some other chemical reducing agents such as aluminium, Zinc, NaBH₄, CaH₂ and imidazole have been extensively used to reduce TiO₂ to black titania.

A representative example of synthesis of black titania through chemical reduction is given here. Wang et al. reported the use of aluminium (Al) as a reducing agent to prepare black titania NPs in an evacuated two-zone vacuum furnace at elevated temperature. To obtain black titania, precursor white TiO₂ and Al were placed in separate zone of a two-zone tube furnace and pressure was set at 0.5 Pa through evacuation (Figure 3a). The Al was heated at 800°C, whereas precursor white titania was heated at 500°C for 7–18 hours. Thus, obtained reduced black titania NPs displayed intense absorption in the visible regions [12].

Figure 3.

(a) Schematic representation of the two-zone furnace. (b) Digital photograph of white and black titania NPs, (c) optical absorption spectra of titania NPs reduced at various temperatures. Reprinted with permission from reference [12]. Copyright 2013, The Royal Society of Chemistry.

2.3 Chemical oxidation

Black titania can be obtained by chemically oxidizing the titanium hydride precursors. For instance, Liu et al. reported the preparation of black titania by oxidising TiH₂ powder with 25% H₂O₂ solution at elevated temperature. In this way, reduced TiO_2-x NPs obtained which possess characteristic blue colour. The obtained NPs are quite stable even in air atmosphere as indicated by the retention of their colour and significant absorption towards the UV to visible light. Similarly, in another report, Grabstanowicz et al. prepared black titania powders following a multistep approach, as indicated in Figure 4. First, H₂O₂ (15 mL) was added into TiH₂ powders (0.96 g) aqueous suspension (10 mL) and stirred for three hours at room temperature to obtain a miscible gel-like slurry, followed by additional H₂O₂ (12 mL and 15 mL) and stirring (4 hours and 16 hours) in forming a yellow gel. Second, the gel was vacuum-desiccated overnight, placed in an oven at 100°C for 12–20 hours to become a yellow powder, and then finally at 630°C for three hours in Ar. The black TiO₂ had a rutile phase and remarkably enhanced absorption in the visible-light and near-infrared regions [13].

Figure 4.

Schematic illustration of the route from precursor to black titania NPs, along with their pictures. Reprinted with permission from reference [13]. Copyright 2013, American Chemical Society.

2.4 Electrochemical reduction

Hydrogenated black titania nanotubes (NTs) were obtained through electrochemical reduction approach by Xu et al. [14]. The NTs were fabricated through two-step anodization at 150 V for an hour with a carbon rod serving as the cathode and Ti serving as the anode. Ethylene glycol, along with 0.3 wt% NH₄F and 10 vol% H₂O, was used as electrolytes. The NTs obtained through first-step anodization were removed using scotch tape and processed for the second anodization. The obtained NTs were then heated at 150°C for three hours and at 450°C for another five hours. The electrochemical doping that leads to reduced titania was achieved using a 5 V electric current for a very short period (5 to 40s) of time in 0.5 M Na₂SO₄ aqueous solution at room temperature. The NTs were used as the cathode, whereas Pt wire was used as the anode, respectively.

Likewise, Li et al. reported the fabrication of black titania NTs through anodization approach followed by electrochemical reduction as indicated schematically in Figure 5 [15]. Precursor titania NTs were first obtained using titanium foil as anode and Pt gauze as the cathode. The voltage or applied was 80 V for 7200 s, or 4 mA for 5000 s. The electrolyte was an “aged” ethylene glycol with 0.2 M HF and 0.12 M H₂O₂ solution. The obtained NTs were further annealed at 450°C in air atmosphere for about 5 hours. The electrochemical reduction was achieved using conditions of 40 V voltage for 200 s in an ethylene glycol and 0.27 wt% NH₄F solution. The samples were treated at a higher voltage for activation before electrochemical doping.

Figure 5.

Schematic of the formation of black titania NTs through an electrochemical reduction approach. Reprinted with permission from reference [15]. Copyright 2014, The Royal Society of Chemistry.

