Highly Conductive Nitrogen-Doped Graphene Grown on Glass toward

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Functional Nanostructured Materials (including low-D carbon)

Highly Conductive Nitrogen-doped Graphene Grown on Glass Towards Electrochromic Applications Lingzhi Cui, Xudong Chen, Bingzhi Liu, Ke Chen, Zhaolong Chen, Yue Qi, Huanhuan Xie, Fan Zhou, Mark H Rümmeli, Yanfeng Zhang, and Zhongfan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11579 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on September 2, 2018

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ACS Applied Materials & Interfaces

Highly Conductive Nitrogen-doped Graphene Grown on Glass Towards Electrochromic Applications Lingzhi Cui, 1Xudong Chen, 1 Bingzhi Liu, 1 Ke Chen, 1Zhaolong Chen, 1Yue Qi, 1Huanhuan Xie, 1

Fan Zhou, 1Mark H. Rümmeli, 4Yanfeng Zhang, *,1,2,3 and Zhongfan Liu*,1,3

1 Center for Nanochemistry (CNC), Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, People’s Republic of China 2 Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China 3 Beijing Graphene Institute, Beijing 100091, People’s Republic of China 4 Soochow Institute for Energy and Materials Innovations (SIEMIS), Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou 215006, P.R. China. KEYWORDS: chemical vapor deposition, nitrogen-doped graphene, direct growth, graphene glass, electrochromic windows

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ABSTRACT: The direct synthesis of low sheet resistance graphene on glass can promote the applications of such intriguing hybrid materials in transparent electronics and energy related fields. Chemical doping is efficient for tailoring the carrier concentration and the electronic properties of graphene that previously derived from metal substrates. Herein, we report the direct synthesis of 5-inch uniform nitrogen-doped (N-doped) graphene on the quartz glass, through a designed lowpressure chemical vapor deposition (LPCVD) route. Ethanol and methylamine were selected respectively as precursor and dopant for acquiring predominantly graphitic-N doped graphene. We reveal that, by a precise control of growth temperature and thus the doping level, the sheet resistance of graphene on glass can be as low as one half that of non-doped graphene, accompanied with relative high crystal quality and transparency. Significantly, we demonstrate that, this scalable, 5-inch uniform N-doped graphene glass can serve as excellent electrode materials for fabricating high performance electrochromic smart windows, featured with a much simplified device structure. This work should pave ways for the direct synthesis and application of the new type graphenebased hybrid material.

INTRODUCTION Graphene, as the first achieved two dimensional material, has attracted tremendous attention because of its superior properties1-5 and versatile applications in transparent electrodes,6 supercapacitors,7 energy storages,8,9 etc. A facile chemical vapor deposition (CVD) route has been developed to achieve large-area uniform graphene films on metal foils.10-12 However, most of the related applications, e.g. transparent electrodes, rely on a complicated transfer process for graphene from the growth substrates to targeting substrates, which customarily introduces


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breakage, wrinkle and contamination in the derived graphene, and affords obvious device performance degradations.6,13 Accordingly, the direct growth of graphene on insulating substrates, such as SiO2,14 Al2O3,15,16 SrTiO3,17 h-BN18 and glass,19-22 have been progressively pursued for their compatibility with the direct construction of high-performance electronic devices or other applications. Particularly, our group have accomplished the direct syntheses of graphene on various solid glass by atmosphericpressure CVD (APCVD) routes, and developed wide range applications based on unusual electrical and thermal properties of grapheme glass.19 Particularly, on high temperature resistance quartz glass, an ethanol-precursor-based LPCVD route was exploited for the fast growth of 25inch uniform graphene on glass, and its application in liquid-crystal-based smart windows was also demonstrated.20 Nevertheless, due to the poor catalytic property of glass and the limited migration of carbon species on it, high density of grain boundaries and defects always appeared to induce relative high sheet resistances of the derived graphene on glass, thus greatly impeded their direct applications.23,24 According to the Drude formula,25 the correlation between conductivity (σ), carrier mobility (μ), and carrier concentration (n) is as follows, σ = neμ. Notably, decreasing the defect density, especially reducing the domain boundaries or increasing the domain size of graphene should be effective for improving the carrier mobility and consequently the conductivity.26 Various metalassisted growth methods, e.g., coated-metallic-film-assisted (Ni, Cu) routes,27-30 remotely placed metal-catalyst (Cu, Ga) assisted routes,31-33 have been developed to grow graphene on insulating substrates (e.g., SiO2, SiO2/Si, quartz, h-BN, Al2O3) with variable domains sizes and layer thicknesses. Particularly, Tour’s group proposed a transfer free method to grow large-area uniform bilayer graphene on insulating substrates by depositing solid carbon feed stocks underneath Ni


