Development of effective TiO₂-based photocatalysts has been a hot topic of research over the last few decades ^{[1, 2, 3]}. Most reports have focused on the effective separation of photoinduced carriers. For example,the formation of a heterojunction in the TiO₂-based photocatalyst is a typical approach to decrease the recombination of photoinduced carriers because it allows the electrons and holes of the catalyst to move in different directions. Heterojunction formation involves the synthesis of polycrystalline TiO₂,such as rutile/anatase,TiO₂(B)/ anatase,and brookite/anatase ^{[4, 5, 6, 7]}. In addition,doping TiO₂ with other elements can also allow effective separation of photocarriers by formation of electron traps. The dopants include metals such as Fe³⁺,Cr³⁺,and V^{4+ [8, 9, 10]} or non-metals,such as N,S,C,and F ^{[11, 12, 13]}. Recently,oxygen vacancies have also been used as a dopant for TiO₂ to improve photocatalytic performance ^{[14, 15]}. TiO₂ doped with oxygen vacancies is black,and it can be easily formed from conventional white TiO₂. Oxygen vacancies in black TiO₂ have an analogous effect to other dopants,serving as a promoter of photocatalytic performance.

Improvements in the photocatalytic performance of TiO₂ have been obtained by all of the methods mentioned above. If two or more of them are coupled together,a more catalytically active TiO₂-based material may be obtained ^{[16, 17, 18]}. For example,it may be prepared by doping oxygen vacancies into polycrystalline TiO₂,which involves coupling of two effects. It is noteworthy that such a TiO₂-based material consisted of only Ti and O elements without any impurities. In our previous work,we prepared bicrystalline TiO₂ composed of TiO₂(B) and anatase nanofibers from potassium dititanate. The resulting material exhibited better photocatalytic activity than that of commercial anatase TiO₂ because of the existence of a heterojunction between TiO₂(B) and anatase ^{[4, 19]}. Herein,we fabricate black TiO₂(B)/anatase nanofibers by annealing a titanate derivative with a Brunauer-Emmett-Teller (BET) surface area ≥ 250 m²/g,a precursor of polycrystalline TiO₂,in hydrogen. The photocatalytic properties of the nanofibers are studied and their structure is also characterized in detail by various methods.

The starting materials of black bicrystalline TiO_2-x were TiO₂.nH₂O and K₂CO₃. First,a mixture of TiO₂/K₂O (molar ratio of Ti to K is 1.9) was prepared by uniformly adding K₂CO₃ to TiO₂.nH₂O prepared by hydrolyzing TiOSO₄ in hot water with vigorous stirring,and then sintering at 860 °C for 4 h to obtain potassium dititanate (K₂Ti₂O₅). K₂Ti₂O₅ was ground with water to form a paste and put into a closed container at 80 °C for 24 h. The paste was then suspended in vigorously stirred aqueous HCl solution (100 mL,0.1 mol/L) until the residual K⁺ was < 0.2 wt% in the final product (detected by inductively coupled plasma-mass spectrometry). The resulting precipitate was separated by filtration,washed with distilled water and dried in a desiccator at 60 °C under vacuum. The powder obtained was hydrated titanate. The hydrated titanate was laid on a quartz boat that placed in the temperature-measurement area of the tubular furnace,and calcined in pure H₂ at 600 °C for 2 h. The black TiO₂(B)/anatase bicrystalline TiO_2-x nanofibers were thus obtained.

For comparison,white TiO₂(B)/anatase bicrystalline TiO₂ nanofibers were prepared from the hydrated titanate derivative by calcination in air at 600 °C for 2 h ^[19]. P25 TiO₂ (Degussa) was used a mixed-phase TiO₂ photocatalyst. Conventional anatase was prepared from TiOSO₄. The hydrolysis of TiOSO₄ was performed as described elsewhere ^[20]. The collected TiO₂.nH₂O was washed with distilled water and dried in a desiccator at 60 °C under vacuum. Finally,pure anatase TiO₂ was obtained by calcination of TiO₂.nH₂O at 500 °C for 2 h in air.

