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Defogging

Direct Growth of Ultrafast Transparent Single-Layer Graphene Defoggers Lifang Tan, Mengqi Zeng, Qiong Wu, Linfeng Chen, Jiao Wang, Tao Zhang, Jürgen Eckert, Mark H. Rümmeli, and Lei Fu*

The idea flat surface, superb thermal conductivity and excellent optical transmittance of single-layer graphene promise tremendous potential for graphene as a material for transparent defoggers. However, the resistance of defoggers made from conventional transferred graphene increases sharply once both sides of the film are covered by water molecules which, in turn, leads to a temperature drop that is inefficient for fog removal. Here, the direct growth of large-area and continuous graphene films on quartz is reported, and the first practical single-layer graphene defogger is fabricated. The advantages of this single-layer graphene defogger lie in its ultrafast defogging time for relatively low input voltages and excellent defogging robustness. It can completely remove fog within 6 s when supplied a safe voltage of 32 V. No visible changes in the full defogging time after 50 defogging cycles are observed. This outstanding performance is attributed to the strong interaction forces between the graphene films and the substrates, which prevents the permeation of water molecules. These directly grown transparent graphene defoggers are expected to have excellent prospects in various applications such as anti-fog glasses, auto window and mirror defogging.

1. Introduction Single-layer graphene is a strictly two-dimentional crystal of carbon atoms, which exhibits an extraordinary thermal conductivity of ca. 5300 W/mK and excellent optical transmittance of 97.3% at a wavelength of 550 nm.[1,2] These properties offer tremendous potential for graphene as a material for transparent defoggers. An ideal defogger should rapidly remove fog for a low input voltage. Thus far numerous carbon nanostructured materials such as carbon nanotubes (CNTs),[3–5] reduced graphene oxide (rGO),[6] and transferred graphene[7,8] have been explored to improve the performance of defoggers and transparent heaters, in particular L. F. Tan, M. Q. Zeng, Q. Wu, L. F. Chen, J. Wang, T. Zhang, Prof. L. Fu College of Chemistry and Molecular Science Wuhan University Wuhan 430072, P. R. China E-mail: [email protected] Prof. J. Eckert, Prof. M. H. Rümmeli IFW Dresden, P.O. Box 270116, 01069, Dresden, Germany DOI: 10.1002/smll.201402427 small 2014, DOI: 10.1002/smll.201402427

to increase the working saturation temperature and to reduce the defogging time. It has been reported that the saturation temperature of defoggers are strongly dependent on the convective heat loss at the solid–air interface, which is affected by the heat-transfer coefficient of the materials.[8] Graphenebased defoggers are reported to have the lowest overall heattransfer coefficient (8 W m−2 °C−1),[8] which is much smaller than that found for other carbon nanomaterials such as CNTs (24,[8] 25,[3] and 53[5] W m−2 °C−1) and graphene oxide (GO) (32 W m−2 ºC−1).[6] The defogging time is inversely proportional to the heating rate, which is dependent on the interface between the substrate and the thermal conductivity of the used materials.[3,8] As compared to other carbon nanomaterials (e.g. CNT and rGO), graphene-based systems achieve a lower convective heat-transfer coefficient due to graphene ideal flat surface, resulting in a higher heating rate.[8] In addition, the high optical transparency of single-layer graphene (97.3%)[2] also makes graphene a promising alternative for next generation transparent defoggers. Previous reports have shown excellent thermal performances for single-layer graphene films.[1] This hints that the future for single-layer graphene as a material for defoggers is bright. After multiple transfers and chemical doping

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processes, the graphene films exhibited superior performance to that of conventional ITO/Cr-based transparent heaters in terms of their temperature response and heat distribution.[7,8] The multiple transferred graphene on glass exhibited a shorter response time and higher saturation temperature than Cr/glass defoggers for a given input voltage. However, all these studies were limited to dry multi-transferred graphene films, and in so far as we are aware, the actual performance of single-layer graphene films as defoggers has yet to be assessed. The resistance of graphene films increases as the surface absorbs water molecules.[9] For transferred graphene, the Van der Waals force between the graphene and the substrate is weak. In addition, cracks and residues from the transfer procedure reduce the adhesion of graphene to the substrate.[10] Water molecules easily permeate into the solid (graphene)– solid (substrate) interface, which causes the resistance of the transferred graphene to sharply increase after its surface is entirely covered by fog. As a result, the saturation temperature of the wet transferred graphene is much lower than that of its initial dry state, and thus it cannot remove fog efficiently. To improve the transfer procedure of graphene, there must be meticulous control of both the contamination and crack formation to minimize the degradation of the transferred graphene's quality.[11,12] However, the lack of a strong interaction force between non−epitaxial graphene with substrates,[13] causes transferred graphene films be soaked by fog. To date, a variety of CVD approaches have been developed to synthesize graphene films on dielectric substrates, such as epitaxial growth with the assistance of Cu vapor[14,15] or without metal catalysts.[13,16–20] Unfortunately, the obtained graphene films are not always continuous or uniform over large-areas, which are key prerequisites for high performance defoggers. Here we present the direct growth of large-area and continuous graphene films on quartz and fabricate the first usable single-layer graphene defogger. The advantages of our single-layer graphene defoggers lie in their ultrafast defogging time for relatively low input voltages and excellent defogging robustness. This outstanding performance is attributed to the strong interaction forces between the graphene films and the substrates, which preventing the permeation of water molecules. Our directly grown transparent graphene defoggers can be considered as the starting point of the graphene application in various defogging fields such as anti-fog glasses, auto window and mirror defogging.

