Tuning the Electronic Structure of Graphite Oxide through Ammonia

Article pubs.acs.org/JPCC

Tuning the Electronic Structure of Graphite Oxide through Ammonia Treatment for Photocatalytic Generation of H2 and O2 from Water Splitting Te-Fu Yeh,† Shean-Jen Chen,‡,§ Chen-Sheng Yeh,∥ and Hsisheng Teng*,†,‡ †

Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan § Department of Engineering Science, National Cheng Kung University, Tainan 70101, Taiwan ∥ Department of Chemistry, National Cheng Kung University, Tainan 70101, Taiwan ‡

S Supporting Information *

ABSTRACT: Graphite oxide (GO) synthesized from the oxidation of graphite powders exhibits p-type conductivity and is active in photocatalytic H2 evolution from water decomposition. The p-type conductivity hinders hole transfer for water oxidation and suppresses O2 evolution. Treating GO with NH3 gas at room temperature tunes the electronic structure by introducing amino and amide groups to its surface. The ammonia-modified GO (NGO) exhibits n-type conductivity in photoelectrochemical analysis and has a narrower optical band gap than GO. Electrochemical analysis attributes the band gap reduction to a negative shift of the valence band. An NGO-film electrode exhibits a substantially higher incident photo-to-current efficiency in the visible light region than a GO electrode. Photoluminescence analyses demonstrate the above-edge emission characteristic of GO and NGO. NH3 treatment enhances the emission by removing nonirradiative epoxy and carboxyl sites on the GO. In half-reaction tests of water decomposition, NGO effectively catalyzes O2 evolution in an aqueous AgNO3 solution under mercury-lamp irradiation, whereas GO is inactive. NGO also effectively catalyzes H2 evolution in an aqueous methanol solution but shows less activity than GO. Under illumination with visible light (λ > 420 nm), NGO simultaneously catalyzes H2 and O2 evolutions, but with a H2/O2 molar ratio below 2. The ntype conductivity of NGO may hinder electron transfer and form peroxide species instead of H2 molecules. This study demonstrates that the functionality engineering of GO is a promising technique to synthesize an industrially scalable photocatalyst for overall water splitting.

1. INTRODUCTION The synthesis of graphite oxide (GO) has received significant attention because of its 2D configuration and versatility of its properties, which can be changed by modifying its functionalities.1 Controlled doping with holes or electrons on the graphene sheets modulate the electronic properties of GO.2,3 The semiconducting property and large accessible surface in aqueous solutions make GO an excellent photocatalytic medium for various applications. The photocatalytic generation of H2 and O2 from water using aqueous semiconductor particle suspensions under solar illumination has become a viable strategy for large-scale H2 productions from renewable resources without carbon dioxide emissions.4−13 GO is advantageous over conventional metal-containing inorganic photocatalysts14−24 because it has a larger surface area for contact with water and does not require noble metal cocatalysts such as Pt or Ru.25−28 GO is a p-doped material; therefore, its charge flow creates negative oxygen atoms and a positively charged carbon grid.29−33 This electronic feature renders the formation of an accumulation layer at the GO/water interface for effective water reduction but creates a barrier for hole © 2013 American Chemical Society

transfer when oxidizing water. The modification or transformation of oxygen functionalities on GO sheets is an important task for extending the applications of GO in photocatalysis processes. Oxygen-containing functionalities are present on GO: epoxy and hydroxyl groups on the basal planes and carbonyl and carboxyl groups along the edges. The presence of these functionalities allows GO to be readily exfoliated and yield a well-dispersed solution of individual sheets in water.34−40 The electronic properties of GO vary with the composition of oxygen bonding on graphene sheets.41−50 Graphene’s valence and conduction bands consist of the π- and π*-orbitals (antibonding π-orbitals), respectively. As oxygen increasingly bonds on graphene, the highest level of the valence band gradually shifts from graphene’s π-orbital to oxygen’s 2p orbital, and the conduction band of GO remains at the π*-orbitals.30,41 GO exhibits p-type conductivity because of oxygen’s high Received: December 21, 2012 Revised: February 18, 2013 Published: March 11, 2013 6516

