DOI: 10.1002/chem.201501742
Communication
& H2 Evolution Catalysts
Surface Functionalization of g-C3N4 : Molecular-Level Design of Noble-Metal-Free Hydrogen Evolution Photocatalysts Yin Chen,[a, b] Bin Lin,*[a] Weili Yu,[a] Yong Yang,[c] Shahid M. Bashir,[d] Hong Wang,[a] Kazuhiro Takanabe,*[b] Hicham Idriss,*[d] and Jean-Marie Basset*[b]
Chem. Eur. J. 2015, 21, 10290 – 10295
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Communication Abstract: A stable noble-metal-free hydrogen evolution photocatalyst based on graphite carbon nitride (g-C3N4) was developed by a molecular-level design strategy. Surface functionalization was successfully conducted to introduce a single nickel active site onto the surface of the semiconducting g-C3N4. This catalyst family (with less than 0.1 wt % of Ni) has been found to produce hydrogen with a rate near to the value obtained by using 3 wt % platinum as co-catalyst. This new catalyst also exhibits very good stability under hydrogen evolution conditions, without any evidence of deactivation after 24 h.
Due to the global energy crisis and environmental pollution, hydrogen is poised to be a very important clean energy vector.[1] Photocatalytic hydrogen production from water by using semiconductor catalysts has received considerable attention in the past few decades,[2] and various kinds of photobased water-splitting catalysts suitable for natural light have been developed. These include inorganic materials such as metal oxides,[3] nitrides,[4] sulfides,[5] phosphides,[2e] and organometallic complexes of Ru, Pt, and Os.[1d, 6a] However, most of these materials contain expensive noble metals or poisonous metals as co-catalysts, which may hinder their widespread application for economic and environmental reasons.[6] In the last few years, a graphite-like organic polymeric carbon nitride (g-C3N4) has attracted increasing interest owing to its photocatalytic activity for hydrogen production from water under visible-light irradiation and in the presence of a sacrificial reagent.[7] This material is cheap, easy to synthesize, and environmental friendly. In fact, g-C3N4 is one of the oldest synthetic polymers, reported for the first time by Liebig in 1834, which was known as melon at first.[8] Unlike graphite, g-C3N4 is a semiconductor material with an optical band gap of 2.7 eV.[7a] However, g-C3N4 itself exhibits a very low activity in water splitting, with a typically efficiency lower than 1 mmol h¢1 for H2 evolution in the presence of triethanolamine as a sacrificial reagent. This is due to the relatively fast electron–hole recom[a] Dr. Y. Chen, Dr. B. Lin, Dr. W. Yu, Dr. H. Wang Physical Sciences and Engineering Department King Abdullah University of Science & Technology 23955-6900 Thuwal (Saudi Arabia) [b] Dr. Y. Chen, Prof. K. Takanabe, Prof. J.-M. Basset Catalysis Center, Physical Sciences and Engineering Department King Abdullah University of Science & Technology 23955-6900 Thuwal (Saudi Arabia) E-mail: [email protected] [email protected] [email protected] [c] Prof. Y. Yang Department of Chemistry, Zhejiang Sci-Tech University Hangzhou, 310018, (P. R. China) [d] S. M. Bashir, Prof. H. Idriss SABIC-CRI at KAUST, 23955-6900, Thuwal (Saudi Arabia) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501742. Chem. Eur. J. 2015, 21, 10290 – 10295
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bination kinetics. Time-resolved fluorescence indicates decays with short and long lifetimes depending on the preparation method,[9] lifetimes that range from 5–10 ns to 50–100 ns. Precious metals such as Pt are always needed as co-catalysts to increase the lifetime of the charge carrier and achieve practical applications.[7a, 10] To make g-C3N4 an economically feasible and useful catalyst, considerable efforts have been devoted to making the catalyst more efficient. Up to now, doping is the most important method to enhance the catalyst efficiency;[7, 11] other methods including increasing the surface area of C3N4,[12] and the utilization of sensitizer.[13] However, the noble metal Pt is essentially the co-catalyst needed in all these method, which greatly limits the application of g-C3N4 in water splitting. Recently, a few examples have shown that co-catalysts other than Pt can be used in the g-C3N4 hydrogen-evolution system. These include [NiII(PPh2{NPhCH2P(O)(OH)2}2)2]Br2,[13a] MoS2,[14] NiS,[15] and Ni(OH)2,[16] yet all these have only moderate hydrogen evolution rates, low stability, and fast loss of reactivity. In the case of NiS or Ni(OH)2, the co-catalyst is needed in large amounts. More recently, one report showed that carbon quantum dots can work as the co-catalyst for g-C3N4 to give stable hydrogen evolution at a moderate rate.[17] The mechanism of carbon nitride photocatalyzed hydrogen evolution usually involves the following: upon absorption of a photon with energy equal to or higher than the band gap of the semiconductor g-C3N4, photogenerated electrons migrate to the surface and are trapped by the Pt nanoparticles deposited on the surface; these produce the hydrogen by electrolysis.[18] The activity of the catalyst comes from the few locations in which the active sites are structurally distinct from the bulk g-C3N4 ; thus, the dispersion of Pt on the surface can greatly influence the catalyst efficiency.[10] However, it is still difficult to improve the catalysts performance based on a reasonable structure–activity relationship alone. Based on a long-term study of the surfaces of organometallic catalysts,[19] we developed a noble-metal-free water-splitting catalyst by using the molecular-level design strategy of grafting single active hydrogen evolution sites onto the surface of g-C3N4. Owing to the specific location and direct chemical bonding, enhanced catalytic efficiency can be expected for the photogeneration of electrons, which can then move directly to the catalytic center. The surface of g-C3N4 is terminated with NH2 groups; these are potential sites for anchoring the catalytic center. However, because of their chemical inertness, no successful design of such a catalyst has been reported yet. g-C3N4 is constructed of repeated tri-s-triazine units, which have similar chemical properties to triazine. Based on our previous experiences with the synthesis of triazine derivatives,[20] we introduced a conjugate triazine unit with a NiS moiety (C3N3S2Ni) to the surface of g-C3N4 (Figure 1). NiS is known to be a good electrolytic hydrogen-evolution material, and a conjugated aromatic ring is poised to enhance the electron transfer. Thus, a noble-metal-free hydrogen-evolution catalyst with enhanced efficiency can be expected. We investigated the reactivity of the NH2 group on the gC3N4 surface at first. The alkylation reaction of g-C3N4 was per-
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Figure 1. Proposed surface functionalized of g-C3N4 for photocatalytic water splitting for hydrogen generation.
