ZINC OXIDE NANOSTRUCTURES
The ZnO nanostructures are known to be prepared broadly via vacuum- based or non-vacuum based approaches. Several synthesis techniques, like chemical vapor deposition (CVD), magnetron sputtering, pulsed laser deposition, atomic layer deposition (ALD), electrochemical deposition, spray pyrolysis and hydrothermal have been reported till date for the fabrication of ZnO nanostructures (Baskoutas, 2014; Dhara and Giri, 2012). But the non-vacuum based hydrothermal (water is used as the solvent to prepare growth solution) approach caught greater attention for synthesis on behalf of its simplicity and cost-effectiveness nature. Mostly by using hydrothermal technique for growth of the ID structure, it became convenient for fabrication of large area. Furthermore, the familiarity of the process caused due to the usage of aqueous solution in the growth process, which ultimately reduces the growth temperature to less than the boiling temperature of water.
Several researchers have reported the synthesis of ZnO by various solution-based approaches namely hydrothermal, solvothermal, spray pyrolysis, electrodeposition, etc. The major advantage of this type of procedures is their low cost, low synthesis temperature. This almost gives a substantial cost reduction in mass production as far as commercialization is concerned. In this technique, H,0 acts as a dopant source along with Zn precursor, and hexamethylenetetramine (HMTA) is used for growing ID nanostructure. Prior to this, most reports suggest coating a seed layer of ZnO of few nanometers. As the purpose of dopant and Zn precursor is to provide the doped type of ZnO NRs only, it is important to discuss about the role of HMTA. It is a non-ionic tetra dentate cyclic tertiary amine usually preferred in hydrothermal technique with regards to its high solubility in water. On the application of temperature, thermal degradation occurs resulting in the formation of hydroxyl ions. They further react with the dopants as well as Zn present in the aqueous solution in order to grow doped ZnO nanostructures. In general, the sealed container containing the seed layer substrate immersed in the aqueous solution is kept at 90-95°C for 4-5 hours. Our group has prepared several doped ZnO NRs through a two-step procedure using a facile sol-gel, spin coating, and hydrothermal method. First is the deposition of the seed layer on cleaned substrates followed by the growth of NRs by dipping the seed layers in growth solution in the second step. All the growth conditions including the growth temperature were kept at 90°C for 5 hrs as per our earlier report.
Two main research approaches have emerged in the development of photocatalytic materials with improved photon-to-current conversion efficiency (PCE). The first approach is to tune the electronic structure of ZnO in order to extend its ability to harvest photons in the visible region of the light spectrum (bandgap engineering). The light-harvesting ability can be enhanced by controlling the morphology of the ZnO ciystal (quantum confinement effect), modifying the carrier concentration within the ZnO ciystal lattice (doping), or by functionalizing the surface of ZnO with photosensitizer molecules. The second approach is to promote effective photogenerated charge separation by controlling the defects in the ciystal lattice or incorporating electron transfer (ET) agents. Alternatively, hybridizing ZnO with metallic or semiconductor co-catalysts has proven successful in accelerating water oxidation reactions.
EFFECT OF MORPHOLOGY ON PEC PERFORMANCE OFZNO NANOSTRUCTURES
Various research groups have investigated different morphologies of Z11O nanostructures in order to enhance their photoelectrochemical performance, hi the following section, we have discussed some useful reports available in the literature.
22.214.171.124.1 Nanorods (NRs)
Babu et al. (2015) reported the photoelectrochemical water splitting properties (under both UY and visible light illumination) of ZnO NRs by changing the diameters from 45 run to 275 run. ZnO NRs with 45 run exhibited maximum photoconversion efficiency of 45.3% under 365 nm UY light illumination and 0.42% under Air Mass 1.5 Global simulated solar light illumination. Higher efficiency for smaller diameter NRs was attributed to the increased light absorption due to a decrease in diameter as verified by the theoretical simulation using the finite-difference time domain. Rokade et al. (2017) studied ZnO NRs and ZnO nanotubes as efficient photoelectrodes for photoelectrochemical water splitting. They reported an enhanced photocurrent density of 0.67 niA/cm2 at 0.5V vs. SCE in the case of ZnO nanotubes in contrast with NRs (0.39 niA/cnr). The difference in these values are due to the high surface area of ZnO nanotubes present in the vicinity of electrolyte which helped in harvesting a large number of photons that significantly increased the generated charge carriers and photocurrent density. Moreover, the ABPE for ZnO nanotubes was 0.50% as compared to NRs (0.29%) under visible light illumination (100 mW/cm2 AM 1.5). In another report, Samad et al. investigated the photoelectrochemical performance of ZnO NRs by changing the applied potential in ZnCl, and KC1 electrolyte. The optimized applied potential was IV which exhibited the highest photocurrent density for UV (17.8 inA/cm2) and visible illumination (12.94 inA/cm2) in contrast with the applied potential of 2V (with UY illumination; 11.78 niA/cm2 and visible illumination; 10.78 mA/cni2). The reason behind the highest photocurrent density is that the applied potential of IV produced ZnO NRs with the highest average aspect ratio. The other two applied potentials (2V and
3V) showed dense porous structures which prevented the charge transfer showing a decreased photocurrent density.
