**1. Introduction**

Global energy consumption has remarkably increased due to the continual increase in world population and lifestyle standards [1]. Transportation and industrial sectors lead to high emissions of greenhouse gases including carbon monoxide, and carbon dioxide, 90% of the global energy and fossil fuel supply amounting to substantial depletion of the environment [2, 3]. Eventual depletion of energy supplies, market uncertainty, and mitigating the consequences of fossil fuel burning demands the development of clean and renewable resources of energy [4–6]. There are several alternative energy sources, including geothermal energy, hydropower, wind power, and solar energy, which are relatively cleaner and more sustainable than fossil fuels. However, the substitution of natural resources with alternative resources is much more challenging due to the limitations of each resource. Such as we cannot store the electricity which is produced by wind turbines. Hydropower undergoes dam construction limitations due to possible adverse environmental effects and high

costs. Geothermal energy is a costly operation because it is a continuous source and is limited in its lifetime [7]. Renewable and free solar energy is unlimited and capable of producing electricity without any maintenance or requirement of having turbines. Half an hour of solar radiation on the earth's surface can be used for 1 year [8].

Though, sunlight is an intermittent energy source that limits the ratio of solar radiation due to its dependence on the geographical season, day, time, and position [9, 10]. Another major disadvantage of solar energy is its low density per unit area of the earth's surface [11]. Therefore, to meet the global energy demand it is very necessary to develop a source of energy that is continuous, storable, and renewable. In this line of research, hydrogen is a profitable fuel for being profuse from different sustainable sources of water and biomass, high energy yield and efficiency, ecofriendly and capable of storage, thus it is considered an ideal alternative source of energy for nonrenewable energy source [12, 13]. Photocatalytic water splitting has attracted considerable attention as a potential renewable energy resource with limited use of fossil fuel and no CO2 emission [1–4, 14]. Recent development in photocatalytic systems for photocatalytic water splitting can be divided into two main approaches. In the first approach, water is split into hydrogen and oxygen by visible light irradiation on the photocatalyst. In this type of system, the photocatalyst should have the suitable thermodynamic potential for photocatalytic water splitting. The narrow band gap of the photocatalyst system harvests visible light photons and provides stability against photo corrosion. Due to the rigorous requirement of photocatalyst, one-step water splitting is limited [5, 6]. The second approach is to apply a two-way mechanism by using two different types of photocatalysts [7]. These mechanisms were inspired by the natural photosynthesis phenomenon in green plants and are called the Z-scheme. The advantage of water splitting under the Z scheme is that a wider range of visible light is available because a change in Gibbs free energy is required for the one-step water splitting for the separation of water molecules and oxygen. Use of semiconductors used for either reduction or oxidation potential for one side of the system. For example, use of metal oxides such as WO3 & BiVO4 act as good oxygen evolution photocatalysts in two-way water splitting systems by using a suitable redox mediator, even though they are unable to reduce water [8, 9]. Overall water splitting via two-step photoexitation by the use of visible light and with different combinations of photo catalysts has been successfully reported [8–13, 15, 16]. However, there are several challenges in the promotion of electron transfer between two semiconductors. In addition, the photocatalytic activity of water splitting is strongly dependent on the physiochemical properties of photocatalysts such as the nature of active sites and the reaction conditions [2, 3, 14]. In past decade there are a number of material have been reported as a visible light active photocatalyst, which produces both oxygen and hydrogen under visible light irradiations [1, 4–17]. However, a number of photocatalysts have successfully achieved water splitting without any reagent. Significant progress has been made in the development of cocatalysts and the interpretation of reaction mechanisms. This perception of the mechanism highlights some important aspects of the recent development in water-splitting research.
