Photocatalytic Activities of Graphitic Carbon Nitride Powder for Water

Mar 5, 2009 - X.W.: phone +49-31-567-9515, fax +49-331-567-9502, e-mail [email protected]. K.D.: phone +81-3-5841-1148, fax +81-3-5841-8838, e...
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J. Phys. Chem. C 2009, 113, 4940–4947

Photocatalytic Activities of Graphitic Carbon Nitride Powder for Water Reduction and Oxidation under Visible Light Kazuhiko Maeda,§,† Xinchen Wang,*,‡,| Yasushi Nishihara,⊥ Daling Lu,# Markus Antonietti,‡ and Kazunari Domen*,† Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Department of Colloid Chemistry, Max-Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Postdam, Germany, Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou UniVersity, Fuzhou 350002, China, DiVision of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama UniVersity, Tsushimanaka, Okayama 700-8530, Japan, and Center for AdVanced Materials Analysis, Tokyo Institute of Technology, 2-12-1 Okayama, Meguro-ku, Tokyo 152-8550, Japan ReceiVed: October 15, 2008; ReVised Manuscript ReceiVed: December 12, 2008

Graphitic carbon nitride (g-C3N4) with a band gap of 2.7 eV is studied as a nonmetallic photocatalyst for H2 or O2 evolution from water under ultraviolet (UV) and visible light. The g-C3N4 catalyst exhibits activities for water reduction into H2 or water oxidation into O2 in the presence of a proper sacrificial electron donor or acceptor, respectively, even without the need for precious metal cocatalysts. When bis(1,5-cyclooctadiene)platinum complex [Pt(cod)2] (a nonionic complex) is used as a precursor of Pt cocatalyst instead of H2PtCl6 (an ionic complex), enhanced H2 evolution activity is acquired. This difference in activity is primarily due to the better dispersion of Pt nanoparticles on g-C3N4, which is considered to originate from the better access of Pt(cod)2 to the g-C3N4 surface, as compared to that of H2PtCl6 in the preparation process. Unmodified g-C3N4 produces O2 from an aqueous silver nitrate solution upon UV irradiation (λ > 300 nm), although N2 release due to self-decomposition of g-C3N4 by photogenerated holes takes place. Modification of g-C3N4 with RuO2 improves not only O2 evolution activity but also stability against the self-decomposition, resulting in stable visible-light-driven O2 evolution (λ > 420 nm). 1. Introduction Direct water splitting using a particulate photocatalyst with visible light is an attractive means of hydrogen production from the viewpoint of large-scale application.1 To date, studies on heterogeneous photocatalysis for visible-light water splitting have focused on developing a material with sufficiently small band gap, an appropriate band edge position for overall water splitting, and the stability necessary for practical applications.2 However, visible-light-responsive photocatalysts identified to date to exhibit water reduction and oxidation ability had been comprised solely of metal-based inorganic solids including metal-oxides,3-8 metal-(oxy)nitrides,9-15 and metal-oxysulfides,16,17 primarily due to the lack of a material that has appropriate band edge positions and adequate stability during the photocatalysis process. Therefore, the development of new materials exhibiting useful electronic structure and high stability for water splitting represents a central challenge of photocatalysis research. Carbon nitride, which is essentially composed of covalent bondings, has received a large interest as the most promising candidate to complement carbon in materials applications, motivated by the theoretical prediction that the β-C3N4 phase * To whom corresponding author should be addressed. X.W.: phone +4931-567-9515, fax +49-331-567-9502, e-mail [email protected]. K.D.: phone +81-3-5841-1148, fax +81-3-5841-8838, e-mail domen@ chemsys.t.u-tokyo.ac.jp. § Research fellow of the Japan Society of Promotion Science (JSPS). † The University of Tokyo. ‡ Max-Planck Institute of Colloids and Interfaces. | Fuzhou University. ⊥ Okayama University. # Tokyo Institute of Technology.

