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Direct CVD-derived graphene glasses targeting wide ranged applications Jingyu Sun, Yubin Chen, Manish Priydarshi, Zhang Chen, Alicja Bachmatiuk, Zhiyu Zou, Zhaolong Chen, Xiuju Song, Yanfeng Gao, Mark H. Rümmeli, Yanfeng Zhang, and Zhongfan Liu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b01936 • Publication Date (Web): 25 Aug 2015 Downloaded from http://pubs.acs.org on August 27, 2015

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Direct CVD-derived graphene glasses targeting wide ranged applications Jingyu Sun1, Yubin Chen1, Manish Kr. Priydarshi1, Zhang Chen3, Alicja Bachmatiuk4,5, Zhiyu Zou1, Zhaolong Chen1, Xiuju Song1, Yanfeng Gao3, Mark H. Rümmeli4,6,7, Yanfeng Zhang1,2*, Zhongfan Liu1* 1

Center for Nanochemistry (CNC), Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China 2 Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China 3 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institution of Ceramics Chinese Academy of Sciences, Shanghai 200050, P. R. China 4 Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, Zabrze 41-819, Poland 5 IFW Dresden, Institute for Complex Materials, P.O. Box 270116, D-01171 Dresden, Germany 6 Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea 7 IBS Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Daejon 305-701, Republic of Korea

ABSTRACT: Direct growth of graphene on traditional glasses is of great importance for various daily-life applications. We report herein the catalyst-free atmospheric-pressure chemical vapor deposition approach to directly synthesizing large-area, uniform graphene films on solid glasses. The optical transparency and sheet resistance of such kinds of graphene glasses can be readily adjusted together with the experimentally-tunable layer thickness of graphene. More significantly, these graphene glasses find a broad range of real applications, enabling the low-cost construction of heating devices, transparent electrodes, photocatalytic plates and smart windows. With a practical scalability, the present work will stimulate various applications of transparent, electrically and thermally conductive graphene glasses in real life scenarios. KEYWORDS: graphene, glass, chemical vapor deposition, catalyst-free, application

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Graphene has attracted vast interests in various fields due to its specific two-dimensional structure and fascinating properties1-4, which have stimulated the rapid development of various synthetic methodologies, mainly including mechanical cleavage4,5, chemical reduction of graphene oxides6-8, liquid exfoliation of graphite9,10, and chemical vapor deposition (CVD)11-16. Among these techniques, CVD on metallic substrates possesses unique advantages for the scalable production of uniform graphene in a controllable manner17-19, where centimeter-size graphene single crystals have been realized over solid Cu substrates11. However, the inevitable transfer of graphene off the metals onto desired substrates is required for further applications. During the transfer procedure, loss of substrate material and the introduction of defects and contaminations are unavoidable. To avoid these issues, efforts have centered on developing transfer-free routes for the direct synthesis of graphene on insulating substrates. To date, the catalyst-free CVD growth of graphene has been demonstrated on a variety of dielectric substrates of interest, such as SiO220-25, Si3N421,26, Al2O327-30, SrTiO331 and h-BN32-35. Notably, the selection of such target substrates is essential since it normally imposes a crucial impact on the potential usage of as-fabricated materials. Recently, we reported the CVD growth of graphene on h-BN for advanced graphene-based electronics, with the underneath as-grown h-BN monolayer serving as a dielectric insulator33. We have also shown the promise of directly-grown graphene on high-κ SrTiO3 substrates for fabricating high-performance field effect transistors (FETs)31. However, the direct growth of graphene on solid glasses, the ordinary transparent insulating materials with broad ranges of daily-life applications has met with limited success36,37. It is no doubt that such kinds of graphene glasses will find wide applications by combing the complementary intrinsic properties graphene and glasses. In the present work, we systematically studied the CVD growth performances of graphene on solid glasses without using metal catalysts. With the optimized atmospheric-pressure

