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Oct 15, 2015 - To further elucidate how such a modification promotes water oxidation, steady-state and time-resolved spectroscopic measurements were c...
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Enhanced Driving Force and Charge Separation Efficiency of Protonated g-C3N4 for Photocatalytic O2 Evolution Chen Ye, Jia-Xin Li, Zhi-Jun Li, Xu-Bing Li, Xiang-Bing Fan, Li-Ping Zhang, Bin Chen, Chen-Ho Tung, and Li-Zhu Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02185 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on October 16, 2015

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Enhanced Driving Force and Charge Separation Efficiency of Protonated g-C3N4 for Photocatalytic O2 Evolution Chen Ye, Jia-Xin Li, Zhi-Jun Li, Xu-Bing Li, Xiang-Bing Fan, 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

ABSTRACT: Photocatalysts based on g-C3N4 by loading cocatalysts or constructing heterojunctions have shown great potential in solar driven water oxidation. However, the intrinsic drawbacks of g-C3N4, such as poor mass diffusion and charge separation efficiency remain as the bottleneck to achieve high-efficient water oxidation. Here we report a simple protonation method to improve the activity of g-C3N4. Studies using valence band X-ray photoelectron spectra and steady-state and time-resolved spectroscopy reveal that the promotion of catalytic ability is originated from the higher thermodynamical driving force and longer-lived charge separation state, which may provide guidance on designing efficient polymeric semiconductor photocatalysts with desirable kinetics for water oxidation. KEYWORDS: g-C3N4, protonation, photocatalytic water oxidation, band structure, charge separation efficiency

INTRODUCTION Semiconductors have attracted much interest for its great potential to split water ever since the phenomenon was discovered in the late 20th century.1 Graphitic carbon nitride (g-C3N4) as a kind of earth-abundant elements composed semiconductor with high chemical and thermal stability exhibits an appealing band alignment capable for water splitting.2-4 Pioneering studies have shown that g-C3N4 can be used as an effective photocatalyst for both H2 and O2 evolution reactions by loading cocatalysts or constructing heterojunctions.5-10 However, g-C3N4 still suffers obstacles to achieve high reactivity, especially in O2 evolution reaction, for the intrinsic drawbacks, particularly the high inclination of recombination.11,12 In this regard scientists have been trying hard modifying g-C3N4 with fine electrical and surface properties, for example by morphology modification13,14 or band structure engineering.15-19 However, the fundamental nature of the photophysics in carbon nitride photocatalysis is not fully understood yet. Direct evidence of the excited state has seldom been mentioned, though some recent works have shed light on the recombination process by analyzing its photoluminescence performance.20,21 Protonation is considered as a facile and effective method to activate or enrich g-C3N4 with specific properties like solubility and ferromagnetism since the early report by Wang et al. in 2008.22-24 Such modification process was recently proposed to affect surface charge properties, which could be utilized to construct

assembled catalysts or biosensing platform.25,26 Particularly, Tang et al. and Wang et al. took advantage of protonation to improve photocatalytic H2 evolution efficiency of g-C3N4.22,27 We questioned how the protonation process could affect the catalytic properties of g-C3N4 for O2 evolution. In the present work, we wish to report that the protonation method significantly affects the electrical properties of g-C3N4 with downshifted valence band positions that is favorable for water oxidation. Further we compared their charge separation efficiency by time-resolved spectroscopic techniques. In addition to the retarded recombination evidenced by the decay curves of photoluminescence, the time scales of the excited states were directly demonstrated by transient absorption spectra. As will be discussed below, the prolonged lifetime of charge separation states is one of the key factors for the promoted catalytic activity. This treatment provides a new strategy of designing lightharvesting substrates with promoted charge separation efficiency, which can be then used to construct more sophisticate and effective solar driven water oxidation catalytic systems.

EXPERIMENTAL SECTION Chemicals and Reagents. All chemicals are of analytical grade and were used as received without further purification. Sample Preparation. The g-C3N4 was prepared by an ever reported thermal polymerization method.2 In a typical procedure, 20 g of dicyandiamide (Aldrich, 99 %) was heated to 550 ℃ in 3 hours and then kept for 4 hours.

