Structure–Activity Relationships in Bulk Polymeric and Sol–Gel

Article pubs.acs.org/cm

Structure−Activity Relationships in Bulk Polymeric and Sol−GelDerived Carbon Nitrides during Photocatalytic Hydrogen Production Dirk Hollmann,*,† Michael Karnahl,† Stefanie Tschierlei,‡ Kamalakannan Kailasam,§ Matthias Schneider,† Jörg Radnik,† Kathleen Grabow,† Ursula Bentrup,† Henrik Junge,† Matthias Beller,† Stefan Lochbrunner,‡ Arne Thomas,*,§ and Angelika Brückner*,† †

Leibniz Institute for Catalysis at the University of Rostock, Albert Einstein-Straße 29A, 18059 Rostock, Germany Institute of Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock, Germany § Institute of Chemistry, Fakultät II, Technische Universität Berlin, Hardenbergstr. 40, 10623 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: Photocatalytic hydrogen evolution rates and structural properties as well as charge separation, electron transfer, and stabilization have been analyzed in advanced sol−gelderived carbon nitrides (SG-CN) pyrolyzed at different temperatures (350−600 °C) and in bulk polymeric carbon nitride reference samples (CN) by XRD, XPS, FTIR, UV−vis, Raman, and photoluminescence as well as by in situ EPR spectroscopy. SG-CN samples show about 20 times higher H2 production rates than bulk CN. This is due to their porous structure, partial disorder, and high surface area which favor short travel distances and fast trapping of separated electrons on the surface where they are available for reaction with protons. In contrast, most of the excited electrons in bulk polymeric CN return quickly to the valence band upon undesired emission of light, which is responsible for their low catalytic activity.

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INTRODUCTION Due to the rising energy consumption in combination with limited fossil fuels, alternative energy sources have to be explored.1,2 In particular, hydrogen, which can be derived from the photocatalytic splitting of water, is considered to be an efficient and sustainable energy carrier of the future.3,4 For realizing energy-conversion schemes5 based on sunlight as the primary energy source, the development of new and abundant semiconductor materials is a major challenge.6 In addition to conventional metal oxides,7 especially organic semiconductors8 are promising materials since they provide effective charge separation in combination with a low-cost preparation. Besides thiophenes9 and polyanilines,10 in particular carbon nitrides (CNx) are a highly interesting class of polymers.11−13 Different carbon nitrides such as β-C3N4, aromatic N-bridged s-triazine, and other tris-s-triazine systems consist of conjugated twodimensional C−N networks, which can form graphite-like structures by π-stacking. These polymers are of high relevance since they combine chemical and thermal robustness (up to 600 °C) with a medium band gap (1.4 eV to −1.3 eV vs NHE). Until now, a wide range of photocatalytic applications is known for these carbon nitrides, including the reduction of protons to molecular hydrogen14−16 and photocatalytic organic reactions.12 As a main advantage, the band gap and the electronic properties can be easily tuned11 by metal17,18 and anion doping (S,19 B20−22) and by chemical modification (specific substituents and functional groups),23 as well as by morphologic templates.24−30 Especially the generation of high surface area was proved to increase activity due to enhanced mass transfer and small diffusion pathways.28,29 © 2014 American Chemical Society

Conversion of cyanamide, dicyanamide, or melamine at elevated temperature results in the formation of polymeric Nbridged tris-s-triazine (Scheme 1), e.g., via melam (Pathway A)31 or melamine−melem composites (Pathway B).32,33 Nevertheless, there is an ongoing discussion of the ideal preparation method, the optimal pyrolysis protocol, and, more important, the best structural composition for high catalytic activity. Moreover, clear correlations between structural features, electronic properties, and photocatalytic activities are still widely missing. In this work, we have explored the influence of structural features of sol−gel derived carbon nitride materials (SG-CN)25 in comparison with bulk polymeric carbon nitride (CN, reference material) on their performance in photocatalytic water reduction. For the first time, charge separation, electron transfer, and trapping has been visualized by in situ EPR spectroscopy in such kind of semiconductors.

