Graphite Oxide with Different Oxygenated Levels for Hydrogen and

ARTICLE pubs.acs.org/JPCC

Graphite Oxide with Different Oxygenated Levels for Hydrogen and Oxygen Production from Water under Illumination: The Band Positions of Graphite Oxide Te-Fu Yeh,† Fei-Fan Chan,† Chien-Te Hsieh,§ and Hsisheng Teng*,†,‡ †

Department of Chemical Engineering, and ‡Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan § Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 320, Taiwan

bS Supporting Information ABSTRACT: Graphite oxide (GO) photocatalysts derived from graphite oxidation can have varied electronic properties by varying the oxidation level. Absorption spectroscopy shows the increasing band gap of GO with the oxygen content. Electrochemical analysis along with the Mott
Schottky equation show that the conduction and valence band edge levels of GO from appropriate oxidation are suitable for both the reduction and the oxidation of water. The conduction band edge shows little variation with the oxidation level, and the valence band edge governs the bandgap width of GO. The photocatalytic activity of GO specimens with various oxygenated levels was measured in methanol and AgNO3 solutions for evolution of H2 and O2, respectively. The H2 evolution was strong and stable over time, whereas the O2 evolution was negligibly small due to mutual photocatalytic reduction of the GO with upward shift of the valence band edge under illumination. The conduction band edge of GO showed a negligible change with the illumination. When NaIO3 was used as a sacrificial reagent to suppress the mutual reduction mechanism under illumination, strong O2 evolution was observed over the GO specimens. The present study demonstrates that chemical modification can easily modify the electronic properties of GO for specific photosynthetic applications.

’ INTRODUCTION Hydrogen generation from water decomposition under light irradiation has received much attention due to the potential for solving energy requirements without environmental contamination.1
7 Photolysis of water using semiconducting photocatalyst powders is considered a prospective candidate because of the low cost in instrumentation and high interfacial contact area for reaction.8
17 The essential requirements for an effective semiconducting photocatalyst are more negative conduction band edge and more positive valence band edge, to facilitate, respectively, reduction and oxidation of water. Additionally, the contact area between the catalyst and water should be large enough to execute photochemical reactions at sufficiently high gas-evolution rates. Most photocatalysts for H2 and/or O2 evolution from water are metal-containing inorganic solids.18
28 High temperature calcination of these metal-containing photocatalysts is generally conducted for high crystallinity,29 but it shrinks the contacting surface area with water. Molecule-like materials with accurate electronic band structure, high surface area, and stability are an alternative to the crystalline solid.19
21,30,31 Graphite oxide (GO) is a polymer-like material made of carbon, oxygen, and hydrogen, having a large exposable surface area after being dispersed in water to molecular scale.32 We demonstrated that r 2011 American Chemical Society

GO can serve as a photocatalyst for H2 evolution from water.33 However, the conduction and valence band levels relative to those for water reduction and oxidation are yet to be explored. GO is derived from extensive oxidation of graphite and contains hydrophilic oxygen functional groups on constituting graphene sheets.34
40 Therefore, GO easily disperses in aqueous solutions and exfoliates in the manner of wrinkled paper. The electronic properties of GO are related to the composition of oxygen bonding on graphene sheets.41
50 The high electronegativity of oxygen atoms on carbon sheets causes the charge flow that exerts p-type semiconductivity to GO.41,47,51
55 As oxygen bonds on graphene, the valence band changes from the π-orbital of graphene to the O 2p orbital, leading to a larger bandgap for a higher oxygenated level of GO. Complete oxidation converts graphene from semiconductor to insulator.41 The conduction band of GO consists of the antibonding π-orbitals (that is, π*-orbitals).52 Understanding the principles of photocatalytic decomposition of water and structural characteristics of GO, we researched the Received: May 25, 2011 Revised: October 1, 2011 Published: October 10, 2011 22587

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electronic band energy levels of GO specimens with various oxidation levels using electrochemical methods along with the Mott
Schottky equation. The results reflect that with sufficient oxidation, the electronic structure of GO is suitable for both reduction and oxidation of water. We also justified these interpreted results using GO as the photocatalyst for production of H2 and O2 gases in the presence of sacrificial reagents. A detailed characterization of GO regarding the photocatalytic activity is discussed in this Article.

