UV-assisted water splitting of stable Cl-doped ZnO nanorod photoanodes grown via facile sol-gel hydrothermal technique for enhanced solar energy harvesting applications

Solar Energy 193 (2019) 148–163

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UV-assisted water splitting of stable Cl-doped ZnO nanorod photoanodes grown via facile sol-gel hydrothermal technique for enhanced solar energy harvesting applications

T

Pooja Sahooa, Akash Sharmaa, Subash Padhanb, G. Udayabhanub, R. Thangavela,

⁎

a b

Solar Energy Research Laboratory, Department of Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, Jharkhand, India Department of Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, Jharkhand, India

ARTICLE INFO

ABSTRACT

Keywords: ZnO nanorods Light harvesting efficiency Solar hydrogen generation Flat band potential Photostability

Vertically aligned pristine ZnO and Cl-doped ZnO nanorod arrays were grown by a simple, cost-effective sol-gel and hydrothermal method. These nanorods (NRs) were fabricated to demonstrate their potential as highly efficient photoelectrodes to be used in photoelectrochemical water splitting applications. XRD measurements indicate that all the fabricated NRs have preferably grown along c-axis (0 0 2) direction. FESEM images confirmed hexagonal shaped NRs grown along (0 0 2) direction. The light harvesting efficiency of pristine ZnO NRs was enhanced with increase in doping concentration which is due to rise in absorbance as verified by UV–Vis absorption spectroscopy. When used as photoanodes in PEC water splitting under UV illumination, these NR arrays exhibits an enhanced photocurrent density of 2.16 mA cm−2 at 1.2 V vs. Ag/AgCl which is much higher than undoped ZnO NRs (0.103 mA cm−2). Cl_Z3 photoanode exhibits a stable photocurrent density even after continuous illumination of 12 h. The significantly improved photoresponse behavior and high photostability with suitable bandgap and high light harvesting efficiency of these photoanodes provides valuable platforms for efficient photoelectrochemical water splitting applications.

1. Introduction Among all the renewable energy sources, photoelectrochemical water splitting also called “artificial photosynthesis” provides an efficient path for pollution free, clean and cost-effective production of hydrogen from solar energy (Walter et al., 2010). Photoelectrochemical (PEC) water splitting devices use photocatalytically active semiconductor materials to absorb solar energy required to break the chemical bond of H2O. The very first report on PEC water splitting was demonstrated in the year 1972 by Fujishima and Honda using TiO2 electrode with an approximate efficiency of 0.1% (Fujishima and Honda, 1972). Apart of this, several other semiconductor materials have been investigated for suitable photocatalytic applications (Kar et al., 2019; Misra et al., 2017, 2015; Prakash et al., 2018; Singh et al., 2017b, 2017a, 2016; Tyagi et al., 2016). But metal oxide semiconductors caught the pace for their application as photoelectrodes in photoelectrochemical cells owing to their excellent features like suitable bandgap, low electrical resistance, appropriate value of flat band potential, non-toxicity, high chemical stability and large scalability for mass production (Wang et al., 2014). Some of these metal oxide

⁎

semiconductors can be listed as ZnO (Govatsi et al., 2019), NiO (Bonomo et al., 2017), Cu2O (Chu et al., 2017), TiO2 (Fujishima and Honda, 1972), SnO2 (Outemzabet et al., 2015), WO3 (Vidyarthi et al., 2011), SrTiO3 (Ng et al., 2010), Fe2O3 (Cha et al., 2011), BiVO4 (Luo et al., 2008), Ta2O5 (Takahara et al., 2001), which have been studied vigorously for PEC water splitting applications. Despite of possessing many features like high chemical and structural stability, non-toxicity and cost-effectiveness, still they are not able to provide complete solutions due to some limitations like difficulty for maintaining stoichiometric ratio, formation of rust, low efficiency due to structural defects, limited light absorption, short diffusion length, poor stability in aqueous solution and diminished photoconversion efficiency caused by recombination of the charge carriers (Sharma et al., 2018). Among all these oxides, ZnO is known to possess similar properties like TiO2 with better electron mobility (Chandiran et al., 2014; Gonzalez-Valls and Lira-Cantu, 2009). ZnO is an n-type wide bandgap (3.37 eV) semiconductor having a hexagonal wurtzite crystal structure. It has high exciton binding energy of 60 meV that ensures efficient excitonic emission at room temperature (Wang, 2004). ZnO has been pervasively used in many optoelectronic devices like sensors (Wei et al.,

Corresponding author. E-mail address: [email protected] (R. Thangavel).

https://doi.org/10.1016/j.solener.2019.09.045 Received 11 July 2019; Received in revised form 5 September 2019; Accepted 12 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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2011), LEDs (No and Akademisk, 2011), thin film transistors (Li et al., 2008) and photodetectors (Boruah et al., 2017). Apart from all these, it has also been used in PEC water splitting devices because of its high electron mobility, simple tailoring of nanostructures to get abundant morphologies, non-toxicity, low cost and eco-friendly nature (Luo et al., 2018). Structural tailoring is considered as one of the key advantages of achieving high efficiency. ZnO can be tailored easily into different growth morphologies like nanosheets, nanoflowers, nanorods, nanotrees, nanotetrapods, nanotriangles, nanotubes, nanopencils etc. (Chandrasekaran et al., 2016; Chen et al., 2010; Hsu et al., 2011; Qiu et al., 2012; Ren et al., 2016; Rokade et al., 2017; Sohila et al., 2016; Wang et al., 2015a, 2015b). Among all of these, NRs have proved their credibility by providing long diffusion length, efficient charge collection, large surface to volume ratio, separation along with transfer of the charge carriers through the photoelectrodes (Muskens et al., 2008; Tang et al., 2008; Yang et al., 2010). Despite its versatility in several aspects, the photocatalytic behavior of ZnO is limited because of its large bandgap of 3.37 eV, low photon absorption, UV sensitivity(which covers only 4–5% of the total solar spectrum) and also high surface recombination rate of charge carriers (Zhang et al., 2015). Various research groups have given different strategies to tackle these important issues (Hamid et al., 2017). Doping of cation or anion, the inclusion of surface plasmon, fabrication of semiconductor composites are the most popular ways used for the absorption of a photon in the visible domain. Surface recombination rate of the charge carriers in ZnO can be minimized by introducing favourable defect states and also by incorporation of mixed phase and composites (Das et al., 2018). Doping semiconductor nanostructures are also been proved as an efficient way of engineering the optical and electrical properties, finally resulting in enhanced photoconversion efficiency of photoelectrochemical cells (Wang et al., 2014). Various doping elements in ZnO nanostructures has also been investigated theoretically by using first principles calculations (Yim et al., 2017). Many research groups have reported water splitting in ZnO with high photoconversion efficiency by doping cations like Cu, B, Si, Al, Fe, N, Sn, Ni, S, Ag, Co whereas doping of ZnO with anions to replace of oxygen has been paid less attention although some results have been reported earlier by doping Cl in ZnO lattice. Wang et al. fabricated efficient photoanodes using chlorine doped ZnO nanowires covered with TiO2 film and reported a photoconversion efficiency of 1.2% at a potential of −0.61 V w.r.t saturated calomel electrode. They have also reported that doping of Cl in ZnO nanowires increases conductivity up to 5 orders of magnitude (Wang et al., 2014). Among the four halogen elements, this non-metallic element is usually favored as a dopant in ZnO nanostructures in order to avoid metallic clusters which leads to inhomogeneous conductivity (Lee et al., 2012). Cl being electronegative and an anionic dopant, it tends to replace oxygen ions and also some of the Cl ions tend to passivate oxygen vacancies which are intrinsically found in ZnO nanostructure during the growth process (Cui et al., 2008). Oxygen vacancies act as recombination centers hence Cl replacing oxygen vacancy will decrease the recombination rate of the photo induced charge carriers (Gautam et al., 2016). Liu et al. have theoretically explained that doping with Cl is a fruitful method to remove oxygen vacancies in ZnO nanostructures during its growth (Liu et al., 2013). The incorporation of Cl in ZnO increases the number of free electrons in the conduction band and also increases the roughness factor on the surface that results in an improvement in its field emission properties (Shao et al., 2015). Later on Fan et al. have observed the enhancement in PEC properties of doping Cl with ZnO nanowires by doping their co-axial view. These homojunction nanowires increase the separation of charge and transfer to the surface, thus serving better substitute to enhance the efficiency of photovoltaic devices (Fan et al., 2011a). Cl doping in ZnO has been reported to have a wide variety of applications. Various synthesis techniques have been employed to achieve Cl-doped ZnO nanostructures (as shown in Table-S1). These methods includes pulsed laser deposition (PLD) (Lee et al., 2012),

