Chem. Mater. 2009, 21, 4093–4095 4093 DOI:10.1021/cm902130z
Ordered Mesoporous SBA-15 Type Graphitic Carbon Nitride: A Semiconductor Host Structure for Photocatalytic Hydrogen Evolution with Visible Light Xiufang Chen,†,‡ Young-Si Jun,‡ Kazuhiro Takanabe,§ Kazuhiko Maeda,§ Kazunari Domen,*,§ Xianzhi Fu,† Markus Antonietti,‡ and Xinchen Wang*,†,‡ †
State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, China, ‡Colloid Chemistry, Max-Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany, and §Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan Received July 14, 2009 Revised Manuscript Received August 12, 2009
Natural photosynthesis feeds nearly all forms of life on the Earth directly or indirectly via conversion of CO2 and H2O to carbohydrates while releasing O2. This has inspired the search for artificial versions of photosynthesis, that is, water splitting and organic synthesis via sunlight. The splitting of water offers an ideal pathway for the sustainable utilization of abundant sunlight by storing it in chemical bonds of H2 and O2, clean fuels for a potential H2-based economy.1,2 To date, numerous photocatalysts have been analyzed, and some have achieved high laboratory quantum efficiencies (LQEs), especially those of consisting of metal oxides.3-5 NiO-promoted La/NaTaO3 photocatalyst developed by Kudo et al., for example, is the present record holder in term of LQEs (56%) for overall water splitting with ultraviolet (UV) irradiation (λ = 270 nm).4 Despite the success of oxide catalysts for photochemistry applications, there remains the general problem that most metal oxides only utilize special UVlight that accounts for ∼4% of total solar irradiation reaching the Earth. It is thus highly desirable to develop photoactive, stable, and particularly abundant materials responsive to visible light, which has been identified as the major goals of artificial photosynthesis.6,7 Graphitic carbon nitride (g-C3N4) and its polymeric precursor, melon, possess high stability with respect to thermal (800 °C and reveals that 99.5% of the silica template was removed by the NH4HF2 treatment. The wide-angle XRD pattern (Figure S1, Supporting Information) of ompg-C3N4 exhibits the typical graphitic interlayer (002) peak with d=0.327 nm. Another pronounced peak is found at 13.2°, which corresponds to (19) Vinu, A.; Ariga, K.; Mori, T.; Nakanishi, T.; Hishita, S.; Golberg, D.; Bando, Y. Adv. Mater. 2005, 17, 1648. (20) Jun,Y. S.; Hong, W. H.; Antonietti, M.; Thomas, A. Adv. Mater. 2009, DOI: 10.1002/adma.200803500.
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Figure 2. (a) Diffuse reflectance absorption spectrum of ompg-C3N4 and g-C3N4 and the optical band gap (Eg) of ompg-C3N4 (inset). (b) Excitation spectrum (1) monitored at 480 nm and PL spectrum (2) of ompg-C3N4 and g-C3N4 at 420 nm excitation at 77 K.
an in-plane structural packing motif. The FTIR spectrum (Figure S2, Supporting Information) shows characteristic bands of aromatic CN heterocycles at 1200 to 1600 cm-1 and heptazine units at 800 cm-1, similar to the results of nonporous g-C3N4. The elemental analysis of ompgC3N4 gives an average atomic C/N ratio value of 0.73. The sample features additional small amounts of hydrogen (∼2 wt %), attributable to uncondensed amino functions and adsorbed water,10 as also confirmed by FTIR analysis. The textural parameters were also characterized by N2sorption measurements. Figure S3 (Supporting Information) shows N2-sorption isotherms and the corresponding pore size distributions for the SBA-15 and ompg-C3N4. Both isotherms are of type IV and exhibit a H1 hysteresis loops, indicating mesoporous characteristics including capillary condensation. Calculations based on the isotherms (Table S1, Supporting Information) show that ompg-C3N4 has a large surface area of 239 m2 g-1 and a pore volume of 0.34 cm3 g-1. The curves already incidate the absence of micropores, that is, contrary to silica, carbon nitrides are inherently dense materials. Pore size distributions determined by the BJH method show that the pore sizes are centered on ∼10.4 nm for SBA-15 and ∼5.3 nm for ompg-C3N4, respectively, which is mainly due to the negative templating. Note that the pore size of ompg-C3N4 is, however, larger than the wall thickness of the SBA-15. The finding can be explained by the volume shrinkage of the filled CN polymers inside the pores during the condensation.20 The textural mesopores are also seen in the SEM (Figure S4, Supporting Information) and TEM images. They show that ompg-C3N4 retains well the rod-like morphology of the SBA-15 template. TEM images of the ompg-C3N4 also reveal the perfect replication of the hexagonal order of the silica pore into the resulting CN material. Figure 1b clearly displays a hexagonal arrangement of the mesopores, whereas Figure 1c shows a view along the [100] direction, further confirming the highly ordered hexagonal arrangement of cylinders made up of carbon nitride. To conclude structural characterization, these findings are in good agreement with the results obtained from SAXS and N2-adsorption measurements. Figure 2a shows the UV/vis absorption spectra of ompg-C3N4 and g-C3N4. A typical semiconductor absorption with a band gap of 2.74 eV is observed for ompgC3N4 as compared to 2.69 eV for bulk g-C3N4. The slight
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Figure 3. (a) Potentiodynamic scans under chopped illumination (λ > 420 nm) for ompg-C3N4 and g-C3N4 electrodes in 0.2 M Na2SO4 aqueous solution. (b) Time-resolved PL spectrum of ompg-C3N4 and g-C3N4 monitored at 480 nm under 420 nm excitation at 77 K. Plotted are the experimental data (points), the exponential fit (heavy line), and the instrument response function (dashed line).
