Graphitic Carbon Nitride Doped with Biphenyl Diimide: Efficient

Sep 9, 2016 - Research Center for Solar Energy Chemistry and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka Universit...
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Graphitic Carbon Nitride Doped with Biphenyl Diimide: Efficient Photocatalyst for Hydrogen Peroxide Production from Water and Molecular Oxygen by Sunlight Yusuke Kofuji,† Satoshi Ohkita,† Yasuhiro Shiraishi,*,†,‡ Hirokatsu Sakamoto,† Shunsuke Tanaka,§ Satoshi Ichikawa,∥ and Takayuki Hirai† †

Research Center for Solar Energy Chemistry and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan ‡ Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Saitama 332-0012, Japan § Department of Chemical, Energy and Environmental Engineering, Kansai University, Suita 564-8680, Japan ∥ Institute for NanoScience Design, Osaka University, Toyonaka 560-8531, Japan S Supporting Information *

ABSTRACT: Photocatalytic hydrogen peroxide (H2O2) production from water and molecular oxygen (O2) by sunlight is a promising strategy for green, safe, and sustainable H2O2 synthesis. We prepared graphitic carbon nitride (g-C3N4) doped with electron-deficient biphenyl diimide (BDI) units by a simple calcination procedure. The g-C3N4/BDI catalyst, when photoirradiated by visible light (λ >420 nm) in pure water with O2, successfully promotes water oxidation by the photogenerated valence band holes and selective two-electron reduction of O2 by the conduction band electrons, resulting in successful production of millimolar levels of H2O2. Electrochemical analysis, Raman spectroscopy, and ab initio calculation results revealed that, upon photoexcitation of the catalyst, the photogenerated positive holes are localized on the BDI unit while the conduction band electrons are localized on the melem unit. This spatial charge separation suppresses rapid recombination of the hole−electron pairs and facilitates efficient H2O2 production. The solar-to-chemical energy conversion efficiency for H2O2 production is 0.13%, which is comparable to that for photosynthetic plants. This metal-free photocatalysis therefore shows potential as an artificial photosynthesis for clean solar fuel production. KEYWORDS: photocatalysis, hydrogen peroxide, carbon nitride, sunlight, water

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INTRODUCTION Hydrogen peroxide (H2O2) is an irreplaceable clean oxidant and is widely used for pulp bleaching and disinfection.1 H2O2 has attracted much attention as an alternative to H2 as a fuel cell energy carrier because it is water-soluble and can be used in a one-compartment cell for electricity generation.2−5 Currently, H2O2 is manufactured by the anthraquinone method on the basis of the hydrogenation of anthraquinone with H2 on Pdbased catalysts followed by the oxidation of anthrahydroquinone with O2.6 Alternative to this high energy-consuming multistep process, direct synthesis with H2 and O2 by Pd-based catalysts has been proposed.7,8 This synthesis quantitatively produces H2O2, but the mixed H2/O2 gas has a potentially explosive nature. A safe and energy-saving process for H2O2 synthesis using earth-abundant resources is therefore desired. Semiconductor photocatalysis is one of the ideal processes. The concept is as follows: the valence band holes (VB h+) formed by the catalyst photoexcitation oxidize water and produce O2 and H+ (eq 1). Two-electron reduction of O2 by the conduction band electrons (CB e−) produces H2O2 (eq 2). © 2016 American Chemical Society

As a result of this, H2O2 can be produced from water and O2 (eq 3) with relatively high energy gain (ΔG° = 117 kJ mol−1).9 H 2O + 2h+ → 1/2O2 + 2H+ O2 + 2H+ + 2e− → H 2O2

H 2O + 1/2O2 → H 2O2

1.23 V vs NHE

(1)

0.68 V vs NHE

(2)

ΔG° = 117 kJ mol−1

(3)