electrodeposition (Fan et al., 2011a), metal organic chemical vapor deposition (MOCVD) (Chikoidze et al., 2008), physical vapor deposition (PVD) (Yousefi and Jamali-Sheini, 2012), thermal evaporation method (Yousefi et al., 2011) and atomic layer deposition (ALD) (Choi et al., 2015). However, these synthesis techniques require highly sophisticated vacuum based environments and high temperature. Not only this but also in the process like electrodeposition, there are problems regarding hydrolysis of water which occurs at the potentials needed for electrochemical growth of the layers. These processes have shown their incapabilities in further commercialization due to high cost as well as chemical instability (Guc et al., 2017). Among the solution based techniques like chemical bath deposition, sol-gel method, spray coating, spin coating, dip coating; the sol-gel method is a simple, cost-effective and non-vacuum technique involving hydrolysis and polycondensation reactions. Sol-gel approach mainly has two advantages. Firstly, the components of the elements are mixed together on a molecular level to form a precursor solution. Secondly, the thin film of this precursor gel is made up of nanoparticles after polycondensation, thus one can obtain crystalline thin films at the lower annealing temperature. During the synthesis process, the precursor solution is first coated on the substrate followed by pre-heating to form a dense nanocrystal thin film (Su et al., 2014). In our present work, we have fabricated pristine ZnO and Cl-doped ZnO photoanodes for water splitting applications. The detailed statistical data of thickness, diameter, light harvesting efficiency and optical bandgap of the grown NRs are presented in this paper. A remarkably enhanced photocurrent density of 2.16 mA/cm2 has been achieved without using any enhancement strategy as stated in the literature survey Table 4. We observed a photoconversion efficiency of ~58% under UV illumination; which is significantly higher than any earlier reported results on Cl-doped ZnO nanorods. Not only this, but also the doped sample (Cl_Z3) shows enhanced light harvesting efficiency with fewer defects, high photocurrent density, lowest carrier transit time within the depletion region as well as better photostability. The detailed photoelectrochemical results obtained in the experiments strongly recommends its ability as a promising candidate in water splitting applications. 2. Experimental details 2.1. Materials Sulphuric acid [H2SO4, Emparta ACS grade, 98% purity], Hydrogen peroxide [H2O2, Emparta grade, 30% purity], Acetone [C3H6O, Emplura grade, 98% purity], 2-propanol [C3H8O, Emplura grade, 99% purity], Zinc acetate dihydrate [Zn(CH3COO)2·2H2O, 98% purity] (ZAD), Zinc nitrate hexahydrate [(Zn(NO3)2·6H2O), 98% purity](ZNH), Hexamethylenetetramine [(C6H12N4), 99% purity](HMT), Sodium Hydroxide pellets [NaOH] were purchased from Merck Chemicals; Monoethanolamine [(C2H7NO), 98% purity] (MEA), Ammonium chloride [(NH4Cl), 98+% purity] and 2-methoxyethanol [(C3H8O2), 99% purity] were obtained from Alfa Aesar. All the chemicals purchased were used without any further purification. De-ionized (DI) water was used throughout the experiment. Glass substrates were cleaned by ultrasonic treatment in piranha solution for 2 h, followed by acetone, 2-propanol and DI water each for 15 min. The Fluorine doped Tin oxide (FTO) substrates were procured from Sigma Aldrich and used only for photoelectrochemical measurements. They were ultrasonicated in acetone, 2-propanol and DI water each for 15 min. All the wet substrates (glass and FTO) so obtained were dried in an oven at 90 °C for 4 h under ambient conditions. 2.2. Fabrication of Cl-doped ZnO photoanodes A 2-step process was followed to carry out the synthesis. Briefly, at first 0.5 M of ZAD was dissolved in 2-methoxyethanol followed by the 149

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KEITHLEY 2450 source meter. Gold coated 3 M clips were used as contacts for the electrical measurements. A UV light (Philips, BLB/T5, 6 W, 365 nm) was used for illumination purpose for the electrical measurements. 2.4. Photoelectrochemical studies The water splitting behavior was investigated by using a commercially available UV light source (30 W, 365 nm). The intensity of the light was kept constant during the entire experiment and was measured to be 0.4 mW/cm2 by using a solar power meter (HTC make). The photoelectrochemical measurements were carried out by a software controlled electrochemical workstation (CH Instruments, Model 660C) by using standard three electrode cell configuration. It comprises of a Pt wire (counter electrode), an Ag/AgCl electrode (reference electrode) and fabricated nanorods (working electrode) respectively. An area of (1.5 × 0.6) cm2 of the sample was exposed to the electrolyte (0.1 M NaOH) for carrying out the measurements. The applied potential w.r.t Ag/AgCl electrode was converted to a reversible hydrogen electrode (RHE) using the formula; VRHE = VAg / AgCl + 0.059 × pH + 0.222 . The linear sweep voltammetry results were obtained with an anodic scan rate of 0.01 Vs−1 within a potential window of 0 to +1.2 V in presence and absence of light. The Mott-Schottky analysis of all the samples was carried out in dark conditions at various frequencies, i.e. 1, 10, 20, 30 kHz. The chronoamperometric (I-t) results were collected without applying bias. The photostability test for the optimized sample was carried out under the similar illumination conditions mentioned earlier for a duration of 12 h.

