Enhanced Charge Separation Efficiency Accelerates Hydrogen

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Enhanced Charge Separation Efficiency Accelerates Hydrogen Evolution from Water of Carbon Nitride & 3, 4, 9, 10Perylenetetracarboxylic Dianhydride Composite Photocatalyst Chen Ye, Jia-Xin Li, Hao-Lin Wu, Xu-Bing Li, Bin Chen, Chen-Ho Tung, and Li-Zhu Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14896 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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ACS Applied Materials & Interfaces

Enhanced Charge Separation Efficiency Accelerates Hydrogen Evolution from Water of Carbon Nitride & 3, 4, 9, 10-Perylenetetracarboxylic Dianhydride Composite Photocatalyst Chen Ye‡, Jia-Xin Li‡, Hao-Lin Wu, Xu-Bing Li, 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 carbon nitride, composite catalyst, photocatalytic water splitting, hydrogen evolution, charge separation, time-resolved spectrum. ABSTRACT: The catalytic ability of graphitic carbon nitride is greatly affected by its intrinsic electronic properties. Though combination with chromophore has been demonstrated to be one of the promising approaches to improve the catalytic performance of carbon nitride, it is imperative to understand the key factors governed the whole process. Here we report a composite photocatalyst CN-P by embedding perylene unit into the matrix of carbon nitride. The composite photocatalyst could catalyze hydrogen evolution with a high rate of 17.7 mmol h-1 g-1, which is 2.8 times faster than pure carbon nitride. The apparent quantum efficiency is high up to be 5.8% at 450 nm. Detailed studies reveal that the light absorption ability and charge separation efficiency are greatly enhanced in the synthesized catalyst. These are the key factors for the improved hydrogen evolution ability of CN-P than pure carbon nitride.

INTRODUCTION Utilizing solar illumination to split water for hydrogen generation is one of the most potential approaches to settle the energetic and environmental problems.1-3 Previous studies have shown the application potential of semiconductor-based heterogeneous systems in photocatalytic hydrogen evolution.4-12 Among these promising materials, graphitic carbon nitride has been in the spotlight of the field for the last several years.13-15 The facile preparation/modification process, earth-abundant elements composition and suitable band alignment render it great potential in solar energy conversion applications like water splitting, photoelectrochemical cells and photocatalysis.16-21 Nevertheless, the limited absorption in visible light region, poor electrical conductivity and high electron-hole recombination probability of carbon nitride materials restrict the practical applications for photocatalytic hydrogen production.22-24 Copolymerization or composition is one of the most widely used methods to tune the constitution of carbon nitride ever since the early stage of carbon nitride application in photocatalysis.25-28 Though many cases have been reported on highly efficient carbon nitride copolymers and composites, detailed studies are still needed to fully unravel the key impacts of the improved activity.29 3, 4, 9, 10-Perylenetetracarboxylic dianhydride (PTCDA) possesses extended π-π electronic interaction, large

exciton diffusion lengths and high electron mobility and thus is considered to be of great potential in solar energy applications.30-32 The large aromatic structure of PTCDA makes it easy to be attached onto the surface of other conjugated 2D materials through supramolecular assembly. We are particularly interested in the possibility of introducing the PTCDA motifs to the matrix of 2D energy materials like carbon nitride to construct highly efficient composite photocatalyst, and expect to understand how this composite structure works during photocatalytic hydrogen evolution and to figure out the key factors that affect the catalytic activity. Herein, we report an modelling and illuminative case of incorporating conjugated motifs of PTCDA into the matrix of mesoporous graphitic carbon nitride (mpg-C3N4) by thermal condensation. The prepared carbon nitride/PTCDA composite photocatalyst (CN-P) could catalyze hydrogen production 2.8 times as faster as that of mpg-C3N4, both with Pt nanoparticles as cocatalyst. We proved that the introducing chromophoric units would obviously extend the absorption properties of carbon nitride in visible light region. By steady state and time resolved spectroscopic measurements, we found that charge separation and trasnfer can be greatly promoted after combining the two kinds of basic units. The improved utilization of photogerated charge carriers is the key factor to the enhanced hydrogen evolution abilility of CN-P photocatalyst.