’ EXPERIMENTAL SECTION GO was prepared from a natural graphite powder (Bay carbon, SP-1, U.S.) using a modified Hummers’ method.56 The graphite powder (5 g) and NaNO3 (2.5 g; Merck, Germany) were introduced to concentrated H2SO4 (18 M, 115 mL; Wako, Japan) in an ice-bath. KMnO4 (15 g; J.T. Baker, U.S.) was added gradually with stirring, so that the temperature of the mixture was maintained below 20 °C. The mixture was then stirred at 35 °C for different periods (4
24 h) of oxidation. After oxidation, deionized water (230 mL) was slowly added to the mixture, followed by stirring the mixture at 98 °C for 15 min. We further diluted the mixture to 700 mL with stirring for 30 min. The reaction was terminated by adding H2O2 (12 mL, 35 wt %; Shimakyu, Japan) while stirring at room temperature. Multiple washings with deionized water (3  500 mL) were conducted, and the GO specimens were obtained by drying the precipitate of the final slurry at 40 °C for 24 h. The symbols GO1, GO2, and GO3 denote the specimens from 4, 12, and 24 h of oxidation, respectively. The crystal structure of GO was characterized by powder XRD using a Rigaku RINT-2000 (Japan) diffractometer with Cu Kα radiation, excited at 40 kV and 40 mA. 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) in diffuse reflectance mode was also conducted to analyze the oxygen functionalities using a Jasco FTIR-4100 (Japan) spectrometer. To obtain the absorption spectra, the GO/ water solutions were placed in a 1 cm quartz cuvette and analyzed using a Hitachi U-4100 (Japan) spectrophotometer. GO electrodes for electrochemical measurements were manufactured using electrophoretic deposition of GO on fluorinedoped SnO2 (FTO) conducting glass substrates (TEC 8, Hartford Glass Co., U.S.).57,58 A GO suspension for electrophoretic deposition was prepared by mixing GO in ethanol to produce a concentration of 5 g/L. An FTO glass substrate and Pt foil, as the anodic deposition substrate and counter electrode, were placed vertically and immersed in the GO suspension 1 cm apart. The deposition process was conducted using constant voltage at room temperature. To increase the film thickness without crack formation, we used a multilayer deposition, consisting of five repetitive depositions at 10 V for 60 s with intermediate drying at room temperature. We subjected the GO electrodes from electrophoretic deposition to air-drying at room temperature for 24 h. The Fermi level potential and donor density of the GO electrodes were analyzed by impedance spectroscopy using an impedance spectrum analyzer (Zahner IM6e, Germany) equipped with Thales software. In the present work, the measurements were performed by applying a sinusoidal potential perturbation with a small amplitude (10 mV), superimposed on a fixed dc potential varying within a potential window from 0 to 1 V (vs Ag/AgCl) in 0.5 M Na2SO4 (Riedel-de Ha€en, Germany) with

Figure 1. X-ray diffraction patterns showing sharp (002) peaks: (a) PG, pristine graphite; (b) GO1, graphite oxide from 4 h oxidation; (c) GO2, graphite oxide from 12 h oxidation; and (d) GO3, graphite oxide from 24 h oxidation.

a Pt foil as the counter electrode. We used a linear potential scan in the same electrochemical system to determine the conduction band edge of the GO electrodes. The measurement was conducted by scanning the electrode potential from 0.4 to
1.2 V at 5 mV/s. Photocatalytic reactions were conducted at approximately 25 °C in a gas-closed 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 filled with flowing temperature-controlled cooling water. For measurements with visible-light irradiation, a temperature-controlled NaNO2 aqueous solution (1 M) was flowed through the jacket to filter the UV light (λ < 400 nm). The GO photocatalysts (0.5 g) were suspended in the reaction chamber containing 1100 mL of methanol solution (20 vol %) for H2 evolution or AgNO3 solution (0.01 M) for O2 evolution tests. The amounts of evolved H2 and O2 were determined using gas chromatography (Hewlett-Packard 7890, U.S.; molecular sieve 5A column, thermal conductivity detector, argon carrier gas). The Ag nanoparticles deposited on the GO surface after the O2 evolution tests were analyzed with TEM (Hitachi H-7500, Japan). 22588

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Table 1. (O 1s)/(C 1s) Atomic Ratio Determined from the Full-Range XPS Spectra (Figures 2 and 9) and Carbon Bonding Composition Determined from the C 1s XPS (Figures 3 and 10) for the GO and irr-GO Specimens carbon bonding composition (%) carbon type

Figure 2. Full-range XPS spectra: (a) GO1; (b) GO2; and (c) GO3.