dx.doi.org/10.1021/jp312613r | J. Phys. Chem. C 2013, 117, 6516−6524

The Journal of Physical Chemistry C

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were measured at ambient temperature using a fluorescence spectrophotometer (Hitachi F-700, Japan). A GO electrode for photoelectrochemical analysis was prepared by drop-casting a GO/water suspension onto fluorine-doped SnO2 (FTO)-conducting glass substrates (TEC 8, Hartford Glass Co.). Thereafter, the GO electrode was air-dried at room temperature for 10 h. The NGO electrode was obtained by treating the as-prepared GO electrode under a NH3 flow of 60 mL/min at room temperature for 12 h in a tube furnace. We subjected the GO and NGO electrodes to photoelectrochemical analysis with a platinum-foil counter and an Ag/AgCl reference. The electrolyte was a 2 M H2SO4 solution. Photocurrent−potential characteristics were recorded under illumination with a solar simulator (Newport, Oriel class A, SP91160A) at 100 mW cm−2 (AM 1.5G) using an electrochemical analyzer (CH Instruments 614B). Using the same electrochemical system without illumination, the conductivity types of the GO and NGO electrodes were 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 −0.6 to 1 V (vs Ag/AgCl). In the same electrochemical system, a linear potential scan (5 mV/s) was conducted to determine the conduction- and valence-band edges of the GO and NGO specimens. The incident photon to current conversion efficiency (IPCE) of the GO and NGO electrodes was analyzed in cells using the 2 M H2SO4 solution as the electrolyte and a platinum-coated FTO substrate as the counter electrode. An IPCE analyzer (Enlitech QE-R3011, Taiwan) was used to scan the cells with illumination from 350 to 1100 nm under ambient conditions. Photocatalytic reactions were conducted at ∼25 °C in a gasenclosed inner irradiation system. The light source was a 450 W high-pressure mercury-lamp (UM452, Ushio, Japan). A jacket between the mercury lamp and the reaction chamber was manipulated with flowing temperature-controlled cooling water. The GO and NGO photocatalysts (0.5 g) were suspended in the reaction chamber with either an 800 mL methanol solution (20 vol %) for the H2 evolution test or with a AgNO3 solution (0.01 M) for the O2 evolution test. The amounts of evolved H2 and O2 were determined using gas chromatography (Hewlett-Packard 7890; molecular sieve 5A column, thermal conductivity detector, argon carrier gas). This study performed overall water splitting reactions by suspending the GO and NGO photocatalysts (0.5 g) in 200 mL of pure water in a Pyrex vessel with side irradiation from a 300 W xenon lamp (Oriel Instruments, model 66901). The incident light wavelength was limited to 420−800 nm by using a UVcutoff filter (Oriel Instruments, 59480) and an IR-cutoff filter (Oriel Instruments, 59044).

electronegativity compared to that of carbon atoms.29−33,41,48 Likewise, n-type conductivity appears when graphene covalently bonds to electron-donating nitrogen functionalities.51−53 Nitrogen doping is one of the simplest synthetic methods for engineering the band structure of GO semiconductors.51 Theoretical calculations show that the N orbital state interferes with the top of the GO valence band and leads to a mixed valence band consisting of O 2p and N 2p orbitals.18 Doping nitrogen functionalities on GO to tune the electronic properties increases the variety of GO applications. In this study, the electronic structure of GO was modified by treating it with ammonia gas, replacing the functionalities on the graphene sheets. The as-synthesized GO and ammoniamodified GO (NGO) specimens were analyzed to determine their electronic structure and photocatalytic activity, with special attention to O2 generation activity from water decomposition. The NGO specimen exhibits n-type conductivity and a narrower band gap than GO for more visible light absorption. It effectively catalyzes O2 evolution from a silver nitrate aqueous solution under mercury-lamp irradiation, demonstrating the high activity and stability of NGO in photocatalytic water oxidation. Visible light (>420 nm) irradiation on NGO also results in simultaneous H2 and O2 evolutions from pure water, but with a H2/O2 molar ratio below 2. This paper elucidates the correlation between the electronic structure and photocatalytic activity of both GO and NGO specimens.

2. EXPERIMENTAL SECTION GO was prepared using a natural graphite powder (Bay carbon, SP-1) through a modified Hummers’ method.54 The graphite powder (5 g) and NaNO3 (2.5 g; Merck, Germany) were introduced to a solution of concentrated H2SO4 (18 M, 115 mL; Wako, Japan) in an ice bath. KMnO4 (15 g; J.T. Baker) 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 H2O2 (12 mL, 35 wt %; Shimakyu, Japan) while 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. The NH3modified GO (NGO) was obtained through nitridation of the as-prepared GO performed at room temperature (25 °C) for 12 h using a flow of NH3 gas (60 mL min−1) in a tubular furnace. For the purpose of comparison, this study synthesized the wellexplored GaN:ZnO catalyst for overall water-splitting tests (see the Supporting Information).6,55,56 A Rh2−yCryO3 cocatalyst was loaded to the as-prepared GaN:ZnO for promoting the photocatalytic activity. X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra DLD, UK) with Al Kα radiation was used to quantitatively analyze the chemical composition of the GO specimens. Fourier transform infrared spectroscopy (FTIR) was conducted in diffuse reflectance mode to analyze the functionalities. A Jasco FTIR-4100 (Japan) spectrometer was used for this purpose. GO/water and NGO/water solutions were placed in a 1 cm quartz cuvette and analyzed using a Hitachi U-4100 (Japan) spectrophotometer to obtain optical absorption spectra. The PL spectra of GO and NGO in water suspensions

3. RESULTS AND DISCUSSION The schematic in Figure 1 shows the interaction of the NH3 lone pair with the epoxy functionality on the GO basal plane; the NH3 lone pair breaks the C−O bond of the epoxy ring through nucleophilic substitution.57,58 This interaction leads to the formation of amino and hydroxyl groups. Figure 1 also shows the interaction of NH3 with the carboxyl functionality in the periphery of the GO sheet, forming the amide linkage ((O)C−N).57,58 Figure 2 shows the whole XPS spectra of the GO and NGO specimens. The GO spectrum (Figure 2a) 6517



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(1.17 + 0.13), which was larger than the number of O atoms on GO (1.25). The nucleophilic attack of NH3 on the epoxy group accounts for the increase in the total number of heteroatoms (O and N) on the graphene sheets because the other interaction (with the carboxyl group) does not change the number of heteroatoms (Figure 1). Therefore, NGO contains 0.05 (1.30 − 1.25) nitrogen atoms per carbon atom from the interaction with the epoxy functionality and 0.08 (0.13 − 0.05) nitrogen atoms from the carboxyl; that is, the interactions with the epoxy and carboxyl groups contributed 40% and 60% of the nitrogen functionalities formed on the NGO, respectively. The C 1s peak, ranging from 280 to 292 eV in the XPS spectra, comprises peaks contributed by several functionalities that have different binding energies. Figure 3a,b shows the C 1s spectra of GO and NGO. These spectra were fit using a symmetric Gaussian function. The C 1s spectrum of GO (Figure 3a) consists of peaks at 284.6, 286.7, 287.7, and 288.9 eV, attributable to the C−C, C−O, CO, and O−CO groups, respectively.59,60 The C 1s spectrum of NGO (Figure 3b) has an additional C−N peak located at 285.3 eV. Table 1 lists the composition of functionalities obtained from the C 1s spectra. The C−N bond composition increased by 10% of the C atoms after NH3 treatment, which was accompanied by a decrease in the C−O bond (also by 10%). Both the interacting substitution mechanisms on the epoxy and carboxyl groups contribute to the bond composition variation. Conversely, carboxyl (O−CO) groups decreased with the increase of carbonyl (CO) groups, verifying the interaction mechanism of NH3 with the carboxyl groups depicted in Figure 1.57,58 The N 1s spectrum of NGO (Figure 3c) shows the C−N and (O)C−N peaks at 399.4 and 401.1 eV, respectively. The former bond was derived from the interaction with the epoxy and the latter from the carboxyl. The curve fitting of Figure 3c indicates that C−N and (O)C−N bonds contributed 40% and 60% of the nitrogen functionalities in NGO, respectively, in agreement with the results interpreted using the (O 1s)/(C 1s) and (N 1s)/(C 1s) ratios. Additional Fourier transform infrared spectroscopy analysis of GO and NGO (see the Supporting Information) verifies these surface modification results of GO after NH3 treatment. Figure 4 shows the photocurrent−voltage curve for GO and NGO films deposited on fluorine-doped SnO2 (FTO) conducting glass under AM1.5 solar illumination. Both the anodic and cathodic scans were performed at a linear rate under chopped illumination of simulated solar light. The GO film exhibits a photocurrent response under a cathodic voltage bias (Figure 4a), indicating its p-type semiconducting property. In contrast, the NGO film shows an anodic photocurrent response that is characteristic of an n-type semiconductor (Figure 4b). The strong electron-donating character of the amino group converts the p-type conductivity of GO to the n-type, despite the N 1s/C 1s ratio of NGO being low relative to the O 1s/C 1s. Electrochemical impedance spectroscopic analysis and the Mott−Schottky equation further identified the conductivity

Figure 1. Mechanistic schematics of ammonia interacting with the epoxy groups on the basal plane and the carboxyl groups in the periphery of a GO sheet to form amino and amide groups, respectively.

Figure 2. Full-range XPS spectra: (a) GO; (b) NGO.

shows the binding energy peaks of C 1s at 286 eV and O 1s at 532 eV. After NH3 treatment on GO, the NGO spectrum (Figure 2b) shows the N 1s peak at 400 eV, accompanied by the C 1s and O 1s peaks. The intensity of the O 1s peak decreases after NH3 treatment, implying a certain degree of GO reduction. Quantitative XPS analysis determined the surface O, N, and C concentrations. Table 1 shows the atomic ratios of (O 1s)/(C 1s) and (N 1s)/(C 1s) for GO and NGO. The total number of O and N atoms per carbon atom on NGO was 1.30

Table 1. (O 1s)/(C 1s) Atomic Ratio Determined from the Full-Range XPS Spectra (Figure 2) and Carbon Bonding Composition Determined from the C 1s XPS (Figure 3a,b) for the GO and NGO Specimens carbon bonding composition (%) carbon type

O 1s/C 1s

GO NGO

1.25 1.17

N 1s/C 1s

C−C

0.13

41 40 6518

C−N

C−O

CO

O−CO

10

44 34

9 12

6 4



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Figure 4. Photocurrent induced by potential scan for the (a) GO and (b) NGO films deposited on the FTO substrate and immersed in an aqueous H2SO4 solution (2 M), under a scan rate of 5 mV s−1 and chopped solar irradiation (AM 1.5G) of 100 mW cm−2.

Figure 3. C 1s XPS spectra of GO and NGO (panels a and b, respectively), and N 1s XPS spectrum of NGO (panel c). These spectra (indicated by the solid lines) were decomposed into several peaks (indicated by the dashed lines) that were fitted using a Gaussian function.

type of GO and NGO.61−64 Figure 5 shows the capacitance values of the space charge region obtained at various applied potentials. According to the Mott−Schottky equation, a linear relationship of 1/C2 versus applied potential can be observed, and its intercept on abscissa corresponds to the material’s Fermi level. The negative slope in the linear region (Figure 5a) verified the p-type conductivity of the GO film. By contrast, the NGO film shows a positive slope in the linear region (Figure 5b), illustrating the n-type conductivity of NGO. Figure 6 shows the optical absorbance spectra of the GO and NGO aqueous solutions. The NGO has a stronger absorption at longer wavelengths than the GO. The inset of Figure 6 shows that the NGO water suspension is dark brown and the GO suspension is light yellow. This color difference implies that the NH3 treatment resulted in a red shift of the GO absorption band edge. A strong electronic coupling between O 2p and N 2p orbitals is present with nitrogen doping for metal oxides.18,65 Some evidence supports that a localized N 2p state generates a midgap level slightly above the top of O 2p valence bands in

Figure 5. Variation of capacitance (C) with the applied potential in 2 M H2SO4 presented in the Mott−Schottky relationship for electrodes of (a) GO and (b) NGO. The capacitance was determined by electrochemical impedance spectroscopy.