2. Results and Discussion Figure 1a schematically illustrates the remote catalyzation process which uses floating vapor phase Ga atoms to decompose the hydrocarbon feedstock. To achieve this, a droplet of Ga (30 mg) was placed on a piece of quartz (1 × 1 cm2) located upstream from the growth substrate (quartz). Both of these quartz substrates were located in the constant temperature zone of a tube furnace. Rather than using metal catalysts as substrates in the conventional thermal CVD process, our approach allows for the direct graphitization of carbon

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Figure 1. Schematic illustrations of the remote catalyzation process for direct growth of single-layer graphene and the graphene defogger. (a) Schematic drawing of Ga vapor-assisted CVD directly grown graphene on a quartz substrate. (b) Graphene defogger fabricated by large area, uniform single-layer gaphene synthesized by Ga vapor-assisted CVD.

fragments over quartz surfaces, forming low-defect graphene films which are not at any possible risk of damage since no post-growth etching or evaporation of the metal catalyst is required. Recently, Ga has been shown to offer excellent catalytic ability for single-layer graphene growth.[21–24] Unlike Cu,[25] Ga absorbs carbon readily,[21] which instills it the special ability to grasp/absorb hydrocarbons, and then to catalytically decompose them into carbon atoms or fragments. Ga has a relatively high sublimation pressure, which is nearly 10 times higher than that of Cu at the same temperature and pressure.[26] This means that Ga particles in sufficient quantity can be provided to the reaction and thus bring about more successful collisions with reactant species, thereby raising the effectiveness of the decomposition of carbon feedstock in the gas phase. After the delivery of carbon atoms or fragments downstream to the quartz substrate, the Ga particles readily sublime again. This sublimation process is efficient that no appreciable Ga residue was found on the as-grown graphene layers, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis which is a highly surface sensitive technique (Figure S1). As a result, no post-growth processing to remove the metal catalysts is required making graphene produced in this manner practical for defoggers (Figure 1b). Figure 2a displays a photograph of a 1.8 × 10 cm2 graphene film that was directly grown on a quartz substrate with excellent optical transparency. Raman spectroscopy is a powerful and fast technique with which to identify the number of layers and presence of defects in graphene over a large area. To evaluate the surface coverage and the layer number distribution of the grown graphene film, we conducted Raman spectroscopy mapping over a large-area region of 7 × 7 mm2 located at the left end of the quartz substrate as marked in Figure 2a (acquisition step = 0.5 mm). In Figure 2b, a 3D-colormap from the dotted areas highlights the perfect uniformity of the directly grown graphene over quartz. The intensity ratio of the 2D to G bands (I2D/IG) lies in the range 1.6 to 2.2, which is characteristic of monolayer graphene.[27,28] In order to further demonstrate the uniformity of the directly grown graphene in larger area, a region in the middle of the quartz substrate was also selected to characterized by Raman mapping, which was exhibited in Figure S2. In addition, the full Raman spectroscopy of the directly grown graphene was shown in Figure S3. A more in-depth inspection of the graphene films using fine Raman mapping and scanning electron microscopy (SEM) confirmed the formation of continuous films without any visible cracks. Figure 2c shows

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Figure 2. Characterization of single-layer graphene directly grown on quartz. (a) Photograph of a 1.8 cm × 10 cm graphene film directly grown on a quartz substrate. (b) A 3D-colormap surface derived from Raman mapping result acquired from the dotted area in (a). (c) Optical microscope image of a graphene film transferred on 300 nm SiO2/Si substrate. (d) HRTEM image and the corresponding selected-area electron diffraction pattern revealing the six-fold symmetry single-crystal nature of the graphene.