Scheme 1. Synthesis of the model compounds.
formed with our previous classical conditions with 13CH3I (99.5 % 13C enriched) for better characterization.[18a] After reaction in THF with 10 wt % 13CH3I and 1.2 equivalents of NaH for 3 h, the solution was filtrated, and solid g-C3N4 was collected and washed with ethanol and water several times until the filtrate showed a neutral pH value. After treatment, the 13C solidstate (SS) NMR (see the Supporting Information, Figure S1) spectrum shows two new peaks at 36 ppm and 29 ppm (Figure 2), assigned to N(CH3)2 and NHCH3 respectively, with the former one being the dominant species. This result clearly indicates the existence of NH2 terminal groups on the surface, and implies that they have comparable reactivity to the NH2 groups present on the triazine ring in the alkylation reaction. Therefore, they are able to be functionalized by a nucleophilic substitution reaction. Then, N,N,N’,N’-tetraethyl-[1,3,5]triazine-2,4,6-triamine 1 (Scheme 1) was used as a model compound to study the possibility of introduction of a C3N3S2Ni moiety onto the surface NH2 group. 1 reacts with 2.4 equivalents of NaH and 2.0 equivalents of cyanuric chloride, resulting in the single triazine substituted product 2 with a yield of 87 %. 2 reacts with NaSH in DMF/H2O to afford SH-substituted product 3 with a yield of 90 % (detailed experimental procedure and characterization data are given in
the Supporting Information, Figures S10–S15). 3 was then allowed to react with NiCl2·6 H2O in methanol at 40 8C for half an hour (the pH was controlled to be around 8), which gave a brown insoluble precipitate. Elemental analysis showed this product to be the expected compound 4. These results indicate that the C3N3S2Ni moiety can be introduced through the NH2 groups on the g-C3N4 surface by such an alkylation reaction. Next, we moved from the model molecule to g-C3N4. Surface C3N3S2Ni-modified g-C3N4 7 was prepared by following the same procedure (Scheme 2, and in the Supporting Information) by a multistep synthesis. Systematic characterizations were performed on 7. Elemental analyses gave 0.03( 0.1 %) of sulfur for 6, and 0.03( 0.1 %) of sulfur and 0.05( 0.02 %) of nickel for 7, with a Ni/S ratio of 1:1–3 (the theoretical ratio based on 4 is 1:2). TEM experiments showed the surface of g-C3N4 remained homogeneous after the treatment and no nanoparticles could be found (Figure S2 in the Supporting Information). Powder-XRD showed that 7 has the same diffraction pattern as raw g-C3N4 (Figure S3 in the Supporting Information), and infrared spectroscopy also gave similar spectra for 7 and raw g-C3N4 (Figure S4 in the Supporting Information). These results indicate that the bulk structure of g-C3N4 remains the same after the treatment. We could not detect the surface species by SS-NMR analysis, probably due to their very low concentration. XPS was then employed to characterize the structure of 7. The Ni 2p, S 2p, C 1s, N 1s, O 1s and valence band regions were scanned with a monochromatized X-ray (AlKa) (see the Supporting Information for more details). In the Ni 2p region, two weak but clear signals at 854.5 0.2 and 873.0 0.2 eV (Figure 3 inset) with splitting of 18.5 0.5 eV are seen, these are evidence for Ni2 + cations. The S 2p region revealed two main components with binding energies at 163 and 169 eV (Figure S5 in the Supporting Information). The small peak at 163 eV can be attributed to sulfur anions bonded to Ni as well as to the aromatic ring. Figure 2. 13C CP-MAS NMR of g-C3N4 after reaction with 13CH3I under the surface modification conditions (the peaks labelled & are the spin side bands). The peak at 169 eV is most likely that of an oxidized Chem. Eur. J. 2015, 21, 10290 – 10295
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Communication Table 1. Surface and near-surface atomic percentages of elements of g-C3N4-N(C3N3S2Ni) determined by XPS.
Element
Sensitivity factor
Corrected area
Atomic %
Ni 2p S 2p N 1s O 1s[a] C 1s[b]
5.40 0.35 0.38 0.63 0.21
158 480 83463 12210 69734
0.09 0.29 (0.23[c] ; 0.06[c]) 50.3 7.4 42.0
[a] Irreversibly adsorbed water and surface hydroxyls. [b] Only the carbon peak at 288.4 eV was taken in account. [c] Individual contribution of the two XPS S 2p peaks at 169.1 eV and 163 eV.