Co-catalyst is a noble metal which when loaded onto the surface of ZnO photocatalyst lowers the recombination rate of the charge carriers, enhances charge separation, increases the number of reactive sites, and reduces activation energy for gas evolution. Co-based catalyst deposited photochemically on the surface of ZnO NRs gives enhanced solar O, evolution (Steinmiller and Choi, 2009). This nanoparticulate morphology (10-30 nrn of Co nanoparticles (NPs) uniformly distributed on the surface of ZnO NRs) increases the surface area of catalysts per unit mass while minimizing the blockage of the ZnO surface. The photocurrents measured at a constant bias of 0.0 V and 0.2 V against the Ag/AgCl REs show that the presence of Co-based catalyst enhanced the steady-state photocurrent by 2.6 and 1.5 times, respectively. ZnO NRs coated with a silver film on a flexible substrate like polyethylene terephthalate (PET) can be used as an efficient photoanode in PEC water splitting as reported by Wei et al. in 2012. They reported a maximum photocurrent density (0.616 niA/cm2) and PCE (0.81%) achieved with an optimized Ag film thickness of 10 mn and a substrate bending radius of 6 mm. The improvement in PEC performance of ZnO NRs coated with Ag film may be attributed to the increase of light absorption capability of ZnO: the localized surface plasmonic effect of Ag islands deposited on the surface of ZnO NRs helps to increase the local field strength, resulting in higher absorption. It also helps to enhance light scattering and lengthen the light path thereby increasing the light trapping. Fabrication of platinum nanoparticles (PtNPs) on the surface of ZnO nanorod (ZnO NR) arrays can also act as an efficient photoelectrode in the process of water splitting as reported by Hsu et al. in 2014. They achieved a twofold enhancement in photocurrent density and greater PEC stability for Pt NPs/ZnO NR arrays as compared to pristine ZnO NR electrodes. Pt decorated ZnO NR arrays showed greater separation efficiency of photogenerated electron-hole pairs and faster charge transfer in comparison with pristine ZnO NRs (Hsu et al., 2014). The hybridization of two semiconductor materials also leads to the formation of heterojunction that proves to be more advantageous in charge transport (Moniz et al., 2015). AgSbS, catalyst on ZnO nanotube arrays plays a crucial rule in the enhancement of photoconversion efficiency and photocurrent density (Han et al., 2015). The three important factors influencing the PEC water splitting performance are photo response region, the stability of the semiconductor, and electrons transmission velocity. In order to broaden the photoresponse spectrum of ZnO/AgSbS2, a suitable energy bandgap of this coupled nanostructure is obtained by using miargyrite AgSbS,. As a result, the light absorption efficiency of ZnO/AgSbS, which ultimately leads to the enhancement of PEC performance of this nanotube arrays. A remarkably higher value of photocurrent density (5.08 mA/cm2) and hydrogen production efficiency (5.76%) was reported for ZnO/AgSbS2 nanotube arrays as compared to ZnO NRs (0.41 mA/cm2 and 0.42%). ID nanostructure (nanotube) without crystal boundary resistance have a very high-speed photo-induced charges transmission and lower recombination rate of photogenerated charge carriers. Thus, ID ZnO/AgSbS, nanotubes help in providing a direct electrical path to ensure the rapid collection of the charge carriers.