the typical optical microscope (OM) image of a graphene voltage, multiple stacking and a wet chemical doping profilm after transfer onto a 300 nm SiO2/Si substrate. One can cess to improve the electrical quality of graphene films was easily observe the graphene films due to the light interfer- required.[30] As an attractive alternative, our directly grown ence effect,[29] which clearly confirm the excellent uniformity single-layer graphene films on quartz have a resistance ∼3 kΩ of the graphene films at a macroscopic level. Low voltage and a transmittance of ∼96.8% at a wavelength of 550 nm aberration-corrected, high-resolution transmission electron without doping, which is very close to the optimal values microscopy (LVAC-HRTEM) was employed to examine the found with mechanically exfoliated single-layer graphene viz. microscopic structures of the directly grown graphene. The 97.3%,[2] indicating that there is still a room for improving HRTEM image in Figure 2d and the corresponding selected the quality of the graphene directly grown by our method. area electron diffraction (inset in Figure 2d) highlights the However, the simple and convenient progress and the strong six-fold symmetry single-crystal nature of the graphene. The adhesion between the graphene film and the substrate meet little hole at the bottom right of the image allows us to con- the requirements for the graphene defoggers. Figure 3a shows firm single-layer graphene. Based on the above characteri- the time evolution with respect to increasing temperature for zation results and analysis, we demonstrate that large-area, the directly grown single-layer graphene over quartz which is continuous single-layer graphene can directly grow on quartz substrates. As compared with directly grown graphene, one needs pay more attention to improve the graphene−substrate contact for transferred graphene films. For example, cracks and residues typically caused by transfer will reduce the adhesion of the graphene to the substrate. In addition, cracks allow water molecules to easily permeate into the graphene–substrate interface, which leads to defogging failure, particularly when the system has Figure 3. Thermal performances of the directly grown single-layer graphene film. a low saturation temperature. In pre- (a) Temperature profiles of a directly grown single-layer graphene defogger at different input vious reports, in order to increase the voltages. (b) Contour map of temperature on the surface of directly grown graphene film on saturation temperature at a given input quartz (1 cm × 1 cm) while applying an input voltage of 20 V. small 2014, DOI: 10.1002/smll.201402427

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supplied by an input voltage. The temperature increased rapidly with time and reached a saturation value within a relative short time compared to the defogger made with transferred CVD grown graphene.[7] Moreover, the saturation temperature increased with increasing supply voltage (Figure S4).[31,32] Figure 3b presents a contour map of the temperature at different locations on the surface of the directly grown graphene film on quartz (1 × 1 cm2) when the input voltage was maintained at 20 V(acquisition step = 1 mm), here the temperature distribution was obtained by using the subminiature K-type thermocouples and digital thermometer. An excessive working temperature (e.g. >100 °C)[7] is not ideal and not necessary for functional defoggers. Both edges of the graphene film were deposited by Cr/Au electrodes with a width of 1 mm using vacuum thermal evaporation (Figure 4a). The surface temperature was 41.3 ± 1.1 °C in the central region, excluding the metal electrode and its contiguous areas. Most of the area exhibits a similar color, indicating a uniform temperature distribution over the graphene defogger, which is attributed to the uniformity of the directly grown graphene over a large area. Both of the edges of the defogger coated with Cr/Au and conductive silver paste show a slightly lower temperature, which may be attributed to the different heat loss, including the radiative and convective heat loss from the surface of different materials.[8]

Ideally defoggers should have a fast defogging speed. As can be seen in Figure 4a, the left photograph is our directly grown single-layer graphene defogger covered with fog before applying a bias voltage. On the right is the same graphene defogger after being supplied a bias of 20 V for 15 s. The fog can be seen to have been completely removed. The screenshot of the defogging progress were shown in Figure S5 and the real time defogging video could be found at Supporting Information. We also measured the starting time and complete time for defogging under different supply voltages, as shown in Figure 4b. The results show that the defogging time decreases with increasing input voltage. In terms of real application in daily life, our experiments were all conducted below the accepted “safe” voltage of 36 V. When the input voltage was increased to 32 V, the complete defogging time was 6 s, which is significantly faster than other graphenebased defoggers. For instance, rGO-based defoggers need 30 s to completely remove fog after an applied voltage of 60 V![5] And the studies of the transferred graphene film in defoggers were limited in dry conditions by far as we know. To the best of our knowledge, our directly grown single-layer graphene films set the record for defogging speed (for an equivalent supply voltage). This may be attributed to its close contact to the substrate which increases the rate of heat transfer and prevents the permeation of water molecules at the interface

Figure 4. Defogging performances of the directly grown graphene defogger. (a) Fog removal performance of the directly grown single-layer graphene defogger before (left) and after (right) heating at 20 V for 15 s. (b) Fog removal time as a function of input voltage. (c) The temperature corresponding to the starting time and complete time for defogging under different input voltage. (d) Defogging robustness of the directly grown single-layer graphene defogger investigated by complete removing fog 50 times at 20 V.

