Synthesis of graphene-like g-C₃N₄/Fe₃O₄ nanocomposites with high photocatalytic activity and applications in drug delivery

Abstract

Graphene-like g-C₃N₄ nanosheet (GCN)/Fe₃O₄ quantum dot (QD) nanocomposites were successfully synthesized by a facile electrostatic self-assembly method. Characterization shows that the GCN is at least several micrometers in size. The GCN/Fe₃O₄ nanocomposites were used as photocatalysts for degradation of Rhodamine B (RhB) under visible light irradiation. After irradiation for 1.5 h, the degradation efficiency was 72.5% for pure g-C₃N₄, 81% for GCN-1 wt% Fe₃O₄, 95% for GCN-2 wt% Fe₃O₄, 60.46% for GCN-3 wt% Fe₃O₄ and 57.2% for GCN-4 wt% Fe₃O₄, indicating that GCN-2 wt% Fe₃O₄ nanocomposites had the highest photocatalytic activity. We deduce that the efficient separation of the photogenerated electron–hole pairs and the high specific surface area of GCN play important roles in the photocatalytic activity of the nanocomposites. In addition, the nanocomposites can be loaded with a model drug (Rhodamine B) and the loading capacity was as high as 108.6 mg g⁻¹, making it a potential candidate for photocatalysis and controlled magnetically targeted drug delivery.

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1 Introduction

Graphite-like carbon nitride (g-C₃N₄), a novel metal-free direct bandgap semiconductor with a bandgap of ca. 2.7 eV, has attracted intense interest for its promising applications in photocatalysis, electrocatalysis, bioimaging and biomedical applications, etc.^1–5 A large amount of studies have reported its ability for organic dye degradation and hydrogen or oxygen production from water splitting under visible light irradiation,^6,7 and more potential properties of g-C₃N₄ are improving rapidly.^3,8–10

Recently, many researches on ultrathin sheet-like nanostructures^11–14 have been conducted since the discovery of graphene (GR) which possesses many unique and inspiring properties different from bulk graphite. In particular, GR–semiconductor nanocomposites show excellent performance in photocatalysis, and GR–magnetic nanoparticle (NP) composite materials show outstanding properties in drug delivery and biosensing.^15,16 So, motivated by the unique nature of graphene in its ultrathin sheet-like nanostructure, researchers have tried to synthesise the graphene-like g-C₃N₄ to obtain similar properties and applications to graphene.^17,18

Different from graphite, the g-C₃N₄ layer is composed of C–N bonds instead of C–C bonds, and there are weak van der Waal’s forces between the layers. Many groups have tried to synthesise graphene-like ultrathin g-C₃N₄ nanosheets. Based on the experience of graphene, Liu et al. tried to exfoliate bulk g-C₃N₄ by the Hummers method, which is widely used in graphene, but failed.¹⁷ This was attributed to the weaker hydrogen bonding between the layers of strands of polymeric melon units in g-C₃N₄, which is different from the planar pure covalent bonding cohesion within graphite.¹⁹ On this basis, researchers have successfully synthesized GCN via many other methods. Liu et al. developed a direct thermal oxidation “etching” process of bulk g-C₃N₄ to get ultrathin g-C₃N₄ nanosheets,¹⁷ Zhu et al. prepared g-C₃N₄ nanosheets with a single atomic layer structure by a simple chemical exfoliation method,²⁰ and Wang et al. obtained ultrathin C₃N₄ nanosheets using a “bottom-up” method.²¹ Ultrathin g-C₃N₄ nanosheets were also successfully prepared by ultrasound exfoliation or liquid exfoliation methods from bulk g-C₃N₄.^18,22 Furthermore, the more synthetic techniques for GCN vis-à-vis graphene including recently developed laser exfoliation²³ and laser reduction techniques^24–26 should be developed and explored in future work.

Although graphene-like g-C₃N₄ nanostructures have already been obtained, few studies have focused on composite materials which are composed of graphene-like g-C₃N₄ nanosheets and magnetic nanoparticles, and the excellent properties of the graphene series composites in drug delivery and photocatalysis.

