Molecular doping of carbon nitride photocatalysts with tunable bandgap and enhanced activity

Journal of Catalysis xxx (2013) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat

Molecular doping of carbon nitride photocatalysts with tunable bandgap and enhanced activity Jinshui Zhang, Mingwen Zhang, Sen Lin, Xianzhi Fu, Xinchen Wang ⇑ Research Institute of Photocatalysis, Fujian Provincial Key Laboratory of Photocatalysis, State Key Laboratory Breeding Base, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, PR China

a r t i c l e

i n f o

Article history: Available online xxxx Keywords: Photocatalysis Doping Carbon nitride Organic photocatalyst Thiophene

a b s t r a c t Molecular doping of conjugated carbon nitride (CN) with thiophene donors was applied for the modification of CN photocatalysts. The incorporation of electron-rich thiophene entities in the conjugated polymer matrix can effectively change the intrinsic bulk and surface features of CN, such as engineering the electronic structure with tunable bandgap and promoting the charge-carrier migration and separation via forming surface dyadic structures. A significant alteration in the texture, morphology, and crystalline was also observed for the thiophene-modified CN samples. The combined benefits of the molecular doping in terms of bulk, surface, and texture properties lead to an improvement in the photocatalytic activity for H2 generation with visible light, which underlines the importance of organic chemistry protocols for the chemical modification of newly developed metal-free, polymeric photocatalysts. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Doping, a process of intentionally introducing foreign impurities into a semiconductor, is regarded as a fundamental strategy to manipulate the electronic structure of semiconductors and thus achieving the control of conductive, optical, luminescent, magnetic, or other physical properties for targeted applications [1–4]. In photocatalysis, especially for wide-bandgap semiconductors, bandgap engineering via cation, anion, or their cooperative doping has been demonstrated as an effective procedure to enhance the optical absorption of the semiconductors and simultaneously adjusting the redox potentials of photoinduced charge carriers for relevant chemical conversions via sunlight [5–9]. Thus far, various inorganic semiconductors have been successfully advanced by elemental doping to narrow the energy bap of both UV-responsive (e.g., TiO2, ZnO, Ta2O5) and visible-light-active (e.g., WO3, CdS, BiVO4) photocatalysts [6–14]. Hence, semiconductor doping is already an extensively used and general strategy for modification of inorganic photocatalysts for solar energy applications [6–10,15,16]. Although the oxidative and reductive doping of conductive conjugated semiconductors has been applied for fabricating p- and n-type organic polymers for plastic optoelectronic devices, their application for photocatalysis has been less widespread as they are unstable undergoing photocorrosion [17]. Recently, a stable organic semiconductor, covalent carbon nitride (CN) with p-conjugated electronic features, has been ⇑ Corresponding author. Fax: +86 591 83920097. E-mail address: [email protected] (X. Wang).

widely recognized as a metal-free photocatalyst for the sustainable utilization of solar energy, offering a new platform to study the heterogeneous photocatalysis and photochemistry [18–21]. Nevertheless, due to the defective polymerization of organic networks, this aromatic p-conjugated system in its pristine form suffers some intrinsic problems, such as insufficient sunlight absorption, low surface area and fast charge recombination, greatly limiting its photochemical functions [22–25]. Thus, control to modulate the p electrons of CN aromatic system, in particular the semiconductive band structure, is regarded as an effective pathway to maximize the photocatalytic performance, and consequently, chemical doping has been already advanced as a good candidate for this purpose by us and others [26–30]. Thiophene and its derivatives with strong electron donor ability have been extensively used as building blocks for the construction of opto-related electronic polymers, acting as the chromophore center to harvest photons [31,32]. On the other hand, a crosslinked conjugated CN semiconductor is an organic material in nature and therefore can be easily modified through grafting organic groups on its surface by organic chemistry protocols [33–35]. Therefore, molecular doping of the CN network via integrating thiophene pigments in a p conjugated system is a feasible manner to modify its bulk electronic features and surface/interface properties as well, which in principle can help to optimize the physical and chemical properties for photocatalysis. In this paper, thiophene-doped CN photocatalysts were successfully synthesized by copolymerizing dicyandiamide (DCDA) with a thiophene derivative, 3-aminothiophene-2-carbonitrile (ATCN). The resultant samples were then carefully examined by theoretical