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RESULTS AND DISCUSSION The designed composite photocatalyst CN-P was synthesized by thermal condensing cyanamide and PTCDA with different ratios. The reaction involved an imidization reaction with the anhydride groups of PTCDA.33 To improve the catalytic activity, colloidal silica was utilized as hard template of introducing mesoporous structure.34 After thermal condensation, the template was removed by NH4HF2. The sample with PTCDA/cyanamide mass ratio of x percent was denoted as CN-P-x%, while the sample in the absence of PTCDA was denoted as mpg-C3N4.

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photocatalytic hydrogen evolution reaction. However, they are not as efficient as triethanolamine (Figure 1a). Further increasing the PTCDA amount, the HER of photocatalyst gradually decreased, which might be ascribed to light hindrance effect. The optimal apparent quantum efficiency (AQE) of CN-P-0.2% was calculated to be 5.8% at 450 nm, which is among the best of the carbon nitride copolymer and composite systems. What’s more, photocatalytic stability was tested for the CN-P samples. The optimal CN-P-0.2% sample is as stable as non-treated mpg-C3N4, which can drive hydrogen evolution reaction efficiently to more than 4 hours (Figure 1b). Control experiments show that hydrogen evolution cannot proceed without CN-P or illumination, confirming the reaction as photocatalysis.

Figure 2. (a) Photograph mpg-C3N4, CN-P-0.2%, CN-P-0.5%, CN-P-2% and CN-P-5% powder. (b) Diffuse reflectance UV-Vis spectrum of mpg-C3N4 and CN-P-0.2% and the photocatalytic hydrogen evolution rates at different wavelength of excitation light. Figure 1. (a) Photocatalytic hydrogen evolution rates (HER) of 5 mg mpg-C3N4 and CN-P-x% samples with different content of PTCDA, using 3 wt% Pt as a hydrogen evolution co-catalyst in 5 mL of TEOA-H2O (v : v = 1 : 9), Methanol-H2O (v : v = 1 : 9), Ethanol-H2O (v : v = 1 : 9) and 0.1 M AA aqueous solution in a Pyrex glass cell under 450 nm LED irradiation. (b) hydrogen evolution control experiments with and without catalyst or illumination. Photocatalytic hydrogen evolution experiments were carried out carefully using various CN-P-x% samples as photocatalysts and 3 wt% platinum as a hydrogen evolution cocatalyst under visible light irradiation (λ = 450 nm) and with triethanolamine (TEOA) as sacrificial reagent. Figure 1a displays the relationship between hydrogen evolution rates (HER) and the initial mass of PTCDA used for condensation. It can be read that the HER of mpg-C3N4 samples was about 6.24 mmol h-1 g-1. However, after PTCDA introduction, the HER of CN-P-0.2% reached to 17.7 mmol h-1 g-1, which is much higher than the excellent g-C3N4-based hydrogen evolution photocatalysts reported previously (Table S1), 2.8 times higher than that of mpg-C3N4. Some other sacrificial reagents like methanol, ethanol and ascorbic acid (AA) can also drive the

The successful incorporation of the aromatic all-carbon cycles into the CN heterocycles skeleton is easily confirmed by the change of colors from yellow for mpg-C3N4 to pale green for CN-P-0.2% as shown in Figure 2. The CN-P-0.2% sample presents a broad absorption peak from about 560 nm to the near infrared region with the peak position at 640 nm, greatly enhancing the visible light response efficiency of carbon nitride.35 The enhancement of photocatalytic activity of composite samples indicates successful chemical engineering of the carbon nitride nanosheets with all-carbon conjugated fragment of PTCDA. Wavelength-dependent hydrogen evolution tests were also demonstrated to show the light response region of the prepared CN-P materials. Thermal condensation with PTCDA enhances the photocatalytic activity of mpg-C3N4, but the hydrogen evolution rate is still limited at long wavelength. To investigate the origin of the improved photocatalytic hydrogen evolution activity, structural characterization tests were first employed to examine chemical relationship between the two basic unites. The mpg-C3N4 and the most active composite CN-P-0.2% samples were characterized by X-ray diffraction (XRD) measurement. As shown in Figure S1a, CN-P-0.2% sample presents similar characteristic XRD peaks as mpg-C3N4. The typical XRD