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layers.27 Choi et al suspended Cu foils above target substrates (quartz, h-BN, SiO2/Si) to catalyze the pyrolysis of methane, achieving almost defect-free single layer graphene.31 However, the subsequent catalysts removal treatments are still very complicated, and the trace residual metals always afford unavoidable contaminations and accidental damages to the derived graphene. Chemical doping (especially, N-doping) was reported to enhance the carrier concentration of graphene, and such doped graphene was customarily synthesized on metal substrates through the facile thermal CVD method, by introducing nitrogen-containing carbon precursors.34-38 Different from the metal-catalyzed CVD routes, the one-step doping and growth of graphene on metal-free insulating substrates is still challenging. Various modified CVD growth routes have been developed to aid the cracking of distinct N-containing precursors (e.g., ammonia), such as rapid heating plasma CVD (RH-CVD) on SiO2/Si with deposited Ni films,39 low-temperature plasma enhanced CVD (PECVD) on various dielectrics.40 Nonetheless, these growth pathways needs complex equipment and delicate operation processes, and various doping types (pyridinic/pyrrolic- /graphitic- N) usually coexist and sometimes decrease the carrier mobility, since only graphitic-N doping is effective to preserve the high carrier mobility of graphene.41,42 In this work, we propose a new and facile method to enhance the conductivity of graphene directly grown on glass by N-substitutional doping, which introduces almost no contaminations and damages to the derived graphene. As known that, for graphitic-N or quaternary doping, N substitutes C atoms in the hexagonal ring and contributes only one n electron to the π system. Therefore, graphitic-N induces minor distortion to the graphene lattice, leading to perfect structural coherence and high carrier mobility of the N-doped graphene. In this unique system, the liquid phase ethanol, and gaseous methylamine which can dissolve in ethanol, were selected as carbon precursor and N-dopant, respectively. Notably, methylamine can easily crack into small N-


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containing molecules (e.g., ammonia) and induce graphitic-N doping to graphene. A LPCVD route was also chose to ensure the large-scale uniformity (5-inch scale), as well as the fast growth of Ndoped graphene on glass by using the ethanol precursor, which is essential to industrial production. The unique N-doping type (graphitic-N) was confirmed by various characterization methods, and its concentration variation with growth temperature was also established to find a suitable growth parameter. Considering the combined low sheet resistance and high transparency, the N-doped graphene on glass hybrids were then directly employed as perfect platforms/electrodes for constructing high-performance electrochromic smart windows, which is featured with a muchsimplified device construction. RESULTS AND DISCUSSION The precursors of liquid phase ethanol (carbon source), and gaseous methylamine (nitrogen source) which can dissolve in ethanol, were separately stored in two stainless steel tanks and pumped into the CVD furnace by the low pressure condition. The flow rates of vaporized ethanol and methylamine were precisely controlled by mass-flow controllers. Figure 1a schematically demonstrates the surface reactions in the LPCVD growth of N-doped graphene on the quartz glass surface. Four main steps can be involved, step 1: ethanol and methylamine thermal decompose into active carbon and nitrogen species, respectively; step 2: active carbon and nitrogen species absorb and desorb on the glass surface; step 3: nucleation happen at the preferred sites under a critical nucleation concentration; step 4: the evolved nucleus/domains prefer to attach C and N atoms at the edges leading to expanded domain sizes, and these domains finally merge with each other to form continuous N-doped graphene films. This growth process is proposed since at relatively high growth temperature (>800 °C),43 ethanol effectively cracks into several types carbon species such as ethylene, acetylene, and CHx (x≤4), and methylamine effectively ACS Paragon Plus Environment