CVD (APCVD) approach, uniform and high quality graphene films with tunable optical transparencies and electrical conductivities have been achieved on various high-temperature resistant solid glasses. More importantly, we have realized the batch production of high quality transparent graphene glasses, which enables our investigations of their applications in various aspects, such as heating devices, low-cost electrodes, photocatalytic plates and smart windows of such unique material. Our study promises novel insight into the practical usage and real applications of catalyst-free, directly-grown CVD graphene as compared to other reports in the relevant field, which have invariably been centering on the synthesis design and quality control of graphene directly on common insulators21-31,36,37. Figure 1a illustrates the schematic diagram of the APCVD equipment used in this study. Specifically, a quartz tube with an inner diameter of 3 inch is fixed to a three-zone electrical furnace. Several representative types of solid glasses, namely, quartz glass, borosilicate glass and sapphire glass are selected as the substrate materials. All these glasses have high softening points to allow the high temperature growth of graphene on the solid-state glass surfaces. The direct growth of graphene on such glasses was carried out by a CH4-APCVD without the aid of any metallic species or catalysts. The growth methodology is described in detail in Methods. Elemental characterization by X-ray photoelectron spectroscopy (XPS) measurements of the as-grown samples were performed to confirm that a complete catalyst-free process had taken place (Supporting Information Fig. S1a). Figure 1b presents a photograph of bare borosilicate glass (leftmost), as well as graphene/borosilicate glasses exposed to different CH4 flow rates from 2 to 10 standard cubic centimeter per minute (sccm) (with the other growth parameters kept identical: Ar/H2: 100/50 sccm at 1000°C for 2 h). One can notice that after CVD growth, the transparency of the samples decreases with increasing CH4 flow rate (from left to right). This indicates that the thickness of the as-grown graphene on glass can be dictated by 2

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adjusting the amount of precursor gas. The photograph in Fig. 1c displays a 6 cm × 4 cm graphene/quartz glass sample. For comparative studies, the as-grown graphene on the right-hand side was subjected to a simple patterning treatment, where graphene was removed by oxygen plasma with the aid of a shadow mask. By performing water dropping tests, one can clearly observe that film-wise water aggregations can be formed on a bare glass surface, whereas small water droplets stand up on the graphene coated glass. This indicates the hydrophobic nature of the graphene surface, similar to the published result for transferred graphene-on-glass system38. This is further evidenced by the photograph in Fig. 1d, which shows a typical concave meniscus for water within a bare quartz test tube but a convex meniscus for a graphene coated one. This phenomenon might inspire the design of novel labware by employing our directly-grown graphene glass. Moreover, the photograph in Fig. 1e presents large-scale plate growth of graphene film directly on sapphire glass (11 cm × 6 cm), which can be achieved with excellent film uniformity. Quality evaluations of representative as-grown graphene samples (undergoing identical APCVD procedures, Ar/H2/CH4: 100/50/6.3 sccm at 1000°C for 2 h) were carried out using Raman spectroscopy (Fig. 1f). Three characteristic peaks can be observed for all graphene glass samples, corresponding to the D, G and 2D band, respectively. Notably, for the quartz glass sample, the intensity ratio of the 2D peak at ~2700 cm-1 and the G peak at ~1580 cm-1 is larger than 2; the 2D peak possesses a symmetric shape with a single Lorentzian profile and the full width at half maximum (FWHM) of 32 cm-1, strongly indicating the formation of predominantly monolayer graphene16. Furthermore, a typical C 1s XPS spectrum of the graphene quartz glass sample (Fig. S1b) can be fitted with three signals from graphene, namely, an sp2 carbon peak (284.8 eV), a C-H peak (285.3 eV) and a broad C-O peak. To further examine the product

