CdS Core

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C: Energy Conversion and Storage; Energy and Charge Transport

Self-Assembled Vertically Aligned Hetero-Epitaxial ZnO/ CdS Core/Shell Array by All CBD Process: A Platform for Enhanced Visible-Light-Driven PEC Performance Rekha Bai, Dinesh Kumar, Sujeet Chaudhary, and Dinesh K. Pandya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04675 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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The Journal of Physical Chemistry

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Self-Assembled Vertically Aligned Hetero-Epitaxial ZnO/CdS Core/Shell Array by all

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CBD Process: A Platform for Enhanced Visible-Light-Driven PEC Performance

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Rekha Bai#, Dinesh Kumar#, Sujeet Chaudhary, and Dinesh K. Pandya*

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Thin Film Laboratory, Physics Department, Indian Institute of Technology Delhi, New Delhi 110016, India

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ABSTRACT

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We report on an all Chemical Bath Deposition (CBD) fabrication of self-assembled vertically

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aligned hetero-epitaxy grown (002)ZnO//(002)CdS ZnO/CdS core/shell nanorod (NR) arrays.

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These single-technique processed NR arrays comprising of CdS QDs conformally and

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epitaxially grown on faceted single crystalline ZnO NRs achieve effective charge separation

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and collection resulting from visible light driven highest ever reported photocurrent density

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without any post-deposition process step unlike previous such studies. The low ion-flux

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controlled ion-by-ion deposition induces chemical-epitaxy growth mechanism, with a 38 tilt

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of CdS c-axis. Structural characterizations confirm the homogeneous growth of (002) CdS QDs

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(size ~30 nm) on hexagonal prismatic c-axis oriented ZnO NRs (400 nm diameter). Compared

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with pristine ZnO NR array, a 20-fold enhancement in photocurrent density (~8.5 mA/cm2 at

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+1.0 V vs. Ag/AgCl) under AM1.5 light illumination is observed without any additional

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passivation/sensitization layer, attributed to the enlarged defect-free sharp ZnO/CdS interface

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and improved optical properties involving high visible light absorption as well as suppressed

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recombination of photoinduced charge carriers due to staircase type-II band alignment and

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hence effective charge separation and transport. Low temperature short time anneal further

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enhances the photocurrent density to 9-10 mA/cm2 and photoconversion efficiency to 4.4%.

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Our results demonstrate that formation of atomically aligned interface is key to realizing high

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performance junction without any post-deposition step, and have significant bearing on

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photoelectrochemical as well as photovoltaic performance of ZnO/CdS radial junction devices.

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*Corresponding Author, E-mail: [email protected]

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#

These authors contributed equally. 1 ACS Paragon Plus Environment

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1. INTRODUCTION

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Photoelectrochemical (PEC) cell water splitting using nanostructured semiconductor

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materials has been attracting great attention for production of clean and high efficiency

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renewable energy.1–5 Broad absorption region, effective charge transfer, and high

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photostability of semiconductor photoanodes are the prerequisite features for the improved

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PEC performance.6–8 Vertically aligned one dimensional (1-D) metal oxide semiconductor

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nanostructure arrays (i.e. nanorods, nanowires and nanotubes) can act as an excellent material

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nano-architectures for various solar energy conversion applications such as solar cells,

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photocatalysis and PEC cells.9–15 Core/shell type aligned 1-D nanostructure arrays are

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potentially superior photoelectrodes for generating high photocurrent owing to their large

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effective junction area, increased light absorption due to light trapping effect, short diffusion

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length, ideal geometrical structures for fast electron transport and less charge recombination

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probability.16,17 The single crystalline vertically oriented nanorods (core) are the key to a

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superior electron conduction through the core of the photoanode. Vertically aligned 1-D metal

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oxides, like ZnO or TiO2, based photoanodes with earth abundant constituents are emerging as

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the promising candidates due to their photostable, inexpensive and environment-friendly

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nature. Moreover, the direct bandgap of ZnO, ease of its crystallization, facile tailoring of

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nanostructures, anisotropic growth along c-axis, higher exciton binding energy, and higher

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electron mobility make it superior than TiO2.14,18,19 However, because of its wide band gap

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(~3.3 eV), bare ZnO cannot efficiently absorb visible light and hence the photoactivity of ZnO

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is restricted in the violet region (~5% of sunlight).20,21 To extend its photoresponse to the visible

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region and to enhance the photovoltaic or photoelectrochemical efficiency of ZnO-based

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devices, different methods such as doping by metals/non-metals, coupling with metal

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nanoparticles (Ag, Au), or decorating with small bandgap semiconductor quantum dots (QDs)

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of PbS, CdS, In2S3, CdSe, PbSe etc. are reported.13,22–30 In case of doping, the dopant-driven

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states facilitate the transfer of excited electrons to the conduction band under illumination.

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However, the confined mid-gap states reduce the oxidation potential of photoinduced holes

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compared to that of pure ZnO as well as provide recombination centres. In this situation, even

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though more visible light can be absorbed by the devices, their PEC performance is usually

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decreased due to enhanced carrier recombination rates.18

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In case of core/shell type device structures, an appropriate approach to efficiently

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absorb light in visible region is to create the staircase band-aligned heterostructures by

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decorating ZnO NRs with QDs of narrow band gap semiconductors such as ZnO/PbS,13,31

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ZnO/In2S3,27,32 ZnO/ZnS,33 ZnO/PbSe,29 ZnO/CdSe,34 ZnO/CdS35–38 etc. Therefore, it may be

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a fascinating approach to enhance the visible light driven photoelectrochemical performance

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based on the structurally superior and suitably band aligned core-shell semiconductors. Among

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various semiconductor QDs, CdS (n-type II-VI semiconductor) is considered to be the most

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appropriate visible sensitizing absorber for ZnO, comprising of direct band gap (~2.4 eV), high

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absorption coefficient, lattice matched similar crystal structure; with potential to facilitate

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better charge transfer in ZnO/CdS nano-architecture device, as required for efficient solar

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energy conversion applications. Furthermore, the staggered energy band alignment between

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ZnO core and CdS shell is favourable for the formation of type-II core/shell heterostructure

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photoelectrodes18,26,37 as in ZnO/CdS heterostructure the band edges of ZnO core are lower

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than that of corresponding band edges of CdS shell. The intrinsic build-in field due to the

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energy offset between conduction band edges will drive the photoinduced electrons (holes)

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towards ZnO core (CdS shell) leading to the rapid separation with suppressed recombination

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of charge carriers. Thus, ZnO/CdS core/shell heterostructure is anticipated as one of the

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promising photoelectrode for efficient solar cells or PEC applications.26,37

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In the NRs based devices, the ZnO NRs have been largely synthesized by hydrothermal

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process,5,10,24,26,36–39 which involves the use of high temperatures (80-200 °C) and high pressure



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(>1 atm.) with a long deposition times that are typically tens of hours. This makes the

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hydrothermal process time-consuming and uneconomical. Further, hexamethylenetetramine

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(HMT) is used in the process to achieve c-axis oriented vertically standing ZnO NRs.5,10,24,26,36–

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38

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to introduce a strong steric hindrance effect for inducing the anisotropic growth along c-axis

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thereby ensuring the vertical arrangement. As widely reported in the literature, the CdS shell

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has been prepared by a different technique, viz. successive ionic layer adsorption reaction

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(SILAR) method10,36,39,40 or pulsed electrodeposition method,37 both of which work on the

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principle of first depositing the alternate cation and anion layers followed by their chemical

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reaction to synthesize CdS. In case of SILAR method, this synthesis is achieved by annealing

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at high temperatures (300-600 °C). Obviously, the throughput and the large area scalability of

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the hydrothermal/SILAR method for realizing ZnO/CdS structures are expected to be low. In

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addition, the CdS is not expected to be suitably stoichiometric and its epitaxial matching with

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ZnO is not straight forward. It seems that these two drawbacks of the synthesis process may be

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responsible for the reported low photocurrent densities, up to 6.0 mA/cm2.26,36–38 There are

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reports of additional layer on top of CdS as passivation layer,5,21,39 viz. Au, NiO, ZnFe2O4, for

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enhancing the photocurrent but the same have not been helpful (see Table 2). Such additional

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layer seems to cause detrimental effects in nature of introducing recombination centres

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affecting the photocurrent generation as well as on account of possible stability issues. Even

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use of triple staircase structure with ZnSe and CdSe has not been much fruitful.10 There is also

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a report on adding ZnS passivation layer and adding 150 C/250 C two-step low temperature

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anneal/sensitization step14 that resulted in ~9.0 mA/cm2 photocurrent up from ~5.0 mA/cm2

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without this step.