2.5 Anodization-annealing

Dong et al. reported the fabrication of black titania NTs using an anodization approach followed by heat treatment at higher temperature as indicated schematically in Figure 6 [16]. Precursor titania NTs were fabricated using 10-hour anodization on a Ti foil for two times at 60 V potential in an ethylene glycol solution having 0.25 wt.% NH₄F and 2 vol% deionized water. The titania NTs were removed from the reaction system after first anodization and then subjected to second anodization. The repeated washing and cleaning of anodized Ti foil was achieved using ethanol and distilled water. The pure cleaned analytes were then dried at 120°C, and then annealed at 450°C for an hour under ambient environment. The removal of top oxide layer from the substrate leads to the development of layer of black titania.

Figure 6.

(a) Representation of the experimental process and images of the stripped titania NTs. (b) Optical absorption spectra of titania NTs. Reprinted with permission from reference [16]. Copyright 2014, American Chemical Society.

3. Characteristic features of black Titania nanostructures

It has been frequently stated that the various black titania nanostructures showed distinct chemical and physical characteristics, as briefly explained in the following sections, since the fabrication techniques and synthesis conditions of black titania nanostructures differ from one another in the literature. The observed black colour of the titania NPs has been attributed to some of these characteristics. The unique features responsible to impart colour to titania are given below.

3.1 Structural disorderness near the surface

According to certain investigations, the surface of black titania nanostructures with a crystalline/disordered core-shell structure shows disordered structural features. However, literature also contains different reports. For instance, Chen et al. reported the existence of a disordered surface layer surrounding the crystalline core in hydrogenated black titania NPs obtained in the conditions of 20 bar hydrogen pressure and 200°C [11]; Lu et al. have observed that hydrogenated titania nanocrystals made by treating commercial Degussa P25 under 35 bar hydrogen pressure and room temperature for up to 20 days also contain disorderness in the structure near surface [17]. Wang and Xu, observed the same structural features in the hydrogenated black titania nanosheets [18]. Therefore, the hydrogenation treatment also suggested a modest lattice expansion. Some groups reported lattice shrinkage in the disordered layer [19]. The surface of the hydrogenated titania NTs, on the other hand, was extremely transparent, according to Lu and Zhou et al. [20].

The disordered phase of the black titania nanostructures may be distinguished from the crystalline phase using high-resolution transmission electron microscopy (HRTEM). For instance, Figure 6 illustrates the comparison of the structure of black and white titania NPs using HRTEM and line analysis [21]. Even at the surface of the nanocrystal, the white titania nanoparticle revealed sharply defined, and well-resolved lattice fringes (Figure 7a), and the spacing between the adjacent lattice planes was uniform throughout the whole nanocrystal and typical for anatase (0.352 nm) (Figure 7b). The crystalline-disordered core-shell structure of the black titania nanoparticle (Figure 7c) revealed a structural divergence from the typical crystalline anatase at the outer layer, where the straight lattice line was curved at the nanoparticle’s edge, and the plane distance was no longer uniform (Figure 7d). The distinction between the amorphous structure and the crystalline phases has occasionally also been made using electron diffraction (ED).

Figure 7.

HRTEM and line analyses of (a & b) one white titania nanoparticle, (c & d) one black titania nanoparticle. The zeros of the axis in b and d correspond to the left ends of the lines in a and c. The red and green curves in b and d correspond to the red and green lines in a and c. Reprinted with permission from reference [21]. Copyright 2013, Nature Publishing Group.

3.2 The presence of Ti³⁺ ion

Ti³⁺ ions can be seen experimentally or not, depending on the synthetic approaches adopted to obtain black titania NPs. With conventional X-ray photoelectron spectroscopy (XPS), synchrotron X-ray absorption, emission, photoelectron spectroscopies, and electron spin resonance spectroscopy, Ti³⁺ ions were not detected in hydrogenated black titania nanocrystals obtained via hydrogen reduction or hydrogen plasma-derived black titania nanostructures. For instance, based on the almost identical Ti 2p XPS spectra of pure and hydrogenated titania nanowires (NWs), Wang et al. proposed the absence of Ti³⁺ in the black hydrogenated titania nanowires processed at 450°C [22]. However, in certain studies, Ti³⁺ ions in the black titania NPs were indicated after hydrogen treatment, chemical reduction, chemical oxidation, and electrochemical reduction, respectively [19]. The existence of Ti³⁺ ions can be detected even using XPS in case of the black titania NTs produced by electrochemical reduction and oxidation of TiH₂ [19].