quality of our direct APCVD-grown graphene on glass, back-gated graphene FET device was fabricated on transferred samples using a Hall-bar contact geometry (Fig. 1g inset). The transfer curve (drain current IDS vs. gate voltage VG) in Fig. 1g shows no observation of the Dirac point in the range of applied gate voltages measured at room temperature under ambient conditions. This is possibly due to water/oxygen adsorption or doping effects from the transfer process17. The carrier mobility extracted from the transfer curve ranges about 553-710 cm2·V-1·s-1, which is superior to those of directly-grown CVD graphene on SiO2 substrates24,25. The transparencies and sheet resistances for the graphene grown on quartz glasses are shown in Fig. 1h, representing our ability to tune the optical and electrical properties of as-fabricated samples by comprehensively tailoring the growth conditions. Herein, the transparency is reflected by the UV-Vis transmittance spectra, and the sheet resistance was measured by using the four-probe method without any pretreatment of the as-grown graphene glass samples (doping and/or annealing). At a wavelength of 550 nm, the transmittance of our thinnest graphene film is ~97.5%, along with an average sheet resistance of 6.1 kΩ·sq-1. In particular, the graphene glass sample possessing a pristine sheet resistance of 2 kΩ·sq-1 can be reproducibly obtained with a transmittance of 80%. At a similar transparency, the electrical performance is comparable to that fabricated by APCVD on SiO2 (79.5% transparency, 1.5 kΩ·sq-1)24, and is superior to those of well-reduced graphene oxides assembled on glasses (79-88% transparency, 8-20 kΩ·sq-1)39,40 or directly grown graphene films on quartz by plasma-enhanced CVD (77-83% transparency, 2.5-4 kΩ·sq-1)22,37. Such easily-obtained samples with good optical and electrical properties are key to their practical applications in transparent electrodes, low-cost heaters and smart windows.

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Figure 1. Catalyst-free APCVD growth of uniform graphene on various solid glasses. (a) Schematic diagram of the catalyst-free APCVD growth method. (b) Photograph of the borosilicate glass substrates before (leftmost) and after graphene growth with different CH4 flow rates at 2, 5, 7.5 and 10 sccm. (c) Demonstration of the hydrophobic and hydrophilic nature of graphene/quartz glass (the left part) and the bare quartz glass, respectively. (d) Photograph showing the differences in water containing behaviors between the graphene-coated and pristine quartz test tubes. (e) Photograph of graphene/sapphire glass plate displaying a good transparency. Scale bar: 4 cm. (f) Representative Raman spectra of directly-grown graphene on different types of solid glasses. (g) Transfer curve of the graphene FET; the inset shows an optical microscope image of an individual device. The graphene was grown on quartz glass under the following APCVD condition: Ar/H2/CH4: 100/50/8 sccm at 1020°C for 3 h. Scale bar: 100 µm. (h) Sheet resistance and UV-Vis transmittance spectra in the wavelength range of 350-800 nm of the graphene/quartz glass. The graphene/quartz glass sample was then selected for detailed structural characterizations because the quartz glass is cheap and readily available. Figure 2a shows a scanning electron microscopy (SEM) micrograph of as-grown graphene on quartz glass. The different contrasts (light gray and dark gray) reveal the differences between the deposited graphene film and the substrate. The obtained graphene was transferred onto the SiO2/Si substrate for optical microscope (OM) inspection (Fig. 2b), which

exhibited a good uniformity of the film at a macroscopic level41. Low-voltage, aberration-corrected, high-resolution TEM (LVAC-HRTEM) was also employed for a detailed characterization of the film thickness, atomic scale structure, and stacking geometry after a sample transfer process31. From the low-magnified TEM view in Fig. 2c, the graphene film is found to be continuous and flat over the TEM grid, where the edges of the minor sheet breakage allow us to directly identify the layer number. The HRTEM 4

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image in Fig. 2d indicates a mixture of mono(accounting for ca. 40-50% area of such sample) and few-layer graphene for the sample with a high optical transmittance (96.3% at 550 nm, Supporting Information Fig. S2). The perfect atomic lattices in Fig. 2e justify the high crystal quality of our synthesized graphene. Figure 2f shows a typical HRTEM image of directly-grown APCVD graphene. The little breakage at the center right of the image allows us to confirm its monolayer thickness. Otherwise, double layer (even few-layer) regions can also be found at several places, forming turbostratic stacking orders, which is in accordance with the HRTEM view observed by Medina et al.22 Fig. 2g illustrates the enlarged atomic-resolution view of a double layer area (marked by the yellow text) in Fig. 2f, where clear Moiré patterns are observed (also see Supporting Information Fig. S3). As shown in Fig. 2g, the Moiré pattern

wavelength is measured to be about 1.08 nm, hence it can be calculated that the relative rotational angle (θ) between two layers is at 13° (See Supporting Information for details)42. This is in good agreement with the measured angle in the fast Fourier transform (FFT) pattern (inset in Fig. 2g). Fig. 2h presents a corresponding structural representation of the superstructure formed by the addition of two graphene layers with a 13° relative rotation. It is of particular interest that the catalyst-free CVD gives rise to the formation of graphene films directly onto a series of glass substrates. To investigate the synthetic process in detail, temperature-dependent and time-dependent growth evolutions were investigated by Raman spectroscopy (Fig. S4). Such direct CVD process on solid glasses is schematically illustrated and microscopically analyzed in Fig. S5 and Fig. S6, respectively.