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dissociates into ammonia, methane, hydrogen cyanide, hydrogen, nitrogen, etc., at temperature higher than 600 °C.44 Consequently, sufficient amounts of active carbon and nitrogen species can be generated under the current growth condition, ensuring the growth of N-doped graphene. Moreover, the low-pressure growth condition allows large-scale uniform feeding of the active species, which guarantees further scaling up of the sample size, now only limited by the size of the CVD furnace. The photographs of quartz glass after LPCVD graphene growth processes are displayed in Figure 1b, by using ethanol plus methylamine (550: 150 sccm) and pure ethanol (550 sccm) precursors at 1040 °C for 8.25 h, respectively, under the same Ar: H2 flow rate of 200: 700 sccm. Herein, sheet resistance was selected as an initial parameter for comparing the overall properties of different samples. Under the above-mentioned conditions, the same sheet resistance (Rs)~1.1 kΩ was achieved for the N-doped and non-doped graphene, followed with different T550 (transparency measured at 550 nm) of 93% and 77%, respectively. The corresponding thicknesses of the related samples are estimated to be bilayers and few layers. Obviously, the N-doped sample seems to possess a more balanced coordination between sheet resistance and transparency, a basic premise for applications in transparent electronic devices. The X-ray photoelectron spectroscopy (XPS) (Figure S1 and Figure 1c) data was then captured to confirm the presence of nitrogen in the N-doped graphene (as shown in Figure 1b). Notably, the C 1s line scan spectrum exhibits the graphite-like sp2 hybridized carbon signal at 284.8 eV, the same as that of the pristine graphene (Figure S1, b).44 Specifically, two peaks located at ~285.8 eV and 287.5 eV appear to be in line with the formation of N-sp2 C bond and N-sp3 C bond, respectively, as similarly reported for N-doped graphene on Si using methane and ammonia as precursors.47 Another two broad peaks located at 287.3 eV and 289.6 eV were attributed to C-O


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bond and C=O bond, respectively (Figure S1, c).22 In pristine graphene, the faint peak at 285.3 eV corresponds to C-H bond.25 Moreover, the characteristic doping-related peak occurs at 401.5 eV (Figure 1c), indicating that the N atoms are bonded mainly in “graphitic” N form, namely substituting C atoms in the hexagonal rings in the graphene lattice.45 In contrast, for the non-doped sample, no obvious peak appears around the binding energy of 401.5 eV. The N atomic percentage (1.7%) is also estimated by the area of N and sp2 C peaks in the XPS spectrum. The representative Raman spectrum of the doped sample (as shown in Figure 1b) is also presented in Figure 1d, wherein the intensity ratio of the 2D/G band is ~1, suggesting the formation of highquality bilayer graphene. As a reference, the Raman spectrum of the non-doped bilayer graphene on glass sample is also shown in Figure 1d. Particularly, the spectrum presents a distinct D' band, which customarily arises from the defect-induced intravalley double resonance scattering process.47 Moreover, a narrow full-width half-maximum (FWHM) value (~26.81 cm-1) also occurs for the G band (at ~1585 cm-1) of the N-doped graphene, whereas a FWHM value of ~29.97 cm-1 appears for G band (at ~1597 cm-1) for the non-doped graphene by Lorentz fitting. Moreover, 2D band (at ~2703 cm-1) for the N-doped graphene shows a light upshift when compared to 2D band (at ~2695 cm-1) for the non-doped graphene. Altogether, the downshift and stiffening of G band are similar with those mentioned in published references for N-doped graphene.45,46 This comparative Raman data confirm the N-doping behavior of the one-pot CVD-derived graphene by using ethanol plus methylamine precursors. The large-scale uniformity and stability of the N-doped graphene grown on quartz glass (6 cm × 10 cm in size, as shown in Figure 1b) were then evaluated by sheet resistance mapping using the macroscopic four-probe technique. As displayed in Figure 1e, a narrow sheet resistance distribution can be noticed from the statistical plot showing an average value 1.1 kΩ•sq-1, as well


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as from its 2D mapping image featured with a uniform green color (Figure 1e inset, collected from 144 points). Moreover, the corresponding Raman mapping image based on the intensity ratio of I2D/IG is almost constant at an average value of ~1, shown in Figure 1f for the N-doped graphene on glass. The homogenous color contrast again justifies its thickness and quality uniformity.