Several inorganic-10−15 or organic-based catalysts16,17 have been proposed but produce only small amounts of H2O2 (420 nm) leads to localization of the photoformed h+ and e− at the 2,6- and 1,4positions of melem (A), respectively.20 The doping of PDI with high electron affinity positively shifts the VB level of g-C3N4, thus promoting water oxidation by h+ (B). In contrast, the e− reduces O2 and creates a superoxo radical (C). This is rapidly reduced by another e− at the para position of melem and forms 1,4-endoperoxide (D), which is readily transformed to H2O2. The efficient 1,4-endoperoxide formation (C → D) suppresses one-electron reduction of O2 (eq 4) and four-electron reduction of O2 (eq 5), thus promoting selective two-electron reduction of O2 (eq 2) with ca. 90% selectivity. These effects facilitate efficient H2O2 production from water and O2. In particular, the catalyst containing 51% PDI units (=PDI/ (melem + PDI) × 100) exhibits the highest activity and produces millimolar levels of H2O2 with an apparent quantum yield of 2.6% (at 420 nm). The purpose of the present work is to enhance the catalytic activity while maintaining high H2O2 selectivity. As shown in Scheme 1b, on g-C3N4/PDI, the photoformed h+ and e− exist on the same melem unit. The close location of these charge

Figure 1. (solid line) TG and (dotted line) DTA data for (black) melem, (green) BTCDA, and (blue) their 1:1 (mol:mol) mixture. The heating rate was 7 K min−1, and the measurements were performed under N2. The red line gives TG data for a mixture of melem and BTCDA (1:1 (mol:mol)) calculated from their respective TG data. 7022

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As shown in Figure S1 in the Supporting Information, the Fourier transform infrared (FTIR) spectrum of g-C3N4/BDI prepared at 653 K shows strong bands at 1200−1700 cm−1 assigned to stretching of the melem framework and broad bands at 2900−3500 cm−1 assigned to N−H stretching of melem, as observed for bare g-C3N4.29 The bands at 1767, 1729, and 736 cm−1 are assigned to asymmetric stretching, symmetric stretching, and bending of the imide CO units, respectively.30 The bands at 1372 and 1111 cm−1 are assigned to stretching and bending of C−N−C moiety in the fivemembered imide ring.30 Figure S2 in the Supporting Information shows the 13C solid-state NMR chart of g-C3N4/ BDI prepared at 653 K. Two strong signals appear at 160−167 ppm assigned to melem carbon and imide CO groups31,32 and at 132−138 ppm assigned to aromatic carbon of BDI units.32,33 As shown in Figure 3, transmission electron

(mol:mol) mixture. Melem (black) shows two endothermic DTA peaks at 720 and 870 K, attributable to the condensation of melem (formation of g-C3N4) and thermal decomposition of g-C3N4, respectively.18 The TG data show substantial weight loss at >800 K by decomposition. BTCDA (green) shows two endothermic DTA peaks at 570 and 670 K due to its melting and evaporation, respectively;22 substantial weight loss by evaporation occurs at >600 K. The 1:1 (mol:mol) mixture of melem and BTCDA (blue) shows a broad exothermic DTA peak at 600−700 K. In this range, its weight loss is smaller than that of the result calculated from the melem and BTCDA data (red). This indicates that the exothermic peak at 600−700 K (blue) is due to the condensation of melem and BTCDA.23,24 A large weight loss of the mixture at >700 K is attributable to the thermal decomposition of the structure, as confirmed by some exothermic DTA peaks in this range.25 These TG-DTA data suggest that the BDI units are incorporated within the g-C3N4 network at 600−700 K, but calcination at higher temperature decomposes the structure. The g-C3N4/BDI catalysts were prepared by calcination of a 1:1 (mol:mol) mixture of melem and BTCDA at different temperatures, where the heating rate was 7 K min−1 and the holding time at the designated temperature was 4 h. Figure 2

Figure 3. TEM images of g-C3N4/BDI50 prepared by calcination at 653 K.

microscopy (TEM) images of the catalyst prepared at 653 K show a sheetlike structure, similar to that of g-C3N4 and gC3N4/PDI.19,34 These XRD, FTIR, 13C solid-state NMR, and TEM images clearly indicate that, on the catalysts prepared by high-temperature calcination, the BDI units are incorporated within the melem sheets and the sheets exist in layers (Scheme 2b), as is the case for g-C3N4/PDI (Scheme 1a).19 However, as shown in Figure 2, calcination at >723 K significantly decreases the intensity for all of the diffractions by the decomposition of the network; this is supported by a large weight loss observed in the TG data at >700 K (Figure 1, blue). The catalysts were used for photocatalytic H2O2 production from water and O2 under visible light. Pure water (30 mL) containing the respective catalyst (50 mg) was photoirradiated by a solar simulator at λ >420 nm with magnetic stirring under O2 (1 atm) at 298 K. Table 1 summarizes the results for H2O2 formation by 24 h reaction. The catalyst prepared at 573 K (entry 1) scarcely produces H2O2 because the BDI units are not incorporated within the g-C3N4 network. In contrast, the catalysts prepared at higher temperature (entries 2−5), which contain BDI units within the network, successfully produce H2O2. Among them, the catalyst prepared at 653 K shows the highest activity (entry 3). In contrast, the catalysts prepared at 723 and 773 K (entries 5 and 6) with partially decomposed structure show decreased activity. These data suggest that 653 K is the best calcination temperature. Effect of BDI Amount. The amount of BDI units strongly affects the catalytic activity. We prepared g-C3N4/BDI by calcination of a mixture of melem and BTCDA with different mole ratios at 653 K and used them for photocatalytic H2O2 production. The results are shown in Table 1 (entries 7−13).