In addition to the AgNO3 solution,59 we used an NaIO3 solution (0.1 M), in which IO3
ions served as an electron scavenger,60 to measure the photocatalytic activity of GO for O2 evolution.

’ RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction (XRD) patterns of the pristine graphite (PG), GO1, GO2, and GO3. The sharp (002) peaks of PG, GO1, GO2, and GO3 near 26.3°, 10.2°, 9.9°,

(O 1s)/(C 1s)

C
C

C
O

CdO

GO1

1.82

49

38

13

GO2

2.00

48

39

13

GO3

2.48

44

43

13

irr-GO1

0.975

75

13

12

irr-GO2

1.06

60

28

11

irr-GO3

1.12

57

30

13

and 9.0° indicate interlayer distances of 0.34, 0.86, 0.89, and 0.98 nm. Oxidation on graphite significantly expands the interlayer spacing between graphene sheets, leading to the disappearance of the 26.3° peak and emergence of peaks with 2θ near 10°. The GO specimens obtained from a long oxidation time exhibited a larger interlayer spacing. This reflects that greater accommodation of oxygen species for more extensively oxygenated GO retained a larger amount of water molecules and led to a larger distance between the two graphene sheets. The variation in the interlayer distance confirms that the population of oxygen functionalities on graphene sheets increased with the duration for oxidation. This study uses XPS to analyze the composition of the oxygen functionalities of the GO specimens. Figure 2 shows that the peaks of the scan spectra have binding energies of approximately 284.6 (C 1s) and 533.5 eV (O 1s). The O 1s peak intensity increases with the degree of graphite oxidation. Quantitative analysis determined the surface O and C concentrations, and Table 1 shows the atomic ratios (O 1s)/(C 1s) for the GO specimens of varying oxidation degrees. The (O 1s)/(C 1s) ratio is an increasing function of the oxidation time, which agrees with the results of XRD analysis. The C 1s peak, ranging from 280
292 eV in the XPS spectra, comprises peaks contributed by several oxygen functionalities that have different binding energies. Figure 3 shows the C 1s spectra (solid lines) of the GO specimens. These spectra were decomposed into three peaks (indicated by the dashed lines) and fitted using a symmetric Gaussian function. These three peaks are due to C
C (284.6 eV), C
O (286.3 eV), and CdO (288.1 eV). Table 1 lists the composition of the functionalities obtained from the C 1s spectra. The proportion of the C
C group shows a constant decrease and that of the C
O group a constant increase with oxidation time, indicating a continuing formation of epoxy and tertiary alcohol on the basal plane caused by oxidation. On the other hand, the proportion of the CdO group remains constant irrespective of oxidation time, indicating that the carboxyl functionalities in the periphery of GO sheets reach saturation in 4 h oxidation. These XPS results reflect that the oxygenation of the graphene basal plane is responsible for the oxygen content increase with oxidation time. Figure 4a shows the optical absorption spectra of the GO specimens. The absorption spectra show a decrease in absorbance and a blue-shift with the prolonged oxidation. This indicates that the oxygen functionalities reduced the symmetry of the π
π* system to open the gap and eventually the top energy level of the valence band to be counterchanged to the O 2p orbital.51 22589

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Figure 4. (a) Optical absorption spectra of GO1, GO2, and GO3; (b) plots of (αhν)2 against the photon energy (hν) for GO1, GO2, and GO3; and (c) plots of (αhν)1/2 against the photon energy (hν) for GO1, GO2, and GO3.

Figure 3. C 1s XPS spectra (indicated by the solid lines): (a) GO1; (b) GO2; and (c) GO3. These spectra were decomposed into three peaks (indicated by the dashed lines) that were fitted using a Gaussian function.