Figure 1. LPCVD growth of large-area uniform N-doped graphene on quartz glass. (a) Schematic diagram of the growth route by using methylamine and ethanol as precursors. (b) Photograph of the N-doped graphene/quartz glass (left, 6 cm × 10 cm) and pristine graphene/quartz glass samples (right, 6 cm × 10 cm) achieved by the LPCVD method, respectively. Growth conditions: Ar/H2/ethanol/methylamine:

200/700/550/150

sccm,

Ar/H2/ethanol:

200/700/550

sccm,

respectively, both under 470 Pa and at 1040 °C for 8.25 h. The same sheet resistance ~1.1 kΩ but different T550 (~93% for N-doped and ~77% for pristine graphene glass, respectively) was attained, respectively. (c) Corresponding N 1s core-level XPS spectra of the two samples. A new peak at 401.5 eV occurs for the N-doped graphene, suggesting the formation of graphitic N. The dopant concentration is also calculated to be ~1.7%. (d) Representative Raman spectra of N-doped and pristine bilayer graphene (grown for 8.25 h, 7 h, respectively, with the same T550 ~93%). (e) Narrow


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distribution of the sheet resistances (collected from 121 points) of the as-grown N-doped sample in (b). The inset shows the sheet resistance mapping image. (f) Raman mapping of I2D/IG ratio intensity over the as-grown N-doped graphene on quartz glass. To understand the initial growth behavior of the N-doped graphene, growth-time dependent AFM observations were then performed (Figure 2a-c). After 4h growth, nearly circular graphene islands formed and randomly distributed on the quartz surface, showing the average diagonal length 300 ± 100 nm (Figure 2a). With increasing growth time to 6 h, the islands expanded their sizes up to 500 ± 50 nm and started to merge with each other (Figure 2b). After 7.5 h growth, a nearly full coverage graphene film evolved, along with the formation of some wrinkles (Figure 2c). Moreover, according to the section views of the three typical AFM images, the heights of the graphene islands (indicated with red dashed lines) showed an average value of 0.84 ± 0.02 nm, consistent with the height of monolayer graphene on SiO2/Si substrates.48 The corresponding growth-time dependent SEM images are also presented in Figure S2. In this regard, a predominant monolayer growth behavior can be inferred from the initial nucleation to near full layer growth regimes. For examining the large-scale uniformity, the N-doped graphene mentioned in Figure 1b with a T550 of 93% was also transferred on SiO2/Si. From the typical optical microscope (OM) image shown in Figure 2d, a uniform color contrast can be noticed, which again indicates the large-area uniformity of the N-doped graphene on glass. This uniformly N-doped graphene on quartz sample (T550 ~93%) was also transferred onto the transmission electron microscopy (TEM) grids for specific thickness and atomic-scale structure characterizations. The film thickness can be measured at the edges for the transferred sample, based on the representative high-resolution TEM (HR-TEM) images shown in Figure 2e. Typically, bi-layer mixed with a few three-layer and monolayer regions can be obtained through random


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imaging of the surface, the thickness measured from the edge of the sheet breakage is ~0.34 nm, which is consistent with the interlayer distance of graphene,

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as shown in Figure 2f. More

significantly, high-angle annular dark-field scanning transmission electron microscopy (STEM) elemental mapping (Figure 2g) images regarding the C and N signals present rather uniform intensities all over the N-doped graphene region, highly suggestive the uniform doping behavior of the CVD-derived sample.