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EXPERIMENTAL SECTION

Synthesis of CN and SG-CN. Bulk CN was prepared by direct pyrolysis of melamine in a semiclosed system according to a reported procedure.34 In a typical run, melamine (Aldrich 99%) was placed in an alumina crucible with a cover and then heated to the corresponding temperature for 4 h with a heating rate of 2.0 °C/min under argon atmosphere. The SG-CN materials were prepared following a modified literature procedure.25 Cyanamide (CA) and tetra-ethylorthosilicate (TEOS) Received: January 6, 2014 Revised: January 28, 2014 Published: January 31, 2014 1727

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Scheme 1. Possible Conversion Pathways (A and B) for Graphitic Carbon Nitrides

were used as carbon nitride and silica source, respectively. Initially, a certain amount of CA was dissolved in 0.01 M HCl (4 g) and ethanol (4 g), while the pH was adjusted to 2 by 1 M HCl solution. Subsequently, the required amount of TEOS was added (TEOS/CA 1/6). The mixtures were stirred for 30 min and poured into Petri dishes. After solvent evaporation, transparent glassy films were formed, which were dried at 80 °C for 24 h and subsequently heated under argon atmosphere with a rate of 2.0 °C/min up to different temperatures and kept for 4 h. To obtain silica-free carbon nitride the composites were treated with 4 M ammonium bifluoride (NH4HF2) solution for 40 h followed by careful washing with water and ethanol. The obtained carbon nitrides were dried in a vacuum oven at 150 °C overnight. The samples are denoted as CN-XXX and SG-CN-XXX, where XXX represents the respective pyrolysis temperature (350−600 °C). Materials Characterization and in Situ EPR Studies. Experimental details of ex situ characterization methods are listed in the Supporting Information. In situ EPR measurements in X-band (microwave frequency ≈ 9.8 GHz) were performed at 300 K with a Bruker EMX CW-micro spectrometer equipped with an ER 4119HSWI high-sensitivity optical resonator with a grid in the front side. The samples were irradiated with a 300 W Xe lamp (LOT Oriel). All CN and SG-CN samples were measured under the same conditions (microwave power: 6.99 mW, receiver gain: 1 × 104, modulation frequency: 100 kHz, modulation amplitude: 3 G, Sweep time: 122.8 s). g values have been calculated from the resonance field B0 and the resonance frequency ν using the resonance condition hν = gβB0. The calibration of the g values was performed using DPPH (2,2-diphenyl-1picrylhydrazyl) (g = 2.0036 ± 0.00004). Photocatalytic Activity Measurements. Catalytic tests were performed in a water/triethanolamine (TEOA) mixture (9/1; v/v) with TEOA acting as sacrificial reductant and K2PtCl6 as cocatalyst applying UV−vis light to ensure a maximum of H2 generation. A detailed experimental description can be found in the Supporting Information (SI-B).

Figure 1. Photocatalytic activity of the selected (a) CN and (b) SGCN samples.

pyrolysis temperature. The best catalytic performance of CN was obtained with a sample pyrolyzed at 600 °C (Figure 1a). In comparison to CN, SG-CN materials exhibit a significantly higher photocatalytic activity (Figure 1b). Again, the lowest H2 evolution rate is found for a pyrolysis temperature of 400 °C (Table SI-1, Supporting Information). In contrast to CN the activity does not continuously increase but passes a maximum at a pyrolysis temperature of 575 °C. A further rise of the pyrolysis temperatures (i.e., to 600 °C) seems to be unfavorable. This may be due to the onset of thermal decomposition of this material above 575 °C. Noteworthy, application of only visible light (400−700 nm) led to a significant activity for the SG-CN samples (Figure SI2a, Supporting Information). For instance, under these conditions the sample SG-CN-525 produces 432 μmol H2 per hour. This hydrogen evolution rate represents 70% of the value obtained with UV−vis light (616.3 μmol H2 h−1). However, since this work is mainly focused on the comparison of CN and SG-CN properties, UV−vis light was used for the majority of the photocatalytic experiments. Pore Characteristics. The pore properties of the samples were analyzed by nitrogen physisorption measurements (for

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RESULTS AND DISCUSSION Photocatalytic Activity. In Figure 1, the H2 evolution rate under irradiation with light in the range from 320 to 500 nm is presented. For all tabulated values and hydrogen evolution curves see Supporting Information (Table SI-1 and Figure SI1a/b). The CN sample pyrolyzed at 400 °C (Table SI-1, Supporting Information) is poorly active but gains activity with rising 1728

