Although ZnO is an n-type material with excess electrons to donate, it can be modified with suitable cationic dopant to appear with shallow acceptors and thus behaving as a p-type. The p-type ZnO NRs has been known to achieve by doping with group I elements. Several research articles have been reported using dopants of group I elements such as Li, Na, K, and Cs. But among these Li is known to possess a smaller ionic radius as compared with Zn as well as it acts as a shallow' acceptor/reactive donor. Thus, it is difficult for the Li doped ZnO to retain p-type nature. Henceforth in order to achieve a stable p-type ZnO semiconductor it is preferable to dope other group I elements with bigger cationic radius. Lee et al. (2016) has reported enhanced photoelectrochemical w'ater oxidation for К and Na doped ZnO NRs synthesized by means of chemical bath deposition (CBD) technique. They showed a photocurrent density of 0.62 and 0.90 mA/crn2 as compared with the undoped ZnO NRs. THE SCANDIUM FAMILY

Qasim et al. (2018) reported the enhancement in photoconversion efficiency of Y doped ZnO NRs synthesized by a two-step process, i.e., sol-gel spin coating of seed layer followed by hydr othermal growth of nanostructures. Lowering of bandgap along with the creation of oxygen vacancies are the preferable reasons for choosing Y3+ as a dopant. They achieved a photocurrent density of 1.2 niA cm-2 for 1.2 rnol% doping; which is nearly five times higher as compared with the pristine ZnO NRs (0.25 mA cm-2) at a potential of 0.2V vs. Ag/AgCl.


The advantage of the ion implantation technique resides within the control of the doping percentage with variation of the ions dose. So the implantation causes modification of the electronic structure within the bulk of the semiconductor. Cai et al. (2015) have reported the doping of vanadium (V) into the ZnO matrix by using an ion implantation technique. Exposing ZnO nanostructures to V4+ ion allows forming an impurity level in the forbidden band and thus expands the optical absorption to visible range. Upon evaluation of their electrochemical performance under visible light illumination, the V ions implanted sample with 2.5 x Ю15. It shows a photocurrent density of 10.5 pA/cm2 at 0.8Y (vs. Ag/AgCl); which is nearly 4 times higher compared to that of the undoped ZnO NRs arrays. On further increase in the ions implantation dose, more number of defects is induced causing recombination of the photo earners. THE CHROMIUM FAMILY

Shen et al. (2013) engineered the intra-bandgap states by using Cr as a dopant in the ZnO nanosheet causing enhanced absorption in the visible range. At the same time, the usage of ID nanostructure provides a facile pathway for earlier transport. Though the efficiency achieved for the isostructure is higher than the individual parent structure, still the overall efficiency is lower for this structure. The authors provide a concept for fabrication of further high efficient photoelectrochemical devices. Cai et al. (2017) doped tungsten (W) by ion implantation technique into hydrothennally grown ZnO NR arrays. The optical absorption gets extended to visible range due to the creation of defect and a photocurrent density of 15.2 pA/cm2 was observed at IV (vs. Ag/AgCl). THE IRON FAMILY

Recently Selloum et al. (2019) electrochemically fabricated n-type Fe doped Zinc oxide NRs on ITO substrates at a temperature of 70°C. The photocurrent values obtained for all the samples under UY illumination showed the optimum Fe doping concentration is 2%. Furthermore, they have presented the change in morphology from NRs to nanoflakes with increase in Fe concentration afterwards.


Lee et al. (2016) also reported with performance enhancement of ZnO NRs by doping with Co. They obtained a photocurrent of 0.58 mAcnr2 as compared with the undoped ZnO nanostructures (0.42 mAcnr2). THE NICKEL FAMILY

Nickel has also been used by Lee et al. (2016) in an attempt to improve the photoconversion efficiency of ZnO NRs. It was also observed that the doping efficiency of Ni was higher which also suggests that Ni is an effective dopant for hydrothennally grown ZnO NRs. A slight increment, i.e., 0.48 mAcnr2 in the value of photocurrent density was observed as compared with pristine ZnO. The lowered value is reported because of the formation of lesser defects after doping (as evident from the onset potential of ZnO and Ni:ZnO as -0.35 and -0.39 VAg/AgC1)- Reddy et al. (2019) also used Ni as a dopant for visible-light-driven hydrogen generation. They obtained a photocurrent density of -3.28 mA/cm2 for 1% Ni-doped sample in a 0.1M NaOH solution. The efficient extraction of photocarriers after doping is the main cause for this enhancement. Upon visible light illumination, numerous charge carriers are produced. The same group also reported that 1.5 mol% of Ni-doped ZnO NRs are capable of providing a photocurrent density of 4.6 mA/cm2, in an electrolyte solution of 0.1 M KOH and thus emphasizes a substantial improvement as compared with undoped ZnO (1.4 mA/cm2) (Neelakanta et al., 2018). THE COPPER FAMILY