Fig. 1. Scheme for synthesis of Cl-doped ZnO NRs using sol-gel, hydrothermal technique.

addition of MEA dropwise. This reaction mixture was stirred at 60 °C for 2 h. A transparent homogenous solution was obtained and further aged for 24 h. The aged solution so obtained was coated on the glass substrates at speed of 3000 rpm for 30 s by using a spin coater. After spin coating each successive layer, the substrates were baked at 300 °C for 10 min on a hot plate for removal of the organic residuals. The coating and drying process was repeated four times to obtain a thin film of desirable thickness. All the samples so obtained were annealed at 500 °C for 1 h to obtain a well-adhered seed layer. In the second step, an equimolar aqueous solution of 0.025 M ZNH and HMT was prepared. Ammonium chloride was used as a doping source of chlorine precursor for the growth of Cl-doped ZnO NRs. Five different molar solutions were prepared by varying Cl concentration ranging from 5 mM to 25 mM. Undoped and Cl-doped ZnO NRs were grown by submerging the seed layer inverted in the growth solution at 90 °C for 5 h. After completion of the desired time interval, the as-grown NRs were washed by DI water to remove the organic residuals as well as to eliminate chloride salt traces from the surface of the samples. A schematic diagram of our synthesis procedure is represented in Fig. 1. In order to prepare photoelectrodes for testing the water splitting performance of the doped ZnO NRs; the entire scheme mentioned above was followed on the cleaned FTO substrates leaving a portion uncoated for electrical contacts. The synthesized NRs were grown with different doping concentrations of Cl as 0 mM, 5 mM, 10 mM, 15 mM, 20 mM, 25 mM and are abbreviated now onwards as ZnO, Cl_Z1, Cl_Z2, Cl_Z3, Cl_Z4, and Cl_Z5 respectively.

3. Results and discussion 3.1. Mechanism of photoelectrochemical performance for Cl-doped ZnO NRs Fig. 2 represents the mechanism of carrier transport in case on ntype electrodes. When the NRs array films immersed within the electrolyte are illuminated; electron-hole pairs are generated. But due to the presence of defects as well as the limited absorption of ZnO the performance is restricted. On the other hand, a comparatively better performance can be obtained for samples with less defects, better lightharvesting properties and charge transport properties (as observed for Cl_Z3, which is discussed afterwards). As per the present case, upon UV illumination with photons having energy greater than the bandgap of the material, the electrons residing at the top of the VB absorbs energy and moves to the CB. During the process, with every electron moved to the CB, an immediate hole is created in the VB. The holes easily get diffused at the semiconductor electrolyte interface due to the built-in

2.3. Material characterization of the films The structural properties of hydrothermally grown NRs were characterized by X-ray diffraction (XRD) (Bruker D8 Advance model) at UGC DAE CSR, Indore. Raman spectra were measured by Jobin Yvon Horiba LABRAM -HR Visible (400–1100 nm) system at UGC-DAE CSR Indore by using a 632.8 nm He-Ne laser as the excitation source. The morphology of the nanorods were visualized by using Field Emission Scanning Electron Microscope (FESEM) (ZEISS Supra 55 model) and Atomic Force Microscope (AFM) (Nanoscope E). Elemental analysis was performed by Energy Dispersive Analysis by X-ray (EDAX) (Oxford Instruments) attached with the above FESEM instrument. Optical measurements of the grown NRs were carried out by using Agilent Cary 5000 model UV–Vis-NIR spectrophotometer. Photoluminescence (PL) measurements were performed by a lifetime spectrophotometer (QM400 HORIBA USA model) using Xe arc lamp. The photocurrent density (J) vs. voltage (V) characteristics curves were measured using

Fig. 2. Schematic diagram presenting the carrier transport pathways of Cl doped ZnO photoanodes. 150

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Fig. 3. XRD patterns of the grown undoped and Cl-doped ZnO NRs along with the magnified XRD pattern representing the lower angle shifting of (0 0 2) peak with an increase in the concentration of NH4Cl.

electric field, thus extending the lifetime of the electrons. At the same instant, the electrons start moving through the external circuit to reach the metal electrode. The holes react with water to generate O2 (Eq. (1)), while the electrons reaching the Pt electrode takes part in water splitting reaction to evolve H2 gas as mentioned in the equation below.

H2 O + 2h+ 2H+

1 O 2 2

+ 2e

+ 2H+ H2

crystallite sizes for the other higher doped samples may be caused due to the further decrease in oxygen vacancies. The gradual decrease in the retarding forces with the incorporation of Cl ions plays the key role in increase in the crystallite size (Ravichandran et al., 2015). As compared to the pristine ZnO NRs, we observed a slight increase in lattice parameter of the doped samples which may have caused due to the amount of Cl ions or their possible inclusion in ZnO NRs. Also, the increase in the lattice parameters may be due to interstitial incorporation of Cl ions into the ZnO lattice (Rousset et al., 2009). For further understanding of phase purity and the substitution of O2 by Cl ions, Reitveld refinement of ZnO nanorods were carried out by FullPROF software. The profile fits for the Rietveld refinement of pristine ZnO and Cldoped ZnO NR samples are displayed in Fig. S1; where the symbol (red circle) represents the experimental data and the solid line (black) represents calculated data. The blue curve at the bottom represents the difference between the experimental and calculated data. The simulated XRD data were observed to be well matched with the experimental data. The results of refinement (Table 1) further demonstrate that neither the host nor the dopant generated any impurity or secondary phases in ZnO. We observed that ZnO shows hexagonal structure having space group P 63 mc (186). Further crystallinity and doping of Cl into ZnO lattice was investigated by Raman spectroscopy. Fig. S2 represents the Raman spectra of both pristine as well as Cl-doped ZnO NRs samples. An intense, sharp and dominant peak was observed at around 439 cm−1 corresponding to E2high mode which reveals the hexagonal wurtzite phase of as grown NRs. All the samples show a similar kind of peak which implies their identical crystal structure as shown by XRD measurements. This is because of the lattice expansion (caused by the inclusion of Cl) along the c-axis of NRs. In addition, the Raman active E2high mode shifts towards higher frequency side with an increase in Cl concentration. This blue shift in Cl-doped ZnO is similar to previously reported results (Boruah et al., 2017; Rousset et al., 2009). The appearance of this mode is a confirmation which presumably relates to the distortions introduced in the lattice by the incorporation of Cl (Rousset et al., 2009).

