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

### **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.*

hydrogen source is considered an important source of hydrogen. Black titania-based photocatalysts have been extensively used for generating hydrogen through a watersplitting 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

### **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.*

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].
