2D-C3N4 heterostructure for high efficiency photocatalytic hydrogen evolution

Journal Pre-proof An all-organic TPA-3CN/2D-C3 N4 heterostructure for high efficiency photocatalytic hydrogen evolution Jiajun Fu (Formal analysis) (Writing - original draft), Zhao Mo (Formal analysis) (Writing - original draft), Ming Cheng (Data curation), Fan Xu (Software), Yanhua Song (Supervision), Xingdong Ding (Formal analysis), Zhigang ChenWriting-review and editing), Hanxiang Chen (Supervision), Huaming LiWriting-review and editing) (Supervision), Hui Xu (Supervision)Writing-review and editing)

PII:

S0927-7757(19)31395-0

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124397

Reference:

COLSUA 124397

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

13 October 2019

Revised Date:

24 December 2019

Accepted Date:

25 December 2019

Please cite this article as: Fu J, Mo Z, Cheng M, Xu F, Song Y, Ding X, Chen Z, Chen H, Li H, Xu H, An all-organic TPA-3CN/2D-C3 N4 heterostructure for high efficiency photocatalytic hydrogen evolution, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124397

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An all-organic TPA-3CN/2D-C3N4 heterostructure for high efficiency photocatalytic hydrogen evolution Jiajun Fua, Zhao Moa, Ming Chenga, Fan Xua, Yanhua Songb, Xingdong Dinga, Zhigang Chena,*, Hanxiang Chena, Huaming Lia, Hui Xua,* a

School of the Environment and Safety Engineering, Institute for Energy Research,

Jiangsu University, Zhenjiang 212013, PR China b

School of Environmental and Chemical Engineering, Jiangsu University of Science

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and Technology, Zhenjiang 212003, PR China

*Corresponding author: Tel.: +86-0511-88799500; Fax: +86-0511-88799500; Email address: [email protected], [email protected]

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Graphical abstract

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TPA-3CN/2D-C3N4 is synthesized by in-situ recrystallization and lowtemperature annealing. The introduction of TPA-3CN can extends the visible solar response range, and the formation of heterostructure can promote the photogenerated charge carrier separation, which exhibits the excellent obvious enhanced photocatalytic H2 evolution rate (311.738 μmol/h) and enhanced external quantum efficiency (~9.32%, λ= 420 nm).

0.6 TPA-3CN 2wt% TPA-3CN/2D-C3N4

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4wt% TPA-3CN/2D-C3N4

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6wt% TPA-3CN/2D-C3N4 2D-C3N4

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Absorbance (a.u.)

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Highlights

TPA-3CN/2D-C3N4 is synthesized via in-situ recrystallization of TPA-3CN on

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1.2

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the surface of 2D-C3N4 and low-temperature annealing. The successful introduction of micromolecule can form the type Ⅰ

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heterostructure, which hugely quicken the electron transition from 2D-C3N4 to TPA-3CN.

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The particular D-A structure take advantage of its efficient electron extraction and transfer ability to exhibit a stable H2 evolution rate of 311.738 μmol/h and the excellent external quantum efficiency up to 9.32% (λ = 420 nm).

Abstract: Photocatalytic hydrogen evolution have emerged as a promising technology to alleviate energy and environmental issues. Numerous inorganic semiconductors have been designed for efficient photocatalytic hydrogen evolution, but which are generally limited by the heavy metal contamination. All-organic semiconductors have attracted great interest due to the low cost, stable properties and tunable chemical structures. In my paper,TPA-3CN,with the superior donor-acceptor (D-A) structure ， was synthesized via the molecular engineering of triphenylamine (TPA) and (3-cyano-

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4,5,5-trimethyl-2(5H)-furanylidene) malononitrile (3-CN). Then an all-organic TPA3CN/2D-C3N4 heterostructure is formed by linking the TPA-3CN of stronger

electron-withdrawing property with two-dimension graphitic carbon nitride (2DC3N4), which realizes the broader solar absorption compared to the pristine CNS, as

well as the quicker electron transfer rate. Significantly, the TPA-3CN/2D-C3N4 shows

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superior photocatalytic performance of 1558.69 μmol (twice as 2D-C3N4) under 5 h

visible light irradiation (λ > 400 nm) and its apparent quantum efficiency (AQE)

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reach 9.32% at 420 nm. This work offers an innovative opinion for rational

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1. Introduction

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development of efficient all-organic photocatalysts for solar energy utilization.

Seeking and perfecting photocatalysts in the field of energy and environment has received widespread attention accounting for their application in solar energy

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conversion [1-4]. Photocatalytic water splitting, as a promise way to transform renewable solar energy into clean hydrogen, has changed our life a lot [5-7].