should be hard as diamond.18 Theoretically, there are several hypothetical phases of carbon nitrides, including the R, β, cubic, pseudocubic, and graphitic (Scheme 1).19 Among these, graphitic carbon nitride is, however, considered the most stable at ambient conditions and has the smallest direct band gap due to the sp2 hybridization of carbon and nitrogen forming the π-conjugated graphitic planes.19 Accordingly, there are an even larger number of reports in the literature approaching the synthesis of different modifications of this material. The authors have adopted the thermal polycondensation of common organic monomers to synthesize graphitic carbon nitride (g-C3N4) by combining melem units (Scheme 1f) with various architectures, which have been successfully used as metal-free heterogeneous catalysts for Friedel-Crafts reactions20 and activation of CO2 with benzene as a sacrificial hydrocarbon.21 Very recently, as our continuing efforts to explore metal-free catalysis with carbon nitride, we briefly reported that g-C3N4 with a band gap of 2.7 eV achieves functionality as a stable photocatalyst for H2 evolution from water containing a proper electron donor under visible light irradiation (λ > 420 nm), even without using a cocatalyst.22 Density functional theory (DFT) calculations suggest that the visible-light-response of the photocatalyst originates from an electron transition from the valence band populated by N2p orbitals to the conduction band formed by C2p orbitals, which contributes to the visible-light-driven H2 evolution from water. This is a successful example of H2 evolution using a nonmetallic semiconductor photocatalyst with adequate stability. The discovery of a nonmetallic material achieving the same function as conventional metal-based

10.1021/jp809119m CCC: $40.75  2009 American Chemical Society Published on Web 03/05/2009

Visible-Light-Responsive C3N4 Photocatalyst

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SCHEME 1: Crystal Structure Models Proposed for Carbon Nitride: (a) r-C3N4; (b) β-C3N4; (c) Cubic C3N4; (d) Pseudocubic C3N4; (e) a Graphitic-C3N4 Sheet Based on Melamine Building Blocks; and (f) a Graphitic-C3N4 Sheet Based on Melem Building Blocks

photocatalysts is expected to offer new opportunities for progress in the field of artificial photosynthesis. In contrast to H2 evolution that is a kinetically simpler process, O2 evolution via water oxidation is a relatively difficult reaction to achieve, because the process involves a complicated mechanism.23,24 In addition, for nonoxide-type photocatalysts, anion components such as N3- and S2- in nonoxides are less stable than those in metal-oxides25,26 and, in some cases, more susceptive to oxidation than water,24 rendering O2 evolution by nonoxide catalysts a challenge. It has been believed that active sites for catalytic water oxidation to produce O2 molecules consist of metals with a complicated structure.23,24 For example, photosystem II in green plants has an oxo-bridged manganese aggregate as the catalytic O2 evolution site.23 At this moment, the fact that

g-C3N4 photocatalyst lacks metal components in the pristine composition stimulates us to examine the possibility of g-C3N4 for water oxidation, i.e., whether water oxidation takes place on the metal-free surface or not, although RuO2-loaded g-C3N4 catalyzes photooxidation of water at a moderate rate.22 To develop a highly efficient photocatalytic system, it is important to know the reaction behavior of a given photocatalyst. As photocatalytic properties are expected to largely differentiate nonmetallic materials from the conventional metal-based materials, detailed investigation on g-C3N4 as a photocatalyst is of interest and will provide useful information for progress in heterogeneous photocatalysis. The present full article reports on a more detailed investigation of photocatalytic activities of g-C3N4 not only for water reduction but also for water oxidation.