(at the semiconductor)

(at the Pt

electrode)

(1)

3.2. Structural properties Fig. 3 depicts the crystalline nature of all the samples investigated through XRD. The observed diffraction peaks were subjected to the hexagonal wurtzite structure of ZnO with JCPDS card no. 36-1451. No impurity phases were discerned in the XRD pattern. For all the grown NRs sample a peak at nearly 34° can be seen which indicates the c-axis orientation of the samples. The strong (0 0 2) peak indicates the purity and crystallinity of all the NRs. With an increase in the concentration of Cl, lower angle shifting of (0 0 2) peak was observed as shown in Fig. 3. This confirms that tensile strain is produced in the crystal structure upon inclusion of this anionic dopant. This may be because of the lattice expansion of ZnO caused by the replacement of small O2 ions having ionic radius 0.14 nm with large Cl ions having ionic radius 0.18 nm (Cui et al., 2008). The average crystallite size (D) , dislocation density ( ) and strain ( ) of all the NRs were estimated from the following equations:

D=

0.9 ; hkl cos

=

1 ; D2

=

cos 4

(2)

where is the wavelength of radiation used, stands for the angle of diffraction and represents the FWHM. The values of these structural parameters are tabulated in Table 1. The value of crystallite size of the NR samples were ranged between 55 nm and 86 nm without particular tendency according to Cl doping. The size of the crystallite increased for Cl_Z1 in comparision with pristine ZnO NRs indicating the increase in crystallinity of the sample. But on further increase in Cl doping concentration it decreased to 81.87 nm. This decrease in crystallite size of Cl_Z2 can be explained by Zener pinning effect. According to this phenomena, the expansion of grain boundaries is restricted by the retarding force originated from the crystal imperfections like vacancies and interstitials (Suwanboon et al., 2011). However, upon further increase in the doping concentration, the crystallite size again increased to 83.95 nm. The minimal change in

3.3. Morphological properties Fig. 4(a–f) and (m–r) exhibit the top and cross-sectional view FESEM images of pristine and Cl-doped ZnO NRs respectively. We observed hexagonal shaped NRs vertically grown on the glass substrate which is in harmony with the c-axis growth results obtained from XRD

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Table 1 The structural parameters calculated from XRD patterns of all the grown NRs. Value of lattice parameters and agreement factors derived from Rietveld refinement analysis of XRD patterns. Sample

ZnO Cl_Z1 Cl_Z2 Cl_Z3 Cl_Z4 Cl_Z5 #

2θ at (0 0 2) plane (°)

34.464 34.424 34.401 34.405 34.390 34.380

Crystallite Size#

Strain (ɛ)

(nm)

(×104)

55.86 86.37 81.87 81.98 83.40 83.95

6.204 4.013 4.233 4.228 4.156 4.129

Dislocation Density (×104) (nm)-2

Lattice parameter (Å) a=b

c

3.20 1.34 1.49 1.48 1.43 1.41

3.2518 3.2342 3.2229 3.2548 3.2398 3.2558

5.2021 5.2461 5.2101 5.2095 5.2112 5.2131

2

1.13 2.19 2.86 4.22 10.9 3.79

Using Debye-Scherrer’s equation.

results. The surface morphologies of the doped samples do not change significantly in contrast with undoped ZnO NRs. In addition, the density of NRs has been somewhat decreased with increase in doping concentration. The value of the average diameter of all the grown NRs has been calculated using Image J software (1.8.0 112 version) and are listed in Table 2. The average diameter distributions estimated from their respective FESEM images are shown in Fig. 4(g–l). The average diameter of doped NRs increased largely as compared to the undoped ZnO NRs i.e. an abrupt increase in nanorod diameter from 124 nm (for ZnO) to 257 nm (for Cl_Z5) was observed. The length of the NRs was calculated from the cross-sectional FESEM images as shown in Fig. 4(m–r). Fig. S3 shows the nanorod diameter and length distribution as a function of Cl concentration. It was observed that the length of NRs decreased rapidly from 2.811 µm for pristine ZnO to 1.57 µm for Cl_Z4 sample. Thus it can be concluded that while the diameter of undoped ZnO NRs increased with NH4Cl concentration, on the other hand, the length of NR was shortened. As reported earlier, NH4Cl decreases the growth along the NR axis while the radial growth of the NRs is increased (Cui et al., 2008). A similar type of growth behavior was reported for ZnO NRs by Tian et al. with varying concentration of Na3C6H5O7 in the growth solution (Tian et al., 2003). The increase in diameter and decrease in length of Cl-doped ZnO NRs can be explained as follows. One of the main reason behind this type of growth behavior is the adsorption of Cl on the (0 0 2) growing surface by substitution of O2 , resulting in suppression of ZnO growth along the NR axis. Since Cl gets adsorbed to the (0 0 2) face, lesser reagents are present for the column tip growth, and a reduced length is observed due to the diffusion-limited growth mechanism. So, the average diameter of Cl-doped ZnO NRs increases with the increment in concentration of chlorine precursor, which is also in suitable agreement with earlier reported literatures (Rousset et al., 2009; Smith and Rodriguez-Clemente, 1999). This trend of decreased length was followed upto Cl_Z4. The length of NRs again increased for Cl_Z5 which may be due to the excess Cl ions incorporated into ZnO lattice are positioned as interstitials. The surface topography of pristine and Cl-doped ZnO NRs was studied by AFM images shown in Fig. S4(a–f). Inset of each figure represents the 3D images of the corresponding samples, which indicates the uniform distribution of NRs on the substrate. The results obtained from the AFM micrographs were found to be in agreement with the FESEM images. Nanoscope software (version 1.4) was used to analyze the AFM data. The roughness factor estimated from AFM images is listed in Table 2. The surface roughness induced in Cl_Z3 is highest among all the doped samples. Doping Cl in ZnO NRs produces a surface roughness that contributes to more incident UV light interaction to significantly enhance the generation of electric field. Moreover, the generation of free electrons in Cl doped ZnO NRs also plays a crucial role in the photoresponse enhancement process. Thus, both the combined participation of UV light interaction and generation of free

electrons helps in the photoresponse behavior of the NRs (Boruah et al., 2017). Furthermore in order to verify the composition as well as to testify the presence of Cl, EDAX analysis was carried out for all the samples as shown in Fig. S5. Elements such as Zinc (Zn), Oxygen (O) were observed along with Cl hence confirming the doping in ZnO. An enhanced intensity of Cl signal was observed from the EDAX spectra when the amount of NH4Cl increased in the growth solution. The Cl content was found to be 0.30, 0.42, 0.55, 0.61, 0.65 at.% for sample Cl_Z1, Cl_Z2, Cl_Z3, Cl_Z4, and Cl_Z5 respectively. Apart from Cl, Zn and O peaks a Pt peak was observed in all samples which are regarded as due to the thin conducting coating of platinum used for imaging. 3.4. Optical properties The optical absorption properties of all the grown NRs were analyzed by UV–visible light absorption spectroscopy. Fig. 5(a) shows the optical absorption spectra of undoped and Cl-doped ZnO NRs. Absorbance spectra show that all the grown NRs exhibited strong absorption at 370 nm. Absorption coefficient (α) was estimated from Lambert’s 2.303 × Absorbance equation (Jiamprasertboon et al., 2018) given by = ; thickness and the band-gap value was estimated by using Tauc’s relation (Stern, 1963).