Figure 2. AFM and TEM characterizations of transferred N-doped graphene on SiO2/Si and on TEM grids. (a-c) AFM images of transferred N-doped graphene on SiO2/Si, achieved at different growth time 4 h (a), 6 h (b), 7.5 h (c), but with other parameters the same as that of Figure 1b. The bottom insets are the height profiles across the red lines in the corresponding AFM images, showing an average thickness ~0.8 nm. (d) Corresponding OM image of the transferred N-doped graphene onto SiO2/Si. (e) TEM sectional-views of the N-doped graphene mentioned in Figure 1b showing the coexistence of monolayer, bi-layer, and three-layer N-doped graphene. Scale bars:


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2.5 nm. (f) Representative HR-TEM image showing the predominant bi-layer graphene film mixed with a few three-layer and monolayer regions. Scale bar: 2 nm. (g) High-angle annular dark-field STEM elemental mapping of C and N elements in N-doped graphene confirming its uniform doping behavior. In order to clarify the effect of doping concentration on the conductivity of graphene glass, growthtemperature-dependent CVD processes (from 1100 °C to 1000 °C with an interval of 20 °C) were then performed on quartz glass, for achieving same transmittance or same graphene thickness samples. In this regard, N-doping level can be correlated with growth temperature. This series samples were well characterized with XPS regarding the C 1s and N 1s signals (Figure 3a, b). Herein, the C 1s peak can be decomposed into at least two components with the strongest peak located at ~284.8 eV, corresponding to sp2-hybridized carbon atoms in graphene. Whereas, the peak at ~285.8 eV is ascribed to sp2 C atoms bonded with N (C=N), and the peak at 287.5 eV relates to sp3 C atoms bonded with N.45 For the current samples, in the N 1s spectra (Figure 3b), only one characteristic peak centered at ~401.5 eV can be noticed in each growth temperature, while other typical doping signals (e.g. pyridinic- /pyrrolic-N) are almost missing. Accordingly, only graphitic-N is proposed to be introduced in the current CVD-derived graphene on glass according to the published reference.45 The atomic percentages of N (N/C) in the N-doped graphene samples grown at 1100 °C, 1080 °C, 1040 °C, 1020 °C, and 1000 °C are then estimated as 1.16%, 1.25%, 1.7%, 2.5% and 3.5%, respectively. Altogether, this series results indicate that, the doping concentration can be precisely tuned by varying the growth temperature. In this temperature range, graphitic-N is the most stable configuration in the graphene lattice when compared to other configurations. When the growth temperature increases, the nitrogen content decreases because of the instability of C-N bonding


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than C-C bonding. And graphitic-N can survive at a relatively high growth temperature at which the graphitization of carbon is relatively high. For a clearer image, Figure 3c displays the schematics of the pyridinic-N, graphitic-N, pyrrolic-N doping types for graphene. For the graphitic N-doped graphene synthesized in this work, a N atom substitutes a C atom in the hexagonal carbon ring, and thus contributes one n electron to the π system, leading to an increased carrier concentration. Additionally, the graphitic-N induces a minor lattice distortion, thus greatly preserving the high carrier mobility of graphene.41,42 However, the pyridinic N bonds with two C atoms at the graphene edge or defect, and contributes one p electron to the π system. The pyrrolic N doping introduces sp3 hybridization into graphene, forms a five-membered heterocyclic ring and contributes two p electrons to the π system. Notably, the pyridinic-N and pyrrolic-N doping are customarily accompanied with the appearance of bonding disorders or vacancies, leading to highly disordered graphene lattices. More significantly, the doping induced disorders may hinder the lateral growth of graphene with the evolution of abundant domains and domain boundaries. Consequently, the two types doping should be avoided in the growth of graphene on glass towards the achievement of high conductivity. Corresponding Raman spectra of the different samples were also collected to establish one-to-one correspondence between the growth parameter (mainly temperature), doping level and crystal quality (reflected by Raman spectra, as shown in Figure 3d). A good parameter for evaluating the defect density ID/IG, is thus achieved to first decrease from 0.90 to 0.57 (from 1100 °C to 1040 °C) and then increase from 0.57 to 0.79 (from 1040 °C to 1000 °C). Notably, at the growth temperature of ~1040°C, ID/IG shows the lowest value ~0.57, indicating the formation of less defects or a lower N-doping concentration. Correspondingly, the intensity ratio of I2D/IG alters with the similar tendency. However, since 2D and G-band characteristics are highly correlated with the number of