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Compared to the traditional metal photocatalysts, polymeric graphite carbon nitride (g-C3N4), as an inorganic non-metal photocatalyst, has unique advantages for its suitable band structure, huge active area, chemical and thermal stability [8-10]. However, the bulk g-C3N4 for the limited active surface severely hinder its practical application, mainly because of the limited solar spectrum range and the serious reorganization of photovoltaic electron and holes.[11-13]. The two-dimension graphite carbon nitride (2D-C3N4) have unprecedented merit of abundant active sites

in its huge reactive site and excellent photochemical properties in comparison with bulk g-C3N4 [14]. Nevertheless, 2D-C3N4 still has crucial obstacles in the photocatalytic performance [15-17]. One of the drawbacks is that photogenerated electron-hole pair will recombine to some extent while being separated. In addition, the narrow solar response further hampers photocatalytic performance. To overcome above problems, a series of organic semiconductors are induced into the 2D-C3N4 to broaden the optical absorption toward widen region and simultaneously perfect charge-carrier dynamics and redox property, which is considered as an effective strategy [18-20].

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Conjugated polymeric compound, for the wide application in new energy field ,especially for solar energy conversion and utilization, have aroused widespread

concerns due to fine-tuned band structure, broad solar absorption range, precise

design of structure [21-26]. A novel donor-acceptor (D-A) structured ETM named TPA-3CN was originally applied to perovskite solar cell (PSCs) as electron transport

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layer (ETL) [21]. Interestingly, cyano group has grafted in the edge of the conjugated

triazine frameworks due to thermal polycondensation at 300 °C in the tube furnace.

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The micromolecule modified conjugated triazine frameworks depending on improved light-induced charge carrier transport and separation characteristic has apparent

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admirable photocatalytic activity compared to 2D-C3N4 nanosheets. Generally, in the D-A form framework, the highest occupied molecular orbital (HOMO) energy level is usually decided on the electron-donating ability of the donor unit, and it is worth

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noting that the electron-withdrawing capability of the acceptor unit seems to have a remarkable influence on the position of the lowest unoccupied molecular orbital (LUMO) energy level. An alternative approach employs D-A dyads for band gap

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tuning. It is conjectured that incorporating appropriate bandgap D-A units into the 2D-C3N4 not only exhibits broader visible solar region but reduce the lifetime of the

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charge carriers to realize more rapid electron transfer speed, which verify the results electron mobility test rigorously from another aspect [19, 27]. Herein, an all-organic TPA-3CN/2D-C3N4 heterostructure was established

through in-situ recrystallization of TPA-3CN on the cover of 2D-C3N4 and lowtemperature annealing. Noteworthily, the introduced TPA-3CN expands the visible light response range and boosts the photogenerated charge carrier separation. As a result, the TPA-3CN/2D-C3N4 exhibits excellent photocatalytic performance of

311.738 μmol h-1 with an AQE of 9.32% (λ = 420 nm). This work opens a new facile solution for the structure of all-organic materials for energy crises.

2. Experimental Preparation of samples TPA-3CN was firstly reported by chen et.al, the detailed synthetic route of TPA3CN are shown in the supporting information [21]. And 2D-C3N4 was prepared with the bulk g-C3N4 as precursor. The bulk g-C3N4 was synthesized in the muffle furnace using melamine as precursor. The chimney in the muffle furnace connects to the atmosphere. In details, 2 g melamine was calcined at 550 °C for 4 h with 2 °C/min

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heating rate. The obtained bulk g-C3N4 was brilliant yellow. Subsequently, 400 mg bulk g-C3N4 ground into powder was putted into the combustion boat, and heated at

550°C for 1 h with a ramp rate 5 °C/min. And 2D-C3N4 was directedly obtained by the two-step calcination approach.

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The synthesis of TPA-3CN/2D-C3N4 was dissolved by ethyl alcohol, whose concentration was 5 mg/mL, 1 mg/mL respectively, then using ultrasonics for 30 min.

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We took 1 mL, 2 ml, 3 mL TPA-3CN solution (1 mg/mL ethyl alcohol ) into 10mL 2D-C3N4 solution (10 mg/mL ethyl alcohol), whose quality of the solubility was 2

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wt% , 4 wt% and 6 wt% respectively. Then the mixture was stirring at 50 °C over 24 h to obtain the powdered mixtures. Next we take these powders into tube furnace, and it would be calcined at 300 °C for 2 h with 5 °C/min heating rate. The final produce

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was grinded in the mortar.