126.96.36.199.2 Nano Rod @ Nano Platelet
Another strategy to enhance the photoelectrochemical conversion efficiency of ZnO nanostructures was reported by Zhang et al. in 2015, i. e., Au-sensitized ZnO nanorod@nanoplatelet (NR@NP) core-shell arrays. The introduction of Au NPs on ZnO NR@NP increases the absorption of light and promotes the charge transfer to the electrode/electrolyte interface resulting in the PEC performance. The maximum PCE for the Au-ZnO NR@NP arrays was found to be 0.69% (at 0.42 V vs. Hg/Hg,Cl,) which is 1.3 times higher than the ZnO NR@NP arrays (0.54% at 0.40 V). Furthermore, an enhanced value of photo- current density was observed for Au NPs sensitized ZnO NR@NP photoelectrode (0.06 mA/cm2) at 0.6 V as compared to the ZnO NR@NP having 0.04 mA/cm2 at a potential of 0.6 V vs. Hg/Hg,Cl,. Heterostructures constructed using metal oxide co-catalyst or carbon-based ET agents like graphene are pervasively investigated materials to improve the transport of charge carriers in semiconductors. Development of photoanodes with enhanced light-harvesting efficiency is a key factor in improving PEC performance. Another approach to enhance the STH efficiency of ZnO photoanodes is the lowering of recombination rate of the photogenerated charge carriers in ZnO on semiconductor surface and improving the capability for instantaneous charge collection, separation, and transportation. ZnO NRs/RGO/ZnIn,S4 heterojunction gives a photoconversion efficiency of 0.46% as reported in previous results (Bai et al., 2015). hr this heterojunction, ZnO NWs acts as core materials, RGO sheets behave as the charge transfer interlayer and ZnIn,S4 serves as a visible light sensitizer. Furthermore, RGO sheets also provide instantaneous pathways for photogenerated charge earners resulting in a decrease of the recombination rate of the charge carriers.
Bakianov et al. (2017) fabricated ZnO nanosheets and ZnO NRs by electrochemical deposition and further modified these morphologies with AgNPs in order to enhance the PEC performance. They observed approximately 1.4 times increase in photocurrent density of ZnO nanosheets after integration of plasmonic AgNPs on the surface. The photocurrent density was furthermore enhanced (~5 times) in case of ZnO NRs decorated by AgNPs. In the case of nanosheets, the charge earner trapping zones could enter into force in ZnO/Ag sheet-like composites which was determined by the presence of structural defects, i.e., the difference observed in values of photocurrent density was due to presence of large number of defects in nanosheets structure. Zhang et al. (2016) also reported the fabrication of ZnO nanosheet films in a scalable manner. The photoelectrochemical properties observed for the solution-processed on ITO photoelectrode was optimized by varying ZnS04 concentration. As compared with all the prepared samples, the 7.5 mM sample performed better photoelectrochemical properties and showed a photocurrent density of 500 pA/cm2. They explained the enhancement as a synergistic effect of high surface to volume ratio along with lowered resistance.
Hsu et al. (2011) synthesized ZnO nanotubes and ZnO nanosheets as photoelectrodes in water splitting cells. They reported that the difference between the relative intensities of XRD (100) and (002) peaks of ZnO nanotubes and ZnO nanosheets is related to the different degrees of polarity in ZnO nanostructures which greatly affects the PEC performance of these photoanodes. A lower value of (100)/(002) ratio reveals the formation of nano tubes along the c-axis and a large fraction of non-polar facets. Conversely, a high (100)/ (002) ratio indicates shortening along c-axis of ZnO nanosheets whose surface is dominated by polar facets. PEC measurements revealed that ZnO nanosheets show high photocurrent density than those of ZnO nanotubes. ZnO nanosheets with polar facets possessed a more negative flat band potential (-0.35V) than those of ZnO nanotubes (-0.23V). This difference in value of flat band potential was due to the fact that polar and nonpolar facets generated different surface states and atomic arrangements, affecting the absorption of reactant ions or molecules. Furthermore, a threefold enhancement was observed in PCE of ZnO nanosheets with polar facets (0.27%) as compared with ZnO nanotubes with non-polar facets (0.08%) because of the high surface energy, spontaneous polarization, and negative flat-band potential of a polar-oriented surface.
