A pure organic heterostructure of μ-oxo dimeric iron(III) porphyrin and graphitic-C₃N₄ for solar H₂ roduction from water

Abstract

Due to the two-dimensional flexible structure and abundant pendant amine, graphitic-C₃N₄ (g-C₃N₄) may be easily modified by organic molecules as a promising photocatalyst for solar H₂ production from water. Here, through a simple liquid chemical reaction between g-C₃N₄ and the precursor of μ-oxo dimeric iron(III) porphyrin [(FeTPP)₂O], we provide a novel route to construct a pure organic heterostructure of g-C₃N₄/(FeTPP)₂O on the basis of the π–π and the Fe–amine interactions. The experimental results demonstrated that (FeTPP)₂O acted not just as a photosensitizer, but also played the role of a charge promotor to prohibit the recombination of the excited electrons and holes of g-C₃N₄. As compared to pure or mixed g-C₃N₄ and/or (FeTPP)₂O, the obtained pure organic g-C₃N₄/(FeTPP)₂O heterostructure exhibited dramatic photocatalytic H₂ production under solar light without any cocatalysts.

---

Introduction

The increasing worldwide energy shortage and environmental issues have stimulated intensive research on searching for green technologies as sustainable ways to address these concerns. Among various potential solutions, solar H₂ production from water splitting offers an environmentally clean energy for the future and exhibits a method for solar energy storage and chemical energy conversion.^1–3 One ideal route for H₂ evolution is semiconductor-based water splitting under solar light irradiation.^4–7 The most challenging task in photocatalytic water splitting is to develop efficient and stable photocatalysts, which are capable of sufficiently absorbing solar light for splitting water. At present, many semiconductors have been widely studied, mainly including inorganic materials such as metal oxides, metal sulfides and metal nitrides.^7–10

g-C₃N₄, a new metal-free polymeric semiconductor,^11,12 stands out from a mass of photocatalysts as a shining star due to its appealing electronic structure with a medium band gap for both water reduction and oxidation, and good chemical and physical stability.¹³ Especially, the polymeric semiconductor is only composed of C and N elements, and can be easily prepared from low-cost and environmentally friendly precursors,^14–17 such as urea,¹⁵ thiourea¹⁶ and melamine.^17,18 The adjustable photocatalytic activity in visible light combining with its low cost make it potentially useful in a variety of applications.¹⁹ The utilization of g-C₃N₄ as a photocatalyst for hydrogen evolution has been intensively studied in recent years, nevertheless, the photocatalytic efficiency of pristine g-C₃N₄ is still not satisfactory due to the rapid recombination of photo-excited electron–hole pairs, low visible utilization efficiency and small surface area.²⁰ To break these limitations, many strategies such as textural managing,^22–24 doping,^25,26 heterostructure construction,^20,27 dye sensitization,^28,29etc., have been developed.^21–29 Among them, the formation of heterostructures holds great potential to separate the electron–hole pairs because it is helpful for the charge transfer across the interface of the heterostructure and restrains the recombination of electrons and holes.^20,30 Now, semiconductor heterostructures are commonly applied in inorganic/inorganic and inorganic/organic semiconductors, but seldom observed in an organic/organic system.^20,30,31 For organic semiconductors, Coulomb binding energies are observed in the range of several hundred meV for the Frenkel exciton, which is higher as compared to that for the Wannier exciton in inorganic semiconductors.^32,33 Thus, the fabrication and study of pure organic semiconductor heterostructures as photocatalysts, for example the g-C₃N₄–P3HT polymer composite^34,35 and CNS–CN heterojunction,³⁶ is very important in order to more efficiently avoid the rapid recombination of photo-generated holes and electrons.

In fact, g-C₃N₄ may be easily modified with organic small molecules as a promising photocatalyst due to its two-dimensional flexible structure and abundant pendant amine. Here, (FeTPP)₂O was chosen to construct a pure organic heterostructure with g-C₃N₄ due to its strong absorption bands in the visible region, large π-conjugated aromatic system and outstanding chemical and thermal stability.³⁷ Furthermore, the (FeTPP)₂O is found to be an active centre in many enzymes, which has proven to be the versatile, potential catalyst for the oxidation of hydrocarbons in air in the absence of co-reductants or solvents.³⁸ Here, through a simple liquid chemical reaction between g-C₃N₄ and a precursor of (FeTPP)₂O, we provide a novel route to construct the heterostructure between g-C₃N₄ and organic small molecules on the basis of the π–π and the Fe–amine interactions. Unlike the similar reported several organic dye-sensitized C₃N₄ systems,^39–41 in which they do not efficiently work without cocatalysts, (FeTPP)₂O in our composite is not only a photosensitizer for wider photon absorption, but also obviously promoted the separation of the photo-induced electrons and holes and the production of photocatalytic H₂ in the absence of a cocatalyst.

