Defect States Control Effective Band Gap and Photochemistry of

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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 27195−27204

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Defect States Control Effective Band Gap and Photochemistry of Graphene Quantum Dots Mauricio A. Melo, Jr. and Frank E. Osterloh* University of California Davis, One Shields Avenue, Davis, California 95616, United States

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S Supporting Information *

ABSTRACT: Graphene quantum dots (GQDs) have emerged as a new group of quantum-confined semiconductors in recent years, with possible applications as light absorbers, luminescent labels, electrocatalysts, and photoelectrodes for photoelectrochemical water splitting. However, their semiconductor characteristics, such as the effective band gap, majority carrier type, and photochemistry, are obscured by defects in this material. Herein, we use surface photovoltage spectroscopy (SPS) in combination with photoelectrochemical measurements to determine the parameters that are essential to the use of GQDs as next-generation semiconductor devices and photocatalysts. Our results show that ordered GQDs (1−6 nm) behave as p-type semiconductors, based on the positive photovoltage in the SPS measurements on Al, Au, and fluorine-doped tin oxide substrates, and generate mobile charge carriers under excitation of defect states at 1.80 eV and under band gap excitation at 2.62 eV. Chemical reduction with hydrazine removes some defects and increases the effective band gap to 2.92 eV. SPS measurements in the presence of sacrificial electron donor and acceptors show that photochemical charge carriers can be extracted and promote redox reactions. A reduced GQDs photocathode supports an unprecedented photocurrent of 50 μA cm−2 using K3Fe(CN)6 as sacrificial electron acceptor. Additionally, while pristine GQDs do not photoreduce protons under visible light, hydrazine-treated GQDs generate H2 from aqueous methanol under visible and UV light (0.04% quantum efficiency at 375 nm) without added co-catalysts. These findings are relevant to the use of GQDs in photochemical and photovoltaic energy-conversion systems. KEYWORDS: graphene quantum dots, photocatalysis, water splitting, surface photovoltage spectroscopy, p-type semiconductor

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encouraging findings, there is still controversy about the transport properties and energetics of graphene quantum dots. For example, it is not clear if the size-dependent luminescent properties are a result of a discrete band gap modified by a quantum size effect or if they are due to defect states associated with surface groups or distortions of the lattice.8,13,17,19,27−33 Similarly, there are conflicting reports about the majority carrier type in the material. Photoelectrochemical measurements on N-doped GQDs seem to suggest that GQDs are intrinsic semiconductors,15 while photoexcitation studies on GQDs coupled to single crystal TiO2(110) surfaces seem to suggest electrons as majority carriers.23 To shed light on these issues, we employ here a combination of optical, surface photovoltage and photoelectrochemistry techniques to investigate energetics and photochemistry of GQDs. Materials for this study were synthesized from pyrene as precursor via nitration to 1,3,6-trinitropyrene, followed by mild hydrothermal treatment, as described by Wang and collaborators in 2014.11 This method produces graphene quantum dots of high crystallinity and with narrow size

INTRODUCTION After their discovery in 2004,1 graphene quantum dots (GQDs) have attracted increasing interest due to their tunable optical and electronic properties.2−6 GQDs are nanoparticles of 400 nm).

To eliminate defect sites in GQDs, chemical reduction with hydrazine was employed. Recently, hydrazine has been shown to improve the fluorescence quantum yield in GQDs,17,45,46 and it has also been shown to be effective for the removal/ reduction of hydroxyl, carbonyl, carboxyl, and epoxy groups in graphene oxide.47−52 Accordingly, an aqueous GQDs suspension was heated at 100 °C in the presence of 0.2 M hydrazine for 24 h to reduced graphene quantum dots (rGQDs). The product was obtained as a black powder after dialysis, followed by 80 °C drying in air. The spectral properties of rGQDs are displayed in Figure 4. The Fourier transform infrared (FTIR) spectrum of rGQDs (Figure 4A) shows reduced intensity of the band at 3300 cm−1 compared to the untreated GQDs, which is ascribed to the νO−H mode. Furthermore, the band at 1766 cm−1 (νCO) was completely suppressed.53 This 27198

DOI: 10.1021/acsami.8b08331 ACS Appl. Mater. Interfaces 2018, 10, 27195−27204

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ACS Applied Materials & Interfaces

