Photocatalysis is considered as one of the most appealing advanced technology in solution to the environmental problems and non-renewable energy resources depletion with a variety of applications such as synthesis, industry, water, and environmental remediation. [1][2][3][4][5] Although enormous research has been carried out with impressive advancement in photoactive materials, the process of photocatalysis still suffers from low efficiency and poor stability that is far below the requisites for practical applications. The three main key steps that governs photocatalysis includes light absorption, generation of electron-hole pairs, followed by their migration from the bulk to the surface and initiation of interfacial redox reactions upon their arrival at the active sites. [6,7] In order to boost up the photocatalytic efficiency, various approaches have been adopted such as doping, crystal facet exposure, synthesis parameters, reaction environments, hybridization, dimensions, and morphology tuning of photocatalysts. [8][9][10][11] However, the efficiency of these photocatalysts is still limited by their cost, wide band gap, lower stability and poor charge transfer kinetics. [12] In order to address the aforementioned issues, extensive research has been conducted to increase the surface area, such as nanowires, nanosheets, nanotubes, and other hierarchical nanostructures are developed with abundant active sites for redox reactions. [13][14][15][16][17] Similarly, to enhance charge separation, hetrojunctions were explored utilizing different semiconductors with appreciable band alignment. [18][19][20] Alternatively, effective light absorption with undoubtedly decreased recombination and improved photocatalytic performance through defect engineering have been reported. [21,22] Defects in semiconductors greatly alter carrier concentration and interface reaction affecting the overall process efficiency of a photocatalyst. Thus, defects are the most frequently investigated scenario for tuning the properties of photocatalyst. [23,24] However, defects are detrimental to photocatalysis in regard of the recombination centers and lack detailed explanation in terms of carrier concentration, transfer dynamics, band structure, and interface profiles. [25] The emerging trend of defect engineering still brought the opportunity of deliberately manipulating the photocatalyst properties. Although the influence of defects on photocatalytic performance has been previously elaborated in some reviews, their optimization and intrinsic role in photocatalysis is still elusive. [12,[25][26][27][28] Therefore, some novel strategies based on advanced modeling theoretical and experimental knowledge are of great need to elucidate the role of defects in photocatalysis. Subsequently, it has been suggested that different synthesis strategies offer different mechanisms thus altering the band structure, carrier mobility, and so reactivity (Figure 1). It therefore remains a challenging task to account the relationship between defect chemistry and per...