Results and discussions

The composite of g-C₃N₄/(FeTPP)₂O was synthesized on the basis of the reduction of tetraphenyl iron(III) porphyrin perchlorate with g-C₃N₄ dispersion (see Experimental section). In order to prove the existence of (FeTPP)₂O in as-obtained samples, firstly, a sample was dissolved in dichloromethane (CH₂Cl₂) to obtain a yellow-green solution via filtration. Then, the UV-vis absorption spectrum and MALDI-TOF Mass Spectrometry (MS) were used to measure the solution, and the results are shown in Fig. S1.† The UV-vis absorption spectrum in Fig. S1a† shows three major absorption bands with λ_max values at 407 nm (the Soret band), 570 nm (Q-band) and 611 nm (Q-band), which belong to typical μ-oxo dimeric iron(III) porphyrin [(FeTPP)₂O].³⁸ The MS of the solution in Fig. S1B† exhibits the existence of the ion of (FeTPP)⁺, which is from the ionic fragment of (FeTPP)₂O. In addition, pure (FeTPP)₂O was also prepared by replacing C₃N₄ dispersion with pure water. The presence of the typical stretching mode of Fe–O–Fe at 870 and 895 cm⁻¹ further proves the dimeric μ-oxo iron(III) porphyrin structure,³⁸ as shown in Fig. S2.†

The Fourier transform infrared spectroscopy (FTIR) spectra of pristine g-C₃N₄, pure (FeTPP)₂O and a series of g-C₃N₄/(FeTPP)₂O composites are obtained and shown in Fig. S3.† For the pristine g-C₃N₄, the intense band at 812.1 cm⁻¹ represents the out-of-plane bending vibration characteristic of heptazine rings. The bands from 1000–1750 cm⁻¹ are from the stretching and bending modes of nitrogen containing heterocycles. The broad feature at 3250 cm⁻¹ is usually assigned to the stretching modes of –NH– groups.^42,43 Unfortunately, FTIR spectra in Fig. S3† exhibits the typical bands of g-C₃N₄, however no noticeable trace of (FeTPP)₂O can be found on account of the low loading amount of (FeTPP)₂O (lower than 10 wt%). Based on the above experimental results and analysis, the existence of (FeTPP)₂O and g-C₃N₄ in as-obtained samples were proved, which indicates the successful synthesis of the composite of g-C₃N₄/(FeTPP)₂O in this work.

To clarify whether the heterostructure is successfully formed in the composite of g-C₃N₄/(FeTPP)₂O, the optical properties of the above-mentioned materials were studied. Fig. 1a shows the UV-vis diffuse reflectance spectroscopy (DRS) of g-C₃N₄/(FeTPP)₂O composites, pure g-C₃N₄ and (FeTPP)₂O. The absorption peak of sole g-C₃N₄ is up to ∼450 nm, and has a small absorption tail which is probably due to n–π* transitions.⁴⁴ The (FeTPP)₂O has strong absorption in the visible region from 400 to 700 nm, mainly including two Q bands around 567 nm and 613 nm, which are similar to that of the (FeTPP)₂O solution (Fig. S1†). The g-C₃N₄/(FeTPP)₂O composites show the absorption features combining g-C₃N₄ and (FeTPP)₂O, and exhibit wider absorption of solar light. As the content of g-C₃N₄ increased, the obvious red-shift of the Q band of (FeTPP)₂O in composites can be found, which suggests π–π stacking interactions between g-C₃N₄ and (FeTPP)₂O.^45,46

Fig. 1 (a) UV-vis diffuse reflection spectra of g-C₃N₄, (FeTPP)₂O and g-C₃N₄/(FeTPP)₂O composites with different (FeTPP)₂O contents (3.3 wt%, 5 wt% and 6.6 wt%); (b) photoluminescence spectra (excited at 400 nm) of g-C₃N₄, (FeTPP)₂O and g-C₃N₄/(FeTPP)₂O dispersion (1 mg mL⁻¹, water was used as the solvent), (c) XRD patterns, and (d) nitrogen adsorption–desorption isotherms with the corresponding pore size distribution curves (inset) for pure g-C₃N₄ and g-C₃N₄/(FeTPP)₂O (5 wt%).

Photoluminescence (PL) is an effective and commonly used method to investigate the electron transfer property of semiconductor materials.^45,46Fig. 1b displays the PL spectra of g-C₃N₄, pure (FeTPP)₂O and g-C₃N₄/(FeTPP)₂O nanocomposites. Obviously, g-C₃N₄ has a strong and wide emission, in contrast, (FeTPP)₂O has negligible PL signal response when the excited wavelength is 400 nm. The emission intensity of g-C₃N₄ remarkably decreased after it was combined with (FeTPP)₂O. It means that the recombination of photo-excited holes and electrons of g-C₃N₄ is largely prohibited by (FeTPP)₂O and that the charge transfer occurs in the composites. The big red shift of PL spectra of g-C₃N₄ in these composites also demonstrates the existence of interaction between g-C₃N₄ and (FeTPP)₂O. On the basis of the above optical properties, it was guessed that their heterojunctions have been successfully prepared and the charge transfer may occur between them.

