Oxygen Vacancy-Induced Structural, Optical, and Enhanced Supercapacitive Performance of Zinc Oxide Anchored Graphitic Carbon Nanofiber Hybrid Electrodes Gowra Raghupathy Dillip, † Arghya Narayan Banerjee,* ,† Veettikkunnu Chandran Anitha, † Borelli Deva Prasad Raju, ‡ Sang Woo Joo,* ,† and Bong Ki Min § † School of Mechanical Engineering and § Center for Research Facilities, Yeungnam University, Gyeongsan 712 749, South Korea ‡ Department of Future Studies, Sri Venkateswara University, Tirupati 517 502, India * S Supporting Information ABSTRACT: Zinc oxide (ZnO) nanoparticles (NPs) anchored to carbon nanofiber (CNF) hybrids were synthesized using a facile coprecipitation method. This report demonstrates an effective strategy to intrinsically improve the conductivity and supercapacitive performance of the hybrids by inducing oxygen vacancies. Oxygen deficiency-related defect analyses were performed qualitatively as well as quantitatively using Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All of the analyses clearly indicate an increase in oxygen deficiencies in the hybrids with an increase in the vacuum-annealing temperature. The nonstoichiometric oxygen vacancy is mainly induced via the migration of the lattice oxygen into interstitial sites at elevated temperature (300 °C), followed by diffusion into the gaseous phase with further increase in the annealing temperature (600 °C) in an oxygen-deficient atmosphere. This induction of oxygen vacancy is corroborated by diffuse reflectance spectroscopy, which depicts the oxygen-vacancy-induced bandgap narrowing of the ZnO NPs within the hybrids. At a current density of 3 A g -1 , the hybrid electrode exhibited higher energy density (119.85 Wh kg -1 ) and power density (19.225 kW kg -1 ) compared to a control ZnO electrode (48.01 Wh kg -1 and 17.687 kW kg -1 ). The enhanced supercapacitive performance is mainly ascribed to the good interfacial contact between CNF and ZnO, high oxygen deficiency, and fewer defects in the hybrid. Our results are expected to provide new insights into improving the electrochemical properties of various composites/hybrids. KEYWORDS: zinc oxide/carbon nanofiber hybrid, oxygen deficiency, bandgap narrowing, supercapacitor 1. INTRODUCTION Supercapacitors (SCs) exhibit many outstanding properties compared to conventional dielectric capacitors and batteries, such as higher energy and power density, fast charging and discharging, and long cycle life for a wide range of applications, such as consumer electronics, medical electronics, memory backup systems, hybrid electric vehicles, transportation, and military defense systems. 1-4 However, to satisfy demands in the rapidly growing field of energy applications, more efforts have to be expended to the development of new electrodes and electrolytes without sacrificing the power density and cycle life. 5,6 Over the past few decades, transition metal oxides/ hydroxides have been explored for use as high energy-density pseudocapacitor electrodes because of their theoretical capacitance and abundance. 7 Among the various metal oxides, capacitors based on ruthenium oxide (RuO 2 ) show remarkably high specific capacitance and power. 8 However, because of high toxicity and cost, the use of RuO 2 -based capacitors in practical large-scale production is limited. 9 Therefore, much effort has been devoted to identifying inexpensive and low-toxicity metal oxide electrode materials with reasonable electrochemical properties as alternatives to RuO 2 . 10 In practice, the most important characteristics required for using a metal oxide as a capacitor electrode are pseudocapacitive behavior, large surface area, high conductivity, and high electrochemical stability. 9 Zinc Received: December 17, 2015 Accepted: February 2, 2016 Research Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/acsami.5b12322 ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX