Photoelectrochemical Properties of N-Doped Graphene/Hematite Composites

Abstract

Photocatalytic and photoelectrochemical (PEC) water splitting are gaining huge interests for its ability to convert solar energy directly into fuel (hydrogen) in a simple, cost-effective and cleaner manner. However, lack of more active, stable and robust semiconductor materials for water oxidation with good light absorption and low charge recombination rate hampers this technology from large-scale application. Hematite (α- Fe2O3) is one of the favorable materials for PEC water oxidation as it possesses a suitable band gap of about 2.0 eV, moderate stability in a broad pH range, absorbs more than 40% of incident solar spectrum, and is less-toxic and less expensive. However, it suffers from poor electronic conductivity, low absorption coefficient, short hole diffusion length (< 10 nm), and high electron-hole recombination rate, which results in a relatively low solar-to-chemical energy efficiency which limits its practical applications.[1 ] At the other end, graphene commands special interest in PEC applications as it is a zero-band-gap material with outstanding electronic conductivity of 106 Scm-1 and optoelectronic properties (98% transmittance).[2 ]In addition, graphene with its exceptional charge carrier mobility can be used as mediators for electron transport thus resulting in better carrier separation[3 ]. In this work, we discuss the development and PEC properties of graphene and nitrogen-doped graphene-based photo anode with non-toxic, low-cost hematite for enhanced water oxidation capability and increased life time. Graphene is used as the conductive layer here to promote charge collection and transport by accepting photogenerated electrons from the semiconductor oxide. Substituting nitrogen on to graphene is found to be particularly interesting as it results in dramatically altered graphene band structure. To prepare graphene, an electrochemical exfoliation route is carried out in a two electrodes system. Graphite flake and Pt rod is used as working and counter electrode respectively. Na2SO4 aqueous solution or (NH4)2SO4 /ammonia solution is used in the exfoliation in order to obtain graphene or N-doped graphene. A constant voltage of +10 V (vs. Ag/AgCl) was applied on the system to initiate the exfoliation. To understand the effect of synthesis route on the PEC properties, α- Fe2O3 is synthesized by three different routes such as flame spray pyrolysis (FSP), e-beam evaporation and hydrothermal method. The physical properties of graphene, N-doped graphene and α- Fe2O3 are characterised by XPS, XRD, SEM and TEM. Photo anodes are fabricated in two steps: i) a conductive layer is formed by coating graphene/N-doped graphene layers on quartz substrates and ii) a photocatalyst layer is sprayed coated or grown over this conductive layer depending on the synthesis method. The effect of synthesis route on the PEC properties of photo anodes such as I-V characteristics and impedance spectroscopy under dark and 1-sun illumination determined using a 3-electrode cell with Pt-foil as counter electrode and Ag/AgCl as reference electrode in 1 M NaOH is discussed. References Kevin Sivula, Florian Le Formal, and Michael Gratzel, Solar Water Splitting: Progress Using Hematite (α- Fe2O3) Photoelectrodes, ChemSusChem 2011, 4, 432Guancai Xie , Kai Zhang , Beidou Guo , Qian Liu , Liang Fang , and Jian Ru Gong , Graphene-Based Materials for Hydrogen Generation from Light-Driven Water Splitting, Adv. Mater. 2013, 25, 3820Zhang, Lili, Hongrui Hu, Mingzai Wu, Xinxin Yu, Zhaoqi Sun, Guang Li, Xiansong Liu, and Xiuwen Zheng, A novel bubbling-assisted exfoliating method preparation of magnetically separable γ- Fe2O3/graphene recyclable photocatalysts, Functional Materials Letters, 2014, 7, 1450056