Figure 4. (A) FTIR spectra of graphene quantum dots (black) and reduced graphene quantum dots (blue); (B) Raman spectrum, (C) X-ray diffraction pattern, (D) UV−vis optical spectrum, and (E) photoluminescence spectra of rGQDs at variable excitation wavelengths. (F) Suspension of rGQDs in water (0.5 g L−1) illuminated by UV light (365 nm wavelength) and polychromatic visible light (inset).

cause for the observed photovoltage decrease.40,43 As a result of the hydrazine treatment, the rGQDs also displayed decreased solubility in water. This was attributed to the loss of oxygen groups on the GDQs surface. The increased hydrophobicity made it possible to fabricate a GQDs electrode that is stable in aqueous electrolyte. An anodic dark scan of that electrode is shown in Figure S7A. An anodic current occurs at positive potentials >1.2 V vs NHE that can be attributed to the oxidation of the rGQDs. Using the tangential method, the onset of the anodic current (1.5 V vs NHE) is extracted. This value is taken as the valence band edge of the

potassium hexacyanoferrate(III) can accept photogenerated electrons from the rGQDs, increasing the hole concentration that is able to reach the FTO substrate.41 On the basis of the energy diagram in Figure 6, the MV2+ and the hexacyanoferrate(III) reduction potentials are −0.45 and −0.37 V vs normal hydrogen electrode (NHE), respectively. The larger photovoltage increase for hexacyanoferrate(III) agrees well with the stronger oxidizing power of this reagent.41,54−56 Methanol is a sacrificial electron donor, and the decrease of the photovoltage confirms that photoholes from rGQDs can oxidize methanol, which is the 27199

DOI: 10.1021/acsami.8b08331 ACS Appl. Mater. Interfaces 2018, 10, 27195−27204

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Figure 5. (A) SPS images recorded on films of the GQDs and rGQDs, prepared on FTO substrates from 2 g L−1 aqueous solutions. (B−D) SPS spectra of films of rGQDs on FTO treated with two drops of (B) aqueous 0.01 mol L−1 potassium hexacyanoferrate(III), (C) 0.01 mol L−1 methyl viologen dichloride, and (D) methanol.

Figure 6. Energy diagram correlating the energy levels of GQDs and rGQDs with the electrochemical redox potentials of the potassium hexacyanoferrate(III)/hexacyanoferrate(II) and MV2+/MV•+ redox pairs. Potentials for methanol oxidation and the FTO work function were obtained from the literature.41,54−56

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DOI: 10.1021/acsami.8b08331 ACS Appl. Mater. Interfaces 2018, 10, 27195−27204

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Figure 7. Chopped-light photocurrent−potential curve of rGQDs cathode on FTO substrate (A) in a 0.5 mol L−1 K2SO4 electrolyte and (B) with addition of 0.1 mol L−1 potassium hexacyanoferrate(III) as sacrificial agent under visible light (100 mW cm−2 and λ > 400 nm).

of the dots, chemical modification, or the addition of cocatalysts.59

material. Using the effective band gap of 2.92 eV from SPS and optical spectroscopy, the conduction band of the rGQDs is estimated as −1.42 V vs NHE. These values are summarized in the energy diagram in Figure 6 together with the defect energy levels established from the SPS and the optical data for both GQDs and rGQDs. It can be seen that the band gap of the asprepared dots is obscured by defect states near the band edges. Hydrazine treatment removes some of these states, which increases the potential energy of the photogenerated electron− hole pairs. Photoelectrochemical scans from this electrode in the absence and presence of potassium hexacyanoferrate(III) as sacrificial agent are shown in Figures 7 and S7B. In aqueous K2SO4 electrolyte, a small (2 μA) cathodic photocurrent is seen at potentials negative of −0.8 V vs NHE. This photocurrent is likely associated with the surface reduction of the GQDs, as seen in the SPS experiments. Figure 7B shows the chopped light scan in the presence of potassium hexacyanoferrate(III) as a fast electron acceptor. The much larger value of the photocurrent (50 μA cm−2 at −0.8 V vs NHE) indicates fast photoreduction of hexacyanoferrate(III). This confirms that the rGQDs are able to generate mobile photoelectrons that can participate in reactions. The photoonset potential of the cathodic scan (Figure S7B) makes it possible to estimate the quasi-Fermi level of the GQDs as +0.27 V vs NHE. The position of this level below the midgap position is consistent with the p-type character of the material that also follows from the SPS data above. Considering the improvements in the photochemistry of the reduced GQDs, the visible light-driven hydrogen evolution test was repeated under the same conditions as before, using methanol as a sacrificial donor (Figure 3). While no hydrogen was detected within 6 h for the pristine quantum dots, the rGQDs supported stable H2 evolution at approximately 1 μmol h−1 and without notable decay after 78 h (Figure S8). Repeat experiments under monochromatic illumination (Figure S9) show that H2 is produced at 0.04% (375 nm). However, the photochemical activity of the reduced GQDs remains 2 orders of magnitude below the activity of inorganic photocatalysts.38,57,58 This shows that although treatment of the dots with hydrazine improves the photochemistry, the performance of this photocatalyst is still limited by defect recombination. This limitation may be overcome through alternative syntheses