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CN-450) could be observed for SG-CN. After pyrolysis at 500 °C, a graphite-like structure is formed, as indicated by the characteristic interlayer stacking reflection around 27.5° of polymeric melon sheets.35 However, a weak feature at 13.1° was also observed, which is consistent with an in-plane structural motif between nitride pores.35 Moreover, a new peak at 17.5° is observed, which has been suggested to belong to a modified disordered melon structure in which every second melon sheet is displaced.35 Thus possibly two different melon structures are present. Note that this peak does not correspond to excess ammonium silicon fluoride36 since no fluoride was detected by elemental analysis. To obtain more detailed information on the nature of possible amorphous phases formed during pyrolysis of SG-CN samples, vibrational spectroscopy was used. In the IR spectrum of SG-CN-400, bands of melem−melamine adducts33 are observed at 1600, 1452, 1399, 803, and 792 cm−1, which likewise occur also in the spectra of CN-400 (Figure 3 and SI-

details see Section SI-A in the Supporting Information). Very small surface areas below 10 m2 g−1 were determined for the CN samples. Compared to these, all SG-CN samples are mesoporous with markedly larger surface areas up to 273 m2 g−1 (Table 1). Table 1. Surface Areas and Pore Properties of the SG-CN Samplesa

a b

sample

SBET [m2 g−1]

Vtp [cm3 g−1]

Dp [nm]

SG-CN-300 SG-CN-400 SG-CN-500 SG-CN-550 SG-CN-575 SG-CN-600

36 54 161 230 273 264

0.15 0.13 0.19 0.30 0.34 0.42

b 3.8 3.0 3.4 3.0 3.4

SBET, surface area; Vtp, total pore volume; Dp, pore diameter. Represents broad pore size distribution.

Surface areas and pore volumes increase with rising pyrolysis temperature. The small drop of the SBET value at 600 °C might be a consequence of the beginning thermal instability of the mesoporous material. For all SG-CN samples similar pore diameters of around 3.2 nm were determined. Structural Characterization. The results obtained by XPS, XRD, FTIR, and Raman spectroscopy for the CN samples (see Supporting Information) correspond to those already published in the literature.31,33 Briefly, three different transformations were found during the pyrolysis of melamine to polymeric CN. First, melamine was converted to a melamine−melem 1:2 adduct at 350−400 °C and subsequently to a pure melem phase at 450 °C. Further heating to 600 °C led in a continuous process to a polymerized melon-sheet structure. Remarkably, no melam phase was detected (Scheme 1, pathway A).33 Thus, it is concluded that under these conditions pyrolysis of melamine proceeds via pathway B. The XRD powder patterns of the SG-CN samples show narrow reflections for samples pyrolyzed below 500 °C, which cannot be assigned to a known crystalline phase and which disappear at higher temperature (Figure 2). In contrast to CN neither melamine−melem adducts33 (Figure 2, compare SGCN-400 with CN-400) nor a pure melem phase33 (Figure 2

Figure 3. Selected FTIR spectra of CN and SG-CN pyrolyzed at 400, 500, and 600 °C (characteristic bands are denoted as △, melamine; ▲, melamine:melem adducts; ●, melem; ■, polymerized melon).

5b, Supporting Information). Pyrolysis at higher temperature resulted in the direct formation of polymeric CN (bands at 1619, 1549, 1425, 1316, 884 cm−1).34 In agreement with previous studies of CN materials, no melam was detected in these SG-CN samples.33 By Raman spectroscopy distinct bands of the CN backbone can be detected. Raman spectra of CN samples are in good accordance with FTIR and XRD results, showing the subsequent conversion of melamine to melem37 and further to polymerized melon (Figure 4). In the case of SG-CN samples, a Raman spectrum could only be measured after pyrolysis at 300 °C (Figure 4). After pyrolysis above 400 °C, no Raman bands were observed for SG-CN samples (Figure SI-6b, Supporting Information). This suggests that the Raman bands of sample SG-CN-300 are due to lattice vibrations of the crystalline phase detected in the XRD patterns below 400 °C (Figure 2). Electronic Properties. To elucidate the electronic properties of the carbon nitrides, diffuse reflectance UV−vis, emission, and EPR measurements were performed. In contrast to the melamine starting material, CN and SG-CN samples absorb light in the visible region up to 600 nm (Figure SI-7a/b, Supporting Information). The samples pyrolyzed at low