The photocatalytic properties of the samples were assessed in a Pyrex reactor attached to a 300-W high-pressure mercury lamp. Thermostated water (27 °C) was circulated through a jacket between the mercury lamp and reaction chamber. At the same time,O₂ or N₂ was introduced into the Pyrex reactor continuously to keep the concentration of oxygen constant and produce a homogeneous suspension of the catalyst in the reaction solution containing methyl orange (MO). Aliquots of the reaction solution were taken out at given time intervals and filtered through a 0.22-μm syringe filter for further analysis. The MO concentration was analyzed by a standard colorimetric method with an ultraviolet-visible spectrophotometer (UV-2802S,UNICO) by measuring the absorbance of MO at 464 nm as a function of irradiation time.

The morphology and microstructure of samples were evaluated by field-emission scanning electron microscopy (FESEM; S-4800,Hitachi). Specific areas were calculated by BET method from adsorption-desorption measurements (TristarII 3020M,Micromeritics) at −196 °C. Raman spectra were obtained using a HR 800 spectrometer (Horiba) equipped with a CCD camera detector. As an excitation source,the 514-nm line of an argon/krypton ion laser system (2018,Spectra Physics) was focused through a microscope (BX41,Olympus) equipped with a 50× magnification objective. The laser power did not exceed 5 mW for each sample. X-ray photoelectron spectroscopy (XPS) was performed using a spectrometer (ESCALAB 250,ThermoFisher Scientific,USA) equipped with Al K_α radiation operated at 300 W. The shift of binding energy was corrected using the C1s level at 284.7 eV as an internal standard. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (SDT 2960,TA Instruments,USA). TGA measurements were conducted over a temperature range of 35-850 °C at a heating rate of 10 °C/min under air. UV-Vis spectroscopy (Lambda 900,Perkin-Elmer) was used to identify the spectral properties of the samples. Photoluminescence (PL) spectra were measured at room temperature on a fluorescence spectrophotometer (Varian Cary Eclipse,Varian,USA) with an excitation wavelength of 325 nm.

Figure 1(a) reveals that the morphology of black bicrystalline TiO_2-x is fiber-like with a uniform width of 0.2 μm and length of 1-5 μm,which is identical to that of white bicrystalline TiO₂ (Fig. 1(b)). The surface area of black TiO_2-x is 28.9 m²/g,which is also to that of white bicrystalline TiO₂ (28.5 m²/g). This demonstrates hydrogen treatment has no influence on the morphology of the titania materials.

Fig. 1. FESEM images of black bicrystalline TiO2-x nanofibers (a) and white bicrystalline TiO2 (b).

The Raman spectra of black bicrystalline TiO_2-x and white bicrystalline TiO₂ are presented in Fig. 2. The six obvious (3E_g+2B1_g+A1_g) Raman-active modes of the anatase phase are detected in both samples. This indicates that the anatase phase is the main component of these TiO₂ samples ^[4]. In the magnified image (inset (b) in Fig. 2),two weak bands from 220 to 260 cm^-1 are observed in both samples,which correspond to Raman-active modes of TiO₂(B) ^[21],confirming the presence of TiO₂(B) in both samples. Furthermore,inset (a) in Fig. 2 indicates that there is a blue shift of the Eg band observed for black bicrystalline TiO_2-x (146 cm^-1) compared with that of white bicrystalline TiO₂ (141 cm^-1). The Eg band of titania is related to the structural properties of the bulk anatase phase. Localized defects in the bulk anatase phase are the main factor responsible for the blue shift of the Eg band of black bicrystalline TiO_2-x ^[22]. Simultaneously,it can also be observed that there is a slight blue shift of bands assigned to the TiO₂(B) phase in black bicrystalline TiO_2-x compared with that of white bicrystalline TiO₂,which illustrates that the TiO₂(B) phase of black bicrystalline TiO_2-x has the defects as well. Thus,defects should be present in both the TiO₂(B) and anatase phases of black bicrystalline TiO_2-x.

Fig. 2. Raman spectra of black bicrystalline TiO2-x (1) and white bicrystalline TiO2 (2).