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Figure 2. Structural characterizations of APCVD graphene films directly grown on quartz glass. (a) SEM image of directly grown APCVD graphene on quartz glass. Scale bar: 2 µm. (b) OM image of the transferred graphene film onto the SiO2/Si substrate. Scale bar: 20 µm. (c) Low-magnification TEM image of the graphene film. Scale bar: 500 nm. (d) HRTEM images showing monolayer, double layer and three layer graphene. Scale bars: 2 nm. (e) Atomic-resolution TEM image of as-grown monolayer graphene. Scale bar: 1 nm. (f) Aberration-corrected HRTEM image of typical directly grown graphene, showing a co-existence of monolayer and double layer, as marked in the image. Scale bar: 2 nm. (g) HRTEM image of the double layer graphene marked by the yellow text in (f). Scale bar: 0.5 nm; the inset is the corresponding fast Fourier transform pattern. (h) Structural representation of two graphene layers with 13° rotation, showing excellent agreement with the superstructure displayed in (g). Figure 3 presents our capability to synthesize uniform graphene films directly on glass at large scale in a controllable manner, which is crucial for practical applications. The photograph in Fig. 3a displays bare sapphire glass plate (6 cm × 11 cm) and graphene films grown on such glass plates with different growth periods from 2, 4 to 6 h (all other conditions were kept identical: Ar/H2/CH4: 500/50/17.5 sccm; growth temperature: 1050°C). The optical transparence of the graphene films decreases accordingly with increasing the growth time, indicating one can tailor the film thickness by simply adjusting the growth time. The photograph was taken on printed letters as a background to illustrate the sample transmittance, as well as reveal the good film uniformity. The corresponding optical microscope images of the graphene grown in 2, 4 and 6 h after transferred onto 300 nm SiO2/Si substrates also verify the excellent uniformity of our graphene films at a macroscopic level (Figs. 3b-d). The color changes of the transferred graphene films are observed from pink to purple and this is due to a light interference effect of increased layer thickness41. Raman mapping of the I2D/IG in Fig.

3e displays a rather uniform color contrast, confirming the uniformity of produced graphene films. To further evaluate the uniformity of the directly-grown graphene films on glass, the sheet resistances at 25 points over a 5 cm × 5 cm graphene/quartz glass plate (Transparency ~ 81% at 550 nm) were measured using the four-probe method. The spatial distribution of the sheet resistance of the graphene film ranges from 1.75 kΩ·sq-1 to 2.35 kΩ·sq-1, as displayed in Fig. 3f. The uniformly distributed green color in most of the areas highlights the good uniformity of our as-grown graphene. Fig. 3g presents a temperature mapping on the surface of as-fabricated graphene/quartz glass (1 cm × 2 cm) which is supplied by a 20 V input voltage. The surface temperature was 43.5 ± 1.3°C in the central region, excluding the edge areas with metallic electrode deposition, further verifying the excellent uniformity of directly grown graphene films over glass. Such large-scale, highly uniform, directly-grown graphene on glasses shows good heating performance, making it an ideal candidate for transparent electrodes, low-cost heating devices and thermochromic windows.