obtained is close to 0.1 %, approximately one Ni atom per 1 nm2 of the surface of g-C3N4 (BET surface area 10.4 m2g¢1). This indicates that Ni most probably exists on the surface as a single site and bridges two nearby thiol groups. The photocatalysis experiments were carried out with surface-functionalized g-C3N4-N(C3N3S2Ni) under previously reported conditions.[7a] The as-prepared g-C3N4-N(C3N3S2Ni) achieved steady H2 production from water with 10 vol % triethanolamine[7a] as a sacrificial reagent under light illumination (l > 420 nm; Figure 4). These results indicate that g-C3N4N(C3N3S2Ni) functions as a stable photocatalyst for visible-lightdriven H2 production. A typical time course of H2 production using 3 wt % Pt-deposited g-C3N4 is also shown in Figure 4. The hydrogen-evolution activity of g-C3N4 increases with Pt loading up to 3 wt % with negligible activity observed with 0.1 wt % Pt loading.[7a, 13] These results show that g-C3N4N(C3N3S2Ni) has comparable activity for H2 production to that of the common Pt-based catalyst (70 % of the rate for Pt-deposited g-C3N4). The evolution rate of H2 with 7 is 5.5 mmol h¢1 (110 mmol h¢1 gcatal, a quantum efficiency of 2.6 % at 420 nm), whereas the value for 3 wt % Pt-deposited g-C3N4 is 8.1 mmol h¢1. To further test the catalyst stability, the reaction
Scheme 2. Synthesis of g-C3N4-N(C3N3S2Ni).
Figure 3. XPS spectrum of N 1s and Ni 2p (inset) of 7.
form of sulfur due to the presence of water and exposure to ambient conditions prior to XPS analysis. Two peaks are found for the C 1s (Figure S6 in the Supporting Information); the peak at 285 eV belongs to the carbon support holder and the other carbon peak at 288.4 eV belongs to the N¢C¢N carbon of g-C3N4, which is similar to the reported C3N4 framework.[9b] In the N 1s region, one main peak at 399.0 eV is found (Figure S7 in the Supporting Information), which can be attributed to the C¢N¢C nitrogen.[9b] The shoulder peak at 401.6 eV can be assigned to a terminal NH group or positively charged N as in zwitterionic structures.[21] The small peak at 404.7 eV is due to a p excitation. The XPS valence band structure of 7 shows the leading edge of the N 2p line intercept the binding energy axis at a value very close to 2 eV (Figure 3) and more information in the Supporting Information) Table 1 presents the atomic percentages from these ex-situ XPS studies. The peak areas of each element were corrected to their ionization cross section and escape depth (correction factors) taken from tables at a take-off angle of 908. Thus, the obtained N-to-C ratio is 1.2 (close to the theoretical ratio 1.33) and the Ni-to-S ratio is around 1:2–3; the amount of Ni Chem. Eur. J. 2015, 21, 10290 – 10295
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Figure 4. A typical time course of H2 production from water containing 10 vol % triethanolamine as an electron donor under visible light (irradiation wavelength longer than 420 nm) by unmodified g-C3N4 with 3 wt % Pt (photodeposition) (squares) and surface-functionalized carbon nitride g-C3N4-N(C3N3S2Ni) (circles). Conditions: 50 mg catalyst; Xe lamp 125 mW cm¢2, for the photon distribution of the light, see Figure S16 in the Supporting Information; 10 vol % triethanolamine aqueous solution, 50 mL.
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Communication Table 2. Rate of hydrogen production over a series of C3N4 catalysts together with the presence/absence of Pt, the sacrificial agents used, light energy and light fluxes.
Catalyst
Pt [wt %]
Rate [mol gcatal¢1 h¢1]
Sacrificial reagent
Surface area [m2g¢1]
l [nm]
Lamp (power)/Flux
Ref.
g-C3N4/Cu2O g-C3N4/CdS
3 0.5
241 4500[b] (2500)[c]
TEOA, 10 vol % ascorbic ac. 0.1 m
– –
> 420 > 420
11h 11i
g-C3N4/Zn
0.5
60
methanol, 25 vol %
–
> 420
g-C3N4/MoS2
1
230[d]
methanol, 25 vol %
–
> 400
g-C3N4 g-C3N4 (S-doped) g-C3N4
3 1 3
140 120 100
TEOA, 10 vol % methanol, 25 vol % TEOA, 10 vol %
10 10 10
> 420 > 400 > 420
mpg-C3N4[e] mpg-C3N4/MoS2 mpg-C3N4 mpg-C3N4/CQD
3 0 3 0
1490 Decrease from 1003 470 575
TEOA, 10 vol % TEOA, 10 vol % TEOA, 10 vol % none
67 145 70 93
> 420 > 420 > 420 > 420
g-C3N4/Ni
0
110
TEOA, 10 vol %
10
> 420
Xe (300 W)/not given Xe (300 W)/not given QY = 8 % Xe (200 W)/0.8 mW cm¢2 QY = 3.2 % (420 nm) Xe (300 W) QY = 2.8 % undefined Hg (500 W)/not given Xe (200 W)/0.8 mW cm¢2 Xe (300 W) QY = 0.1 % (420–460 nm) Xe (300 W) Xe (300 W) Xe (300 W)/57 mW cm¢2 Xe (300 W)/ QY = 16 % (410–430 nm) Xe (300 W)/100 mW cm¢2 QY = 2.6 % (420 nm)
11j 11b 12a 11k 7a 12a 14 11L 17 This work
[a] The rate quoted in the reference is per 3 h based on Figure 6. [b] Hydrogen production is not linear (decreased) with time. [c] Hydrogen production over CdS alone. [d] Hydrogen production rate decreased with time. [e] mpg-C3N4 = mesoporous g-C3N4.