4942 J. Phys. Chem. C, Vol. 113, No. 12, 2009 2. Experimental Section 2.1. Preparation of Graphitic Carbon Nitride. The graphitic carbon nitride (g-C3N4) was prepared by heating cyanamide (99%, Aldrich Chemical Co.) in air at 823 K for 4 h according to our previous report.22 Before use for photocatalytic reactions, the thus-obtained g-C3N4 was ground well into a powder with a mortar. The band gap energy of the material was estimated to be ca. 2.7 eV from the onset of the diffuse reflectance spectrum. The specific surface area of g-C3N4 was determined to be approximately 10 m2 · g-1 based on the Brunauer-Emmett-Teller (BET) method at liquid nitrogen temperature. 2.2. Modification with Cocatalysts. Various metals as cocatalysts for H2 evolution are loaded onto the as-prepared g-C3N4 catalysts by an in situ photodeposition method27 with use of (NH4)2RuCl6 (Aldrich), Na3RhCl6 · 2H2O (Kanto Chemicals, 97% as Rh), (NH4)2PdCl4 (Kanto Chemicals, 37% as Pd), Na2IrCl6 · 6H2O (Kanto Chemicals, 97% as Ir), H2PtCl6 · 2H2O (Kanto Chemicals, 97% as Pt), and HAuCl4 · 4H2O (Kanto Chemicals, 99.0%) as precursors. To examine the effect of loading method, Pt was also loaded by an impregnation method by using bis(1,5-cyclooctadiene)platinum complex [Pt(cod)2].28 The g-C3N4 powder was immersed in a tetrahydrofuran (THF) solution containing dissolved Pt(cod)2 and stirred at 333 K for 4 h. The solution was then dried under reduced pressure, followed by heating in air at 373 K for 1 h to remove THF. The resulting powder was finally heated in air at various temperatures for 1 h. The Pt-loaded g-C3N4 samples prepared by using different precursors are studied by high-resolution transmission electron microscopy (HR-TEM; Jeol JEM-2010F). Ruthenium(IV) oxide as a cocatalyst for O2 evolution was loaded onto g-C3N4 by an impregnation method in a similar manner to that described above, but with dodecacarbonyltriruthenium, [Ru3(CO)12] (99%, Aldrich Chemical Co.), as the precursor.13b,14a,29,30 2.3. Photocatalytic Reactions. Reactions were carried out in a Pyrex top irradiation-type vessel connected to a glass closed gas circulation system. Photoreduction of H+ to H2 was performed by dispersing 0.1 g of the Pt-loaded catalyst powder in an aqueous triethanolamine solution (TEA, 10 vol %, 100 mL) as the sacrificial reagent without pH control (pH ∼10), while photooxidation of H2O to O2 was conducted by dispersing 0.1 g of the as-prepared g-C3N4 powder in an aqueous silver nitrate solution (AgNO3, 0.01 M, 100 mL) containing 0.2 g of La2O3, which buffers the pH of the reactant solution at pH 8-9 during the reaction.9-12,15-17 The reactant solution was evacuated several times to remove air completely prior to irradiation under a 300 W xenon lamp fitted with a cutoff filter and a water filter. The temperature of the reactant solution was maintained at room temperature by a flow of cooling water during the reaction. The evolved gases were analyzed by gas chromatography. 3. Results and Discussion 3.1. Effect of Metal-Loading on the H2 Evolution Activity of g-C3N4. In the absence of cocatalysts, the as-prepared g-C3N4 produces H2 from water containing TEA as an electron donor. Although the H2 evolution activity of bare g-C3N4 is somewhat fluctuant, metal-loading can solve this problem by improving the activity and minimizing the experimental error within 10-15%.22 Metals loaded on the surface of g-C3N4 effectively capture photogenerated electrons in the conduction band of g-C3N4 and host active sites for H2 evolution, thereby enhancing the catalytic performance. It is known that photocatalytic activity of a given material for H2 evolution from aqueous solution

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Figure 1. Dependence of steady rate of H2 evolution by Pt-loaded g-C3N4 under visible light (λ > 420 nm) on loading amount of Pt. Reaction conditions: catalyst, 0.1 g; reactant solution, aqueous triethanolamine solution (100 mL); light source, xenon lamp (300 W) with cutoff filter; reaction vessel, top-irradiation type.

TABLE 1: Photocatalytic H2 Evolution Activities of g-C3N4 Modified with Various Metal Cocatalystsa entry

cocatalystb

steady rate of H2 evolution/µmol h-1

1 2 3 4 5 6

Ru Rh Pd Ir Pt Au

2.1 1.6 5.7 0.2 7.3 3.7

a Reaction conditions: catalyst, 0.1 g; reaction solution, aqueous triethanolamine solution (100 mL); light source, xenon lamp (300 W) with cutoff filter; reaction vessel, top-irradiation type. b 0.5 wt % loaded by an in situ photodeposition method.

containing an electron donor is dependent on the kind of metal cocatalysts employed.31-33 As listed in Table 1, it was found that H2 evolution activity of g-C3N4 is also dependent on the loaded cocatalysts. Among metals examined as cocatalysts, Pt loading was most effective for enhancing H2 evolution by g-C3N4, while Ir had little promotional effect. To achieve efficient H2 evolution, migration of photogenerated electron to the loaded cocatalyst and reduction of surface-adsorbed H+ followed by H2 formation on the metal-cocatalyst surface have to proceed smoothly. Platinum possesses an excellent ability to act as a catalytically active site for H2 evolution due to the smallest overvoltage, while iridium has the largest values among metals employed.34 Therefore, it appears that the rate of H2 evolution by g-C3N4 is related to the overvoltage of metals for H2 evolution, although other factors cannot be excluded. 3.2. Effect of Pt-Loading Method on Water Reduction Activity of Pt-Loaded g-C3N4. In the previous section, we have applied an in situ photodeposition method for metal loading as a H2 evolution site on the g-C3N4 surface. To examine the effect of the loading method of Pt, we attempted to apply an impregnation method with Pt(cod)2 that is a nonionic platinum metal complex as a new precursor. Figure 1 shows the dependence of the steady rate of H2 evolution on the amount of loaded Pt for samples calcined at 423 K after impregnation from a THF solution containing Pt(cod)2. The rate of H2 evolution increased markedly with Pt content to a maximum at 1.0 wt %, then decreased upon further loading. Reproducibility tests showed that experimental error is within 10%. Although the effect of calcination temperature after impregnation with Pt(cod)2 on activity was also investigated, the steady rate of H2 evolution with catalysts calcined at 373-623 K remained almost unchanged (ca. 22 µmol · h-1). As the melting point of Pt(cod)2 is 383-388 K, it appears that calcination at temperatures in