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The yellow solid was milled into powder for the subsequent modification. The protonation procedure was undertaken by stirring 1 g g-C3N4 with 12 mL hydrochloric acid (37 %) for 3 hours at room temperature and centrifugal washing until neutral. Then the product was dried at 100 ℃ overnight. The final white powder was donated as p-g-C3N4. Cocatalysts (RuO2, Co3O4 and Co-Pi) loading was performed according to the procedure in the literature.28,29 Characterization. The X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 Advance diffractometer using Cu Kα1 radiation. The UVVis diffuse reflectance spectra (DRS) were undertaken on a Varian Cary 500 Scan UV/vis system. The attenuated total reflectance Fourier transform infrared spectroscopy (FTIR) spectra were measured on Excalibur 3100 system (Varian, USA). The Solid-state 13C nuclear magnetic resonance (NMR) was performed on a Bruker Advance III 400 WB spectrometer. The X-ray photoelectron spectroscopy (XPS) and the valence band X-ray photoelectron spectra (VBXPS) were obtained on an ESCALAB 250 spectrophommeter with Al K-α radiation. The Brunauer-Emmett-Teller (BET) surface area was calculated based on nitrogen adsorption-desorption isotherm measurements at 77 K. The scanning electron microscope (SEM) images were collected on a Hitachi S4800 FE-SEM system. Photoluminescence spectra (PLS) were gauged by time-correlated-single photon counting (Edinburgh Instruments, FLS-920). The transient absorption spectra (TAS) was determined on Edinburgh LP920 specteophotometer with an OPO laser at 410 nm for excitation and a xenon lamp as probe. The elemental analysis was conducted on Flash EA 1112 elemental analyzer from Thermo Scientific. Photocatalytic Test. The O2 evolution rate of the initial stage were calculated from the dissolved O2 concentration recorded by a Clark electrode (Oxygraph System, Hansatech Instruments Ltd) under ambient conditions. In a typical experiment 1 mg of catalyst, 4 mg La2O3 as the pH stabilizer and 2 mL 0.01 M AgNO3 aqueous solution as the sacrificial electron acceptor were added into the reaction vessel under the illumination of a 450 nm LED. The reactant were sonicated for 10 min for better disperision and then purged with Argon for 5 min to expel the dissolved O2. Electrochemical Analysis. The electrochemical analysis was carried out with a Zennium electrochemical workstation (Zahner, Germany) using a standard threeelectrode quartz cell. To make a working electrode, gC3N4 or p-g-C3N4 powder was deposited on a 10 × 10 mm fluorine-doped tin oxide (FTO) substrate. In a typical procedure, 4 mg g-C3N4 or p-g-C3N4 powder was sonicated in a mixture of 1 mL acetone and 0.05 mL Nafion. 200 μL slurry was then transfer onto a FTO electrode with an electroactive area of 1 cm2 and dried by IR lamp to form the final electrode.

RESULTS AND DISCUSSION

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Structural and morphological characterization. GC3N4 was prepared by thermal condensation method2, while p-g-C3N4 was obtained by immersing g-C3N4 in concentrated HCl. The surface morphologies of g-C3N4 and p-g-C3N4 were imaged by scanning electron microscopy (SEM). Note that the in plane packing pattern of 2D lamellar structure turned amorphous after protonation, while the layered structure maintained (Figure S1).30 This transformation was also confirmed by the XRD pattern in Figure S2, where p-g-C3N4 retained the characteristic (002) peak corresponding to the interlayer stacking of graphitic-like structures while the (100) peak vanishes.31 The chemical states of g-C3N4 and pg-C3N4 were verified by solid-state 13C NMR (Figure 1a). Besides the two signals at ca. 164.3 and 155.6 ppm assigned to a poly(tri-s-triazine) structure, p-g-C3N4 exhibits a new peak at ca. 149.7 ppm which could be assigned to the carbon adjacent to a quaternary ammonium moiety.32 The structure of tris-s-trizaine building blocks can be also confirmed by the stretching modes of CN heterocycles at 1200-1600 cm-1 and the breathing mode of triazine units at 810 cm-1 in FTIR spectrum (Figure S3).22,33

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Figure 1. (a) C NMR spectrum and (b) high resolution N 1s XPS spectra of g-C3N4 and p-g-C3N4.