The HMT has two-fold role, as a source of OH- ions (through its thermal degradation) and

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The originality of our present work lies in achieving seamless growth of ZnO/CdS

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interface via epitaxy and atomic matching (in sharp contrast to previous reports on this system)


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and demonstrated significant improvements in PEC photoresponse of as-prepared samples

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alike previous studies that necessitate additional layer/process steps. Our novel approach is to

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use the simple and single process of CBD free of additives that has potential for ion-flux

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controlled chemical epitaxy (atomically matched interface, free of interface states), layer-by-

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layer growth (sharp step-junction) and stoichiometric material yield (free of bulk

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impurities/defects), to achieve significantly enhanced performance of photoanode. This helps

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in the growth of atomically matched interface between CdS (as well as other potential II-VI

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chalcogenides) and ZnO by chemical epitaxy made possible by the above-mentioned features

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unique to CBD. In addition, this technique also results in synthesis of almost stochiometric

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CdS as process works on atomic (ionic) level rather than on cluster level. The CBD technique

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also yields the hexagonal single crystal c-axis oriented vertically aligned ZnO NRs without

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using any additional precursor.13 Thus, whereas the similarity of the current work with those

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reported previously lies in obtaining high quality ZnO NRs by employing CBD as well, the

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difference lies in terms of topotaxial growth of CdS shell via chemical epitaxy on these ZnO

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NRs in the present case. Thus, we employed a simple but novel change in approach exploiting

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the bottom-up atomistic epitaxial growth process and that too by using a single process (also

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commercially viable) to complete the active device structure under ambient conditions. This

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work also demonstrates that the CBD growth process (Figure 1) efficiently couples the 1-D

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ZnO NRs core, as the electron mediator to facilitate charge transport with CdS QDs shell, as

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the visible light absorber, in fabricating the stochiometric and highly oriented hetero-epitaxial

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ZnO/CdS core/shell NR array photoanodes that possess sufficiently high photocurrent density

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desired in PEC cells. Additionally, the enhancement in photocurrent density that necessitates

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concurrently large effective junction area with minimal surface and volume defects is achieved

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in these aligned NR arrays. In particular, we have been able to achieve the hexagonal single

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crystal prismatic c-axis oriented ZnO NR arrays with topotaxial conformal coverage of ~30 nm



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thick (002) CdS QDs shell; a defect-free sharp ZnO NR/CdS QDs interface; exceptionally

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improved PEC response vis-à-vis hitherto reported in such ZnO/CdS core/shell NR arrays

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without any additional passivation/sensitization layers (e.g. Au, Ag, graphene, etc.); and

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maximum photocurrent density Jlight = 8.5 mA/cm2 with IPCE ~60% This work provides the

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first reported evidence for hetero-epitaxy of CdS QDs on ZnO NRs (002)ZnO//(002)CdS with

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very high photocurrent density. The process-structure-property-performance correlation is also

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established.

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2. EXPERIMENTAL DETAILS

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2.1. Fabrication of ZnO NRs:

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In a typical growth process, ZnO NR array were prepared on indium doped tin oxide

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(ITO) coated glass substrate in alkaline aqueous medium by CBD technique. Prior to the

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deposition of ZnO NRs, the ITO substrates were sequentially cleaned ultrasonically in

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deionized water, acetone and propanol for 15 min each. All analytical grade reagents (Merck)

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were used for synthesis without any further purification. The typical aqueous solution was

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prepared from 100 mM zinc chloride and the bath pH=10 was adjusted by slowly adding the

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ammonia solution.13 The reaction bath was kept immersed into a constant temperature water

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bath placed on a magnetic stirrer. Firstly, a uniform and thin ZnO seed layer (thickness ~30-40

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nm) was deposited on the ultrasonically cleaned ITO substrate by CBD to provide the

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nucleation sites for ZnO NRs growth. Thereafter, the ZnO seed layer covered substrates were

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mounted vertically into the reaction bath with constant magnetic stirring and kept at 75oC bath

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temperature for 90 min. After deposition, the ZnO NR array samples were thoroughly rinsed

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with deionized water to remove loosely adhered particles from the surface and dried in ambient

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air. The as-prepared ZnO NRs samples were further annealed in vacuum at 300 oC for one hour

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before CdS coating to improve NRs crystallinity as well as to provide defect free surface for

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tight ZnO/CdS interface.


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2.2. Preparation of CdS QDs on ZnO NR array:

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In order to extend the light absorption region, ZnO NR arrays were further decorated

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with CdS QDs by an optimized CBD process.41 Though this process can result in the formation

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of wurtzite or cubic phase of CdS, we have confirmed the formation of wurtzite phase via

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HRTEM studies. The optimized growth solution contained 100 mM cadmium acetate

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[Cd(CH3COO)2.2H2O] as a source of cadmium ions (Cd2+) and 200 mM thiourea [SC(NH2)2]

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as a source of sulfide ions (S2–) to deposit CdS QDs onto ZnO NRs.42 Ammonia solution was

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added slowly to adjust the bath pH to 12. The ITO substrates coated with ZnO NR array were

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suspended vertically into the reaction bath for a desired time (t = 0 to 120 s) at 75 °C with

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continuous slow stirring during deposition to fabricate ZnO/CdS core/shell NR array. After

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deposition the substrates were rinsed with deionized water and then left to dry at room

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temperature. The ZnO/CdS core/shell NR array samples with CdS QDs shell deposition times

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t = 0, 30, 60, 90, and 120 s were referred to as ZC0 (t = 0 s), ZC30 (t = 30 s), ZC60 (t = 60 s),

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ZC90 (t = 90 s), and ZC120 (t = 120 s). Low temperature annealed ZC90 samples are referred

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as ZC90_A150_10 (150 °C anneal for 10 min), ZC90_A150_20 (150 °C anneal for 20 min)

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and ZC90_A250_10 (250 °C anneal for 10 min).

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2.3. Characterization:

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The surface morphology and definite microstructure of ZnO/CdS core/shell NR array

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photoanodes were investigated by field emission scanning electron microscopy (FESEM, FEI

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Quanta 200F SEM) equipped with an energy dispersive spectroscope and high-resolution

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transmission electron microscopy (HRTEM, Tecnai G2F20 Twin). The X-ray diffraction

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(PANalytical X’pert PRO) of NRs samples was recorded with monochromatic CuKα radiation

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(λ = 1.54 Å) in 2θ range from 20° to 80° using Bragg-Brentano geometry to determine the

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crystal structure and phase purity of the samples. The optical absorbance spectra were recorded

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using Perkin Elmer Lambda 1050 UV–VIS–NIR spectrophotometer. Raman spectra and



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photoluminescence (PL) data were acquired by spectrometer (Labram HR Evolution, Horiba

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Scientific) with 325 nm He-Cd laser excitation at room temperature. The photoelctrochemical

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(PEC) measurements were performed at room temperature using an AM1.5 solar simulator in

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a typical three-electrode electrochemical workstation.

