Synthesis of Polymeric Carbon Nitride Films with Adhesive Interfaces

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The synthesis of polymeric carbon nitride films with adhesive interfaces for solar water splitting devices yuanxing fang, Xiaochun Li, and Xinchen Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02549 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Catalysis

The synthesis of polymeric carbon nitride films with adhesive interfaces for solar water splitting devices Yuanxing Fang, Xiaochun Li and Xinchen Wang* State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, P. R. China.

Abstract

Polymeric carbon nitride (PCN) films are synthesized for solar water splitting devices. Herein, five precursors and their mixture are attempted, achieving PCN films in ca. 40 different morphologies. We find that sulfur (S) containing and non-S precursors must be mixed to grow PCN films on fluorine doped tin oxide (FTO) glasses. By the investigations of the PCN films, S is found only existing at the interfaces between the PCN and FTO films.

In this case, S not

only acts as the initialization for the growth of the PCN films, but also as connections to assistant charge migration for water splitting devices. As a result, the best performance ca. 100 uA/cm2 is achieved at 1.23 VRHE under AM 1.5 illumination in NaOH electrolyte solution without sacrificial agent. This result can be attributed to the reduced defects along the interfaces and the lessened charge recombination. This synthesis of PCN films with the improved chemicaladhesion to the FTO layer envisage such organic material for the constructions of other flexible functional devices.

ACS Paragon Plus Environment

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Keywords: Polymeric carbon nitrides, Films, Interfaces, Solar water splitting devices, molten mediate approach.

Introduction

Film is fundamental to devices, ranging from electronics, optoelectronics, electrochemical cell and

photocatalytic

devices.1-4

As

an

example,

solar

water

splitting

device

or

photoelectrochemical (PEC) cell is capable of storing solar energy by photocatalysis.5-7 The structure of the device generally consists of two functional layers, namely, the charge collector films and photocatalytic films.8-9 Specifically, the films of charge collector are normally transparent conductor oxides (TCOs).10-13 The photoactive films are semiconductor to serve multiple tasks,14-16 including harvest of solar energy, generation of charge carriers and proceeding of surface redox reactions.17-18 Beyond the layers themselves, the interfaces between them are also significant to influence the performance, because the photoinduced free carriers must transfer across the interfaces to the electrode that on the other side. The defects along the interfaces are very likely to affect the migration of charge and thus promote recombination. To improve the quality of the interfaces, it is general to deposit photoactive films on charge collector layer through a in situ approach, especially for the metal-based PEC cells. The methods, such as hydrothermal, vapor liquid solid, chemical bath deposition and more, were broad developed.19-22 Polymeric carbon nitride (PCN) has received increasing attention as a photocatalyst, due to its advantages of a proper band gap, reliability, sustainability and cost-effectiveness.23-25 However, thermodynamic challenge highly restricts the polymerization to in situ achieve PCN films on a conductor through conventional methods.26-27 To date, two approaches were developed, called in situ vapor condensation and molten mediate polymerization.28 In


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ACS Catalysis

comparison, molten mediate approach is an ease to achieve PCN films. As a typical example, sulphur (S) can act as molten mediate to facilitate the growth of PCN films.29 But in the reported literatures, the performance is generally low, and many of PCN films formed by this approach are even solar inactive.30 This situation is because of asiding from film quality, the interfaces between the functional films are also important to determine the charge recombination. The insufficient binding between the PCN films and conductive substrate formed by the molten mediate approach is thus considered as a major issue to hinder PCN films for solar water splitting devices. However, none of research group yet investigates and optimizes the interfaces previously. In addition, the role of the molten organic for the formation of PCN films is still quite unclear. Nevertheless, a variety of PCN precursors are yet attempted to synthesize PCN films by this approach. Herein, five kinds of nitrogen abundant precursors are attempted to grow PCN films, including melamine (M), trithiocyanuric acid (TA), thiosemicarbazide (TSC), thiourea (TU) and urea (U) (Scheme S1). We find that S-containing and non-S precursors must be mixed to grow PCN films on fluorine doped tin oxide (FTO) glasses. By the investigations of the PCN films, S is found only existing at the interfaces between the PCN and FTO films. On one hand, S initializes the growth of PCN films. On the other hand, S facilitates charge migration for water splitting devices. As a result, it contributes an improved solar conversion efficiency with respect to the other reports, achieving the best performance ca. 100 uA/cm2 at 1.23 VRHE under AM 1.5 illumination.29, 31-33 This result can be accredited to the reduced defects along the interfaces, and therefore significantly lessening the charge recombination. This approach is an ease to synthesize PCN films, and form fundamental to use such organic materials for the construction of other


