Supporting Information

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1 Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, Supporting Information for Adv. Mater., DOI: /adma Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water-Splitting under Visible Light Illumination Te-Fu Yeh, Chiao-Yi Teng, Shean-Jen Chen, and Hsisheng Teng*

2 Supporting Information Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water Splitting under Visible Light Illumination By Te-Fu Yeh, Chiao-Yi Teng, Shean-Jen Chen, and Hsisheng Teng* [*] Prof. Hsisheng Teng, Mr. Te-Fu Yeh, Mr. Chiao-Yi Teng Department of Chemical Engineering and Center for Micro/Nano Science and Technology National Cheng Kung University, Tainan (Taiwan) Prof. Shean-Jen Chen Department of Engineering Science and Center for Micro/Nano Science and Technology National Cheng Kung University, Tainan (Taiwan) Supporting information for experimental methods, transmission electron microscopy (TEM) images of NGO-QDs, atomic force microscopy (AFM) image and height profile of NGO-QDs, electron energy-loss spectroscopy (EELS) spectrum of NGO-QDs, diffuse reflectance spectrum of the NGO-QD powder, results of long-time photocatalytic reactions, and photoluminescence (PL) emission spectra of the NGO-QD and Rh 2-y Cr y O 3 /GaN:ZnO, and GO-QD suspensions. 1. Experimental methods GO was prepared using a natural graphite powder (Bay carbon, SP-1, USA) through a modified Hummers method. [1] The graphite powder (5 g) and NaNO 3 (2.5 g; Merck, Germany) were introduced to a solution of concentrated H 2 SO 4 (18M, 115 ml; Wako, Japan) in an ice bath. KMnO 4 (15 g; J.T. Baker, USA) was gradually added with stirring; therefore, the temperature of the mixture remained below 20 C. The mixture was stirred at 35 C for 4 h to allow oxidation. Thereafter, deionized water (230 ml) was slowly added to the mixture and stirred at 98 C for 15 min. The mixture was further diluted to 700 ml and stirred for 30 min. The reaction was concluded by adding H 2 O 2 (12 ml, 35 wt %; Shimakyu, Japan) while 1 1

3 stirring at room temperature. Multiple washings were conducted with deionized water (3 500 ml), and the precipitate of the final slurry was dried at 40 C for 24 h to obtain the GO specimens. We synthesized nitrogen-doped graphene (NG) by treating the as-prepared GO in a flow of NH 3 gas at 500 C for 3 h. The NGO-QDs were obtained from oxidizing the synthesized NG with the modified Hummers method described above, followed by centrifugation to remove larger particles. Multiple washings of NGO-QDs were conducted with ethanol and centrifugation was used to collect the specimens. In addition to NGO-QDs, nitrogen-free GO-QDs were synthesized in the same manner as that for NGO-QDs except that the NH 3 treatment was replaced by Ar treatment. We also modified NGO-QDs with NH 3 treatment at 25 C for 12 h to obtain NH 3 -NGO-QDs. Previous studies reported that ammonia gas can strongly interact with the epoxy and carboxylic groups on graphite oxide at room temperature and form C-N bonds. [2,3] For the purpose of photocatalytic activity comparison, this study synthesized the well-explored GaN:ZnO catalyst for overall water-splitting tests. A GaN:ZnO catalyst was prepared by nitridation of a Ga 2 O 3 -ZnO mixture with NH 3. [4-8] The Ga 2 O 3 component was obtained from calcination (1000 C for 6 h) of a crystalline Ga(OH) 3 powder, which was derived from autoclaving an aqueous solution of gallium nitrate (Alfa Aesar, USA) and ammonium hydroxide (NH 4 OH, 25 vol %, Sigma Aldrich, Germany) at 120 C for 6 h. [9] We mixed the Ga 2 O 3 powder with ZnO (Kanto Chemical, Japan) at a Ga:Zn molar ratio of 1:1 by grinding and then treated the mixture in a NH 3 flow at 775 C for 15 h. To load Rh 2 y Cr y O 3 co-catalyst onto the as-prepared GaN:ZnO catalyst, a slurry containing 0.5 g of GaN:ZnO and suitable amounts of Na 3 RhCl 6 12H 2 O (Alfa Aesar, USA) and Cr(NO 3 ) 3 9H 2 O (Alfa Aesar, USA) was stirred in a crucible at 90 C to evaporate the water. The dried mixture was then calcined in air at 350 C for 1 h to produce a Rh 2-y Cr y O 3 /GaN:ZnO catalyst. Rhodium and 2 2