( h ) 2 = A(h

Eg )

(3)

Fig. 5(b) represents Tauc’s plot that gives the approximated bandgap of all the grown NRs. The observed bandgap of the pristine and Cl-doped ZnO NRs are listed in Table 2. As shown in Tauc’s plot, a slight shift in the absorption edge towards higher photon energy was observed for Cl_Z1 (3.256 eV) as compared with the undoped ZnO NRs (3.252 eV). ZnO is an n-type semiconductor having natural electron donors generated by O vacancies and Zn interstitials. The addition of donor Cl ions shifts the fermi level above the CB due to which absorption edge shifts to energies higher than the actual band gap of pristine ZnO (Lee et al., 2013). However, with further increase in doping concentration, the band gap decreases for Cl_Z2 and Cl_Z3. This band gap narrowing may be due to the presence of defect states below the conduction band (Antony et al., 2019; Gunasekaran et al., 2018; Ratheesh Kumar et al., 2005). For all other NR samples, we observed an increase in bandgap with increase in doping concentration. The fermi level of Cl-doped ZnO is shifted above the minimum of conduction band, therefore the optical bandgap increases as shown in Fig. 5(b). DFT calculations have proved that the replacement of O by Cl atom instigates a shallow donor state acquired from Cl 3 s states, which causes the Fermi level to shift towards the CB, consequently increasing the bandgap as per BM effect (Jiamprasertboon et al., 2018). The conduction band minima (CBM) and valence band maxima (VBM)

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Fig. 4. FESEM images of NRs grown for different concentrations of NH4Cl (a) 0 mM, (b) 5 mM, (c) 10 mM, (d) 15 mM, (e) 20 mM and (f) 25 mM respectively along with the graphs (g-l) representing diameter distribution, (m-r) shows the cross-sectional FESEM acquired from the same samples.

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Table 2 Average diameter, approximate length of pristine and Cl-doped ZnO NRs obtained from FESEM images. Roughness is calculated from topographical images. The optical bandgap (Eg) , light harvesting efficiency (LHE), absolute electronegativity (χ), conduction band minima (CBM) and valence band maxima (VBM) levels estimated from absorption spectrum results. The decay time results were estimated from the TRPL data by fitting monoexponentially. Sample

Average Diameter (nm)

Approx. length (μm)

Roughness (nm)

Eg (eV)

χ

CBM (eV)

VBM (eV)

LHE (%)

Decay Time t1 μs

ZnO Cl_Z1 Cl_Z2 Cl_Z3 Cl_Z4 Cl_Z5

122.48 140.61 177.78 161.79 178.94 257.87

2.811 3.052 3.008 1.725 1.578 1.840

20.5 12.2 12.9 22.2 17.8 12.8

3.252 3.256 3.223 3.211 3.271 3.288

5.792 5.820 5.848 5.876 5.904 5.933

−0.193 −0.308 −0.263 −0.229 −0.230 −0.210

3.059 2.948 2.959 2.981 3.040 3.077

94.535 97.978 97.868 97.650 91.256 94.699

0.367 0.330 0.309 0.265 0.221 0.195

levels were calculated in a similar fashion as reported earlier (Sharma et al., 2018) and their values are listed in table 2. Light harvesting efficiency (LHE) is an essential parameter which is used to investigate the generation of electron-hole pairs in optoelectronic devices (Chen et al., 2009; Deka Boruah and Misra, 2016a). In other words, it is the measure of the light absorption capability of the semiconductor material and defined asLHE = (100 Transmittance Reflectance )%. Fig. 5(c) represents the LHE of undoped ZnO and Cl-doped ZnO NRs with wavelength ranging from 400 nm to 800 nm. The quantitative value of LHE is calculated and listed in Table 2. The high value of LHE in the wavelength range 400–550 nm shows the strong light absorption in all the NRs. Interestingly, we observed a significant increase in LHE of doped samples in comparison with pristine ZnO NRs. Cl_Z1 shows the highest LHE (97.97%) at around 400 nm in contrast with undoped ZnO (94.53%) and all other doped samples. This is mainly due to the increased light absorption efficiency through more light interaction on rough Cl-doped ZnO NRs surface (Boruah et al., 2017). Further, from the absorbance plot, it can be understood that Cl_Z1 has the highest absorbance due to which it gives the maximum light absorption efficiency. However, for high concentration of Cl, absorbance of the NRs decreases which leads to decline of its light absorption efficiency. PL spectroscopy is an important characterization technique which gives us an idea about the transfer and recombination processes of the photogenerated charge carriers in a semiconductor material. The defects present due to doping of Cl in ZnO NRs was further confirmed by photoluminescence (PL) analysis. Fig. 5(d) shows the steady state PL spectra of all the grown NRs excited with an excitation wavelength of 325 nm. PL spectra clearly show UV emission peaks at 395 nm, 411 nm, a violet blue band at 451 nm, a strong blue band at 470 nm, a greenish blue emission at 483 nm, a weak blue emission at 493 nm. The emission peak appearing at around 395 nm and 411 nm are related to near band edge emission (NBE) of ZnO. The strong blue band at 470 nm appeared because of the transition of electrons from zinc interstitial (Zni) to zinc vacancy (VZn) (Sharma et al., 2018). Additional emission peaks at around 550 nm and 575 nm are mainly due to oxygen vacancies (Vo). Distinct emissions at 528 nm and 483 nm may be attributed to surface defects in Cl-doped ZnO NRs as reported earlier (Gautam et al., 2016). Fig. 5(d) clearly indicates that, in case of Cl_Z4 sample, the emission peak at around 550 nm is enhanced as compared with pristine ZnO NRs; that results in the increase of oxygen vacancy related defects (Sharma et al., 2018). As per the PL graph, the intensity of the visible band decreases as we increase the amount of Cl concentration. Cl_Z3 and Cl_Z5 exhibited the lowest defect states. This observation clearly signifies the importance of Cl in reducing Vo in ZnO NRs. This may be due to the saturation of surface growth by Cl, which restricts the evolution of Vo in ZnO NRs (Gautam et al., 2016). The lesser number of defect states results in the decrease of recombination rate of photogenerated charge carriers improving the photocurrent density (Cha et al., 2011).