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functional devices, such as (photo)catalytic32,

34-35

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and electrochemical devices,36-38 sensors39

photovoltaics40-41 and others.42-43 Experimental The synthesis of PCN films. M, TA, TSC, TU and U, and their mixture were used as precursors to synthesize PCN films, and the synthesis procedures were illustrated in Scheme S1. The mixed precursors were prepared by blending a S-containing compound into a non-S precursor in the weight ratio of 1:1, 1:2, 1:5 and 1:10, and grinding the mixture in a mortar with pestle. The asprepared precursors were transferred into a crucible, and a clean FTO glass was buried into the powder. The clean FTO glass was prepared by washing with acetone in sonication for 30 minutes to remove pollutants. Afterwards, the FTO glass was rinsed with deionized water and dried using nitrogen flow. The crucible was annealed at 500°C for 4 hours with a raping rate of 3°C/minute under a constant nitrogen flow (100 cm3/minute) and cooling down in ambient condition. The PCN films were sonicated in water for 2 hours to remove the residual aggregates on the surface, and they can be directly used as photoanodes. Characterizations. The morphological and structural studies of the photoanodes were performed through scanning electron microscopy (SEM, Jsm-6700F). The crystallinity of the samples was investigated using powder X-ray diffraction (XRD, Bruker D8 Advance with Cu Kα1 radiation, k = 1.5406Å), and data were collected with a rate of 0.02°/2Ɵ in the range of 20° to 80°. The elementary studies of the PCN samples were conducted using X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250 instrument with a monochromatized Al Kα line source, 200W). The samples for XPS were prepared by cutting an electrode into 2 mm × 2 mm and placed in a sample holder for the optical measurements. Depth profile of the XPS was


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conducted to study S2p, and the Ar+ ion gun with the energy of 4 keV was used for sputtering. Ultraviolet-visible diffusion reflector spectrometers (UV-Vis DRS, Varian Cary 500 Scan UVvisible system) were used to study the optical properties of the photoanodes. UV-Vis DRS was used to probe the photoanodes, a pristine FTO glass was used as the references for the measurements. Photoluminescence (PL) and the time-resolved PL were conducted using a Hitachi F-7000 FL spectrophotometer. An electrode was cut into 1 cm × 1 cm, and it can be settled into a sample holder, an emission spectrum was then acquired with the excitation wavelength of 380 nm. With same excitation wavelength, the time-resolved PL spectra could be obtained for the calculation of the decay time of the photoexcited charge. Photoelectrochemical measurements. PEC measurements (BAS epsilon Electrochemical system) were conducted with a three-electrode system, including a PCN photoanode, a Pt cathode and a Ag/AgCl reference electrode, in 1.0 M NaOH electrolyte solution with an air mass 1.5 (AM 1.5) solar simulator (Newport, USA). The electrolyte solution with the pH values of 3, 7, 11, 14 were adjusted using 1.0 M H2SO4, Na2SO4 and NaOH aqueous solutions. The electronic impedance spectroscopy (EIS) was conducted with Mott-Schottky and Nyquist plots. MottSchottky plots were achieved without any irradiation to obtain flat-band potentials of the PCN films. Meanwhile, Nyquist plots were obtained from the same system under the situations of the light on (AM 1.5) and off. Applied bias photon-to-current efficiency (ABPE) and incident lightto-electron conversion efficiency (IPCE) were measured using a Xenon (300 W) with monochromator (Newport, USA) to accomplish a certain wavelength. The intensity of each wavelength from the input light source was initially measured, then with the applied voltage of 1.23 VRHE, the photocurrent densities were measured under the certain input wavelength. Results and discussion