188.8.131.52.5 Nanowires (NWs)
Zhifeng Liu and group studied photoelectrochemical properties of three different morphology of ZnO NWs by varying their dipping time in the growth solution. They reported that dipping time of 5,10, and 15 days yields NRs, nanotubes, and nanodisks respectively. Among these four morphologies, ZnO nanotubes exhibits the highest photocurrent density of 0.49 niA/ cm2 at 1.2V (vs. RHE) in comparison with ZnO NWs (0.38 mA/cm2), ZnO NRs (0.38 mA/cm2) and ZnO nanodisks (0.26 mA/cm2) respectively. The tubular structure of ZnO nanotubes helps them to possess excellent light scattering ability for harvesting light through multiple reflections between the nanostructures. Thus, they could utilize sunlight folly to generate more electron-hole pahs for PEC water splitting (Liu et al., 2017). Similar efforts were carried out by Govatsi et al. (Govatsi et al., 2018) who explored the role of morphology of ZnO NWs on the photoelectrochemical performance. They classified the nanowire arrays into five different average diameters ranging from 40-260 mn. ZnO NWs with average diameter of 120 nm exhibited the maximum applied bias photoconversion efficiency of 6.3% upon illumination 11.5 mW at 365 nm. Moreover, the photoanodes displayed a stable performance up to 10 hours representing its stability for a long duration. Other attempts to enhance the PEC performance of ZnO NWs was reported by Hamandez et al. they deposited a shell of anatase TiO, on to the arrays of ZnO NWs. TiO,@ZnO core-shell structure shows an enhancement by 15 tunes in photocurrent density and 5 times in photo conversion efficiency. Anatase shell of TiO, protects the ZnO NWs structure as a result of which long term lifetimes of the core-shell photoanodes are obtained for water splitting reaction. Furthermore, these core-shell structure NWs when annealed in air increases the PEC performances favoring charge separation and lowers the recombination rate of the photogenerated charge earners. Similar results were reported by other researchers using Ti02 shell (Liu et al., 2013). The ultrathin TiO, shell chemically protects the Z11O NWs and allows the PEC water splitting of ZnO in a strongly alkaline environment, which is very important for efficient mass transport near the photoanode and to achieve high PEC activity. Also, ТЮ, shell passivates ZnO arrays surface states through partially removal of deep hole traps, without affecting the minority carrier diffusion due to its almost negligible thickness, resulting in an increase of photocurrent density. Chen et al. demonstrated a photodevice based on photosensitization of ZnO NWs with CdTe quantum dots to enhance photocurrent and photoconversion efficiency. They achieved an enhancement of more than 200% in photoconversion efficiency (1.83%) for ZnO@ CdTe as compared to bare ZnO NWs. CdTe sensitization improves visible light absorption, and also the amount of photogenerated charge earners hi CdTe quantum dot can be easily transferred to ZnO nanowire as a result of which efficiency increases.
Another type of enhancement strategy was reported by GuO and group which used Graphene quantum dots to enhance the PEC performance of ZnO NWs (Guo et al., 2013). Zheng et al. (2016) also reported a heterostructure where ZnO NWs are synthesized via MOCVD. They put carbon on the NWs and upon illumination with a 300 Watt Xe lamp obtained 8.6 pmol of El, (2.5 tunes higher than the pristine ZnO). Heejin Kim and group developed a stable and environment-friendly photoelectrode for PEC hydrogen generation by using carbon nanodots coupled with a 3D ZnO structure. This 3D ZnO structure comprises of ID ZnO nano wire (core) and 2D ZnO nanosheets on the surface to enhance the effective surface area for C-dot sensitization. The result reveals a 4 fold increase in photocurrent density of C-dot modified ZnO nanostructure (0.72 mA/cm2) as compared to bare ZnO nanostructure (0.18 mA/cm2) at a potential of 1,23V vs. RHE. This increment in photocurrent density of C-dot modified ZnO nanostructure may be attributed to the cascade band structure that enables efficient charge transfer between C-dots and ZnO nanostructures through their interface due to C-dot sensitization. Furthermore, they achieved a significantly increased without the use of any sacrificial reagents for the overall water splitting system. The photocurrent density was stable for more than 8000 seconds under 1 sun illumination. The enhanced stability of the photoelectrode conies from the amine passivation of C-dots that removes the surface defect states and provides a chemical shield to protect undesirable oxidation to form strong amide bonding with the intrinsic surface carbonyl groups (Kim et al., 2015).