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3. RESULTS AND DISCUSSION:

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3.1. Field Emission Scanning Electron Microscopy (FESEM):

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Figure 2 shows the FESEM images of the pristine ZnO and ZnO/CdS NRs revealing

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the NR morphology before and after decoration of ZnO NRs with CdS QDs. As seen from

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Figure 2a, the pristine ZnO NRs (ZC0) grow almost vertical (normal) to the substrate. All the

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NRs possess smooth and flat lateral surfaces as well as top hexagonal terraces. The prismatic

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ZnO NRs have a typical size of ∼300-400 nm with preferential growth along c-axis (supported

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by XRD shown later). Further the NRs appear to consist of hexagonal sheets packed on top of

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each other and thus eventually forming almost prismatic hexagonal NRs. The formation of such

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flat terraces and nano-sheets can be correlated with the low ion flux induced layer-by-layer

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Frank van der Merwe growth mechanism43 and is indicative of the ion-by-ion nucleation

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controlled deposition process. The length of ZnO NRs is ~5.7 µm as determined from cross-

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sectional FESEM image (Figure 2b), which is quite beneficial for enhanced light absorption

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via optical trapping. Figs. 2c-f display the surface morphology of the NR samples decorated

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with CdS QDs grown for 30 s (ZC30), 60 s (ZC60), 90 s (ZC90) and 120 s (ZC120),

20

respectively. It can be observed that in ZC30 and ZC60 samples (t < 90 s), the surface of ZnO

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NRs is partially covered with isolated small sized CdS QDs and these NRs exhibit rougher

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surface compared to ZC0. For t ≥ 90 s, the CdS QDs get uniformly decorated on the surface of

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ZnO NRs with QDs being compact and larger in size (supported by TEM analysis given later).

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The uniform compact coverage along the whole length of NRs is attributed to nucleation-

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controlled growth of CdS QDs under optimized conditions leading to large nucleation rate. The

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CdS QDs formation follows heterogeneous nucleation mechanism; which can be illustrated 8 ACS Paragon Plus Environment

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from the fact that the CdS bath solution remains clear during the entire CBD process, while

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ZnO NRs gradually turn into yellow with continued CdS QDs decoration on NRs. It may be

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noted that in the event of homogeneous nucleation in the bath solution precipitation of CdS

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would have happened with increasing time. The increase in diameters and surface roughness

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of NRs with increase in CdS deposition time demonstrates the formation of thicker CdS shells.

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The elemental composition of all the samples prepared at different shell deposition time

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is obtained by EDX measurement and is shown in Table 1. In ZC0, only Zn and O elements

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are present. The slightly greater extent of O over Zn could be due to contribution from

9

conducting ITO glass substrate. In ZnO NRs core/CdS QDs shell samples Cd and S elements

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are also observed. As expected on moving from ZC30 to ZC120 sample, the atomic

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concentrations of Zn and O are continuously decreasing while that of Cd and S is increasing

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with increase in CdS shell thickness. It is noteworthy that the Cd/S ratio stays ≈ 1 for all

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deposition times. This observation highlights the novelty of the CBD process in realizing the

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stochiometric deposition. The EDX spectrum and the elemental mapping of the Zn, O, Cd and

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S elements for ZC90 are shown in Figure 3, which suggests the uniform distribution of

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constituent elements. These results are further supported by structural and electron microscopy

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analyses.

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3.2 X-Ray Diffraction (XRD):

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The typical crystal structure of pristine ZnO (ZC0) and ZnO/CdS core/shell NR array

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samples (ZC30–ZC120) was examined by XRD as shown in Figure 4a. The XRD pattern of

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ZC0 sample shows the characteristic diffraction peaks of wurtzite structure of ZnO (JCPDS

22

05-0664). The sharp and very intense (002) ZnO diffraction peak at 34.4° confirms that ZnO

23

NRs are almost single crystalline and preferably oriented along c-axis normal to the substrate,

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well in consonance with the FESEM images (Figs. 2a,b). The very weak peaks corresponding

25

to other planes of ZnO may be attributed to the slight tilt in the NRs. The crystallite size of



1

ZC0 calculated from the Scherrer equation is found to be ~20 nm. Moreover, it is observed

2

that, besides the diffraction peaks of ZnO and ITO substrate (marked by * in Figure 4a), a broad

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peak at 26.7° corresponding to (002) plane of wurtzite structure of CdS (JCPDS 41-1049)

4

appeared in all ZnO/CdS NRs samples. This (002) plane diffraction of CdS QDs reveals that

5

the CdS shell is formed with wurtzite structure (also confirmed by Raman spectroscopy) and

6

grows topotaxially with c-axis preferential orientation. This broad CdS peak indicates that the

7

crystallite size of decorated CdS QDs on the surface of ZnO NRs is quite small. The bare CdS

8

film is also textured oriented along (002) direction (Figure S1). Furthermore, the CdS (002)

9

peak becomes sharper and intense with increase in deposition time of CdS shell, revealing the

10

improvement in size as well as crystallinity of CdS QDs. (Figure 4b). The crystallite size of

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CdS QDs increases from 8.5 nm for ZC30 to 10.5 nm for ZC120. Additionally, the decrease in

12

intensity of ZnO (002) peak with increase in CdS deposition time (Figure 4c) is attributed to

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the enhancement of CdS shell thickness covering the ZnO NRs, in consonance with the FESEM

14

images (Figure 2). Also, no peak shift is observed for ZnO (002) peak after the fabrication of

15

ZnO/CdS heterostructures. This indicates the growth of CdS shell on ZnO NRs is in the form

16

of hetero-epitaxial core/shell NR array, which will be further evidenced by electron microscopy

17

studies.

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3.3. Raman Spectroscopy:

19

Raman spectroscopy is employed to further evidence the crystal structure and phase

20

purity of ZnO NRs and CdS QDs. Figure 5 shows the Raman spectra of pristine ZnO and

21

ZnO/CdS NR array samples recorded by utilizing 325 nm excitation wavelength. For ZC0

22

sample, two strong peaks are observed centred at 575 and 1150 cm-1, which are attributed to

23

the longitudinal optical mode (LO) and two-phonon scattering process (2LO) of hexagonal

24

ZnO, respectively.44 For ZnO/CdS core/shell NRs, one additional peak at ~300 cm-1 is observed

25

apart from the wurtzite ZnO peaks. This peak is attributed to the first-order longitudinal optical


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(1LO) phonons of wurtzite CdS.26,45,46 The absence of Raman peak at ~243 cm-1 corresponding

2

to cubic CdS14 confirms the hexagonal phase purity of CdS in all NR array samples, consistent

3

with the XRD results. The intensity of 1LO scattering mode of hexagonal CdS increases with

4

increase in CdS shell thickness from ZC30 to ZC120 sample with an associated continuous

5

decrease in intensity of ZnO peaks due to covering of NRs by the shell. This reduction in

6

intensity of Raman peaks of ZnO may be possibly due to enhancement in absorption of exciting

7

photons and scattered photons by the CdS shell on ZnO NRs. Thus, Raman analysis provides

8

the evidence for the hexagonal structure of ZnO NRs cores and CdS QDs shells well in

9

consonance with XRD results.

