One-step growth of reduced graphene oxide on arbitrary substrates

Abstract

Reduced graphene oxide (rGO) has inherited the outstanding electronic, optical, thermal and mechanical properties of graphene to a large extent, while maintaining sufficient chemically active sites. Therefore, it has attracted a great deal of research attention in the fields of energy storage, electronics, photonics, catalysis, environmental engineering, etc. Currently, the most popular way to prepare rGO is to reduce graphene oxide, which is obtained by modified Hummer methods using tedious treatments in a harsh environment, to rGO flakes. Industrial applications demand advanced preparation methods that can mass produce highly uniform rGO sheets on arbitrary substrates. In this work, a one-step growth process is introduced that utilizes cellulose acetate as a precursor, without any catalysts, to produce uniform ultrathin rGO films on various substrates and free-standing rGO powders. Systematic spectroscopic and microscopic studies on the resulting rGO are performed. Prototypes of electronic and optoelectronic devices, such as field effect transistors (FETs), photodetectors, and humidity sensors, are fabricated and tested, demonstrating the intriguing applications of our rGO materials across a wide range of fields.

Introduction

Reduced graphene oxide (rGO) has attracted a great deal of attention from researchers thanks to its anticipated applications in transparent and flexible electronics [1,2], supercapacitors [3], batteries [4], sensors [5], photodetectors [6], electromagnetic shielding [7], etc. It possesses higher in-plane electrical conductivity than graphene oxide (GO) and contains more active sites (e.g., hydroxyl groups and defects) for chemical functionalization and catalysis than pristine graphene [[8], [9], [10], [11]]. Currently, modified Hummers’ methods are the most popular way to prepare GO and its derivatives, such as rGO [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. Briefly, graphite is oxidized and intercalated using a mixture of potassium permanganate (KMnO₄), NaNO₃, and concentrated H₂SO₄. The product is purified to obtain GO. Then, an aqueous dispersion of the obtained GO is treated with reducing agents, such as a solution of hydrazine and ammonia, to partially restore the graphitic sp [2] carbon. Finally, these rGO dispersions has to be transferred onto target substrates via spray or filtration for thin-film applications. Due to the strong oxidizers and various reducing agents involved in these processes, it is difficult to control the size, roughness, and thickness of the end rGO product. In this work, we report a facile way of growing ultrathin rGO films on arbitrary substrates and free-standing rGO powders, where the solid precursor (cellulose acetate) is transformed by thermal annealing. Regarding the selection of solid precursors, small organic molecules such as glucose need to go through polymerization process under certain conditions to form oxygen-containing polymers first before transformed into rGOs, otherwise they will sublime away completely under vacuum at high temperature. Therefore, in our procedure of one-step growth of rGO, polymers such as cellulose acetate are our first choice considering the cost, simplicity and practicability. Here, we refer to rGO derived from cellulose acetate as CA-rGO. Cellulose acetate is an ester of cellulose, which is abundant in natural resources such as cotton, wood, and hemp. It can be readily dissolved in a wide range of solvents, such as acetone, dioxane, and methal acetate [13,14], and spin-coated onto arbitrary substrates, which has facilitated its use in both academic and industrial applications [[14], [15], [16]].