Photocatalytic Applications of Heterostructure Graphitic Carbon Nitride: Pollutant Degradation, Hydrogen Gas Production (water splitting), and CO2 Reduction

In 1972, Fujishima and Honda discovered the water photolysis on a TiO 2 electrode [ 1 ] as the response to the steady increase of energy shortage and environmental pollution caused by industrialization and population growth in 1970 [ 2 ]. Their discovery was recognized as the landmark event that stimulated the investigation of photonic energy conversion by photocatalytic methods [ 2 ]. Due to population growth, high industrialization, and improvements in agricultural technologies, till the twenty-first century, energy shortage and environmental pollution are still challenges [ 3 ]. In recent decades, photocatalysis has become one of the most promising technologies owing to its potential applications in solar energy conversion to solve the worldwide energy shortage and environmental pollution alleviation [ 4 ]. Photocatalysis is the process that involves photocatalyst. “A photocatalyst is defined as a substance which is activated by adsorbing a photon and is capable of accelerating a reaction without being consumed” [ 5 ]. Photocatalysts are invariably semiconductors. Several semiconductors such as TiO 2 , ZnO, Fe 2 O 3, CdS, and ZnS are used as photocatalysts in environmental pollutants treatment and solar fuel production such as methane (CH 4 ), hydrogen (H 2 ), formic acid (HCOOH), formaldehyde (CH 2 O), and methanol (C 2 H 5 OH) [ 6 ]. Due to its photocatalytic and hydrophilic high reactivity, reduced toxicity, chemical stability, and lower costs [ 7 ], TiO 2 has been mostly studied as having the high ability to break down organic pollutants and even achieve complete mineralization [ 8 ]. Due to its large band energy, TiO 2 can only absorb solar energy in the UV regions which only constitutes 4% of the total solar energy irradiated [ 9 , 10 ]