Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu Optimization strategy for CdSe@CdS core–shell nanorod structures toward high performance water splitting photoelectrodes Nguyen Duc Quang a,1 , Sutripto Majumder a,1 , Gyu Seok Choi b , Chunjoong Kim a, *, Dojin Kim a, * a Department of Materials Science and Engineering, Chungnam National University, Daejeon, 34134 Republic of Korea b Gumi Electronic and Information Technology Research Institute, Gumi, Gyeongbuk, 39171 Republic of Korea ARTICLE INFO Keywords: CdS nanorod CdSe Hydrothermal Water splitting ABSTRACT Photoelectrochemical (PEC) water splitting offers a promising strategy for converting solar energy to chemical fuel of hydrogen. Herein, we report about the successful synthesis of hexagonal CdSe@CdS core–shell nanorod (NR) structures by facile two-step chemical synthesis of hydrothermal and chemical bath growth. Structural, morphological, optical, and electrical properties of the NR structures are thoroughly investigated, thereby heterojunction characteristics can be elaborated. Compared with the bare CdS NRs, the CdSe@CdS core–shell NR-heterostructures exhibit far enhanced photoelectrode performance mainly owing to the small bandgap en- ergy of CdSe and the optimized CdSe layer thickness. The CdSe@CdS core–shell NRs reveal the PEC current density of 11.0 mA/cm 2 at 0 V vs. SCE, which is more than twice of that of the bare CdS NRs (4.3 mA/cm 2 ). The enhancement in the PEC performance is elucidated by the energy band diagrams of the various heterostructures, which can be assessed by the Mott-Schottky and electrochemical impedance spectroscopy measurements. 1. Introduction The concept of water splitting to produce hydrogen and oxygen by the photo-generated electron-hole carriers in semiconductor junctions was proposed by Fujishima and Honda in 1972 [1]. Since then many studies were carried out to develop cost-effective semiconductor thin films such as TiO 2 , ZnO, SnO 2 , Fe 2 O 3 , WO 3 , and BiVO 4 [2–8]. However, their water splitting efficiencies remained far lower than the economic efficiency (10%) as well as the low photoelectrochemical (PEC) current density, less than a few mA/cm 2 . The limitation of the performance is mainly originated from the large bandgap energy of the photoelec- trodes, which are mainly metal oxides. Other than the metal oxide semiconductors, several metal chalcogenides were reported to show the outstanding water splitting properties. Among them, n-type CdS with 2.42 eV bandgap energy showed a high PEC current density [9,10] even though it was demonstrated in the solutions with Cd scavengers. However, CdS nano-structures showed relatively fast recombination of photogenerated charge carriers at the interface, which limits its water splitting efficiency [11,12]. The engineering of heterojunction can be the efficient way to solve this problem. The heterojunction en- ables semiconductors to have lower recombination rate by suitable band edge positions, therefore the charge transfer characteristics are enhanced by increase in the lifetime of the photogenerated charge carriers [13,14]. Therefore, selection of the material, of which bandgap energy is smaller than that of CdS, is critical to improve the PEC per- formance. Among the candidates such as Bi 2 S 3 , Bi 2 Se 3 , Bi 2 Te 3, Hg 2 S, CdSe, Ag 2 S, PbS, and MoS 2 [15–21], CdSe that has the bandgap energy of 1.7 eV is considered one of the promising candidates as a composite member for the CdS-based heterojunction [22] Along with the development of the photoelectrode materials, na- notechnology can be implemented for further enhancement of the photo-current density. Contrary to the simple flat thin films, nano- structures that show higher surface-to-volume ratio lead to higher current densities. The nanostructured films can be prepared to have different morphologies of nanoparticles, nanoflakes, and nanorods (NRs). Particularly, NRs exhibited improved charge carrier transporta- tion, larger optical cross-sectional absorption, and less recombination loss at the grain boundaries [23–25]. Lindgren et al. reported that the perpendicular growth of NRs on the substrate facilitated electron transport to the backside current contact [26]. In this study, we fabricated CdSe@CdS NR photoanodes with the various CdSe layer thicknesses on CdS NRs. CdS were grown by a hy- drothermal method and then CdSe were deposited using a chemical bath deposition method. The growth of CdSe layer over CdS NRs was identified by structural, morphological and optical characterizations. The PEC current densities of the photoanodes were measured while https://doi.org/10.1016/j.materresbull.2020.110914 Received 18 February 2020; Received in revised form 28 April 2020; Accepted 28 April 2020 ⁎ Corresponding authors. E-mail addresses: ckim0218@cnu.ac.kr (C. Kim), dojin@cnu.ac.kr (D. Kim). 1 These authors contribute equally. Materials Research Bulletin 129 (2020) 110914 Available online 04 May 2020 0025-5408/ © 2020 Elsevier Ltd. All rights reserved. T