**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,

### **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.*

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

### *Perspective Chapter: Black Titania – From Synthesis to Applications DOI: http://dx.doi.org/10.5772/intechopen.110545*

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].