Furthermore, to have an insight into the charge carrier kinetics of the doped ZnO nanostructures time-resolved photoluminescence (TRPL) was carried out. Fig. S6 depicts the TRPL spectrum of pristine and Cl-doped ZnO NRs. The decay time for all the samples were calculated by fitting the decay curves monoexponentially and are enlisted in Table 2. We observed a lowest decay lifetime of 0.195 μs for the Cl_Z5 sample as compared against pristine ZnO (0.367 μs). This significant reduction in the decay time value of the doped sample may be due to oxygen defect passivation by Cl dopant (Lee et al., 2016). As observed in the Fig. 5(a) a clear reduction in the PL spectra indicates faster transfer of the photoexcited electrons resulting in the shorter decay lifetime of the excited states. Thus quenched photoluminescence and reduced decay lifetime are ought to demonstrate the faster interfacial transfer which is also in agreement with the earlier reports (Ge et al., 2016; X. Wang et al., 2015). The above results strongly recommends the samples with lower decay times can be used for further water splitting applications as the carriers can be transferred to the neighboring electrolyte medium leaving either of the carrier from the exciton to take part in current generation. 3.5. Electrical properties The photoresponse behavior of as-synthesized undoped and Cldoped ZnO NRs have been studied by I-V measurement. Fig. 6 shows the J–V characteristic curves of all the grown NR samples measured at room temperature, under both dark and illumination within the voltage range of −5 to +5 V. The linear graph depicts the ohmic nature of all the synthesized NRs samples. Moreover, with an increase in dopant concentration, we observed a significant increase in the photocurrent density under UV illumination. The sample Cl_Z3 shows the highest value of photocurrent density of 20.54 µA/cm2. This result is in harmony with the PL graph (Fig. 5(d)) which shows the less defect states for Cl_Z3. Furthermore, we noticed a huge increment in the values of current density in the light as compared to dark. This can be explained as follows. Under UV illumination, electrons in the valence band absorb sufficient energy and e + h+) . Consequently, the photo moves to the conduction band(h induced charge carriers (holes in VB and electrons in CB) are generated in the ZnO nanostructure (Deka Boruah and Misra, 2016b). So, the sudden increase in photocurrent density under UV illumination is mainly due to the generation of charge carriers that dominate the process of scattering and recombination. Generation of free electrons in Cl-doped ZnO NRs also contributes in the photoresponse enhancement process (Boruah et al., 2017). Hence, increment in photocurrent density under illumination is because of the combined contributions of light interaction and generation of free electrons. But surprisingly, we observe a lower value of photocurrent density when the doping concentration was increased upto an extent. This may be due to the

154

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Fig. 5. (a) UV–Visible absorption spectra of pristine ZnO and Cl-doped ZnO NRs with variation in doping concentration, (b) Tauc’s plot indicating the optical bandgap of all the grown nanorod samples, (c) Light harvesting efficiency (LHE) of pristine ZnO and Cl-doped ZnO NRs and (d) Steady state PL spectra of pristine ZnO and Cl-doped ZnO NRs respectively.

Fig. 6. Current-voltage characteristics of all the ZnO nanorods samples (a) with and (b) without illumination (designated as D for dark and L for illumination along with the name of the samples).

reduced grain size and increased grain boundaries that adds to the scattering of charge carriers (Sharma et al., 2018). The values of photocurrent under dark and light illumination alongwith the enhancement (as reported earlier in Sharma et al., 2018) are listed in Table 3.

3.6. Photoelectrochemical properties 3.6.1. Mott-Schottky analysis Mott-Schottky measurements were carried out within an electrolyte environment (0.1 M NaOH) in absence of light (to exclude the effect of

155

4.03 0.27 0.20 8.22 0.46 6.77

=

JL @ Cl _Z 3 JL

JL

(from J V data) ;

339.33 43.14 151.45 – 433.50 128.02 LSV

=

JL @ ClZ 3 JL

JL

156

NH4Cl ZnCl2, KCl, NH4Cl, AlCl3 NH4Cl

NH4Cl

KCl

NH4Cl

NH4Cl

FTO FTO

FTO

FTO

FTO

FTO

ITO

Precursor

Substrate

Electrodeposition

Electrodeposition

Electrodeposition

Electrodeposition

Electrodeposition

Sol-gel and Hydrothermal Hydrothermal

Fabrication Technique

ZnO Shell homojunction

–

–

ZnS Core-Shell

TiO2 Core-shell

Cl doping TiO2 Heterojunction

Enhancement Strategy

0.103 0.428 0.467 2.161 1.835 1.014

JL

(from LSV data) .

0.042 0.392 0.392 1.977 1.51 0.844

UV Illumination (HgXe lamp) 100 mW/cm2

–

UV illumination (HgXe lamp and solar irradiation (Xe Lamp) UV Illumination (HgXe lamp) 100 mW/cm2 –

UV Light (30 W) Xe Lamp; 100 mW/cm2

Pin (mW/cm2)

Table 4 Summary of the Photocurrent density achieved for Cl doped ZnO nanostructures.

JV

4.66 14.31 8.11 20.54 3.87 8.93

(%)

(%) 0.758 0.777 0.870 0.825 0.749 0.690

0.1 M Na2SO4

0.1 M KCl

3.1

–

–

1.5

1.15

2.16 2.0

JL (mA/ cm2)

Vfb (V vs. RHE) @1 kHz

0.1 M Na2SO4 and 0.5 M Na2S 0.05 M Zn(NO3) and 0.05 M ZnClO4 and KClO4

0.1 M Na2SO4

0.1 M NaOH 1 M KOH

Electrolyte

1998 173 362 – 17.7 113

LSV

JD

JV

JD

JL

J (mA/cm2) values at +1.2 V from LSV curve

J (µA/cm2) values at +5 V from J ~ V curve

J: Photocurrent density;

ZnO Cl_Z1 Cl_Z2 Cl_Z3 Cl_Z4 Cl_Z5

Sample

Ag/AgCl

7 × 1018

MSE Ag/AgCl

7.4 × 10 Nitrate and 3.7 × 1019 for perchlorate solution 4.2 × 1020 1016

MSE

Ag/AgCl

7 × 1018

17

Ag/AgCl SCE

7.48 × 1019 –

0.859 0.917 0.797 7.480 2.766 1.281

Nd (cm−3) (×1019)

Reference electrode

1.290 1.249 1.340 0.437 0.719 1.057

LD (nm)

Carrier conc. (cm−3)

−0.757 −0.776 −0.869 −0.824 −0.748 −0.689

Ef (eV)

(Fan et al., 2011b) (Fan et al., 2011b)

(Rousset et al., 2009)

(Fan et al., 2013)

Present Work (Wang et al., 2014) (Fan et al., 2012)

Ref.

16.17 16.67 15.42 20.83 21.24 15.50

(s)

d

Table 3 Photocurrent density values obtained from J ~ V curve and LSV graph. The flat band potential, Fermi energy, Debye length, and carrier density were estimated from MS analysis and transient decay time calculated from chronoamperometric data of pristine ZnO and Cl doped ZnO NRs.

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Fig. 7. Mott Schottky plots of Cl doped ZnO nanorods.

photo generated charge carriers) to elucidate the behaviour of fundamental parameters like flat band potential(Vfb) , carrier density(Na) , and width of space charge layer(W ) of ZnO NRs after doping Cl. The flat band potential can be estimated from the Mott –Schottky plots based upon the famous Mott-Schottky equation stated below (Fan et al., 2011b; Karmakar et al., 2016).