The Ti 2p XPS measurements for black bicrystalline TiO_2-x and white bicrystalline TiO₂ are shown in Fig. 3. The Ti region of black bicrystalline TiO_2-x is regular,and the binding energies of Ti 2p_3/2 and Ti 2p_1/2 at 459.4 and 464.9 eV,respectively,compared well with data for Ti⁴⁺ in TiO₂ ^[23]. The same XPS peak shape of Ti 2p for both samples demonstrates the similar electronic state of Ti atoms in them. This means that the defects are not distributed on the surface of black bicrystalline TiO_2-x. Similar results were obtained Chen et al. ^[24] and Pen et al. ^[25]. According to simulations by Chen et al. ^[26],the energy barrier for oxygen vacancies of anatase TiO₂ to diffuse from surface sites to subsurface ones is sufficiently low to ensure rapid equilibration of the vacancy distribution at typical annealing temperatures. That is,the oxygen vacancies of anatase TiO₂ can spontaneously diffuse from surface to subsurface sites during the preparation process. Thus,combined with the Raman analysis,it can be confirmed that the oxygen vacancies (i.e.,defects) are located in the bulk of black bicrystalline TiO_2-x.

Fig. 3. XPS spectra of the Ti 2p peaks (a) and O 1s peaks (b) for black bicrystalline TiO2-x (1) and white bicrystalline TiO2 (2).

The content of oxygen vacancies in TiO_2-x is difficult to determine directly,unlike the analysis of other conventional elements. Herein,we tried to measure the change in the mass of samples by TGA to estimate the content of oxygen vacancies in black bicrystalline TiO_2-x. As shown in Fig. 4,the mass loss of white bicrystalline TiO₂ is mainly ascribed to the desorption of physically adsorbed H₂O and hydroxyl groups ^[27]. The mass of TiO₂ remained constant until the temperature exceeded 600 °C. For black bicrystalline TiO_2-x the TGA curve below 300 °C is the same as that of white bicrystalline TiO₂,which indicates that both samples have the same content of physically adsorbed H₂O and hydroxyl groups. However,there is an obvious difference in mass loss between them when the temperature exceeds 300 °C. An increase of mass is observed for black bicrystalline TiO_2-x,which is finished at about 750 °C. Because black TiO_2-x consists of only Ti and O elements,the mass increase can be attributed to the oxidation of the sample. However,the mass change process of black TiO_2-x from 300 to 750 °C may also involve the desorption of hydroxyl groups according to the TGA of white bicrystalline TiO₂. It is difficult to confirm the accurate mass change due to oxidation of black bicrystalline TiO_2-x. Comparison of the two TGA curves indicates that the maximum mass difference between them is about 0.58 wt%. Thus,it can be confirmed that the mass change ascribed to oxidation of oxygen vacancies is less than 0.58 wt%; that is,“x” of TiO_2-x is calculated to be less than 0.03.

Fig. 4. TGA curves of black bicrystalline TiO2-x (1) and white bicrystalline TiO2 (2).

Figure 5 depicts the UV-Vis diffuse reflection spectra of black bicrystalline TiO_2-x and white bicrystalline TiO₂. A large absorption peak is observed for white bicrystalline TiO₂ below 400 nm,but there is no absorption in the visible region. This is identical to the typical characteristics of general TiO₂ ^[28]. Black bicrystalline TiO_2-x exhibits an absorption peak in the UV region but the intensity is a little lower than that of white bicrystalline TiO₂. More importantly,a broad absorption spanning the whole visible region is observed,which makes the sample black.

Fig. 5. UV-Vis diffuse reflection spectra of black bicrystalline TiO2-x (1) and white bicrystalline TiO2 (2).