A novel approach was reported by Wang et al. (2019) who fabricated ZnO nanowires overlapping junction (ZnO NWs-OLJ) by a facile and low- cost method in order to enhance the PEC water splitting of ZnO nanowire photoanodes. The NWs overlap and touch each other during their growth process to form ZnO NWs-OLJ. The photocurrent density of these overlapping junction (OLJ) was found to be 57 pA/cm2 which was almost double than that of the ZnO NR vertical arrays at a potential of OV versus Ag/AgCl. The unproved PEC performance of ZnO NWs-OLJ may be because of two reasons. The first reason for enhancement is the existence of inner electric field at the junctions. The OLJs can result in tunneling effects due to an infinitely small gap between ZnO NWs. This provides a potential barrier and an inner electric field is formed. So, the separation efficiency of the photo- induced charges can be improved with the help of inner electric field and further enhanced the PEC performance. Secondly, nanowire junction arrays can effectively increase the ratio of multiple reflections which proves to be beneficial for capturing more light. Further, to enhance the PEC performance even more, they decorated ZnO NWs-OLJ with AuNPs. The average photo- current density of this heterostructure was calculated to be 87 pA/cm2 at OV vs. Ag/AgCl which was 1.5 times higher than the pure ZnO NWs-OLJ. They proposed a mechanism for electron-transfer in Air/ZnO NWs-OLJ hetero- structure during water splitting. Upon UV illumination, electron-hole pairs are generated where electrons gets excited and jumps from VB to CB of ZnO equal amount of holes in VB. The newly formed Fermi energy level of the heterostructure will be lower than the bottom of the CB of ZnO. Due to the energy difference, the photo-induced electrons will be transferred fr om ZnO to Au and effectively reduce the recombination rate of the charge carriers. Moreover, due to surface plasmon resonance (SPR) excitation, AuNPs can absorb the resonant photons to generate hot electrons. These electrons will be injected into the platinum electrode via the CB to generate hydrogen. The holes present in VB of ZnO will be immediately consumed by producing oxygen.
Photosensitization of semiconductors with noble metals helps in the increment of its PEC performance as reported by various researchers. One of the previous, reports reveal that Au nanoparticle sensitized ZnO nanopencil arrays yields a high photocurrent density of 1.5 rnA/cm2 at IV versus Ag/ AgCl (Wang et al., 2015). Gold does not undergo corrosion during photoreaction and has the capability to strongly interact with light in visible and infrared region because of its localized surface plasmon resonance (LSPR) properties. The enhancement in photocurrent density is due to the formation of long tips of the nanopencil arrays which provides semiconductor-electrolyte interface where the space charge layer is developed. Furthermore, it has been earlier reported that ZnO NRs having long tips have better electrical properties because of the presence of oxygen vacancies.
The facility to tune the physiochemical properties of graphene between a semiconductor and a semimetal makes this carbon sheet an important subject of investigation for increment in PEC performance of ZnO nanostructures. ZnO nanotriangles on graphene oxide (GO) at proper condition (pH = 9) yields high photocurrent density and photoconversiorr efficiency due to optimum amount of oxygen vacancies as investigated by the experimental results and various deformation models (Chandrasekaran et al., 2016).
Sohila et al. (2016) synthesized ZnO nano flowers by hydrolysis of zinc acetate in the presence of DMF (dimethyl formarnide) with traced amount of water and explored its usage as ZnO nanostructure based photoanode for PEC water splitting. They evaluated the PEC performance by measuring photocurrent density at the different applied voltage and achieved the maximum photocurrent density of 0.39 niA/cm2 at 0.6V vs. Ag/AgCl. The improved PEC performance of ZnO nanoflowers based photoanode was attributed to its mesoporous nature constituted by well-connected ultra-small ZnO NPs with high surface to volume ratio that provides a larger area of electrode/electrolyte junction and more number of water oxidation sites at the electrolyte-ZnO interface. Also, the ultra-small ZnO NPs in the flowerlike morphology promoted the efficient separation of photogenerated charge carriers.
Apart from these ID morphologies, branched ZnO nanotetrapods also facilitates better electron transport and network forming ability for efficient photoelectrochernical water splitting (Qiu et al., 2012). The PEC behavior of these photoanodes experiences even more dramatic enhancement in photocurrent density (0.99 mA/cm2 at 0.31V vs. Ag/AgCl) by doping Nitrogen. There are basically three main reasons responsible for this significant enhancement. Firstly, the branched growth of nanotetra- pods helps in increasing the light-harvesting capacity. Secondly, N-doping increases the roughness factor which boosts up the light-harvesting associated with the ZnO nanotetrapod branching. Furthermore, N-doping also shifts the absorption spectra of ZnO towards visible light region by narrowing its bandgap.