3.3 Oxygen vacancies

Oxygen vacancies have been continuously documented in black titania nanostructures obtained via hydrogen thermal treatment, electrochemical reduction, chemical reduction, and chemical oxidation [19]. For instance, oxygen vacancies were detected by ESR spectroscopy in the black titania NTs formed through thermal hydrogen treatment [23], and electrochemical reduction [16], as well as in the black titania NPs synthesized through Al reduction approach [24]. Like Ti³⁺ ions, oxygen vacancies can also not always be detected. For instance, in the report of Xia et al. [25] no oxygen vacancy was found with ESR in the black titania NPs synthesized with thermal treatment.

3.4 The existence of Ti: OH groups

The hydrogenation treatment leads to a change in the OH content in the black titania nanostructures. A satellite peak characteristic of Ti–OH in the O 1 s XPS spectrum was observed in black titania NPs obtained via hydrogen treatment at 200°C for five days [11], in the hydrogen-treated titania NWs [22] and in the hydrogenated titania NTs obtained using ultrapure H₂ atmosphere and 200–600°C temperature for one hour [26]. However, the hydrogenated titania NTs arrays treated at 450°C for one hour in a reducing atmosphere of 5% H₂ and 95% argon did not manifest any alteration in the Ti-OH peak in the O 1 s XPS spectra, and the hydrogenated black titania NPs treated at 450°C for four hours under five bar H₂ displayed a decreased OH signal in the O 1 s XPS spectrum [25].

Black hydrogenated titania NPs demonstrated a change in the strength of the OH vibrational band in the Fourier transform infrared (FTIR) spectrum [25]. In hydrogenated titania nanosheets treated at 400°C for two hours in a pure H₂ environment, more surface OH groups were seen. Titania NPs that had been hydrogenated and subjected to a hydrogen plasma at 500°C for 4–8 hours showed additional peaks at wave numbers of 3685, 3670, and 3645 cm⁻¹ [27]. The intensity of the OH peak was significantly lower in hydrogenated titania microspheres than in pure titania after being treated at 500°C for 4 hours with a flow of H₂ (5% in N₂, 300 sccm). Based on the weaker bands at 3446 and 1645 cm⁻¹, less water and/or hydroxyl groups were adsorbed onto the hydrogenated titania NTs when they were heated to 450°C for one hour in a reducing environment of 5% H₂ and 95% Ar [20]. Black hydrogenated titania treated at 200°C for five days showed a drop in O-H intensity, and hydrogenated titania NPs treated at 450°C for 4 hours under 5 bar H₂ showed no OH absorption bands [25]. When hydrogenated titania NPs were exposed to hydrogen plasma at 500°C for 4 to 8 hours, the proton nuclear magnetic resonance (NMR) spectra revealed a higher peak at 5.5 ppm from bridging hydroxyl groups and additional signals at 0.01 and 0.4 ppm from the internal and terminal hydroxyl groups [27]. In the hydrogenated black titania NPs treated at 200°C for five days, a peak at chemical shift +5.7 ppm with two additional tiny, narrow peaks at chemical shifts 0.03 and 0.73 ppm were observed [21]. The hydrogenated titania NPs, on the other hand, showed much smaller OH signals after being heated at 450°C for 4 hours under 5 bar H₂ [25].

3.5 Hydrogen doping

It has occasionally been noted that hydrogenated black titania NPs possess Ti-H groups. 106,117,134,137 Hydrogenated titania nanowire microspheres obtained via treatment at 500°C for four hours under a flow of H₂ (5% in N₂, 300 sccm) and hydrogenated titania NPs treated with hydrogen plasma at 500°C for 4–8 hours, a satellite peak at about 457.3 eV in the Ti 2p XPS spectrum was observed, which refers to the presence of surface Ti-H bonds [27]. The surface of hydrogenated titania nanosheets produced at 400°C for two hours under a 10-bar pure H₂ atmosphere was hypothesized to be converted to Ti-H bonds [18]. The diffraction peak at about 59.281 in the X-ray diffraction (XRD) pattern of hydrogenated titania NTs obtained via treatment in ultrapure H₂ atmosphere at 200–600°C for one hour also referred to the presence of the Ti-H bond [28].