Figure 2. XRD patterns for melem, BTCDA, and the g-C3N4/BDI catalysts prepared by calcination of a 1:1 (mol:mol) mixture of melem and BTCDA at different temperatures.

shows the X-ray diffraction (XRD) of the catalysts. The catalyst prepared at 573 K shows peaks at 2θ = 13.4, 27.2°, similar to those observed for BTCDA, suggesting that condensation of melem and BTCDA scarcely occurs at this temperature. In contrast, high-temperature calcination leads to the disappearance of these peaks with an appearance of new peaks. The new three peaks at 2θ = 14.1, 15.8, 16.7° (d = 0.628, 0.560, 0.529 nm) are assigned to the in-plane and out-of-plane packing of the melem-BDI chain in a liquid-crystal-like ordered domain.26 The peaks at 2θ = 24.4° (d = 0.345 nm) and 26.8° (d = 0.331 nm) are assigned to π stacking27 of BDI and graphitic stacking of melem units, respectively. The peak at 2θ = 31.3 (d = 0.286 nm) is assigned to donor−acceptor stacking28 between the melem and BDI units. 7023

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100) can be determined by integration of the N 1s XPS signals (Figure S4 in the Supporting Information). As summarized in Table 1 (entry 9), the catalyst showing the highest activity contains 50% BDI units. Other catalysts containing smaller or larger amounts of BDI (entries 3 and 12) show decreased activity, indicating that the catalyst consisting of equimolar amounts of melem and BDI exhibits the best activity. This is a tendency similar to that of g-C3N4/PDI catalyst, where the catalyst containing 51% PDI units shows the highest activity.19 Photocatalytic Activity. Figure 5 shows the time-dependent change in the amounts of H2O2 formed on the respective

Table 1. Results of Photocatalytic H2O2 Production on the gC3N4/BDI Catalysts Prepared under Different Conditions.a entry

calcination temp (K)

melem:BTCDA (mol:mol)

1 2 3 4 5 6 7 8 9 10 11 12 13

573 623 653 673 723 773 653 653 653 653 653 653 653

1:1 1:1 1:1 1:1 1:1 1:1 1:1.5 1:2 1:2.5 1:3 1:3.5 1:4 1:5

BDI units (x) (%)b

32

50

58

H2O2 (μmol) 420 nm) at a bias of 0.5 V vs Ag/AgCl. (b) Normalized decay of the transient photocurrent.

the photocurrent response of the catalysts loaded on a fluorine tin oxide (FTO) electrode obtained under photoirradiation (λ >420 nm). The photocurrent density of g-C3N4/BDI50 is lower than that of g-C3N4/PDI51. This is probably due to the weaker light absorption of g-C3N4/BDI50 in the visible region (Figure 8). This is inconsistent with the higher photocatalytic activity of g-C3N4/BDI (Figure 5). As shown in Figure 10a, both profiles show a photocurrent spike by the sudden generation of charge carriers.50 The decay rate of the transient photocurrent was studied to clarify the charge recombination behavior. The decay rate is usually normalized with the parameter D:51−53 I − Ist D= t Iin − Ist (7) where Iin is the initial photocurrent at t = 0, It is the photocurrent at a time t, and Ist is the steady-state photocurrent, respectively. As shown in Figure 10b, the decay of the transient photocurrent for g-C3N4/BDI50 is much slower than that for g-C3N4/PDI51, indicative of slower charge recombination. This may result in higher photocatalytic activity of g-C3N4/BDI50. Ab Initio Calculations. The slower charge recombination on g-C3N4/BDI is because the photoformed h+ is localized on the BDI moiety, while the e− is localized on the melem moiety. This spatial charge separation suppresses their recombination. To clarify this, ab initio calculations were performed on the basis of density functional theory (DFT) within the Gaussian 03 program using simple melem-PDI and melem-BDI models, as often used for the determination of the electronic structure for g-C3N4-based photocatalysts.54,55 As shown in Figure 11, 7026

