Carbon Nanotubes Bridged with Graphene Nanoribbons and Their Use in High‐Efficiency Dye‐Sensitized Solar Cells

Abstract

To improve the practical application of functional nanomaterials, it is critically important, but often challenging, to extend their excellent properties from the nanoscale to the macroscopic scale. For example, carbon nanotubes (CNTs) have been widely studied as a new family of electrode materials for various optoelectronic and electronic devices, owing to their unique structure and remarkable electronic and catalytic properties. However, CNTs were typically aggregated to form networks with many boundaries, which significantly inhibited rapid charge transport. The resulting CNT-based electrodedevices showed much lower efficiency than expected. To solve the above problem and achieve high performance at the macroscopic scale, a general strategy is to design novel structures to realize effective interactions among CNTs at the molecular scale. To this end, nature provides excellent models for efficiently performing complex functions through the creation of elaborate structures. Well-known examples are blood vessels, which are constructed with trunks interconnected by a lot of branches, a paradigmatic structure to rapidly deliver nutrients throughout our bodies. Inspired by nature, herein we discuss the development of a new structure in which CNTs are bridged by graphene nanoribbons. Briefly, multiwalled CNTs are partially unzipped to form nanoribbons with one end on the mother CNT and the other on a neighboring CNT. Due to the strong p–p interaction between nanotube and nanoribbon, and the high charge mobility in nanoribbons, produced electrons can be rapidly transported among CNTs to macroscopically achieve high performance. As a demonstration, dye-sensitized solar cells (DSCs) with graphene-nanoribbon-bridged CNTs as counter electrodes (Figure 1) showed an energy conversion efficiency of up to 8.23%, compared with 7.61% for a conventional platinum counter electrode under similar conditions. Multiwalled CNTs with a diameter of 20–40 nm and a wall number of 20–30 were primarily studied (Supporting Information, SFigure S1). The CNTs were chemically unzipped to produce graphene oxide nanoribbons (GONRs; Figure S2). The degree of unzipping could mainly be increased by increasing the amount of potassium permanganate. X-ray diffraction (XRD) analysis was undertaken to quantitatively determine the degree of unzipping. Figure S3 shows typical XRD patterns for the mixtures of GONR and CNT with different GONR weight percentages. As the amount of GONR in the mixture increased, the characteristic GONR peak (2q= 11.28) gradually increased while the CNT peak (2q= 26.18) gradually decreased. Based on the intensity of their characteristic peaks, a relationship curve between GONR weight percentage and intensity ratio was obtained. This curve could be then used to calculate the weight percentage of GONRs in the resulting hybrid (Figure S4). Figure 2 compares the XRD patterns of pristine CNTs, GONR/CNT hybrids with different GONR weight percentages, and pure GONR. The GONR weight percentages in three hybrid samples were calculated as approximately 16%, 55%, and 85%. For simplicity, they were defined as GONR16%/CNT, GONR55%/CNT and GONR85%/CNT. CNTs could be also completely unzipped to form pure nanoribbons. Raman spectroscopy was used to monitor structural changes during the unzipping process of CNTs (Figure S5). The D band at ca. 1350 cm 1 gradually broadened and increased in intensity as the reaction progressed, a result which corresponds to increasing GONR weight percentage. As a result, the ratio between the intensities of the D and G bands (ID/IG) was enhanced, which indicates a decrease in Figure 1. a) the structure of dye-sensitized solar cells based on R-GONR-bridged CNTs as counter electrode. b) the mechanism of rapid electron transport in the counter electrode.