3.6 Valence band edge modifications

The black titania nanostructures sometime exhibit a shift in the valence band. For instance, when hydrogenated titania was obtained via treatment at 200°C for four days under 20 bar H₂ [11] or heated at 500°C for one hour under a hydrogen atmosphere [29], a redshift of the valence band was observed as indicated from the valence band XPS spectrum. A similar shift was observed in the black brookite titania NPs, rutile titania NPs, and titania NTs prepared with the Al reduction method, in the black anatase titania NTs obtained through NaBH₄ reduction, in the black titania NPs fabricated via electron beam treatment, in the hydrogenated titania NTs treated at 450°C for an hour in a reducing 5% H₂ and 95% Ar environment and in the electrochemically hydrogenated black titania NTs [19]. It was discovered that Ti³⁺ ions did not contribute to the extra bandgap states of the hydrogenated TiO₂ nanocrystals because when Ti³⁺ appeared, these mid-gap states vanished [21]. On the other hand, pure and hydrogenated TiO₂ NWs treated for three hours at 200–550°C in an ultrapure hydrogen atmosphere (ambient pressure) showed similar valence-band structures. Hydrogenated titania nanosheets exhibit similar valence band shifts.

4. Photocatalytic applications of black titania nanostructures

There are three main steps that are involved in photocatalysis. These are absorption of light in the form of photons, electrons, and holes excitation, separation of charges and migration to the surface of photocatalysts, and then the transfer of charges between the photogenerated carriers and the reactant. Prior to the discovery of hydrogenated black titania NPs, it had been demonstrated that hydrogen thermal treatment of titania could improve its photocatalytic properties. Harris and Schumacher discovered that hydrogen reduction at high temperatures decreased recombination canters and prolonged the lifespan of the holes [30]. Oxygen vacancies, Ti³⁺ species, and hydroxyl groups produced by this process are probably what contributed to the increased photoactivity. Black titania nanostructures exhibit superior performance towards various photocatalytic applications because of the distinct optical and charge transport features as compared to the pristine white titania. Various photocatalytic applications include photocatalytic removal of contaminations, photocatalytic hydrogen production through photoelectrochemical water splitting, photoelectrochemical sensor and photocatalytic CO₂ reduction.

4.1 Photocatalytic degradation of environmental pollutants

It has been estimated that about 20% of dye was wasted while the dyeing process in the textile industry and released as effluent in the water. The release of such a large number of coloured dyes into water causes it to be polluted and become a major source of environmental pollution [31, 32]. This polluted water can be photocatalytically detoxified using high-intensity solar energy. Titania is usually thought to be the best photocatalyst and has the tendency to treat wastewater, but only restricted to the ultraviolet region. However, this limitation can be overcome by using coloured titania.

Chen et al. first time prepared the hydrogenated black titania NPs from white titania. The detailed synthetic strategy involves the treatment of white titania with 20 bar of pure H₂ at 200°C for five days [11]. The obtained black titania had core-shell structure in which the core is well crystalline, and shell is amorphous. The much-enhanced photocatalytic performance towards methylene blue (MB) degradation was observed under simulated sunlight. For instance, the complete photodegradation of MB was achieved in a very short period of eight minutes under simulated solar light by using black titania as compared to white titania as indicated in Figure 8a and b.

Figure 8.

a) Photocatalytic removal of MB from aqueous medium by white and black hydrogenated titania under stimulated solar light illumination, (b) cycling tests of solar-driven photocatalytic removal of aqueous MB of the black hydrogenated titania, c) stability tests of photocatalytic H₂ evolution of the black hydrogenated titania, and the H₂ evolution rate was calculated to be 10 mmol h⁻¹ g⁻¹. Reproduced with permission from reference [11]. Copyright 2011, American Association for the Advancement of Science.

4.2 Photocatalytic hydrogen generation

The recent energy and environmental situation suggests that hydrogen will be the ultimate source of clean and green energy. The photocatalytic water splitting facilitated by solar energy in which natural sunlight and water are employed as the hydrogen source is considered an important source of hydrogen. Black titania-based photocatalysts have been extensively used for generating hydrogen through a water-splitting reaction [33]. For instance, black hydrogenated titania obtained using the high-pressure H₂ can generate H₂ from water at a rate of 10 mmol h⁻¹ g⁻¹ with exceptional stability under sunlight illumination, as displayed in Figure 8c [11]. The H₂-plasma assisted black titania also exhibited enhanced photocatalytic H₂-production rate of 8.2 mmol h⁻¹ g⁻¹, about 13.5 times greater than the white titania.

