Introduction

Fundamentals and thermodynamics of photocatalysis

Advancements in photocatalysts for water splitting

Heterojunction construction

Factors that influence photocatalyst activity

Advancements in photoreactors

Conclusion and future perspectives

References

Abstract

Water splitting for hydrogen production under light irradiation is an ideal system to provide renewable energy sources and to reduce global warming effects. Even though significant efforts have been devoted to fabricate advanced nanocomposite materials, the main challenge persists, which is lower efficiency and selectivity towards H2 evolution under solar energy. In this review, recent developments in photo-catalysts, fabrication of novel heterojunction constructions and factors influencing the photocatalytic process for dynamic H2 production have been discussed. In the mainstream, recent developments in TiO2 and g-C3N4 based photo-catalysts and their potential for H2 production are extensively studied. The improvements have been classified as strategies to improve different factors of photocatalytic water splitting such as Z-scheme systems and influence of operating parameters such as band gap, morphology, temperature, light intensity, oxygen vacancies, pH, and sacrificial reagents. Moreover, thermodynamics for selective photocatalytic H2 production are critically discussed. The advances in photo-reactors and their role to provide more light distribution and surface area contact between catalyst and light were systematically described. By applying the optimum operating parameters and new engineering approach on photoreactor, the efficiency of semiconductor photocatalysts for H2 production can be enhanced. The future research and perspectives for photocatalytic water splitting were also suggested.

Introduction

Natural resources such as coal and petroleum products as a source of energy are nearly exhausted [1]. The reduction of fossil fuel reserves has prompted substantial research efforts toward the usage of hydrogen (H2) as an environmentally friendly energy carrier for the post fossil fuel regime [2]. It is currently generally agreed that H2 may be the best option for tackling the triple issues of exhaustion, pollution and climate change effects [3]. One of the technologies for H2 production is photocatalytic water splitting, since it entails photonic energy, which is the most abundant energy resource on the Earth [4]. Previous research states that solar based H2 generation by photocatalysis provides near zero global warming and air pollutants [5], and can be stored easily [6]. Therefore, H2 is considered as a possible important energy in future, since it is free from toxic and it can produce high energy content from natural resources such as light (photon) energy and water, which are clean, long lasting sources of energy, and renewable resources [7]. Pioneer work as early as 1972 by Fujishima and Honda [8] reported water splitting for H2 production over TiO2 semiconductor. Since then, various types of semiconductors for photocatalytic H2 productions are under investigation. Among all, titanium dioxide (TiO2) with band gap 3.2 eV is a recognized photocatalyst and it has been extensively studied because of numerous advantages such as low cost, high photochemical stability and non-toxic [6,9]. On the other hand, wide band gap limits its applications under visible light and faster charges recombination rate lowers its photocatalytic activity [6,10]. Coupling TiO2 with visible light semiconductors can narrowing the band gap with faster charges separation, thus could enables enhanced photo-catalytic activity. Among the low band gap semiconductors, polymeric graphitic carbon nitride (g-C3N4) has attracted more attentions as metal-free polymeric semiconductor in photocatalytic water splitting. It is a visible light responsive with lower band gap and low cost semiconductor. It can be synthesized from cheap precursors such as melamine and urea by simple thermal approach. In addition, g-C3N4 has numerous advantages such as high thermal and chemical stability and appropriate band structure (2.7 eV) to absorb visible light irradiation [11]. Among the limitations, g-C3N4 has low surface area and small active sites for interfacial (photon) reaction, moderate oxidation reaction of water to Hþ and low charge mobility which disrupt the delocalization of electrons.