metal oxides.66 Therefore, the N 2p orbital of the NGO amino and amide groups may reduce the band gap of GO and promote light absorption at longer wavelengths. Because GO and NGO comprise graphene molecules of various oxygenated or nitridated levels, the optical absorption spectra (Figure 6) do not show sharp absorption edges for obtaining a representative gap energy. Incident photon to current conversion efficiency (IPCE) analysis remedies this shortcoming of the absorption spectroscopy by revealing the action spectra of the GO and NGO specimens. The IPCE action spectra relate to electronic band gap transition and electron injection. We subjected the GO and NGO films to IPCE analysis at various excitation wavelengths. Figure 7a 6519



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(Figure 7b) shows that band gaps are 2.7 eV for the direct and 2.5 eV for the indirect band gaps. However, the NGO spectrum in Figure 7c shows 2.6 eV for the direct and 2.3 eV for the indirect band gaps. Although the gap energies of both GO and NGO satisfy the theoretical energetic requirement for water decomposition (1.23 eV), the knowledge of electronic band characteristics, highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) of the specimens is more critical for O2 and H2 evolution from aqueous solutions. This study subjected the conduction band edge (LUMO) and valence band edge (HOMO) positions of the GO and NGO films to analysis with linear potential scans.68,69 Figure 8a,b shows abrupt cathodic and anodic currents increase

Figure 6. Optical absorption spectra of GO and NGO dispersed in water. The inset shows the photographs of the GO and NGO dispersions, which exhibit light yellow and dark brown colors, respectively.

Figure 8. Cathodic and anodic linear potential scan for determining the conduction band edge (CB) and valence band edge (VB) of the GO and NGO specimens at 5 mV/s: (a) GO with the cathodic scan; (b) GO with the anodic scan; (c) NGO with the cathodic scan; (d) NGO with the anodic scan.

resulting from the formation of inversion and accumulation layers. The results show that GO has a conduction band edge at −0.6 eV and a valence band edge at 1.9 eV (vs Ag/AgCl). Furthermore, Figure 8c,d shows that NGO also has a conduction band edge at −0.6 eV, but a valence band edge at 1.6 eV (vs Ag/AgCl). The band gap energies determined from this potential scan method are consistent with those from the IPCE spectra. The results shown in Figure 8 illustrate that both GO and NGO have similar conduction band edges and that NH3 treatment elevates the valence band edge of GO. The valence band edge elevation of NGO may result from the amino group’s N 2p orbital replacing the O 2p orbital as the HOMO level. Based on the preceding data (Figures 5 and 8), Figure 9 shows the energy level diagrams of GO and NGO compared with the potentials for water reduction and oxidation and with the vacuum level. Photoluminescence (PL) analysis provides further interpretation of the charge transfer in the GO and NGO catalysts.17 The optoelectronic characteristics of carbon materials are determined by the π state of the sp2 clusters,70 which usually

Figure 7. (a) IPCE spectra of the GO and NGO films. The inset shows the magnified onset region of the IPCE spectra. (b) Plots of (ηE)2 and (ηE)1/2 against the photon energy (E) for the GO film, where η is the IPCE value. (c) Plots of (ηE)2 and (ηE)1/2 against the photon energy (E) for the NGO.

shows the IPCE spectra of the GO and NGO films immersed in 2 M H2SO4 aqueous solutions and countered by platinum. The inset in Figure 7a shows the magnified onset region of the action spectrum. In the light-sensitive regime the NGO film exhibits a larger photocurrent than the GO film, indicating a more efficient charge separation in the NGO specimen. The action spectrum edges for the GO and NGO films are approximately 480 and 530 nm, respectively. NH3 treatment promotes GO photoactivity and narrows the band gap. We plotted the square and square root of the converted energy (ηE, where η is the IPCE value) against the photon energy (E) to determine the direct and indirect gap transition energies of the GO and NGO.67 The linear extrapolation of the GO spectrum 6520