Figure 2. Selected XRD patterns of CN and SG-CN pyrolyzed at different temperatures with characteristic reflections indicated: ▲, melamine:melem 1:2; ▼, melamine:melem 2:1; ●, melem; ■, polymerized melon; ◆, disordered CN structure. 1729

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with rising pyrolysis temperature from 350 to 600 °C. This is easily understood considering that emission and absorption are mirror images in molecular systems.40 The emission intensity of the CN samples pyrolyzed at 350 to 450 °C, which contain melamine−melem adducts, is quite strong (Figure SI-8a, Supporting Information) and becomes weaker for increasing pyrolysis temperature with hardly any emission for the catalytically most active sample CN-600. Interestingly, almost no steady state emission intensity has been detected for SG-CN samples pyrolyzed above 400 °C, which are catalytically much more active than the CN samples (compare Figures 1 and SI-8b, Supporting Information). This suggests that emission might be a general energy loss channel, which is unfavorable for catalysis. It implies that in the less active CN samples most electrons after light-driven excitation return quickly to the valence band where they are no longer available for reaction with protons. For more information about this decay process, time-resolved studies with a streak camera system were performed and the respective emission decay times were determined (Table 2 and Figure SI-9, Supporting Information). The decay curves (Figure SI-10a/b, Supporting Information) were fitted with a double exponential function,28 yielding two different decay times τ1 and τ2. We assume that these two processes result from the excitation of two different phases in the material as found by XRD or from a branching into two different channels after the absorption event. Since reasonable catalytic activity is only obtained on samples loaded with platinum particles, the decay times of the most active sample SG-CN-575/Pt are compared with those of its Pt-free counterpart in Table 2. Interestingly, τ1 and τ2 are almost not affected by depositing Pt on SG-CN-575, despite the fact that the catalytic activity increases dramatically in the presence of Pt (Figure SI-2b, Supporting Information). This indicates that both emission decays are not associated with catalytic activity. Instead, the most important issues for this might be short transport pathways of electrons to the surface combined with their stabilization right after charge separation to prevent the detrimental emission process. To analyze these charge separation processes, in situ EPR spectroscopy has been applied, which is a versatile tool to detect paramagnetic species such as radicals41 or conduction band electrons (CB-e−) during irradiation. These CB-e− can be trapped at oxygen vacancies42 and carbon defects43 or delocalized in a polymer chain.44 As seen from Figures SI-11a/b (Supporting Information), in both materials, CN as well as SG-CN, a narrow isotropic EPR signal with a g value of g = 2.0041 and Lorentzian line shape was detected even in the dark. This confirms the presence of paramagnetic centers inside the carbon nitrides. The g value suggests that this signal arises from surface trapped electrons that bear preferentially carbon character.23,45,46 Note that the conduction band is formed by carbon 2p orbitals.16 Information about the charge separation efficiency and, thus, the generation of photoactive, trapped CB-e− can be obtained by analyzing the EPR signal intensity during irradiation (Figure 5, red spectra). Furthermore, the recombination of the CB-e− with holes observed after light switch-off (Figure 5 gray spectra) is of high interest since this can give information about the extent to which conjugation within the polymers influences charge separation and transport. For this purpose, the spectra under irradiation with light (Figure 6, full symbols) and after light switch-off (Figure 6, empty symbols) were double integrated, and the corresponding

Figure 4. Selected Raman spectra of CN and SG-CN pyrolyzed at 400, 500, and 600 °C recorded with an excitation wavelength of 785 nm (characteristic backbone bands are denoted as △, melamine; ●, melem; ■, polymerized melon; ○, melem-like backbone; □, no clear assignment).