The gap nature of graphene also gradually changes from direct to indirect with increasing accommodation of oxygen functionalities. Figure 4b and c shows the square and square root of the ab-

sorption energy (αhν, where α is the absorbance) against photon energy (hν) to determine the energies for direct and indirect gap transitions. Because GO comprises graphene molecules of various oxygenated levels, the converted plots do not show a sharp absorption edge for a precise gap of energy. The oxidation time for GO synthesis simply affects the proportion distribution of graphene sheets of different oxygenation levels. This renders all three GO/water solutions similar in color (light brown). From approximate linear extrapolation, Figure 4b and c displays apparent energies of 3.2
4.2, 3.5
4.5, and 3.8
4.6 eV for direct transition in the GO1, GO2, and GO3 specimens, respectively, and 2.3
2.8, 2.5
3.3, and 2.7
3.6 eV for indirect transition. The band gap energy is sufficiently large to overcome the theoretical 22590

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Figure 5. Variation of capacitance (C) with the applied potential in 0.5 M Na2SO4 presented in the Mott
Schottky relationship for: (a) GO1; (b) GO2; and (c) GO3. The capacitance was determined by electrochemical impedance spectroscopy.

endothermic 1.23 eV requirement of the water-splitting reaction. However, to generate H2 or O2, the conduction or valence band edges of GO must have sufficient overpotentials for water reduction or oxidation. To identify the electronic band characteristics of GO, we deposited the GO specimens on the FTO substrate and determined the Fermi level (EF) potentials of the GO films using electrochemical impedance spectroscopic analysis along with the Mott
Schottky equation,61
63 that is   1 2 kT ¼ E
E
F C2 eεε0 NA e where C represents the capacitance of the space
charge region, ε0 is the vacuum permittivity, ε is the dielectric constant of the GO, e is the electron charge, E is the applied potential, k is the Boltzmann constant, T is the absolute temperature, and NA is the acceptor density (or donor density). The temperature term is

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Figure 6. Cathodic linear potential scan for determining the conduction band (CB) edge of the p-type GO specimens at 5 mV/s: (a) GO1; (b) GO2; and (c) GO3.

generally small and can be neglected. The capacitance values of the space-charge region obtained at various applied potentials are shown in Figure 5. According to the Mott
Schottky equation, a linear relationship of 1/C2 versus E can be observed. The negative slope of the straight lines justifies the p-type conductivity of the GO films. The intercept on abscissa was used to calculate the Fermi levels, as indicated in Figure 5 for the GO specimens. The dielectric constant of GO may vary with the oxygen content on graphene or even with temperature,64 but the variation does not influence the Fermi level determination. The Fermi level of the GO specimens shows a positive shift with the population of oxygen functionalities on the graphene layers. The EF levels are 1.4, 1.5, and 1.6 V (vs Ag/AgCl) for GO1, GO2, and GO3. This indicates that a higher GO oxidation level results in a more positive potential in water oxidation. We subjected the conduction band edge position of the GO specimens to analysis with a linear potential scan. Figure 6 shows 22591

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Figure 7. Schematic energy level diagrams of the GO specimens in comparison with the potentials for water reduction and oxidation.

the results of the scan from 0.4 to
1.2 V (vs Ag/AgCl). An abrupt current increase due to formation of an inversion layer occurs at approximately
0.75 V (vs Ag/AgCl) for all GO1, GO2, and GO3. This indicates that the GO specimens had similar conduction band edge levels at
0.75 V (vs Ag/AgCl), regardless of the difference in the oxygen contents. On the basis of the preceding data including the band gap energies, Figure 7 shows the energy level diagrams of the GO specimens as compared to the potentials for water reduction and oxidation. We adopt the lower limits of the bandgap energies to construct these energy level diagrams. The conduction and valence bands of the GO specimens have suitable potential levels for water reduction and oxidation. We used sacrificial reagents for water splitting half-reactions, to evaluate photocatalytic activity of the GO specimens under illumination. Figure 8 shows the evolution of H2 and O2 gases over time, from the methanol and silver nitrate solutions over the GO catalysts under mercury-lamp irradiation. H2 gas generation was stable, and with no activity decay, for all catalysts during the experimental period (Figure 8a). As shown in Figure 7, the conduction band levels of the GO photocatalysts are high enough for water reduction to produce H2. The activity of the catalysts in H2 generation shows an order of GO1 > GO2 > GO3. More extensively oxygenated GO catalysts contain a greater proportion of wide-gap molecules, thus exhibiting a lower degree of light absorption (Figure 4a), and have a lower activity. A cocatalyst, serving as an electron trap, is generally required to assist water reduction for H2 production.60,65
67 Platinum is generally used as a cocatalyst for H2 generation.68
70 Auxiliary experiments showed that in situ photodeposition of Pt did not promote the H2 evolution rate for all catalysts.33 This indicates that photoinduced charges in molecule-like GO catalysts can easily access the catalyst
liquid interface to react with the species in the liquid phase. The inset of Figure 8a shows the evolution of H2 from the methanol solution containing the GO1 specimen under visible-light irradiation. The H2 evolution rate was stable and comparable with that of graphitic C3N4,19 but much lower than those with mercury-lamp irradiation were. Obviously, a great proportion of the GO molecules is not visible-light active in terms of photocatalytic activity. This indicates that the GO specimens