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Figure 3. Large-area uniformity characterizations of graphene films produced by catalyst-free APCVD on solid glasses. (a) Photograph of the bare sapphire glass plate (6 cm × 11 cm) and graphene films grown on such plates with different growth times from 2 to 6 h. (b-d) Corresponding optical microscope images of transferred graphene films onto SiO2/Si substrates for (a), demonstrating the excellent uniformity of full coverage samples. Scale bars: 20 µm. (e) Raman mapping of I2D/IG ratio over the graphene film directly grown on sapphire glass. Scale bar: 5 µm. (f) Sheet resistance mapping on a 5 × 5 cm2 area of the as-grown graphene film on quartz glass plate. Scale bar: 1 cm. (g) Contour map of surface temperature on a 1 cm × 2 cm graphene/quartz glass sample under an input voltage of 20 V. Scale bar: 5 mm. Note that the mapping results in (e-g) were obtained on the as-grown sample surfaces after CVD reactions without any involvement of graphene transfer process. En-route to real applications that could benefit from the directly grown, transparent, conductive graphene glasses, our obtained samples were firstly tested in a series of heating devices. Fig. 4a shows the schematic of a transparent heater using such kinds of graphene glasses. To demonstrate its use in thermochromic displays, a 4 cm × 6 cm APCVD graphene on quartz glass plate sample was subjected to shadow mask patterning to form a ribbon-shaped configuration (1.5 cm × 6 cm across the center of the plate) with the aid of an oxygen plasma, followed by

the selective deposition of color reversible thermochromic ink (color change: dark red to light yellow at 39°C) and metal electrodes. By applying a relatively low DC voltage (30 V) to the device, a heat-induced color change within 35 s can be observed (inset in Fig. 4b, Supporting Information Movie S1). A switching temperature of 39°C occurs to our device. The vertical axis in Fig. 4b represents the time needed for the completion of color-change by applying 20 V in a single run. Remarkably, no noticeable deterioration can be observed even after repetitive tests of one 7

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robustness are the two key advantages of our graphene defoggers. Furthermore, our directly grown graphene glasses were utilized as transparent heating electrodes for the construction of thermal-induced, light modulating windows. The typical windows are normally composed of electrochromic or thermochromic materials that are sealed between two transparent conductive films (TCFs), such as indium tin oxide (ITO). In our experiments, both the conventional ITO TCFs were replaced by the directly grown graphene glass plates (4 cm × 6 cm in size for one piece). The fabrication procedure of our prototype window is detailed in Methods. When kept at ambient temperature (23°C), the as-fabricated window presents a variable optical transmittance during bleached/interval/colored final cycles (Fig. 4d), changing from T550 = 76.3%, T550 = 51.5%, to T550 = 0.4%, respectively. In a cycle-running manner, stable coloration/bleaching processes can also be realized. The photographs (inset in Fig. 4d) present the corresponding states of our light modulating window, clearly revealing a homogeneous optical modulation process.

hundred cycles (Fig. 4b, input voltage: 20 V). Our directly grown graphene glass can also be integrated into high-performance defoggers. By employing a 2 cm × 2 cm defogger, we have measured the complete time for defogging under different input voltages (all below the safe voltage of 36 V). When the input voltage was set at 30 V, the complete defogging time was 21 s (Fig. 4c), superior to that of other graphene-based defoggers43,44 (e.g. needing 30 s under 60 V). Meanwhile, no discernable conductivity loss during the measurement is observed. To further investigate the defogging robustness, the defogging behavior of transferred graphene (TG) was also introduced to compare with the directly grown graphene (DG). As depicted in the inset of Fig. 4c, the TG-based defogger shows a far more degradation during defogging (18s to 57s) than that of the DG-based defogger after an ultrasonication treatment for 2 min. The outstanding robustness of our DG defogger is attributed to the good adhesion between the directly-grown graphene and glass substrate. Consequently, a fast defogging response under relatively low input voltages and excellent defogging