4 chromium were loaded at ratios of 2.5 and 2.0 wt % (metallic content), respectively. [4-8] X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra DLD, UK) with Al K radiation was used to quantitatively analyze the chemical composition of NGO-QDs. High-resolution transmission electron microscopy (HRTEM; Jeol 2100F, Japan), equipped with a field-emission gun operating at 200 kev and a Gatan Enfina electron energy-loss spectroscopy (EELS) spectrometer, was used to explore the microstructure of NGO-QDs. The samples were deposited on carbon substrates for measurements. The thicknesses of NGO-QDs were analyzed using EELS carried out with a dispersion of 1 ev per channel. We also analyzed the topography of NGO-QDs with atomic force microscopy (AFM; Nanoscope IIIa, Digital Instrument, USA) conducted in a tapping mode. The samples for AFM measurements were deposited on mica substrates. The optical absorption spectrum of a NGO-QDs/water suspension was obtained by placing the solution in a 1-cm quartz cuvette and analyzed using a Hitachi U-4100 (Japan) spectrophotometer. The diffuse reflection spectrum of the NGO-QD powder was also measured and was converted from reflection to absorbance. The PL spectra of the NGO-QD and Rh 2-y Cr y O 3 /GaN:ZnO catalysts in water suspensions were measured at ambient temperature using a fluorescence spectrophotometer (Hitachi F-700, Japan). QD electrodes for electrochemical analysis were prepared by drop-casting QD/water suspensions onto a screen-printed carbon electrode (Zensor R&D Co., USA). We subjected the QD electrodes to electrochemical analysis in 0.5 M Na 2 SO 4 solution with a Pt foil counter and an Ag/AgCl reference. The conductivity type of the QD electrodes was analyzed through impedance spectroscopy (Zahner IM6e, Germany) equipped with Thales software. The measurements applied a sinusoidal potential perturbation with a small amplitude (10 mv), superimposed on a fixed DC potential varying within a potential window from 0.4 to 1 V (vs. Ag/AgCl). In the same electrochemical system, a linear potential scan (5 mv/s) was 3 3

5 conducted to determine the CBM and VBM of the NGO-QD specimen. Photocatalytic reactions were conducted at approximately 25 C in a gas-enclosed side irradiation system. We suspended the QD catalysts (1.2 g) in 200 ml of pure water (ph = 3) in a Pyrex vessel with side irradiation from a 300 W Xenon lamp (Oriel Instruments, model 66901, USA). The incident light wavelength was limited to nm by using a UV-cutoff filter (Oriel Instruments, 59480, USA) and an IR-cutoff filter (Oriel Instruments, 59044, USA). The amounts of evolved H 2 and O 2 were determined using gas chromatography (Hewlett-Packard 7890, USA; molecular sieve 5A column, thermal conductivity detector, argon carrier gas). 4 4

6 2. TEM images of NGO-QDs Figure S1. a) Wide-range TEM image of NGO-QDs. b) High-resolution TEM image of the NGO-QD shown in Figure 2b. The electron beam direction was adjusted for revealing the crystalline lattice fringes of the NGO-QDs. 5 5

7 3. AFM image and height profile of NGO-QDs Figure S2. a) AFM image of NGO-QDs distributed on a mica substrate. b) The height profile along the line in panel a. 6 6

8 4. EELS spectrum of NGO-QDs Figure S2a shows a high concentration of NGO-QDs deposited on a carbon substrate for analysis. Figure S2b presents the EELS spectrum acquired over the area indicated in Figure S2a. We determined the mean thickness of the NGO-QDs using the equation I t ln( t I 0 ) (1) where t represents the specimen thickness, is the mean free path of inelastic electron-scattering in the specimen, and I 0 and I t are the intensities of the zero loss peak and total signal of an EELS spectrum, respectively. We treated the NGO-QDs as amorphous carbon because they were too small to be regarded as graphite crystals. The mean free path of inelastic electron scattering in amorphous carbon is approximately 20 nm at 200 kev. [10] Based on the spectrum in Figure S2 and carbon substrate thickness (4.4 nm), we used Equation (1) to calculate a mean thickness of 1.8 nm for the NGO-QDs. 7 7

9 Figure S3. Thickness analysis of NGO-QD layers. a) TEM image of NGO-QDs. b) An EELS spectrum acquired over the area in panel a. 8 8

10 5. Diffuse reflectance spectrum of the NGO-QD powder Submitted to Figure S4. Diffuse reflectance spectrum of the NGO-QD powder. The inset shows the photograph of the NGO-QD powder, which exhibit dark brown color. 9 9

11 6. Results of long-time photocatalytic reactions Figure S5. a) Time course of gas production over 1.2 g of NGO-QDs suspended in 200 ml of pure water under visible-light (420 nm 800 nm) irradiation. A Xe-lamp combined 10 10

12 with UV- and IR-cutoff filters served as the light source. The steady H 2 and O 2 evolution from pure water in the stoichiometric ratio of 2:1 for 72 h has strongly demonstrated the capability and stability of NGO-QDs in photocatalytic overall water-splitting. b) Time course of H 2 evolution from a 20 vol% aqueous methanol solution (900 ml) suspended with 0.3 g of NGO-QDs under inner mercury lamp (UM452, Ushio, Japan) irradiation. The number of hydrogen atoms in the H 2 evolution of 45 h was more than twice that of carbon atoms in the NGO-QD photocatalyst. This supports that NGO-QDs can steadily generated H 2 through photocatalytic process, rather than self-decomposition. An auxiliary blank test has shown that the amount of H 2 evolved from a photocatalyst-free methanol solution (20 vol%) was negligibly small relative to that from the NGO-QDs-containing system. c) Time course of H 2 evolution from a 20 vol% aqueous methanol solution (900 ml) suspended with 0.3 g of NGO-QDs under inner irradiation with UV-blocked mercury lamp (UM452, Ushio, Japan), in which a NaNO 2 aqueous solution (1 M) was used filter out the UV light ( < 400 nm)

13 7. PL emission spectra of the NGO-QD, Rh 2-y Cr y O 3 /GaN:ZnO, and GO-QD suspensions Figure S6. Photoluminescence emission spectra of the NGO-QD, Rh 2-y Cr y O 3 /GaN:ZnO, and GO-QD aqueous suspensions subjected to a 420 nm excitation

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