Wide-angle X-ray diffraction (XRD) patterns also confirm the presence of g-C₃N₄ in as-prepared samples. As shown in Fig. 1c, XRD patterns of g-C₃N₄ with different (FeTPP)₂O contents exhibit a typical diffraction peak at 27.41°, which corresponds to the interlayer stacking of aromatic segments and indexed as (002). As compared to pure g-C₃N₄, no obvious (FeTPP)₂O characteristic peaks can be found, indicating that g-C₃N₄/(FeTPP)₂O retains the original crystal structure of g-C₃N₄. The same conclusion can also be made from BET experiments. The nitrogen adsorption–desorption isotherms of pure g-C₃N₄ and (FeTPP)₂O-mediated g-C₃N₄ show type IV with hysteresis loops, indicating the presence of mesopores (Fig. 1d). The specific surface areas of pure g-C₃N₄ and g-C₃N₄/(FeTPP)₂O (5.0 wt%) composites are 74 and 64 cm² g⁻¹, respectively. The inserted image in Fig. 1d indicates the similar mesopore structure between pure g-C₃N₄ and g-C₃N₄/(FeTPP)₂O. These data suggest that the introduction of (FeTPP)₂O into g-C₃N₄ lowers the specific surface areas of g-C₃N₄, however, it does not have any obvious effect on the mesopore structure of g-C₃N₄, which is closely related to its photocatalytic activity.

In order to further investigate the components and structure of g-C₃N₄/(FeTPP)₂O composites, scanning electron microscopy (SEM, JSM-6700F, JEOL, Inc.) and transmission electron microscopy (TEM, F20, FEI) were used. The corresponding images are shown in Fig. 2. As compared to g-C₃N₄ shown in Fig. S4,† which has smooth folded nanosheets besides some fragments, (FeTPP)₂O in Fig. S5† is composed of a lot of aggregations of amorphous nanoparticles (NPs) or naonoplates. After (FeTPP)₂O was introduced into the as-obtained g-C₃N₄, many small NPs can be found on g-C₃N₄ sheets and quite a lot of aggregations are attached on g-C₃N₄, as shown in Fig. 2a and b. The TEM image in Fig. 2b exhibits that the g-C₃N₄/(FeTPP)₂O heterostructure still retains the morphology of g-C₃N₄. Fig. 2c clearly displays a typical layer-by-layer structure of g-C₃N₄ according to the difference of image contrast, and the inserted selected area electron diffraction (SAED) image indicates a low crystallinity of g-C₃N₄ in the composite. A clue to the existence of (FeTPP)₂O can be found in the TEM image shown in Fig. 2d. In order to further prove the existence of (FeTPP)₂O, the corresponding energy dispersive X-ray spectroscopy (EDS) was also performed on the sample. The result in Fig. 2d shows that Fe–K peaks exist in the spectrum and their percent content is calculated to be ∼0.11 at%, which is close to the content in theory (5 wt%). At the same region of the sample, TEM-assisted elemental mappings were carried out and the results are shown in Fig. 2e. The Fe–K mapping in Fig. 2e further indicates that the NPs of (FeTPP)₂O exist and they should be equably distributed on the surface of g-C₃N₄ nanosheets. The above results further demonstrate the successful formation of the g-C₃N₄/(FeTPP)₂O heterostructure and the intimate contact between g-C₃N₄ and (FeTPP)₂O. Interestingly, although at least two obvious NPs can be found in the orange square in Fig. 2e, as compared to Fig. 2d, only one (FeTPP)₂O nanoparticle can be found in the Fe–K mapping. It means that most of the NPs or nanoplates should be fragments of g-C₃N₄ as a result of the reaction between g-C₃N₄ and the precursor of (FeTPP)₂O.

Fig. 2 (a) SEM, (b) TEM, (c) HRTEM images and SAED pattern, (d) TEM images and the corresponding EDS (right), and (e) TEM images and the corresponding mapping images (right) of the g-C₃N₄/(FeTPP)₂O heterostructure including 5 wt% (FeTPP)₂O.