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The large surface-to-volume ratio of graphene’s structure makes the PL properties of graphene-related materials strongly dependent on their surface states. Both GO and NGO have the characteristic PL at 436 nm (corresponding to 2.8 eV). This emission energy was greater than the gap-transition energy obtained from the IPCE action spectrum, indicating that the emission resulted from excited electron transitions involving quantized discrete levels higher than the LUMO or those lower than the HOMO. This above-edge emission by NGO indicates that the photogenerated electrons can remain at the higher quantized levels without relaxation. The removal of the charge relaxation sites by NH3 treatment represents a significant advantage for using NGO in photosynthetic systems. This study used a gas-enclosed circulation system with inner mercury-lamp irradiation to analyze the photocatalytic activity of GO and NGO in half-reactions of water decomposition.75,76 The half-reactions were conducted in the presence of sacrificial reagents: electron donor (methanol) for photoreduction and an acceptor (Ag+) for photo-oxidation. No metal-containing materials were used as cocatalysts in these photocatalytic reactions. Figure 11 shows the GO and NGO catalysts’ time

Figure 9. Schematic energy level diagrams of GO and NGO in comparison with the potentials for water reduction and oxidation.

serve as the recombination sites (π*−π transition) for luminescence. GO is a 2D network of sp2- and sp3-bonded carbon, and sp2 luminescence centers are embedded within an sp3 matrix. The oxygen-containing functional groups of GO have contributed to luminescence,71 in addition to the sp2 clusters. The fingerprinting PL bands of GO include σ*−n and π*−π transitions.71 Under UV-light excitation the π*−π transition is mainly associated with the electron relaxation from the higher σ* orbital to the π* orbital. Epoxy and carboxyl groups on GO usually induce nonradiative charge relaxation.72−74 Figure 10 shows the PL spectra of the GO and NGO

Figure 11. Time course of H2 evolution from an aqueous methanol solution (20 vol %) containing 0.5 g of the GO and NGO photocatalysts under mercury-lamp irradiation.

course of H2 gas evolution from a methanol solution (20 vol %) over 0.5 g under mercury-lamp irradiation. Both GO and NGO steadily catalyzed H2 evolution. The H2 evolved with GO was ∼3 times that of NGO. The depletion layer that formed on the n-type NGO surface may have hindered the injection of photogenerated electrons into the solution phase for H2 evolution. In the photo-oxidation half-reaction, GO cannot catalyze O2 evolution from an AgNO3 aqueous solution because the p-type conductivity of GO is not favorable for hole transfer into the solution and the valence band edge is not positive enough after light irradiation.25,26 Conversely, Figure 12 shows that NGO effectively catalyzes O2 evolution from a AgNO3 aqueous solution. The decay of the O2 evolution rate may be due to a gradual Ag-metal coverage of the NGO surface.77 The high activity of NGO indicates that amino groups on the graphene sheets introduced donor levels near the conduction band to attract hole migration toward the interface region for O2 evolution. During O2 evolution, a miniature (but steady) evolution of H2 was also detected (the inset of Figure 12). The reduced Ag atoms may have served as an electron-trapping site, reducing the surrounding water for H2 evolution.78 The evolution of H2 indicates that NGO is capable of catalyzing water decomposition into H2 under light irradiation.

Figure 10. Photoluminescence emission spectra of the GO and NGO dispersed in water with an excitation wavelength of 330 nm. The inset shows the photographs of the GO and NGO dispersions under 365 nm light irradiation, which emit weak yellow-green and strong blue colors, respectively.

specimens with a 330 nm excitation. Both spectra contain a sharp peak of water Raman at 373 nm.73 The PL emission peaks of GO appear at 436 and 548 nm, which can be assigned to the σ*−n and π*−π transitions, respectively. The spectrum of NGO only exhibits a strong peak at 436 nm. The NH3 treatment may passivate the GO surface by removing the nonradiative sites, causing the enhanced σ*−n PL emission of NGO. The inset of Figure 10 shows blue (strong) and yellowgreen (weak) emissions under 365 nm light irradiation from NGO and GO suspensions, respectively. These colors and intensities complement the PL spectra data. 6521



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of Figure 13 shows the H2 and O2 evolution data of the test. The comparison indicates that the activity of NGO in water splitting was not as high as that of Rh2−yCryO3/GaN:ZnO, but their activities were within a comparable scale. The NGO’s positive response to visible-light irradiation for simultaneous H2 and O2 evolutions is encouraging. This reflects the important role nitrogen atoms play on graphene oxide sheets. Further studies are planned with a more detailed investigation of the correlation between the NGO photocatalytic activity and the nitridation level of graphene oxide sheets.