Ideal g-C3N4 possess a C/N stoichiometric ratio of 0.75 which is however difficult to obtain. General synthetic routes always yield g-C3N4 consisting of incompletely condensed tri-s-triazine units.34 The generating defects and multiple surface terminated groups would render the C/N molar ratio deviating from the ideal composition. Elemental analysis of g-C3N4 and p-g-C3N4 prepared by us were shown in Table S1. The C/N molar ratio of g-C3N4 and p-g-C3N4 can be calculated at 0.69 and 0.65, meaning that there are many amine groups present in the matrix due to incomplete condensation.35 The fraction of terminal groups were further determined by high-

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resolution X-ray photoelectron spectra (XPS) calibrated with the reference carbon at 284.6 eV. The main N 1s signal could be deconvoluted into four parts with three main peaks at a binding energy (BE) of 398.6, 399.8 and 401.5 eV assigned to sp2-hybridized nitrogen (C=N-C), sp3hybridized nitrogen tertiary nitrogen (N-(C)3) and amino functional groups with one hydrogen atom (C-N-H), respectively (Figure 1b).27 The broad peaks at about 404.1 eV due to either charging effect or π excitation are not under consideration.36 The sp2 N/ sp3 N ratios for g-C3N4 and p-g-C3N4 were calculated to be 1.11 to 0.678 (Table S2 and S3), indicating the introduction of more defects and terminal groups during this process. Because a suitable amount of defects in g-C3N4 can adjust the bandgap and reduce surface inertness, we envisioned that the protonation would introduce more defects into the carbon nitride matrix, and thus improving the catalytic performance.37

the UV region. The rate of initial O2 evolution in 50 s under the irradiation at 410 nm was calculated to be 0.445 μmol g-1 s-1 and 3.22 μmol g-1 s-1, respectively. When cocatalysts such as RuO2, Co3O4 and Co-Pi were introduced on these carbon nitride substrates, the O2 evolution efficiency could be greatly enhanced with the apparent quantum efficiency (AQE) reaching 2.1% at 410 nm (Figure S5). The comparison of samples loaded with some common O2 evolution cocatalysts like Co3O4 further indicated that p-g-C3N4 is more efficient than g-C3N4 for photocatalytic O2 evolution (Table S4, Figure S6).

Figure 3. Polarization curves in the dark and under illumination of g-C3N4 and p-g-C3N4 in 0.1 M Na2SO4 Solution. Electrochemical O2 evolution reactions of g-C3N4 and pg-C3N4 were further explored to confirm the water oxidation results. All the electrodes were carefully prepared under the same procedure to guarantee electrode quality (Figure S7). The linear sweep in the potential ranging from 0 to 1.6 V vs NHE was measured in the anodic direction, as shown in Figure 3. The positive current in the range of 1.2−1.6 V vs NHE could be ascribed to O2 evolution.38 Evidently, p-g-C3N4 exhibited reduced the current-onset overpotential both in the dark and under illumination than g-C3N4, which could promote multiple-electron water oxidation kinetics. Figure 2. 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 (a) 450 nm, (b) 410 nm. Photocatalytic and electrochemical Activities. Photocatalytic water oxidation experiments were carried out in deoxygenated aqueous solution using AgNO3 as electron acceptor, La2O3 as pH buffer agent under monochromatic light from 410 and 450 nm LEDs. Figure 2 shows the dissolved O2 content in the reactant solution monitored at room temperature. Under the irradiation at 450 nm, p-g-C3N4 exhibited higher catalytic ability than gC3N4, and the rate of O2 evolution for g-C3N4 and p-g-C3N4 determined from the slope of the linear fitting for the first 50 s was 0.307 μmol g-1 s-1 and 0.985 μmol g-1 s-1, respectively. The difference of O2 evolution rate would be more tremendous when illuminated by the light closer to

Mechanistic Studies. To elucidate the catalytic essence of these samples, a series of characterizations have been involved to investigate the difference between the two samples. Figure 4a shows the absorption profiles of g-C3N4 and p-g-C3N4. The pure g-C3N4 and p-g-C3N4 exhibited fundamental absorption edges at 450 nm and 410 nm. According to the Kubelk-Munk relation, the bandgaps were calculated at 2.72 eV and 2.92 eV (Figure 4b). This blue-shift phenomenon can be attributed to the

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Figure 4. (a) Ultraviolet–visible diffuse reflectance spectra of g-C3N4 and p-g-C3N4. (b) (Fhυ)0.5 as a function of photon energy (hυ), where F is the Kubelka–Munk function. (c) High resolution valence band XPS spectra of g-C3N4 and p-g-C3N4. (d) Schematic band structures of g-C3N4 and p-g-C3N4. at -1.12 V, -0.55 V and 1.60 V, 2.37 V respectively, as shown in Figure 4d. The trend of the Valence Band edge downshift was also evidenced in Mott–Schottky plots for g-C3N4 and p-g-C3N4 (Figure 5a). The flat-band potentials were calculated to be -1.44 V and -1.14 V for g-C3N4 and pg-C3N4, respectively, from the intercept on the abscissa. The positive slopes for the linear plots reveal the typical n-type characteristics for both the semiconductors which could be used as photoanodes for the photoelectrolysis of water.41