was allowed to proceed for a total of 24 h under visible-light irradiation (l > 420 nm) and continuous H2 evolution was observed with no noticeable change in the production rate and no noticeable degradation of the carbon nitride. A blank test was performed to make sure the hydrogen was produced by the surface bonded nickel. When NiCl2 (3 wt % relative to g-C3N4) was added to a suspension of unfunctionalized g-C3N4 in 10 vol % triethanolamine aqueous solution, even under ultraviolet light (l > 300 nm), only negligible amounts of H2 were produced. In most cases where nitrogen-containing materials are used as photocatalysts, a low level of N2 evolution is detected in the initial stage of the photocatalytic reaction. After the photocatalysis reaction with g-C3N4-N(C3N3S2Ni) for 24 h, no increase in the N2 or CO2 levels (oxidation of the sacrificial reagent) was found in the system.[3, 7] Additionally, no color change of the catalyst was observed after the reaction. On the contrary, when Ni was impregnated onto g-C3N4 in the form of NiS2 or Ni(OH)2 as co-catalysts,[14, 15] significantly more nickel (3 wt %) was needed to achieve a moderate performance, and the g-C3N4 was noticeably darker after the reaction due to decomposition of the co-catalysts. This provides further evidence to show that the surface N(C3N3S2Ni) moiety is very stable compared to the heterogeneous NiS or Ni(OH)2 co-catalyst. To confirm the surface N(C3N3S2Ni) moiety is active for hydrogen evolution, we did the hydrogen-evolution reaction (HER) test with the model compound 4 under similar conditions. An on-site HER potential of ¢0.65 V was observed, which indicates a good hydrogen-evolution activity (Figure S9 in the Supporting Information). It is worth comparing the observed results of this work to those of previous ones. We have incorporated in Table 2 the normalized rates, the presence or absence of Pt, the light Chem. Eur. J. 2015, 21, 10290 – 10295
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flux, and other parameters needed for a comparison of the catalysts’ performance. In summary, by modifying the surface of g-C3N4 in a controlled manner, conjugated thiol ligands have been successfully introduced to the surface NH2 groups of g-C3N4, and this led to a noble-metal-free hydrogen-evolution catalyst after reaction with Ni2 + (with less than 0.1 % nickel overloading). This catalyst has an activity in hydrogen evolution near to that attained with a 3 wt % platinum photodeposited g-C3N4 catalyst. These results indicate that g-C3N4-based hydrogenevolution catalysts can be designed at the molecular level to achieve better atomic catalytic efficiency (more than ten times previous), which opens a new way to develop these catalysts.
Acknowledgements The authors are grateful for financial support from KAUST. Y.Y. thanks the Chinese Natural Science Foundation (No. 51102107), Zhejiang NSF (Grant LY12B02021) and “521” talent program of ZSTU for support. We thank Prof. Karl Leo for support and careful reading of the manuscript. Keywords: catalyst design · graphite hydrogen evolution · noble-metal functionalization
carbon nitride · free · surface
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[10] K. Maeda, X. Wang, Y. Nishihara, D. L. Lu, M. Antonietti, K. Domen, J. Phys. Chem. C 2009, 113, 4940 – 4947. [11] a) Q. Xiang, J. Yu, M. Jaroniec, J. Phys. Chem. C 2011, 115, 7355 – 7363; b) L. Ge, C. Han, X. Xiao, L. Guo, Int. J. Hydrogen Energy 2013, 38, 6960 – 6969; c) Z.-Y. C, B. Yuan, T.-N. Yan, Int. J. Inorg. Mater. 2014, 16, 785 – 794; d) S. Chu, Y. Wang, Y. Guo, J. Feng, C. Wang, W. Luo, X. Fan, Z. Zou, ACS Catal. 2013, 3, 912 – 919; e) J. Zhang, G. Zhang, X. Chen, S. Lin, L. Mçhlmann, G. Dołe˛ga, G. Lipner, M. Antonietti, S. Blechert, X. Wang, Angew. Chem. Int. Ed. 2012, 51, 3183 – 3187; Angew. Chem. 2012, 124, 3237 – 3241; f) Y. Zhang, T. Mori, J. Ye, M. Antonietti, J. Am. Chem. Soc. 2010, 132, 6294 – 6295; g) Y. Wang, Y. Di, M. Antonietti, H. Li, X. Chen, X. Wang, Chem. Mater. 