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Protonated Graphitic Carbon Nitride with Surface Attached Molecule as Hole Relay for Efficient Photocatalytic O2 Evolution Chen Ye, Xu-Zhe Wang, Jia-Xin Li, Zhi-Jun Li, Xu-Bing Li, LiPing Zhang, Bin Chen, Chen-Ho Tung, and Li-Zhu Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02664 • Publication Date (Web): 03 Nov 2016 Downloaded from http://pubs.acs.org on November 3, 2016
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Protonated Graphitic Carbon Nitride with Surface Attached Molecule as Hole Relay for Efficient Photocatalytic O2 Evolution Chen Ye, Xu-Zhe Wang, Jia-Xin Li, Zhi-Jun Li, Xu-Bing Li, Li-Ping Zhang, Bin Chen, Chen-Ho Tung, and Li-Zhu Wu* Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China. KEYWORDS: graphitic carbon nitride, water splitting, photocatalytic O2 evolution, BODIPY, hole relay, transient absorption spectrum, photogenerated holes.
ABSTRACT: Carbon nitride has been considered as one of the most promising materials to catalyze O2 evolution from water, however its intrinsic drawbacks of surficial inertness and poor charge separation efficiency are serious. In contrast to the ordinary modification method of cocatalyst loading, we describe here that a simple modification method of molecule loading can greatly improve the activity of carbon nitride for O2 evolution. A mechanistic investigation by timeresolved transient absorption spectroscopy reveals that the BODIPY molecule can extract photogenerated holes in carbon nitride to promote the oxidation of water. It is the first case of a dye molecule acting as hole relay to promote photocatalytic water oxidation. These results provide a new approach to the modification for the better catalytic performance of carbon nitride on O2 evolution.
INTRODUCTION Water oxidation is considered as a bottleneck to water splitting for its energetically and mechanistically high demands.1-3 Graphitic carbon nitride, a kind of metal free organic polymeric semiconductors, has attracted much interest in photocatalytic water oxidation due to its proper bandgap, chemical and thermal stability, widespread availability and facile preparation.4-6 Since the early report of Wang et al. in 2009, structural modifications by protonation, doping and copolymerization have been developed to enhance its activity profile.7-11 Surface modification with a range of cocatalysts have been also shown to promote its catalytic performance, including silver phosphate, cobalt oxide and selenide, layered double hydroxide (LDH) and carbon nano-dot.12-18 However, the intrinsic drawbacks of graphitic carbon nitride, particularly the high inclination of recombination restrict seriously the catalytic vigor.4,19 Thus, to separate and lead the charge carriers would be a key factor to keep carbon nitride as one of the most promising candidates for the photocatalysts of O2 evolution. In this contribution, we extend these studies by reporting a case of carbon nitride catalyzed solar water oxidation with the assistance of boron-dipyrromethene, a famous dye of interest for labeling reagents, fluorescent switches, chemosensors and laser dyes.20-22 Photocatalytic
tests showed that O2 evolution of carbon nitride was accelerated obviously after loading BODIPY. Mechanistic investigation by transient absorption methods gave us the most direct evidence of BODIPY playing as a hole relay to promote O2 evolution. Though dye sensitizing to enhance the photocatalytic ability of carbon nitride has been realized,23-26 it is the first time that dye molecule is used as hole relay to promote water oxidation. It is true that this kind of composited catalysts is still unstable, however we can get the enlightenment that exporting the holes in carbon nitride is crucial to the high efficiency for the final O2 evolution.
RESULTS AND DISCUSSION As a starting point, two kinds of carbon nitride, graphitic carbon nitride (g-CN) and protonated graphitic carbon nitride (p-CN) were fabricated according to the ever reported procedure.8 Zeta potential tests showed that the two kinds of carbon nitride have the same kind of Scheme 1. Idealized structures of (a) g-CN, (b) p-CN, and (c) BODIPY.