3. Results and discussion

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3.1 Structural and Morphological Characterization As displayed in Fig. 1a, TPA-3CN showed a sheet-like structure. Meanwhile, 2D-C3N4 holds very similar sheet-like morphology with TPA-3CN (Fig. 1b). As

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shown in Fig. 1c, compared with 2D-C3N4, the morphology of TPA-3CN/2D-C3N4 has not changed. The transmission electron microscopic (TEM) image of TPA3CN/2D-C3N4 in Fig. 1d is carried out to further verify the graphite-like layered structure. The prepared samples were analyzed by X-ray diffraction (XRD). As displayed in Fig. 2a, no significant diffraction summits were seen in TPA-3CN, implying that our samples belong to amorphous phase. Remarkably, it is worth noting that TPA-3CN/2D-C3N4 have two typical peaks at 27.7° and 13.0° that can be

attributed to the (002) and (100) planes of g-C3N4 with graphitic-like structure (PDF No. 87-1526, JCPDS) [28, 29], but the relevant intensities of the two representative peaks of TPA-3CN/2D-C3N4 increase slightly with increasing proportion of the TPA3CN,probably due to the introduction of TPA-3CN into the CN [27]. The chemical element of the synthesized samples was also characterized by FTIR spectroscopy. Seen from the Fig. 2b, the typical tensile mode of the C-N heterocycle is contributed to a series of peaks in the range of 1600 - 1200 cm-1, while the peak at 813 cm-1 is related the bending vibration of heptazine in the CN. [30, 31]. The peak value of 3200 to 3000 cm-1 mainly belong to N-H vibration attributing to partial condensation and O-H vibration ascribed to adsorbed H2O on the surface [32].

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Obviously, the sharp peak at 2233 cm-1 is attributed to unsaturated C≡N [33].

Surprisingly, the C≡N disappeared after connecting with CN, exhibiting that the structure of TPA-3CN has been destroyed in the calcination. This was mainly because

the 2D-C3N4 would deaminize and generate a part of H2O in the process of

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recombination, further brought about the formation of the amide bond in the acidic

atmosphere [44]. Subsequently, from the Raman spectra (Fig. S1), the adding of the

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TPA-3CN perfected strong fluorescence interference of the 2D-C3N4 indicated that TPA-3CN has been grafted onto the surfaces of 2D-C3N4. To verify the structure

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stability of TPA-3CN after low-temperature annealing, the origin TPA-3CN was put into tube furnace and calcined in the same condition. The samples were taken measured to the FTIR spectroscopy and Raman spectra (Fig. S2). It turned out that the

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summit of the C≡N still existed. Therefore, it is expected that TPA-3CN is stable in the process of calcination.

Fig. 3a shows the survey XPS spectra of the 2D C3N4 and TPA-3CN/2D-C3N4,

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which exhibits the similar functions. C, N and O elements are found in the both samples, manifesting that the TPA-3CN/2D-C3N4 maintains the same chemical

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composition and chemical states with the 2D-C3N4. For the C 1s fine XPS spectrum of 2D-C3N4 (Fig. 3b), one peak at 284.60 eV is ascribed with the sp2 C-C bonds and sp2 hybridized C (N-C=N) appears at 288.22 eV [34, 35]. And other peak with the low intensity at 285.4 eV is related to C-NH2 [36]. Obviously, the adding of TPA-3CN makes a little shift to the high binding energy. For N 1s spectrum of samples, the result could be able to divided into three peaks. On the basis of the reported literatures, the N 1s peaks at 398.7 eV, 399.8 eV corresponded to the sp2-hybridized aromatic N bonded to carbon atoms (C=N-C) and sp3-hybridized nitrogen (N-(C)3) in the 2D-

C3N4 [37]. What’s more, the weaker peak at 401.0 eV could be ascribed to N-H side groups [38]. Interestingly, the N 1s peaks of TPA-3CN/2D-C3N4 were transferred to the higher binding energy in contrast with CN. Such changes could be put down to the replacement for N atoms in heptazine rings after the combination of the micromolecule through condensation polymerization. In the C 1s, the peak area of the C-C bond has expanded from 2D-C3N4 to TPA-3CN/2D-C3N4, which demonstrated that partial carbonization happened in the reaction process (Table S2). 3.2 Photocatalytic performance In the Fig. 4, the photocatalytic activity of TPA-3CN/2D-C3N4 in hydrogen

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evolution reaction under 10 vol% of triethanolamine was compared under visible light. As presented in the Fig. 4a, TPA-3CN/2D-C3N4 obtained the highest hydrogen

production activity, and the mass ratios of the polymer was optimized to 4%. Nevertheless, as the mass of the organic matter further increased, the photocatalytic