Figure 8. (a) Time course of H2 evolution from a 20 vol % aqueous methanol solution suspended with 0.5 g of GO photocatalysts under mercury-lamp irradiation. (b) Time course of O2 evolution from a 0.01 M aqueous AgNO3 solution suspended with 0.5 g of GO photocatalysts under mercury-lamp irradiation. The inset in panel (a) shows H2 evolution from the methanol solution suspended with 0.5 g of the GO photocatalyts under visible light irradiation.

with these oxidation levels were not effective to absorb visible light for energy conversion. Figure 8b shows the evolution of O2 from the silver nitrate solution over GO3, whereas the evolution of O2 over GO1 and GO2 was not detectable. The band energy diagram in Figure 7 indicated that the valence band edges of all GO specimens are low enough for water oxidation. The low activity of GO1 and GO2 in O2 production may be associated with the reduction of GO with light irradiation and the corresponding narrowing of the band gap.33 This reduction effect must have resulted in changes in the electronic structure of GO. During photocatalytic reaction, we observed color change and aggregation of GO sheets due to GO reduction.33,71,72 We therefore subjected the GO specimens with 6 h mercury-lamp irradiation in the methanol solution (that is, irr-GO) to analysis with XPS, optical absorption, and electrochemical spectroscopy. Our previous study has pointed out that color change of the GO-suspended solution was significant in the 22592

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Figure 9. Full-range XPS spectra: (a) irr-GO1; (b) irr-GO2; and (c) irrGO3.

first 2 h mercury-lamp irradiation, and the change became minor afterward.33 The irr-GO specimens, which have been irradiated for 6 h, therefore represent the photocatalysts nearly saturated with irradiation treatment. Figure 9 shows the full-range XPS spectra of the irr-GO specimens. When compared to the results of the GO specimens (Figure 2), Figure 9 reflects an obvious reduction in the O 1s

Figure 10. C 1s XPS spectra (indicated by the solid lines): (a) irr-GO1; (b) irr-GO2; and (c) irr-GO3. These spectra were decomposed into three peaks (indicated by the dashed lines) that were fitted using a Gaussian function.

peak intensity for GO after irradiation in the methanol solution. Table 1 compares the atomic ratios (O 1s)/(C 1s) for the GO 22593

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Figure 12. Variation of capacitance (C) with the applied potential in 0.5 M Na2SO4 presented in the Mott
Schottky relationship for: (a) irrGO1; (b) irr-GO2; and (c) irr-GO3. The capacitance was determined by electrochemical impedance spectroscopy. Figure 11. (a) Optical absorption spectra of the irradiated GO specimens: irr-GO1, irr-GO2, and irr-GO3; (b) plots of (αhν)2 against the photon energy (hν) for irr-GO1, irr-GO2, and irr-GO3; and (c) plots of (αhν)1/2 against the photon energy (hν) for irr-GO1, irr-GO2, and irrGO3.