temperature show absorption below 400 nm resulting from the excitation of conjugated electrons within the triazine rings. With increasing polymerization degree the absorption in the visible range, which belongs to the band gap transition from nitrogen 2p to carbon 2p orbitals,16 becomes more intense. Moreover, a shoulder around 450−600 nm in the UV−vis spectra of SG-CN samples pyrolyzed at temperatures above 500 °C suggests the presence of carbon impurities remaining in the CN network after preparation. These C dopants obviously improve the ability of the SG-CN samples to absorb visible light.38 A similar band was reported for mesoporous carbon nitride−silica composites25 and samples with enhanced carbon content39 or heteroatom doped systems.19 Steady state emission spectra of the different CN and SG-CN samples were recorded after photoexcitation (Figure SI-8a/b, Supporting Information). The detected luminescence of CN occurs in the visible region and consists of a single broad peak, the maximum of which is listed in Table 2. As already observed for the absorption edge (Figure SI-7a, Supporting Information), these maxima are gradually red-shifted from 441 to 513 nm Table 2. Emission Wavelengths and Decay Times of the Different CN and SG-CN Samples

a

sample

emission [nm]

decay time τ1a [ps]

decay time τ2a [ps]

CN-350 CN-400 CN-450 CN-500 CN-550 CN-600 SG-CN-300 SG-CN-400 SG-CN-500 SG-CN-525 SG-CN-550 SG-CN-575 SG-CN-575/Pt SG-CN-600

441 445 464 465 469 513 422 520 561 561 571 560 550 575

315 (50) 277 (50) 1283 (42) 809 (22) 622 (30) 269 (50) 205 (45) 254 (45) 119 (72) 138 (67) 92 (70) 99 (70) 126 (83) 65 (53)

1586 (50) 1527 (50) 5766 (58) 3743 (78) 2809 (70) 1477 (50) 1454 (55) 1541 (55) 1056 (28) 1054 (33) 652 (30) 624 (30) 654 (17) 550 (47)

The corresponding ratios (%) of the decay times are in parentheses. 1730

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nanoparticles on the support surface (Figure SI-2b, Supporting Information). To prove the impact of those particles on the electron transfer properties of the catalyst, SG-CN-575 loaded with platinum nanoparticles was isolated after 3 h from the catalytically active reaction mixture and measured by EPR (Figure 6, SG-CN-575/Pt, black dots). In this sample, the signal of CB-e− was much smaller than in the poorly active Ptfree support SG-CN-575. This suggests that CB-e− formed by light-induced charge separation within the support moves quickly to the Pt nanoparticles, from which they are transferred to the protons to form hydrogen. This process is in excellent agreement with our recent observations on Au/TiO 2 catalysts.42 Structure−Activity Relationship. In general, CN samples pyrolyzed below 400 °C are only poorly active. At this temperature, the corresponding melamine/melem adducts were formed. Pyrolysis at higher temperatures (450 to 550 °C) leads to improved hydrogen formation of up to 29.6 μmol H2 h−1. At these temperatures pure melem starts to convert into polymerized melon as indicated by the gradually growing interlayer stacking peak at 2θ = 27.6° in the XRD powder patterns (Figure SI-4, Supporting Information). However, the properties of melem, such as low charge separation and strong emission (as reflected by the low in situ EPR signal in Figure SI-11a, Supporting Information, and the strong signal in Figure SI-8a, Supporting Information) still dominate over the properties of polymerized melon. This agrees well with the similar and rather low catalytic activities of CN samples pyrolyzed between 450 and 550 °C (Table SI-1, Supporting Information). Raising the pyrolysis temperature to 600 °C, where full polymerization was detected by XRD, causes a further increase of the hydrogen production rate up to 39.3 μmol H2 h−1. This indicates that only the fully polymerized phase exhibits reasonable activity. In comparison to CN, mesoporous SG-CN materials show a significantly different pyrolysis and activity behavior. At 300 and 400 °C FTIR spectroscopy points to the formation of a melem backbone while XRD and Raman measurements indicate a completely different lattice and structural environment compared to CN. No pure melem or melem−melam adducts are formed. Furthermore, as evidenced by the characteristic XRD reflection around 27.5° (Figure 2), stacked polymeric melon sheets with long-range in-plane order are formed already at 500 °C, yet the stacking order of these sheets is partly disordered (peak at 17.5°, Figure 2). This may be due to the templating effect of SiO2 liberated during synthesis, which creates pores and increases the BET surface area (Table 1). In contrast to CN samples with their well-ordered graphite-like structure, this should lead to markedly shorter distances, which separated CB-e− have to travel until they reach stabilizing traps on the surface. The much more effective charge separation in SG-CN samples is also confirmed by the in situ EPR results (Figures 5 and 6). This is considered as a major reason for the markedly higher activities of the SG-CN catalysts.

