Lei Shib,
Lin Liangc,
Fangxiao Wangb,
Mengshuai Liub,
Tao Liangb,
Kunlong Chenb and
Jianmin Sun*ab
aState Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150080, China. E-mail: sunjm@hit.edu.cn; Tel: +86 451 86403715
bThe Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080, China
cSchool of Life Science and Technology, Harbin Institute of Technology, Harbin 150080, China
First published on 14th July 2015
Porous graphitic carbon nitride (pg-C3N4) was facilely and economically prepared through in situ bubble templates such as (NH4)2S2O8, and the surface area of the resultant pg-C3N4 was controlled from 6.4 to 55.0 m2 g−1 by adjusting the mass ratio of (NH4)2S2O8/melamine. Moreover, pg-C3N4 was also obtained by other gas-generating porogens of ammonium salts, displaying the generality of the preparation method. The as-prepared g-C3N4 presented a porous structure with a higher surface area and displayed an improved separation efficiency for photogenerated electron–hole pairs. These integrative positive factors contributed to pg-C3N4 possessing more excellent photocatalytic activity for degrading Rhodamine B and phenol pollutants, and splitting water to H2 than bulk g-C3N4 under visible-light. Moreover, the as-prepared pg-C3N4 exhibited unexceptionable stability and reusability even after four photocatalytic runs. The simple, economical and general fabrication strategy with bubble-generated porogen agents for porous graphitic carbon nitride with superior visible-light photocatalytic performance is attractive for environmental and energy application fields.
Photocatalysts with porous structures and large surface areas are vital for satisfactory catalytic efficiency, which could increase the mass transfer, offer more surface reactive sites and suppress the recombination of photoinduced electron–hole pairs.22,24 Generally, porous g-C3N4 with a large surface area is obtained by a template-induced method. However, the synthesis and the catalytic activity of porous g-C3N4 are limited because soft-templating methods result in some carbon residue in the product during template calcination. Besides, the weak interaction between a soft template and the precursors of g-C3N4 leads to a poor porous structure, and moreover, the necessary organic mesoscaled soft template in synthesis leads to the high cost of the catalyst.25 Using silica as a hard template is an alternative and successful pathway for porous g-C3N4 synthesis.26–28 However, toxic ammonium bifluoride or aqueous hydrogen fluoride is necessarily involved for template removal, and the removal process is also complex and time-consuming. Hence, the facile and economic synthesis of porous g-C3N4 with large surface area is attractive to practical applications.
In this contribution, porous g-C3N4 (pg-C3N4) with a large surface area was facilely fabricated by in situ direct pyrolysis of (NH4)2S2O8 and melamine in air. Using (NH4)2S2O8 as a bubble-generating template resulted in the formation of a porous structure during the condensation process of the melamine precursor. Moreover, the porous structure and photocatalytic activity of the resultant pg-C3N4 were easily controlled by adjusting the mass ratio of (NH4)2S2O8 to melamine. Due to the low recombination rate of photogenerated electron–hole pairs and the large surface area, the resultant pg-C3N4 exhibited much higher efficiency than bulk g-C3N4 for decomposing Rhodamine B (RhB) and phenol, together with water splitting to evolve H2. Promisingly, the preparation method for porous g-C3N4 was versatile for other bubble-generated porogens such as NH4Cl and (NH4)2S2O3 besides (NH4)2S2O8. The developed strategy provided a simple and economic avenue for the fabrication of porous g-C3N4 with superior visible-light photocatalytic performance, which is appealing to wide application fields of g-C3N4.
The visible-light-induced catalytic H2 evolution by the as-prepared photocatalyst was carried out in a Pyrex top-irradiation reaction vessel connected with a closed glass gas-circulation system. H2 production was performed in a solution of 50 mg photocatalyst and 10 mL triethanolamine in 90 mL distilled water. 3 wt% Pt was loaded on the surface of the photocatalyst by an in situ photodeposition method using H2PtCl6 as the starting material. The reactant solution was evacuated several times to remove the air prior to irradiation by a 300 W Xe lamp with a 400 nm cut-off filter and a water filter. The evolved gases were analyzed by gas chromatography (SP7800) with N2 as the carrier gas.
Fig. 2A shows the N2 adsorption–desorption isotherms of the as-prepared pg-C3N4 samples and bulk g-C3N4. Except for the bulk g-C3N4, all the samples exhibited type IV isotherms characteristic of mesoporous materials. And as shown in Table 1, the surface areas of pg-C3N4 increased progressively with increasing mass fractions of (NH4)2S2O8, due to the greater number of bubbles produced in the condensation process. Hence, by adjusting the mass ratio of (NH4)2S2O8/melamine, the surface area of pg-C3N4 could be successfully controlled, and the surface area of pg-C3N4-3 was approximately 8.5 times higher than that of bulk g-C3N4. The large surface area would benefit the photocatalytic activity. In addition, the wide distributions of BJH pore size in the range of 20–100 nm further confirmed the formation of different sized pores, which was consistent with the TEM and SEM results.