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ACS Applied Materials & Interfaces peaks of graphitic-like layer structures were observed for both samples. The stronger one located at 27.4°, corresponding to an interlayer distance of 0.328 nm, is a characteristic (002) interlayer stacking of 2D conjugated system. And the peak at 13.0° (d = 0.675 nm) can be attributed to the in-plane structural repeating units of tri-s-triazine.13 The introduction of PTCDA units into carbon nitride matrix decreases the signal intensity, indicating the disturbance of graphitic structure by inserting PTCDA units in the layered structure. The Fourier transform infrared spectroscopy (FTIR) (Figure S1b) of the two samples exhibit all the characteristic stretch modes of aromatic CN heterocycles at 1200-1600 cm-1 together with the breathing mode of the heptazine units (810 cm-1).36 These results demonstrated the well preservation of the structure and core chemical skeleton of s-triazine and PTCDA in CN-P samples.

Figure 3. XPS characterization of (a) mpg-C3N4, (b) CN-P-0.2%, (c) CN-P-0.5% and (d) CN-P-2% samples demonstrated in the energy regions of C 1s. The relative ratio is between the peak areas of the cyan peak (C=N-C) and the red peak (C=C-C). X-ray photoelectron spectroscopy (XPS) measurements were conducted to get insights into the chemical composition and chemical status of the elements in the synthesized samples (Figure 3, Table S2). Two major peaks were observed in the C 1s region. The peak at 288.2 eV was assigned to C=N-C coordination of the basic s-triazine units in carbon nitride, while the one at 285.0 eV was attributed to the sp2 C=C-C bonds of the graphitic carbon atoms.37-38 This can be considered as the proof of graphitic carbon nitride. Considering that the PTCDA units have higher ratio of graphitic carbon atoms, the increasing relative intensity of all-carbon aromatic carbon peak with the increasing amount of PTCDA in CN-P samples confirms the successful introduction of large conjugated structure into the skeleton of mpg-C3N4. In the XPS spectra of N 1s (Figure S2), the contribution at 398.7, 400.3 and 401.4 eV were ascribed to sp2-bonded N in the triazine rings (C-N=C), the ternary nitrogen N-(C)3 and the amino functions (C-N-H) in the heterocycles, also confirming the formation of

graphitic carbon nitride structure.37, 39 The emerging of O 1s peak and the increasing the C/N ratio in CN-P samples can be considered as another proof to the incorporation of PTCDA into carbon nitride matrix (Figure S3). We can also predict the existence of covalent copolymerization of triazine and PTCDA units from the above analysis of chemical states. Similar mesoporous morphology was observed in the transmission electron microscopy (TEM) images of the mpg-C3N4 and CN-P-0.2% (Figure S4a, b), indicating that both samples derived the similar porous structure from the silica nanoparticles template. Template-free g-C3N4 and CN-P materials were also prepared and characterized by TEM (Figure S4c, d). Scanning electron microscope (SEM) images were also used to demonstrate the morphological information of the mpg-C3N4 and CN-P-0.2% on larger scale (Figure S5). Elemental mapping of the two samples showed that C, N and O atoms were uniformly distributed in both mpg-C3N4 and CN-P-0.2% (Figure S6). The sample surface information was obtained by N2 adsorption and desorption experiments (Figure 4), which provided the surface areas as 177.5 and 173.6 cm3/g for mpg-C3N4 and CN-P-0.2% respectively, by Brunner−Emmet−Teller (BET) mode. Pore size distribution (Figure S7) for these two photocatalysts is also similar. We thus conclude that the surface properties are not the main reason for the enhanced photoactivity of CN-P samples.