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3.4. High Resolution Transmission Electron Microscopy (HRTEM):

11

To further investigate the morphology and microscopic structures of NRs, TEM images

12

are recorded as shown in Figure 6. The bright field TEM image of ZC0 sample in Figure 6a

13

shows a smooth and clean surface with flat terraces at the tip of ZnO NRs consistent with the

14

observed FESEM in Figure 2a. Both the high-resolution TEM (HRTEM) image in Figure 6b

15

and the diffraction spots in selected area electron diffraction (SAED) pattern in Figure 6c

16

evidence the excellent single crystal structure of pristine ZnO NRs. The interplanar spacing of

17

2.6 Å corresponds to the (002) plane of the wurtzite ZnO (JCPDS 05-0664), which is well in

18

consonance with the XRD results (Figure 4). The crystallite size of ZnO ~20 nm obtained from

19

XRD seems to agree with the thickness of the hexagonal terraces seen in FESEM and HRTEM.

20

Figure 6d displays the bright field TEM image of ZC90 NR, in which the white dashed line

21

indicates the interface between the ZnO NR and CdS shell, which appears to be flat and smooth.

22

The figure reveals that the ZnO NRs are uniformly decorated with a thin layer (~30 nm) of CdS

23

QDs shell, confirming the formation of topotaxial core/shell heterostructure of NR array and

24

consistent with the FESEM (Figure 2e). The low ion-flux of Cd2+ and S2– ions reaching on the

25

surface of vertically aligned ZnO NRs, promotes the topotaxial growth of CdS QDs followed



1

by controlled heterogeneous ion-by-ion growth mechanism during chemical bath deposition.

2

The HRTEM image in Figure 6e clearly resolves QDs and their lattice fringes of the ZC90

3

heterostructure. The interplanar spacing of ~3.3 Å in shell region is attributed to the (002) plane

4

of wurtzite CdS (JCPDS 41-1049) on the well oriented ZnO NR core. Moreover, the angle

5

between the lattice fringes of (002) planes of ZnO core (d002 ~ 2.6 Å) and CdS shell (d002 ~ 3.3

6

Å) is found to be ~38°, which suggests tilted-epitaxy growth - (002)ZnO//(002)CdS hetero-epitaxy

7

- of wurtzite CdS QDs on ZnO NRs.47,48 Furthermore, the corresponding SAED pattern of

8

ZC90 in Figure 6f is the combination of two sets of diffraction patterns from the ZnO core as

9

well as CdS shell. The clearly observed diffraction spots correlate to the [002] growth direction.

10

The observed spotted-rings in SAED pattern of ZC90 sample exhibit an intense spotted-ring

11

corresponding to (002) planes and another weak spotted-ring for (101) planes of hexagonal

12

CdS. This points to the CdS preferential growth along c-axis for QDs with in-plane

13

rotation/out-of-plane tilt among the QDs giving rise to the weak ring. Thus, the preferred (002)

14

orientation of both ZnO NRs as well as CdS QDs may produce an appropriate ZnO/CdS

15

interface with less lattice mismatch and low density of associated trap states, which can

16

facilitate the efficient charge transfer from CdS QDs to ZnO NRs as essential for the improved

17

photoelectrochemical properties (will be discussed later). This nucleation controlled epitaxial

18

growth is thus responsible for the formation of sharp and high quality ZnO/CdS interface.

19

3.5. Optical Properties:

20

UV-vis absorption measurement is utilized to investigate the effect of the CdS shell

21

deposition time on the optical absorption in ZnO/CdS NR array. The absorption spectra of

22

pristine ZnO and ZnO/CdS NRs with various deposition time of CdS shell are shown in Figure

23

7. For ZC0 sample, a clear and abrupt absorption edge at ~375 nm is ascribed to the direct band

24

gap of wurtzite ZnO (Eg ~3.3 eV), and there is low absorbance in visible region. With

25

deposition of CdS on ZnO NRs absorbance in visible is observed, which continues to increase


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1

with the eventual evolution of the second absorption edge when the CdS QD-shell becomes

2

conformal. This sharp increase in absorption beyond UV, which shows a systematic increase

3

as CdS shell thickness increases, attains a high near flat absorbance till ~470 nm followed by

4

a drop terminating at ~540 nm corresponding to the band edge of wurtzite CdS (Eg ~2.3 eV).

5

The sudden jump in CdS absorption as we go from ZC60 to ZC90 sample and consequent

6

evolution of the absorption edge seems to indicate an almost complete coverage of ZnO NRs

7

consistent with the FESEM study. To determine the band gap of our samples, we carried out

8

Tauc’s plot analysis for all photoanodes (Figure S2). For ZC0, the observed direct band gap is

9

~3.3 eV. While for ZnO/CdS NRs arrays, two band gaps are observed; one at ~3.3 eV for ZnO

10

and other at ~2.3 eV for CdS.

11

Furthermore, the additional absorption extending lower than the CdS band gap, is

12

ascribed to the interfacial transitions corresponding to the formation of effective band-gap

13

between the ZnO NRs core and CdS QDs shell. Therefore, ZnO/CdS core/shell NRs samples

14

have quite broad absorption range (from UV to visible) as compared to pristine ZnO NRs due

15

to the expected formation of type-II band alignment, which will be further confirmed by

16

photoluminescence measurement as well as by an empirical relation later in the article. Due to

17

high optical absorption along with broad spectral range, an enhanced photovoltaic or

18

photoelectrochemical performance is expected from the core/shell NRs as compared to that

19

from pristine ZnO NRs.

20

3.6. Photoluminescence Spectra (PL):

21

PL measurement is employed to further explore the effect of CdS shell on the optical

22

behaviour aspects of CdS coated ZnO NRs. Figure 8a illustrates the room temperature PL

23

spectra of ZC0–ZC120 core/shell NR arrays utilizing the excitation wavelength of 325 nm.

24

Pristine ZnO NRs emit a luminescence with an intense and sharp ultraviolet (UV) emission

25

band centred at ~378 nm and a weak visible emission band spreading over a broad spectral



1

range. The strong UV emission is ascribed to the near band edge (NBE) emission due to the

2

radiative recombination of charge carriers in ZnO and visible emission band is commonly

3

attributed to the presence of defect states in ZnO.49,50 The sharp and strong NBE emission peak

4

reveals the high crystallinity and good optical response of ZnO NRs possessing direct bandgap

5

transition (~3.3 eV). The PL intensity of NBE emission reduces continuously with increase in

6

surface coverage of ZnO NRs by CdS QDs shell from ZC30 to ZC120 NRs and as shell

7

thickness increases. It is observed that for ZC90 the PL intensity significantly reduces to ~38%

8

of that of ZC0. On uniform and full coverage of ZnO NRs surface by CdS QDs for t ≥ 90 s

9

(Figs. 2 and 6) there is significant reduction in NBE emission peak intensity. The reduction in

10

observed PL intensity in ZnO/CdS NRs may be possibly due to the absorption of exciting

11

photons and the emitted PL radiation from ZnO NRs cores by the CdS shell. With increased

12

coverage and shell thickness such absorptions are higher, consistent with the observed

13

increased visible absorption of incident photon flux (Figure 7). In addition, the spatial charge

14

separation due to staggered energy band alignment in type-II heterostructure composed of ZnO

15

NRs and CdS QDs may efficiently suppresses the radiative recombination of photoinduced

16

carriers and quenching the luminescence from ZnO, which is the dominant contribution to the

17

reduction in NBE peak intensity of ZnO/CdS NRs.13,15,26,45 Such a suppression of charge carrier

18

recombination suggests the occurrence of prolonged carriers’ lifetime in ZnO/CdS NRs

19

heterostructure, beneficial for their photoelectrochemical cell (PEC) performance.