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layers, strains, doping, and defects, etc., it is not suitable for evaluating the doping level in graphene.44 The doping concentration and corresponding sheet resistance are also summarized in Figure 3e as a function of growth temperature. Notably, by adjusting growth temperature from 1000 °C to 1100 °C (with an interval of 20 °C), the N-percentage varies from 3.5%, 2.5%, 1.7%, 1.25%, to 1.16%. Accordingly, the sheet resistances are modulated from 16.6 kΩ, 10.5 kΩ, 1.1 kΩ, 3.5 kΩ to 5.5 kΩ, respectively. In this regard, the graphene glass obtained at 1040 °C with 1.7% N-doping has the best electrical conductivity, which ensures a reasonable crystal quality and a moderate doping level of graphene, as in sharp contrast with the pristine graphene (Rs =3.6 kΩ at the similar transparency). Corresponding FET results are shown in Figure S3 and Table S1. With the increase of nitrogen percentage, the carrier concentration increases and carrier mobility decreases. The flow rate ratio of ethanol and methylamine can also influence the N doping effect. The results show that as the flow rate ratio of methylamine increases within certain range, the N concentration in graphene increases. (Figure S4) In order to address the superior conductivity of the N-doped graphene glass, a comparison of the sheet resistance with the published references was also made, covering the CVD-grown pristine graphene on dielectric substrates with different precursors,18,19 with the assistance of copper vapor catalysts32 and interfacial segregation by nickel.28 In general, increasing graphene layer thickness (reducing the transparency) is usually followed with the decrease of sheet resistance of graphene. More importantly, the overall sheet resistance for the Ndoped graphene is much lower or even comparable with the metal-catalyzed CVD graphene on dielectric substrates (Figure 3f).18,19,28,32


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Figure 3. Doping level modulations and the corresponding conductivity evaluations. (a) XPS C 1s and (b) N 1s spectra of as-grown bilayer N-doped graphene obtained at growth temperature of 1000 °C, 1020 °C, 1040 °C, 1080 °C, 1100 °C, respectively, all with the similar T550 of 93%. Growth conditions: Ar/H2/ethanol/methylamine: 200/700/500/150 sccm, 470 Pa. (c) Schematics of three N-doping modes of pyridinic-N, graphitic-N, and pyrrolic-N doping, respectively. (d) Corresponding Raman spectra of N-doped graphene samples achieved with different growth temperature. (e) Nitrogen content and corresponding sheet resistance plotted as a function of growth temperature. (f) Sheet resistance comparison between this work and the published references. As mentioned above, excellent optical transparency and conductivity are highly desired for fulfilling the wide application potentials of graphene glass. By varying the growth time from 8.25 h, 8.75 h, 9.15 h, to 9.45 h at the optimized growth temperature of 1040 °C, different graphene


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glass samples were achieved to show different graphene thicknesses, along with disparate transparencies and conductivities, as shown in Figure 4a. The thermal stability of graphene on glass should be another important parameter for direct applications. To address this, the stability of CVD-derived N-doped graphene on glass was also carefully compared with that of HNO3-doped and HCl-doped graphene on glass by step-by-step heating the samples to 400 °C and checking the sheet resistance variations. Obviously, the doping routes through post-treatments with acids offered prominent reductions in conductivity with increasing the annealing temperature.6 Specifically, after being heated to 400 °C, the sheet resistance for HNO3 and HCl treated samples became 2.6 and 2 times larger than that of the one-pot CVD-derived N-doped samples synthesized in this work (Figure 4b). In this regard, the N-doped graphene on glass demonstrates superior thermal stability with regard to that of the chemically doped ones. Furthermore, a transparent heating device (Figure 4c) was also fabricated with the N-doped graphene glass, as demonstrated in Figure 1b (size: 6 cm × 10 cm, transmittance: ~93%, sheet resistance: ~1.1 kΩ/sq). Under the input voltage of ~30 V, the N-doped graphene glass shows uniform temperature distribution with an average value 58.3 ± 1.0 °C, as presented in the infrared (IR) image in Figure 4c. This addresses the highly uniform conductivity even in a large scale. Based on these, several advantages of the N-doped graphene glass can be inferred, e.g., reduced sheet resistance at specific transparency or graphene layer thickness, extra high stability with regard to other samples derived from chemical doping routes, and compatibility with large-scale production via the facile one-pot thermal CVD route. Except for thermal stability, the wettability of N-doped graphene glass is another essential property that may afford intriguing applications. This property was then examined by a simple dropping water test on a 6 cm × 10 cm sample, as displayed in Figure 4d (left image), with bare quartz glass