In the present work, we synthesized graphene-like ultrathin g-C₃N₄ nanosheets by an ultrasound exfoliation method from bulk g-C₃N₄, and then prepared graphene-like g-C₃N₄ nanosheet/Fe₃O₄ quantum dot nanocomposites for the first time via an easy electrostatic self-assembly method, and their structural properties were systematically characterized. The effect of the Fe₃O₄ content in the g-C₃N₄ nanosheet/Fe₃O₄ nanocomposites on the photocatalytic activity under visible light irradiation was investigated. Moreover, we also conducted a pilot study on the drug loading capacity of the material.

2 Experimental

2.1 Preparation

Preparation of graphene-like g-C₃N₄ nanosheets (GCNs). Briefly, 10 g urea (A.R., Sinopharm Chemical Reagent Co., Ltd) was dissolved in 20 ml deionized water under stirring for 15 min at room temperature, then the solution was put into an alumina crucible, and heated to 550 °C in a tube furnace for 2 h. The synthetic yellow powder was collected for use. Finally, the as-prepared g-C₃N₄ was sonicated and washed with deionized water several times to get graphene-like ultrathin g-C₃N₄ nanosheets. Finally, the sample was dried at 60 °C for 24 h.

Preparation of Fe₃O₄ QDs. Firstly, we dissolved FeCl₂·4H₂O (A.R., Sinopharm Chemical Reagent Co., Ltd) and FeCl₃·6H₂O (A.R., Sinopharm Chemical Reagent Co., Ltd) with a molar ratio of 1 : 2 into deionized water, and stirred the mixed solution for 5 min. Then, we added an NH₃·H₂O aqueous solution (25%, A.R., Sinopharm Chemical Reagent Co., Ltd) into the above solution at 60 °C, drop by drop. The color of the solution changed from dark orange to black about 40 min later, which indicated the formation of Fe₃O₄ QDs. The as-prepared Fe₃O₄ QDs were magnetically separated and washed with deionized water and ethanol several times, and then dried in vacuum at 60 °C for 24 h.

Preparation of GCN/Fe₃O₄ nanocomposites. GCN/Fe₃O₄ nanocomposites were prepared by the electrostatic self-assembly method: 100 mg GCNs were dispersed in 50 ml of deionized water by ultrasound for 30 min. Meanwhile, different amounts (1 mg, 2 mg, 3 mg and 4 mg) of Fe₃O₄ QDs were added into 10 ml of deionized water by ultrasound for 15 min. Then, we added the suspension of Fe₃O₄ QDs to the suspension of GCN drop by drop under vigorous stirring. After that, the obtained mixed suspension was sealed and stirred for 24 h. Finally, the product was dried at 60 °C for 24 h in a vacuum drying oven and then annealed at 150 °C in the vacuum drying oven for 2 h.

2.2 Characterization

The characterizations of the samples were carried out via X-ray diffraction (XRD) patterns collected on a MAC Science MXP-18 X-ray diffractometer using a Cu target radiation source; field-emission transmission electron microscopy (FETEM) was performed on a JEOL JEM-2100F; Fourier transform infrared (FTIR) spectroscopy was recorded on a Bruker Vertex 70 spectrophotometer in KBr pellets; X-ray photoelectron spectrum measurement was performed on a Vgescalab MK II X-ray photoelectron spectrometer (XPS) using Mg Kα radiation (hν = 1253.6 eV) with a resolution of 1.0 eV. The UV-Vis diffuse reflectance spectra (UV-Vis DRS) were obtained for the dry-pressed disk samples using a Specord 2450 spectrometer (UV-3100) equipped with the integrated sphere accessory for diffuse reflectance spectra, using BaSO₄ as the reflectance sample. The surface areas of samples were measured by TriStar 3000-BET/BJH Surface Area.