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considering that the amount of dye molecules on the surface is quite small.
charge on their surface (Figure S1). Then a simple borondipyrromethene derivative, 4,4-difluoro-8-phenyl-4-bora3a,4adiaza-s-indacene (denoted as BODIPY in this work) with strong fluorescence, was synthesized upon the reports of Wu et al.27 And the composite catalysts were prepared by a simple liquid-phase immersion with g-CN or p-CN into BODIPY solution of 2% weight ratio followed by rotary evaporation and annealing at a low temperature. The transmission electron microscope (TEM) images showed layered morphology of these samples (Figure S2), which was confirmed by X-ray diffraction (XRD) with the typical (002) peaks (Figure S5).28,29 The UV-Vis absorption and infrared spectra displayed the typical patterns of BODIPY both in p-CN/BODIPY and gCN/BODIPY (Figure S3 and Figure S4), suggesting the combination of carbon nitride matrix with BODIPY molecules. This result is largely due to a consequence of the π-π stacking interactions for the aromatic nature of both BODIPY molecules and the carbon nitride matrix.30 The chemical structure of carbon nitride can be confirmed by high-resolution X-ray photoelectron spectra (XPS) calibrated with the reference carbon at 284.6 eV (Figure 1). The intensity increase of peak at 284.6 eV implied the loading of BODIPY. The C 1s signal could be deconvoluted to two parts at binding energy (BE) of 288.2 and 284.6 eV assigned to sp2-hybridized carbon (C=N-C) and graphitic carbon absorbed on the surface.31 The higher ratio of sp3- hybridized nitrogen at 399.8 and 401.0 eV for p-CN is in common with Tang’s work as a result of the introduction of terminal groups by protonation.32 The existence of BODIPY on the surface of carbon nitride cannot be verified by XPS signals of either N 1s or B 1s,
Figure 1. High resolution B 1s, N 1s and C 1s XPS spectra of (a) g-CN, (b) p-CN, (c)BODIPY, (d) g-CN/BODIPY and (e) p-CN/BODIPY. These photocatalysts were tested for O2 evolution in an aqueous sacrificial solution of 0.01 M AgNO3 at room temperature and atmospheric pressure with La2O3 powder added to keep neutral conditions. The comparison of photocatalytic characteristics of these two kinds of carbon nitrides with and without BODIPY demonstrates that a striking improvement could be achieved when BODIPY was loaded on the surface of p-CN while g-CN loaded with the same amount of BODIPY seemed O2 consuming at the initial stage (Figure 2a and Figure S6). Quantitative analysis shows that with BODIPY loaded on the surface, the photocatalytic ability at the initial stage of 100 s of pCN is enhanced by 5.61 and 6.27 times upon illumination at 410 nm and 450 nm, respectively, which can even rival the most widely used metal oxides like CoOx and RuO2 as cocatalysts loaded on carbon nitride.7,9 Notably, upon illumination at 525 nm an appreciable apparent quantum efficiency (AQE) of 0.26% was observed for pCN/BODIPY, which greatly extends the photoresponsive range of carbon nitride that rare case under illumination above 500 nm has been reported.33 Optimization tests with higher concentration of sacrificial reagent or molar ratio of BODIPY exhibited impaired activity due to light shielding effect of the larger Ag particles and the excess dye (Figure S7).34
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Figure 3. (a) High resolution valence band XPS spectra and (b) Surface voltage spectra of g-CN and p-CN.
Figure 2. (a) Dissolved O2 concentrations measured with a Clark electrode in 2 mL deoxygenated 0.01 M AgNO3 aqueous solutions containing 1 mg catalysts and 4 mg La2O3 as buffer (pH 7) under irradiation at 410 nm, (b) Wavelength dependent apparent quantum efficiency (AQE) of p-CN and p-CN/BODIPY. (c) O2 production of p -CN/BODIPY and CN-B under irradiation at 410 nm. Carbon nitride - BODIPY copolymer (CN-B) was also prepared by thermal method and tested for water oxidation (Figure 2c). Though XRD, IR and XPS spectra showed the typical chemical and structural features of carbon nitride matrix (Figure S8), p-CN/BODIPY has much better activity than CN-B with the same content of BODIPY in precursor. Thus, we inferred that the efficient interfacial interaction between BODIPY molecule and carbon nitride is crucial to the high photocatalytic ability. Pioneer studies on the modification of carbon nitride prefer to use molecules as building blocks or doping precursors to introduce special units or atoms into carbon nitride matrix and see whether the catalytic performance can be improved.35-39 We have proved here some interface association can also lead to the promotion of O2 evolution activity for carbon nitride. This method can prevent the molecules from the strict requirements of thermal copolymerization and thus maintain their original structure.