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The products on FTO substrate achieved by pristine precursors were investigated by SEM. Unfortunately, none of them could achieve films. M formed nothing on the substrate (Figure S2). The other three S-containing precursors induce small islands anchoring on the FTO substrates, as shown in Figure 1a, 1c and 1e. The morphologies of the islands were observed from the corresponding magnificent images in Figure 1b, 1d and 1f. Porous morphology could be noticed from products achieved by TA and TU, but irregular structures were induced by TSC. Digital photographs of the photoelectrodes are shown in Figure 1g, and they are almost clean glass by naked eyes.

Figure 1. The PCN islands formed by (a) and (b) TSC, (c) and (d) TA, and (e) and (f) TU. (g) The digital photographs of the films. (h) the corresponding S2p spectrum by XPS.

To investigate the chemical features of the islands, XPS was attempted. The products can be identified as PCN by resolving C1s and N1s spectra (Figure S3 and S4).44 The two peaks of


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C1s at 286.4 and 288.3 eV are attributed to C in the group of N‒C=N, and the peak locating at 284.8 eV is due to the C=C group. Three main peaks are allocated in N1s spectrum. The peak at 398.8 eV and 400.5 eV can be assigned to N in the group of C‒N=C and (C)3‒N, respectively. The peak at 401.3 eV implies the formation of the amino group of C‒N‒H. Moreover, it is notable that the weak S single from XPS spectra (Figure 1h). The main peak at 161.8 eV can be assigned to metal sulphide, which is Sn-S in this case, and it is only possible existing on the surface of FTO glass. In addition, the shoulder peak at164 eV can be accredited to S-C group in PCN.29 A shallow peak at ca. 169.0 eV by TU induced films can be assigned to metal sulfate, which is possibly due to contaminations on FTO glass. The results, with respect the films by M, indicate that S is the key to grow and anchor of PCN islands on the FTO substrate.

Figure 2. The PCN films achieved by the mixed precursor. (a) to (d) correspond to the PCN films achieved by the mixture of M and TA, (e) to (h) correspond to the PCN films achieved the mixture of M and TU, and (i) to (l) correspond to the PCN films achieved mixture of M and TSC with certain ratio marked in the images.


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We then tried mixed precursors containing both S-containing and non-S compounds. Theoretically, S-containing precursor can serve to form PCN islands, on which, further polymerization is able to be induced by non-S precursor. As the result shown in Figure 2, films gradually appeared when the ratio of M to S-containing precursor were increased. As the series of the SEM images from Figure 2a to 2d, they were induced by using the mixture of M and TA. When the ratio was 1:1, a few naked FTO area still could be observed. With slightly increasing the composition of M, high-porous films could be observed to fully cover the substrate (Figure 2b). If the ratio became to 1:5, the growth of films was further improved to form nonporous morphology (Figure 2c), and further increase leads to the films to be irregular with residues on it (Figure 2d). Similar situations can be observed from the other two series of the PCN films, the films kept growing with increasing the quantity of M. Figure 2e to 2h present the results using the mixture of M and TU as precursor, the morphology was quite different to the above example using M and TA. Large islands formed with the ratio of 1:1 (Figure 2e), and films can be obtained when the ratio becomes 1:2 (Figure 2f). The PCN films become even thicker when the ratio is further increased (Figure 2g and 2h). The film morphology achieved by the mixture of TSC and M is quite different to the other two sets, as shown from Figure 2i, 2j, 2k to 2l. Inhomogeneous morphology were observed from all the products. Such phenomenon reveals that the S-containing precursor possibly determines the morphologies of the PCN films. TSC initially induced irregular PCN islands (Figure 1a and 1b), and the further polymerization was based on such spotty structures. We also introduced U to replace M as the non-S precursor to prove this concept, and a ratio of 1:2 between S-containing precursors to U were attempted. The results are shown in Figure S5, and the growth excellently follows this hypothesis. While, the yield of film growth is lower than that by using M, which can be attributed to the even more complicated