To further understand the interaction of the two components, X-ray photoelectron spectroscopy (XPS) was performed on these samples. The binding energies of Fe 2p_3/2 of (FeTPP)₂O and g-C₃N₄/(FeTPP)₂O are 711.5 eV and 711.1 eV (Fig. 3a), respectively, the decrease of 0.4 eV in the binding energy of Fe 2p_3/2 might be caused by the interaction between (FeTPP)₂O and g-C₃N₄, which indicates the formation of an Fe–N bond because the N element has lower electron affinity than O.⁴⁷ The binding energy of Fe 2p_1/2 also has the same tendency. To prove the interaction, N 1s XPS spectra of pure g-C₃N₄ and g-C₃N₄/(FeTPP)₂O were also measured and compared, and the results are exhibited in Fig. 3b. Three peaks at ca. 398.3, 399.4 and 400.9 eV are observed for single g-C₃N₄. The main N 1s peak at 398.3 eV corresponds to the sp² hybridized aromatic N bonded to carbon atoms (C–N–C). The peak at 399.4 eV is assigned to the tertiary N bonded to carbon atoms in the form of N–(C)3 or H–N–(C)2. And the weak peak with a high binding energy at 400.9 eV is attributed to the quaternary N bonded to three carbon atoms in the aromatic cycles.^15,42,43 After forming the heterostructure with (FeTPP)₂O, all the N 1s peaks of g-C₃N₄ shift to a higher binding energy, corresponding to the decrease of the Fe 2p binding energy. Apparently, a strong interaction between (FeTPP)₂O and g-C₃N₄ is built through the formation of the Fe–N bond.⁴⁸ As compared to the relative atomic concentration of Fe (1.29 at%) in the pure (FeTPP)₂O composite shown in Fig. 3c, that of the g-C₃N₄/(FeTPP)₂O composite is 0.21 atom% estimated from the XPS data. The result is higher than that from the EDS and our theoretical data because the XPS data are usually collected from the sample surface. The above XPS data indicate that in the as-obtained samples, (FeTPP)₂O is on the surface of g-C₃N₄ and they form the heterostructure through π–π and the Fe–amine interactions.

Fig. 3 (a) Fe 2p XPS spectra of pure (FeTPP)₂O and g-C₃N₄/(FeTPP)₂O; (b) N 1s XPS spectra of pure g-C₃N₄ and the g-C₃N₄/(FeTPP)₂O heterostructure; (c) and (d) XPS spectra of pure (FeTPP)₂O and g-C₃N₄/(FeTPP)₂O.

Between g-C₃N₄ and (FeTPP)₂O, the charge transfer has been revealed in the PL spectra, the SEM, TEM and XPS data have also demonstrated the formation of the heterostructure. However, only the relative band positions of the two components can tell us the direction of charge transfer and the type of the heterojunction. Based on the UV-vis diffuse reflectance data, the band-gap energy was calculated according to Fig. 4a and c. The band gap of g-C₃N₄ and (FeTPP)₂O are 2.73 eV and 2.09 eV, respectively. The valence band XPS (VB XPS) spectra of g-C₃N₄ and (FeTPP)₂O were measured, as shown in Fig. 4b and d, the valence band (VB) of (FeTPP)₂O is more negative about 1.56 eV than that of g-C₃N₄. Together with their band gaps we can deduce that the conduction band (CB) of (FeTPP)₂O is more negative about 0.96 eV than that of g-C₃N₄. As illustrated in Scheme 1, the band alignment of g-C₃N₄ and (FeTPP)₂O forms the typical type II heterojunction.^49,50 The large CB and VB offsets between g-C₃N₄ and (FeTPP)₂O make the photo-generated electrons transfer from (FeTPP)₂O to g-C₃N₄, while the photo-generated holes migrate from g-C₃N₄ to (FeTPP)₂O in a thermodynamically favorable manner, which leads to an efficient charge separation and significant enhancement in photocatalytic activity.^36,50

Fig. 4 (a) Band-gap evaluation from the plots of (αE_photon)²vs. the energy of the absorbed light E_photon for pristine g-C₃N₄; (b) VB XPS spectrum of pristine g-C₃N₄; (c) band-gap evaluation from the plots of (αE_photon)²vs. the energy of the absorbed light E_photon for pure (FeTPP)₂O; (d) VB XPS spectrum of pure (FeTPP)₂O.

Scheme 1 The band-structure diagram of the g-C₃N₄/(FeTPP)₂O heterostructure.