To get a detailed comprehension of the mechanistic origin of this effect, thermodynamic information is discussed with the help of a series of spectral analysis. Valence band XPS shows that the valence band maximum (Evb) locates at 1.7140,41 and 2.22 V vs. normal hydrogen electrode (NHE) for g-CN and p-CN, respectively (Figure 3a). Our previous results8 have shown that the optical band gaps of g-CN and p-CN are 2.72 eV and 2.92 eV, and the conduction band minimum (Ecb) is then calculated to be -1.08 V and -0.7 V. Surface photovoltage spectroscopy, a method that is sensitive and wavelength dependent to probe the photoinduced charge separation in semiconductors, was used herein to determine the energetic positions of defect states on carbon nitride (Figure 3b).42,43 Since the sub-gap photovoltage of g-CN and p-CN appears at 476 nm and 436 nm, corresponding Scheme 2 Energy Level Diagrams Describing the possible process of charge carriers in carbon nitride and BODIPY molecule (abbreviated as B in this Scheme).
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to the excitation energy of 2.62 eV and 2.84 eV, the major defect states which holds long-lived holes to oxidize water are assigned to 2.14 V and 1.61 V vs. NHE of trapped state position (Ets) for p-CN and g-CN, respectively, quite close to edge of the intrinsic valence band. Scheme 2 illustrates an energy level diagram of BODIPY molecule and carbon nitride semiconductors and thus describes the possible movement of charge carriers between them. Because both the valence band top and the surface trap states of p-CN lie more positive than the oxidation potential of BODIPY, holes can provide enough thermodynamical driving force to oxidize the ground state BODIPY at the surface of p-CN to fulfill its role as hole relay. Furthermore, a model of dye sensitizing is also proposed that the excited BODIPY molecule is able to inject electron into the conduction band of p-CN to reduce the sacrificial reagent, a possible origin of the pronounced activity of p-CN/BODIPY at where p-CN doesn’t absorb much light. However, the amount of excited BODIPY molecules is rather small compared to its ground-state, the contribution of dye sensitizing process to the overall activity is not as significant as hole relay, especially at light response region of carbon nitride. The hole relay is consequently considered as the major mechanical process of this system, which can be traced by the transient absorption experiments (see below). On the contrary photogenerated holes of g-CN, especially those in the surface trap states, cannot afford the energetic requirement for hole transfer to BODIPY very well. The
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dye sensitized process is also thermodynamically forbidden since the conduction band of g-CN and the reduction potential of BODIPY cannot match. Kinetic studies by time-resolved techniques are further used to elucidate the mechanism. Upon the illumination of pulse laser at 375 nm, both g-CN and pCN solid powder can produce strong luminescence corresponding to the band to band recombination of photogenerated holes and electrons (Figure S9). The lower intensity and longer decay lifetime of p-CN reveal that the recombination process in carbon nitride is retarded after the modification of protonation. Given that the intensity of luminescence is reduced and the decay course is accelerated by loading 2% BODIPY, it is reasonable to consider that charge transfer occurs between carbon nitride and BODIPY and promotes the charge separation efficiency of the composite catalyst. The decay curves of their luminescence further suggested that charge transfer between p-CN and BODIPY is much more efficient than that between g-CN and BODIPY.
Scheme 3. Illustration of (a) transient absorption experiments of carbon nitride substrates, (b) carrier dynamics in carbon nitride substrates.
Figure 4. Transient absorption spectrum under laser excitation (λexc = 410 nm) of (a) p-CN, (b) p-CN/BODIPY, (c) g-CN and (d) g-CN/BODIPY. Transient absorption spectra (TAS) is one of the most powerful methods to investigate photogenerated charge carrier dynamics of semiconductors especially at the material-reactant interface.44 For the last two decades, some well accepted models have been proposed and developed on titania, hematite and some other famous photocatalytic semiconductors.45-47 It has been known that long-lived photogenerated holes in carbon nitride can be traced in visible light region in the time range from ns to ms, in accordance with the cases of solid photocatalyst like TiO2, Fe2O3, BiVO4 and LaTiON.47-52 We introduced this method in this work to compare the two kinds of composite catalysts in terms of hole kinetics.8,34 The long-lived photogenerated holes which are primarily trapped by surface defect states are considered to be crucial to water oxidation since they have sufficient lifetime to accumulate and drive the sluggish kinetics of O2 formation on the surface of semiconductors (typically
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< 1 s-1).51,53 As shown in Figure 4, the transient absorption signal of p-CN was greatly reduced by the deposited BODIPY while g-CN showed a complicated feature after loading BODIPY and the intensity stayed at the same level, which is in accordance with the above discussion that higher thermodynamical driving force facilitates the effective hole transfer from p-CN to BODIPY. Spectroelectrochemical experiments of BODIPY membrane showed no apparent new peaks at a positive voltage form 1.2 V to 2 V vs. SCE, indicating a negligible difference in absorption between BODIPY and its oxidized species (Figure S10). It explains why we haven’t observed any direct signal of BODIPY accepting positive charges in the transient absorption patterns.
