Fig. 1 illustrated the XRD patterns (normalization based on the intensity of diffraction peak (110)) of different TiO2. The main phase of samples hydrogenated at different temperatures all maintained rutile TiO2, and no obvious change was observed among 700℃-H-TiO2, 750℃-H-TiO2, 800℃-H-TiO2, and white TiO2. Higher hydrogenation temperatures led to the formation of suboxide titanium, reflecting as tiny splits in the main diffraction peak (27.4°), and a reconstruction process induced by titanium suboxides during reduction processes brought other effects which were detailed discussed in supporting information [25, 26].
The information about appearance, absorbance, and bandgaps of different TiO2 was given in Fig. 2. According to Fig. 2a, the colors of black TiO2 were gradually darkened as hydrogenation temperatures went up. The deepening of colors brought photocatalysis marked significance that visible light could not be absorbed by white TiO2 but black TiO2 broke through this limitation. The UV-Vis DRS showed in Fig. 2b exhibited a similar result that the visible-light absorbance of white TiO2 was almost 0, while black TiO2 had significantly stronger absorbance proportional to hydrogenation temperatures. The values of bandgaps shown in Fig. 2c were obtained by mathematical processing Fig. 2b and the following equation:
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Here, α, υ, and Eg represented absorption coefficient, frequency, and bandgaps respectively. This formula was the main calculation method of black TiO2 bandgap and was widely used by researchers [27,28]. The bandgap of white TiO2 (rutile) in our experiments was 2.94 eV which was closed to experienced data 3.0 eV, while black TiO2 had constantly narrowing bandgaps with hydrogenation temperatures raising. 800℃-H-TiO2 had a markedly narrowed bandgap of 2.71 eV, and furtherly the bandgap of 900℃-H-TiO2 was down to 2.05 eV. These characteristics indicated that white TiO2 nanoparticles were successfully transformed to black TiO2.
Fig. 3 presented the micromorphology information of black TiO2 hydrogenated under different temperatures. White TiO2 nanoparticles without treatment were in the shape of spindles with a length-diameter of nearly 50 nm, while black TiO2 nanoparticles were ellipsoidal in obvious sphericity due to the thermal effect in annealing processes, and the size of nanoparticles also slightly increases continuously with annealing temperature raising. In this paper, the nanoparticle sizes of different black TiO2 were mostly between 70-110 nm, so the interference of large particle size on photocatalytic performance could be eliminated. The lattice-disordered shell was a significant feature of traditional black TiO2, which was interestingly not observed in our hydrogenated materials. Lattice stripes at the edge of black TiO2 nanoparticles were still clear, indicating quite low surface disorder and defect concentration. White TiO2 had an interplanar spacing of 0.319 nm in (110) face, while this parameter in black TiO2 expanded to 0.342 nm. This 7.2% expansion was due to the existence of VO enriched in the body of black TiO2, and subsequent analysis of crystal defects further verified this conclusion.
The photocatalytic activity of different TiO2 was determined by the time-depend residual of RhB, as shown in Fig. 4. RhB itself did not degrade under visible light, so its concentration change was caused by the photocatalytic effect of black TiO2. When the different mixtures of TiO2 powders and RhB water solution had just been exposed to light, their absorption-dissociation balance reached in the dark would be disturbed, causing an abnormal upward change in the C/C0 curve of RhB in the first 20 min and this phenomenon disappeared with time growth. As expected, white TiO2 had no visible-light photocatalytic activity due to its poor ability in utilizing visible light, while black TiO2 had obvious enhanced photocatalytic activity driven by visible light, further suggesting that black rutile TiO2 were successfully prepared. RhB degradation rates of black TiO2 increased with hydrogenation temperatures and visible-light absorbance before 800℃, but an abnormal phenomenon of sharply decreased photocatalytic activity was observed in the experiments of 850℃-H-TiO2 and 900℃-H-TiO2, although they had darker colors and narrower bandgaps. This anomaly could be attributed to the changes in crystal planes of different black TiO2 and the difference in photocatalysis induced by crystal planes [29,30]. This would be detailly discussed in supplemental information. In general, 800℃-H-TiO2 had the best performance that it could degrade more than 70% RhB in 2 hours, and we also chose another substrate Cr6+ that was more difficult to decompose to further illustrate the excellent photocatalytic activity of our material, which was revealed in Fig S1.