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activity decreased. This result can be put down to excess TPA-3CN influencing the

reaction site and light gone through the suspension system [39]. Under 5 h visible

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light irradiation (λ > 400 nm), the total amount with the optimal proportion of H2 reaches 836.71, 1558.69 μmol for the 2D-C3N4, TPA-3CN/2D-C3N4, respectively. The function of molecular engineering is efficient, about twice as high as that of the

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2D-C3N4 may due to its wider light absorption range and more efficient photogenerated charge segregation. Meanwhile, as shown in Fig. S3a, without 3% Pt as co-

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catalyst, the 2D-C3N4 and molecular engineering of TPA-3CN/2D-C3N4 exhibit the similar sluggish hydrogen evolution kinetics, because the produced charge carriers are unavailable to be made full use of. In addition, a several of blank experiments was

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taken, aiming at the light, sacrificial gent and catalyst. As expected in the initial design, the sample without light and no catalyst all exhibited no hydrogen production

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activity, but the sample without triethanolamine had a decent activity (Fig. S3b), about 133.47 μmol in 5 h, which demonstrated that the sample itself has a superior photogenerated charge carrier separation ability through the molecular regulation. To further explore the photocatalytic stability, six runs of accumulatively 30 h irradiation had been carried out (Fig. 4c). After 4 runs of experiment, 5 mL fresh TEOA were replaced in the reactor. The experimental data suggest that the photocatalytic performance of TPA-3CN/2D-C3N4 has no obvious deactivation for six cycles, confirming TPA-3CN/2D-C3N4 has excellent long-term stability. Moreover, FTIR

and XRD tests were characterized on recovered TPA-3CN/2D-C3N4 (Fig. S4). It follows that the FTIR spectra and XRD spectra of TPA-3CN/2D-C3N4 did not change apparently before and after photocatalysis. It shows that the organic photocatalyst have an unprecedented effect on the prospects for the pattern of solar energy utilization. 3.3 Possible photocatalytic mechanism The UV/vis diffuse reflectance spectra (DRS) of samples are displayed in Fig. 5. All the different quality ratios of TPA-3CN/2D-C3N4 showed broader absorption range in the visible region. With the increasement of the ratio, the absorption edge of

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TPA-3CN/2D-C3N4 moved gradually towards infrared region, which means that the

inducing of TPA-3CN broaden the solar absorption range, thereby accounting for the promotion of hydrogen production performance.

To further study the internal electron transfer kinetics of TPA-3CN/2D-C3N4,the

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samples were tested by photoluminescence emission (PL) and time-resolved fluorescence (FL) spectra .In the PL spectrum (Fig. 6a), it is obvious to find a strong

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emission peak at 390 nm, mainly because electron-hole pairs occurred to restructure at the interface [40]. As we can see, quenched emission peak can be clearly monitored

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when loaded with 4 wt% TPA-3CN/2D-C3N4, implying that the process of electron extraction took place during its transport process. The PL intensity for TPA-3CN/2DC3N4 is lower than that of 2D-C3N4, suggesting the enhanced electron extraction

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ability and faster transfer speed for the introduction of TPA-3CN, consistent with the higher electron mobility of doped TPA-3CN. Moreover, as shown in Fig. 6b, the life time reduced dramatically by inducing 4 wt% doped TPA-3CN may owning to rapid

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electron migration, which is consistent with the electron migration test [5]. The photocurrent responses of the 2D-C3N4, TPA-3CN and TPA-3CN/2D-C3N4 were also

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carried out (Fig. 6c). Obviously, the photocurrent responses can be observed during intermittent illumination, and further depicted the good stability of the three materials. More significantly, TPA-3CN/2D-C3N4 exhibited the stronger photocurrent response, three times as high as the 2D-C3N4. That result revealed that TPA-3CN/2D-C3N4 has a better efficiency in the separation of the photon-generated electrons and holes [41]. Moreover, the electrochemical impedance spectroscopy (EIS) were carried out in Fig. 6d, and TPA-3CN/2D-C3N4 exhibits the smallest semicircle diameter compared with that of 2D-C3N4 and TPA-3CN. It suggests TPA-3CN/2D-C3N4 possesses the best

separation efficiency of the photon-generated electrons and holes [42]. Therefore, based on the above analysis, the construction of all-organic TPA-3CN/2D-C3N4 heterostructure not only extends the visible light response range, but also boosts the photogenerated charge carrier separation, resulting in the increased photocatalytic activity of evolution production. The band alignment between TPA-3CN and 2D-C3N4 like the interlaminar structure plays a good role in enhancing photocatalytic activity of TPA-3CN/2D-C3N4 toward H2 evolution. According to the previous report, the conduction band (CB) and valence band (VB) positions of TPA-3CN were calculated to be -4.07 and -5.92 eV,