Solar-light-induced H₂ production was investigated to examine the photocatalytic activity of the as-prepared samples. Fig. 5a shows the varied amount of H₂ evolution under solar light for various photocatalysts. Almost no H₂ can be detected when only g-C₃N₄ was used as the photocatalyst. It may be due to the detrimental electron-hole recombination and the Frenkel exciton effect. In contrast, the g-C₃N₄/(FeTPP)₂O composite exhibited dramatic photocatalytic H₂ production. Here, the loading amount of (FeTPP)₂O plays an important role in tailoring the photocatalytic activity of the heterostructures. With the content of (FeTPP)₂O increased, the amount of H₂ first increases and then decreases. The experiments in Fig. 5 show that the optimal amount of (FeTPP)₂O is about 5.0 wt%, and the best sample produces 59.2 μmol H₂ under solar light irradiation for 4 h. As we know, for the photocatalytic activity of the heterostructure nanocomposites, two factors have a serious effect on the hydrogen evolution. One is the capability of light absorption, the other is the amount of activity sites on the surface of the composite. In our system, the (FeTPP)₂O plays a dual role of dye-sensitization and charge transfer for g-C₃N₄. In some ranges, more doping of the guest means the formation of more heterojunctions (activity sites) and stronger absorption in visible-light. With the further increase of doping beyond some amount, however, surface activity sites and/or absorption actually decrease. Therefore, there is always an optimal doping ratio. As far as the g-C₃N₄/(FeTPP)₂O composite is concerned, the decrease of its specific surface areas with the increase of the amount of (FeTPP)₂O (see Fig. 1d) means that the optimal ratio of (FeTPP)₂O in the heterostructure is determined by its visible-light absorption. UV-vis DRS in Fig. 1a clearly shows that the ratio should be ∼5.0 wt%, which is consistent with our experimental results in Fig. 5.

Fig. 5 Under solar light, (a) time course of photocatalytic hydrogen evolution over g-C₃N₄ and g-C₃N₄/(FeTPP)₂O heterostructures with different contents of (FeTPP)₂O (3.3 wt%, 5 wt% and 6.6 wt%); (b) time course of photocatalytic hydrogen evolution over C₃N₄/(FeTPP)₂O heterostructures (5 wt%) in the presence of various electron donors; (c) photocatalytic H₂ production rates over pure (FeTPP)₂O, the g-C₃N₄ + (FeTPP)₂O (5 wt%) mixture and g-C₃N₄/(FeTPP)₂O (5 wt%); (d) photocatalytic H₂ production rates over commercial (FeTPP)₂O, the g-C₃N₄/commercial (FeTPP)₂O) mixture and g-C₃N₄/(FeTPP)₂O heterostructure (5 wt%).

Furthermore, it was found that sacrificial electron donors are also essential for the consumption of photo-generated holes and the regeneration of (FeTPP)₂O. As shown in Fig. 5b, without any electron donors, no H₂ is produced. The presence of ascorbic acid (AA) also has little effect on promoting the activity of g-C₃N₄/(FeTPP)₂O heterostructures, while the addition of triethanolamine (TEOA) significantly enhances the photocatalytic H₂ production. The reason must be ascribed to the highest redox potential and the created basic circumstance of TEOA.⁵¹ Apparently, sole (FeTPP)₂O shows no activity toward water splitting, as shown in Fig. 5c. It must be noted that the physical mixture of g-C₃N₄ and (FeTPP)₂O failed to produce the heterostructure, thus the mixture also shows negligible photocatalytic activity.⁵² In addition, commercial (FeTPP)₂O was also thought to combine with g-C₃N₄ in a physical manner. Through impregnation of g-C₃N₄ with a dichloromethane solution of commercial (FeTPP)₂O overnight, the composite of g-C₃N₄/(FeTPP)₂O was prepared. However, Fig. 5d shows that the corresponding composite also exhibits negligible H₂ production. Therefore, the formation the Fe–N bond between g-C₃N₄ and (FeTPP)₂O should play a key role in the solar H₂ production from water splitting.

Hydrogen evolution measurement under UV-vis light irradiation with a 300 W Xenon lamp was also investigated. The durability of g-C₃N₄/(FeTPP)₂O (5.0 wt%) acting as the photocatalyst for H₂ evolution was evaluated by three consecutive operations. As shown in Fig. S6,† UV-vis-light can stimulate the photocatalyst to produce H₂ at a rate of ∼40 μmol h⁻¹ and the amount of H₂ increases with irradiation time. Followed by the first run, some deactivation was observed in the second run. Interestingly, after the second cycle and lighting off for several hours, some photocatalytic activity was recovered in the third cycle. The deactivation should have occurred due to the non-renewable consumption of the (FeTPP)₂O in the photocatalytic process,⁴⁷ while the recovery is ascribed to the partial regeneration of (FeTPP)₂O with TEOA in the dark. In addition, under visible light (λ > 420 nm) generated by using an Xenon lamp with a UV-cutoff filter, H₂ evolution is 11 μmol h⁻¹ (see Fig. S7†). At 420 nm, a quantum efficiency (QE) of 0.0415% was obtained.⁵³

