# Highly Active under VIS Light M/TiO\({}_{2}\) Photocatalysts Prepared by Single-Step Synthesis
## 1 Introduction  
Methylene blue (MB) is an organic compound that is known as a significant water pollutant, contributing to the deterioration of the quality of water in oceans, seas, lakes, rivers, etc. It is a toxic and non-degradable chemical that is discharged from the textile industry. The photodegradation of MB is proven to be an effective method for pollutant removal, and it can be used to protect aquatic environments [1]. Methylene blue is also regarded as a benchmark substance for the performance testing of various photocatalysts. Methylene blue is a thiazine dye used in the textile industry, and it has regularly been exploited for comparing the effectiveness of synthesized photocatalysts by measuring the latter's ability to stimulate MB photodegradation. Titanium dioxide or titania (TiO\({}_{2}\)) is an established photocatalyst with good capabilities concerning MB photodegradation; however, it is afflicted by a significant drawback: the limitation that restricts it to advantageously making use of the ultra-violet (UV) portion of solar radiation, which amounts to only 5% of solar radiation [2, 3, 4, 5]. The adsorption-desorption equilibrium that is in effect during the degradation process when the catalyst is pure TiO\({}_{2}\) has been investigated, as well as the effects of the wavelength of the light in use, the concentration of TiO\({}_{2}\), and the initial MB concentration [5]. To remedy the aforementioned disadvantage of pure TiO\({}_{2}\) photocatalysts, various efforts have been made by using different metals, such as dopants, while also studying the effect of their concentration in the catalysts. The aim of experimenting with different doping agents is to facilitate the activation of the TiO\({}_{2}\) catalysts by visible light alongside UV radiation. The role of the photocatalyst in the degradation of MB is absorbing light radiation, containing photons to enable electrons to migrate and create positively charged holes. These holes then cause the oxidation of water, forming hydroxyl radicals that target and degrade organic pollutants, such as MB [5]. In the literature, there have been several studied attempts at synthesizing photocatalysts for the degradation of methylene blue apart from using TiO\({}_{2}\), such as the dopingof ZnS nanoparticles with Cu under both visible and UV radiation [6]. The magnetite (Fe\({}_{3}\)O\({}_{4}\))/H\({}_{2}\)O\({}_{2}\) catalyst has also been studied under UV irradiation with the concentrations of magnetite, H\({}_{2}\)O\({}_{2}\), and MB, as well as the initial pH and the source of light being parameters whose influence was evaluated through experimentation [7]. ZIF-8, a metal-organic framework (MOF), was studied on its use as a photocatalyst for the degradation of MB, and the effects of initial dye concentration and pH were considered [8]. Experimental studies with a Co\({}_{3}\)O\({}_{4}\)/ZnO nanocomposite prepared through precipitation have taken place in the literature for the evaluation of a photocatalyst for MB degradation [1]. A ZnO/Eu nanocomposite system has also been investigated as a possible catalyst for the degradation of MB [9]. A ZnO/SnO\({}_{2}\) nanocomposite has been reported in the literature as being able to boost the performance of pure ZnO [10]. On the other hand, the application of Co has been tested as a dopant for TiO\({}_{2}\) using a sol-gel method which exhibited better efficiency than pure TiO\({}_{2}\) under visible light; however, results were not equally promising for UV radiation [11]. Another case of TiO\({}_{2}\) doped with graphene has also been studied, aiming to create a porous composite that would be able to improve the degradation efficiency to a significant degree, as it exhibited a 6.5 times higher rate constant over Degussa P25 [12]. Furthermore, graphene oxide has been widely used as the precursor of graphene (GR) to synthesize graphite-based hybrid photocatalysts for solar-to-chemical energy conversion [13].