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Figure 4. Applications of as-grown graphene glasses as heating devices. (a) Structural schematic of a directly-grown graphene glass heater. Silver paste or copper electrodes were deposited on the edges of the sample. (b) Cycling performance of the thermochromic display made with graphene/quartz glass (4 cm × 6 cm; T550 = 85.7%) under an input voltage of 20 V, showing a stable thermochromic behavior over a hundred runs; the inset presents photographs of the display before and after applying voltage. (c) Fog removal time and defogger resistance as a function of input voltage; the inset shows the different defogging behaviors of the DG-based and TG-based defoggers before and after experiencing 2 min ultrasonication treatment. (d) Optical transmittance spectra of initial, bleached, interval and colored final states of the light modulating window; the inset shows photographs of the window in initial, interval and colored final states with a background featuring the West Gate of Peking University. photocatalysis systems with the presence of graphene45,46. We have also explored the performances of our graphene photocatalytic plates by employing other visible-light-responsive photocatalysts (BiVO4, Bi2WO6) and found the same trend. For practical usage, we have examined the photocatalytic repeatability and stability of our graphene glass based photocatalytic plates. The degradation efficiency of RhB can maintain at 96% during each cycle (at the duration of 3 h). It is worth-noting that after five cycling runs, the photocatalytic capacity of our plate has exhibited no significant loss (Fig. 5c). Moreover, our graphene glasses also show great promise in constructing energy-saving smart windows when combined with thermochromic VO2 coatings47 (Fig. 5d). To achieve this, VO2 thin films were magnetron sputtered onto 2 cm × 2 cm graphene glass substrates. Corresponding Raman features of our VO2 coated graphene glasses show several peaks at 194, 226, 262, 310, 342, 389, 445 and 615 cm-1 (Fig. 5e inset), which correspond to the characteristic vibration modes of monoclinic VO248. To gain insight into the thermochromic properties, we performed temperature-dependent measurements of optical transmittance of the samples (Fig. 5e). The main difference in transmittance in the infrared (800-2500 nm) range at room temperature (25°C) and at a higher temperature (90°C) can be explained by the phase transition of VO2 films (undergoing monoclinic to rutile phase transition at a theoretical transition temperature of 68°C). Fig. 5f presents the temperature-dependent transmission (at the wavelength of 2000 nm) and thermal hysteresis loops of

To further demonstrate the great application potentials of transparent graphene glasses, we have attempted to fabricate prototype devices such as photocatalytic plates and energy-saving smart windowpanes. In such devices, the directly grown, highly uniform graphene glasses serve as suitable platforms for constructing hybrid thin films, rendering the functions and performances of devices better than those based on bare glass. By integrating a visible-light-responsive photocatalyst BiOF with graphene glasses, we were able to construct photocatalytic plates that can be used for dye wastewater treatment (Fig. 5a for schematic illustration and Fig. S7 for detailed synthetic route). The performance of our device was evaluated by the photocatalytic degradation of Rhodamine B (RhB) under natural sunlight irradiation. Notably, comparative investigations of the photocatalytic performances were carried out amongst our chemically coated BiOF/graphene glasses, physically coated BiOF/graphene glasses, physically coated BiOF/pure glasses, pure graphene glasses, and pure glasses. It is noted from Fig. 5b that, under natural sunlight irradiations (duration: 180 min), the degradation efficiency of RhB by our chemically-coated BiOF/graphene glasses reaches at 96.2% (red curve), superior to that achieved by physically coated graphene glass samples (59.5%, blue curve), whilst the BiOF samples (without the presence of graphene) even displays poor degradation performance (38.3%, pink curve). Such enhanced visible-light-driven photocatalytic activity of graphene glass-based plates may be attributed to an effective suppression of electron-hole recombination rates within the 9

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VO2/graphene glass (red curve) and VO2/bare glass (black curve). It can be found that, through heating and cooling cycles, both samples display thermochromic behaviors by undergoing reversible phase transitions. However, graphene glass-supported VO2 samples show two different transition temperatures of Theating = 70.4°C on heating and Tcooling = 53.6°C on cooling, realizing the critical transition temperature Tc = 62°C (Tc = (Theating + Tcooling)/2), which is lower than the

measured Tc for VO2/bare glass (~64°C). Such reduced transition temperature reveals the unique function of graphene platform, and might be attributed to the surface strain effect due to the distinct thermal expansion coefficients between graphene (negative) and VO2 (positive)49. These results suggest that our graphene glass-supported VO2 film could be readily applicable to a new type of energy-saving smart windows.