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As shown in Figure 5 (shown in red), during photoirradiation of g-C3N4/BDI50 in pure water with O2, H2O2 is produced rapidly. However, as shown by blue points, photoirradiation of water containing ca. 100 μmol of H2O2 produces H2O2 more slowly. These data suggest that oxidation of H2O2 by the photoformed VB h+ scarcely occurs at low H2O2 concentration but occurs at higher concentration.

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CONCLUSION We found that g-C3N4 doped with electron-deficient BDI units successfully produces H2O2 from water and O2 under sunlight, via water oxidation and selective two-electron reduction of O2. Photoexcitation of g-C3N4/BDI leads to localization of h+ and e− on the BDI and melem units, respectively, and suppresses rapid recombination of these charge carriers. This results in efficient H2O2 production with a solar to chemical energy conversion efficiency of 0.13%, comparable to the highest levels achieved by powdered semiconductor water-splitting catalysts. Recently, g-C3N4-based photocatalytic systems have attracted growing interest for artificial photosynthesis.61−63 The present metal-free g-C3N4-based system has potential to be a new artificial photosynthesis system for safe, clean, and sustainable solar fuel production. The basic concept presented here based on spatial charge separation by doping of aromatic diimide moieties into the g-C3N4 network may contribute to the design of more efficient systems for H2O2 production and may provide a new strategy toward artificial photosynthesis.

Figure 12. Raman spectra for (a) g-C3N4/BDI50 and (b) the g-C3N4/ BDI50 recovered after visible light irradiation (12 h) in a 2-PrOH/O2 system.

Scheme 3. Proposed Mechanism for H2O2 Formation on the Photoexcited g-C3N4/BDI Catalyst

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EXPERIMENTAL SECTION General Considerations. All reagents were supplied from Wako, Tokyo Kasei, and Sigma-Aldrich and used as received. gC3N4 was prepared by calcination of melamine (9.0 g) at 823 K for 4 h under N2 at a heating rate of 2.3 K min−1.29 Melem was prepared by heating melamine at 698 K for 4 h under N2,21 at a heating rate of 7 K min−1. The g-C3N4/PDI51 catalyst was prepared by calcination of a mixture of melem (2.0 g) and pyromellitic anhydride (4.0 g) at 598 K for 4 h under N2 at a heating rate of 7 K min−1 followed by grinding of the resultant product.19 Preparation of g-C3N4/BDI50. The catalyst was prepared as follows: a mixture of melem (1.0 g, 4.6 mmol) and BTCDA (3.4 g, 11.5 mmol) was calcined at 653 K for 4 h at a heating rate of 7 K min−1 under N2. Grinding of the resultant product afforded gray-brown powders of g-C3N4/BDI50. Photoreaction. Catalyst (50 mg) was added to water (30 mL) within a borosilicate glass bottle (φ 35 mm; capacity, 50 mL), and the bottle was sealed with a rubber septum. The catalyst was dispersed by ultrasonication for 5 min, and O2 was bubbled through the solution for 15 min. The bottle was photoirradiated at λ >420 nm by a solar simulator (XES-502S, Kansai Chem. Eng. Co., Ltd.)19 in a temperature-controlled water bath (298 ± 0.5 K).64 For SCC efficiency determination, an AM1.5G Solar Simulator (SX-UID502XQ, USHIO Inc.) was used as a light source, where 250 mg of catalyst and 50 mL of water were used. In that, a λ >420 nm cutoff filter was used to avoid decomposition of the formed H2O2 by UV light.65 The water oxidation reaction with AgNO3 as a sacrificial electron acceptor was performed with catalyst (100 mg) in a buffered La2O3 (30 mg) solution (30 mL, pH 8−9) with AgNO3 (10 mM) under Ar.18 The O2 reduction reaction with 2-PrOH as a sacrificial electron donor was performed with catalyst (50 mg) in a 2-PrOH/water mixture (9/1 v/v, 30 mL) under O2.20 After