2010, 22, 5119 – 5121; h) J. Chen, S. Shen, P. Guo, M. Wang, P. Wu, X. Wang, L. Guo, Appl. Catal. B 2014, 152 – 153, 335 – 341; i) S.-W. Cao, Y.-P. Yuan, J. Fang, M. M. Shahjamali, F. Y. C. Boey, J. Barber, S. C. J. Loo, C. Xue, Int. J. Hydrogen Energy 2013, 38, 1258 – 1266; j) B. Yue, Q. Li, H. Iwai, T. Kako, J. Ye, Sci. Technol. Adv. Mater. 2011, 12, 034401; k) L. Ge, C. Han, X. Xiao, L. Guo, Y. Li, Mater. Res. Bull. 2013, 48, 3919 – 3925; l) J. Liu, Y. Zhang, L. Lu, G. Wu, W. Chen, Chem. Commun. 2012, 48, 8826 – 8828. [12] a) X. C. Wang, K. Maeda, X. F. Chen, K. Takanabe, K. Domen, Y. D. Hou, X. Z. Fu, M. Antonietti, J. Am. Chem. Soc. 2009, 131, 1680 – 1681; b) K. Schwinghammer, M. B. Mesch, V. Duppel, C. Ziegler, J. Senker, B. V. Lotsch, J. Am. Chem. Soc. 2014, 136, 1730 – 1733. [13] a) Y. J. Zhang, A. Thomas, M. Antonietti, X. Wang, J. Am. Chem. Soc. 2009, 131, 50 – 51; b) C. A. Caputo, M. A. Gross, V. W. Lau, C. Cavazza, B. V. Lotsch, E. Reisner, Angew. Chem. Int. Ed. 2014, 53, 11538 – 11542; Angew. Chem. 2014, 126, 11722 – 11726. [14] Y. D. Hou, A. B. Laursen, J. S. Zhang, G. G. Zhang, Y. S. Zhu, X. Wang, S. Dahl, I. Chorkendorff, Angew. Chem. Int. Ed. 2013, 52, 3621 – 3625; Angew. Chem. 2013, 125, 3709 – 3713. [15] a) J. Hong, Y. Wang, Y. Wang, W. Zhang, R. Xu, ChemSusChem 2013, 6, 2263 – 2268; b) L. S. Yin, Y. P. Yuan, S. W. Cao, Z. Y. Zhang, C. Xue, RSC Adv. 2014, 4, 6127 – 6132. [16] J. G. Yu, S. H. Wang, B. Cheng, Z. Lin, F. Huang, Catal. Sci. Technol. 2013, 3, 1782 – 1789. [17] J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S-T. Lee, J. Zhong, Z. Kang, Science 2015, 347, 970 – 974. [18] J. H. Yang, D. G. Wang, H. X. Han, C. Li, Acc. Chem. Res. 2013, 46, 1900 – 1909. [19] a) Y. Chen, E. Callens, E. Abou-Hamad, N. Merle, A. J. P. White, M. Taoufik, C. Coperet, E. Le Roux, J.-M. Basset, Angew. Chem. Int. Ed. 2012, 51, 11886 – 11899; Angew. Chem. 2012, 124, 12056 – 12059; b) Y. Chen, R. Credendino, E. Callens, M. Atiqullah, M. A. Al-Harthi, L. Cavallo, J.-M. Basset, ACS Catal. 2013, 3, 1360 – 1364; c) Y. Chen, S. Ould-Chikh, E. Abou-Hamad, E. Callens, J. C. Mohandas, S. Khalid, J.-M. Basset, Organometallics 2014, 33, 1205 – 1211; d) J.-M. Basset, R. Psaro, D. Roberto, R. Ugo, Modern Surface Organometallic Chemistry, WILEY-VCH, Weinheim, 2009; e) C. Cop¦ret, M. Chabanas, R. P. Saint-Arroman, J. M. Basset, Angew. Chem. Int. Ed. 2003, 42, 156 – 181; Angew. Chem. 2003, 115, 164 – 191; f) D. Astruc, Nat. Chem. 2012, 4, 255. [20] a) Y. Chen, D.-X. Wang, Z.-T. Huang, M.-X. Wang, J. Org. Chem. 2010, 75, 3786 – 3796; b) M.-X. Wang, Acc. Chem. Res. 2012, 45, 182 – 195. [21] a) G. J. Fleming, K. Adib, J. A. Rodriguez, M. A. Barteau, H. Idriss, Surf. Sci. 2007, 601, 5726 – 5731; b) G. J. Fleming, K. Adib, J. A. Rodriguez, M. A. Barteau, J. M. White, H. Idriss, Surf. Sci. 2008, 602, 2029 – 2038.
Received: May 5, 2015 Published online on June 12, 2015
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& H2 Evolution Catalysts
Surface Functionalization of g-C3N4 : Molecular-Level Design of Noble-Metal-Free Hydrogen Evolution Photocatalysts Yin Chen,[a, b] Bin Lin,*[a] Weili Yu,[a] Yong Yang,[c] Shahid M. Bashir,[d] Hong Wang,[a] Kazuhiro Takanabe,*[b] Hicham Idriss,*[d] and Jean-Marie Basset*[b]
Chem. Eur. J. 2015, 21, 10290 – 10295
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Communication Abstract: A stable noble-metal-free hydrogen evolution photocatalyst based on graphite carbon nitride (g-C3N4) was developed by a molecular-level design strategy. Surface functionalization was successfully conducted to introduce a single nickel active site onto the surface of the semiconducting g-C3N4. This catalyst family (with less than 0.1 wt % of Ni) has been found to produce hydrogen with a rate near to the value obtained by using 3 wt % platinum as co-catalyst. This new catalyst also exhibits very good stability under hydrogen evolution conditions, without any evidence of deactivation after 24 h.