0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2013.01.008

Please cite this article in press as: J. Zhang et al., Molecular doping of carbon nitride photocatalysts with tunable bandgap and enhanced activity, J. Catal. (2013), http://dx.doi.org/10.1016/j.jcat.2013.01.008

2

J. Zhang et al. / Journal of Catalysis xxx (2013) xxx–xxx

Scheme 1. Synthesis of 3-aminothiophene-2-carbonitrile: Conditions: (A) NaN3/DMPU, 35 °C; (B) hydroxylamine-o-sulfonic acid/H2O, 40–50 °C; (C) H2, Pd-C/EtOH, RT.

Fig. 1. Electronic structure of polymeric trimer models, including the optimized HOMO and LUMO for CN trimer (a) and CNA trimer (b), and their corresponding DFTcalculated HOMO–LUMO gap (c).

calculations and physical analyses to study the influence of thiophene donors on their morphology, optical, and electronic properties. The visible-light-driven photocatalytic performance of the samples toward H2 evolution was also investigated in detail.

completion of the reaction, the reaction mixture was filtered and evaporated to dryness to give 3-aminothiophene-2-carbonitrile (4), denoted as ATCN. 2.2. Synthesis of photocatalysts

2. Experimental 2.1. Synthesis of 3-aminothiophene-2-carbonitrile As illustrated in Scheme 1, 3-aminothiophene-2-carbonitrile is synthesized step by step according to the reported procedure [36]. First, a suspension of 1 (0.2 mol) and sodium azide (0.8 mol, Caution!) in DMPU (1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, 100 mL) was stirred at 35 °C for 24 h. Then, it was poured into water and extracted with ether. The combined extracts were dried over magnesium sulfate and evaporated to dryness to give 3-azidothiophene-2-carbaldehyde (2). Note, due to the toxicity of sodium azide, the aqueous waste was treated with a diluted ethanol solution of iodine to neutralize the remaining azide. Second, a suspension of 2 (0.1 mol) and hydroxylamine-o-sulfonic acid (0.12 mol, Caution!) in water (100 mL) was stirred at 50 °C for 24 h. After cooling, the precipitate was filtered off and washed with water, giving 3-azidothiophene-2-carbonitrile (3). Third, a suspension of 3 (0.05 mol) and 10% Pd-carbon (0.3 g) in ethanol (50 mL) was stirred under hydrogen at room temperature for 4 h. After

3 g DCDA was mixed with different amounts of ATCN in 15 mL water with stirring at 60 °C to remove water. The resultant solids were calcined at 550 °C for 4 h in air to obtain the final samples. The samples thus obtained were denoted as CNAx, where x (0.005, 0.01, 0.03, 0.05, 0.07, 0.1, 0.15, and 0.3 g) is the weight-in amount of ATCN. The pristine sample CN was obtained by directly heating 3 g dicyandiamide in air at 550 °C for 4 h. 2.3. DFT calculations The density-functional-theory (DFT) calculations were carried out by using Gaussian03 program [37]. The B3LYP functions were employed, and the 6-31 G(d,p) basis set was used for all the atoms in the DFT calculations [38]. Actually, in this work, only the qualitative but not the quantitative results are required to interpret the experimental observations. Therefore, a minimum unit cell (CN or CAN trimer) with a pore is selected as our calculation model. The structural optimizations were performed without any symmetry constraints. The highest occupied molecular orbital (HOMO) and

Please cite this article in press as: J. Zhang et al., Molecular doping of carbon nitride photocatalysts with tunable bandgap and enhanced activity, J. Catal. (2013), http://dx.doi.org/10.1016/j.jcat.2013.01.008

