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X-ray absorption spectroscopy (c) Fourier transform infrared spectroscopy (d) Raman spectroscopy 2. Physical properties (i) Physical structure (a) Electron microscopy (b) Atomic force microscopy (c) Gas adsorption–desorption analysis (ii) Crystallographic properties (a) X-ray diffraction (b) Transmission electron microscopy (iii) Optical absorption (a) Diffuse reflectance spectroscopy (b) Finite-difference time-domain method (iv) Charge dynamics (a) Photoluminescence spectroscopy (b) Transient absorption spectroscopy (c) Surface photovoltage and photocurrent spectroscopy (d) Electrochemical impedance spectroscopy (v) Defects (a) Electron spin resonance (b) Positron annihilation spectroscopy (c) XPS valence-band spectrum (vi) Colloidal stability (a) ζ- potential (vii) Thermal stability (a) Thermo gravimetric analysis 3. Band structure (i) Band gap (a) Tauc plot (b) Photoluminescence and surface photovoltage spectroscopy (ii) Band edges and band edge offsets (a) Density functional theory (b) XPS spectrum (c) Ultraviolet photoelectron spectroscopy (iii) Fermi level (work function and flat-band potential) (a) Kelvin probe force microscopy (b) Secondary electron cutoff (c) Photoelectrochemical methods

Figure 1.4 Strategies for the fabrication of photocatalysts [15].

      1.3.5 Design Challenges of Photocatalytic Reactors

Designing a suitable reactor set up for a photocatalytic reaction is not an easy task due to lack of proper research about various challenges and section criteria. Scaling and optimization studies related with photocatalysis are not elaborated in order. Only oxidation and reduction steps are considered while material balance is not into the real manner along with unidentified side reactions or by-products. The necessary information regarding side products is still missing in the literature related with such reactor design studies. Therefore, the researchers should be focused on suitable reactor kinetics so that hidden valuable aspects can come into the scene and poisoning of photocatalysts can be avoided to enhance the activity or stability of those catalysts will for lowering down the activation energy [5].

      Further, the design of any reactor is dependent upon the rate limited step in the reaction mechanism. Separation of the catalyst from reaction mixture is also a quite challenging task but fluidized bed and immobilized catalysts are found to be two possible solutions for resolving separation issues. On a small scale, a laboratory set up has been installed to carry out research on slurry reactors which can be an option to raise the performance of system than immobilized catalytic reaction system. Rather, the researchers are working to minimize water and air pollution problems occurring during process but large scale applications on optimization of reaction systems are still absent.

      For continuous flow in organic synthesis, there are some types of reactors for multicomponent-multiphase reactions such as single channel reactor, membrane micro reactor, and falling film micro reactor. In some cases, a photocatalytic pervaporation reactor has been designed for enhancing the yield of vanillin as double. On the other hand, Alfano et al. [18] complied valuable information about reactor design for the photocatalytic degradation of contaminants present in wastewater streams via heterogeneous photocatalysis mechanism.

      Over oxidation of the reactant is also an important lack in photocatalysis which is responsible of lowering down the selectivity even high conversion. It is necessary to attain high selectivity and high conversion in such conditions but this issue can be resolved by involvement of flow reactors as batch reactor are not capable for achieving desired output. Therefore, a research should be done in the direction of flow reactors to gain complete conversion or other necessary products because continuous output is achieved all time in flow reactors.

      For designing reactors for photocatalysis, the selection of light source (UV, visible, natural solar, etc.) is important consideration to industries for the maximization of desired products. Using mercury or xenon lights are conventional in nature in photocatalysis but their efficiency is very low. For reactions in microfluidics reactors, light emitting diode is used in photocatalysis which is quite popular nowadays because it minimizes heat loss and optimizes energy. Quartz and glass materials are very common in making photocatalytic reactor. Quartz utilizes both UV and visible

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