and irr-GO specimens, explicitly exhibiting the removal of oxygen functionalities by irradiation. We also decomposed the C 1s spectra of irr-GO into three peaks, that is, C
C (284.6 eV), C
O (286.3 eV), and CdO (288.1 eV), using a symmetric Gaussian function (Figure 10), and Table 1 lists the composition of the functionalities for both the GO and irr-GO specimens. The proportion of the CdO group remained constant after irradiation, indicating that the CdO group resisted the photocatalytic reduction. The proportion of the C
O group shows a significant decrease after irradiation, indicating an extensive removal of epoxy and tertiary alcohol on the basal plane caused by irradiation. The oxygen content of the irr-GO specimens is still an increasing

function of oxidation time. The band gap of irr-GO3 would therefore be wide enough for O2 evolution, whereas those of irrGO1 and irr-GO2 would be too narrow because of the low oxygenation degrees. Figure 11a shows the absorption spectra of the irradiated GO specimens and reveals that the irradiation resulted in extension of the GO absorption spectra (Figure 4a) to longer wavelengths. The converted spectra in Figure 11b and c show that the irradiation reduced the direct transition gap to 1.7
2.8, 2.0
2.9, and 2.4
3.3 eV for irr-GO1, irr-GO2, and irr-GO3, respectively, and the indirect gap to 1.5
2.0, 1.6
2.2, and 1.8
2.2 eV. The XPS spectra of the GO and irr-GO specimens have shown that the irradiation led to oxygen functionality removal, which causes band gap reduction. Additional FTIR analysis of the GO and irrGO specimens (see the Supporting Information) reflect reduced absorption intensities for all of the oxygen functionalities on GO because of irradiation. For electronic band characterization, 22594

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Figure 14. Schematic energy level diagrams of the irr-GO specimens in comparison with the potentials for water reduction and oxidation.

Figure 13. Cathodic linear potential scan for determining the conduction band (CB) edge of the p-type irr-GO specimens at 5 mV/s: (a) irrGO1; (b) irr-GO2; and (c) irr-GO3.

Figure 12 shows the capacitance values of the irr-GO film electrodes obtained with the electrochemical impedance analysis at varying applied potentials in a Mott
Schottky plot. The Fermi levels of irr-GO specimens, corresponding to the intercepts on abscissa, are indicated in the figure. The data of Figures 5 and 12 show that the Fermi levels of GO exhibit a strong negative shift because of the reduction from irradiation. The EF levels are 0.75, 0.89, and 1.0 V (vs Ag/AgCl) for irr-GO1, irr-GO2, and irr-GO3. Figure 13 shows the linear potential scan results for the irr-GO electrodes. The scan measurements provided the irr-GO specimens a conduction band edge of approximately
0.7 V (vs Ag/AgCl), representing an insignificant positive shift relative to that of the GO specimens. The humps appearing in the results of irr-GO1 and irr-GO2 may be associated with the pseudocapacitive response of the reduced GO that are electrically conductive.33,71,72 By incorporating the results from the optical and electrochemical measurements, we constructed the band energy diagrams for the

Figure 15. TEM image of Ag nanoparticles dispersed on the graphene oxide sheets of GO3.

irr-GO specimens, as shown in Figure 14. The diagrams show that the conduction band of the irr-GO specimens remained high enough for H2 evolution. This explains why the H2 evolution rate remained stable during the entire period of light irradiation (Figure 8a). By contrast, the valence band edge of irr-GO1 and irr-GO2 lies above the potential level for O2 evolution, in agreement with the observation that no O2 evolved from the AgNO3 solution during irradiation. Regarding irr-GO3, the valence band edge is only marginally more positive than the water oxidation level. This explains why O2 evolution over GO3 was detected (Figure 8b), but the amount was much smaller than that of H2 evolution. Figure 15 shows the transmission electron microscopy (TEM) image of irr-GO3 that was irradiated in the AgNO3 solution. Ag nanoparticles, which were identified with energy dispersive X-ray spectroscopy of 2
10 nm, are dispersed on the GO sheets. This indicates that the reduced Ag atoms serve as the electron traps to reduce surrounding Ag+ ions for nanoparticle growth.73 This also 22595

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modification or stabilization of oxygen functionalities on graphene sheets.