Due to the global energy crisis and environmental pollution, hydrogen is poised to be a very important clean energy vector.[1] Photocatalytic hydrogen production from water by using semiconductor catalysts has received considerable attention in the past few decades,[2] and various kinds of photobased water-splitting catalysts suitable for natural light have been developed. These include inorganic materials such as metal oxides,[3] nitrides,[4] sulfides,[5] phosphides,[2e] and organometallic complexes of Ru, Pt, and Os.[1d, 6a] However, most of these materials contain expensive noble metals or poisonous metals as co-catalysts, which may hinder their widespread application for economic and environmental reasons.[6] In the last few years, a graphite-like organic polymeric carbon nitride (g-C3N4) has attracted increasing interest owing to its photocatalytic activity for hydrogen production from water under visible-light irradiation and in the presence of a sacrificial reagent.[7] This material is cheap, easy to synthesize, and environmental friendly. In fact, g-C3N4 is one of the oldest synthetic polymers, reported for the first time by Liebig in 1834, which was known as melon at first.[8] Unlike graphite, g-C3N4 is a semiconductor material with an optical band gap of 2.7 eV.[7a] However, g-C3N4 itself exhibits a very low activity in water splitting, with a typically efficiency lower than 1 mmol h¢1 for H2 evolution in the presence of triethanolamine as a sacrificial reagent. This is due to the relatively fast electron–hole recom[a] Dr. Y. Chen, Dr. B. Lin, Dr. W. Yu, Dr. H. Wang Physical Sciences and Engineering Department King Abdullah University of Science & Technology 23955-6900 Thuwal (Saudi Arabia) [b] Dr. Y. Chen, Prof. K. Takanabe, Prof. J.-M. Basset Catalysis Center, Physical Sciences and Engineering Department King Abdullah University of Science & Technology 23955-6900 Thuwal (Saudi Arabia) E-mail: [email protected] [email protected] [email protected] [c] Prof. Y. Yang Department of Chemistry, Zhejiang Sci-Tech University Hangzhou, 310018, (P. R. China) [d] S. M. Bashir, Prof. H. Idriss SABIC-CRI at KAUST, 23955-6900, Thuwal (Saudi Arabia) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501742. Chem. Eur. J. 2015, 21, 10290 – 10295
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bination kinetics. Time-resolved fluorescence indicates decays with short and long lifetimes depending on the preparation method,[9] lifetimes that range from 5–10 ns to 50–100 ns. Precious metals such as Pt are always needed as co-catalysts to increase the lifetime of the charge carrier and achieve practical applications.[7a, 10] To make g-C3N4 an economically feasible and useful catalyst, considerable efforts have been devoted to making the catalyst more efficient. Up to now, doping is the most important method to enhance the catalyst efficiency;[7, 11] other methods including increasing the surface area of C3N4,[12] and the utilization of sensitizer.[13] However, the noble metal Pt is essentially the co-catalyst needed in all these method, which greatly limits the application of g-C3N4 in water splitting. Recently, a few examples have shown that co-catalysts other than Pt can be used in the g-C3N4 hydrogen-evolution system. These include [NiII(PPh2{NPhCH2P(O)(OH)2}2)2]Br2,[13a] MoS2,[14] NiS,[15] and Ni(OH)2,[16] yet all these have only moderate hydrogen evolution rates, low stability, and fast loss of reactivity. In the case of NiS or Ni(OH)2, the co-catalyst is needed in large amounts. More recently, one report showed that carbon quantum dots can work as the co-catalyst for g-C3N4 to give stable hydrogen evolution at a moderate rate.[17] The mechanism of carbon nitride photocatalyzed hydrogen evolution usually involves the following: upon absorption of a photon with energy equal to or higher than the band gap of the semiconductor g-C3N4, photogenerated electrons migrate to the surface and are trapped by the Pt nanoparticles deposited on the surface; these produce the hydrogen by electrolysis.[18] The activity of the catalyst comes from the few locations in which the active sites are structurally distinct from the bulk g-C3N4 ; thus, the dispersion of Pt on the surface can greatly influence the catalyst efficiency.[10] However, it is still difficult to improve the catalysts performance based on a reasonable structure–activity relationship alone. Based on a long-term study of the surfaces of organometallic catalysts,[19] we developed a noble-metal-free water-splitting catalyst by using the molecular-level design strategy of grafting single active hydrogen evolution sites onto the surface of g-C3N4. Owing to the specific location and direct chemical bonding, enhanced catalytic efficiency can be expected for the photogeneration of electrons, which can then move directly to the catalytic center. The surface of g-C3N4 is terminated with NH2 groups; these are potential sites for anchoring the catalytic center. However, because of their chemical inertness, no successful design of such a catalyst has been reported yet. g-C3N4 is constructed of repeated tri-s-triazine units, which have similar chemical properties to triazine. Based on our previous experiences with the synthesis of triazine derivatives,[20] we introduced a conjugate triazine unit with a NiS moiety (C3N3S2Ni) to the surface of g-C3N4 (Figure 1). NiS is known to be a good electrolytic hydrogen-evolution material, and a conjugated aromatic ring is poised to enhance the electron transfer. Thus, a noble-metal-free hydrogen-evolution catalyst with enhanced efficiency can be expected. We investigated the reactivity of the NH2 group on the gC3N4 surface at first. The alkylation reaction of g-C3N4 was per-
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Figure 1. Proposed surface functionalized of g-C3N4 for photocatalytic water splitting for hydrogen generation.
Scheme 1. Synthesis of the model compounds.
formed with our previous classical conditions with 13CH3I (99.5 % 13C enriched) for better characterization.[18a] After reaction in THF with 10 wt % 13CH3I and 1.2 equivalents of NaH for 3 h, the solution was filtrated, and solid g-C3N4 was collected and washed with ethanol and water several times until the filtrate showed a neutral pH value. After treatment, the 13C solidstate (SS) NMR (see the Supporting Information, Figure S1) spectrum shows two new peaks at 36 ppm and 29 ppm (Figure 2), assigned to N(CH3)2 and NHCH3 respectively, with the former one being the dominant species. This result clearly indicates the existence of NH2 terminal groups on the surface, and implies that they have comparable reactivity to the NH2 groups present on the triazine ring in the alkylation reaction. Therefore, they are able to be functionalized by a nucleophilic substitution reaction. Then, N,N,N’,N’-tetraethyl-[1,3,5]triazine-2,4,6-triamine 1 (Scheme 1) was used as a model compound to study the possibility of introduction of a C3N3S2Ni moiety onto the surface NH2 group. 1 reacts with 2.4 equivalents of NaH and 2.0 equivalents of cyanuric chloride, resulting in the single triazine substituted product 2 with a yield of 87 %. 2 reacts with NaSH in DMF/H2O to afford SH-substituted product 3 with a yield of 90 % (detailed experimental procedure and characterization data are given in