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CONCLUSIONS In summary, we have used a combination of surface photovoltage and optical spectroscopy to observe the electronic structure and carrier dynamics in thin films of graphene quantum dots. As-prepared GQDs are p-type semiconductors with an effective band gap of 2.62 eV, whose ability to donate photogenerated charge carriers is limited by defect states near the band edges. Reaction of the dots with hydrazine eliminates near band-edge defects and widens the effective band gap of the material to 2.92 eV. These reduced dots photoreduce methyl viologen cation and ferricyanide ion under visible light illumination and they photooxide methanol. The reduced dots also support photochemical hydrogen evolution under visible light with up to 0.04% quantum efficiency at 375 nm without the need for a co-catalyst. The ability to control the electronic properties of GQDs through chemical modifications is relevant to applications of the material in solar energy conversion and electronics.

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EXPERIMENTAL SECTION

Chemicals. Pyrene (98%, Acros Organics), analytical-grade nitric acid (70%, Sigma-Aldrich), sodium hydroxide (97.0%, EM Science), and hydrazine monohydrate (98%, Sigma-Aldrich) were of reagent grade and used as purchased. The same applies to the use of potassium sulfate (99.0%, EMD), methyl viologen dichloride hydrate (98%, Acros Organics), potassium hexacyanoferrate(III) (99%, Sigma-Aldrich), and methanol (99.9%, Fisher Scientific). The water used in the syntheses and photocatalytic tests was purified to 18 MΩ cm resistivity using a Nanopure system. Syntheses of Graphene Quantum Dots (GQDs). The synthesis of pure graphene quantum dots was carried out by following the methods published by Takahashi et al.60 and Wang et al.11 First, 26.7 mL (0.42 mol) of concentrated nitric acid was slowly added to a 100 mL two-neck round-bottom flask containing 0.3329 g (1.65 mmol) of pyrene. This system was kept at 80 °C under reflux and stirring for 12 h. After cooling down to room temperature, the yellow suspension was diluted to 1 L, by pouring it into a beaker containing deionized water. The resulting yellow precipitate was filtered through a 0.22 μm poly(vinylidene fluoride) (PVDF) microporous membrane and washed with three fractions of 20 mL of pure deionized water to remove the excess of acid. A mass of 0.5496 g of 1,3,6-trinitropyrene was obtained, which corresponds to a yield of 99%. 27201