2.3 Photocatalytic reaction

The photocatalytic degradation of rhodamine B (RhB) was carried out at room temperature in a 250 ml glassy reactor containing catalyst (60 mg) and RhB aqueous solution (60 ml, 7 mg l⁻¹). Before the photocatalytic reaction, the catalyst was stirred thoroughly in the dark to reach the adsorption equilibrium. The catalyst was exposed to visible light using a 350 W Xe lamp with cutoff of 420 nm. After different irradiation intervals, the solution concentration of RhB was analyzed by a UV-vis spectrophotometer (UV-3100) at room temperature.

2.4 Conjugation of GCN/Fe₃O₄–RhB

Firstly, 10 mg GCN/Fe₃O₄ nanocomposites were added into an RhB solution with the desired concentration, sonicated for 30 min and then stirred for 24 h at room temperature in the dark. After that, all solutions were ultracentrifuged at 14 000 rpm for 20 min and the RhB concentration in the upper layer was measured via a standard RhB concentration curve generated by a UV-vis spectrophotometer from a series of RhB solutions with different concentrations. The amount of RB loaded on the GCN/Fe₃O₄ (Φ) was determined using following equation: Φ = (M_RhB − M_RhB′)/M_GCN/Fe3O4, where M_RhB is the initial amount of RhB, M_RhB′ is the amount of RhB in the upper layer, and M_GCN/Fe3O4 is the amount of the nanocomposite added.

3 Results and discussion

The XRD patterns of pure g-C₃N₄ and GCN/Fe₃O₄ nanocomposites are shown in Fig. 1. We found a strong peak at about 27.3° in all samples which represents the stacking of the conjugated aromatic system. It is indexed as the (002) peak of g-C₃N₄, which corresponds to an interlayer d-spacing of 0.336 nm. The small peak in all samples at 12.9° is indexed as the (100) plane of g-C₃N₄, which is associated with interlayer stacking.²⁷ Fe₃O₄ characteristic peaks can be observed in the nanocomposites, which can be indexed as the pure face centered cubic structure of Fe₃O₄ according to JCPDS card no. 19-0629,²⁸ the low intensity of the peaks is due to the small amount of Fe₃O₄. These results indicate that the nanocomposites are composed of both g-C₃N₄ and Fe₃O₄. The small difference in the g-C₃N₄ peaks between pure g-C₃N₄ and the nanocomposites suggests that the presence of Fe₃O₄ does not influence the crystal nature of g-C₃N₄, which is advantageous for the photocatalytic activity of the nanocomposites.

Fig. 1 XRD patterns of pure g-C₃N₄ and GCN/Fe₃O₄ nanocomposites.

Typical TEM and HRTEM images of the GCN/Fe₃O₄ nanocomposites are shown in Fig. 2. The thin silk-like structure of the graphene-like g-C₃N₄ nanosheets can be clearly observed in Fig. 2a, the lateral size of the transparent sheets is as large as several micrometers, the darker part in the image is the wrinkle or overlap of GCNs. Fig. 2b shows an enlarged image of the nanocomposite. From Fig. 2b, the Fe₃O₄ QDs were found to be dispersed on the surface of the GCN, which had agglomerated to some extent. The HRTEM image of the composite is shown in Fig. 2c, the diameter of Fe₃O₄ QDs is about 10 nm, and we can observe a lattice of Fe₃O₄ with d-spacing of about 0.297 nm and 0.258 nm, which conforms to the (220) and (311) planes, respectively, of face centered cubic Fe₃O₄.²⁸ The image also shows the intimate interfacial contact between the GCN and Fe₃O₄ QDs. As the interfacial interaction has a great impact on the transfer process of charge carriers in the nanocomposites, it could be expected that there would be a good charge transfer during the photocatalysis process. All of the above results show that the as-prepared sample is composed of two separate phases of Fe₃O₄ QDs and GCN.

Fig. 2 (a and b) TEM images of GCN/Fe₃O₄ nanocomposites; (c) HRTEM image of Fe₃O₄ QDs; (d) a model of the GCN/Fe₃O₄ nanocomposite.

