**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 TiO2-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

### **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.*

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

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 vacuumdesiccated 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].

### **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 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.*

### **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.*

### **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.*