DOI: 10.1021/acsami.8b08331 ACS Appl. Mater. Interfaces 2018, 10, 27195−27204

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ACS Applied Materials & Interfaces In a second step, 1,3,6-trinitropyrene was dispersed in 100 mL of a 0.2 mol L−1 aqueous sodium hydroxide solution and sonicated for 2 h. The suspension was transferred to a 130 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 10 h. After cooling down to room temperature, the suspension containing the water-soluble graphene quantum dots was filtered through the 0.22 μm PVDF membrane to remove insoluble side products. The filtrate containing the dispersion of graphene quantum dots was dialyzed (retained molecular weight: 3500 Da) in nanopure water for 7 days to remove sodium salt and molecular impurities, with water being refreshed every 8 h. The resulting black solution was dried in an oven at 80 °C for 12 h to obtain 0.3590 mg (yield, 63%) of a shiny black powder, named GQDs. Synthesis of Reduced Graphene Quantum Dots (rGQDs). Typically, 100 mg of GQDs was suspended in 100 mL of deionized water in a 200 mL round-bottom flask, followed by the addition of 1.2 mL (24.7 mmol) of hydrazine monohydrate. The solution was heated at 100 °C under reflux and stirring for 24 h. The product was purified by dialysis for 7 days with high-purity water, which was refreshed every 8 h. The resulting black suspension was dried in an oven at 80 °C for 12 h, resulting in 88.4 mg (yield of 88%) of a black powder, named rGQDs. Characterization. Infrared spectra were acquired on a Bruker Alpha FTIR spectrophotometer by accumulating 40 scans, in the 4000−400 cm−1 range with a resolution of 4 cm−1. Powder X-ray diffraction patterns were recorded on a Rigaku Miniflex 600 diffractometer at wavelength of λ = 0.15418 nm (Cu Kα radiation, with a Ni Kα filter) in the 2Θ range of 10−90° and a step scan of 0.02° min−1. A Renishaw RM1000 Raman spectrometer with λexc = 785 nm was employed for the collection of the Raman spectra. Ultraviolet−visible absorption spectra of 0.01 g L−1 solutions were recorded using a Thermo Scientific Evolution 220 Spectrometer. Photoluminescence (PL) spectra of aqueous solutions of GQDs and rGQDs were recorded at room temperature on an Agilent Cary Eclipse fluorescence spectrophotometer. High-resolution transmission electron microscopy (HRTEM) images of the samples were obtained by using a JEOL JEM-2100F model at an accelerating voltage of 200 kV. The samples were prepared by dispersing the solids in water followed by the dripping onto copper grids (400 mesh) covered with ultrathin carbon film and dried in air. The thicknesses of the films were measured using a Veeco Dektak profilometer. Photoelectrochemical measurements were recorded on a Gamry Reference 600 potentiostat in a three-electrode cell with a saturated calomel reference electrode connected to the cell through a 3 mol L−1 KCl salt bridge and a Pt counter electrode. K2SO4 (0.5 mol L−1) was employed as electrolyte, using 0.1 mol L−1 K3[Fe(CN)6] as sacrificial agent in some measurements. Photocurrents were recorded under the illumination of a Xe lamp with a power density of 100 mW cm−2 and an aqueous NaNO2 long-pass filter (λ > 400 nm). The scans were performed in the cathodic direction with a rate of 10 mV s−1. The cell was calibrated using the standard reduction potential of K3[Fe(CN)6]. The working electrode was prepared by drop-coating a 2 g L−1 rGQDs dispersion onto an area of 1 cm2 delimited on a fluorinedoped tin oxide (FTO)-coated glass slide, dried in air, and annealed at 450 °C for 2 h under Ar flow. Surface Photovoltage Spectroscopy (SPS) Measurements. SPS measurements were conducted under vacuum (8.6 × 10−5 mbar) on GQD and rGQD films deposited on fluorine-doped tin oxide (FTO), gold, and aluminum substrates. A gold Kelvin probe (Delta PHI Besocke) mounted inside a vacuum chamber served as reference electrode. Samples were illuminated with monochromatic light from a 150 W Xe lamp filtered through an Oriel Cornerstone 130 monochromator (0.1−0.3 mW cm−2). Spectra plot the contact potential difference (CPD) under illumination after subtraction of a dark scan (no light) of the respective sample. The samples were prepared by drop-coating aqueous dispersions of the samples with concentrations of 0.5, 2, 4, and 8 g L−1 onto the different substrates. After drying in air, the films were annealed at 150 °C for 90 min under Ar flow. In the case of aluminum substrates, the films were prepared inside a glovebox, followed by drying, but without high-

temperature annealing step. For the time-dependent experiment, the films of GQDs and rGQDs prepared on FTO prepared from a concentration of 2 g L−1 were irradiated with monochromatic light with a photon energy of 3.54 eV (28 571 cm−1). After the acquisition of a stable baseline in the dark, the film was irradiated for 15 min and then the light was turned off for 30 min. This cycle was repeated twice to collect three consecutive photovoltage signals. Hydrogen Evolution. Hydrogen evolution experiments were carried out with dispersions of approximately 100 mg of the photocatalysts in 100 mL of a 30% aqueous methanol solution in a round-bottom glass flask. The container was evacuated down to 50 torr and purged with argon gas several times until the chromatogram of the atmosphere above the solution indicated that no hydrogen, oxygen, or nitrogen was detected. The suspension, under stirring, was irradiated with >400 nm light (998 mW cm−2) from a 300 W xenon arc lamp and using a 0.22 mol L−1 NaNO2 solution as long-pass filter. The air-tight irradiation system was connected to a vacuum pump and to a Varian 3800 gas chromatograph (with a 60/80 Å molecular sieve column and a thermal conductivity detector) to quantify the amount of evolved hydrogen, using area counts of the recorded peaks. For the apparent quantum efficiency experiments, the same setup was used. In this case, 100 mg of rGQDs was suspended in 100 mL of a 30% aqueous methanol solution. This suspension was irradiated by a 375 nm light-emitting diode with a cross-sectional area of 2.01 cm2 and a power density of 141 mW cm−2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08331. Characterization details; photoemission spectrum of rGQDs; SPS data; electrochemical scans; and photochemical H2 irradiation results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Frank E. Osterloh: 0000-0002-9288-3407 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0015329, for financial support of this work.