The absorption range of light is very important in the visible light photodegradation of contaminants, therefore, the UV-Vis diffuse reflectance spectrum was measured and shown in Fig. 3a. As a comparison, the spectrum of pure g-C₃N₄ was also measured. As Fig. 5a indicates, both spectra have broad absorption in the UV-visible region, which demonstrate that g-C₃N₄ and the nanocomposites possess visible-light absorption ability. An obvious red shift in the absorption edge and enhanced absorption intensity of GCN/Fe₃O₄ was also observed. This phenomenon may be attributed to a charge-transfer transition between the Fe₃O₄ species and the g-C₃N₄ conduction or valence band, which can lead to a stronger redox ability and higher photocatalytic activity.²⁹

Fig. 3 Optical measurement of pure g-C₃N₄ and the GCN/Fe₃O₄ nanocomposite, (a) UV-Vis diffuse reflectance spectra; (b) FTIR spectra.

The bond structure of the GCN/Fe₃O₄ nanocomposites was studied by FTIR spectroscopy, and as a comparison, the spectrum of pure g-C₃N₄ was also measured. The results is shown in Fig. 3b. Several main characteristic peaks are visible in both spectra, the peak at 809 cm⁻¹ is related to the tri-s-triazine ring modes, several strong bands in the 1200–1650 cm⁻¹ region correspond to the typical stretching modes of CN heterocycles, and the peaks at 1635, 1569 and 1411 cm⁻¹ were attributed to stretching vibration modes of heptazine-derived repeating units, which agree with the XRD result of pure g-C₃N₄. The peaks at 1316 and 1238 cm⁻¹ are assigned to the stretching vibration of connected trigonal units of C–N(–C)–C or bridging C–NH–C (partial condensation).³⁰ In addition, the wide band between 3000–4000 cm⁻¹ can be assigned to the absorption of water or O–H groups. For the GCN/Fe₃O₄ nanocomposite, the characteristic absorption of Fe₃O₄ centered at 585 cm⁻¹ was observed, which corresponds to the vibration of the Fe–O bonds.^31,32 It can be clearly seen that all of the main characteristic peaks of g-C₃N₄ and Fe₃O₄ appeared in the Fe₃O₄/GCN, and there is no obvious peak shift between the pure g-C₃N₄ and the GCN/Fe₃O₄ nanocomposite. The intensity of the peaks shows some variation compared with the pure g-C₃N₄, indicating that there are close interfacial connections between g-C₃N₄ and Fe₃O₄ rather than simply a physical adsorption,³³ which is consistent with the HRTEM results, and these connections may serve as electron migration paths to promote the charge separation, leading to an improved photoactivity.³⁴

XPS spectrum analysis was employed to investigate the valence states and chemical environment of the constituent elements on the surface of the GCN/Fe₃O₄ nanocomposite, which is shown in Fig. 4. Fig. 4a shows the survey spectrum of the GCN/Fe₃O₄. O, C, N and Fe elements were detected in the nanocomposites. Fig. 4b shows the regional spectra of C 1s, which can be deconvoluted into three peaks at 284.1, 286.8 and 287.8 eV. The peak at 284.6 eV arises from adventitious carbon, the peak at 286.1 eV can be assigned to C–N–C coordination, and the other peak at 288 eV can be attributed to the C–(N)³ group of g-C₃N₄.^35–38 The N 1s region can be fitted into three peaks (Fig. 4c), ascribed to C–N–C (398.4 eV), N–(C)₃ (400.1 eV) and C–N–H groups (401.3 eV), respectively.³⁹ The XPS data also gives evidence for the existence of a graphite-like sp²-bonded structure in the GCNs.

Fig. 4 (a) XPS survey spectrum of the GCN/Fe₃O₄ nanocomposite; (b–d) high-resolution binding energy spectra of C 1s, N 1s, and Fe 2p for the nanocomposite, respectively.

The Fe 2p region of the spectrum is shown in Fig. 4d, two peaks were observed centered at about 725 and 711 eV, which were assigned to Fe 2p_1/2 and Fe 2p_3/2, respectively.^40,41 These are almost equal to the standard binding energy of Fe₃O₄, suggesting the Fe element exists in a Fe₃O₄ form in the nanocomposite.⁴² Moreover, neither of the peaks of Fe–C or Fe–N were detected in the spectra, indicating that no chemical bonds formed between the Fe₃O₄ QDs and the GCNs, which is consistent with the above work.