Figure 5. (a) Mott–Schottky plots and of g-C3N4 and p-gC3N4. (b) Nyquist impedance plots (scatters) for g-C3N4 and p-g-C3N4 and simulation (lines). The frequency range is from 30000 to 0.01 Hz with perturbation at 10 mV. Inset: the equivalent circuit mold.

weakening of the π conjugated system due to decreased condensation.39,40 Valence band XPS was then carried out to determine the relative valence band maximum. Compared with g-C3N4, an obvious downshift for the valence band edge of p-g-C3N4 could be observed in Figure 4c, indicating that the protonation process greatly reduced the VB level, which resulted in higher thermodynamical driving force of photooxidation. Combined with the differences in bandgap, the CB edges and VB edges of g-C3N4 and p-g-C3N4 could be calculated

Figure 6. (a) Photoluminescence spectra and (b) timeresolved photoluminescence spectra monitored at 450 nm of g-C3N4 and p-g-C3N4. Both samples were excited with 340 nm light from pulsed laser.

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To exclude the possibility that the enhanced catalytic rate was induced by a higher surface area, the exact values of surface area were estimated by recording N2 adsorption-desorption isotherms (Figure S8). The Brunauer-Emmett-Teller (BET) specific surface areas obtained for g-C3N4 and p-g-C3N4 are 6.73 m2 g-1 and 13.0 m2 g-1, respectively. Both samples exhibit rather small surface areas, which are much lower than the corresponding carbon material.13,42 Since p-g-C3N4 exhibited an O2 evolution ability 10 times better than gC3N4 at 410 nm, yet absorbs less light and only has a surface area twice as big, surface area is not the dominant factor for this marked difference of the catalytic ability. So the better performance of p-g-C3N4 may be attributed to the improved electronic properties. The kinetics of O2 evolution reaction (OER) at electrode/electrolyte interface was investigated by the electrochemical impedance spectroscopy (EIS) technique (Figure 5b).43 According to the semicircle in the low frequency zone, the charge transfer resistances (Rct) of gC3N4 and p-g-C3N4 were calculated to be 407 and 301 kΩ, which means that p-g-C3N4 has the faster charge transfer rate and the more favorable OER kinetics.44

the photoluminescence peaks. In addition, p-g-C3N4 exhibited much weaker photoluminescence intensity than that of g-C3N4, indicating that the radiative recombination in p-g-C3N4 was greatly suppressed. The photoluminescence decay kinetics was investigated by time-resolved spectra. As shown in Figure 6b, p-g-C3N4 has the longer lifetime corresponding to the slower charge recombination of the excited state. Moreover, the decay curves of both g-C3N4 and p-g-C3N4 obviously deviate from the single-exponential decay.47,48 By multiexponential fitting, the average lifetimes of the entire decay for g-C3N4 and p-g-C3N4 were estimated to be 9.86 and 18.4 ns, respectively. The decreased photoluminescence intensity and prolonged lifetime are apparently associated with improved charge localization on the carbon nitride matrix, which is believed to facilitate the heterogeneous photocatalysis. Some recent works also suggested the promoted charge separation efficiency evidenced by the suppressed photoluminescence.49-51 Furthermore, when the sacrificial agent AgNO3 was added to the colloidal suspension of gC3N4 and p-g-C3N4 prepared by sonication, the photoluminescence was dramatically quenched (Figure 7). AgNO3 showed greater quenching capacity in p-gC3N4, meaning that p-g-C3N4 could react with AgNO3 more efficiently during photocatalysis. This provided another evidence for this viable modification.

Figure 7. Photoluminescence spectra of (a) g-C3N4 and (b) p-g-C3N4 colloid by sonicating (0.02 mg / mL) with different concentrations of AgNO3. To further elucidate how such kind of modification promotes water oxidation, steady-state and time resolved spectroscopic measurements were carried out to discuss charge carrier course. Photoluminescence spectroscopy is considered to be one of the most powerful techniques for characterizing the recombination processes of the photogenerated excitons.45,46 The results in Figure 6a clearly suggest that the protonation process effectively widen the bandgap which confirmed by the blue shift of

Figure 8. (a) Transient absorption spectrum collected at a delay time of 1 μs after excitation (λexc = 410 nm) and (b) decay curves monitored at 700 nm of g-C3N4 and p-g-C3N4.