Figure 4. N2 adsorption-desorption isotherms obtained for mpg-C3N4 and CN-P-0.2%. Furthermore, we compared the mpg-C3N4 and the high PTCDA ratio sample CN-P-5% by solid state carbon Nuclear Magnetic Resonance (13C NMR) (Figure 5). The signal at 165 ppm and 157 ppm can be assigned to the triazine carbon atoms adjacent to amino group and triazine carbon atoms with all sp2-bonded N atoms surrounded, respectively.33 Because of the low concentration of PTCDA precursor, it was hard to observe the aromatic carbon in PTCDA unit at the region from 100 ppm to 140 ppm. Thermal Gravimetric Analysis (TGA) result (Figure S8) showed that the PTCDA units would decompose through decarboxylation reaction at the thermal preparation process of CN-P composite. The decomposed products consist mainly of perylene and some other condensed carbon materials. It is in accordance with our analysis above that the thermal condensation greatly enhanced the absorption efficiency of CN-P at long wavelength region (Figure S9). These ingredients have the

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potential to increase the charge conductivity of CN-P composite photocatalyst and even play as charge transfer sites.

Figure 5. Solid state CN-P-5% and PTCDA.

13C

NMR spectrum of mpg-C3N4,

Electron spin-resonance spectroscopy (ESR) is conducted to explore the electronic properties of the prepared polymeric photocatalysts. As shown in Figure 5a, only one single Lorentzian line centered at a g value of 2.0034 was observed for the mpg-C3N4 and CN-P-0.2% samples, demonstrating a well-established carbon nitride structure even for the composite samples.40 These Lorentzian lines are originated from the unpaired electrons in the aromatic rings of carbon atoms.41 Compared with mpg-C3N4, the ESR intensity of CN-P-0.2% are dramatically enhanced, implying the effective extension of delocalized systems of CN heterocycles. The extended structure in the matrix of carbon nitride materials would greatly affect the charge distribution and thus affect the catalytic performance.28, 41-44 Both mpg-C3N4 and CN-P-0.2% samples were evaluated by electrochemical impedance spectroscopy (EIS), an evidently decreased diameter of the semicircular Nyquist plots was revealed for CN-P-0.2% in the dark (Figure 6b). The charge transfer resistance of CN-P-0.2% is much smaller, which means the transfer of photo-generated charge carriers from CN-P-0.2% to aqueous phase is much more favorable. The result indicates reduced electronic impedance and improved charge mobility, which may be very beneficial for photogenerated charge separation during photocatalytic process.25

Figure 6. (a) ESR spectra of mpg-C3N4 and CN-P-0.2% samples in the dark. (b) Electrochemical impedance spectroscopy Nyquist plots of mpg-C3N4 and CN-P-0.2%. Energy level analysis by diffuse reflectance spectrum and XPS valence band spectrum reveal the both carbon nitride and CN-P sample are capable of hydrogen evolution from water (Figure S9 – S11). The behaviors of photogenerated charge carrier were further investigated by steady state and time resolved spectroscopic measurements. Figure 7 shows the photoluminescence (PL) spectra of mpg-C3N4 and CN-P-0.2%. We chose 375 nm as excitation wavelength to make sure that the light is mainly absorbed by carbon nitride part. Combined with the fact that the major part of CN-P composite photocatalysts is triazine unit, the PL emission signals in mpg-C3N4 and CN-P samples can both be assigned to the charge recombination in the C3N4 structure. Compared with mpg-C3N4, CN-P-0.2% presented significantly quenched PL intensity. And the average PL lifetimes of mpg-C3N4 and CN-P-0.2% were calculated to be 4.81 ns and 3.73 ns (Table S2), respectively. The declined PL intensity and shortened lifetime indicate the transfer of photogenerated electrons from carbon nitride units to PTCDA units. This process can serve as a competitive route to the charge recombination and thus facilitates the charge separation and migration within the CN-P sample. We also observed that Pt loading could quench the PL intensity of these carbon nitride materials. The decreased PL lifetime of mpg-C3N4 (4.81 ns to 4.03 ns) and CN-P-0.2% (3.73 ns to 3.07 ns) suggests the effective photogenerated electron transfer from carbon nitride photocatalysts to the Pt nanoparticles for hydrogen production (Table S3).34, 45 The electron transfer efficiency from mpg-C3N4 and CN-P-0.2% to surface loaded Pt cocatalyst is calculated as 16.2% and 17.7%, respectively. Therefore, it is reasonable to conclude that the thermal condesation with PTCDA can not only enhance the charge separation efficiency within CN-P composite photocatalyst,

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ACS Applied Materials & Interfaces but also promote the electron transfer from light-harvesting substrate to the surface cocatalyst.

excited charge carriers. According to the transient absorption decay kinetics analysis, the lifetime of trapped excited states was significantly prolonged in CN-P. Hence, we speculate that the composite photocatalyst derived from PTCDA and cyanamide presents better charge separation and migration efficiency for enhanced photocatalytic activity.