The at. % of each element was found by calculating the peak areas, and shown in Table 1.

Table 1 The at. % of each element

Name	Peak BE	Height counts	FWHM eV	Area (N)	at. %
C 1s	284.6	12074.45	1.42	0.37	38.36
N 1s	398.43	15284.8	1.39	0.45	47.74
O 1s	532.54	8411.49	2.55	0.18	12.88
Fe 2p	710.83	938.6	0.85	0	0.25

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According to Table 1, the N : C ratio is 1.22, which is lower than the theoretical value (1.33). This may mainly be due to the structure defects resulting from the existence of chemically oxygenic functional groups like C=O and –COOH, and the amount of oxygen may come from these groups and H₂O which is adsorbed to the surface of the sample.

The photocatalytic activity of GCN/Fe₃O₄ nanocomposites with different loading amounts of Fe₃O₄ and pure g-C₃N₄ was evaluated by degrading the well-known organic dye RhB under visible-light irradiation. To fully consider the high adsorption ability of g-C₃N₄, an adsorption equilibrium experiment was also performed before the photocatalysis. The adsorption capacities are shown in Fig. 5a, it can be observed that all samples can achieve adsorption equilibrium within 60 min in the dark, and 59–83% of the RhB was adsorbed. Pure g-C₃N₄ exhibited the best adsorptivity, and the adsorptivity of the GCN/Fe₃O₄ nanocomposites decreased with increasing amounts of Fe₃O₄, which is due to the lower surface area of Fe₃O₄. The photodegradation process of RhB was recorded by the temporal evolution of the spectra and is shown in Fig. 5b. In order to observe the photocatalytic activity of the samples more visually, the degradation efficiency of the samples after irradiation for 1.5 h is plotted as a histogram (Fig. 5c). From Fig. 5b and c, the degradation efficiency after irradiation for 1.5 h is 72.5% for pure g-C₃N₄, 81% for GCN-1 wt% Fe₃O₄, 95% for GCN-2 wt% Fe₃O₄, 60.46% for GCN-3 wt% Fe₃O₄ and 57.2% for GCN-4 wt% Fe₃O₄. It is obvious that the photocatalytic activity was gradually enhanced with the increasing proportion of Fe₃O₄ at first, and the GCN-1 wt% Fe₃O₄ and GCN-2 wt% Fe₃O₄ can degrade RhB by nearly 100% within 90 min and 120 min, respectively. The as-prepared GCN-2 wt% Fe₃O₄ showed the highest photocatalytic activity. However, the photocatalytic activity of the nanocomposites decreased with further increased proportions of Fe₃O₄. Remarkably, the GCN-3 wt% Fe₃O₄ and GCN-4 wt% Fe₃O₄ showed lower photocatalytic activity than pure g-C₃N₄.

Fig. 5 (a) The adsorption of pure g-C₃N₄ and GCN/Fe₃O₄ nanocomposites in the dark; (b) photocatalytic degradation of RhB under the irradiation of visible light with pure g-C₃N₄ and GCN/Fe₃O₄ nanocomposites; (c) degradation efficiency of pure g-C₃N₄ and GCN/Fe₃O₄ nanocomposites after irradiation for 1.5 h; (d) reusability of the GCN-2 wt% Fe₃O₄ nanocomposite for the visible light degradation of RhB.

The mechanism of the photocatalytic activity of the samples under visible-light irradiation is proposed to be as follows: under visible light irradiation, the GCN was excited to generate photo-generated electrons from HOMO to LUMO. Without the presence of other materials, electrons will undergo a quick transition back to the valence band owing to the instability of excited states. Then, with the introduction of Fe₃O₄ QDs, the photogenerated e⁻ in the GCN could easily transfer to Fe₃O₄ (CB) through their interfacial interaction because the energy level of the CB of GCN is higher than the Fermi level of Fe₃O₄, thus hindering electron–hole recombination.^43–45 Therefore, the greatly enhanced photocatalytic activity can be attributed to the promotion of electron–hole separation by the electron transfer process, in addition to the large surface area of GCNs. GCN-2 wt% Fe₃O₄ exhibited the best photocatalytic activity, indicating that the adsorption capacities, photogenerated holes and electron transfer are optimal. However, with further increasing proportions of Fe₃O₄, the excess Fe₃O₄ QDs cover the active sites on the g-C₃N₄ surface and thereby reduce the efficiency of charge separation.⁴⁶