One may suspect whether BODIPY can directly drive water oxidation like those reported molecular metal complexes.55-57 To exclude this possibility, we tested the catalytic ability of chemical water oxidation with cerium (IV) ammonium nitrate (CAN) as the oxidant and found no activity of BODIPY at all (Figure S11). Therefore, the hole relay effect of surface associated BODIPY are the key factor of the dramatically enhanced activity for O2 evolution as compared to bare p-CN with limited rate. The successful detection of H2O2 in the reaction solution of p-CN/BODIPY upon irradiation (Figure S12) further indicated that the surface associated BODIPY can accelerate the O2 evolution by H2O2 route reported in literature.58,59 It may also be worth pointing out that O2 evolution rate of p-CN/BODIPY decreased dramatically to the same level of bare p-CN after an initial stage of about 100 seconds, indicating instability of the composite structure. Stability tests show that BODIPY in acetone solution can be degraded rapidly by the photocatalysis of both g-CN and p-CN (Figure S13), which agrees with pioneer studies where carbon nitride is used as photocatalyst to drive organic transformation in the presence of O2.60-63 Since BODIPY detaches from the active sites gradually and silver particles reduced from the sacrificial silver nitride accumulate and aggregate to shield light, the composite catalyst finally loses its activity.
CONCLUSIONS
Figure 5. Decay curves of transient signal monitored at 660 nm of (a) g-CN and g-CN/BODIPY, (b) p-CN and pCN/BODIPY. The kinetic courses of transient signal monitored at 660 nm in Figure 5 showed that the decay process of photogenerated holes in p-CN is dramatically accelerated with the average lifetime of 2.53 ms to 0.87 ms, while the difference hole decay in g-CN and g-CN/BODIPY is quite smaller. These results demonstrate us with detailed information of time range about hole dynamics. Based on the recent literature,54 carbon nitride generates holes and free electrons immediately after excitation. Some of photogenerated holes are trapped into defect states within a picosecond and can survive from the fast band to band recombination. These holes with lifetime of several microseconds which can be traced by nanosecond transient absorption techniques have a higher chance to oxidize water. Evidenced by transient absorption techniques, BODIPY can extract the trapped holes and release them from their most probable destiny of recombination. This modification increases the possibility of the photogenerated holes in p-CN to participate O2 evolution on the time range of several seconds and greatly increases the light conversion efficiency.
In summary, we have applied a kind of molecular dye for the first time to enhance the photocatalytic ability for O2 evolution of carbon nitride. The comparison of g-CN and p-CN show that only p-CN with more positive valence band can be activated by surface loading of BODIPY. A detailed investigation of the thermodynamics and kinetics reveals that the larger driving force of p-CN appears to be the main factor promoting the catalytic performance. These observations raise an important issue that some molecules can be introduced as guides for the photogenerated holes in carbon nitride to participate in specific reactions. Since the poor charge separation efficiency is among the major hinders of photocatalytic water oxidation for carbon nitride, it is anticipated that this method would bring some new insights into the more effective design and modification for its utilization in photocatalytic O2 evolution. Further study is undergoing to strengthen its stability and finally achieve durable and highly efficient photocatalytic O2 evolution.
ASSOCIATED CONTENT Supporting Information. Experimental Sections, Optimal Tests, Photoluminescence Spectra, Spectroelectrochemical Spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
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Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT We are grateful for financial support from the Ministry of Science and Technology of China (2014CB239402, 2013CB834505 and 2013CB834804), the National Science Foundation of China (91427303, 21390404, 51373193 and 21403260), Strategic Priority Research Program of the Chinese Academy of Science (XDB17030200), and the Chinese Academy of Sciences.
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ACS Paragon Plus Environment
