## 1 Introduction  
Increasing populations and modernization, human dependence on energy is increasing continuously, and this is becoming a burden on the global environmental system, as most of the energy is generated from fossil fuels [1]. Various nations have been raising issues of pollution and energy crisis, which is why alternative means of developing renewable and clean energy systems are constantly being explored [1, 2]. Hydrogen (H\({}_{2}\)) gas, which acts as a secondary energy source, holds a significant position in this aspect [3]. H\({}_{2}\) has advantages over other energy resources, because of its lightweight, high calorific value, and pollution-free clean combustion [4]. Since hydrogen gas can be a green source of energy, new and easier methods of production are constantly being discovered; however, currently with the existing technologies, the H\({}_{2}\) production method is quite expensive [5]. Various methods have been adopted for H\({}_{2}\) production such as petroleum-based petrochemical processes [6], steam reforming of methane or propane [7], thermal decomposition of water by nuclear energy [8], solid electrolysis [9], biomass gasification [10] and splitting of water [11]. H\({}_{2}\) production via water electrolysis or water splitting holds a clear edge over other contemporary methods since it does not emit secondary pollution [9; 11; 12]. Production of H\({}_{2}\) by the water-splitting process on the surface of the photocatalyst in the presence of light is being recognized as the most advanced and resourceful technology [12]. The most commonly used materials to streamline this process are nanosized semiconductors that act as photocatalysts and prove to be promising candidates due to their unique physical and optical properties [13]. Despite having many properties, the drawback of these nanomaterials is their inability to work better in the presence of visible or natural solar light [14; 15]. Generally, a photocatalyst directly absorbs a major portion of sunlight for efficient H\({}_{2}\) production. This requires a suitable band gap. A larger-band-gap photocatalyst absorbs sunlight mainly in the UV region. For example, TiO\({}_{2}\) was determined to be a stable and efficient photocatalyst for producing H\({}_{2}\) but its larger band gap (\(>\)3 eV) limits light absorption [14; 15]. It is known that photocatalysts produce electrons and holes in the presence of light, which are responsible for the photolysis process; however, due to the large band gap in the photocatalyst or due to reduced charge separation or high electron-hole recombination, their efficiency is nullified [15]. These shortcomings ultimately reduce the yield potential and photocatalytic properties of such materials under natural solar light. This may not prove useful considering H\({}_{2}\) production requirements [15]. In view of the above deficiency, many upgradations have been made, the most useful is the development of heterojunction composite materials [16; 17; 18]. Heterojunction composites have better optical properties due to new crystallographic phases than the original oxides, the creation of defect energy levels in the band gap region, and changes in the surface characteristics of the individual oxides due to the formation of new sites at the interface of the components, increasing the stability of the photoactive crystalline phase [16; 17; 18]. From a photocatalytic point of view, coupling two materials leads to the formation of a heterojunction with different positions of energy bands. Previous studies have suggested that there can be three types of heterojunctions depending on the position of the band edge such as straddling gap (type I), staggered gap (type II) or broken gap (type III) [19]. Currently, a type I heterojunction is considered attractive and advanced, responsible for many benefits such as a narrow energy band gap, prevention of recombination of photo-generated electron holes, and charge separation [19].