1 = e Cs2

2 2 0 Nd A

V

Vfb

kT e

constant of the semiconductor material (10 for ZnO) and permittivity of vacuum (8.85 × 10 14 F/cm) , A is the area of the semiconductor in contact with the electrolyte, Nd is the carrier density (cm 3) , V equals to the applied potential w.r.t the Ag/AgCl electrode, Vfb represents the flat band potential, kT is the temperature dependent correction term e (Karmakar et al., 2016). Vfb can be derived from the MS plot by extrapolating the linear part of the graph as shown in Fig. 7. The values so obtained from the MS plots at 1 kHz has been presented in Table 3. Higher value of Vfb implies towards larger values of band bending and thus a smaller potential is required to generate photocurrent. Ideally, the Vfb does not depend on frequency. In order to ensure this, for the

(4)

where Cs is the space charge capacitance, e represents elementary electronic charge(1.6 × 10 19 C) , and 0 stands for the dielectric 157

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Fig. 8. (a) Flat band potential of all the ZnO Nanorods samples with and without doping. The values represented against each concentration of Chlorine has been calculated from the mean of the Mott Schottky plots obtained at 1, 10, 20, and 30 kHz. (b) CBM and VBM levels estimated from the electrochemical analysis. (c) Variation in the Debye length for Cl-doped ZnO photoanodes with different concentration. (d) Variation in the space charge width (W ) with respect to the applied potential (measured against RHE scale) for photoanodes with different Cl doping concentration.

hydrothermally grown ZnO NRs samples the experiment was also conducted at various frequencies like 10, 20 and 30 kHz. Fig. 8(a) represents the mean values obtained from the various frequencies along with the standard deviation represented as error bar. The slight variation in the flat band potential values with varied frequency indicates the complicated nature of contributions involved in the Cs (Dutoit et al., 1975; Nozik, 1978). The positive nature of the slope confirms the n-type nature of all the photoelectrodes (Karmakar et al., 2016). The donor carrier density (Nd ) of the photoelectrodes can also be calculated from the same slope of MS plot. The fermi level also can be calculated from the equation given by: Ef = eVfb . Here Ef is the Fermi level of the semiconductor film before contact with the electrolyte. The conduction band minima as well as the valence band maxima were calculated (shown in Fig. 8(b)) by the following equations:

ECB = EF

kT ln

and Eg = ECB

with increase in Cl concentration as compared with pristine ZnO NRs i.e. by changing the doping concentration in growth solution, the carrier concentration of Cl-doped ZnO NRs could be tuned from 0.85 × 1019 to 7.48 × 1019 cm−3. The increase in donor density of the NRs sample after doping upto 15 mM of Cl are considered as the degree of band bending at the vicinity of the semiconductor surface. Also the charge separation efficiency gets improvised which helps in achieving better photocurrent density under illumination (Li et al., 2017). The diffusion lengths of charge carriers also known as Debye length(LD) , for the photoanodes have been estimated by using the equation:

LD =

ND NC

(5)

where ECB , EVB , Eg are the CBM, VBM and bandgap respectively.

NC = 2

(

)

2 mh kT 3/2 h2

0 kT e 2Nd

1/2

. It was observed that with an increase in the donor

density the Debye length decreases. In addition to this, the charge transit time through the depletion region has been reported to be directly proportional to the square of the Debye length (Karmakar et al., 2016). Thus a material in possession with a lower LD value has a minimal transit time for an efficient electron hole separation. With regard to these properties it can be clearly understood that upon doping ZnO with a Cl concentration of 15 mM, the carrier separation occurs effectively within a comparatively short span of time and the synergistic effect helps in achieving better photoelectrochemical performance. The width of the space charge region (W ) is given by

( ) EVB

( )

represents the effective density of the states at the

bottom of the conduction band and mh = 0.24m 0 (Rasouli et al., 2019). An increase in carrier concentration of photoanodes was observed 158

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W=

2

0 (V

eNd

Vfb )

3.6.3. Linear sweep voltammetry analysis In order to put insight into the electrochemical performance of all the grown NRs arrays in the presence of light, linear sweep voltammograms were recorded as shown in Fig. 10(a). From the potentiodynamic curves an increase in photocurrent density was observed with the increase in positive potential values, thus confirming the n-type nature of the material (also confirmed from the MS plots shown in Fig. 7). The enhancement of photocurrent density indicates the generation of carriers due to illumination. For the pristine ZnO NRs sample, a photocurrent density of 0.103 mA/cm2 was observed in contrast with the dark current density of 0.042 mA/cm2 in 0.1 M NaOH at +1.2 V vs. Ag/ AgCl. Enhanced photoresponse values (depicted in Table 3) were further obtained in our case with a subsequent increase in the Cl concentration (upto 15 mM). The Cl_Z3 sample was found to exhibit the highest photocurrent density of 2.16 mA/cm2, which is remarkably (~20 times) enhanced than undoped ZnO NRs. The highest photocurrent values observed after doping Cl can be attributed to the optical properties like increased light harvesting efficiency along with reduced surface defects. The better PEC properties observed here demonstrates the efficient separation of photogenerated carriers as well as a reduced recombination probability at the semiconductor electrolyte interface (Wang et al., 2017). But on further increasing the dopant concentration more surface defects were formed in the ZnO host matrix as observed from the PL spectra (Fig. 5(d)). The defects act as recombination centres and cause a reduction in the photocurrent density values (Sharma et al., 2018).

1/2

(6)

Here the symbols have their usual meaning as mentioned earlier (Eq. (4)). The width of the space charge region for all the photoanodes are plotted against the applied potential (vs. RHE) as sown in Fig. 8(d). 3.6.2. Electrochemical impedance spectroscopy analysis Further in order to investigate the carrier transport properties at the semiconductor electrolyte interface electrochemical impedance spectroscopy (EIS) study was carried out. Fig. 9(a) shows the Nyquist plot of undoped and Cl-doped ZnO NRs. The arc of the semicircle in the Nyquist plot represents the interfacial charge transfer resistance or faradaic charge transfer resistance and hence predicts the ability of carrier separation. From the graph (Fig. 9(a)) it can be clearly seen that after illumination, the arc radius of the impedance curve decreases for all the samples. This clearly indicates the light-sensitive nature of the NRs arrays. Furthermore, the lowest values for charge transfer resistance were observed for Cl_Z4 sample, which indicates the efficient separation as well as suppressed recombination of photogenerated charge carriers. The above discussion also implies the reduction in charge separation efficiency of ZnO NRs at the interface with Cl doping. From the Bode plot (Fig. 9(c) it can be observed that at higher frequency region the phase angle was almost zero. Thus the impedance can be considered to be almost independent of frequency in the high frequency region.

Fig. 9. (a) Nyquist plot for all the photoanodes in absence and presence of light (inset shows the equivalent circuit used for fitting). (b) Zoomed presentation of the earlier mentioned Nyquist plot. (c) Bode plots for Cl doped ZnO nanorods. 159

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Fig. 10. (a) Photocurrent density vs. applied potential V (vs. Ag/AgCl) with and without UV illumination. (b) Photoconversion efficiency as a function of potential (vs. RHE).