the Supporting Information, Figures S10–S15). 3 was then allowed to react with NiCl2·6 H2O in methanol at 40 8C for half an hour (the pH was controlled to be around 8), which gave a brown insoluble precipitate. Elemental analysis showed this product to be the expected compound 4. These results indicate that the C3N3S2Ni moiety can be introduced through the NH2 groups on the g-C3N4 surface by such an alkylation reaction. Next, we moved from the model molecule to g-C3N4. Surface C3N3S2Ni-modified g-C3N4 7 was prepared by following the same procedure (Scheme 2, and in the Supporting Information) by a multistep synthesis. Systematic characterizations were performed on 7. Elemental analyses gave 0.03( 0.1 %) of sulfur for 6, and 0.03( 0.1 %) of sulfur and 0.05( 0.02 %) of nickel for 7, with a Ni/S ratio of 1:1–3 (the theoretical ratio based on 4 is 1:2). TEM experiments showed the surface of g-C3N4 remained homogeneous after the treatment and no nanoparticles could be found (Figure S2 in the Supporting Information). Powder-XRD showed that 7 has the same diffraction pattern as raw g-C3N4 (Figure S3 in the Supporting Information), and infrared spectroscopy also gave similar spectra for 7 and raw g-C3N4 (Figure S4 in the Supporting Information). These results indicate that the bulk structure of g-C3N4 remains the same after the treatment. We could not detect the surface species by SS-NMR analysis, probably due to their very low concentration. XPS was then employed to characterize the structure of 7. The Ni 2p, S 2p, C 1s, N 1s, O 1s and valence band regions were scanned with a monochromatized X-ray (AlKa) (see the Supporting Information for more details). In the Ni 2p region, two weak but clear signals at 854.5 0.2 and 873.0 0.2 eV (Figure 3 inset) with splitting of 18.5 0.5 eV are seen, these are evidence for Ni2 + cations. The S 2p region revealed two main components with binding energies at 163 and 169 eV (Figure S5 in the Supporting Information). The small peak at 163 eV can be attributed to sulfur anions bonded to Ni as well as to the aromatic ring. Figure 2. 13C CP-MAS NMR of g-C3N4 after reaction with 13CH3I under the surface modification conditions (the peaks labelled & are the spin side bands). The peak at 169 eV is most likely that of an oxidized Chem. Eur. J. 2015, 21, 10290 – 10295
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Communication Table 1. Surface and near-surface atomic percentages of elements of g-C3N4-N(C3N3S2Ni) determined by XPS.
Element
Sensitivity factor
Corrected area
Atomic %
Ni 2p S 2p N 1s O 1s[a] C 1s[b]
5.40 0.35 0.38 0.63 0.21
158 480 83463 12210 69734
0.09 0.29 (0.23[c] ; 0.06[c]) 50.3 7.4 42.0
[a] Irreversibly adsorbed water and surface hydroxyls. [b] Only the carbon peak at 288.4 eV was taken in account. [c] Individual contribution of the two XPS S 2p peaks at 169.1 eV and 163 eV.
obtained is close to 0.1 %, approximately one Ni atom per 1 nm2 of the surface of g-C3N4 (BET surface area 10.4 m2g¢1). This indicates that Ni most probably exists on the surface as a single site and bridges two nearby thiol groups. The photocatalysis experiments were carried out with surface-functionalized g-C3N4-N(C3N3S2Ni) under previously reported conditions.[7a] The as-prepared g-C3N4-N(C3N3S2Ni) achieved steady H2 production from water with 10 vol % triethanolamine[7a] as a sacrificial reagent under light illumination (l > 420 nm; Figure 4). These results indicate that g-C3N4N(C3N3S2Ni) functions as a stable photocatalyst for visible-lightdriven H2 production. A typical time course of H2 production using 3 wt % Pt-deposited g-C3N4 is also shown in Figure 4. The hydrogen-evolution activity of g-C3N4 increases with Pt loading up to 3 wt % with negligible activity observed with 0.1 wt % Pt loading.[7a, 13] These results show that g-C3N4N(C3N3S2Ni) has comparable activity for H2 production to that of the common Pt-based catalyst (70 % of the rate for Pt-deposited g-C3N4). The evolution rate of H2 with 7 is 5.5 mmol h¢1 (110 mmol h¢1 gcatal, a quantum efficiency of 2.6 % at 420 nm), whereas the value for 3 wt % Pt-deposited g-C3N4 is 8.1 mmol h¢1. To further test the catalyst stability, the reaction
Scheme 2. Synthesis of g-C3N4-N(C3N3S2Ni).
Figure 3. XPS spectrum of N 1s and Ni 2p (inset) of 7.
form of sulfur due to the presence of water and exposure to ambient conditions prior to XPS analysis. Two peaks are found for the C 1s (Figure S6 in the Supporting Information); the peak at 285 eV belongs to the carbon support holder and the other carbon peak at 288.4 eV belongs to the N¢C¢N carbon of g-C3N4, which is similar to the reported C3N4 framework.[9b] In the N 1s region, one main peak at 399.0 eV is found (Figure S7 in the Supporting Information), which can be attributed to the C¢N¢C nitrogen.[9b] The shoulder peak at 401.6 eV can be assigned to a terminal NH group or positively charged N as in zwitterionic structures.[21] The small peak at 404.7 eV is due to a p excitation. The XPS valence band structure of 7 shows the leading edge of the N 2p line intercept the binding energy axis at a value very close to 2 eV (Figure 3) and more information in the Supporting Information) Table 1 presents the atomic percentages from these ex-situ XPS studies. The peak areas of each element were corrected to their ionization cross section and escape depth (correction factors) taken from tables at a take-off angle of 908. Thus, the obtained N-to-C ratio is 1.2 (close to the theoretical ratio 1.33) and the Ni-to-S ratio is around 1:2–3; the amount of Ni Chem. Eur. J. 2015, 21, 10290 – 10295
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Figure 4. A typical time course of H2 production from water containing 10 vol % triethanolamine as an electron donor under visible light (irradiation wavelength longer than 420 nm) by unmodified g-C3N4 with 3 wt % Pt (photodeposition) (squares) and surface-functionalized carbon nitride g-C3N4-N(C3N3S2Ni) (circles). Conditions: 50 mg catalyst; Xe lamp 125 mW cm¢2, for the photon distribution of the light, see Figure S16 in the Supporting Information; 10 vol % triethanolamine aqueous solution, 50 mL.