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polymerization processes of U.45 Also, U is likely to evaporated-off under a high temperature. Subsequently, we tested the robustness of the PCN films by ultrasonic treatment of them. They remained intact for more than 4 hours, revealing the superior mechanical-stability, which also confirms the stable connection between the FTO and PCN films.

Figure 3. Depth XPS study of the S 2p of the PCN films induced by (1) TA:M=1:2, (2) TU:M=1:2, and (4) Of: U=1:2.

Subsequently, a few optical characterizations were acquired for typical samples, including TA:M=1:2, TU:M=1:2, TSC:M=1:2 and TA:U=1:2, to determine their chemical properties. XRD were directly conducted to the PCN films on FTO glass. In the XRD spectra, they presented similar peaks, but mainly attributing to the FTO glass (JCPDS #46-1088) (Figure S6a). Shallow peaks at 27.4º could be observed in the magnificent spectra (Figure S6b), and this peak was due to the crystal plane of the PCN interlayer structure. To further confirm the formation of PCN, the sample of TSC: M = 1:5 was also conducted. As shown in Figure S7, the shoulder peak at 27.4o is even stronger than FTO peak at 26o. In addition, a minor peak at 13o can also be observed due to the π-π stacking of heptazine unit. XPS was also conducted, and the C1s and N1s spectra are shown in Figure S8 and S9, respectively. Their spectra are very similar


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to Figure S3 and S4. In addition, oxygen content can be estimated from XPS result to determine the impurity of the PCN films (Figure S10). The main peak of O1s at 431.5 eV corresponds to C-O, and the shoulder peak can be considered as C=O. The area can be considered as the oxygen defects in the films. The integrated surface areas of the oxygen peaks are similar to each other. This result, together with the data from XRD (Figure S6), implies the quality of PCN films of the typical samples are similar in amorphous state. It is notable that S cannot be observed from TA: M = 1:2, TU: M = 1:2 and TA: U=1:2 (Figure S11). This result reflects that S only exists among the films but not on the surface. In contrast, S signal appeared in TSC: M = 1:2, probably due to its inhomogeneous structure. Subsequently, we attempted XPS depth profile of S2p for TA: M = 1:2, TU: M = 1:2 and TA: U=1:2 (Figure 3). It can be observed that a peak gradually appeared with the increase of the etching time. Relatively weak intensity is observed at 20 minutes, and it becomes even stronger for 30 minutes. These spectra can be assigned into two peaks at 164 eV and 161.8 eV, which are defined as S-C and Sn-S, respectively. These results are in accordance with Figure 1h. Therefore, in this case, S can be considered only existing on the surface of the FTO and binding PCN films, but not doped into the PCN films. The band gaps of the films were investigated using UV-DRS (Figure S12a). They presented a similar value of ca. 2.75 eV (Figure 12b), and again confirming that S is absent from the PCN films. In addition, Mott-Schottky plots of EIS were conducted to identity their flat-band potentials (Figure S13), and the results are also in similar manners, implying that the electronics properties of the PCN films are similar to each other and in accordance to the state of the art of pristine PCN material.46

Table 1. The PCN precursors, film morphologies and PEC performances.