Incorporation of Cu2" within the ZnO matrix has also been known to cause a remarkable impact on the photoelectrochemical performance of ID ZnO nanostructures. On behalf of the very small difference in ionic radii of Cu and Zn, the Cu2+ ion can substitutionally replace Zn2+ in ZnO hexagonal wurtzite structure. Thus, it can be a suitable dopant for ZnO for photoelectrochemical hydrogen generation. Hsu et al. (Hsu and Lin, 2012) synthesized Zrij Cu О NRs by means of electrodeposition technique with varying concentration from 1% to 10%. Better photoresponse in the visible range was observed due to the bandgap narrowing effect. For 4% doping the maximum use of solar spectrum was observed resulting with an enhanced photoconversion efficiency of 0.21% as compared with undoped ZnO (-0.1%). Babu et al. (2018) also reported the fabrication of Cu doped ZnO NR arrays synthesized via thermal deposition technique. Enhanced photoelectrochemical performance was evident for the sample from photocunent density of 0.92 mA/ cm2 and photon conversion efficiency of 0.349%. Wang et al. (2014) also doped Cu into the host matrix of ZnO by ion implantation technique. They achieved a significant enhancement (11 times) in photocunent density of 18 pA/'cnr at 0.8V (vs. SCE) after inclusion of Cu. The visible-light-driven photoelectrochemical process observed in case of Cu is expected to help in realizing efficient use of solar energy. Rasouli et al. (2019) reported the enhanced photoelectrochemical performance after doping of Cu in a graded manner upon ZnO NRs. The fabrication was carried out by means of elec- trodepositiorr technique. The doping causes reduction of bandgap, which allows the efficient charge transfer and enhanced photocunent density as well. Khurshid et al. (2019) fabricated rGO coated Ag-doped ZnO NRs and a photocunent density of 206 rrA/cm2 was obtahred.


hr order to enhance the optical properties all the group III elements have proved beneficial as dopant for ZnO. Among all these В possess the highest electronegativity and lowest electronic radii. Our group reported the enhanced photoelectrochemical performance achieved via boron-doped ZnO NRs synthesized via facile hydrothermal technique. For a 6% В doped ZnO sample the almost five fold enhancement in photoconversion efficiency (i.e., 2.054%) was obtained as compared with undoped ZnO (0.491%). Wang et al. (2017) also fabricated В doped ZnO nanostructures on flexible PET substrates by means of hydrothermal method. Change morphology from sheet to sphere was observed hr their case along with the increase hr boron (B) dopant concentration. The B-dophrg causes efficient separation of charge carr iers which is observed as an enhancement in photocunent density of 0.055 mA/crn2 from 0.016 rnA/cm2 (for undoped ZnO NRs) under UY illumination. Kant et al. (2018) reported the fabrication of AZO NRs from doped seed layer solution. For Z11O photoanode the photocurrent density was ~19 pAcnr2, which becomes 182 pAcnr2 after Al inclusion. They presented that the enhancement in PEC performance is due to the extended absorption as well as improved morphology. Also, Al doping removes the trap centers from the ZnO host matrix and facilitates better charge transport. Indium (In) has been reported to be a dopant of interest in order to increase the electrochemical performance of ZnO NRs. Heimi et al. (2016) synthesized the ID nanostructure via electrodeposition. A photocurrent of 18 mA was obtained for ZnO while it increases up to 44 mA for 4% dopant concentration, showing the effect of doping In.


We have also studied photoelectrochemical performance of Si-doped ZnO (Shanna et al., 2018a). It was observed that doping with Si increases the stability of the ZnO NRs in a chemical environment. A PEC efficiency of -0.87% was observed after doping.


Several reports have also suggested the importance of Nitrogen as a dopant to enhance the photoelectrochemical performance of ZnO. Yang et al. (2009) carried out the fabrication of N doped ZnO via facile hydrothermal technique. They also reported that the N substitution occurs at О sites up to - 4% doping. By means of these non-vacuum based photoanodes, they achieved a photo to hydrogen conversion efficiency of 0.15% at +0.5 V (vs. Ag/AgCl). Moreover, the IPCE measured for the samples demonstrated the appreciable enhancement at visible region. Wang et al. (2015) followed an ion implantation technique to fabricate N doped ZnO structure. They found a remarkable enhancement in visible light-driven PEC photocurrent density of 160 pA.cnr2 at +1.1V vs. SCE, for an ion dose of 1015 ions/cm2. A gradient distribution of N dopants occurred throughout the structure which served the purpose of extending the optical band edge to visible range. Not only this but also a terraced structure was introduced due to this, which promotes efficient transfer of photo earners finally resulting with an enhanced efficiency. Another important aspect is the post-growth doping strategy which can help in further designing solar energy harvesting devices. THE HALOGEN FAMILY

Halogens are also known to contribute in the photoelectrochemical enhancement of ZnO NRs. Wang et al. (2014) designed Cl doped ZnO NRs along with TiO, in a core-shell maimer and achieved a photon to current PEC efficiency of 1.2% at -0.61V versus SCE; which is almost 3 times higher than the figures obtained for pure ZnO.

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