4. CONCLUSIONS The NGO obtained from ammonia treatment showed promising photocatalytic activity for generating H2 and O2 from water decomposition (especially the latter). Ammonia interacted with GO through lone-pair attack on the epoxy and carboxyl functionalities to form amino and amide groups. The introduction of nitrogen functionalities converted the p-type GO to an n-type NGO and narrowed the band gap. The IPCE action spectra showed that NGO was active with irradiation wavelengths up to 530 nm and the GO to only 480 nm. Thus, the N 2p orbital may have replaced the O 2p orbital for the HOMO level. The PL emission spectra indicated that NH3 treatment passivated GO by removing the nonradiative epoxy and carboxyl sites, which led to the enhanced above-edge PL emission of NGO. Photocatalytic reaction with mercury-lamp irradiation showed that the n-type characteristic of NGO promoted O2 evolution in an AgNO3 aqueous solution and suppressed H2 evolution in an aqueous methanol solution. NGO exhibited the ability to simultaneously catalyze H2 and O2 evolutions from pure water under a light irradiation of >420 nm because of the active response to visible light irradiation. The stronger hole injection tendency of the n-type NGO may have caused the rate ratio of H2 to O2 to be smaller than 2 to 1. This study demonstrates the graphite oxide functionality engineering flexibility for tuning the electronic structure and its potential as a photocatalytic medium for water splitting.

Figure 12. Time course of O2 evolution from an aqueous AgNO3 solution (0.01 M) containing 0.5 g of NGO photocatalysts under mercury-lamp irradiation. The inset shows H2 evolution from the NGO-containing AgNO3 solution under mercury-lamp irradiation.

The IPCE action spectra (Figure 7a) revealed that NGO was active with visible light irradiation; thus, the NGO catalyst was subjected to overall water splitting analysis using a gas-enclosed system with external visible-light irradiation (λ > 420 nm). Pure water was the only reactant contained in the reaction vessel. Figure 13 shows simultaneous evolutions of H2 and O2 from

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Figure 13. Time course of H2 and O2 evolutions from pure water containing 0.5 g of NGO under visible-light (420 nm < λ < 800 nm) irradiation. A Xe lamp combined with UV and IR cutoff filters served as the light source. For purposes of comparison, the inset shows the H2 and O2 evolutions from pure water containing 0.5 g of the Rh2−yCryO3/GaN:ZnO catalyst.

S Supporting Information *

FTIR spectra of the GO specimens and the synthesis procedure of the GaN:ZnO catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.

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the reacting system, with rates that remained constant for 14 h. The molar ratio of the evolved H2 to O2 was smaller than the ideal ratio of 2 to 1. This deviation from the ideality may be associated with n-type conductivity that favors hole injection for O2 generation (R1), whereas the hindered injections of photogenerated electrons led to the formation of peroxide species (R2 and R3) in addition to H2 (R4). 2H 2O + 4h+ → O2 + 4H+

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax 886-6-2344496. Notes

The authors declare no competing financial interest.

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(R1)

H 2O + H + e → H 2O2

(R2)

H 2O + H+ + e− → 2·OH

(R3)

2H+ + 2e− → H 2

(R4)

ACKNOWLEDGMENTS This research is supported by the National Science Council of Taiwan (101-2221-E-006-243-MY3, 101-2221-E-006-225-MY3, 102-3113-P-006-012, and 102-3113-E-006-002) and the “Aim for the Top-Tier University and Elite Research Center Development Plan” of National Cheng Kung University.

Creating electron trap sites on the NGO surface is a means to promote H2 evolution from reaction R4. This study tested the well-characterized Rh2−yCryO3/GaN:ZnO catalyst in the present reacting system for comparison purposes.55 The inset

(1) Liu, H.; Liu, Y.; Zhu, D. Chemical Doping of Graphene. J. Mater. Chem. 2011, 21, 3335−3345.

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Tuning the Electronic Structure of Graphite Oxide through Ammonia

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