Visible-Light-Responsive C3N4 Photocatalyst

Figure 2. Time courses of H2 evolution on g-C3N4 modified with (a) 1.0 wt % Pt from Pt(cod)2 by impregnation and (b) 3.0 wt % Pt from H2PtCl6 by photodeposition (optimized in the previous study) under visible light (λ > 420 nm). Reaction conditions: catalyst, 0.1 g; reactant solution, aqueous triethanolamine solution (100 mL); light source, xenon lamp (300 W) with cutoff filter; reaction vessel, top-irradiation type.

the above range is enough to eliminate the coordinating 1,5cyclooctadiene ligands from the Pt metal and to produce an active H2 evolution catalyst. It has been reported by many researchers that the rate of H2 evolution from aqueous solution containing a sacrificial electron donor by a given photocatalyst is enhanced with increasing the amount of loaded Pt, while excess loading results in lowering the catalytic activity.32,35-39 This is because an increase in Pt loading amount contributes to increasing the density of active sites for H2 evolution, while excess loading can hinder light absorption of the photocatalyst and/or cause enhanced recombination between photogenerated electrons and holes.38 The appearance of a maximum in the curve of the H2 evolution activity of Pt-loaded g-C3N4 with respect to Pt loading amount (Figure 1) indicates that an appropriate amount of Pt loading is essential to maximize efficiency. This is an identical tendency to the previous reported metal-based photocatalysts for H2 evolution from an aqueous solution containing sacrificial electron donors.32,35-39 Figure 2 shows the time course of H2 evolution on 1.0 wt % Pt-loaded g-C3N4 catalyst prepared by an impregnation with Pt(cod)2 under visible light (λ > 420 nm). The data obtained by an in situ photodeposition with H2PtCl6 (optimized in the previous study22) are also shown for comparison. Although each catalyst produced H2 steadily upon visible light, the steady rate of H2 evolution achieved by the impregnation method is about 2 times faster than that achieved by the photodeposition method. HR-TEM images of each optimized sample are shown in Figure 3. The Pt deposits are clearly distinguishable because of the difference in electron density between Pt and g-C3N4. Aggregation of Pt nanoparticles is observed in the sample prepared by the photodeposition method in addition to ∼3 nm nanoparticles (Figure 3A). In contrast, the impregnation method sample exhibits relatively good dispersion of Pt nanoparticles with 1-3 nm in size (Figure 3B). This difference in the arrangement of Pt nanoparticles is considered to affect the activity of Pt-loaded g-C3N4 for H2 evolution. It is a general trend in H2 evolution by a particulate photocatalyst that highly dispersed cocatalysts such as Pt are essential for enhanced activity.32,35-39 In the case of the photodeposition method, Pt deposits are formed by the reduction of PtCl62- anions with photogenerated electrons that come from g-C3N4, and tend to aggregate because photogenerated electrons migrate easily to the previously deposited Pt nanoparticles. Since g-C3N4 is essentially made of a covalent bond, the access of PtCl62- anions (ionic species) to