Renewable catalytic activity is also very important for a photocatalyst, so the stability and durability of the GCN-2 wt% Fe₃O₄ nanocomposite was further studied by a recycling test which is shown in Fig. 5d. There is no significant loss of activity after three cycles of the degradation reaction, which indicates the superior stability and durability of the nanocomposites.

We also conducted a pilot study on the drug loading capacity of the material. Because the Brunauer–Emmett–Teller (BET) specific surface area and porous structure play an important role in drug load and delivery, so N₂ adsorption measurement was conducted. Fig. 6a shows the nitrogen adsorption–desorption isotherm and the corresponding pore size distribution curve of the GCN-2 wt% Fe₃O₄ nanocomposite (inset in Fig. 6a). From the adsorption isotherm, the BET specific surface area of the sample was estimated to be 70 m² g⁻¹. It has lower specific surface area than graphene (200 m² g⁻¹), which is due to the stacking of graphene-like sheets. The isotherm of the nanocomposite is type IV (Brunauer–Deming–Deming–Teller classification) and exhibits H₃ hysteresis loops, which suggests it has sheet-like morphology and slit-like mesopores,⁴⁷ which is consistent with the TEM results (Fig. 2). As the inset shows, the pore-size distribution of the sample is very broad, indicating the existence of mesopores and macropores. Due to their especially large external surface area, the nanocomposites may show a large capacity for drug loading and delivery.

Fig. 6 (a) Nitrogen adsorption–desorption isotherm and pore size distribution profile of the GCN-2 wt% Fe₃O₄ nanocomposite; (b) loading capacity of RhB on the GCN-2 wt% Fe₃O₄ nanocomposite in different initial RhB concentrations.

To investigate the loading capacity of the nanocomposites, we used the GCN-2 wt% Fe₃O₄ nanocomposite to absorb RhB as a model drug and measured the UV spectrum at 554 nm. The loading capacity was calculated by the differences in RhB concentration between the original RhB solution and the supernatant solution after loading. The saturated loading amount of RhB on the nanocomposite is shown in Fig. 6b, which can reach 108 mg g⁻¹ at a RhB concentration of 20 mg l⁻¹. Besides physical adsorption, hydrogen bonds and electrostatic interactions⁴⁸ between GCNs and RhB may also play important roles in loading. Therefore, the loading capacity of the GCN/Fe₃O₄ nanocomposites means that they may be used as potential candidates for controlled magnetically targeted drug delivery.

4 Conclusion

In this paper, we synthesized graphene-like ultrathin g-C₃N₄ nanosheets by an ultrasound exfoliation method from bulk g-C₃N₄, and graphene-like g-C₃N₄ nanosheet (GCN)/Fe₃O₄ quantum dot (QD) nanocomposites were obtained via an electrostatic self-assembly method. The photocatalytic activity of the GCN/Fe₃O₄ nanocomposites was evaluated under visible light irradiation, it was found that the GCN-2 wt% Fe₃O₄ nanocomposite had the highest photocatalytic activity. The mechanism of the highly enhanced photocatalytic activity is attributed to the efficient separation of the photogenerated electron–hole pairs and the high specific surface area of the graphene-like g-C₃N₄ nanosheets. Moreover, the nanocomposites can load a model drug (Rhodamine B) and the loading capacity was as high as 108.6 mg g⁻¹, so the nanocomposites possess great potential applications in photocatalysis and controlled magnetically targeted drug delivery, perhaps even in many newly born properties for their special structure.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51479220), the National Programs for High Technology Research and Development of China (863) (Item no. 2013AA032202) and the National Youth Program Foundation of China (Grant no. 61308095).

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