Conclusions

In conclusion, through a simple liquid chemical reaction between g-C₃N₄ and the precursor of (FeTPP)₂O, we presented a novel route to construct a pure organic heterostructure of g-C₃N₄/(FeTPP)₂O as a photocatalyst for solar H₂ production from water splitting. The (FeTPP)₂O was found attached to g-C₃N₄ through the π–π interaction and Fe–N bond. The composite exhibited a significant enhancement in photocatalytic activity due to efficient light utilization and the charge separation which arises from the band offsets. Besides the loading amount of (FeTPP)₂O and the type of sacrificial electron donors, it was found that the formation of Fe–amine interactions between g-C₃N₄ and (FeTPP)₂O should play a key role in the solar H₂ production from water splitting. A further study on the effect of the Fe–N bond between g-C₃N₄ and (FeTPP)₂O on photocatalytic activity is in progress. In a word, an efficient chemical path to significantly promote light harvesting and the charge transfer of g-C₃N₄ was achieved through the construction of a heterostructure between organic small molecules and g-C₃N₄. This work is expected to promote the research and application of g-C₃N₄ for solar H₂ production in the absence of cocatalysts.

Experimental

Materials

Silver perchlorate, tetraphenyl iron(III) porphyrin chloride (FeTPPCl), anhydrous dichloromethane and acetonitrile (water < 30 ppm) were purchased from J&K Chemicals Co. Ultrapure water with a resistivity of 18.2 MΩ cm was produced using a Water Purifier apparatus (WP-UP-IV-20). Urea (10 g, AR) was obtained from Sinopharm Chemical Reagent Co., China.

Synthesis of g-C₃N₄

Graphitic carbon nitride (g-C₃N₄) was synthesized by thermal treatment of urea (10 g, AR) in a lidded high quality alumina crucible covered with aluminium foil under ambient pressure in air. The crucible was placed in a high temperature box furnace (KSL-1100X-S) and heated to 600 °C for 3.0 h, and then raised to 650 °C for 0.5 h to complete the reaction. The synthesized yellow mass was crushed to powder (330 mg) for further experiments.

Synthesis of tetraphenyl iron(III) porphyrin perchlorate

A precursor of tetraphenyl iron(III) porphyrin perchlorate was produced via reaction (1):

FeTPPCl + AgClO₄ = AgCl + FeTPP⁺·ClO₄⁺ (1)

In a typical synthesis, a solution of dry silver perchlorate in a minimum amount of anhydrous acetonitrile (0.6 mL, 0.30 mM) was added into a 16 mL solution of 0.27 mM FeTPPCl in anhydrous dichloromethane with stirring. After stirring for about 3 h, the mixture was separated by centrifuging at 10 000 rpm for 10 min. Relying on the difference in solubility between silver chloride and FeTPPClO₄ in acetonitrile, a brown FeTPPClO₄ solution is separated from the admixture. The solution was poured into 50 mL n-hexane and placed in a fridge at −22 °C for 24 h. Then the precipitation was obtained through centrifuging at 10 000 rpm for 2 min and further washed with n-hexane two times. The as-obtained product was dried for further use.

Synthesis of C₃N₄/(FeTPP)₂O (5 wt%) nanocomposites

Typically, g-C₃N₄ (80 mg) powder was added into 100 mL of ultrapure water and sonicated at 800 W for 2 h. 4 mg of FeTPPClO₄ was dissolved in 8 mL of anhydrous acetonitrile. Under stirring, the solution of FeTPPClO₄ was added dropwise into the C₃N₄ dispersion, and kept stirring for 1.5 h. The composite was obtained through vacuum filtration and washed twice with ultrapure water.

Characterization

FESEM images were recorded with a Hitachi SU8010 instrument. Samples casted on a copper grid were observed via a transmission electron microscope (TEM, F20). The UV-vis absorption spectra were obtained on a Perkin Elmer Lambda 950 UV-vis spectrophotometer equipped with an integrating sphere. BaSO₄ was used as a reflectance standard in the diffuse reflectance experiments. FT-IR spectra were recorded on a VERTEX70 FTIR spectrometer when samples were embedded in KBr pellets. Photoluminescence (PL) spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer. X-Ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific ESCALAB 250Xi system. X-ray powder diffraction patterns were recorded with a Rigaku MiniFlex II diffractometer with a Cu sealed tube. N₂ physisorption isotherms of g-C₃N₄ and g-C₃N₄/(FeTPP)₂O were measured using a Micromeritics ASAP 2020 surface area and porosimetry analyser.