J. Phys. Chem. C, Vol. 113, No. 12, 2009 4943 the surface of g-C3N4 is expected to be prevented to some extent. This situation also assists the aggregation of Pt nanoparticles on g-C3N4, which contributes to a decrease in activity. On the other hand, it appears that the access of a nonionic precursor, Pt(cod)2, to the g-C3N4 surface is better than that of PtCl62anions. This would lead to better dispersion of the precursor in the preparation process and the resulting Pt nanoparticles. It was thus found that an impregnation method with Pt(cod)2 is better for loading Pt cocatalysts on g-C3N4 than an in situ photodeposition method with H2PtCl6. 3.3. Water Oxidation on Unmodified g-C3N4. As mentioned earlier, water oxidation by nonoxide catalysts remains a challenge. In fact, there are some cases where N2 evolution and sulfur deposition are observed as a result of oxidative decomposition of the photocatalyst.25,26 When g-C3N4 was examined as a water oxidation photocatalyst under visible light (λ > 420 nm), no gas evolution was detected even in the presence of AgNO3 as an electron acceptor, being different from the good visible-light activity for H2 evolution. This result implies that photogenerated electrons and holes in the g-C3N4 underwent recombination before they reach the surface region of g-C3N4. Upon ultraviolet irradiation (λ > 300 nm), however, production of O2 was clearly observed, as shown in Figure 4. No gas evolution was detected in the dark. From the results of H2 and O2 evolution, it is concluded that g-C3N4 meets the thermodynamic requirements for overall water splitting; that is, the tops of valence bands of the material are located at a more positive level than the water oxidation potential, whereas the bottoms of the conduction bands are located at a more negative level than the water reduction potential. This is consistent with the DFT calculations.22 However, the fact that N2 evolution is simultaneously detected in the O2 evolution reaction indicates that the material is decomposed by photogenerated holes. N2 evolution is sometimes observed in photocatalytic reactions with nitrogen-containingphotocatalystssuchasmetal-(oxy)nitrides.9-15,25 This decomposition consumes photogenerated holes that would otherwise be consumed in the oxidation of water, and is described as follows.

2N3- + 6h+ f N2

(1)

3.4. Improved Water Photooxidation by Modification with RuO2. It is expected that the oxidative decomposition of g-C3N4 accompanied by N2 evolution may be suppressed by constructing a suitable catalytic active site that efficiently promotes O2 evolution on the g-C3N4 surface. We thus attempted to introduce RuO2 using Ru3(CO)12 as the precursor.29,30 RuO2 is well-known as a good oxidation catalyst for O2 evolution. In the case of water splitting, RuO2 has been demonstrated by many researchers to be effective as an oxidation site for the evolution of O2.40-43 Figure 5 shows the rate of O2 evolution with RuO2-modified g-C3N4 catalyst under UV irradiation (λ > 300 nm) as a function of the loading amount of RuO2 with a common calcination temperature of 573 K. As will be shown in Figure 6, the activity for O2 evolution decreased gradually with reaction time, although an induction period was observed during the initial stage. The decrease in activity is primarily attributable to the deposition of metallic silver on the catalyst surface, which blocks light absorption and obstructs active sites.9-12,15-17 Therefore, the fastest O2 evolution rate was taken as the activity. The activity was improved with an increase in the RuO2 content to reach a maximum at around 3.0 wt %, then decreasing gradually.

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Figure 3. HR-TEM images of g-C3N4 loaded with (A) 3.0 wt % Pt from H2PtCl6 by photodeposition (preparation time, 7 h) and (B) 1.0 wt % Pt from Pt(cod)2 by impregnation.

Figure 4. Time course of O2 evolution on g-C3N4 under UV light (λ > 300 nm) from a silver nitrate solution containing 0.2 g of La2O3. Reaction conditions: catalyst, 0.1 g; reactant solution, aqueous silver nitrate solution (0.01 M, 100 mL); light source, xenon lamp (300 W) with cutoff filter; reaction vessel, top-irradiation type.

At the optimal loading condition, the activity was 5 times as high as that achieved by unmodified catalyst. The enhancement of activity with loading amount of RuO2 is attributed to an increase in the density of active sites for O2 evolution, while the decreased activity at larger loading amounts than 3.0 wt % is associated with excess loading which reduces the density of photocatalytic active sites and/or hinders light absorption of the g-C3N4 component. A similar volcano-like relationship between the loading amount of RuO2 and the activity has been observed in other photocatalytic systems.13b,14a,30 It should be noted that N2 evolution due to the decomposition of g-C3N4 became

Figure 5. Dependence of initial rate of O2 evolution from a silver nitrate solution containing 0.2 g of La2O3 by RuO2-loaded g-C3N4 under visible light (λ > 420 nm) on loading amount of RuO2 with a common calcination temperature of 573 K. Reaction conditions: catalyst, 0.1 g; reactant solution, aqueous silver nitrate solution (0.01 M, 100 mL); light source, xenon lamp (300 W) with cutoff filter; reaction vessel, top-irradiation type.