3

J. Zhang et al. / Journal of Catalysis xxx (2013) xxx–xxx Table 1 Physicochemical properties and photocatalytic activity of CNA catalysts for hydrogen evolution with visible light (k > 420 nm).

a b c

Catalyst

C/N molar ratio

S cont. (wt.%)

Surface areaa (m2 g

CN CNA0.005 CNA0.01 CNA0.03 CNA0.05 CNA0.07 CNA0.1 CNA0.15 CNA0.3

0.73 0.73 0.73 0.73 0.73 0.74 0.74 0.75 0.78

/ / 0.11 0.14 0.16 0.20 0.25 0.43 1.2

9 11 16 13 12 16 12 15 17

1

)

Band gapb (eV)

HERc (lmol h

2.72 2.70 2.66 2.62 2.55 2.49 2.31 2.05 1.72

13.4 53 131 94 78 74 57 22 4.9

1

)

Calculated from N2-sorption isotherms. Estimated from optical measurements. H2 evolution rate.

the lowest unoccupied molecular orbital (LUMO) of CN and CNA models were then constructed. 2.4. Characterization 1 H NMR and solid-state 13C NMR spectra were recorded using a Bruker Advance III 500 spectrometer. X-ray photoelectron spectroscopy (XPS) data were measured in powder form by using a Thermo ESCALAB250 instrument with a monochromatized Al Ka line source (200 W) at 3.0  10 10 mba. And all the binding energies were calibrated to graphitic carbon at 284.6 eV. UV–Vis diffuse reflectance spectra (UV–Vis DRS) were performed on Varian Cary 500 Scan UV–visible system. Photoluminescence (PL) spectra were recorded on an Edinburgh FI/FSTCSPC 920 spectrophotometer. Electron paramagnetic resonance (EPR) measurements were recorded using a Bruker model A300 spectrometer with a 300 W Xe lamp equipped with an IR-cutoff filter (k < 800 nm) and an UV-cutoff (k > 420 nm) as visible light source. Powder X-ray diffraction (XRD) measurements were performed on Bruker D8 Advance diffractometer with Cu Ka1 radiation (k = 1.5406 Å). Fourier transformed infrared (FTIR) spectra were recorded on BioRad FTS 6000 spectrometer. Transmission electron microscopy (TEM) was obtained by Zeis 912 microscope. The nitrogen adsorption–desorption isotherms were collected at 77 K using Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. Thermogravimetric analysis (TGA) was performed on TG209 (NETZSCH Co.). Elemental analysis (EA) results were collected from a Vario MICRO. Electrochemical analysis was carried out at Zahner Zennium Electrochemical Workstation and BAS Epsilon workstation.

(a)

CNA

CN

300

200

100

0

δ / ppm S 2p3/2

(b) S 2p1/2

2.5. Photocatalytic test Photocatalytic H2 production was carried out in a Pyrex top-irradiation reaction vessel connected to a glass-closed gas circulation system. For each reaction, 100 mg well-ground catalyst powder was dispersed in an aqueous solution (100 mL) containing triethanolamine (10 vol.%) as sacrificial electron donor. 3 wt.% Pt was photodeposited onto the catalysts using H2PtCl6 dissolved in the reactant solution. The reactant solution was evacuated several times to remove air completely prior to irradiation under a 300 W Xe lamp and a water-cooling filter. The wavelength of the incident light was controlled by using an appropriate long pass cut-off filter. The temperature of the reactant solution was maintained at room temperature by a flow of cooling water during the reaction. The evolved gases were analyzed by gas chromatography equipped with a thermal conductive detector (TCD) with argon as the carrier gas.

CNA

CN

168

164

160

BE / eV Fig. 2. Solid-state 13C NMR spectra (a) and high-resolution S2p XPS spectra (b) for CN and CNA samples.  indicates the spinning side bands in the 13C spectra.

