Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX
www.acsami.org
Controllable Synthesis of Tunable Microstructures of Self-Supporting Graphene Films from Opened Bubble to Cube via in Situ TemplateModulating Na Li,*,† Qiao Yang,† Xing Liu,† Xuankai Huang,† Haiyan Zhang,*,† and Chengxin Wang*,‡ †
School of Material and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China State Key Laboratory of Optoelectronic Materials and Technologies, School of Material Science and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China
‡
S Supporting Information *
ABSTRACT: Three-dimensional (3D) microstructured building units have replaced layer-to-layer stacked designs in transparent graphene films to fully exploit the advantages of two-dimensional graphene. However, it is still challenging to precisely control the size and microstructures of these building blocks to develop multifunctional graphene-based materials that satisfy the performance requirements of diverse applications. In this study, we propose a controllable method to regulate the microstructures of building units to form structures ranging from opened bubbles and cubes, while the size decreased from 20 to 3 μm, via an in situ templatemodulating technology. NaCl was used as either a liquid or solid template by changing the dc bias. The reduced size and dense arrangement of the building units not only provide an improved mass loading for the transparent films but also build multiple pathways for fast ion/electron transmission, enhancing their promise for various practical applications. Generally, we provide a convenient protocol for finely regulating the microstructure and size of these building units, resulting in multifunctional films with a controllable transmittance, which enables the use of these graphene-based architectures as transparent electrodes in various applications and extends the family of multifunctional materials that will present new possibilities for electronics and other devices. KEYWORDS: tunable microstructure, graphene opened-bubble, graphene opened-cube, template-modulating, transparent film
1. INTRODUCTION Smart wearable electronic devices are developing rapidly and are being applied in healthcare (as electronic skin), bodybuilding (in smart watches), and aerospace (as wearable antennas). Clearly, the current electronic components (such as sensors and energy supply systems), which are of large volume, hard, and completely opaque, cannot meet the flexibility, transparency, and stretchability requirements of smart wearable devices.1−5 Graphene has attracted great attention for its potential applications in sensors6−8 and energy-storage devices9−12 because of its excellent physicochemical properties such as strong mechanical strength, high transparency, and large surface area.13−18 However, the primary problem preventing the progress is the development of effective, macroscopic, transparent, stretchable, and flexible graphenebased materials for diverse applications (particularly energystorage devices such as supercapacitors). In previous reports, the transmittance goals were commonly achieved by adjusting the thickness of the graphene film. For example, the thicknesses of both the vacuum filtration film19 and air−liquid self-assembly film20,21 were dependent on the © XXXX American Chemical Society
volume of graphene solution used, and the layers of the reduced graphene oxide (RGO)22 film and chemical vapor deposition (CVD)-graphene film23 were determined by a layer-by-layer electrostatic interaction process22 and growth conditions,23 respectively. Briefly, the desired thickness was obtained mainly by changing the number of two-dimensional (2D) graphene layers added, using a layer-by-layer stacking method. However, increasing the load (thickness) sacrifices the transmittance but does not improve the performance of the supercapacitor device, as the corresponding surface area of the graphene film remains unchanged. Additionally, stretchability was realized mainly through adjusting the macroscopic morphology, that is, by designing the shape of the substrate. For example, crumpled graphene papers24 and multiscale/hierarchically patterned graphene films25 were prepared using similar prestretched (skin-patterning25) elastomer substrates.24 Wavy-shaped and omnidirectionally stretchable graphene electrodes were fabReceived: August 30, 2017 Accepted: November 10, 2017 Published: November 10, 2017 A
DOI: 10.1021/acsami.7b13117 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 1. Preparation Process and Different Growth Stages process step 1 step 2
NaCl melt and C-groups dissolved in NaCl melt bubble units: NaCl supersaturate, precipitate, and nucleate graphene growth and NaCl etching cube units: NaCl recrystallization and surface segregation graphene growth and NaCl etching
gases (sccm)
pressure (Torr)
power (W)
determined by MW−power. dc bias determined by MW−power. dc bias (>150 V)
Ar/H2/CH4 = 100:10:10
70
1000
270
5
Ar/H2/CH4 = 100:10:5
70
1000
150, 230, 300
20
determined by MW−power. dc bias (150 V). During the graphene bubble growth process, the NaCl liquid-drop template vaporizes or sputters under the heavy bombardment of the high-energy plasma. The diameter of the openings depends on the strength of the plasma etching; a strong, directional electric field caused by the high dc bias will result in rapid sputtering, and the Na/Cl plasma will escape from the liquid template and deposit onto the cool area of the system. Finally, NaCl was etched off and a pure graphene film was
Figure 3. Morphology characterizations of bubble-based graphene films. Low/high-resolution SEM micrographs of the microstructured films obtained at 300 V (a,b), 230 V (c−e), and 150 V (f,g), which are marked as bubble-1, bubble-2, and bubble-3, respectively.
formed. The microstructures of the films were indeed kept intact according to the SEM characterization of bubble-1-based TMGF before and after being immersed into DI water (Figure S1b,c), and pure graphene without residual NaCl was acquired, according to the XRD result (Figure S1a). Furthermore, the effect of the amount of NaCl on the microstructure of the TMGF was investigated. Take the maximum dc bias value (300 V) as an example; as shown in the SEM micrographs, the height of the graphene wall of the bubble units decreased obviously when the amount of NaCl was reduced from 0.65 g (Figure S2b) to 0.2 g (Figure S2a), which results in a thinner film. However, there is no obvious change in the opening size as the recrystallized NaCl grains and the etching effect (dc bias) remained almost unchanged. When the amount of NaCl was continuously reduced (