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as a reference (right image). One can clearly notice that, film-like water forms on the bare glass surface, while small water droplets accumulate/stand up on the N-doped graphene glass, highly indicative its hydrophobic behavior. Additionally, the hydrophilicities of thicker N-doped graphene are also measured, as displayed in Figure 4e. Notably, with increasing graphene thickness, the contact angle is enhanced accordingly from 78° to 86° with the transmittance variable from 93% to 58%. However, this N-doped graphene glass is not as hydrophobic as pristine graphene synthesized on quartz glass, the latter presenting a variable contact angle from 83° to 115° under a transmittance range from 91.5% to 61%.19 This can be explained that, N doping moves the Fermi level of graphene upward with regard to the Dirac point, leading to increased electron density of state. Negatively charged graphene tends to attract the hydrogen atoms of water molecules, resulting in stronger interaction between water and graphene, followed with a small contact angle.50 Representative XPS spectra of pristine and N-doped bilayer graphene in Figure S5 showed that the surface oxygen content in N-doped graphene is much larger than that of pristine graphene. Table S2 illustrated the ratio of carbon and oxygen results collected from N-doped graphene grown at different temperatures. In general, with the increase of nitrogen doping content, the surface oxygen content of graphene monotonically increases. And this can serve as an indirect evidence for explaining why N-doped graphene is not as hydrophobic as pristine graphene. This unique hydrophobic property should facilitate its applications in multi-functional, eco-friendly, self-cleaning windows and displays.


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Figure 4. Enhanced conductivity and varied surface properties of N-doped graphene glass. (a) Sheet resistance and UV−vis transmittance spectra in the wavelength range of 250−800 nm of the N-doped graphene on quartz glass obtained through 8.25 h, 8.75 h, 9.15 h, 9.45 h growth, respectively. (b) Changes in the sheet resistance of the as-synthesized N-doped graphene, HNO3doped graphene, and HCl-doped graphene under a step-wise thermal treatment from room temperature to 400°C, leading to intact or varied sheet resistances, respectively. (c) Heating demonstration of the 6 cm × 10 cm N-doped graphene glass under an input voltage of 30V leading to a uniform temperature increase up to ~58.3 °C. (d) Demonstration of the hydrophobic and hydrophilic characteristics of 6 cm × 10 cm N-doped graphene on quartz glass (left) and bare quartz glass (right), respectively. (e) Contact angle versus optical transmittance plot for the asgrown N-doped graphene on quartz glass (for the samples from Figure 4a). Considering the broadband transparency, good electrical conductivity, remarkable stability, the Ndoped graphene on glass hybrids were then directly used as transparent electrodes in electrochromic devices. Such electrochromic devices can realize the reversible change of


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transparency, reflectivity and color by applying a voltage of several volts, thus possessing broad application prospects in energy-saving building and transportation, etc.51 Specifically, in this work, a transparent electrochromic smart window was fabricated using the CVD-derived N-doped graphene glass as transparent electrodes, with poly (3,4-ethyl-enedioxythiophene) –poly (styrene sulphonate), (PEDOT-PSS)