Although photoluminescence can reveal the information on recombination, the relaxation course of carrier dynamics remains vague. Transient absorption spectroscopy (TAS) can be used to investigate photogenerated charge carrier behaviors, especially the

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events of photoinduced electrons and holes at the material-reactant interface.52-54 This technique is based on the pump-probe method by using a pulse laser as the excitation source and a second beam as the probing light. With the relaxation course of transient absorption recorded, the dynamics of the photogenerated transient species were obtained.55 When excited by pulse laser at 410 nm, both g-C3N4 and p-g-C3N4 showed broad and continuous absorption from 400 nm to more than 800 nm (Figure 8a), which could be attributed to the absorption of charge separation state formed by photogenerated electrons or holes locating in different traps in these semiconductors.56-58 As the transient absorption intensity at the same monitoring wavelength reflects the relative charge separation efficiency, we can infer that the charge separation process is improved after protonation.59 This can be also corroborated by the kinetic courses of the transient signal at 700 nm (Figure 8b) that p-g-C3N4 exhibited greatly retarded decay kinetics. The transient signal remained notable even after 10 ms. For semiconductors, the fast recombination process are usually considered as the main fate of photogerenerated charge carriers, and only a few fraction of the charge carriers can survive to the micro- or millisecond range by trapping effect.60 The charge carriers in micro-second to second window are more important in the discussion of water oxidation mechanism, as the short-lived charge carriers may be not able to afford the slow kinetics of water oxidation (typically < 1 s-1).56,61 The relative longlived charge separation state of p-g-C3N4 can increase the probability of charge carriers to participate in catalytic redox reactions.

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The transient signal of p-g-C3N4 was further assigned to photogenerated holes by applying an anodic electrical bias and adding scavenger. When applying 0.8 V between the anode of FTO/p-g-C3N4 and the cathode of platinum sheet, the transient absorption intensity greatly increased (Figure 9a), and the time-resolved spectra also showed the prolonged relaxation process (Figure 9b). These transient signals can be then identified as the features of photogenerated holes, for that applied anodic bias can effectively retard electron-hole recombination by reducing the background electron density of p-g-C3N4 and forming space charge layer (SCL), and thus increase the both transient absorption intensity and lifetime of photogenerated holes.60 The experiments also explain why significant overpotential is needed to generate higher photocurrent by p-g-C3N4 photoanodes for water oxidation. This assignment, on the other hand, was also confirmed by chemical scavengers. With ethanol as hole scavenger, the transient absorption intensity was partially reduced, which could be attributed to the reaction between ethanol and the photogenerated holes migrated to the surface of p-g-C3N4 (Figure S9).62,63 And this decay process could be accelerated by loading cocatalysts like Co3O4, due to the extraction of holes by cocatalysts for O2 evolution (Figure S10). However there was no apparent strengthen of the transient absorption signal in the whole monitoring range, the identical absorption signal of photogenerated electrons may be located in the NIR and IR regions like LaTiO2N reported by Furube et al. and Yamakata et al.64,65 Based on the above observations, we can conclude that the prolonged lifetime of photogenerated holes play a crucial role for the improvement of the solar driven water oxidation.

Conclusion In the present article we have verified that protonated g-C3N4 showed much more efficient water oxidation reactivity than g-C3N4. As proved by time-resolved spectroscopy, this simple protonation method can significantly restrain the charge recombination rate and thus enhance the possibility of the photogenerated charge carries participating in the surface reaction. According to transient absorption spectrum, the photogenerated holes can remain in the millisecond window. By weakening the π conjugate and introducing an appropriate amount of terminal groups and defects, the valence band positions tremendously downshift. Since the poor charge separation efficiency and sluggish surface kinetics are commonly considered as the bottlenecks of photocatalytic water oxidation, it is anticipated that this treatment would provide a flexible choice to enhance the solar energy utilization of carbon nitride photocatalysts.

ASSOCIATED CONTENT Figure 9. (a) Transient absorption spectrum collected at a delay time of 1 μs after excitation (λexc = 410 nm) and (b) decay curves monitored at 700 nm of p-g-C3N4 at open circuit and under 0.8 V in 0.2 M Na2SO4 electrolyte.

Supporting Information. SEM images, XRD patterns, FTIR spectrum, elemental compositions, deconvolution result of N 1s XPS spectra, O2 evolution performance with cocatalyst, N2 adsorption-desorption isotherms, fitting results of EIS

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experiments, multi-exponential fitting results of time resolved photoluminescence, transient absorption spectrum immersed in EtOH and transient absorption spectrum decay curve with cocatalyst. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

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 and 51373193), the Key Research Programme of the Chinese Academy of Sciences (KGZD-EW-T05) and the Chinese Academy of Sciences.

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