Figure 16. Time course of O2 evolution from a 0.1 M aqueous NaIO3 solution suspended with 0.5 g of GO photocatalysts under mercurylamp irradiation.

shows that the Ag+ ions cannot serve as a charging agent to improve dispersion of GO sheets and, thus, to avoid mutual photocatalytic reduction. In addition to the AgNO3 solution, we also suspended the GO specimens in an NaIO3 solution for O2 evolution under mercurylamp irradiation. The IO3
ions serve as a sacrificial reagent to scavenge electrons generated from light irradiation on GO according to74
76 IO3







þ 3H2 O þ 6e f I




þ 6OH




Figure 16 shows that the evolution of O2 was strong from the NaIO3 solutions containing GO1, GO2, and GO3. During the photocatalytic reaction, the GO specimens remained dispersed in the solution, indicating that the reduction of GO was minimized in the NaIO3 solution. We inferred that the adsorbed Na+ ions on the GO sheets introduced repulsion force between the sheets and facilitated GO dispersion. This prevented the mutual photocatalytic reduction of GO. Under this circumstance, the valence band levels of all of the GO specimens were positive enough for O2 evolution from water, as shown in Figures 7 and 16. Similar to H2 evolution, the amount of O2 evolution shows an order of GO1 > GO2 > GO3, due to a stronger absorption spectrum for less oxygenated GO. However, the rate of O2 evolution exhibited a decreasing trend with the reaction time due to the inevitable mutual reduction between GO sheets under illumination. This study demonstrated that GO materials offer the potential for engineering the band gap and electronic characteristics of photocatalyts. Although stable photocatalytic performance was observed in both of the half reactions, especially for H2 evolution, it did not lead to a similar photocatalytic activity in overall water splitting in the absence of sacrificial reagents. Nevertheless, the study of the GO catalyst half-reactions and their physicochemical properties is still valuable for developing light sensitive photocatalysts. To obtain a high activity for O2 evolution, reduction of GO during irradiation must be avoided. This can be achieved by binding the oxygen functionalities with nitrogen.77 Further studies are conducted along with a more detailed investigation into the correlation between the oxidation activity and the

’ CONCLUSIONS In this study, polymer-like GO showed potential as a photocatalyst for H2 and O2 evolution from water with the presence of sacrificial reagents. Optical absorption and electrochemical impedance analysis, along with the Mott
Schottky theory, demonstrated that the band gap energy of GO increases with the population of oxygen functionalities on GO sheets. The downward shift in the valence band edge was predominantly responsible for the enlargement of the band gap in the GO sheets, and the change in the conduction band edge was negligible. With sufficient oxygen content, the electronic structure of GO proved suitable for both the reduction and the oxidation of water under illumination. During photocatalytic reaction, the mutual reduction between GO sheets narrowed the band gap, leading to activity decay of GO in catalyzing O2 evolution from an AgNO3 solution because of the upward shift of the valence band edge, whereas the activity for H2 evolution from a methanol solution remained unchanged. A strong evolution of O2 from an NaIO3 solution under illumination was observed, probably due to more effective GO dispersion to suppress mutual reduction. The electronic and photocatalytic analysis of GO demonstrated that GO has a tunable feature for its properties and can serve as effective media for photosynthetic reactions including water decomposition. ’ ASSOCIATED CONTENT

bS

Supporting Information. FTIR spectra of the GO specimens. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

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

’ ACKNOWLEDGMENT This research is supported by the National Science Council of Taiwan (98-2221-E-006-110-MY3, 99-2622-E-006-010-CC2, 1003113-E-006-001, 100-3113-E-006-012, 100-3113-E-007-008, and 98-2221-E-006-112-MY2), and the Bureau of Energy, Ministry of Economic Affairs, Taiwan (100-D0204-2). ’ REFERENCES (1) Khaselev, O.; Turner, J. A. Science 1998, 280, 425–427. (2) Gr€atzel, M. Nature 2001, 414, 338–344. (3) Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295–295. (4) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253–278. (5) Kitano, M.; Hara, M. J. Mater. Chem. 2010, 20, 627–641. (6) Zhang, G.; Ding, X.; Hu, Y.; Huang, B.; Zhang, X.; Qin, X.; Zhou, J.; Xie, J. J. Phys. Chem. C 2008, 112, 17994–17997. (7) Liu, J. W.; Chen, G.; Li, Z. H.; Zhang, Z. G. Int. J. Hydrogen Energy 2007, 32, 2269–2272. (8) Zhou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625–627. (9) Sayama, K.; Mukasa, K.; Abe, R.; Abe, Y.; Arakawa, H. Chem. Commun. 2001, 23, 2416–2417. (10) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Am. Chem. Soc. 2004, 126, 13406–13413. 22596

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