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Communication Table 2. Rate of hydrogen production over a series of C3N4 catalysts together with the presence/absence of Pt, the sacrificial agents used, light energy and light fluxes.
Catalyst
Pt [wt %]
Rate [mol gcatal¢1 h¢1]
Sacrificial reagent
Surface area [m2g¢1]
l [nm]
Lamp (power)/Flux
Ref.
g-C3N4/Cu2O g-C3N4/CdS
3 0.5
241 4500[b] (2500)[c]
TEOA, 10 vol % ascorbic ac. 0.1 m
– –
> 420 > 420
11h 11i
g-C3N4/Zn
0.5
60
methanol, 25 vol %
–
> 420
g-C3N4/MoS2
1
230[d]
methanol, 25 vol %
–
> 400
g-C3N4 g-C3N4 (S-doped) g-C3N4
3 1 3
140 120 100
TEOA, 10 vol % methanol, 25 vol % TEOA, 10 vol %
10 10 10
> 420 > 400 > 420
mpg-C3N4[e] mpg-C3N4/MoS2 mpg-C3N4 mpg-C3N4/CQD
3 0 3 0
1490 Decrease from 1003 470 575
TEOA, 10 vol % TEOA, 10 vol % TEOA, 10 vol % none
67 145 70 93
> 420 > 420 > 420 > 420
g-C3N4/Ni
0
110
TEOA, 10 vol %
10
> 420
Xe (300 W)/not given Xe (300 W)/not given QY = 8 % Xe (200 W)/0.8 mW cm¢2 QY = 3.2 % (420 nm) Xe (300 W) QY = 2.8 % undefined Hg (500 W)/not given Xe (200 W)/0.8 mW cm¢2 Xe (300 W) QY = 0.1 % (420–460 nm) Xe (300 W) Xe (300 W) Xe (300 W)/57 mW cm¢2 Xe (300 W)/ QY = 16 % (410–430 nm) Xe (300 W)/100 mW cm¢2 QY = 2.6 % (420 nm)
11j 11b 12a 11k 7a 12a 14 11L 17 This work
[a] The rate quoted in the reference is per 3 h based on Figure 6. [b] Hydrogen production is not linear (decreased) with time. [c] Hydrogen production over CdS alone. [d] Hydrogen production rate decreased with time. [e] mpg-C3N4 = mesoporous g-C3N4.
was allowed to proceed for a total of 24 h under visible-light irradiation (l > 420 nm) and continuous H2 evolution was observed with no noticeable change in the production rate and no noticeable degradation of the carbon nitride. A blank test was performed to make sure the hydrogen was produced by the surface bonded nickel. When NiCl2 (3 wt % relative to g-C3N4) was added to a suspension of unfunctionalized g-C3N4 in 10 vol % triethanolamine aqueous solution, even under ultraviolet light (l > 300 nm), only negligible amounts of H2 were produced. In most cases where nitrogen-containing materials are used as photocatalysts, a low level of N2 evolution is detected in the initial stage of the photocatalytic reaction. After the photocatalysis reaction with g-C3N4-N(C3N3S2Ni) for 24 h, no increase in the N2 or CO2 levels (oxidation of the sacrificial reagent) was found in the system.[3, 7] Additionally, no color change of the catalyst was observed after the reaction. On the contrary, when Ni was impregnated onto g-C3N4 in the form of NiS2 or Ni(OH)2 as co-catalysts,[14, 15] significantly more nickel (3 wt %) was needed to achieve a moderate performance, and the g-C3N4 was noticeably darker after the reaction due to decomposition of the co-catalysts. This provides further evidence to show that the surface N(C3N3S2Ni) moiety is very stable compared to the heterogeneous NiS or Ni(OH)2 co-catalyst. To confirm the surface N(C3N3S2Ni) moiety is active for hydrogen evolution, we did the hydrogen-evolution reaction (HER) test with the model compound 4 under similar conditions. An on-site HER potential of ¢0.65 V was observed, which indicates a good hydrogen-evolution activity (Figure S9 in the Supporting Information). It is worth comparing the observed results of this work to those of previous ones. We have incorporated in Table 2 the normalized rates, the presence or absence of Pt, the light Chem. Eur. J. 2015, 21, 10290 – 10295
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flux, and other parameters needed for a comparison of the catalysts’ performance. In summary, by modifying the surface of g-C3N4 in a controlled manner, conjugated thiol ligands have been successfully introduced to the surface NH2 groups of g-C3N4, and this led to a noble-metal-free hydrogen-evolution catalyst after reaction with Ni2 + (with less than 0.1 % nickel overloading). This catalyst has an activity in hydrogen evolution near to that attained with a 3 wt % platinum photodeposited g-C3N4 catalyst. These results indicate that g-C3N4-based hydrogenevolution catalysts can be designed at the molecular level to achieve better atomic catalytic efficiency (more than ten times previous), which opens a new way to develop these catalysts.
Acknowledgements The authors are grateful for financial support from KAUST. Y.Y. thanks the Chinese Natural Science Foundation (No. 51102107), Zhejiang NSF (Grant LY12B02021) and “521” talent program of ZSTU for support. We thank Prof. Karl Leo for support and careful reading of the manuscript. Keywords: catalyst design · graphite hydrogen evolution · noble-metal functionalization
carbon nitride · free · surface
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Received: May 5, 2015 Published online on June 12, 2015
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