as the electrochromic layer and ACN/PC/PMMA/LiClO4 as

electrolyte, as schematically shown in Figure 5a. In short, an electrochromic polymer layer and an ion conducting layer are sandwiched between two transparent conducting electrodes, in this case, N-doped graphene glass. In order to clearly see the contrast of on and off states, the colorchange region (PEDOT-PSS region) was printed as an array of squares. Figure 5b shows the photos of PEDOT-PSS/N-doped graphene glass (6 cm ×10 cm) based electrochromic devices worked in the “off state” (left) and “on state” (right). Herein, a voltage of −2.0 V was applied to achieved the “on state”. A similar electrochromic device was also constructed only by replacing the electrodes with graphene glass achieved from an ethanol-precursor-based CVD route (1.1 kΩ, 77%). Notably, when the applied voltage changes between +2.0 V and −2.0 V, the UV/Vis transmittance response of the device (measured from the logo region at 550 nm) for the N-doped graphene switches from 80% to 50%, while the other one switches between 55% and 45%. That means, under the similar sheet resistance of graphene glass, N-doped graphene-glass-based electrochromic device presents a larger transmittance variation (30%) between the on/off states than that of the pristine-grapheneglass based device (10%). Moreover, when the potential changes between +2.0 and −2.0 V, the two type devices switch with the same switching time (90% of state change) of 20 s for bleaching and 155 s for coloring, respectively. Except for contrast and switching time, cycle life is another critical standard to evaluate the performance of an electrochromic device.52 As shown in Figure 5e,


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PEDOT/N-doped graphene shows good stability through 500 cycles with only 3% transmittance decay, which highlights its robustness. However, the electrochromic devices with PEDOT presented an optical contrast ~15% initially, and loses almost 10% contrast after 500 cycles.51 In this regard, our N-doped graphene-based device exhibits a larger contrast and a better cycle life than the non-doped graphene glass. Herein, the N-doped graphene on glass can serve as a protection layer, as well as a conductive layer for the construction of structurally simplified, high performance electrochromic windows.

Figure 5. Application of highly conductive N-doped graphene glass in electrochromic windows as protection/conductive layers. (a) Schematic demonstration of the electrochromic device structure. (b) Photographs of the 6 cm × 10 cm N-doped graphene-glass-based electrochromic device in the off (left) and on state (right) by applying a voltage of −2.0 V. The color change region (PEDOTPSS printed region) was printed as an array of squares. The black dot indicates the color change area, where the transmittance at 550 nm was measured. (c) Comparison of the UV/Vis transmittance responses (measured from the logo region) by applying 2.0 V (a, b) and −2.0 V (d,


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c) between the N-doped graphene-glass-based (NG, 1.1 kΩ, 93%) and pristine-graphene-glassbased (PG, 1.1 kΩ, 77%) electrochromic devices. (d) Corresponding transmittance changes for the two electrochromic devices by applying potential steps from 2.0 V to −2.0 V with the same switching time (90% of state change) of 20 s for bleaching and 155 s for coloring (measured at 550 nm), respectively. (e) Cycling test of the electrochromic window made by N-doped grapheneglass under a periodic voltage for 500 cycles. In summary, we have accomplished the direct growth of 5-inch uniform N-doped graphene on glass by a catalyst-free LPCVD approach, by a unique design of methylamine and ethanol precursors. A specific graphite-N doping type is achieved to induce much lower sheet resistance for the derived graphene glass than that of the non-doped one. Particularly, the sheet resistance is even comparable with the metal-catalyzed CVD graphene on dielectric substrates. Thanks to its balanced transparency, conductivity and extra high stability, the graphite-N doped graphene glass was also utilized to fabricate high-performance, structurally simplified electrochromic smart windows, with graphene serving as protection/conductive layers. This work should hereby shed light on the scalable production of highly conductive N-doped graphene glass by a designed onepot CVD route, as well as promote their versatile daily-life related applications in smart windows, optoelectronic devices and energy-related applications.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Materials and method details, XPS data, SEM image, FET curves (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Z.L.). * E-mail: [email protected] (Y.Z.). ORCID: Zhongfan Liu: 0000-0001-7896-7156 Yanfeng Zhang: 0000-0003-1319-3270 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (Nos. 2016YFA0200103), and the National Natural Science Foundation of China (NSFC) (51432002, 5290272, 51520105003, 51861135201), and the Beijing Municipal Science and Technology Commission (No. Z161100002116020). Thanks Xu Zhou from PKU for the help of the FET measurements.


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Highly Conductive Nitrogen-Doped Graphene Grown on Glass toward

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