Green preparation of reduced graphene oxide for sensing and energy storage applications

Abstract

Preparation of graphene from chemical reduction of graphene oxide (GO) is recognized as one of the most promising methods for large-scale and low-cost production of graphene-based materials. This study reports a new, green, and efficient reducing agent (caffeic acid/CA) for GO reduction. The CA-reduced GO (CA-rGO) shows a high C/O ratio (7.15) that is among the best rGOs prepared with green reducing reagents. Electronic gas sensors and supercapacitors have been fabricated with the CA-rGO and show good performance, which demonstrates the potential of CA-rGO for sensing and energy storage applications.

Figure 1. Structure characterizations of GO and CA-rGO.

(a) TEM and (b, c) HRTEM images of 24h-CA-rGO. Inset: SAED patterns. (d) Digital photographs of aqueous dispersions of GO before and after reduction by CA for different reaction time. (e) Water droplet on the surface of GO and CA-rGO sheets. The error in the contact angle measurements is on the order of 0.1% of the measured values. (f) Tapping-mode AFM images and the corresponding height profiles of GO and 24h-CA-rGO dispersed on a mica substrate. About twenty 24h-CA-rGO sheets were characterized by AFM (Supplementary Information, Fig. S1). The height and size distributions of the 24h-CA-rGO sheets were obtained from the data shown in Fig. S1.

Figure 2. XRD and Raman data of GO and CA-rGO.

(a) XRD patterns of pristine graphite, GO, and 24h-CA-rGO. (b) Raman spectra of GO before and after CA reduction for different reduction time.

Figure 3. XPS data of GO and CA-rGO.

(a) XPS survey spectra of the as-prepared GO and CA-rGOs (CA:GO = 50:1) for different reaction times. (b) XPS survey spectra of the as-prepared GO and CA-rGOs with different CA to GO ratios (reaction time: 24 hours). Gaussian line fitted C1s spectra of (c) GO and (d) 24h-CA-rGO.

Figure 4. Gas sensor application of CA-rGO.

(a) Schematic diagram of the rGO-based gas sensor device. (b) SEM image of 24h-CA-rGO sheets bridging a pair of gold sensor electrodes. (c) Direct current measurement results of 24h-CA-rGO with drain-source potential ramping from -1.0 to +1.0 V. (d) FET results (V_d = 0.5 V) of the 24h-CA-rGO. Dynamic gas sensing results of the 24h-CA-rGO gas sensors for (e) 100 ppm NO₂ and (f) 1% NH₃ tested under room temperature.

Figure 5. Supercapacitor application of CA-rGO.

(a) Schematic diagram of the rGO-based double-layer supercapacitor. (b) Digital photographs of an LED light powered by a supercapacitor cell. Inset: digital photograph of a bare Ni foam before and after being coated with 24h-CA-rGO sheets. CV curves of the supercapacitors using GO and 24h-CA-rGO working electrodes in (c) 1.0 M KCl and (d) 1.0 M TEABF₄/AN electrolytes tested at a scan rate of 100 mV/s.