Figure 5. Applications of as-grown graphene glasses in photocatalytic plates and energy-saving smart windows. (a) Schematic of graphene glass based photocatalytic plates for degradation of dye wastewater in a recyclable manner. (b) Comparison of the degradation efficiencies of RhB-contained wastewater by using chemically coated BiOF/graphene glass (red), physically coated BiOF/graphene glass (blue), physically coated BiOF/pure glass (pink), pure graphene glass (green), and pure glass (black) as photocatalysts under natural sunlight irradiation; the inset displays a photograph of prototype reactors after photo-degradation by employing the marked catalysts. (c) Cycling photo-degradation of RhB-contained solution using chemically coated BiOF/graphene glass plates. (d) Schematic of energy-saving window based on VO2/graphene glass. (e) Transmission spectrum of VO2/graphene glass samples taken at 25 and 90°C, respectively, displaying a prominent thermochromic behavior; the inset shows the typical Raman spectrum of VO2 film on graphene glasses. (f) Temperature-dependent optical transmission (at λ = 2000 nm) of VO2/graphene glasses and VO2/bare glasses. In summary, by using catalyst-free APCVD approach, we have demonstrated for the first time the direct well-controlled growth of high-quality graphene on insulating solid glasses. Large area, uniform graphene films have been achieved on high-temperature resistant glasses, giving fascinating graphene glasses. We found that, the optical

transparency and sheet resistance of such kinds of graphene glasses can be easily tuned together with the experimentally-tunable layer thickness of graphene. Such directly-grown graphene glasses are readily applicable to a wide range of applications in heating devices, low-cost electrodes, photocatalytic plates and smart windows. In essence, the present work 10

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demonstrates the scalable production route of graphene glasses and their great potential to benefit novel daily-life applications.

measuring meter (Guangzhou 4-probe Tech Co. Ltd., RTS-4) based on four-point probes method to eliminate contact resistance. Four metal probes were aligned in a line at intervals of 1 mm. The outer pair probe was current-carrying electrode and the inner pair probe was voltage-sensing electrode. For the temperature measurements of graphene-based heaters, a K-type thermocouple with a digital thermometer (HH11B, Omega) was employed. The temperature was kept constant during the measurements. FET device fabrication and measurement. Back-gated graphene Hall-bar FET devices were fabricated by standard photolithography technique on silicon substrates with 300 nm SiO2 as the gate insulator. Graphene samples were prepared by transferring, Cr/Au source-drain electrodes (5/50 nm thick) were designed on the graphene sheets by thermal evaporation. To improve the contact resistance, post-annealing was performed for 1 h in an inert Ar atmosphere at 200°C. The electrical measurements of FET device were carried out in air at room temperature with the aid of an MM6200-Keithley4200 semiconductor characterization system. Prototype light modulating window fabrication. A hydrogel containing hydroxypropyl methyl cellulose (HPMC), sodium chloride, and pure water with a mixed proportion (2/5/100 w/w/w) was used as the thermochromic layer. Such hydrogel is non-toxic, cost-effective, and possesses a reversible switching temperature at 33-35°C (water-clear state at low temperature, whilst paper-white state at high temperature). A spacer was used at the sealing part to guarantee the thickness of the hydrogel layer to be about 1 mm. Silver paste in conjunction with copper foil electrodes was applied on the edges of the graphene glass plates. Photocatalytic experiment. Photocatalytic reactions were carried out in a 20 mL borosilicate photochemical batch reactor and 1 cm × 2 cm sized quartz graphene glass or bare glass samples were used. In each experiment, photocatalytic plate was placed into 10 mL RhB aqueous solution (5 mg·L-1). No pH adjustment was used during the entire course of the degradation process. During each photocatalytic run, a

Methods CVD growth of large-area uniform graphene films directly on different solid glasses. The graphene films were directly grown on different high temperature resistant glass substrates using a catalyst-free APCVD method. In a typical CVD process, quartz glass, sapphire glass and borosilicate glass were thoroughly cleaned with deionized water, acetone, and ethanol before loaded into a horizontal quartz tube (1 or 3-in.-diameter) placed inside a three-zone high-temperature furnace. The CVD system was flushed with 500 sccm Ar to remove air prior to ramping the temperature. The furnace was then heated to the desired growth temperature and stabilized for about 10 min. Typical growth conditions (for the case of employing 3-in.-diameter tube) were optimized with a gas mixture of 500 sccm Ar, 50 sccm H2, and 15.5-26.5 sccm CH4, growth temperatures of 1000-1100°C and growth times of 1-7 h. The thickness and uniformity of graphene samples can be controlled by varying the growth time, the flow rate of gases or altering the growth temperature. Characterizations. The prepared samples were systematically characterized using optical microscopy (Olympus DX51), SEM (Hitachi S-4800; operating at 1 kV), Raman spectroscopy (Horiba, LabRAM HR-800; 514 nm laser excitation), UV-Vis spectroscopy (Perkin-Elmer Lambda 950 spectrophotometer), AFM (Vecco Nanoscope IIIa; working at a tapping mode), XPS (Kratos Analytical Axis-Ultra spectrometer using a monochromatic Al Kα X-Ray source), and TEM (FET Tecnai F20; operating at 200 kV). A Cu Quantifoil TEM grid was used for TEM characterization, onto which graphene film was transferred with the aid of poly-methylmethacrylate coating. The atomically-resolved TEM investigations were performed on a third-order aberration corrected (objective lens) FEI Titan3 300-80 operating with an acceleration voltage of 80 kV. The sheet resistances of the films were measured using a four-probe resistance 11