800℃-H-TiO2 was selected for more tests and a series of stability experiments because of its optimal performance, and we adopted EPR measurement to reveal the changes from white TiO2 to black TiO2 from the angle of lattice defects. As shown in Fig. 4a, for 800℃-H-TiO2, a small peak at g=2.005 indicated the generation of a small number of VO [24,31], while the spectrum of white TiO2 with few crystal defects appeared horizontal. This result explained the variation in bandgaps and absorbance from white TiO2 to 800℃-H-TiO2. VO produced donor levels under the conduction band [17] and then extended the bottom of the conduction band downward, thereby narrowing bandgaps of black TiO2. It was worth noting that different VO peaks correspond to diverse positions of existence, and black TiO2 reported previously usually had the peaks at both nearly g=1.98 (in the surface and in coexistence with Ti3+) and nearly g=2.005 (in the body) [24], while the peak at g=1.98 in 800℃-H-TiO2 in this paper seemed not obvious. This analysis meant that in 800℃-H-TiO2 VO was concentrated mainly inside nanoparticles. According to Fig. 5b, the double peaks at 458.7 eV and 464.5 eV in white TiO2 could be all assigned to Ti4+ [14]; the spectrum of 800℃-H-TiO2 showed a similar result that the total peak was almost identical to the Ti4+, also indicating that defects Ti3+ barely existed in the surface of black TiO2 nanoparticles. Fig. S2 further illustrated that only a few surface defects existed in our material, and those characteristics may contribute to the excellent stability of 800℃-H-TiO2.
VB-XPS spectra shown in Fig. 5c gave the levels of valance bands (VB) in different TiO2. For white TiO2, the maximum of VB (VBM) was at about 2.65 eV toward the vacuum level, and 800℃-H-TiO2 had a similar VBM of 2.63 eV. The improvement effect of VO on black TiO2 was mainly through introducing donor levels in its conduction band (CB), so their valence band (VB)did not change significantly. The position of CB could be obtained by combining bandgaps with VB. The minimum of CB in white TiO2 was at -0.29 eV, while that of 800℃-H-TiO2 was extended to -0.08 eV. This change coincided with the generating of donor levels in the edge of CB, so the bandgaps of black TiO2 were narrowed and the excitation driven by visible light was also promoted.
Fig. 6 gave the appearance and photocatalytic activity of 800℃-H-TiO2 after natural aging treatment for 18 months in the air atmosphere. As shown in Fig 6a, 800℃-H-TiO2 after 18 months of aging treatment still retained a typical appearance of black TiO2, with no significant difference from initial samples. Fig 6b showed the comparison of photocatalytic performance of 800℃-H-TiO2 before and after aging treatment. 800℃-H-TiO2 after aging treatment could degrade 65.3% RhB in 2 h, and its photocatalytic activity was only 10.5% lower than that of the samples before aging treatment, indicating that our black TiO2 contains great stability and were only slightly oxidized after long-term storage in air. Corresponding to our works, black TiO2 mainly depending on surface VO experienced a similar degree of photocatalytic performance attenuation in only 6 months [22]. The circulation experiment result of 800℃-H-TiO2 after aging treatment was shown in Fig. 6c. Residual efficiency was calculated by the following equation:
![](data:image/png;base64,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)
The suspension (include the part which was used to measure absorbance) during catalytic experiments was all collected, after that, black TiO2 powder was filtered from the suspension, cleaned in ultrasonic, and dried in argon. RhB concentration was correspondingly reduced in each subsequent cycle experiment due to the slight quality loss of catalyst, while other experimental parameters of photocatalysis were the same as those of Fig. 3. The photocatalytic activity of 800℃-H-TiO2 during 5 cycles of experiments did not change significantly, which further demonstrated its good storability.
To highlight the effect of pH on the material itself, the black TiO2 powders were pretreated with acid-base instead of directly being put into reaction solutions with different pH as commonly used. Aqueous solutions with different pH were regulated by HCl or NaOH, and then the 800℃-H-TiO2 powder after 18 months of aging treatment was dispersed in different water for 180 h, respectively. The parameters of subsequent processes were the same as above, and relevant results were shown in Fig. 7. Long-term immersion in acid solution did not decrease the photocatalytic activity of black TiO2, even that the photocatalytic activity of black TiO2 was improved due to changes in surface states driven by protonation [32]. A large number of H+ interacted with black TiO2 for a long time, which might improve the absorption capacity of materials to RhB and thus increased their photocatalytic activity. The alkali group gave results of slightly reduced photocatalytic properties of black TiO2 after being placed in alkali solutions for a long time. In the group of pH=8, 800℃-H-TiO2 remained 91.4% photocatalytic activity after 180 h treatment, and 800℃-H-TiO2 had nearly 80% residual efficiency after being immersed for 180 h at a stronger alkali solution of pH=10 or 12. The logarithmic diagram in Fig. 7c showed that the photocatalytic reactions of black TiO2 before and after treatment were all first-order reactions. In conclusion, our black TiO2 had excellent acid resistance, while when they were long-term used in alkali water, a little risk of performance decrease may be concerned.