negligibly slow (less than detectable limit) upon RuO2 modification even in lower RuO2 content (1.0 wt %). These results indicate that the loaded RuO2 nanoparticles capture photogenerated holes migrated from g-C3N4, exhibiting the functionality as efficient O2 evolution sites. It was thus found that, with an appropriate kinetic control in photooxidation reaction, the stability of this catalytic system is much improved against the degradation of the nitride material. Although RuO2 on some photocatalysts has been reported to act not only as an oxidation

Visible-Light-Responsive C3N4 Photocatalyst

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Figure 6. Time courses of O2 evolution on 3.0 wt % RuO2-loaded g-C3N4 calcined at (a) 523, (b) 573, and (c) 623 K under UV light (λ > 300 nm) from a silver nitrate solution containing 0.2 g of La2O3. Reaction conditions: catalyst, 0.1 g; reactant solution, aqueous silver nitrate solution (0.01 M, 100 mL); light source, xenon lamp (300 W) with cutoff filter; reaction vessel, top-irradiation type.

Figure 7. Time course of O2 evolution on 3.0 wt % RuO2-loaded g-C3N4 calcined at 573 K under visible light (λ > 420 nm) from a silver nitrate solution containing 0.2 g of La2O3. Reaction conditions: catalyst, 0.1 g; reactant solution, aqueous silver nitrate solution (0.01 M, 100 mL); light source, xenon lamp (300 W) with cutoff filter; reaction vessel, top-irradiation type.

catalyst but also as a reduction catalyst,13a,14b it appears that the loaded RuO2 on g-C3N4 works as an O2 evolution site rather than reduction sites for Ag+, judging from the fact that N2 evolution derived from the decomposition of the nitride catalyst is markedly suppressed upon modification with RuO2. A similar result has been observed for visible-light-driven O2 evolution with a LaTiO2N photocatalyst modified with IrO2 nanoparticles as a cocatalyst for efficient O2 evolution.11a Our preliminary experiment using RuCl3, which is an ionic precursor, revealed that the RuCl3-derived catalyst is also effective for promoting O2 evolution from aqueous silver nitrate solution, but releases N2 continuously (see Figure S1 in the Supporting Information). Although the system with RuCl3 has yet to be optimized, the detection of N2 evolution from the RuCl3-derived catalyst is a characteristic feature, as compared to the Ru3(CO)12-derived catalysts that do not release N2 during the reaction at any of the preparation conditions. This result indicates that the use of a nonionic precursor is essential for promoting not only H2 evolution but also O2 evolution. To optimize the O2 evolution activity of RuO2-loaded g-C3N4, dependence of O2 evolution activity on calcination temperature after impregnation of Ru3(CO)12 was also investigated, because the calcination process is important in that the impregnated Ru3(CO)12 is converted to catalytically active RuO2.13b Figure 6 shows time courses of O2 evolution under UV irradiation (λ > 300 nm) using RuO2-loaded g-C3N4 catalysts calcined at different temperatures with a common loading amount of 3.0 wt % RuO2. All catalysts produced O2 without releasing N2 upon UV irradiation. The order of the activity was 573 > 623 > 523 K. Our previous X-ray absorption spectroscopy studies using RuO2-loaded (Ga1-xZnx)(N1-xOx) catalyst have indicated that the generation of RuO2 upon calcination starts at 523 K, increasing the size of RuO2 and finally forming large RuO2aggromerates.13b Accordingly, it appears that the enhancement of activity from 523 to 573 K is attributed to the generation of catalytically active RuO2, while the decrease in activity at temperatures higher than 573 K is likely to be due to aggregation of the nanoparticles. However, it should be stressed that the activity is dropped by 50% from 573 to 623 K. This is a somewhat different feature as compared to our previous system, RuO2-loaded (Ga1-xZnx)(N1-xOx), where the maximum activity is obtained at a calcination temperature of 623 K.13b If calcination itself is the culprit for decreasing the activity, photocatalytic activity of g-C3N4 even for H2 evolution, where additional calcination is not necessarily needed for cocatalystloading, should be dependent strongly on the calcination time

