**4.3. Photoelectrochemical water splitting**

Hydrogen energy is a key issue to cope with the present global energy crisis and environmental complication exploiting clean and inexhaustible energy [128]. Photoelectrochemical (PEC) water splitting is a promising technique to create hydrogen fuel by utilizing solar energy. Within the nonstop efforts in developing efficient photoelectrodes, the major challenge researchers presently face is to explore cost effective, nontoxic, and earth-abundant photoelectrodes with high efficiency [129]. In a recent study, Ag/Ag2WO4 was fabricated on ZnO nanorods using 0.05 M AgNO3 and 0.05 M Na2WO4 as the cationic and anionic precursors, respectively, by following SILAR technique and the composite material demonstrated outstanding performances in PEC water splitting with 3 mAcm<sup>2</sup> at 1.23 V versus RHE in the presence of 0.1 M Na2SO4 electrolyte. Based on these results, a brief possible updated mechanism of the PEC activity was demonstrated by Adam et al. for the better understanding of the technique with **Figure 6** [130]. The development of PEC activity of the semiconductors was principally attributed to electrons and hole transfer at the interfaces of the photoelectrodes. The band edge potentials of the Ag/Ag2WO4 and the ZnO materials showed a significant role in the efficiency of growth and separation technique of the electron (e) and hole (h<sup>+</sup> ) pairs. The energy of valence band (EVB) of ZnO and Ag2WO4 is calculated as +2.86 and + 3.03 eV, whereas energy of conduction band (ECB) of them is projected as 0.34 and 0.07 eV, respectively [131].

In sunlight, both the semiconductors absorb light and the electrons in the VB become excited up to a higher potential of 0.34 and 0.07 eV for the ZnO and Ag2WO4, respectively. Consequently, due to high photon energy, within the semiconductor, the effective charge transfer process proceeds. Ag<sup>0</sup> nanoparticles (NPs) cause active separation of h<sup>+</sup> or e pairs upon the absorption of light owing to the surface plasmon resonance (SPR) effect. Electrons from the Ag NPs are transported to the CB of the Ag2WO4 and the ZnO, while holes persist in the Ag NPs. In the meantime, to occupy the vacant holes created by the plasmonic absorption, the

#### **Figure 6.**

*(a) and (b) FM-SEM images of the ZnO NRs and the ZnO/Ag/Ag2WO4 heterostructure. (c) Curves of the ZnO NRs, and the ZnO/Ag/Ag2WO4 photo-electrodes under light and dark conditions using linear sweep voltammetry. Schematic diagram presenting the energy band structure and probable electron-hole separation as well as transportation in ZnO/Ag/Ag2WO4 heterostructure with the SPR effect [130].*

photogenerated electrons in the CB of ZnO will be transported to the Ag NPs [132]. The photogenerated charge carriers can be proficiently separated to enhance the PEC performance by following this mechanism. Further, the photogenerated electrons will eventually reach at the Pt electrode (counter) and contribute to H2 generation. Also, the photogenerated holes in the VB of Ag2WO4 and ZnO will contribute on O2 production *via* H2O oxidation. Hence, these outcomes validate the modification *via* Ag/Ag2WO4, which is an active technique to attain a high PEC activity by means of ZnO NRs arrays.

Like the above example, many studies based on SILAR had been devoted toward exploring the potential of semiconductor thin films as photoelectrodes for water splitting as shown in **Table 6** with their potential applications. In terms of low-cost, simplicity, and theoretically high solar to H2 efficiency, PEC water splitting is much more favorable than solar photobiological, photochemical, and thermochemical generation of hydrogen [146]. The most investigated semiconductor materials include BiVO4, Fe2O3, CuCoO2, WO3, and TiO2 [147]. Other semiconductor materials such as Cu2O [24, 148, 149], ZnO [150, 151], TiO2 [152, 153], and CdO [28] were also produced using SILAR method but the PEC performances are quite low under visible light due to their wide bandgap.

### **5. Factors affecting SILAR deposition**

A lot of research work has been done on the deposition and optimization of the SILAR thin films for optoelectronic device applications. Solution concentration, composition of precursors, the number of SILAR cycles, pH, annealing, and doping will absolutely affect the quality and quantity of thin films, which directly influence the cell performance. The effect of different parameters used in SILAR deposition on the performance of thin films is reviewed based on the contemporary research work.

#### **5.1 Solution concentration**

Solution concentration of the used precursor is one of the key factors in governing the properties as well as the performances of SILAR grown thin films. From a general viewpoint, depositing through a more concentrated solution results with bigger grain size and higher surface roughness during deposition. Consequently, thinner, smoother, and probably pinhole-free deposition can be attained using multiple SILAR cycles with a lower concentrated precursor solution.

With the increase of molar concentration (0.03, 0.05, and 0.1 M) of the cationic solution prepared by Cd(CH3COO)2 and H2O2, the surface morphology of the SILARdeposited nanostructured CdO thin films was improved toward the crack free and homogeneous nature [154]. On the other hand, the structural change such as nanorods, nanoflowers, and nanoflakes morphologies was observed by altering only the concentration of anionic precursors NaOH (high, 0.05 M; moderate, 0.01 M; and low, 0.001 M) with fixed Zn precursor concentration (0.005 M) [155]. A comparative study of CdS films deposited by SILAR and CBD techniques revealed that the S/Cd ratio in the sample increases (0.83 to 1.04) for SILAR deposited films with the molar concentration of sulfur (1:1, 3:1, 5:1 and 7:1) in the starting solution increases, while it was almost constant (0.80) for CBD films [156]. During the investigation of the effect of the molar concentration of pyrrole monomer on the electrochemical behavior