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REFERENCES

(1) Xu, X.; Ray, R.; Gu, Y.; Ploehn, H. J.; Gearheart, L.; Raker, K.; Scrivens, W. A. Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments. J. Am. Chem. Soc. 2004, 126, 12736−12737. (2) Baker, S. N.; Baker, G. A. Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem., Int. Ed. 2010, 49, 6726−6744. (3) Shen, J.; Zhu, Y.; Yang, X.; Li, C. Graphene Quantum Dots: Emergent Nanolights for Bioimaging, Sensors, Catalysis and Photovoltaic Devices. Chem. Commun. 2012, 48, 3686−3699. (4) Zhu, S.; Zhang, J.; Qiao, C.; Tang, S.; Li, Y.; Yuan, W.; Li, B.; Tian, L.; Liu, F.; Hu, R.; Gao, H.; Wei, H.; Zhang, H.; Sun, H.; Yang, 27202

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(24) Yu, S.; Zhong, Y. Q.; Yu, B. Q.; Cai, S. Y.; Wu, L. Z.; Zhou, Y. Graphene Quantum Dots to Enhance the Photocatalytic Hydrogen Evolution Efficiency of Anatase TiO2 with Exposed {001} Facet. Phys. Chem. Chem. Phys. 2016, 18, 20338−20344. (25) Guo, C. X.; Dong, Y.; Yang, H. B.; Li, C. M. Graphene Quantum Dots as a Green Sensitizer to Functionalize ZnO Nanowire Arrays on F-Doped SnO2 Glass for Enhanced Photoelectrochemical Water Splitting. Adv. Energy Mater. 2013, 3, 997−1003. (26) Tsai, K.-A.; Hsu, Y.-J. Graphene Quantum Dots Mediated Charge Transfer of CdSe Nanocrystals for Enhancing Photoelectrochemical Hydrogen Production. Appl. Catal., B 2015, 164, 271−278. (27) Liu, F.; Jang, M. H.; Ha, H. D.; Kim, J. H.; Cho, Y. H.; Seo, T. S. Facile Synthetic Method for Pristine Graphene Quantum Dots and Graphene Oxide Quantum Dots: Origin of Blue and Green Luminescence. Adv. Mater. 2013, 25, 3657−3662. (28) Kim, S.; Hwang, S. W.; Kim, M.-K.; Shin, D. Y.; Shin, D. H.; Kim, C. O.; Yang, S. B.; Park, J. H.; Hwang, E.; Choi, S.-H.; Ko, G.; Sim, S.; Sone, C.; Choi, H. J.; Bae, S.; Hong, B. H. Anomalous Behaviors of Visible Luminescence from Graphene Quantum DotsInterplay Between Size and Shape. ACS Nano 2012, 6, 8203−8208. (29) Yan, X.; Li, B.; Cui, X.; Wei, Q.; Tajima, K.; Li, L. S. Independent Tuning of the Band Gap and Redox Potential of Graphene Quantum Dots. J. Phys. Chem. Lett. 2011, 2, 1119−24. (30) Li, Y.; Shu, H.; Wang, S.; Wang, J. Electronic and Optical Properties of Graphene Quantum Dots: The Role of Many-Body Effects. J. Phys. Chem. C 2015, 119, 4983−4989. (31) Zhu, S.; Song, Y.; Zhao, X.; Shao, J.; Zhang, J.; Yang, B. The Photoluminescence Mechanism in Carbon Dots (Graphene Quantum Dots, Carbon Nanodots, and Polymer Dots)- Current State and Future Perspective. Nano Res. 2015, 8, 355−381. (32) Cayuela, A.; Soriano, M. L.; Carrillo-Carrion, C.; Valcarcel, M. Semiconductor and Carbon-Based Fluorescent Nanodots: The Need for Consistency. Chem. Commun. 