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S-containing

Non-S

Ratio of

Film

Current density

precursor

precursor

the precursors

morphology

(µA/cm2)

Entry 1

TA

1:0

Porous islands

8.0

2

TU

1:0

Porous islands

3.5

3

TSC

1:0

Porous islands

5.0

M

0:1

None

None

4 5

TA

M

1:1

Porous islands

40

6

TA

M

1:2

Porous films

100

7

TA

M

1:5

Compact films

27

8

TA

M

1:10

Irregular films

24

9

TU

M

1:1

Large islands

18

10

TU

M

1:2

Nodulous films

50

11

TU

M

1:5

Nodulous films

35

12

TU

M

1:10

Irregular films

8

13

TSC

M

1:1

NC films

16

14

TSC

M

1:2

NC films

35

15

TSC

M

1:5

NC thick films

10

16

TSC

M

1:10

NC thick films

4.0

None

None

17

U

18

TA

U

1:2

Porous films

55

19

TU

U

1:2

Compact films

28

20

TSC

U

1:2

Bulky islands

10


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*NC=non-continuous, All the films were conducted for PEC water splitting in 1.0 M NaOH electrolyte solution under AM 1.5 illumination without any sacrifice agent. The performances were collected at the 1.23 VRHE, as shown in Table 1. The details of the photocurrent density were exhibited in Figure S14 to Figure S18. Among them, the highest photocurrent density was ca. 100 µA/cm2 by entry 6, corresponding to the morphology shown in Figure 2b, and high porosity can be noticed from the films. When the compact films formed (entry 7, Figure 2c), the performance was significantly reduced to 27 µA/cm2. Further increasing the ratio of M would lead to even lower performance. The other two series of entries presented similar trends, indicating that, with respect to the compact films, the porous structure along the PCN films have an increased amount of active site to promote the surface water oxidation. We also attempted to investigate the photoresponse by varying the pH value of the electrolyte solution, adjusting by 1.0 M H2SO4NaSO4 and NaOH aqueous solution, as shown Figure S19. By reducing the pH value, the photocurrent density would significantly decrease. This result is in accordance with theoretical understandings of the band structure of PCN, as the valance band is only little lower than water oxidation potential.19, 21


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Figure 4. (a) IPEC and (b) PL decay time of the photoanodes based on enter 6, 10, 14 and powder-based films.

All the films induced by TSC are inhomogeneous, leading to performances of the series of samples are poor. We also synthesized PCN powders using all the mixed precursors, and the PCN powders were dip-coated on FTO glasses as photoanodes. The performances were shown in Table S1, however, they contributed similar results in low values, which reflects that the interface between the PCN films and FTO is of-particular important to determine the solar conversion efficiency. One typical curve achieved by powder PCN (entry S6) is shown in Figure S20. The photocurrent densities can be observed ca. 2~3 uA/cm2 with rapid reduction. In comparison, stability measurement was also conducted for entry 6 as shown in Figure S21. In this case, the photocurrent density can only be preserved for about 15 minutes, but this value is much better than the dip-coated samples. In addition, the performance is generally an order of magnitude lower than to the result by molten mediate approach. In specific, enter 6 achieved the photocurrent density around 30-fold to the powder-based photoanode. We also compared our result with recent publications involving PCN photoanodes in Table S2, and entry 6 still presented as an outstanding performance, and is comparable to the boron doped PCN films.32


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Entries of 6, 10, 14 and a powder-based photoanodes were selected as typical samples for further investigations. The IPEC (Figure 4a) and ABPE (Figure S22) spectra were acquired at 1.23 VRHE, and they are presented in same trend in accordance with the UV-DRS spectrum (Figure S12a). With respect to the powder sample, all three photoanodes prepared by in-situ molten approach show much higher conversion throughout all the wavelength. The highest conversion is at 340 nm in this case by entry 6 (16%), and this result is even double to entry 10.

Figure 5. (a) The growth mechanism of the PCN using the mixture precursor. (b) The photocatalytic mechanism for PEC water splitting.


