Mesoporous Metal-Containing Carbon Nitrides for Improved Photocatalytic Activities

Graphitic carbon nitrides (g-C 3 N 4 ) have attracted increasing interest due to their unusual properties and promising applications in water splitting, heterogeneous catalysis, and organic contaminant degradation. In this study, a new method was developed for the synthesis of mesoporous Fe contained g-C 3 N 4 (m-Fe-C 3 N 4 ) photocatalyst by using SiO 2 nanoparticles as hard template and dicyandiamide as precursor. The physicochemical properties of m-Fe-C 3 N 4 were thoroughly investigated. The XRD and XPS results indicated that Fe was strongly coordinated with the g-C 3 N 4 matrix and that the doping and mesoporous structure partially deteriorated its crystalline structure.TheUV-visible absorption spectra revealed thatm-Fe-C 3 N 4 with a unique electronic structure displays an increased band gap in combination with a slightly reduced absorbance, implying that mesoporous structure modified the electronic properties of g-Fe-C 3 N 4 .The photocatalytic activity of m-Fe-C 3 N 4 for photodegradation of Rhodamine B (RhB) was much higher than that of g-Fe-C 3 N 4 , clearly demonstrating porous structure positive effect.


Introduction
Carbon nitride has been widely regarded as the most promising candidate to complement carbon in materials applications.Among various carbon nitride compounds, graphitic carbon nitride (g-C 3 N 4 ) is the most stable allotrope, which has attracted much attention for its potential application in splitting water, decomposing organic pollutants, and photosynthesis under visible light [1][2][3][4][5].The polymeric g-C 3 N 4 material contains graphitic stacking of C 3 N 4 layers, which are constructed from tri-s-triazine units connected by planar amino groups [2,6].To further improve the performance of g-C 3 N 4 , many methods such as introducing functional atoms (B, F, S, P, etc.), controlling shape, and oxidation reaction have been used for its modification [1,[7][8][9][10].
One important way to improve its light utilization efficiency is to reduce the band gap and extend the light absorption range.Metal element dopants are usually introduced to prepare functional organic-metal hybrid material based on g-C 3 N 4 .Wang et al. firstly reported the synthesis of metal (Fe 3+ )-containing carbon nitride compounds using dicyandiamide and metal chloride as precursors, and such synthesized carbon nitride showed high photocatalytic activities for the degradation of various organic dyes [11].g-C 3 N 4 photocatalyst was also modified with other transition metal elements (Co, Ni, Mn, Cu [12], or Zn [13]) to obtain the high photocatalytic activity and good stability.It also was active for the direct oxidation of benzene to phenol using hydrogen peroxide [14].
The porous structure can increase the semiconductor surface area, which contributes to an enhancement in energy conversion efficiency [15,16].Herein, we report the synthesis of porous m-Fe-C 3 N 4 photocatalysts by using SiO 2 nanoparticles as template and dicyandiamide as precursor, and the physicochemical properties of synthesized m-Fe-C 3 N 4 and g-Fe-C 3 N 4 were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), UV-visible spectrophotometer (UV-vis), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectra, and N 2 adsorption-desorption measurement.Moreover, their performance for photodegradation of Rhodamine B (RhB) was evaluated.

Materials and Methods
2.1.Preparation of g-Fe-C 3 N 4 .Graphitic metal-containing carbon nitride compounds were synthesized according to the procedure reported previously [11].Dicyandiamide was mixed with 0.12 g FeCl 3 in water (20 mL) under stirring, and then the mixed solution was heated at 100 ∘ C to remove water.The resulting powder was then heated at a rate of 2.3 ∘ C min −1 for about 4 h to reach a temperature of 550 ∘ C and then held at this temperature for an additional 4 h under flowing nitrogen gas.The sample was then cooled to room temperature and was denoted as g-Fe-C 3 N 4 .

Preparation of m-Fe-C
3 N 4 .A 5 wt% dispersion of 15 ± 5 nm SiO 2 particles in water (20 mL) was heated and stirred at 100 ∘ C with dicyandiamide (4 g, Aldrich) and FeCl 3 (0.12 g) added.The mixed solution was treated following the same procedures as mentioned above to obtain high-temperature power.The resulting power was then stirred in 200 mL of 4 mol/L NH 4 HF 2 for 12 h, followed by filtration and washing with water.After that, the sample was cooled to room temperature and was denoted as m-Fe-C 3 N 4 .