Figure 5. EPR signals (background signal in the dark subtracted) of the most active samples CN-600 and SG-CN-575 as well as SG-CN575 loaded with Pt during irradiation (red) and after light switch-off (gray).

Figure 6. Double integral of the CB-e− EPR signal of CN (blue), SGCN (red), and SG-CN-575/Pt (black) during UV−vis irradiation (full symbols) and after light switch-off (empty symbols, background signals in the dark subtracted).

background signals before starting the experiment (Figure SI11a/b, Supporting Information) were subtracted. Irradiation of the CN samples pyrolyzed between 450 to 600 °C results in a rise of the signal of trapped CB-e− (Figure 6, blue full triangles), which points to increasing polymerization and, thus, a better charge separation inside the carbon nitride material. However, after switching off the light, the EPR signal vanished. This indicates that fast charge recombination occurs in this bulk material. This may also be the reason for the low catalytic activities of these samples. In contrast, the SG samples show a more than 3−4 times higher intensity of the CB-e− signal under irradiation (Figure 6, red full dots). This suggests that charge separation is much more efficient in this material. Possibly, fast recombination of charge carriers is suppressed due to the transfer and stabilization of the electrons at the surface, which persists to a minor extent even after light switch-off as reflected by the still visible EPR signal (Figure 6, empty red dots). The same trend of a steady increase of the CB-e− signal as in CN, yet on a markedly higher level, is evident, too, for the SG-CN samples with a maximum at 600 °C. Interestingly, only very low hydrogen production was observed with the bare CN supports without the K2PtCl6 cocatalyst, which decomposes to Pt

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CONCLUSIONS Photocatalytic hydrogen evolution rates as well as structural and electronic properties of advanced sol−gel-derived carbon nitrides (SG-CN) pyrolyzed at different temperatures (300− 600 °C) have been comprehensively analyzed in comparison to bulk polymeric carbon nitride reference samples (CN). In both materials, stacked graphite-like melon sheets are formed during pyrolysis. However, in SG-CN samples this process starts 1731

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already at a markedly lower temperature (500 °C) compared to CN (600 °C) and leads to a porous structure with high surface area and partial disorder. In photocatalytic water reduction with UV−vis light, SG-CN samples provide about 20 times higher hydrogen production rates than CN materials, and most remarkably, their activity with pure visible light reached still about 70% of the value under UV−vis light. This is due to differences in charge recombination, transfer, and stabilization which are governed by structural features of both materials. In bulk CN samples electrons and holes recombine quickly via emission (identified as an undesired loss channel) before they can reach stabilizing traps at the surface, due to long pathways within the bulk CN material. Even though the electron transfer mechanism is similar in SG-CN catalysts, the pore structure, partial disorder, and high surface area in these materials favor short travel distances and fast trapping of separated electrons on the surface where they are available for reaction with protons. For the first time, electron separation and trapping has been visualized by in situ EPR spectroscopy in these materials. Consequently, our results contribute to a better understanding of structure−activity relationships in carbon nitride photocatalysts. They show that, even though a polymeric melon structure is needed for efficient electron transport, any synthesis strategies leading to tailored disorder and high surface area are desired to optimize electron transfer and stabilization in surface sites.

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ASSOCIATED CONTENT

* Supporting Information S

General details, additional photocatalytic measurements, XPS measurements, IR, Raman, UV−vis diffuse reflectance, steadystate and time-resolved PL, and EPR spectroscopy. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(D.H.) E-mail: [email protected]. Fax: +49 381 1281 51352. *(A.T.) E-mail: [email protected]. Fax: +49 30 314 29271. *(A.B.) E-mail: [email protected]. Fax: +49 381 1281 51244. Author Contributions

The manuscript was written and approved through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Petra Bartels for her support with the catalytic experiments. Financial support from the Federal Ministry of Education and Research of Germany and the state of Mecklenburg-Western Pomerania within the project Light2Hydrogen is gratefully acknowledged.

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REFERENCES

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