Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Improving optoelectrical properties of photoactive anatase TiO 2 coating using rGO incorporation during plasma electrolytic oxidation Sema Ebrahimi, Aidin Bordbar-Khiabani, Benyamin Yarmand ⁎ , M. Amin Asghari Nanotechnology and Advanced Materials Department, Materials and Energy Research Center (MERC), Karaj, Iran ARTICLE INFO Keywords: Photoactive TiO 2 coating Anatase phase rGO incorporation Plasma electrolyte oxidation ABSTRACT In this study, photoactive anatase titanium dioxide (TiO 2 )/reduced graphene oxide (rGO) composite coatings were successfully developed on titanium substrate using plasma electrolytic oxidation (PEO) process to evaluate the effect of incorporated rGO on their photoactivity and optoelectrical properties. Structural and morphological examinations verified the incorporation of the rGO sheets in the anatase TiO 2 coatings with pancake-like morphology. Optical spectroscopy revealed that a higher content of rGO enhanced the light absorption in UV and visible regions due to the high photo-trapping behavior of the composite samples. Photoluminescence declined by increasing rGO content, which suggested that the recombination of the photogenerated electron-hole pairs was suppressed. As a result, the photocurrent response of the TiO 2 /rGO coatings was enhanced significantly under UV radiation compared to the pure one, by which the photoactivity response revealed an over twenty-fold increase for a high concentration of rGO. Moreover, the electron transfer pathways created by the 2D structure of rGO sheets inside the anatase TiO 2 network, promoted the photoresponsivity and photoswitching behavior more than 60%, interestingly. 1. Introduction Titanium dioxide (TiO 2 ) is a wide band gap semiconductor (E g ~ 3.2 eV) that is activated under UV radiations. Due to its excellent physical and chemical properties such as the chemical stability, tunable microstructure, photocorrosion resistance, non-toxicity, and low prices, TiO 2 is extensively used in photoactive systems including water split- ting, energy conversion, and air and water purification [1]. As well as can be a promising substrate because of inherent optoelectronic prop- erties. TiO 2 is found with three crystal structures, namely; anatase, rutile, and brookite, among which anatase is the most suitable phase to be used into photoactive systems because it features photoexcited charge carriers with longer lifetime and lighter average effective mass [2]. However, the low electrical conductivity and the high re- combination rate of the photogenerated charge carriers, which result in a low quantum efficiency, can be named as the drawbacks of anatase phase that diminish its performance [3]. One solution to these limitations is to incorporate carbon nanos- tructures such as reduced graphene oxide (rGO) into the anatase TiO 2 matrix as a reinforcement [4,5]. The rGO is a two-dimensional (2D) carbon sheet with various oxygenated functional groups which has notable properties including high electron mobility, large specific sur- face area, optical transparency at room temperature, inertness, strong mechanical and great stability [1,6]. Accordingly, coupling the favor- able properties of anatase TiO 2 with marvelous features of rGO improve the performance of this substance in photoactive systems [7,8]. Plasma electrolytic oxidation (PEO) process is a novel method de- veloped in recent years for preparing TiO 2 coatings with tailored properties. In this method, the TiO 2 layer is grown on a titanium sub- strate during oxidation in an alkaline electrolyte at voltages exceeding the dielectric breakdown limit of the initial passive layer. The layer grows as a result of the consecutive melting and solidification due to sparking over the substrate immersed in an aqueous electrolyte [9]. This method is very effective in fabricating photoactive system com- ponents due to its facility, low cost, environmentally-friendly char- acteristics, and high adhesion strength, as well as the control of this, provides over the chemical composition, phases, morphology, and surface porosity [10,11]. For example, Chu et al. [12] employed the PEO process to develop nanostructured TiO 2 films to be used in the working electrode of dye-sensitized solar cells. Stojadinović et al. [10] prepared TiO 2 /WO 3 coatings using the PEO process to evaluate their photocatalytic properties. Yao et al. [13] employed PEO to create mi- croporous Ni-doped TiO 2 photocatalyst coatings. Moreover, Lo et al. [14] used composite TiO 2 -CNT coatings produced by PEO in fabricating anodes for lithium batteries. Few studies have thus far addressed the incorporation of https://doi.org/10.1016/j.ceramint.2018.10.057 Received 20 September 2018; Received in revised form 7 October 2018; Accepted 8 October 2018 ⁎ Corresponding author. E-mail address: byarmand@merc.ac.ir (B. Yarmand). Ceramics International 45 (2019) 1746–1754 Available online 12 October 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved. T