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Figure 5. The different behaviors of the directly grown single-layer graphene and the transferred single-layer graphene after exposure to fog. (a) The resistance profiles of directly grown graphene defoggers and transferred graphene defoggers before and after exposure to fog. (b) Directly grown graphene films perfectly adhered on the surface of the quartz substrate. (c) Upper surface of directly grown graphene absorbed by water molecules and the current carriers passing through the under surface protected by the quartz substrate. (d) Inevitable cracks and residues caused by the transfer procedure in transferred graphene. (e) Both sides of transferred graphene absorbed by water molecules for lack of strong interaction force between the graphene and the quartz substrate.

of the graphene films and the substrates. Figure 4c presents the temperature of the graphene films corresponding to the starting time and completion time for defogging for different supply voltages. It indicates that the fog starts to disappear once the graphene defogger reaches ∼25 °C and it will be completely removed once a temperature of ∼30 °C has been reached. The defogging robustness of our directly grown single-layer graphene films were further investigated by completely covering them with fog and then defogging 50 times for an input voltage of 20 V (Figure 4d). It is important to note that after 50 defogging cycles there is no discernable change in the defogging time. This outstanding performance is attributed to the strong interaction forces between the graphene films and the substrates. In order to compare the defogging behavior between directly grown graphene and transferred graphene, we transferred single-layer graphene films grown on Cu foil by low pressure chemical vapor deposition (LPCVD) onto quartz substrates (cut into 1 × 1 cm2) and measured their defogging performance under the same input voltage of 20 V. Surprisingly, the transferred graphene defoggers can only efficiently remove the fog in the first cycle. Thereafter, they are unable to remove fog in less than 5 min (Figure S6a). The directly grown graphene defoggers completely remove fog in 15 s to 16 s with very good repeatability. Figure S6b shows the resistance changes of the directly grown graphene and the transferred graphene before and after being covered by fog. There is a slight increase in the resistance of the directly grown graphene after it becomes wet. The resistance always recovers to the initial level after being dried at room temperature. However, the resistance of the transferred graphene defoggers rapidly increases after each defogging test and does not return to its initial value even when dried for a considerable time. Previous reports have shown that the resistance of transferred graphene increases after exposure to high humidity,[10,33] which is in agreement to our observations with transferred graphene. To understand the disparity between our directly grown graphene defoggers and those implementing transferred graphene defoggers, we measured the change of resistance before and after exposure to fog as shown in Figure 5a. The initial resistance of the directly small 2014, DOI: 10.1002/smll.201402427

grown graphene and the transferred graphene are similar. When their surfaces were entirely covered by fog, the resistance of the directly grown graphene increased by ∼10%, which then returned to its initial value after naturally drying at room temperature. In contrast, the resistance of the transferred graphene had a significant increase of ∼220%, and its resistance would further increase upon drying. As illustrated in Figure 5b, the directly grown graphene film perfectly adheres on the surface of the quartz substrate. When the surface is covered with fog, the water molecules hardly permeate into the graphene–substrate interface where there is a strong bonding force. The resistance of the directly grown graphene defogger slightly increases because only its upper surface has absorbed water molecules and current carriers still can pass through its lower surface which is protected by the quartz substrate as illustrated in Figure 5c. In the case of defoggers based on transferred graphene, cracks and residues caused by the transfer process cannot be completely avoided (Figure 5d). Thus, after a transferred graphene defogger is exposed to water vapor, its resistance will sharply increase because both sides absorb water molecules since water molecules can traverse the cracks and intercalate between the graphene and substrate. This reduces the interaction force between the graphene and the substrate (Figure 5e). As a result, the saturation temperature of the wet transferred graphene is too low to remove the fog efficiently. During the drying process, the cracks in the transferred graphene become larger due to liquid contraction, leading to a further deterioration in conductivity.

3. Conclusion In summary, the direct growth of large-area, continuous single-layer graphene films on quartz by the catalysis of floating Ga vapor enables the successful direct fabrication of single-layer graphene on insulating substrates to serve as defoggers for the first time. The advantages of our singlelayer graphene defoggers lie in their ultrafast defogging speed for relatively low input voltages and excellent defogging robustness. They can completely remove fog within 6 s when supplied a safe voltage of 32 V. No discernable

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changes in the full defogging time after 50 defogging cycles are observed. This outstanding performance is attributed to the strong interaction forces between the graphene films and the substrates. Our directly grown transparent graphene defoggers can be considered as the starting point of the graphene application in various defogging fields such as anti-fog glasses, auto window and mirror defogging.