3. Results and discussion DFT calculations were carried out to study the semiconductive band structure of thiophene-doped CN [18,34,35]. In Fig. 1, both

the HOMO and LUMO are greatly changed after introducing strong electron donor groups in the CN network. For the pristine trimer, HOMO is predominantly derived from the combination of nitrogen

Please cite this article in press as: J. Zhang et al., Molecular doping of carbon nitride photocatalysts with tunable bandgap and enhanced activity, J. Catal. (2013), http://dx.doi.org/10.1016/j.jcat.2013.01.008

4

J. Zhang et al. / Journal of Catalysis xxx (2013) xxx–xxx

CNA0.3

(a)

CN

3480

3500

3520

3540

X (G)

(b) CNA0.1

CNA0.07

I / a.u.

CNA0.01 CN

CN

CN

dark vis

dark dark

vis vis

3480

3500

3520

3540

3480

3500

X/G

3520

X/G

3540

3480

3500

3520

3540

X/G

Fig. 3. Room-temperature EPR spectra of CN and CNA samples. (a) In the dark; (b) under visible-light irradiation (k > 420 nm).

pz orbitals, while LUMO mainly localizes in C–N bond orbitals (Fig. 1a) [18,34,35]. Grafting thiophene hetero-molecules on CN can effectively relocalize the p electrons to modify the corresponding HOMO and LUMO. For example, HOMO of doped-trimer shifts to locate between the tri-s-triazine subunit and the copolymerized thiophene segment, whereas LUMO still retains in C–N bond orbitals [34,35]. In Fig. 1c, this reconstruction of p-conjugated electrons evidently induces the up-shift of HOMO with 0.36 eV and simultaneously the down-shift of LUMO with 0.18 eV, reducing the bandgap energy by 0.54 eV altogether. In addition, the resulting non-coplanar HOMO and LUMO in principle facilitate the separation of photoinduced electron–hole pairs, resulting in an obvious promotion of photocatalytic performance [34,35]. Experimentally, the incorporation of thiophene in CN-conjugated system is very challenging. Because the chemical inertness of thiophene is comparable to that of benzene, directly mixing dicyandiamide (DCDA) with thiophene to undergo polymerization/ condensation at high temperatures fails to generate any thiophene-doped CN (Fig. S1). Thus, chemically activating thiophene molecular with amino and cyano side groups (changing thiophene as ATCN) to undergo nucleophilic reaction is considered as a helpful alternative protocol to graft thiophene motifs in the CN skeleton (Fig. S2) [35]. In this study, the ATCN monomer was synthesized from 3-bromothiophene-2-carbaldehyde precursor (Scheme 1) and had been well identified by FTIR, 1H NMR, and XPS spectra (Fig. S3) [36]. To prepare thiophene-doped samples, typically, 3 g DCDA was copolymerized with different amounts of ATCN at 550 °C to promote the construction of two-dimensional conjugated

CN frameworks functionalized with thiophene, and thus, creating surface dyadic junctions known to promote charge collection and separation at semiconductor interfaces. The successful combination of thiophene hetero-molecular in CN network gradually increases the C/N molar ratio from 0.73 for pristine CN to 0.78 for CNA0.3 as well as resulting in the sulfur species (Table 1). These additional carbon and sulfur species were further characterized by solid-state 13C NMR spectra and XPS analysis, respectively. In Fig. 2a, a new broad peak centering at 105 ppm is clearly observed for CNA sample, suggesting the incorporation of aromatic thiophene carbon species in the CN-conjugated network [35,36,39]. In addition, as another result of thiophene doping, evident disturbance of poly(tri-s-triazine) structure is revealed both by the lower resolution of the peaks at chemical shifts of 155.6 and 164.3 ppm and the reducing signal to noise ratio of 13C NMR spectra [21,22,35,39]. The presence of sulfur species in CN matrix is easily verified by XPS spectra. In Fig. 2b, the binding energies (BEs) collected for S2p3/2 and S2p1/2 are determined as 164.0 and 165.2 eV with an intensity ratio (IR) of 1:2.2. As expected, these BEs together with the IR are very similar to the values obtained from the ATCN monomer (Fig. S3c) and reported by previous literatures for thiophene-based molecules [40,41], where S2p3/2 and S2p1/2 with IR of 2.1 arise at 163.8 and 165.0 eV. Note that these BEs are quite different from those (163.9 and 168.5 eV) of sulfurdoped CN [27]. Thus, basing on the above characterizations, especially the S2p XPS spectra, we can conclude that doping CN matrix with ATCN monomer is more likely recognized as a molecular doping with full thiophene groups, rather than the co-doping of carbon