2016, 52, 1311−1326. (33) Ehrat, F.; Bhattacharyya, S.; Schneider, J.; Lof, A.; Wyrwich, R.; Rogach, A. L.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Tracking the Source of Carbon Dot Photoluminescence: Aromatic Domains versus Molecular Fluorophores. Nano Lett. 2017, 17, 7710−7716. (34) Sun, H.; Wu, L.; Gao, N.; Ren, J.; Qu, X. Improvement of Photoluminescence of Graphene Quantum Dots with a Biocompatible Photochemical Reduction Pathway and its Bioimaging Application. ACS Appl. Mater. Interfaces 2013, 5, 1174−1179. (35) Wu, X.; Ma, L.; Sun, S.; Jiang, K.; Zhang, L.; Wang, Y.; Zeng, H.; Lin, H. A Versatile Platform for the Highly Efficient Preparation of Graphene Quantum Dots: Photoluminescence Emission and Hydrophilicity-Hydrophobicity Regulation and Organelle Imaging. Nanoscale 2018, 10, 1532−1539. (36) Zhu, S.; Shao, J.; Song, Y.; Zhao, X.; Du, J.; Wang, L.; Wang, H.; Zhang, K.; Zhang, J.; Yang, B. Investigating the Surface State of Graphene Quantum Dots. Nanoscale 2015, 7, 7927−7933. (37) Khan, S.; Gupta, A.; Verma, N. C.; Nandi, C. K. Time-Resolved Emission Reveals Ensemble of Emissive States as the Origin of Multicolor Fluorescence in Carbon Dots. Nano Lett. 2015, 15, 8300− 8305. (38) Melo, M. A., Jr.; Wu, Z.; Nail, B. A.; De Denko, A. T.; Nogueira, A. F.; Osterloh, F. E. Surface Photovoltage Measurements on a Particle Tandem Photocatalyst for Overall Water Splitting. Nano Lett. 2018, 18, 805−810. (39) Zhao, J.; Nail, B. A.; Holmes, M. A.; Osterloh, F. E. Use of Surface Photovoltage Spectroscopy to Measure Built-in Voltage, Space Charge Layer Width, and Effective Band Gap in CdSe Quantum Dot Films. J. Phys. Chem. Lett. 2016, 7, 3335−3340. (40) Ma, X.; Cui, X.; Zhao, Z.; Melo, M. A.; Roberts, E. J.; Osterloh, F. E. Use of Surface Photovoltage Spectroscopy to Probe Energy Levels and Charge Carrier Dynamics in Transition Metal (Ni, Cu, Fe, Mn, Rh) Doped SrTiO3 Photocatalysts for H2 Evolution From Water. J. Mater. Chem. A 2018, 6, 5774−5781. (41) Wang, J.; Zhao, J.; Osterloh, F. E. Photochemical Charge Transfer Observed in Nanoscale Hydrogen Evolving Photocatalysts