Characterization
The morphology of the samples were observed using transmission electron microscopy (TEM, JEM-2011, JEOL Co., Japan).The nitrogen adsorption-desorption isotherms and Brunauer-Emmett-Teller (BET) surface areas were measured using a TriStar II 3020M instrument at 77 K.The crystal properties of the samples were identified by an 18 kW rotating anode X-ray diffractometer (MAP18AHF, MAC Sci.Co., Japan).The optical absorbance spectra of the samples were recorded using a UV-visible spectrophotometer (SolidSpec-3700, Shimadzu Co., Japan).The samples were ground with KBr power and pressed to form a uniform disk prior to FTIR analysis (Magna-IR 750, Nicolet Instrument Co., USA).

Photocatalytic Activity Measurement
Photocatalytic activities of samples for RhB degradation were evaluated with irradiation by a 500 W Xe lamp.In a Pyrex glass reactor, a total amount of 20 mg photocatalyst powder was dispersed in 40 mL of 5 mg L −1 aqueous solution of RhB.One mL suspension was sampled at fixed time intervals during the reaction.The suspension was centrifuged to remove the photocatalyst and then the concentration variation of RhB was examined by UV-vis spectrophotometer (UV-2450, Shimadzu Co., Japan).

Results and Discussion
XRD data obtained from both g-Fe-C 3 N 4 and m-Fe-C 3 N 4 are illustrated in Figure 1.All these patterns indicate a structural similarity between g-Fe-C 3 N 4 and m-Fe-C 3 N 4 .There were two peaks in both samples.The strongest peak at 27.6, corresponding to an interlayer distance of 0.33 nm, could be indexed as the (002) peak of the stacking of the conjugated aromatic system [17].However, XRD peak originated from iron species was not found in both samples.This indicates that mesoporous structure of metal-containing carbon nitride compounds did not disrupt the crystal structure of graphitic metal-containing carbon nitride compounds.Thus, it could be inferred that the iron species in m-Fe-C 3 N 4 was chemically coordinated to the g-C 3 N 4 host [12].
To get insight into the surface functionalities created by mesoporous structure, XPS measurements were performed.XPS survey scan spectra show C1s and N1s peaks at 288.1 eV and 398.7 eV in both the g-Fe-C 3 N 4 and m-Fe-C 3 N 4 samples (data not shown).The C1s binding energy shows mainly one carbon species, corresponding to a C-N-C coordination.In the N1s spectrum the main signal shows the existence of C-N-C groups [18].The assignment of peaks in the C1s and N1s spectra of two samples was in good agreement with the literature report [12].It could be seen that C-N bonds were They all fell within the range of binding energy of Fe(III) state, and only a small amount of Cl could be detected.This result implies that Fe(III) is connected to C 3 N 4 framework mainly through Fe-N bonds, while little was used for charge balance by Cl − ions.Thus, Fe(III) in m-Fe-C 3 N 4 might also be stabilized in the electron-rich C 3 N 4 structure like g-Fe-C 3 N 4 .
Figure 3 shows the FTIR spectra of the g-Fe-C 3 N 4 and m-Fe-C 3 N 4 samples.They exhibit a peak at 808 cm −1 and several other major bands from 1200 to 1700 cm −1 .The two samples show stretching modes in the 1200-1700 cm −1 region, which were typical stretching modes of CN heterocycles originating from the extended C 3 N 4 network [19], whereas the sharp band at about 808 cm −1 should be attributed to ring-sextant bending vibration characteristic of triazine or heptazine ring systems [20,21].The results indicate that both the samples were chemically coordinated to the C 3 N 4 host and formed no metal nitrides or metal carbides.
Figure 4 shows TEM images of the g-Fe-C 3 N 4 and m-Fe-C 3 N 4 samples.These results indicate that the pore size and connectivity of those powders exactly reflect the geometric properties of the original template [2].The g-Fe-C 3 N 4 shows a graphitic-like and two-dimensional layer structure (Figure 4(a)), which is similar to that of pure g-C 3 N 4 .The TEM image of m-Fe-C 3 N 4 in Figure 4(b) shows disordered pore system of spherical pores with diameter of 15-20 nm, which is consistent with the size of SiO 2 template.Compared to g-Fe-C 3 N 4 , m-Fe-C 3 N 4 exhibited a slightly less dense structure and a higher surface area.