in the synthesis of g-C3N4 from cyanamide. However, the activity remained unchanged even for the sample synthesized by 10 h of calcination in air (see Figure S2 in the Supporting Information). This result clearly indicates that the pristine g-C3N4 is stable for calcination in air at that temperature. It should be also noted that calcination of the Ru3(CO)12-impregnated g-C3N4 powder at temperatures higher than 573 K results in significant loss of the powder weight, even though the present g-C3N4 powder possesses adequate durability against calcination in air as discussed above. The weight losses were 10-20% for the 573 K sample and 40-50% for the 623 K sample. On the other hand, such significant weight loss was not observed in our previous studies on RuO2-loaded metal-based materials such as (Ga1-xZnx)(N1-xOx) and β-Ge3N4. Thus, the observed weight loss in the case of g-C3N4 is no doubt caused by the loaded Ru-species. It has been reported that RuO2 acts as a catalyst for combustion of hydrocarbon.44 It is well-known that the destruction of the catalyst structure contributes directly to decreasing photocatalytic activity. Presumably, g-C3N4 underwent combustion by the loaded Ru-species including RuO2 during the calcination process, thereby leading to the weight loss and the decreased activity. Similar weight loss is considered to take place when other cocatalysts, which are active for catalytic combustion of carbon material, are loaded on g-C3N4. Accordingly, it is considered that the less-heat-tolerant property of g-C3N4 in the presence of certain catalytic metal species is also responsible for a decrease observed at higher temperatures. This is a striking difference between g-C3N4 and metalcontaining materials. It was thus found that an appropriate choice of loading amount and calcination temperature is very important for efficient O2 evolution with RuO2-loaded g-C3N4 catalyst. As shown in Figure 7, the optimized RuO2-loaded g-C3N4 catalyst exhibited visible-light activity (λ > 420 nm) for O2 evolution with no N2 evolution, although the activity was an order of magnitude lower than that achieved under UV irradiation (λ > 300 nm). Table 2 lists photocatalytic activities of Ptand RuO2-loaded g-C3N4 catalyst for H2 and O2 evolution under irradiation with UV and visible light. The activity for O2 evolution is about an order of magnitude lower than that for H2 evolution regardless of irradiation wavelength. Interestingly, for metal-based nitride photocatalysts such as Ta3N510 and GaN,45 the activity for H2 evolution is an order of magnitude lower than that for O2 evolution. Therefore, the relatively low O2 evolution activity of g-C3N4 is a unique feature in the photocatalytic property, as compared to the previous reported metalbased nitride photocatalysts.

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TABLE 2: Photocatalytic Activities of Modified g-C3N4 Catalysts for H2 and O2 Evolutiona activity/µmol h-1 entry

catalyst

irradiation wavelength/nm

1 2 3 4

RuO2 (3.0 wt %)/g-C3N4

>420 >300 >420 >300

Pt (1.0 wt %)/g-C3N4

b

H2

O2c 1.2 11.4

21.6 183

a Reaction conditions: catalyst, 0.1 g; reaction solution, aqueous solution (100 mL); light source, xenon lamp (300 W) with cutoff filter; reaction vessel, top-irradiation type. b Steady rate of H2 evolution from an aqueous triethanolamine solution (10 vol %). c Initial rate of O2 evolution from an aqueous silver nitrate solution (0.01 M).

Materials that are active for light-induced O2 evolution have been comprised solemnly of metal-based molecular assemblies and inorganic solids so far.23 The finding that O2 evolution occurs as a result of water oxidation on the metal-free g-C3N4 surface is therefore of interest, although a proper modification such as RuO2 loading is essential to stabilize the catalytic system against the oxidative decomposition of the nitride component. The present result is expected to stimulate research on new metal-free photocatalysts for light energy conversion from the viewpoint of both fundamental science and practical application. Further research on the mechanisms of O2 evolution by g-C3N4 needs to be done, and is now being pursed in our laboratories. Overall water splitting with g-C3N4 modified with RuO2 or Rh2-yCryO313c was also attempted, but no simultaneous production of H2 and O2 was observed even under UV irradiation. In all cases, H2 and N2 were evolved with no O2 evolution, indicating that photogenerated holes in the valence band of g-C3N4 are consumed by oxidation of N3- instead of water oxidation; this N2 evolution was not suppressed when RuO2 was loaded on g-C3N4 as a cocatalyst for O2 evolution. It is considered that the observed N2 evolution during the reaction occurs near the catalyst surface, while O2 evolution will take place on the loaded RuO2 and/or g-C3N4 surface. Therefore, photogenerated holes to be consumed in O2 evolution must migrate over a longer distance compared to those involved in N2 evolution.25b If the crystallinity of a nitride catalyst is too low, the photogenerated electrons and holes have less ability to migrate longer distances, resulting in a higher likelihood that N2 evolution will take place. Our previous studies on β-Ge3N4 and GaN for overall water splitting have revealed that similar gas evolution behavior is observed when the crystallinity of the catalyst is relatively low.25b,45 Improvement of the preparation method to increase the crystallinity of g-C3N4 may therefore be one of the key routes for achieving overall water splitting that is still a big challenge. Another possible explanation for the unavailability of RuO2 to suppress N2 evolution in the overall water splitting condition is that RuO2 on g-C3N4 promotes not water oxidation but reduction of H+ into H2. In fact, it has been reported that RuO2 functions not only as a water oxidation promoter but also a water reduction promoter, depending on reaction condition.14b To achieve overall water splitting with g-C3N4, investigating the function of the loaded cocatalysts also appears to be an essential subject. Work along this line is now in progress. 4. Conclusion Photocatalytic properties of g-C3N4 powder for H2 and O2 evolution from water containing sacrificial electron donor or acceptor were examined. The g-C3N4 catalysts modified with Pt or RuO2 cocatalyst achieved functionalities as stable photocatalysts for water reduction or oxidation, respectively, under visible light (λ > 420 nm). For efficient and stable H2 evolution,