B. Strongly Green-Photoluminescent Graphene Quantum Dots for Bioimaging Applications. Chem. Commun. 2011, 47, 6858−6860. (5) Li, Y.; Hu, Y.; Zhao, Y.; Shi, G.; Deng, L.; Hou, Y.; Qu, L. An Electrochemical Avenue to Green-Luminescent Graphene Quantum Dots as Potential Electron-Acceptors for Photovoltaics. Adv. Mater. 2011, 23, 776−780. (6) Ritter, K. A.; Lyding, J. W. The Influence of Edge Structure on the Electronic Properties of Graphene Quantum Dots and Nanoribbons. Nat. Mater. 2009, 8, 235−242. (7) Ye, R.; Xiang, C.; Lin, J.; Peng, Z.; Huang, K.; Yan, Z.; Cook, N. P.; Samuel, E. L.; Hwang, C. C.; Ruan, G.; Ceriotti, G.; Raji, A. R.; Marti, A. A.; Tour, J. M. Coal As an Abundant Source of Graphene Quantum Dots. Nat. Commun. 2013, 4, No. 2943. (8) Pan, D.; Zhang, J.; Li, Z.; Wu, M. Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots. Adv. Mater. 2010, 22, 734−738. (9) Peng, J.; Gao, W.; Gupta, B. K.; Liu, Z.; Romero-Aburto, R.; Ge, L.; Song, L.; Alemany, L. B.; Zhan, X.; Gao, G.; Vithayathil, S. A.; Kaipparettu, B. A.; Marti, A. A.; Hayashi, T.; Zhu, J. J.; Ajayan, P. M. Graphene Quantum Dots Derived from Carbon Fibers. Nano Lett. 2012, 12, 844−849. (10) Lin, L.; Zhang, S. Creating High Yield Water Soluble Luminescent Graphene Quantum Dots via Exfoliating and Disintegrating Carbon Nanotubes and Graphite Flakes. Chem. Commun. 2012, 48, 10177−10179. (11) Wang, L.; Wang, Y.; Xu, T.; Liao, H.; Yao, C.; Liu, Y.; Li, Z.; Chen, Z.; Pan, D.; Sun, L.; Wu, M. Gram-scale synthesis of singlecrystalline graphene quantum dots with superior optical properties. Nat. Commun. 2014, 5, No. 5357. (12) Yan, X.; Cui, X.; Li, L.-s. Synthesis of Large, Stable Colloidal Graphene Quantum Dots with Tunable Size. J. Am. Chem. Soc. 2010, 132, 5944−5945. (13) Fu, M.; Ehrat, F.; Wang, Y.; Milowska, K. Z.; Reckmeier, C.; Rogach, A. L.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons. Nano Lett. 2015, 15, 6030−6035. (14) Li, X.; Rui, M.; Song, J.; Shen, Z.; Zeng, H. Carbon and Graphene Quantum Dots for Optoelectronic and Energy Devices: A Review. Adv. Funct. Mater. 2015, 25, 4929−4947. (15) Yeh, T. F.; Teng, C. Y.; Chen, S. J.; Teng, H. Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall WaterSplitting under Visible Light Illumination. Adv. Mater. 2014, 26, 3297−3303. (16) Li, Y.; Zhao, Y.; Cheng, H.; Hu, Y.; Shi, G.; Dai, L.; Qu, L. Nitrogen-Doped Graphene Quantum Dots With Oxygen-Rich Functional Groups. J. Am. Chem. Soc. 2012, 134, 15−18. (17) Wang, S.; Cole, I. S.; Zhao, D.; Li, Q. The Dual Roles of Functional Groups in the Photoluminescence of Graphene Quantum Dots. Nanoscale 2016, 8, 7449−7458. (18) Zhang, W.; Liu, Y.; Meng, X.; Ding, T.; Xu, Y.; Xu, H.; Ren, Y.; Liu, B.; Huang, J.; Yang, J.; Fang, X. Graphenol Defects Induced Blue Emission Enhancement in Chemically Reduced Graphene Quantum Dots. Phys. Chem. Chem. Phys. 2015, 17, 22361−22366. (19) Li, L.; Wu, G.; Yang, G.; Peng, J.; Zhao, J.; Zhu, J. J. Focusing on Luminescent Graphene Quantum Dots: Current Status and Future Perspectives. Nanoscale 2013, 5, 4015−4039. (20) Li, H.; Kang, Z.; Liu, Y.; Lee, S.-T. Carbon Nanodots: Synthesis, Properties and Applications. J. Mater. Chem. 2012, 22, 24230−24253. (21) Zhuo, S.; Shao, M.; Lee, S.-T. Upconversion and Downconversion Fluorescent Graphene Quantum Dots- Ultrasonic Preparation and Photocatalysis. ACS Nano 2012, 6, 1059−1064. (22) Zhu, Z.; Ma, J.; Wang, Z.; Mu, C.; Fan, Z.; Du, L.; Bai, Y.; Fan, L.; Yan, H.; Phillips, D. L.; Yang, S. Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction: The Role of Graphene Quantum Dots. J. Am. Chem. Soc. 2014, 136, 3760−3763. (23) Williams, K. J.; Nelson, C. A.; Yan, X.; Li, L.-S.; Zhu, X. Hot Electron Injection from Graphene Quantum Dots to TiO2. ACS Nano 2013, 7, 1388−1394. 27203