20

Furthermore, a weak and broad visible emission band in the wavelength range of 500–

21

750 nm (having two shoulder peaks P1 ~570 nm and P2 at ~630 nm) is observed in measured

22

PL spectra (inset of Figure 8a). The defect states in ZnO such as oxygen vacancies or surface

23

states (i.e. dangling bonds) would normally promote charge carrier trapping and reduce the

24

intrinsic PL response and cause visible emission in a wide spectral region45,50 due to such

25

assorted defects in the band gap of ZnO. The very weak visible luminescence in our as-grown


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1

samples (in comparison to normal polycrystalline ZnO in which defect PL peak is almost of

2

same intensity as NBE peak)51–53 indicates very low concentration of such defects in our ZnO

3

NRs. Moreover, it is worth noting that the deposition of CdS QDs on the surface of ZnO NRs

4

may further reduce the PL intensity of this defect emission, and defect peak intensity is thus

5

minimum for ZC90 sample possessing conformal coverage. This behaviour may be attributed

6

to the fact that the decorated CdS QDs can alleviate the carrier recombination at defect states

7

by passivating the oxygen vacancies and/or surface states (i.e. dangling bonds) of ZnO through

8

adsorption of sulfur atoms or CdS QDs, which efficiently reduces the visible emission of ZnO

9

NRs.45,50 Additionally, a weak blue shift in shoulder corresponding to ZnO defect peak P1 and

10

slight red-shift in peak P2 are observed with increase in CdS shell deposition time. Figure 8b

11

shows the PL spectrum of bare CdS layer, in which the sharp NBE peak is seen at ~520 nm

12

and weak defect bands are observed in the broad wavelength 550-750 nm region. The shift in

13

defect peaks P1 and P2 of ZnO are observed due to the involvement of defect emission of CdS.

14

3.7. Photoelectrochemical Properties:

15

To investigate the effect of CdS shell deposition time on the photoelectrochemical

16

(PEC) properties of ZnO/CdS core/shell NRs samples, PEC measurements are performed in a

17

three-electrode electrochemical cell comprising the as-prepared photoanode (ZC0–ZC120

18

NRs) as working electrode (active area ~0.4 cm2), a Platinum mesh as counter electrode and

19

the standard Ag/AgCl as reference electrode. The PEC performance of all photoelectrodes are

20

examined by measuring the photocurrent response under AM1.5 solar illumination in an

21

aqueous solution of 0.1 M Na2SO4 (pH ~ 7) as electrolyte. Figure 9a depicts the typical curves

22

of current density (J) as a function of applied potential (varying from -1.0 to +1.0 V w.r.t.

23

Ag/AgCl) of the five different photoanodes in dark and under illumination. The linear sweep

24

voltammograms (J−V curves) show negligible dark current density (Jdark) for all the samples

25

(Jdark ~0.1 mA/cm2), while photocurrent density grows significantly to large value (Jlight = 8.5



1

mA/cm2) under illumination. The cathodic current is very small (Jlight < 0.1 mA/cm2) and

2

shown in Figure S3. The generation of anodic photocurrent from all the samples by applying

3

electrochemical bias confirms the n-type semiconducting behaviour of both ZnO NRs as well

4

as ZnO/CdS core/shell photoelectrodes. Under illumination, the ZC0 NR array reaches small

5

Jlight ~0.39 mA/cm2 at +1.0 V (vs. Ag/AgCl) linked to wide band gap (~3.3 eV) and poor

6

photoresponse of ZnO NRs in visible light. After decoration with CdS QDs, the photocurrent

7

density (Jlight) increases significantly from 1.6 mA/cm2 for ZC30 to 8.5 mA/cm2 for ZC90.

8

However, when CdS deposition time t > 90 s, the Jlight reduces to 6.9 mA/cm2 for ZC120 NRs.

9

Our study reveals that the ZnO/CdS NRs demonstrate a high PEC activity in visible light and

10

largest Jlight for ZC90 is more than 20 times that of bare ZnO NRs. The observed enhancement

11

in PEC performance is attributed to the additive/linker free growth of uniformly distributed

12

CdS QDs on the surface of highly crystalline c-axis oriented ZnO NRs by optimized

13

heterogeneous ion-by-ion growth mechanism. In addition, the increased photocurrent with

14

increasing deposition time of CdS shell signifies the improvement in visible light absorption

15

by CdS QDs and also more efficient generation and separation of photoinduced charge carriers,

16

which are consistent with the results obtained by optical absorbance and PL measurements

17

(Figs. 7 and 8). The significantly large photogeneration despite low CdS thickness of 30-50 nm

18

can be ascribed to the phenomena of optical trapping on account of multiple reflections in the

19

inter NW space of the array. Since the carrier generation effectively occurs in the junction

20

region, it amounts to efficient separation. Moreover, the larger sized CdS QDs obtained with

21

increase in deposition time reduce the grain boundaries in CdS shell, which then shortens the

22

transport path of the photogenerated electron-hole pairs and inhibit the charge carrier

23

recombination. This evidences that the high generation as well as separation and collection of

24

charge carriers promotes the production of higher photocurrent in our NRs array. Further, the

25

decrease in Jlight for ZC120 sample indicates that larger CdS shell thickness is detrimental for


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1

photocurrent collection due to associated enhancement in bulk recombination loss in thicker

2

CdS. It may also be pointed out that photogenerated charge carriers themselves start getting

3

generated farther away from the ZnO/CdS interface. Our PEC analysis evidences that the ZnO

4

NRs decorated with CdS QDs for t = 90 s is the optimum sample with maximum Jlight of 8.5

5

mA/cm2 owing to optimal optical absorption and suppressed photoinduced electron-hole pair

6

recombination.

7

The enhanced PEC response of ZnO/CdS core/shell NRs can be further understood in

8

terms of the staggered band alignment of ZnO and CdS in type-II heterostructure composed of

9

ZnO NRs and CdS QDs. The relative band edge positions of ZnO and CdS are explored, since

10

the band-edge potential has a vital role for controlling the photoinduced charge transport in a

11

core/shell heterojunction. The lower bound of conduction band (ECB) is computed empirically

12

via relation:11

13

ECB = X - 0.5Eg + Eo

14

where X is the electronegativity of semiconductor (which can be deduced from the geometric

15

mean of the absolute electronegativities of the constituent atoms); Eg is the optical band gap

16

(which is estimated from the onset of the absorption edge); and Eo is the constant energy factor

17

linking the reference electrode redox level to the absolute vacuum level (Eo = −4.5 eV for

18

normal hydrogen electrode (NHE)). The valence band edge potential (EVB) can be calculated

19

as EVB = ECB + Eg. The band gap energies estimated from the optical spectra (Figs. 7 and 8) are

20

found to be ~3.3 eV and ~2.3 eV for ZnO and CdS, respectively. From the above equation, the

21

obtained conduction band potentials (ECB) of ZnO and CdS are −4.33 and −4.03 eV,

22

respectively. Correspondingly, the valence band potentials (EVB) of ZnO and CdS are −7.63

23

and −6.33 eV, respectively. Thus, the band edge positions ECB and EVB of CdS are at higher

24

potentials, respectively, than that of ZnO. The lower bound of conduction band (ECB) and the

25

upper bound of valence band (EVB) of ZnO lie at −0.17 and 3.13 V with respect to NHE, and 17 ACS Paragon Plus Environment


1

for CdS the values are −0.47 and 1.83 V, respectively. Since the band gap of CdS (Eg ~2.3 eV)

2

is less than that of ZnO (Eg ~3.3 eV) but the ECB and EVB of CdS are higher than those of ZnO,

3

a core/shell heterostructure forms helping the efficient separation of photoexcited charge

4

carriers due to suitable band offsets between the ZnO and CdS semiconductors. Figure 9b

5

shows the schematic energy band diagram of the ZnO NR/CdS QD core/shell array exhibiting

6

type-II band alignment as well as basic PEC operation steps. This type of band alignment

7

promotes the easy separation as well as effective transfer of the photogenerated charge carriers.