4. Experimental Section Direct Growth of Single-Layer Graphene on Quartz: Single-layer graphene films were directly grown on quartz substrates under ambient pressure. Typically, the commercial quartz substrates were cut into small pieces (1 cm × 1 cm), and then they were ultrasonicated and rinsed with acetone, alcohol and ultrapure water prior to being dried under a nitrogen stream. After a droplet of Ga (about 30 mg) was be placed on one of the quartz substrates at 50 °C, they were loaded into a horizontal quartz tube mounted inside a high-temperature furnace (HTF 55322C Lindberg/Blue M). The growth protocol consisted of three steps: (1) heating the quartz substrates to 1050 °C at a rate of 30–40 °C/min under the flow of Ar and H2; (2) exposure of the quartz substrates to a carbon source at 1050 °C for 2 hours under 300 sccm Ar, 20 sccm H2 and 5 sccm CH4; and (3) cooling the quartz substrates to room temperature at a rate of 15 °C /min under Ar and H2. Growth of Single-Layer Graphene on Copper and Transferring it to a Target Substrate: Single-layer graphene films were grown on Cu foils under low pressure. Typically, the commercial Cu foils (25 µm) were cut into small pieces (1 cm × 1 cm), and then they were soaked with the mixture of ultrapure water, alcohol and glacial acetic acid (1:1:1) for 3 min prior to being dried under a nitrogen stream. Then the Cu foils were loaded into a horizontal quartz tube mounted inside a high-temperature furnace (HTF 55322C Lindberg/Blue M). The growth protocol consisted of three steps: (1) heating the Cu foils to 1000 °C at a rate of 30–40 °C/ min under 7 sccm H2 and annealing the Cu foils for 15 min; (2) exposure of the Cu foils to a carbon source at 1000 °C for 30 min under 7 sccm H2 and 20 sccm CH4; and (3) cooling the Cu foils to room temperature at a rate of 15 °C /min under H2 and CH4. Then the graphene films were transferred from the Cu foils to quartz substrates. A typical transferring procedure involved spin coating a poly(methylmethacrylate) (PMMA) film onto the graphene-grown Cu foils and releasing the PMMA/graphene film by etching out Cu foils in an iron (III) chloride (FeCl3) aqueous solution (∼1 M) overnight. This was followed by a rinse in HCl/ultrapure water to remove the metal ions. The PMMA layer was dissolved with hot acetone after the PMMA/graphene film was transferred onto the target substrates (quartz). Characterization: Optical images were taken with an optical microscopy (Olympus DX51, Olympus), and Raman spectroscopy was performed with a laser micro-Raman spectrometer (Renishaw in Via, Renishaw, 532 nm excitation wavelength). The transmission electron microscopy (TEM) images were taken with an aberrationcorrected, high-resolution TEM (AC-HRTEM, JEOL 2010F) operating at 80 kV with graphene samples directly transferred onto a copper grid. XPS was performed on a Thermo Scientific, ESCALAB 250Xi. The transmittance of the films was characterized by a UV-Vis spectrophotometer (MAPADA UV-6).

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Temperature Measurements: Both edges of graphene films were deposited by Cr/Au electrodes with a width of 1 mm by the vacuum thermal evaporator (ZHD-300, Beijing Technol Science Co.). Cu wires were pasted on Cr/Au electrodes with conductive silver paste. The input voltages were applied to the graphene defoggers by DC power supply (DH1720A-5, Beijing Dahua Radio Instrument Co.). The temperature was measured with Omega K-type thermocouples and digital thermometer (HH11B, Omega). The room temperature was kept constant during the measurements. Defogging Tests: The directly grown transparent graphene defoggers were placed on the above of the beaker. The laboratory temperature was 23 °C and the water temperature in the beaker was 50 °C. A constant water level was maintained, and the atmosphere between the film and the water bath was assumed to be saturated with water vapor.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The research was supported by the Natural Science Foundation of China (Grants 51322209), the Sino-German Center for Research Promotion (Grants GZ 871) and the Ministry of Education (Grants 20120141110030). We thank N. Geiβler for obtaining the TEM images.

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Direct growth of ultrafast transparent single-layer graphene defoggers.

The idea flat surface, superb thermal conductivity and excellent optical transmittance of single-layer graphene promise tremendous potential for graph...
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