The development of sustainable and environmentally friendly energy sources is considered one of the biggest challenges humanity must face nowadays [1, 2]. Among the sustainable energy sources proposed, such as those originating from sunlight and wind, require them to store the electrical energy produced [3]. An interesting option for storing energy is the production of hydrogen as a secondary energy carrier [4].
A great deal of work has been carried out on splitting the water by direct incidence of solar light [5–8], since Fujishima and Honda observed that illuminating TiO2 resulted in the generation of hydrogen and oxygen [9]. TiO2, in particular, is a semiconductor that has been shown to be an interesting candidate because of its low cost, low toxicity, good position of the relative bands to the water oxidation-reduction potentials, and good stability against photo-corrosion [10]. However, its large band gap (3.0 ~ 3.2 eV) [11], high recombination occurrence, and slow charge transfer hinder the high output generation of H2 [12]. An attractive morphology that has been shown to increase the material surface area and its charge transfer is by the growth of self-aligned nanotubes (NTs) [13]. The NTs are currently used in various fields, such as electrochemical sensing [14, 15], photoreduction of CO2 [16], water treatment [17], Li-ion batteries [18], and photo-catalysis [19], to list the main ones. Several routes were developed for the NTs synthesis, such as the use of molds, sol-gel, solvothermal, and anodization [20, 21]. Among the various techniques for growing NTs, the anodization path has the greatest advantages, as it produces spatially ordered NTs and is an economically feasible and scalable method for their production. However, the synthesis of NTs does not solve all the problems of TiO2. In order to improve their photoelectrochemistry, the NTs are typically doped with metals, other semiconductors, and non-metals [22]. Nonetheless, some of the doping materials bring other problems, such as the creation of structural defects that produce charge recombination [23]. An attractive solution that was first discovered in 2011 by Chen et al. [24] is the modification of TiO2 by the partial reduction of Ti4+ to Ti3+, decreasing the material's band gap and improving its conductivity. This new energy levels are located 0.73 and 1.18 eV below the CB [25]. The reduction causes the material to darken, taking on a bluish/black color, thus referring commonly to "black titania". Several reduction methods can be found in the literature, such as hydrogen atmosphere reduction, reduction with metals, and electrochemical reduction, among others [26, 27]. The later one presents several advantages compared to the others, as it presents a mild process that is economically feasible and can also be scalable.
Titanium is commercially available in six different grades (1, 2, 3, 4, 7, and 11) and several alloys. Among the pure grades, grade 2 presents a titanium content of up to 99.3% and is the most common for corrosion resistance applications [28]. Furthermore, titanium of higher purity, with contents above 99.6%, might be obtained from specialty chemical vendors for research activities. Thus far, few reports have been published on the effect of titanium substrate purity on the photoelectrochemical response of a photoanode [29].
In the present work, nanotubes were grown over Ti sheets electrochemically and they were subjected to a thermal treatment in order to improve crystallinity. A comparison of nanotubes with and without thermal treatment was made in the effect of water splitting. Subsequently, both samples were converted to black titania by electrochemical reduction and mechanistically explanation is given for the photoresponse obtained. Finaly, a comparison of two different grades of Ti sheets were used as substrate: high purity (HP) Ti and commercial grade 2 (G2) Ti, to compare its effect in the photoelectrocemical measurements. The structural characterization of the NTs was carried out by scanning electron microscopy (SEM), diffuse reflectance, Raman, and X-ray diffraction (XRD). Photoelectrochemical characterization for the generation of H2 through water splitting was carried out by linear sweep voltammetry (LSV) and chronopotentiometry (CP) with and without light. Moreover, the photoelectrochemical characterization was carried out with the use of a Xe lamp (UV-Vis) and a monochromatic light source.