**2. Mechanistic aspect for water splitting**

In the development of overall photocatalytic water splitting under visible light a number of photocatalytic materials and preparation methods have been studied. A number of studies have focused on the material development of materials that

#### *New Strategy to Improve Photocatalytic Activity and Mechanistic Aspect for Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.109960*

are suitable for overall water splitting and their visible light absorption properties, crystallographic quality, phase purity, band edge position, and particle morphology. However, it is very challenging to identify the factor which dominates the net photocatalytic activity based on physical properties. Photocatalytic reactions proceed through an intricate sequence of competing for multistep processes. This multistep process establishes the kinetics and dynamics of photocatalytic water-splitting reactions for future applications [18]. The rate of water splitting with the modified photocatalyst was proportional to light intensity under solar irradiations. Excess loading of the co-catalyst did not improve the photocatalytic water splitting rate. The experimental results define the shortage of photo-exited carriers available for surface redox reactions under visible irradiations, this indicates that the balance between the rates of redox reactions on the surface of photocatalyst and charge carrier recombination in the bulk photocatalyst determines the charge concentration in the photocatalyst. The kinetic model of photocatalytic water splitting also determines the rate of reaction and probability of photoexcited holes for oxygen evolution and recombination. It was easy to assume two different co-catalyst distinctly facilitate hydrogen and oxygen evolution thereby stimulating overall water splitting in harmony. As stated earlier, visible light-responsive photocatalyst has been devoted to the development of active sites on photocatalyst and elucidating reaction mechanisms, which leads to significant progress in the field of heterogeneous photocatalytic water splitting [18–31]. Conversion of solar energy most efficiently can achieve overall water splitting under longer wavelength irradiation; this is because the number of accessible photons in the solar spectrum increased with an increased wavelength of the solar spectrum.

In this line of research, the development of a photocatalyst having a wider absorption band is highly desirable for overall water splitting. Although, a photocatalyst with an absorption edge of 600 nm would be optimal for the activation of the surface barrier and so surface reactions that can produce hydrogen and oxygen. The most promising candidates in this category have already developed, such as LaTiO2, Ta3N5, and Ti2S2O5 with a band gap of 2 eV which means they have an absorption edge near 600 nm [2]. However, the photocatalytic activities of these materials are not sufficient to achieve overall water splitting. Recent progress has been also made in material chemistry towards reducing the density defects [13, 31]. Consequently, it is very important to study the nature of defects, which can facilitate the undesirable electron–hole pair recombination in the photocatalytic system. In a two-step, a watersplitting system for BaTaO2N12 and Ta3N5 the absorption, the wavelength has been increased to 660 nm [16]. The research in this area is underway in direction of both photo catalyst preparations and mechanistic aspects of water splitting processing in harmony.