water and O2 under visible light. It must be noted that the formed H2O2 is subsequently decomposed at higher concentration by the photoformed VB h+. This is confirmed by the half reactions. As shown in Figure S8 in the Supporting Information, visible light irradiation of water containing ca. 100 μmol of H2O2 with g-C3N4/BDI50 under an Ar atmosphere with 2-PrOH as a sacrificial electron donor scarcely decomposes H2O2. This indicates that reductive decomposition of H2O2 by the CB e− (eq 8)59 scarcely occurs. In contrast, photoirradiation of the H2O2 solution with AgNO3 as a sacrificial electron acceptor promotes decomposition, indicating that, as shown by eq 9, the VB h+ promotes oxidative decomposition of H2O2.60 H 2O2 + 2H+ + 2e− → 2H 2O H 2O2 + 2h+ → O2 + 2H+

1.78 V vs NHE

0.68 V vs NHE

(8) (9) 7027

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ACS Catalysis the reactions, the gas-phase products were analyzed by GCTCD (Shimadzu, GC-14B). The catalyst was recovered by centrifugation, and the liquid-phase products were analyzed by GC-FID (Shimadzu, GC-2010 system). The amount of H2O2 was determined by redox titration with KMnO4.19,20,66 Action Spectrum Analysis. Photoreactions were performed in water (30 mL) with catalysts (50 mg). The reaction bottle was photoirradiated by a solar simulator for 12 h, where the incident light was monochromated by the band-pass glass filters (Asahi Techno Glass). The full width at half-maximum of the lights was 11−16 nm.67 The temperature of the solutions was kept at 298 ± 0.5 K. The photon number entered into the reaction bottle was determined with a spectroradiometer (USR40, USHIO Inc.). Electrochemical Analysis. The measurements were carried out in a three-electrode cell on an Electrochemical Analyzer (SI 1280B, TOYO Co.). An Ag/AgCl electrode and a Pt-wire electrode were used as the reference and counter electrodes, respectively. The working electrode was prepared using an FTO glass.19 Catalyst (50 mg) was mixed with acetone (2 mL), and the mixture was ultrasonicated for 30 min. The slurry was put onto the FTO and annealed at 623 K for 30 min in air for strong adhesion. A 0.1 M Na2SO4 solution was used as the electrolyte, where its pH was adjusted to 6.6 and N2 was bubbled through the solution for 10 min prior to use. For photocurrent response measurements, an AM1.5G solar simulator (HAL-320, Asahi spectra Co., Ltd.) was used as the light source with a 20 wt % NaNO2 solution as a cutoff filter (λ >420 nm).68 The working electrode was photoirradiated from the back side (FTO glass/semiconductor interface) to minimize the effect of the thickness of the semiconductor layer.69 The exposed area for irradiation was 0.25 cm2. The perturbation signal for Mott−Schottky measurements was set at 10 mV. Calculation Details. Calculations were carried out at the DFT level with the Gaussian 03 package using the B3LYP/631G(d) basis set. The excitation energies and the oscillator strengths were calculated by TD-DFT using the polarizable continuum model (PCM). Cartesian coordinates of the models are summarized in the Supporting Information. Other Analyses. XRD was measured on a Philips X’PertMPD spectrometer with Cu Kα radiation. DR UV−vis spectra were obtained on a V-550 UV−vis spectrophotometer (JASCO Corp.) equipped with an Integrated Sphere Apparatus (ISV469) using BaSO4 as a reference. A 13C solid-state NMR chart was recorded on a JEOL ECA-400 spectrometer equipped with a 9.39 T magnet. FTIR spectra were measured on a FT/IR6100 spectrometer (JASCO Corp.). TG-DTA analysis was performed by DTG-H60 (Shimadzu). XPS measurements were performed on a JEOL JPS-9000MX spectrometer using Mg Kα radiation as the energy source.70 TEM observations were performed using an FEI Tecnai G2 20ST analytical electron microscope operated at 200 kV.71

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with AgNO3, electrochemical Mott−Schottky plots of the catalysts, results of photocatalysis of H2O2, and Cartesian coordinates for melem-PDI and melem-BDI models (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by PRESTO from JST. REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b02367. FTIR, 13C solid-state NMR, XPS charts for C 1s and N 1s levels for catalysts, time-dependent change in the amounts of H2O2 during reaction with 2-PrOH, timedependent change in the amounts of O2 during reaction 7028

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ACS Catalysis

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DOI: 10.1021/acscatal.6b02367 ACS Catal. 2016, 6, 7021−7029