The photoconversion efficiency ( %) of the samples were estimated in the presence of external applied potential to derive a favourable condition to make maximum use of light energy along with a substantial reduction of external bias (Babu et al., 2015; Hou et al., 2010). Fig. 10(b) shows the photoconversion efficiency as a function of applied potential (V vs. RHE) for the ZnO NRs with and without Cl doping under illumination. A maximum photoconversion efficiency of 57.75% was obtained for the Cl_Z3 sample, against undoped ZnO (2.46%) at an applied potential of 2.06 V (vs. RHE).

the Cl_Z3 sample, minimal recombination is assured. To ensure the lowest recombination rate of the Cl_Z3 as compared with the other samples the transient decay time was calculated by (I I ) means of the formula: d = (I t Ist ) ; where Iin and It are current values at in st I t 0 and s, while st is the steady state current. The transient decay time is defined as the value where ln d = 1 (Chakraborty et al., 2019). The calculated values of transient decay time was listed in Table 3. From Fig. 11(b), a decay time of 20.7 s was obtained for Cl_Z3; which is the highest among all the samples. Thus the slowest recombination was observed for the aforementioned sample also signifies the reason behind the enhanced photocurrent density of Cl_Z3. Electrode stability is a crucial parameter to develop electrochemical cells. Electrolytes, best known for their interaction with the electrodes for carrying out fundamental process always ends up with corrosion of the photoelectrodes. Also, systems exposed under continuous illumination undergoes photo corrosion after a certain duration. This necessitates the stability test for photoelectrodes before carrying out any fabrication of photoelectrochemical cells. In order to do so, we have conducted a stability test of Cl_Z3 under continous illumination for 12 h without applying bias voltage and the I-t curve hence obtained is shown in Fig. 11(c). At the very beginning of the experiment, a sharp increase in photocurrent density was observed, which almost decreased to a stable state after a very short span of time. Very negligible fluctuations in photocurrent density values of the photoanode were observed even after illumination of 12 h. More precisely after 8 h of constant illumination, a 6% decrease in the photocurrent density was observed. While by the end of 12 h a decrease of 11.6% has been noted. These results clearly state the ability of the Cl-doped ZnO NRs in further usage for the fabrication of photoelectrochemical cells.

3.6.4. Chronoamperometry and stability test Chronoamperometry tests were carried out for all the samples, to evaluate the photogenerated carrier separation, charge transfer behavior at the interface and photocurrent response under discontinuous UV illumination. Fig. 11(a) represents the photocurrent transient curves of all the prepared photoanodes at an applied voltage of 0 V vs Ag/AgCl. It can be clearly noted that the I-t curves of all the photoanodes exhibit similar nature i.e. upon illumination, the photocurrent density increases to a maximum value and remains almost constant. As soon as the light is chopped off, current decreases suddenly to the steady state current. In spite of these similarities, we also observed certain differences in photocurrent density. We observed a higher photocurrent density for all the doped NRs in contrast with pristine ZnO photoanode. Among all, Cl_Z3 shows the highest photocurrent density which reveals that it has a lower recombination rate and higher separation efficiency of photogenerated charge carriers under UV light illumination. Furthermore, transient photoresponse result of Cl_Z3 is in harmony with optical absorption and photoluminescence spectra as described in optical properties (Fig. 5(a) and (d)). On behalf of the low defects present in case of

Fig. 11. (a) Photocurrent transients of undoped and Cl-doped photoanodes at an applied bias of 0 V vs. Ag/AgCl under chopped illumination. (b) Transient decay time curves. (c) I-t curve of Cl_Z3 after 12 h of photoelectrochemical measurement under illumination measured without applying an external bias. 160

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4. Conclusion

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In a nutshell, we fabricated a simple, low cost, highly efficient undoped and Cl-doped ZnO NRs by hydrothermal technique. We discovered that Cl played an important role in structural, morphological, optical, electrical and photoelectrochemical properties of the prepared ZnO photoanodes. From morphological properties we observed that Cl ions prefer to adsorb on (0 0 2) surface of ZnO NRs, that leads to the reduced length along the NR axis. All the grown NRs are vertically aligned, dense, uniform with hexagonal wurtzite structure having perpendicular orientation on the substrate. UV–Vis measurements shows that Cl_Z3 has high light harvesting efficiency and suitable bandgap. From PL data, we concluded that the incorporation of Cl into ZnO lattice reduces the oxygen vacancies as a result the recombination rate of the charge carriers is low. Cl_Z3 shows the highest photocurrent density as measured from I–V characteristic curve of all the grown NRs. LSV measurements showed an enhanced photocurrent density of 2.16 mA/cm2 along with a photoconversion efficiency of 57.75% upon illumination as compared to undoped ZnO photoanode (0.10 mA/cm2). The carrier concentration of NRs was tuned from 0.85 × 1019 to 7.48 × 1019 cm−3 by changing the doping concentration. The reduced Debye length put insights into the reduced charge transit time along with effective electron hole separation. The photoresponse of the 15 mM doped sample was found to be significantly better than the other samples under intermittent UV illumination. A transient decay time of 20.7 sec was estimated for the optimized sample. To ensure the photostability, the sample was tested under continuous illumination for 12 h and observed a decrease in photocurrent values (6% after 8 h and 11.6% after 12 h). The enhanced photoelectrochemical properties of Cl_Z3 makes it a suitable photoanode for solar water splitting applications in a cost effective manner. This study not only states the importance of anionic dopant for tuning electronic and optical properties but also signifies the facile synthesis technique for achieving better PEC performance. The report clearly indicates that 15 mM doping concentration of Cl gives better results for water splitting application. Acknowledgement The authors express their deep gratitude to Dr. Mukul Gupta, Scientist-F, Dr. V Ganesan, Centre Director and Dr. Vasant Sathe, Scientist-G UGC-DAE CSR, Indore for providing XRD, AFM and Raman facilities. P.S would like to acknowledge Central Research Facility (CRF), IIT(ISM) Dhanbad, India for the experimental measurements supporting this work. The authors would like to acknowledge DST-FIST facility (project no. SR/FST/PSI-004/2013) for using life time spectrometer. The authors are also thankful to Prof. Kaushal Kumar and Mr. Sachin Maurya, IIT(ISM), Dhanbad for their help and useful discussions regarding analysis of TRPL spectra. P.S, A.S and S.P are thankful to IIT(ISM) for the research fellowship. Declaration of Competing Interest The authors declare no conflict of interest. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.09.045. References Antony, A., Poornesh, P., Ozga, K., Rakus, P., Wojciechowski, A., Kityk, I.V., Sanjeev, G., Petwal, V.C., Verma, V.P., Dwivedi, J., 2019. An electron beam induced study in fluorine doped ZnO nanostructures for optical filtering and frequency conversion application. Opt. Laser Technol. 115, 519–530. https://doi.org/10.1016/j.optlastec. 2019.03.003. Babu, E.S., Hong, S.K., Vo, T.S., Jeong, J.R., Cho, H.K., 2015. Photoelectrochemical water

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