Please cite this article in press as: J. Zhang et al., Molecular doping of carbon nitride photocatalysts with tunable bandgap and enhanced activity, J. Catal. (2013), http://dx.doi.org/10.1016/j.jcat.2013.01.008

J. Zhang et al. / Journal of Catalysis xxx (2013) xxx–xxx

5

F (R)

(a)

CNA 0.3 CN 400

500

600

700

800

λ / nm

(b)

465 nm

I / a.u.

CNA0.1

I / a.u.

CNA0.15

CNA0.3 450

CN

550

CNA0.3

λ / nm

650

590 nm

450

550

650

750

λ / nm Fig. 4. Semiconductive optical features of CNA samples. (a) UV–Vis DRS spectra; (b) PL spectra under 400 nm excitation. Inset (a) shows the color of the samples.

and sulfur atoms, which is validated by the pi-network-extension as confirmed by the following optical characterizations. The intrinsic semiconductor properties of CN solids, such as electronic structures, optical absorption and emission features, were evidently changed by integrating thiophene in the CN skeleton, which was in good agreement with the prediction of DFT calculations (Fig. 1). For instance, the intensity of EPR spectra, originating from unpaired electrons, increases progressively with integrating more and more thiophene in the CN network to extend the delocalization of p-conjugated system (Fig. 3) [33,35]. The shoulder peak determined for heavy doped samples at 3500 G should be assigned to the presence of sufficient p-conjugated sulfur species in the CN matrix. As expected, this extended p-conjugated system can efficiently promote the generation of photochemical radical pairs and thus exhibiting an enhanced EPR signal when irradiated with visible light [21,35]. Accompanied with the evolution of electronic band structure is the remarkable red shift of optical absorption from 460 nm for pristine CN to 600 nm for CNA0.07 and finally to 720 nm for CNA0.3, corresponding to the change of sample color from yellow1 to red and to brown (Fig. 4a). Because the combination of strong electron donors in the CN matrix can efficiently extend the conjugated p-electron system and thus reducing the semiconductive bandgap in the way as already illustrate by DFT calculations (Fig. 1 and Table 1). In addition, the photoluminescence (PL) spectra of CN solids were also greatly changed by the molecular doping. In Fig. 4b, the emission peak of CN solids gradually shifts toward longer wavelength (from 465 nm to 590 nm) with continuing increasing

1 For interpretation of color in Figs. 4 and 7, the reader is referred to the web version of this article.

Fig. 5. TEM images for CN (a) and CNA (b) samples.

ATCN content to reduce the HOMO–LUMO energy gap (Fig. 1). Simultaneously, evident fluorescence quenching is also observed for thiophene-doped samples, indicating that the extended p-conjugation system with a CN/thiophene heterostructure at material interfaces can effectively reduce the recombination of photoinduced electron–hole pairs [35]. In addition, a residual signal at 465 nm is observed for the moderated doped CN, due to the insufficient amount of ATCN monomer to copolymerize with DCDA. Evidently, the surface structure, even the local packing motifs of CN subunits, is greatly modified by the incorporation of thiophene molecules, because the doping procedure in principle can alter the traditional route of thermally induced condensation/polymerization of DCDA precursors. In Fig. 5, the surface morphology is greatly changed with integrating thiophene donors in the CN skeleton, where the compact stacking of tri-s-triazine-based layered sheets (for CN) becomes much looser, just like the agglomeration of gauze (for CNA). This texture alternation is further investigated by other physical characterizations. In Fig. S4, both of the XRD patterns and the FT-IR spectra become broader and gradually less resolved with increasing the ATCN content, again underlining the doping effect (with the large S atom) to decrease structural ordering [42]. In addition, the slight increase in specific surface area (Table 1) and the reduction of thermal stability (Fig. S5) are observed for the CNA samples, potentially due to the altered local packing motifs with thiophene doping [42]. The charge generation, migration, and separation within CN/H2O interfaces are examined by (photo)electrochemical