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In the mechanistic study, Nyquist plots of EIS were attained to study the interface reactions, as shown in Figure S23. Entry 6 presented smallest diameter among the entries, indicating the high charge transport and suppressed recombination. It is notable that the result from entry 10 is similar to entry 14, which also reflects their performances. While, the powerbased photoanode presented extensively high resistance, about two orders of magnitude to the value of entry 6. This result could be accredited to the non-continuous structure and less interaction between the PCN powder to the FTO layer by dip-coating approach. Ambient PL was further conducted to investigate the charge carrier behavior. The emission spectra of the PL by the excitation wavelength at 380 nm are shown in Figure S24. The emission peak at 470 nm confirms the band gap of the PCN films. Also, entry 6 presents the emission intensity considerably decreases with respect to the other samples, implying its efficient charge transport with respect to the others. In addition, time-resolved PL spectra were acquired with the same excitation wavelength (380 nm). As the results shown in Figure 4b, the fitting decay time of charge collectors by entry 6 is 7.3 ns, which is the shortest among the entries. In contrast, the powder-based sample presents a long decay time with respect to the others. This result also confirms the high charge transport and lessened recombination effect by entry 6, revealing the significance of the interfaces along the device. In the aspect of developing PCN films for solar water splitting device, S plays important roles. As the growth mechanism illustrated in Figure 5a, S initializes the formation of the PCN by bonding with Sn (stage I). This pre-deposited product allows the non-S precursor to proceeding a cascade of polymerization on the formed PCN (stage II) until the reaction terminates (stage III). However, the PCN films are amorphous, which cannot be precise to illustrate the growth direction. In state-of-the-art, the parallel growth of the PCN films to the


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interface is preferable for photoexcited charge separation and transport. In addition, the intimate PCN films on the FTO glass connecting by S significantly improved efficiency for the solar to fuel conversion, as the photoelectron-catalytic mechanism illustrated in Figure 5b. In detail, two interfaces determine the photo-efficiency, i.e. the solid-solid interface between charge collector layer and photocatalytic layer, and the solid-liquid interface between the photocatalytic layer and electrolyte solution. In this case, the solid-solid interface is greatly improved, and thus allowing the photo-separated electron and hole to readily migrate to the cathode and anode, respectively. As such, the photorecombination probabilities would be extensively suppressed, and the solar conversion efficiency is improved. Conclusions In conclusion, PCN films in ca. 40 different morphologies were achieved and investigated for solar water splitting devices. It was found that the S-containing and non-S PCN precursors must be mixed to allow the formation of PCN films. By the investigations of the PCN films, S was only observed at the interfaces between the PCN and FTO films. S on one hand was the initialization for the growth of PCN films, and on the other hand assisted charge migration for water splitting devices. The optimal films achieved the best PEC efficiency ca. 100 uA/cm2 (at 1.23 VRHE) under AM 1.5 illumination in NaOH electrolyte solution without sacrificial agent. This result presented as a significantly improvement with respect to the PCN films by other approaches. However, in a comparison of metal oxide base solar water splitting devices, the result was still relatively poor. This phenomenon was mainly due to the nature of polymer properties, such as the challenges to form crystallized structure, poor adhesion with substrate, strong Columbus force in exciton and more.47-48 On basis of the wide investigations of the powder-based system for photocatalytic water splitting, it is very potential to further improve the


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performance of PCN based solar water splitting devices.49-50 In addition, this demonstration should be merited not only for photocatalytic devices, but also for other functional devices that based on organic semiconductor.51 Associated content Supplementary Information. Additional details such as characterizations and the measurements of performance, including SEM images, XRD spectra, XPS spectra, UV/vis DRS spectra, Mott-Schottky and Nyquist plots of EIS, ABPE data, PL and PEC performance data. Corresponding Author *Xinchen Wang, E-mail: [email protected]; webpage: https://wanglab.fzu.edu.cn. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgement We acknowledge the financial support from the National Key R&D Program of China (2018YFA0209301), National Natural Science Foundation of China (21425309, 21761132002, 21703040 and 21861130353), China Postdoctoral Science Foundation (2017M622051 and 2018T110639) and 111 Project.

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Synthesis of Polymeric Carbon Nitride Films with Adhesive Interfaces

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