Figure 6(a) shows the degradation curves of MO in the presence of different TiO₂ samples. The influence of absorption (control sample) and self-degradation of MO (blank sample) on the photocatalytic performance of the catalysts can be ignored. It reveals that the degradation rate of conventional anatase TiO₂ is about 50% after 15 min. Meanwhile,75% of MO is degraded after 15 min by white TiO₂(B)/anatase bicrystalline TiO₂,which is similar to the efficiency of commercial titania P25. This enhanced performance can be attributed to the heterojunction of TiO₂(B) and anatase limiting the recombination of photoinduced carriers ^[29]. As expected,black bicrystalline TiO_2-x exhibits better photocatalytic performance than white TiO₂(B)/anatase bicrystalline TiO₂,with the degradation ratio reaching up to 98% after 15 min. This indicates that the introduction of oxygen vacancies improves the photocatalytic activity of black bicrystalline TiO_2-x. As shown in the inset of Fig. 6(a),the reaction rate constant (k) of black bicrystalline TiO_2-x (0.38 min^-1) is 4.2 times higher than that of white bicrystalline TiO₂ (0.09 min^-1) and 10.5 times higher than that of anatase (0.04 min^-1). Thus,black bicrystalline TiO_2-x exhibits excellent photocatalytic performance because of the formation of bicrystalline structure and incorporation of oxygen vacancies.

Fig. 6. (a) Comparison of photocatalytic degradation of MO by various samples. (1) Black bicrystalline TiO2-x; (2) White bicrystalline TiO2; (3) Conventional anatase TiO2; (4) P25; (5) Control sample (absorption of anatase TiO2 in the dark); (6) Blank sample (without catalytic materials). (b) The recyclability of black bicrystalline TiO2-x in photocatalysis of MO.

Furthermore,recycling experiments were carried out to study the photocatalytic stability of black bicrystalline TiO_2-x; the results are presented in Fig. 6(b). Taking into account the mass loss of the catalyst,the photocatalytic activity of black bicrystalline TiO_2-x shows only a very slight decrease after ten cycles. Therefore,compared with impurity dopants,the oxygen vacancies in TiO₂ show a marked advantage in terms of photocatalytic stability.

PL analysis can provide the information about the separation of photogenerated electron-hole pairs. Fig. 7 shows that the PL spectra of black bicrystalline TiO_2-x and white bicrystalline TiO₂ are characterized by two peaks at around 360 and 380 nm corresponding to the direct and indirect bandgap transitions of TiO₂,respectively ^[30]. The PL intensity of black bicrystalline TiO_2-x is obviously lower than that of white bicrystalline TiO₂. This indicates that in addition to the heterojunction effect of white bicrystalline TiO₂,the photoinduced carriers can be further separated by the oxygen vacancies in black bicrystalline TiO_2-x. The effect of oxygen vacancies on the band structure of anatase and rutile TiO₂ has been reported ^{[31, 32]}. Oxygen vacancies can narrow the bandgap of TiO₂ and form a similar impurity level in the bandgap of two-phase TiO₂. However,how to influence the band structure of TiO₂(B) using oxygen vacancies has not been reported. Thus,we were unable to confirm the band structure of black bicrystalline TiO_2-x and their charge-hole transfer. The PL results reveal that incorporation of oxygen vacancies into black bicrystalline TiO_2-x is beneficial for hole-charge separation,providing direct evidence for the origin of its improved photocatalytic performance.

Fig. 7. PL spectra of white bicrystalline TiO2 (1) and black bicrystalline TiO2-x (2).

We prepared black TiO₂(B)/anatase bicrystalline TiO_2-x nanofibers from a porous titanate derivative by calcination in hydrogen. Hydrogen treatment had no influence on the morphology of black bicrystalline TiO_2-x. XPS results showed that there was no Ti³⁺ on the surface of black bicrystalline TiO_2-x. According to Raman analysis,the oxygen vacancies of black bicrystalline TiO_2-x were distributed in both the anatase and TiO₂(B) phases. The results suggested that the content of oxygen vacancies in black bicrystalline TiO_2-x was about 0.58 wt%,according to which the O/Ti atom stoichiometric ratio was 1.97. The photocatalytic activity of black bicrystalline TiO₂ was 4.2 times higher than that of white bicrystalline TiO_2-x and 10.5 times higher than that of anatase TiO₂. There was no deactivation of black bicrystalline TiO_2-x after recycling ten times. Because of its unique characteristics,our group is undertaking further studies of the use of this catalyst in the fields of environment and energy.