Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Bio-template assisted hierarchical ZnO superstructures coupled with graphene quantum dots for enhanced water oxidation kinetics Suhaib Alam a , Tushar Kanta Sahu a , Devipriya Gogoi b , Nageswara Rao Peela b , Mohammad Qureshi a, ⁎ a Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India b Department of Chemical Engineering, Indian Institute of Technology Guwahati, Assam 781039, India ARTICLE INFO Keywords: Hierarchical ZnO superstructures Photoelectrochemical water oxidation Polygalacturonic acid Hole extracting agent Graphene quantum dots ABSTRACT Due to anisotropic growth behavior and tunable electrical properties, ZnO nanostructures having dimensions such as 0-D, 1-D, 2-D and 3-D are actively studied for their optoelectronic properties. However, ZnO based photoanodes suffer from unfavorable recombination of electron hole pair, which hinders its use in photoelec- trochemical (PEC) water oxidation. Herein, we demonstrate a strategy to enhance the PEC performance using bio-template assisted in-situ grown hierarchical ZnO superstructures directly over fluorine-doped tin oxide (FTO) modified by graphene quantum dots (GQDs). GQDs decorated hierarchical ZnO superstructures displayed a significant increment of ~77% in photocurrent density value compared to pristine ZnO with an impressive carrier density of 3.19 × 10 20 cm -3 , which is ~1.8 orders of magnitude higher than that of pristine ZnO. It is observed that GQDs acts as an efficient hole extractor, which improves the carrier separation on ZnO surface and reduces the hole trapping probability. 1. Introduction Splitting of water into hydrogen and oxygen is an attractive alter- native to harvest solar energy for future energy crisis. In this regard, photo electrochemical (PEC) water splitting has received much atten- tion to effectively couple solar irradiation with the electrochemical processes (Ma et al., 2018; Li et al., 2013). During the past few decades, several metal oxides such TiO 2 , ZnO, WO 3 , Fe 2 O 3 , BiVO 4 , etc have been used as photo-electrodes for PEC water splitting (Zhang et al., 2014; Zhang et al., 2018; Tang et al., 2019; Marelli et al., 2014; Hegner et al., 2017). Among these n-type metal oxides, Zinc oxide (ZnO), a wide bandgap (~3.2 eV) semiconductor photoanode has attracted much at- tention due to its attractive optical properties, high electron mobility, low toxicity and its anisotropic growth behavior (Zhan et al., 2018). Although ZnO has many advantages over similar n-type materials, still the PEC activity is inferior to many photoanodes due to its sluggish water oxidation kinetics. Recently, ZnO has been modified with dif- ferent strategies to overcome the above drawbacks (Chen et al., 2014). Recent developments in the synthetic strategies of inorganic semi- conductors having different morphologies with controlled shape and size have gained significant interest due to their catalytic and charge transport properties (Li and Yu, 2019; Dong et al., 2012). The significant scientific and technological importance of nanomaterials and their synthetic procedures are still challenging and require addi- tional efforts for the complete utilization of their potential for appli- cation (Shi et al., 2013; Guo et al., 2008; Shevchenko et al., 2006; Liu et al., 2013). Different structures of ZnO such as 3D branched nano- wires (Kargar et al., 2013), star like structures (Marlinda et al., 2019), ZnO Nano tree and nano cluster structures (Ren et al. 2016), nanowires (Li et al., 2015a, 2015b), nanorods (Wang et al., 2015), nanoplates assemble sphere like structures (Emil et al., 2018), have been utilized for PEC water oxidation. Semiconductors having hierarchical morphologies can be an effective way to enhance the PEC performance because of their improved charge transfer, surface area and sensitizer loading capabilities apart from their strong light scattering effects and efficient electron transport (Memarian et al., 2011; Ko et al., 2011). Compared to the 1D nanostructures, hierarchical superstructures are more advantageous as they provide long optical pathways for efficient light absorption and multiple reflections, short channels for faster charge transport and a large interfacial area for water redox reactions (Li et al., 2015a, 2015b). Several growth patterns based on their crystal growth behavior have been used to synthesize a regular ordered structure such as template derived homo or heteroepitaxial growth behavior (Xu et al., 2009). Here, we have utilized a naturally occurring https://doi.org/10.1016/j.solener.2020.02.015 Received 22 November 2019; Received in revised form 17 January 2020; Accepted 4 February 2020 ⁎ Corresponding author. E-mail address: mq@iitg.ac.in (M. Qureshi). Solar Energy 199 (2020) 39–46 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T