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certain amount of the suspension was collected at predetermined time intervals for analysis. The concentration of RhB was analyzed by measuring the absorption intensity at its maximum absorbance wavelength of 553 nm using a UV-Vis spectrophotometer (UV-1700, SHIMADU) with a spectrometric quartz cell, and was calculated from the calibration curve. The degradation efficiency of the RhB dye wastewater was determined according to the recipe published in previous report50-52.

Haigh, S. J.; Geim, A. K.; Tartakovskii, A. I.; Novoselov, K. S. Nat. Mater. 2015, 14, 301-306. (2) Novoselov, K. S.; Fal'ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. Nature 2012, 490, 192-200. (3) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197-200. (4) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666-669. (5) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Proc.

ASSOCIATED CONTENT Supporting Information

Natl. Acad. Sci. USA 2005, 102, 10451-10453.

Elemental and structural characterization of graphene glass, investigation on the graphene growth evolution by Raman spectroscopy, schematic illustration of catalyst-free growth process, and supporting figures and movie. This material is available free of charge via the Internet at http://pubs.acs.org.

Nanotechnol. 2008, 3, 270-274.

(6) Eda, G.; Fanchini, G.; Chhowalla, M. Nat. (7) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558-1565. (8) Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, 3394-3398. (9) Paton, K. R.; Varrla, E.; Backes, C.; Smith, R. J.;

AUTHOR INFORMATION Corresponding Authors

Khan, U.; O'Neill, A.; Boland, C.; Lotya, M.; Istrate, O.

*E-mail: [email protected]. *E-mail: [email protected].

Puczkarski, P.; Ahmed, I.; Moebius, M.; Pettersson, H.;

Notes

Sanchez, B. M.; Duesberg, G. S.; McEvoy, N.;

The authors declare no competing financial interest.

Pennycook, T. J.; Downing, C.; Crossley, A.; Nicolosi, V.;

M.; King, P.; Higgins, T.; Barwich, S.; May, P.; Long, E.; Coelho, J.; O'Brien, S. E.; McGuire, E. K.;

Coleman, J. N. Nat. Mater. 2014, 13, 624-630.

ACKNOWLEDGMENTS

(10) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.;

This work was financially supported by the Ministry of Science and Technology of China (Grants 2013CB932603, 2012CB933404, 2011CB921903, 2013CB934600), the National Natural Science Foundation of China (Grants 51432002, 51290272, 51121091, 51222201, 11222434), the Ministry of Education (20120001130010) and the Beijing Municipal Science and Technology Planning Project (Z151100003315013). AB and MHR acknowledge the Sino-German Center for Research Promotion (Grants GZ 871).

Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun'ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. Nat. Nanotechnol. 2008, 3, 563-568. (11) Hao, Y.; Bharathi, M. S.; Wang, L.; Liu, Y.; Chen, H.; Nie, S.; Wang, X.; Chou, H.; Tan, C.; FaIlahazad, B.; Ramanarayan, H.; Magnuson, C. W.; Tutuc, E.; Yakobson, B. I.; McCarty, K. F.; Zhang, Y.-W.; Kim, P.; Hone, J.; Colombo, L.; Ruoff, R. S. Science 2013, 342, 720-723. (12) Teng, P.-Y.; Lu, C.-C.; Akiyama-Hasegawa, K.; Lin, Y.-C.; Yeh, C.-H.; Suenaga, K.; Chiu, P.-W. Nano Lett.

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