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respectively [21]. Meanwhile, the CB and VB positions of 2D-C3N4 was determined to be -3.23 and 6.14 eV, respectively [15]. Hence, as shown in Fig. 7, the TPA3CN/2D-C3N4 may obey the rule of the type Ⅰ structure, the photoinduced electrons

on the CB of 2D-C3N4 transfer to the CB of TPA-3CN, and the photoinduced holes on

the VB of 2D-C3N4 should also flow to the VB of TPA-3CN. Then the transferring

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electrons react with H2O to form H2, and the holes also oxidize TEOA into the oxidation products. In this structure, the photoinduced electrons and holes all

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generated in the surface of the TPA-3CN, and it would be quickly separated profit from the efficient electron extraction and transfer ability in the D-A structure of TPA-

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3CN, which is consistent with the electron mobility test result (Fig. 6). Besides, the ππ stacking of TPA-3CN has increased the level of 2D-C3N4, and the π-π stacking aggregation between the intermolecular of 2D-C3N4 is quite beneficial for the electron

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transfer in the type Ⅰ system [43].

4. Conclusion

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In summary, an all-organic TPA-3CN/2D-C3N4 heterostructure has been successfully constructed via in-situ recrystallization and low-temperature annealing. The introduction of TPA-3CN can extends the visible light response range, and the

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formation of heterostructure can promote the photogenerated charge carrier separation. Hence, the TPA-3CN/2D-C3N4 exhibits 2 times higher H2 evolution activity than 2DC3N4 under visible light, resulting in high AQY of 9.32% and 7.94% at 420 and 450 nm. The construction of all-organic heterostructure could propose a hopeful method for growing effective and stable all-organic semiconductors for photocatalytic hydrogen evolution and even other fields of photocatalysis research.

Author contribution section Jiajun Fu: Formal analysis, Writing - original draft. Zhao Mo: Formal analysis, Writing - original draft. Ming Cheng: Data curation. Fan Xu: Software. Yanhua Song: Supervision. Xingdong Ding: Formal analysis. Zhigang Chen: Writingreview & editing. Hanxiang Chen: Supervision. Huaming Li: Writing-review & editing, Supervision. Hui Xu: Supervision, Writing-review & editing.

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Acknowledgements

This study was supported by National Natura Science Foundation of China (21776118,

21878134),

Natural

Science

Foundation

of

Jiangsu

Province

(BK20190981), Jiangsu Fund for Distinguished Young Scientists (BK20190045),

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High-tech Research Key laboratory of Zhenjiang (SS2018002), A Project Funded by

the Priority Academic Program Development of Jiangsu Higher Education Institutions,

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the high-performance computing platform of Jiangsu University.

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Fig. 1. SEM images of (a) the TPA-3CN, (b) 2D-C3N4 and (c) TPA-3CN/2D-C3N4, (d)

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TEM image of the TPA-3CN/2D-C3N4.

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Fig. 2. (a) XRD patterns and (b) FTIR spectra of the TPA-3CN/2D-C3N4.

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Fig. 3. XPS spectra of the TPA-3CN/2D-C3N4 (a) survey; (b) C 1s; (c) N 1s and (d) O

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Fig. 4. (a) Photocatalytic H2 evolution on three ratio TPA-3CN/2D-C3N4 samples

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under visible light irradiation, using Pt (3%) as a co-catalyst (λ > 400 nm); (b) the TOF of TPA-3CN/2D-C3N4; (c) Recyclability of TPA-3CN/2D-C3N4 for 6 cycles

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over 30 h; (d) Wavelength-dependent AQE of TPA-3CN/2D-C3N4.

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Fig. 5. DRS spectrum of TPA-3CN/2D-C3N4.

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Fig. 6. (a) PL spectra and (b) FL spectra excited by the incident illumination of 390

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nm of TPA-3CN/2D-C3N4; (c) Photocurrent responses of the TPA-3CN/2D-C3N4 under visible-light irradiation ([Na2SO4] = 0.1 mol/L); (d) Nyquist plots of the TPA-

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3CN/2D-C3N4 in the dark ([Na2SO4] = 0.1 mol/L).

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Fig. 7. Schematic illustration of the proposed mechanism for photocatalytic H2

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evolution over the TPA-3CN/2D-C3N4.