Figure 5 shows nitrogen adsorption/desorption isotherms of the g-Fe-C 3 N 4 and m-Fe-C 3 N 4 samples.The isotherms of both samples exhibit type III behavior according to the IUPAC classification, indicating the existence of porous structures in the samples [22].The BET surface area of the m-Fe-C 3 N 4 sample (35.9 m 2 g −1 ) was much higher than that of m-Fe-C 3 N 4 sample (5.7 m 2 g −1 ).The larger pore volume of m-Fe-C 3 N 4 sample (0.17 cm 3 g −1 ) suggested that it was more porous compared with that of g-Fe-C 3 N 4 sample (0.02 cm 3 g −1 ).An average pore diameter of 18.4 nm for m-Fe-C 3 N 4 can be estimated from the BJH pore size distribution (Figure 5(b)).The pore size distribution of the two samples obtained from its absorption isotherm is consistent with the observed pore sizes from the TEM image.These results illustrate that m-Fe-C 3 N 4 sample has been introduced with porous structure, which results in an increased surface area, enlarged pore volume, and narrow pore size distribution.
Figure 6 shows the optical absorbance spectra of the g-Fe-C 3 N 4 and m-Fe-C 3 N 4 samples.The absorption edge was at about 480 nm for g-Fe-C 3 N 4 and 460 nm for m-Fe-C 3 N 4 , corresponding to the calculated band gap ca.2.58 and  The photocatalytic performance of the resultant samples for RhB degradation under irradiation is shown in Figure 7.
The results indicate that the mesoporous structure greatly influences the photocatalytic performance of g-Fe-C 3 N 4 materials.The g-Fe-C 3 N 4 showed a moderate photoreactivity toward organics degradation, which is consistent with the literature report [11].Only ∼42% of RhB was degraded in 60 min by g-Fe-C 3 N 4 under irradiation, which is much lower than that of mesoporous samples.The m-Fe-C 3 N 4 sample exhibits a higher photocatalytic activity, with nearly 88% and 98% of RhB degraded after 60 and 90 min irradiation, respectively.The m-Fe-C 3 N 4 sample showed much higher efficiency in RhB degradation than g-Fe-C 3 N 4 under irradiation.It is known that the photooxidation reactions of organic molecules can directly utilize the generated valence band holes or the main generated active species, • OH radicals, from the reaction of holes with surface adsorbed water or hydroxyl groups [10].The mesoporous structure is beneficial for promoting the mass transfer of reactants and products, enhancing the photocatalytic activity by facilitating access to the reactive sites on the surface photocatalyst.Generally, a larger surface area of photocatalysts is favorable for photocatalytic reaction by providing more possible reaction sites [23].In addition, the unique electronic structure of m-Fe-C 3 N 4 may also contribute considerably to the excellent photooxidation reactivity.

Conclusions
In summary, in situ Fe-doped mesoporous g-C 3 N 4 was synthesized from a single precursor, dicyandiamide, by using SiO 2 nanoparticles as the template.The XRD results indicate that Fe is strongly coordinated with the g-C 3 N 4 matrix and that the doping and mesoporous structure partly deteriorates its crystalline structure.The XPS analysis confirmed that Fe species was coordinated with the g-Fe-C 3 N 4 and m-Fe-C 3 N 4 framework through Fe-N bonds.The UV-visible absorption spectra reveal that the absorption edge of g-Fe-C 3 N 4 was redshifted from 460 nm of the m-Fe-C 3 N 4 sample to 480 nm together with a stronger light absorbance, implying that the formation of mesoporous structure changes the electronic properties of g-Fe-C 3 N 4 .The obtained Fe-doped mesoporous g-C 3 N 4 shows much higher activity than the g-Fe-C 3 N 4 for RhB degradation.This activity enhancement could be attributed to the increased surface area and unique mesoporous structure.

Figure 1 :
Figure 1: XRD patterns of the g-Fe-C 3 N 4 and m-Fe-C 3 N 4 samples.