4.3 Photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is an important strategy to generate hydrogen following a green solar-to-hydrogen route. Significant research is being done to enhance its efficiency. The black titania is thought to be an emerging candidate for PEC water splitting because of its ideal band structure [33]. For instance, black titania NTs fabricated via Al reduction approach exhibited much-enhanced photocurrent as compared to unreduced white titania NTs [15]. The applied bias photon-to-current efficiency (ABPE) of black titania NTs attained 1.20% at a greater bias of 0.68 V (vs. Pt), remarkably greater than that of white titania NTs, 0.25% at 0.49 V (vs. Pt). The incident-photon-to-current-conversion efficiency (IPCE) of black titania NTs was also impressively improved as compared to white titania NTs, along both UV and visible light regions. The superior photocatalytic performance of black titania NTs was credited to the greater electron density and the subsequent better electric conductivity, as indicated from the Mott-Schottky plot. On the basis of this effect, enhanced PEC water-splitting activities were extensively observed in black titania NTs arrays synthesized via other reduction approaches. Moreover, Kim et al. reported that the black titania NTs displayed considerably unique electrocatalytic performance in producing OHs and Cl₂ in comparison with white anatase titania NTs [34].

4.4 Photoelectrochemical sensors

Titania can be employed as a photochemical sensor to measure the concentration and type of different organics found in an aqueous medium. It can be done by estimating the photocurrents generated from the dissociation process in PEC cells. The arrays of hydrogenated black titania NTs or nanorod fabricated through annealing in the H₂ atmosphere were used as a photoelectrochemical sensor to detect and quantify various organic compounds in solar light. For instance, the hydrogenated black titania nanorods arrays (H-TNRs) exhibited a much more sensitive and steady photocurrent response (~100-folds greater than the white titania nanorods (TNR)) in the NaNO₃ solution under solar light (Figure 9a). Under solar light illumination, the estimated photocurrents of the H-TNRs exhibited better linear correlations with the concentrations of most organics such as glucose, malonic acid and potassium hydrogen phthalate (Figure 9b), indicating that H-TNRs can sensitively and steadily quantify the given organic compounds. The enhancement in the photocurrent response was credited to the enhancement of conductivity [35].

Figure 9.

a) The voltammograms of TNR and H-TNR photoanodes obtained at a scan rate of 5 mV s⁻¹ under visible light. Inset shows the photocurrent responses for the TNR and H-TNR photoanodes. b) Relationships between the photocurrent related to the oxidation of the organics net and the concentrations of the organic at the H-TNR electrode. Reproduced with permission from reference [35]. Copyright 2014, Elsevier.

4.5 Photocatalytic CO₂ reduction

Liu et al. reported the fabrication of oxygen-deficient blue titania NPs with two exposed (101) (001) facets. The obtained coloured titania showed enhanced photoreduction towards CO₂ under visible light illumination. A comparatively higher quantum efficiency (0.31% under full spectrum solar light and 0.134% under visible light region) for CO₂ reduction to CO was achieved using water vapour. This quantum yield was almost four times greater than titania having single exposed facets and P25 (Figure 10b). The superior performance of titania was attributed to exposure of more active sites, the facilitated electron transfer between (001) and (101) planes, and Ti³⁺ induced mid-gap states to extend the visible light response (Figure 10a) [36].

Figure 10.

a) Upper: Estimated relative band edges of the (101) and (001) facets of the anatase phase of titania, lower: Spin densities around the oxygen vacancy in (101) and (001) slab model of anatase TiO2 under antiferromagnetic alignment. The yellow and cyan isosurfaces refer to up-spin and down-spin densities, respectively. b) CO production over TiO₂ and TiO_2-x nanocrystals with different exposed facets under visible light. Reproduced with permission [36]. Copyright 2016, American Chemical Society.

5. Conclusions

Black titania nanostructures have attracted extensive interest, and several reductive and oxidative approaches have been established to successfully fabricate the black or coloured titania. A variety of structural and chemical modifications are in practice to impart unique features to black titania, like surface amorphousity, oxygen vacancy/Ti3+, hydroxyl groups and Ti–H bonds. These modifications lead to the change in the electronic structure of titania, for example, decreasing bandgap, which is responsible for the improved visible-light absorption features of black titania. Furthermore, the accompanying charge transport features are enhanced because of the decreased electron-hole recombination. Additionally, the most defective structure of black titania assists the adsorption and dissociation of the reactant on its surface, as well as the consequent charge transfer process. Such a novel optical and electrical features endow the material with enhanced photocatalytic activities as compared to white titania.

Conflict of interest

The authors declare no conflict of interest.