Reports: ND1051799-ND10: Synthesis and Characterization of Separated Graphene Sheets Without Substrates

Yun Hu, Michigan Technological University

Narrative Progress Report


            During this year (09/01/2012-8/31/2013), several important findings were obtained in this project: (1) a novel synthesis approach of 3D graphene, which is based on a reaction between Li2O and CO, was invented, (2) The excellent performance of 3D graphene as counter electrodes for dye-sensitized solar cells (DSSCs) was obtained, (3) Ion-induced precipitation of graphene oxide in its aqueous solution was revealed, and (4) a novel approach was invented to introduce TiO2 as photo-electrodes for DSSCs.

            So far, 4 papers have been published in reputed peer-reviewed journals.

2. 3D graphene from the reaction between Li2O and CO

            Li2O is widely exploited as a promoter in catalysts to inhibit carbon formation. However, this general recognition was challenged by our finding in this project, in which Li2O reacted with CO to form graphene-structured carbon:

Li2O+2CO=graphene+Li2CO3       (1)

Furthermore, simultaneous formation of Li2CO3 with graphene can isolate graphene sheets from each other to prevent graphite formation during the process. On the other hand, the Li2CO3 particles will also play a role in determining the locally curved shape of graphene sheets. As a result, curved graphene sheets with thickness of about 2 nm, which connect to each other and form a 3D honeycomb-like structure, were obtained (Fig.1a and 1b). The cell size of graphene honeycombs lies in the range of 50-500 nm. The micro-structure of honeycomb cells was further evaluated by transmission electron microscopy (TEM). The TEM image showed the intrinsic wrinkles or corrugations of the cell sheets (Fig.1c). Furthermore, the curved shapes of honeycomb-structured graphene (HSG) were further supported by electron diffraction that shows poly-crystalline ring patterns (Fig. 1d). In addition, the dye-sensitized solar cell (DSSC) with the honeycomb-structured graphene counter electrode exhibited energy conversion efficiency as high as 7.8%, which is even comparable to that of DSSCs with an expensive Pt counter electrode. Our paper based on those important findings was published in Angew. Chem. Int. Ed. 52, 9210(2013).

Figure 1. FESEM and TEM images of honeycomb-structured graphene (HSG). (a) FESEM image, (b) enlarged FESEM image for the marked square area in (a), (c) TEM image, and (d) electron diffraction pattern.

3. Ion-induced precipitation of graphene oxide in its aqueous solution

         In this work, it was found that a strong electrolyte (HCl, LiOH, LiCl, LiBr, KCl, or KBr) can destabilize the graphene oxide (GO) solution, causing GO precipitation (Fig.2). This indicates that the electrostatic repulsion plays a critical role in stabilizing aqueous GO solution. The electrolyte-induced precipitates were characterized by transmission electron microscopy (TEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). The oxygen-containing functional groups of GO sheets, which are carboxyl, epoxy, and hydroxyl groups, remained unchanged during acid(HCl) and salt(LiCl)-induced precipitations. In contrast, during the GO precipitation induced by a base (LiOH), the carboxyl group of GO sheets disappeared with a remarkable increase in hydroxyl group and aromatic C=C bonds. This indicates that the LiOH-induced GO precipitation resulted in the partial reduction of GO sheets. This would be the first time to reveal that the GO solution possesses both colloid and solution properties. A paper based on those findings was published in J. Colloid Interface Sci. 391, 21(2013).

Figure 2. Precipitation in aqueous solutions of GO, 0.4 M LiCl-GO, 0.4 M HCl-GO, and 0.4 M LiOH-GO (from left to right) after (a) 0 min, (b) 10 min, (c) 2 hr, (d) 24 hr, (e) 48 hr, and (f) 72 hr.

4. Promoting effect of graphene on dye-sensitized solar cells

         A simple approach without a pre-reduction of GO was exploited to prepare a graphene-doped TiO2 film for dye-sensitized solar cells (DSSCs) in this work. The performance measurement of the DSSCs showed that the incorporation of graphene could increase the short-circuit current density and power conversion efficiency by 52.4 and 55.3 %, respectively. Furthermore, it was demonstrated that the performance enhancement was due to the promoting effect of graphene on electron transfer instead of the increase of dye loading in TiO2/graphene composite films. However, graphene can also absorb solar light, which could lead to the decrease of light harvest of dye molecules and thus a negative effect on the power conversion efficiency of DSSCs. Furthermore, graphene might decrease the actual dye loading on TiO2 in a TiO2/graphene film, which can also make a negative contribution to the conversion efficiency. As a result, the promoting effect of graphene is strongly dependent on its content, namely, the efficiency of DSSCs increases to the maximum value and then decreases with increasing graphene-content in TiO2/graphene composites. A paper based on those findings was published in Ind. Eng. Chem. Res. 51, 10613 (2012).

5. Literature investigation for graphene counter electrodes for DSSCs

         A systematic investigation of literature about graphene-based counter electrodes for dye-sensitized solar cell (DSSCs) was carried out. As a result, a review paper was formed and published in Energy Environ. Sci., 5, 8182(2012).