Photocatalytic activity measurements

Photocatalytic water splitting reactions were carried out in a Pyrex top-irradiation reaction vessel connected to a closed glass gas circulation system (Labsolar III AG, Beijing Perfectlight Technology Co. Ltd). 80 mg of C₃N₄, C₃N₄/(FeTPP)₂O composites or 4 mg of (FeTPP)₂O was dispersed in 90 mL of ultrapure water by sonication, and then 10 mL of TEOA was added into it. The reaction system was evacuated for 30 min to remove the dissolved gases in water prior to irradiation under a 300 W Xe lamp (PLS-SXE 300). A continuous magnetic stirrer was applied at the bottom of the reactor in order to keep the photocatalytic particles suspended in water during the whole experiment. The wavelength of the incident light was controlled by using a solar simulator filter for solar light irradiation. The temperature of the reactant solution was maintained at room temperature by a flow of cooling water during the reaction. The evolved gases were analyzed by gas chromatography (GC 7900, Shanghai Techcomp Instrument Ltd.). H₂ evolution under visible light (λ > 420 nm) was measured by using a 300 W Xe lamp combined with a UV-cutoff filter. The quantum efficiency (QE) was obtained by applying a Xe lamp (300 W) with a 420 nm bandpass filter. The number of incident photons was measured using a radiant power energy meter (MC UV-A). The QE was calculated using the following equation:

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21252001, 21473204), the Special Project of National Major Scientific Equipment Development of China (No. 2012YQ120060), the Natural Science Foundation of Fujian Province (2015J01070), and the Science and Technology Planning Project of Fujian Province, Grant No. 2014H2008.