8

Indeed, the energy offset (∆E ~ 0.3 eV) between the conduction band edges of the ZnO NR

9

core and CdS QD shell is the driving force for the transfer of photoinduced electrons from CdS

10

into ZnO. The type-II band alignment in similar ZnO NR core/CdS shell structure has been

11

analysed in detail54 and our results are consistent with that energy band picture. The scheme of

12

Figure 9b depicts the photogeneration of electron-hole pairs under illumination, electron

13

injection from conduction band of excited CdS QD shell into the conduction band of ZnO NR

14

core and subsequently to the ITO substrate via direct pathway through the length of ZnO NRs

15

(inter-conduction band transfer), hole movement from the valence band of ZnO NR to the

16

valence band of CdS QD and then finally scavenging of holes in electrolyte. The ZnO NRs

17

architecture normal to the substrate facilitates an easy and direct conduction path to the electron

18

transport from ZnO NRs to the ITO substrate and then through external circuit to the load.

19

Thus, this scheme evidences the effective separation and suppressed recombination of

20

photogenerated charge carriers at the photoanode/electrolyte interface, which result into

21

considerable enhancement in PEC performance of ZnO/CdS NRs.

22

Furthermore, to estimate quantitatively the photoactivity of as-prepared samples, the

23

photoconversion efficiency (η) of our photoanodes constituting the PEC cell is evaluated from

24

the J−V curves shown in Figure 9a using the following equation:13,26

25



J P (1.23  VRHE ) Pin 18

ACS Paragon Plus Environment

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1

where, JP is photocurrent density (JP = Jlight − Jdark), VRHE is the applied potential versus

2

reversible hydrogen electrode (RHE) and Pin is the incident light intensity on the photoanodes.

3

VRHE is related with VAg/AgCl according to the following Nernst equation:13,26

4

VRHE = VAg/AgCl + 0.059 pH + 0.1976

5

Figure 9c represents the photoconversion efficiency (η) versus applied potential curves for all

6

photoelectrodes from ZC0 to ZC120. It is observed that η is considerably affected by CdS shell

7

deposition time. Pristine ZnO NRs (ZC0) sample reached its maximum efficiency of 0.19% at

8

applied potential of +0.48 V vs. Ag/AgCl (or +1.09 V vs. RHE). After decoration of ZnO NRs

9

with CdS QDs, the η value increases with increase in CdS deposition time maximizing at +0.72

10

V vs. Ag/AgCl (or +1.33 V vs. RHE) from 0.52% for ZC30 (t = 30 s) to a maximum value of

11

2.75% for ZC90 (t = 90 s), ~17 times larger than that for ZC0 at +0.72 V. The enhanced

12

photoconversion efficiency with increase in CdS shell deposition time can be correlated with

13

the relative narrow band gap of CdS (~2.3 eV) and the extended absorption in visible region as

14

well as efficient separation and collection of photoinduced charge carriers (due to preferred

15

type-II band alignment) in ZnO/CdS NRs. In contrast, at higher deposition time (t = 120 s) for

16

ZP120 sample, η slightly decreases to 2.26%, which can be illustrated in terms of the lesser

17

collection due to increased recombination during their transport through the thicker CdS shell.

18

Thus, the variation of η in our as-prepared core/shell NRs samples corroborates the

19

photocurrent and UV-vis absorption results discussed above.

20

In addition to high efficiency a practical photoanode must also possess a good

21

photostability as a measure of good PEC performance. To investigate the photostability of our

22

NRs samples, the transient photocurrent response (J−t) of all samples (ZC0 to ZC120) is

23

recorded at a fixed potential of +0.8 V vs. Ag/AgCl in an aqueous electrolyte of 0.1 M Na2SO4

24

(pH ~ 7) for 1200 seconds under AM1.5 pulsed illumination. Figure 9d shows that the

25

photocurrent density for all the samples remains almost zero in dark (Light OFF), while all NR 19 ACS Paragon Plus Environment


1

photoanodes produce instantaneous transient photocurrent under illumination (Light ON). This

2

observed behaviour of J−t curves reveals that our as-prepared ZnO/CdS NR array exhibit a fast

3

photoresponse and good photostability. Also, the trend of measured photocurrent density of J−t

4

curves is well in agreement with the results shown in Figure 9a.

5

The electrochemical impedance spectroscopy (EIS) measurement has been carried out

6

under AM1.5 illumination in 10 mHz−100 kHz frequency range to further investigate the

7

kinetics of interfacial charge transfer processes within all our ZnO/CdS core/shell NR arrays

8

photoanodes. The arc radius of the semicircle in the EIS plots describes the interface properties

9

and the charge-transfer resistance (Rct) of the photoelectrode. The intercept on real axis of the

10

Nyquist plot corresponds to the value of Rct at electrode/electrolyte interface which is

11

associated with the charge transport and recombination occurring at the interface. The smaller

12

Rct value usually represents an improved charge separation efficiency with longer lifetime of

13

photogenerated charge carries, resulting in an enhanced PEC performance.13 Figure 10a shows

14

the Nyquist plots of pristine ZnO NRs (ZC0) and core/shell NR arrays (ZC30-ZC120). It is

15

clearly observed that the Rct decreases from ~1065  for ZC0 to ~930  for ZC30. With

16

increase in CdS shell deposition time it becomes smallest at ~430  for ZC90 representing the

17

lowest contact resistance, and thus fastest carrier transport with possible low recombination

18

rate at photoanode/electrolyte interface leading to observed the high photocurrent density Jlight

19

= 8.5 mA/cm2. Further increase in CdS deposition time in ZC120 sample leads to the increase

20

in Rct ~515  and linking to the decreased photocurrent density of Jlight = 6.9 mA/cm2. Thus,

21

the EIS results of the photoanodes are in agreement with the observed photocurrent (J-V)

22

response (Figure 9a).

23

To investigate the origin of photocurrent generation as a function of incident light

24

wavelength, the incident photon to current conversion efficiency (IPCE) measurements have

25

also been performed using 0.1 M Na2SO4 electrolyte solution. The IPCE curves of all ZnO NR 20 ACS Paragon Plus Environment

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1

(ZC0–ZC120) photoanodes recorded in the visible light region are shown in Figure 10b. The

2

observed IPCE results are well in accordance with their corresponding J–V curves (Figure 9a).

3

The pristine ZnO NR array shows negligible IPCE in visible region, as the pure ZnO is photo-

4

active only in violet region. After the decoration of ZnO NRs with CdS QDs, the core/shell NR

5

arrays show pronounced photoresponse in the visible light region (for wavelength < 550 nm).

6

The IPCE values observed at wavelength of 430 nm increase substantially from ~4.5% in ZC30

7

to the maximum of ~60% in ZC90 photoanode. The large enhancement of the IPCE at small

8

wavelengths can be ascribed to the improved carrier transport ability as well as valuable

9

utilization of charge carriers at lower energy induced by CdS shell over ZnO NRs. Thereafter,

10

further increase in CdS shell thickness in ZC120 sample results in the decrease in IPCE to

11

~40% possibly due to reduced carrier collection due to increased bulk recombination, which is

12

consistent with the results of photocurrent and PL.