Please cite this article in press as: J. Zhang et al., Molecular doping of carbon nitride photocatalysts with tunable bandgap and enhanced activity, J. Catal. (2013), http://dx.doi.org/10.1016/j.jcat.2013.01.008

6

J. Zhang et al. / Journal of Catalysis xxx (2013) xxx–xxx

(a)

(a) 150

CN

CNA0.07

CNA 0.01

4

2

CNA 0.07

100

CN

50

F (R)

H2 evolution rate / μmol.h

Z'' / KΩ

-1

6

× 0.6

0 0

2

4

6

8

10

Z / KΩ

(b) 2.0

0

10 s

400

500

CNA0.01 dark

1.5

(b)

90 455 nm

CN

light

i / μA

CNA0.07

600

λ / nm

CNA0.07

1.0

0.5

0.0

On / Off cycles

Produced H2 / μmol

60

495 nm 30

Fig. 6. Photoelectrochemical properties of CNA samples at 0.4 V bias potential vs. Ag/AgCl in a 0.2 M Na2SO4 aqueous solution. (a) Electrochemical impedance spectroscopy plots in the dark; (b) periodic on/off photocurrent response under visible-light irradiation (k > 420 nm).

550 nm 0

experiments. In Fig. 6a, significant decrease in semicircular Nyquist plots is observed for thiophene-doped samples in the dark, clearly demonstrating that incorporating aromatic thiophene in CN skeleton indeed can effectively improve the electronic conductivity of polymer matrix to promote the charge separation [35]. Thus, based on this improvement, enhanced anodic photocurrents are obtained on CNA photocatalysts (Fig. 6b). Moreover, in comparison with CNA0.01, the slight photocurrent response at CNA0.07 is presumably caused by the reduction of the redox potential of the electron–hole pairs. Table 1 lists the photocatalytic performance of CNA samples toward H2 production from H2O/triethanolamine with visible light (k > 420 nm) and 3 wt.% Pt as co-catalyst. Indeed, as the result of thiophene doping to extend the p conjugation, almost all the modified samples show a remarkable improvement over the pristine CN, except for the most heavily doped sample of CNA0.3. The excessive dopant seems to turn the material in part to a degenerate semiconductor by spoiling the electronic structure, which is detrimental to photocatalysis. Interestingly, the most active sample is optimized as the extrinsic semiconductor of CNA0.01, and thereafter, with further increase in the ATCN content, gradual decrease in the activity is observed but they are still more active than the un-doped reference. This is not surprising as a co-catalyst is generally applied to a photocatalyst to promote surface kinetics for

0

2

4

6

t/h Fig. 7. (a) Wavelength-dependent H2 evolution rate on CN and CNA0.07. (b) Time courses of H2 evolution on CNA0.07 as function of the wavelength of incident light.

hydrogen evolution, and it is well-known that the promotional effect is very sensitive to the surface composition and structure of a photocatalysts. This however necessitates the optimization of cocatalysts (type, amount, and size) for a certain thiophene-modified CN sample, especially those responsive to longer wavelengths. Work along this line is in progress in our lab. These thiophene-doped organic photocatalysts exhibit excellent catalytic stability for hydrogen evolution. After the first run, the reaction solution was exposed to the air for 3 weeks, and no noticeable deterioration of activity is observed in the following repeat runs. In addition, wavelength-dependent H2 production was also operated for the characterization of thiophene-doped samples. In Fig. 7, for the red-colored sample, the trend of H2 production matches well with the optical absorption of the photocatalyst, suggesting that the extended visible-light absorption indeed contributes to the photocatalytic production of H2 fuels. Meanwhile, an overall enhancement across the whole absorption spectrum is achieved on modified samples, with an active wavelength