Notes and references

1. A. J. Bard and M. A. Fox, Acc. Chem. Res., 1995, 28, 141.
2. S. S. Penner, Energy, 2006, 31, 33; A. J. Esswein and D. G. Nocera, Chem. Rev., 2007, 107, 4022.
3. K. Maeda and K. Domen, J. Phys. Chem. Lett., 2010, 1, 2655.
4. R. F. Service, Science, 2007, 315, 789.
5. H. G. Kim, D. W. Hwang and J. S. Lee, J. Am. Chem. Soc., 2004, 126, 8912.
6. K. Maeda and K. Domen, J. Phys. Chem. C, 2007, 111, 7851.
7. X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503.
8. F. E. Osterloh, Chem. Mater., 2008, 20, 35.
9. K. Maeda and K. Domen, Chem. Mater., 2010, 22, 612.
10. F. E. Osterloh, Chem. Soc. Rev., 2013, 42, 2294.
11. X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76.
12. S. W. Cao, J. X. Low, J. G. Yu and M. Jaroniec, Adv. Mater., 2015, 27, 2150.
13. Y. Zheng, J. Liu, J. Liang, M. Jaroniec and S. Z. Qiao, Energy Environ. Sci., 2012, 5, 6717.
14. A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J. O. Müller, R. Schlögl and J. M. Carlsson, J. Mater. Chem., 2008, 18, 4893.
15. F. Dong, L. W. Wu, Y. J. Sun, M. Fu, Z. B. Wu and S. C. Lee, J. Mater. Chem., 2011, 21, 15171.
16. Y. W. Zhang, J. H. Liu, G. Wu and W. Chen, Nanoscale, 2012, 4, 5300.
17. H. J. Yan, Y. Chen and S. M. Xu, Int. J. Hydrogen Energy, 2012, 37, 125.
18. Y. J. Zhong, Z. Q. Wang, J. Y. Feng, S. C. Yan, H. T. Zhang, Z. S. Li and Z. G. Zou, Appl. Surf. Sci., 2014, 295, 253.
19. Y. Wang, X. C. Wang and M. Antonietti, Angew. Chem., Int. Ed., 2012, 51, 68.
20. Z. W. Zhao, Y. J. Sun and F. Dong, Nanoscale, 2015, 7, 15.
21. S. Cao and J. Yu, J. Phys. Chem. Lett., 2014, 5, 2101.
22. X. C. Wang, K. Maeda, X. F. Chen, K. Takanabe, K. Domen, Y. D. Hou, X. Z. Fu and M. Antonietti, J. Am. Chem. Soc., 2009, 131, 1680.
23. J. S. Zhang, M. W. Zhang, C. Yang and X. C. Wang, Adv. Mater., 2014, 26, 4121.
24. J. H. Sun, J. S. Zhang, M. W. Zhang, M. Antonietti, X. Z. Fu and X. C. Wang, Nat. Commun., 2012, 3, 1139.
25. Z. Z. Lin and X. C. Wang, Angew. Chem., Int. Ed., 2013, 52, 1735.
26. G. G. Zhang, M. W. Zhang, X. X. Ye, X. Q. Qiu, S. Lin and X. C. Wang, Adv. Mater., 2014, 26, 805.
27. L. Ge, F. Zuo, J. K. Liu, Q. Ma, C. Wang, D. Z. Sun, L. Bartels and P. Y. Feng, J. Phys. Chem. C, 2012, 116, 13708.
28. Y. F. Zhang, F. Mao, H. J. Yan, K. W. Liu, H. M. Cao, J. G. Wu and D. Q. Xiao, J. Mater. Chem. A, 2015, 3, 109.
29. X. H. Zhang, T. Y. Peng, L. J. Yu, R. J. Li, Q. Q. Li and Z. Li, ACS Catal., 2015, 5, 504.
30. H. L. Wang, L. S. Zhang, Z. G. Chen, J. Q. Hu, S. J. Li, Z. H. Wang, J. S. Liu and X. C. Wang, Chem. Soc. Rev., 2014, 43, 5234.
31. S. J. A. Moniz, S. A. Shevlin, D. J. Martin, Z. X. Guo and J. W. Tang, Energy Environ. Sci., 2015, 8, 731.
32. M. Muntwiler, Q. Yang and X. Zhu, J. Electron Spectrosc. Relat. Phenom., 2009, 174, 116.
33. C. Piliego and M. A. Loi, J. Mater. Chem., 2012, 22, 4141.
34. H. J. Yan and Y. Huang, Chem. Commun., 2011, 47, 4168.
35. X. J. Bai, C. P. Sun, S. L. Wu and Y. F. Zhu, J. Mater. Chem. A, 2015, 3, 2741.
36. J. S. Zhang, M. W. Zhang, R. Q. Sun and X. C. Wang, Angew. Chem., 2012, 124, 10292.
37. T. Higashino and H. Imahori, Dalton Trans., 2015, 44, 448.
38. Q. Liu, Y. Z. Gong, C. J. Gong, Q. H. Li and C. C. Guo, J. Porphyrins Phthalocyanines, 2009, 13, 854.
39. K. Takanabe, K. Kamata, X. C. Wang, M. Antonietti, J. Kubota and K. Domen, Phys. Chem. Chem. Phys., 2010, 12, 13020.
40. J. Y. Xu, Y. X. Li, S. Q. Peng, G. X. Lu and S. B. Li, Phys. Chem. Chem. Phys., 2013, 15, 7657.
41. L. J. Yu, X. H. Zhang, C. S. Zhuang, L. Lin, R. J. Li and T. Y. Peng, Phys. Chem. Chem. Phys., 2014, 16, 4106.
42. J. H. Liu, T. K. Zhang, Z. C. Wang, G. Dawson and W. Chen, J. Mater. Chem., 2011, 21, 14398.
43. D. J. Martin, K. P. Qiu, S. A. Shevlin, A. D. Handoko, X. W. Chen, Z. X. Guo and J. W. Tang, Angew. Chem., Int. Ed., 2014, 53, 9240.
44. A. B. Jorge, D. J. Martin, M. T. S. Dhanoa, A. S. Rahman, N. Makwana, J. W. Tang, A. Sella, F. Corà, S. Firth, J. A. Darr and P. F. McMillan, J. Phys. Chem. C, 2013, 117, 7178.
45. D. M. Chen, K. W. Wang, W. Z. Hong, R. L. Zong, W. Q. Yao and Y. F. Zhu, Appl. Catal., B, 2015, 166, 366.
46. M. S. Zhu, Z. Li, B. Xiao, Y. T. Lu, Y. K. Du, P. Yang and X. M. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 1732.
47. D. D. Wang, J. Huang, X. Li, P. Yang, Y. K. Du, C. M. Goh and C. Lu, J. Mater. Chem. A, 2015, 3, 4195.
48. D. H. Wang, L. Jia, X. L. Wu, L. Q. Lu and A. W. Xu, Nanoscale, 2012, 4, 576.
49. S. S. Chen, Y. Qi, T. Hisatomi, Q. Ding, T. Asai, Z. Li, S. S. K. Ma, F. X. Zhang, K. Domen and C. Li, Angew. Chem., Int. Ed., 2015, 29, 8498–8501.
50. M. R. Gholipour, C. T. Dinh, F. Bélandb and T. O. Do, Nanoscale, 2015, 7, 8187.
51. X. H. Zhang, L. J. Yu, C. S. Zhuang, T. Y. Peng, R. J. Li and X. G. Li, ACS Catal., 2014, 4, 162.
52. S. W. Cao, Y. P. Yuan, J. Barber, S. C. J. Loo and C. Xue, Appl. Surf. Sci., 2014, 319, 344.
53. The measurement of quantum efficiency at 420 nm: k = 1.01 × 10⁻⁹ mol s⁻¹, k is numbers of H₂ evolution per second. q_p = p × λ/(h × c), in which q_p is the number of photons per second, P is illumination intensity (1.387 J s⁻¹), λ is incident wavelength of 420 nm, c is the speed of light (3 × 10⁸ m s⁻¹), h is Planck’s constant (6.626 × 10⁻³⁴ J s⁻¹). Thus, QE = 2k/q_p = (2 × 1.01 × 10⁻⁹ mol s⁻¹)/(4.87 × 10⁻⁶ mol s⁻¹) = 0.0415%.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ta07278f

---