blue shift in the band gap reflects, to our opinion, the smaller structural size of ompg-C3N4. Note that ompgC3N4 exhibited a stronger optical absorption than bulk g-C3N4, especially in the 430-550 nm region. The enhanced light-trapping effect in the visible was caused by the reflection or transmission of light scattered by the mesostructure in carbon nitride body. Figure 2b shows the PL spectra of ompg-C3N4 and bulk g-C3N4. Under 420 nm excitation, ompg-C3N4 shows a broad PL peak at 505 nm. Compared to bulk g-C3N4, the PL intensity of ompg-C3N4 was greatly suppressed, indicating the electron localization on the surface terminal sites.11,15 The time-resolved PL experiments (Figure 3b) detecting the recombination kinetics of photoinduced charge revealed that a charge separation of ompg-C3N4 can persist as long as ∼11 ns at 77 K, shorter than that of bulk one (∼13 ns). The slightly faster PL decay for ompgC3N4 can be attributed to the introduction of nanoporous structure. When performed in an electrolyte solution, the organized system can function not only as a “highway” for electrolyte (within the pores) but also for free charge carriers (in the walls) generated by illumination of the electrode. This is indeed confirmed by analyzing the photocurrent response of the ompg-C3N4 electrode in a Na2SO4 aqueous solution as shown in Figure 3a. These results indicate that the mesostructure is favorable for the interfacial separation of electron-hole pairs but also enable charge transport over longer distances. Thus, it is expected to promote heterogeneous photocatalysis. Figure 4a displays the time course of H2 evolution by ompg-C3N4 in the presence of triethanolamine as a sacrificial reagent and Pt as a cocatalyst. In the first run, the H2 evolution rate was relatively low (69 μmol h-1), presumably due to the activation of ompg-C3N4 by photoreduction of Pt4þ to Pt in the initial experiment. After the induction period, the sample remained a high H2 evolution rate of ∼85 μmol h-1 in the consecutive four runs, without noticeable deactivation. The results indicate that ordered mesoporous carbon nitride is a stable photocatalyst for water splitting. The total evolution of
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Figure 4. (a) Stability test for Pt/ompg-C3N4 under visible light irradiation (λ > 420 nm) and (b) wavelength dependence of H2 evolution rate on Pt/ompg-C3N4. The inset in (b) is the H2 evolution by Pt/ompg-C3N4 with visible light (λ > 590 nm).
H2 reaches 2.1 mmol during the course of 25 h of visible light irradiation. Reference experiments showed that no reaction occurs when the system was illuminated in the absence of catalyst or in the presence of the catalyst without illumination. The H2 evolution on the ompgC3N4 was about 5 times higher than that of bulk g-C3N4.15 The H2 evolution rate of ompg-C3N4 was also found to be dependent on the cutoff wavelength of the incident light. The filters were used to cut off the light with shorter wavelengths than those of the indicated numbers. As shown in Figure 4b, the trend of H2 evolution rates matches with that of absorption in the optical spectra, indicating a rather clean semiconductor structure. The H2 evolution was observed up to ∼590 nm (Figure 4b, inset), corresponding to the band gap transition of ompg-C3N4. To conclude, ordered mesoporous carbon nitride, possessing a large surface area, uniform pore size, and a 2D accessible framework, shows improved activity for photochemical reduction of water with visible light in the presence of Pt as a cocatalyst and electron donors. The structure is particularly promising as a host semiconductor scaffold for the design of hybrid visible-light photocatalysts, as it enables the convenient functionalization by surface reaction or deposition which is per definition always in closest proximity to the active sites of the catalytic support. We expect that a wide variety of chromophoric antenna molecules, water-reduction cocatalysts (here exemplified by Pt), and/or water-oxidation complexes can be coassembled into the ompg-C3N4 host matrix, thus generating a new type of biomimetic photocatalyst system for water splitting chemistry but also for selective organic synthesis. Acknowledgment. This work was supported by ENERCHEM project of MPI, AvH Foundation, NSF of China and Fujian Province (20603007 and 2008H0089), 973 Program (2007CB613306), NCET (07-0192), and PCSIRT(0818). The authors thank the reviewers, Prof. A. Thomas and R.Q. Sun, for their helpful suggestions and help. Supporting Information Available: Experimental details and more characterizations (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