Figure 7. (a) PL spectrum of mpg-C3N4 and CN-P-0.2% upon excitation at 375 nm. (b) Decay curves of PL at 490 nm. The charge transfer process can be proven by transient absorption (TA) spectroscopy since it is considered as one of the most powerful strategies to investigate the behaviors of charge carriers.46-47 Previous work and some other related work on photocatalysts like TiO2, Fe2O3 LaTO2N and g-C3N4, transient absorption signal in the visible light region of carbon nitride materials in the time range from nanoseconds to milliseconds was assigned to the hole trapping states.48-52 This absorption process of relatively long lifetime may be explained as the inter sub-band transition of holes trapped excited states of mpg-C3N4 in our former publications and some other related works.53-56 Some latest papers have even clearly elucidated the faster decay process of carbon nitride materials excited states in the timescale from picosencond to nanosecond and will definitely bring some deeper insights into the relationship between charge carriers dynamics and photocatalytic reactions.57-58 Taking our practical conditions and previous experiences into account, we demonstrated the results of nanosecond transient absorption here as an auxiliary method to investigate the charge separation efficiency. As shown in Figure 8, mpg-C3N4 possesses a continuous broad TA region from 450 nm to more than 800 nm upon excitation at 410 nm. By monitoring the change of these TA signals, we could reveal the destiny of the long-lived excited states of these carbon nitride materials after the introduction of PTCDA into the framework of carbon nitride. The TA pattern of CN-P-0.2% was divided and the intensity at above 650 nm increased with respect to mpg-C3N4. We can believe that the introduction of aromatic all-carbon cycles into the skeleton of mpg-C3N4 could change the distribution of

Figure 8. (a) Transient absorption spectra of mpg-C3N4 and CN-P-0.2% upon excitation at 410 nm. (b) Decay curves of transient absorption at 750 nm upon excitation at 410 nm. The effects of Pt co-catalyst loading on the behavior of photogenerated charge carriers were further examined. As shown in Figure S12 – S13, the signal intensity of mpg-C3N4 and CN-P-0.2% were both significantly increased. This can be explained by the electron extraction effect of Pt co-catalyst, resulting in the accumulation of trapped holes in the polymeric composite. For both mpg-C3N4 and CN-P-0.2%, the decay lifetime of hole trapping states in Pt-loaded samples were both significantly prolonged. The above results demonstrate that the loaded Pt cocatalyst could prevent recombination and enhance the spatial separation of electrons and holes by electron extraction for hydrogen evolution.

CONCLUSION We have synthesized a new kind of graphitic carbon nitride composite photocatalyst to reveal the inner mechanism of how chromophore affects the catalytic activity of carbon nitride. The prepared sample presents efficient hydrogen evolution activity in the presence of Pt as cocatalyst and TEOA as sacrificial electron donor under visible light irradiation. The HER of CN-P-0.2% reached to 17.7 mmol h-1 g-1, with AQE = 5.8% at 450 nm. Mechanism studies demonstrated that the incorporation of large conjugated motifs into the framework of carbon nitride could change the distribution of excited charge carriers

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and promote the charge separation. The results point out that the improved charge separation efficiency is the key factor to the better HER performance of the CN-P photocatalyst.

SUPPORTING INFORMATION Detailed experimental procedure, characterization, comparison with literature, calculation methods, and more results of chemical characterization (FTIR, XRD), Elemental Mapping, SEM Images, Pore Size Distribution and more TA spectrum are available in supporting information (PDF).

AUTHOR INFORMATION Corresponding Author [email protected]

Author Contributions ‡These authors contributed equally.

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 and 2017YFAD206903), the National Science Foundation of China (91427303, 21390404 and 51373193), the Strategic Priority Research Program of the Chinese Academy of Science (XDB17000000), and the Chinese Academy of Sciences.

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Enhanced Charge Separation Efficiency Accelerates Hydrogen

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