Please cite this article in press as: J. Zhang et al., Molecular doping of carbon nitride photocatalysts with tunable bandgap and enhanced activity, J. Catal. (2013), http://dx.doi.org/10.1016/j.jcat.2013.01.008

J. Zhang et al. / Journal of Catalysis xxx (2013) xxx–xxx

extended to as long as 600 nm. These results clearly illustrate the promotional nature of semiconductor doping to support visible photocatalysis. 4. Conclusions To conclude, both electronic features and textural structure of CN photocatalyst were greatly modified via molecular doping of the conjugated frameworks with p-electron-rich thiophene entities. The incorporation of thiophene in CN skeleton can effectively extend the aromatic p-conjugated system, adjusting its intrinsic semiconductor properties, such as engineering the band structure with tunable bandgap and promoting the charge migration and separation. Meanwhile, as local-packing motifs were altered by molecular doping, obvious change in texture, morphology, and crystalline was also observed for thiophene-doped samples. Hence, as a result of thiophene doping, an overall enhanced H2 evolution rate, which also matches well with the extended optical absorption, is achieved. Currently, expanding the work by using optimized CN for other solar energy applications is ongoing, especially for the selective organic photosynthesis, because the manipulation of p-conjugated system will tune surface properties of the materials, such as polarities, chemical affinities, and acid– base functions which can in principle help to modify not only the catalytic activity but also the selectivity. Acknowledgments This work is financially supported by the National Basic Research Program of China (2013CB632405), the National Natural Science Foundation of China (21033003, 21173043 and 21203026) and of Fujian Province (2012J05022). We also thank the Department of Education of Fujian Province in China for funding. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2013.01.008. References [1] S.C. Erwin, L. Zu, M.I. Haftel, A.L. Efros, T.A. Kennedy, D.J. Norris, Nature 436 (2005) 91. [2] J.D. Bryan, D.R. Gamelin, in: K.D. Karlin (Ed.), Progress in Inorganic Chemistry, John Wiley & Sons, Inc., 2005, pp. 47–126. [3] K.F. Berggren, B.E. Sernelius, Phys. Rev. B 24 (1981) 1971. [4] M. Pfeiffer, K. Leo, X. Zhou, J. Huang, M. Hofmann, A. Werner, J. BlochwitzNimoth, Org. Electron. 4 (2003) 89. [5] Y. Gai, J. Li, S.-S. Li, J.-B. Xia, S.-H. Wei, Phys. Rev. Lett. 102 (2009) 036402. [6] X. Chen, S. Shen, L. Guo, S.S. Mao, Chem. Rev. 110 (2010) 6503.