13

Finally, the ZC90 photoanode with best PEC performance has been tested for its

14

stability under AM1.5 solar illumination in the same electrolyte solution for long term

15

application in PEC cell. Figure 11 shows the photocurrent density of ZC90 photoanode under

16

illumination over much longer duration to confirm good photostability. Light irradiance gives

17

rise to a very small hump in photoresponse, however, the photocurrent density returns back to

18

a steady state within a few minutes. It is seen that the Jlight of ZC90 has become almost steady

19

at ~6.2 mA/cm2 (at +0.8 V vs. Ag/AgCl). The observed high photoconversion efficiency and

20

good photostability of these photoanodes indicate that the ZnO/CdS core/shell NR arrays are

21

potential photoelectrodes for long term PEC water splitting reactions.

22

In light of a previous study of significant enhancement in photocurrent from that of 5

23

mA/cm2 in as-prepared sample to 9 mA/cm2 on 150−250 C annealing,14 we also explored the

24

role of annealing in our epitaxial ZnO/CdS samples. Our best performance pristine sample

25

ZC90 was annealed at 150 C for 10 and 20 min, and at 250 C for 10 min to examine the role



1

of the effect of low temperature annealing process on junction optimization. The J-V curves in

2

dark and under illumination from -1.0 to +1.0 V w.r.t. Ag/AgCl for all these samples are shown

3

in Figure 12a. The observed Jlight for ZC90 NRs array is slightly improved from the value of

4

8.5 mA/cm2 to 9-10 mA/cm2 on annealing. Further, the photoconversion efficiencies increase

5

from η = 2.75% for pristine sample to 3.71% for ZC90_A150_10 to the maximum of η = 4.44%

6

for ZC90_A150_20 NRs array photoanode (Figure 12b). To the best of our knowledge, these

7

are the highest PEC values ever reported for the similar ZnO/CdS NRs array without employing

8

any passivation/sensitization layer. Moreover, it is also observed that in maximum efficiency

9

sample, the peak shifted to the somewhat lower potential on annealing for more time of 20 min

10

as well as light current was slightly lower at 9.6 mA/cm2 in comparison to 10.1 mA/cm2

11

obtained on 250 C 10 min anneal. Thus, annealing apparently works favourably but with

12

concurrent detrimental effects on junction quality. This observation seems to point to the

13

possibility of further scope of improvement in junction formation, not by anneal, but by

14

improved junction formation in-situ to the ion-by-ion deposition process itself by achieving

15

better epitaxial alignment at atomistic scale.

16

As there are few reports on the hetero-epitaxial ZnO/CdS core/shell heterostructure,47

17

we compare our observed results first in general with the earlier reported PEC data on ZnO/CdS

18

based core/shell systems (Table 2). There are a few studies on ZnO/CdS core/shell NR arrays

19

covered with some passivation/sensitization layer employed to enhance the absorption as well

20

as photostability of ZnO/CdS NR photoanodes. Though this approach seems to enhance the

21

photocurrent, however the additional passivation layer is also detrimental for the PEC

22

efficiency due to its poor conductivity and causing inappropriate band-alignment. More

23

recently Iyengar et al.39 have fabricated NiO-coated ZnO/CdS core/shell NRs exhibiting very

24

less Jlight = 2.18 mA/cm2 due to wide bandgap of flake-like NiO used as passivation layer. Cao

25

et al.5 modified ZnO/CdS core/shell NRs with partial coverage of ZnFe2O4 to enhance visible


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1

light absorption employing stair-case band alignment approach and reported a better Jlight =

2

3.88 mA/cm2. Guo et al.21 constructed a photoanode by inserting Au nanoparticle layer

3

between ZnO core and CdS shell and achieved higher Jlight = 4.5 mA/cm2. Here Au core offers

4

strong electronic interaction with both ZnO and CdS resulting in a unique nanojunction to

5

facilitate rapid charge transfer. Xu et al.10 fabricated triple shell ZnSe/CdS/CdSe sensitized

6

ZnO nanowire arrays by time consuming hydrothermal method and reported a still higher Jlight

7

= 5.5 mA/cm2. This photocurrent value is the highest in this kind of passivation/sensitization

8

layer approach. Furthermore, there are some reports on ZnO/CdS core/shell NRs systems

9

without any additional sensitization layer. These systems have a uniform coverage of shell

10

layer (but no epitaxial growth) on single crystal ZnO NRs, in turn exhibiting improved PEC

11

response. Recently, Nan et al.38 have investigated the PEC response of ZnO/CdS core/shell

12

nanoarrays calcined at 550 °C to improve the interface as well as CdS crystallinity and achieved

13

Jlight = 5.1 mA/cm2, due to the formation of (Cd0.8Zn0.2)S interfacial layer (because of high

14

calcining temperature) in between ZnO and CdS. The likely introduction of defects at the

15

interface in this approach can be detrimental for good charge transport. Similarly Mali et al.36

16

fabricated ZnO/CdS core/shell NRs possessing Jlight = 5.61 mA/cm2 with low η of 1.61% due

17

to poor crystallinity of CdS shell.

18

Now we compare the PEC performance of our CBD prepared expectedly trap(defect)-

19

free (junction-traps free due to atomic matching at interface and bulk-defects free due to

20

avoidance of additives to deposition bath) and topotaxially-grown ZnO/CdS samples (Jlight =

21

8.5 mA/cm2 & η = 2.75% vs Ag/AgCl) with that of some recently reported PEC data in the

22

literature for the similar heterostructures. Tang et al.37 have investigated the PEC response of

23

ZnO/CdS core/shell nanoarrays (where ZnO NRs were prepared by CBD and non-epitaxial

24

growth of CdS nanoparticles deposited by pulsed electrodeposition) reporting a highest Jlight ~

25

6.0 mA/cm2. Further, Yang et al.26 have also reported Jlight ~ 6.0 mA/cm2 by employing time



1

consuming hydrothermal method to grow ZnO NRs and very expensive and low throughput

2

pulse laser deposition for controlled growth of CdS shell. However, no epitaxial growth is

3

reported. More recently, Nie et al.14 investigated the PEC response in same heterostructure by

4

employing ZnS passivation layer and 150 °C/250 °C two-step low temperature additional

5

annealing/sensitization step that resulted in Jlight ~9.0 mA/cm2 up from Jlight ~5.0 mA/cm2

6

without this step. In our present work, we have achieved the highest Jlight = 8.5 mA/cm2 with a

7

concurrently high η = 2.75% due to trap-free sharp hetero-epitaxial (002)ZnO//(002)CdS interface

8

achieved by better epitaxial alignment at atomistic scale without the need of post-deposition

9

junction alignment/activation/annealing.

10

4. CONCLUSIONS

11

1-D vertically aligned hetero-epitaxial ZnO NRs/CdS QDs core/shell arrays having

12

very high visible photoelectrochemical response are fabricated, by inducing chemical epitaxy

13

via low ion-flux, utilizing the nucleation-controlled all-step chemical bath deposition process.