highly dispersed nanoparticulate cocatalysts on the g-C3N4 surface are shown to be essential. In the case for O2 evolution, the presence of RuO2 cocatalysts loaded on the g-C3N4 surface is indispensable not only for enhancing O2 evolution activity but also for suppressing the oxidative decomposition of the nitride catalyst. Acknowledgment. This work was supported by the Research and Development in a New Interdisciplinary Field Based on Nanotechnology and Materials Science programs of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan and the Max Planck Society within the framework of the project ENERCHEM. Acknowledgement is extended to Tokyo Metropolitan Collaboration of Regional Entities for the Advancement of Technological Excellence, Japan Science and Technology Agency (JST). K.M. gratefully acknowledges the support of a Japan Society for the Promotion of Science (JSPS) Fellowship. X.W. thanks the special funding support from the National Basic Research Program of China (973 Program, Grant No.2007CB613306), NSFC (Grant Nos. 20537010 and 20603007), the program of New Century Excellent Talents in University of China (NCET-07-0192), and the AvH Foundation. Supporting Information Available: Time courses of O2 evolution on 1.0 wt % RuO2-loaded g-C3N4 prepared with RuCl3 as the precursor under UV light (λ > 300 nm) and H2 evolution on 0.5 wt % Pt-photodeposited g-C3N4 synthesized at different calcination times under visible light (λ > 420 nm). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141–145. (2) (a) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851– 7861. (b) Lee, J. S. Catal. SurV. Asia 2005, 9, 217–227. (c) Kudo, A.; Kato, H.; Tsuji, I. Chem. Lett. 2004, 33, 1534–1539. (3) Yoshimura, J.; Ebina, Y.; Kondo, J.; Domen, K.; Tanaka, A. J. Phys. Chem. 1993, 97, 1970–1973. (4) Kudo, A.; Mikami, I. Chem. Lett. 1998, 27, 1027–1028. (5) Kato, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 5029–5034. (6) Hosogi, Y.; Shimodaira, Y.; Kato, H.; Kobayashi, H.; Kudo, A. Chem. Mater. 2008, 20, 1299–1307. (7) Kim, H. G.; Hwang, D. W.; Lee, J. S. J. Am. Chem. Soc. 2004, 126, 8912–8913. (8) Kim, H. G.; Borse, P. H.; Choi, W.; Lee, J. S. Angew. Chem., Int. Ed. 2005, 44, 4585–4589. (9) Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Chem. Commun. 2002, 1698–1699. (10) Hitoki, G.; Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K. Chem. Lett. 2002, 31, 736–737. (11) (a) Kasahara, A.; Nukumizu, K.; Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Phys. Chem. A 2002, 106, 6750–6753. (b) Kasahara, A.; Nukumizu, K.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Phys. Chem. B 2003, 107, 791–797. (12) (a) Nukumizu, K.; Nunoshige, J.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Chem. Lett. 2003, 32, 196–197. (b) Maeda, K.; Shimodaira, Y.; Lee, B.; Teramura, K.; Lu, D.; Kobayashi, H.; Domen, K. J. Phys. Chem. C 2007, 111, 18264–18270.

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