7

[7] G. Liu, L. Wang, H.G. Yang, H.-M. Cheng, G.Q. (Max) Lu, J. Mater. Chem. 20 (2010) 831. [8] X. Yang, C. Cao, L. Erickson, K. Hohn, R. Maghirang, K. Klabunde, J. Catal. 260 (2008) 128. [9] X. Yang, C. Cao, K. Hohn, L. Erickson, R. Maghirang, D. Hamal, K. Klabunde, J. Catal. 252 (2007) 296. [10] F.E. Osterloh, Chem. Mater. 20 (2008) 35. [11] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [12] Q. Mi, Y. Ping, Y. Li, B. Cao, B.S. Brunschwig, P.G. Khalifah, G.A. Galli, H.B. Gray, N.S. Lewis, J. Am. Chem. Soc. 134 (2012) 18318. [13] X. Chen, X. Wang, Y. Hou, J. Huang, L. Wu, X. Fu, J. Catal. 255 (2008) 59. [14] H. Li, S. Yin, Y. Wang, T. Sato, J. Catal. 286 (2012) 273. [15] K. Maeda, K. Domen, J. Phys. Chem. Lett. 111 (2010) 2655. [16] K. Maeda, K. Domen, J. Phys. Chem. C 1 (2007) 7851. [17] M. Schwab, M. Hamburger, X. Feng, J. Shu, H. Spiess, X. Wang, M. Antonietti, K. Müllen, Chem. Commun. 46 (2010) 8932. [18] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76. [19] X. Wang, S. Blechert, M. Antonietti, ACS Catal. 2 (2012) 1596. [20] J. Zhang, M. Grzelczak, Y. Hou, K. Maeda, K. Domen, X. Fu, M. Antonietti, X. Wang, Chem. Sci. 3 (2012) 443. [21] Y. Cui, Z. Ding, X. Fu, X. Wang, Angew. Chem. Int. Ed. 51 (2012) 11814. [22] J. Sun, J. Zhang, M. Zhang, M. Antonietti, X. Fu, X. Wang, Nat. Commun. 3 (2012) 1139. [23] J. Zhang, M. Zhang, R.-Q. Sun, X. Wang, Angew. Chem. Int. Ed. 51 (2012) 10145. [24] J. Zhang, J. Sun, K. Maeda, K. Domen, P. Liu, M. Antonietti, X. Fu, X. Wang, Energy Environ. Sci. 4 (2011) 675. [25] J. Zhang, F. Guo, X. Wang, Adv. Funct. Mater. (2013), http://dx.doi.org/10.1002/ adfm.201203287. [26] X. Wang, X. Chen, A. Thomas, X. Fu, M. Antonietti, Adv. Mater. 21 (2009) 1609. [27] G. Liu, P. Niu, C. Sun, S.C. Smith, Z. Chen, G.Q. Lu, H.M. Cheng, J. Am. Chem. Soc. 132 (2010) 11642. [28] S.C. Yan, Z.S. Li, Z.G. Zou, Langmuir 26 (2010) 3894. [29] J. Hong, X.Y. Xia, Y.S. Wang, R. Xu, J. Mater. Chem. 22 (2012) 15006. [30] Z. Lin, X. Wang, Angew. Chem. Int. Ed. (2013), http://dx.doi.org/10.1002/anie. 201209017. [31] K. Zhang, B. Tieke, J.C. Forgie, F. Vilela, P.J. Skabara, Macromolecules 45 (2012) 743. [32] K. Zhang, B. Tieke, Macromolecules 44 (2011) 4596. [33] H. Zheng, J. Zhang, X. Wang, X. Fu, Acta. Phys. -Chim. Sin. 28 (2012) 2336. [34] Y. Guo, S. Chu, S. Yan, Y. Wang, Z. Zou, Chem. Commun. 46 (2010) 7325. [35] J. Zhang, G. Zhang, X. Chen, S. Lin, L. Möhlmann, G. Dołe˛ga, G. Lipner, M. Antonietti, S. Blechert, X. Wang, Angew. Chem. Int. Ed. 51 (2012) 3183. [36] Y.H. Song, B.S. Jo, J. Heterocyclic Chem. 46 (2009) 1132. [37] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision D.01, Gaussian, Inc., Wallingford CT, 2004. [38] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [39] J. Zhang, X. Chen, K. Takanabe, K. Maeda, K. Domen, J.D. Epping, X. Fu, M. Antonietti, X. Wang, Angew. Chem. Int. Ed. 49 (2010) 441. [40] A. Sathyapalan, S.C. Ng, A. Lohani, T.T. Ong, H. Chen, S. Zhang, Y.M. Lam, S.G. Mhaisalkar, Thin Solid Films 516 (2008) 5645. [41] A. Nelson, S. Glenis, A. Frank, J. Chem. Phys. 87 (1987) 5002. [42] J. Zhang, M. Zhang, G. Zhang, X. Wang, ACS Catal. 2 (2012) 940.

Please cite this article in press as: J. Zhang et al., Molecular doping of carbon nitride photocatalysts with tunable bandgap and enhanced activity, J. Catal. (2013), http://dx.doi.org/10.1016/j.jcat.2013.01.008