DOI: 10.1021/acsami.8b08331 ACS Appl. Mater. Interfaces 2018, 10, 27195−27204

Research Article

ACS Applied Materials & Interfaces Using Surface Photovoltage Spectroscopy. Energy Environ. Sci. 2015, 8, 2970−2976. (42) Nail, B. A.; Fields, J. M.; Zhao, J.; Wang, J.; Greaney, M. J.; Brutchey, R. L.; Osterloh, F. E. Nickel Oxide Particles Catalyze Photochemical Hydrogen Evolution from Water-Nanoscaling Promotes p-Type Character and Minority Carrier Extraction. ACS Nano 2015, 9, 5135−5142. (43) Zhao, J.; Osterloh, F. E. Photochemical Charge Separation in Nanocrystal Photocatalyst Films: Insights from Surface Photovoltage Spectroscopy. J. Phys. Chem. Lett. 2014, 5, 782−786. (44) Michaelson, H. B. The Work Function of the Elements and its Periodicity. J. Appl. Phys. 1977, 48, 4729−4733. (45) Kim, Y. H.; Lee, E. Y.; Lee, H. H.; Seo, T. S. Characteristics of Reduced Graphene Oxide Quantum Dots for a Flexible Memory Thin Film Transistor. ACS Appl. Mater. Interfaces 2017, 9, 16375−16380. (46) Feng, Y.; Zhao, J.; Yan, X.; Tang, F.; Xue, Q. Enhancement in the Fluorescence of Graphene Quantum Dots by Hydrazine Hydrate Reduction. Carbon 2014, 66, 334−339. (47) Pei, S.; Cheng, H.-M. The Reduction of Graphene Oxide. Carbon 2012, 50, 3210−3228. (48) Park, S.; An, J.; Potts, J. R.; Velamakanni, A.; Murali, S.; Ruoff, R. S. Hydrazine-Reduction of Graphite- and Graphene Oxide. Carbon 2011, 49, 3019−3023. (49) Park, S.; Ruoff, R. S. Chemical Methods for the Production of Graphenes. Nat. Nanotechnol. 2009, 4, 217−224. (50) Chua, C. K.; Pumera, M. Chemical Reduction of Graphene Oxide: A Synthetic Chemistry Viewpoint. Chem. Soc. Rev. 2014, 43, 291−312. (51) Gao, X.; Jung, J.; Nagase, S. Hydrazine and Thermal Reduction of Graphene Oxide- Reaction Mechanisms, Product Structures, and Reaction Design. J. Phys. Chem. C 2010, 114, 832−842. (52) Chua, C. K.; Pumera, M. The Reduction of Graphene Oxide with Hydrazine: Elucidating its Reductive Capability Based on a Reaction-Model Approach. Chem. Commun. 2016, 52, 72−75. (53) Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of Organic Compounds, 7rd ed.; John Wiley & Sons, Inc.: New York, 2005. (54) O’Reilly, J. E. Oxidation-Reduction Potential of the FerroFerricyanide System in Buffer Solutions. Biochim. Biophys. Acta, Bioenerg. 1973, 292, 509−515. (55) Stombaugh, N. A.; Sundquist, J. E.; Burris, R. H.; OrmeJohnson, W. H. Oxidation-Reduction Properties of Several Low Potential Iron-Sulfur Proteins and of Methylviologen. Biochemistry 1976, 15, 2633−2641. (56) Anderson, A. B.; Asiri, H. A. Reversible Potentials for Steps in Methanol and Formic Acid Oxidation to CO2; Adsorption Energies of Intermediates on the Ideal Electrocatalyst for Methanol Oxidation and CO2 Reduction. Phys. Chem. Chem. Phys. 2014, 16, 10587− 10599. (57) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253−278. (58) Osterloh, F. E. Inorganic Nanostructures for Photoelectrochemical and Photocatalytic Water Splitting. Chem. Soc. Rev. 2013, 42, 2294−2320. (59) Fernando, K. A. S.; Sahu, S.; Liu, Y.; Lewis, W. K.; Guliants, E. A.; Jafariyan, A.; Wang, P.; Bunker, C. E.; Sun, Y.-P. Carbon Quantum Dots and Applications in Photocatalytic Energy Conversion. ACS Appl. Mater. Interfaces 2015, 7, 8363−8376. (60) Takahashi, K.; Asanoma, M.; Yoshida, S.; Ning, G.; Mori, H.; Horibe, T.; Watanabe, T.; Hirayama, T.; Nukaya, H.; Mizutani, T. Identification Of 1,3,6-Trinitropyrene as a Major Mutagen in Organic Extracts of Surface Soil from Nagoya City. Genes Environ. 2006, 28, 160−166.

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DOI: 10.1021/acsami.8b08331 ACS Appl. Mater. Interfaces 2018, 10, 27195−27204