184.108.40.206.10 Nanofibers (NFs)
Qiang Li and group reported a facile synthesis protocol for caterpillarlike branched ZnO nanofibers (BZNs) and explored their use as photoanodes in photoelectrochemical water splitting cells. They observed a significantly enhanced photocurrent density (151% higher) in the case of caterpillar-like BZNs (0.524 mA/cm2 at 1.2V vs. Ag/AgCl) as compared with ZnO NW arrays (0.348 mA/cm2). Also, they achieved a PCE of 0.165% (at 0.89V vs. RHE) for caterpillar-like BZNs which was 147% higher than those of the vertically aligned ZnO NWs (0.112% at 0.85 V vs. RHE). This improvement in PEC performance of caterpillar-like BZNs was attributed to three main reasons. Firstly, the ultra-dense secondary nanowire branches radially grew up on the parental ZnO NFs and offered a high spatial occupancy of the nanocomponents, which greatly increased the surface to volume ratio and roughness factor of the caterpillar like BZNs, and hence help capture more sunlight. Secondly, the nanowoven network consisting of caterpillar-like BZNs possesses an open micrometer porous structure which is beneficial to penetration of solar light and trapping the light via multireflections. Third, the fine branches with high length-to-diameter aspect ratios provided an advantage of efficient charge separation and hole diffusion at the electrode/electrolyte interface. With the elongated branches that generate joint sites among ZnO nanobranches to connect the parental ZnO NFs, the improved electron migration also reduces charge recombination to ensure efficient charge transport (Li et al„ 2014).
Sun et al. (2014) reported unique 3D ZnO nano forests as a potential candidate to enhance the PEC water splitting performances of these ZnO nanobranches synthesized by hydrothermal method. ZnO nanoforests showed a high photocurrent density of 0.919 mA/cm2 at 1.2 V vs. Ag/AgCl and PCE of 0.2999% at 0.89V vs. RHE. The large surface area and roughness factor of ZnO nanoforests resulted in increase of photocurrent density associated with efficient light and photon harvesting. Similar to nature trees, the upstanding nanotree arrays provided straight-forward light path and longer penetration depth, avoiding the thickness limitations of densely stacked NPs or thin films. Also, the nanosized branches of ZnO nanoforests helped to extend the light propagation and improved light trapping because of the multiple internal light reflections and scattering on the surface of branches. It was also worth noting that ZnO nanotrees have broader absorption in solar spectrum in comparison with ZnO NWs, because tree-like morphology can activate maximized excitonic band gaps of wurtzite ZnO. Due to the large surface area and high light trapping capacity, PCE was of ZnO nanoforests was greatly improved.
Alin et al. (2008) synthesized ZnO nanocorals and investigated their usage as photoanodes in PEC water splitting cells. ZnO nanocorals showed enhanced PEC performance (10 fold increment) as compared to ZnO compact and nanorod films because of their suitable electrical pathways for efficient charge collection as well as large surface area.
Wolcott et al. reported that the photoelectrochemical properties strongly depend on the porosity and morphology of ZnO photoelectrodes (2009). They prepared ZnO thin films by three different methods and compared the PEC performance of all the three electrodes. Pulsed layer deposition (PLD) with the substrate normal to vapor flux used to prepare ZnO thin film and the other two methods oblique-angle deposition (OAD) and glancing-angle deposition (GLAD) generated nanoplatelet. Similar efforts were put by Hamid et al. to enhance the applied bias photon to current efficiency (ABPE) of ZnO thin films by hybridization with reduced graphene oxide (RGO). Growing Z11O thin films on rGO increased the photocurrent density from 1.0 to 1.8 mA/ cm2 and ABPE from 0.55% (ZnO) to 0.95% (rGO/ZnO). Hybridization of ZnO with rGO increases the active surface area, increases light absorption range, decreases ciystal strain, reduces defect based recombination, and increases ET at the semiconductor-electrolyte interface. These factors help in enhancing the PEC properties of ZnO thin films when hybridized with rGO.