14

FESEM and HRTEM micrographs reveal the formation of sharp interface between the c-axis

15

oriented ZnO NRs and CdS QDs (thickness ~30 nm) and confirm the epitaxial correlation of

16

(002)ZnO//(002)CdS between ZnO core and CdS shell. The highest visible photocurrent density

17

Jlight = 8.5 mA/cm2 and concurrently high PEC efficiency η = 2.75% is observed for hetero-

18

epitaxial ZC90 core/shell NR arrays without any additional sensitizing layer. After low

19

temperature annealing, the photocurrent density achieved for ZC90_A150_20 NRs array is

20

9.61 mA/cm2 at +1.0 V vs. Ag/AgCl and the corresponding photoconversion efficiency

21

increases to 4.44%. These are the highest ever reported values for the similar ZnO/CdS NRs

22

photoanodes. The annealing apparently works favourably but with concurrent detrimental

23

effects on junction quality. Our results seem to point to the scope of improvement in junction

24

formation, not by anneal, but by improved junction formation in-situ to the ion-by-ion

25

deposition process itself by achieving better epitaxial alignment at atomistic scale. This


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1

significantly high current density is attributed to the defect-free sharp hetero-epitaxial

2

(002)ZnO//(002)CdS interface, aligned NRs induced photon flux trapping leading to high visible

3

light absorption, efficient charge carrier transport with minimum recombinations involving

4

type-II staggered band alignment and good photostability of ZnO/CdS NR photoanode. Thus,

5

our results consisting of high photocurrent density at +1.0 V vs. Ag/AgCl as well as good

6

photoconversion efficiency and photostability are of great significance for efficient

7

photoelectrochemical or photovoltaic performance of ZnO NR/CdS QD heterostructures

8

obtainable simply by all-CBD process without the need of post-deposition junction

9

alignment/activation.

10

ASSOCIATED CONTENT

11

Supporting Information

12

XRD pattern of bare CdS thin film prepared by CBD; Tauc’s plot and Zoomed view of J−V

13

curves before 0 V vs. Ag/AgCl measured under AM1.5 illumination of ZC0 (pristine ZnO NRs)

14

and ZC30−ZC120 (ZnO/CdS NRs) photoanodes.

15

AUTHOR INFORMATION

16

Corresponding Author: *E-mail: [email protected]

17

Tel: +91-11-26591347 (O)

18

Notes

19

The authors declare no competing financial interest.

20

ACKNOWLEDGEMENTS

21

One of the authors Rekha Bai thankfully acknowledges CSIR, India for Senior Research

22

Fellowship. We also acknowledge Nano Research Facility as well as Central Research Facility

23

of I.I.T. Delhi for the use of various characterization techniques. We would like to acknowledge

24

the support from Aalto University for the cross-sectional FESEM measurements.

25

REFERENCES

Fax: +91-11-26581114 (O)



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1


Table 1. EDX elemental composition of ZnO NR arrays with different shell deposition time. Sample ZC0 ZC30 ZC60 ZC90 ZC120

Zn (At%) 45.0 43.0 42.1 40.5 38.4

O (At%) 55.0 54.3 53.8 51.1 48.8

Cd (At%) --1.4 2.1 4.3 6.5

2 3


S (At%) --1.3 2.0 4.1 6.3

Cd/S --1.08 1.05 1.05 1.03


1

Page 34 of 47

Table 2. Comparison of PEC data reported in the present work and in the earlier reports. Core/Shell System ZnO/CdS ZnO/CdS ZnO/CdS ZnO/CdS ZnO/CdS@Au ZnO/CdS/NiO ZnO/CdS/ZnFe2O4 ZnO/ZnSe/CdS/CdSe ZnO/CdS/ZnS

Epitaxial ZnO/CdS

Electrolyte 0.25 M Na2S & 0.35 M Na2SO3 Na2S 0.25 M Na2S & 0.35 M Na2SO3 1 M Na2S 0.25 M Na2S & 0.35 M Na2SO3 0.25 M Na2S & 0.35 M Na2SO3 0.5 M Na2S 0.24 M Na2S & 0.35 M Na2SO3 0.1 M Na2S & 0.2 M Na2SO3 0.1 M Na2SO4

Potential (V) 1.0 V vs. SCE --

Jlight (mA/cm2) 6.0

ηmax (%) 1.36

Reference

5.61

1.61

[36]

0 V vs. Ag/AgCl 0.4 V vs. Ag/AgCl 1.0 V vs. Ag/AgCl 1.23 V vs. RHE 0 V vs. Ag/AgCl 0.5 V vs. Ag/AgCl 1.0 V vs. SCE

6.0

--

[37]

5.1

--

[38]

4.5

--

[21]

2.18

--

[39]

3.88

--

[5]

5.5

--

[10]

5.0 (without anneal)

--

[14]

9.0 (with 150/250 °C two-step anneal) 8.5 (no anneal or passivation step); 9-10 (with anneal)

2.75

Present work

1.0 V vs. Ag/AgCl

2


4.44

[26]

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1

Figure 1. Schematic of the fabrication process of ZnO nanorods decorated with CdS QDs. 2



Figure 2. FESEM images of the (a) pristine ZnO NRs (ZC0), (b) cross-sectional view of ZC0 and ZnO/CdS core/shell NR arrays with (c) 30 s (ZC30), (d) 60 s (ZC60), (e) 90 s (ZC90) and (f) 120 s (ZC120) deposition time of CdS shell. 1 2


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Figure 3. EDX elemental mapping of ZC90 core/shell NR array sample. 1 2



Figure 4. (a) XRD patterns of pristine ZnO NRs (ZC0) and ZnO/CdS core/shell NR array (ZC30−ZC120) samples. Magnified XRD patterns of (b) CdS (002) peak and (c) ZnO (002) peak. Peaks marked with * are from ITO substrate. 1 2


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Figure 5. Room temperature Raman spectra of as-prepared ZnO/CdS core/shell NRs with t = 0, 30, 60, 90, 120 s deposition time of CdS (ZC0–ZC120) excited by 325 nm wavelength. 1 2



Figure 6. Microstructure characterization of (a-c) pristine ZnO NRs and (d-f) ZC90 core/shell NRs. (a,d) Low-magnification TEM images. (b, e) High-resolution TEM images. (c,f) Selected area electron diffraction (SAED) patterns. 1 2


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Figure 7. Optical absorption spectra of ZC0 (pristine ZnO NRs) and ZC30−ZC120 (ZnO/CdS core/shell NR arrays) samples. 1



Figure 8. (a) Room temperature PL spectra of pristine ZnO and ZnO/CdS NR arrays. The inset shows two weak ZnO defect emission peaks P1 at ~570 nm and P2 at ~630 nm in the spectra. (b) The PL spectra of bare CdS film showing the sharp main NBE peak at 520 nm and weak defect bands in 550-750 nm region. 1


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Figure 9. (a) Linear sweep voltammograms (J−V) of ZC0 (pristine ZnO NRs) and ZC30−ZC120 (ZnO/CdS NRs) measured in dark and under AM1.5 illumination. (b) Schematic of staggered type-II energy band alignment of ZnO/CdS NRs for PEC cell. (c) Photoconversion efficiency and (d) Potentiostatic current densities (J−t) at a fixed bias of +0.8 V (vs. Ag/AgCl) for all samples (ZC0−ZC120 NRs) under AM1.5 illumination. 1



Figure 10. (a) EIS Nyquist plots measured at an open bias condition under AM1.5 illumination (b) IPCE spectra of pristine ZnO NRs (ZC0) and ZnO/CdS NR arrays (Z30ZC120) samples. 1


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Figure 11. Current–time curve of ZC90 photoanode at +0.8 V vs. Ag/AgCl under AM1.5 illumination. 1



Page 46 of 47

Figure 12. (a) Linear sweep voltammograms (J−V curves) measured in dark and under AM1.5

illumination

(b)

corresponding

photoconversion

efficiencies

of

ZC90,

ZC90_A150_10, ZC90_A150_20 and ZC90_A250_10 NRs array calculated from J-V curves. 1


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1


Table of Contents Graphic

2 3 4

Hetero-epitaxial ZnO/CdS core/shell nanorod arrays as an efficient

5

photoanode for enhanced photoelectrochemical performance

6


CdS Core

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