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Fig. 2 (A) N2 adsorption–desorption isotherms and (B) pore size distributions of (a) bulk g-C3N4, (b) pg-C3N4-1, (c) pg-C3N4-2 and (d) pg-C3N4-3. |
Samples | Surface area (m2 g−1) | Average pore size (nm) |
---|---|---|
Bulk g-C3N4 | 6.5 | — |
pg-C3N4-1 | 10.4 | 38.7 |
pg-C3N4-2 | 34.4 | 33.4 |
pg-C3N4-3 | 55.0 | 29.5 |
pg-C3N4(NH4Cl) | 41.8 | 32.7 |
pg-C3N4((NH4)2S2O3) | 24.2 | 30.4 |
The typical g-C3N4 structure was detected by XRD in Fig. 3 and no other impurity phase was found in all the resultant samples. The main peaks at 27.6° and 13.0° were indexed to (002) and (100) planes of hexagonal g-C3N4 (JCPDS card no. 87-1526), corresponding to the graphite-like stacking and in-plane structural repeating motifs of the conjugated aromatic units of g-C3N4.29 Compared with the bulk g-C3N4, the peak intensities of the pg-C3N4 samples became weaker, which were induced by the generated pores preventing the formation of extended graphitic layers during the thermal condensation process of melamine.26 In order to check other ammonium salts as bubble-generating porogens for the synthesis of porous g-C3N4, NH4Cl and (NH4)2S2O3 were also applied instead of (NH4)2S2O8. The XRD patterns of g-C3N4 obtained from the former two templates also indicated the typical g-C3N4 diffraction peaks (Fig. 3e and f), and the N2 sorption isotherms further evidenced the porous structure; the surface areas were 41.8 and 24.2 m2 g−1, respectively (Fig. 4). These results implied that the ammonium salts were used as porogens since they decompose with gas evolution without the possibility of generating additional carbon, which might alter or interfere with the properties of the g-C3N4 formed from melamine.
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Fig. 3 XRD patterns of (a) bulk g-C3N4, (b) pg-C3N4-1, (c) pg-C3N4-2, (d) pg-C3N4-3, (e) pg-C3N4 (NH4Cl) and (f) pg-C3N4 ((NH4)2S2O3). |
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Fig. 4 N2 adsorption–desorption isotherms of (a) pg-C3N4 (NH4Cl) and (b) pg-C3N4 ((NH4)2S2O3), (inset is the pore size distributions). |
Fig. 5 shows the FTIR spectra of the bulk g-C3N4 and pg-C3N4 samples. All the samples exhibited the typical IR patterns of g-C3N4, the broad bands in the 3000–3500 cm−1 region were attributed to the adsorbed O–H bands and N–H components.30 The absorption peak at 809 cm−1 was considered as the out-of-plane skeletal bending modes of triazine.31 The absorption bands in the range of 1200–1700 cm−1 were assigned to the typical stretching modes of C3N4 heterocycles.32,33 The peak at 1642 cm−1 was ascribed to the CN stretching vibration mode, while the peaks at 1242, 1320 and 1410 cm−1 were attributed to aromatic C–N stretching vibration modes.
To further determine the surface composition and chemical state, XPS spectra of pg-C3N4-3 were also investigated. XPS survey spectra displayed only two peaks of C 1s and N 1s (Fig. 6A). In the C 1s spectrum, two peaks were distinguished to be centered at 284.6 and 288.1 eV. The peak at 284.6 eV was exclusively assigned to carbon atoms (C–C bonding) in a pure carbon environment, and the peak at 288.1 eV was identified as sp2-bonded carbon (CN).34 The N 1s spectrum was divided into three peaks at 398.6, 399.3 and 400.8 eV in Fig. 6C. The peak at 398.6 eV was typically attributed to an sp2-bonded N atom to two carbon atoms (C–N
C), and the two peaks at 399.3 and 400.8 eV were assigned to tertiary nitrogen (N–(C)3) and amino functional groups with a hydrogen atom (N–H).35,36 And as seen from the S 2p spectrum in Fig. 6D, there was no S residue in the pg-C3N4 sample, which further confirmed that (NH4)2S2O8 only acted as the bubble template during the g-C3N4 preparation process, and then (NH4)2S2O8 was removed completely after condensation, consistent with the EDS elemental mapping analysis of the sample.
The element contents in the as-prepared samples were further confirmed by elemental analyses (Table 2). The atomic ratios of carbon to nitrogen were almost 0.64, lower than 0.75 for the ideal g-C3N4 sample, and the hydrogen elements detected were indicative of the incomplete thermal condensation of melamine. Combined with the FTIR and XPS results, the residual hydrogen atoms were bonded to the edges of the graphene-like C–N sheet in the forms of C–NH2 and 2C–NH bonds.37,38
Samples | N wt% | C wt% | H wt% | C/N atomic ratio |
---|---|---|---|---|
Bulk g-C3N4 | 61.32 | 33.90 | 1.60 | 0.64 |
pg-C3N4-1 | 60.00 | 33.08 | 1.79 | 0.64 |
pg-C3N4-2 | 60.22 | 33.28 | 1.75 | 0.64 |
pg-C3N4-3 | 59.32 | 33.14 | 1.84 | 0.65 |
The UV-visible DRS of the samples was investigated and shown in Fig. 7. Bulk g-C3N4 displayed absorption from ultraviolet to visible-light, and its absorption edge was around 463 nm, corresponding to the band gap at 2.68 eV and consistent with a previous report.12 It was reported that if S was doped in the g-C3N4 sample, the corresponding band gap was narrowed.39 However, the as-prepared pg-C3N4 samples displayed similar band gaps to bulk g-C3N4, proving that pg-C3N4 was not doped by elemental S.
The photocatalytic capability of the pg-C3N4 samples was evaluated by degrading RhB and phenol under visible-light. As shown in Fig. 8A, prior to illumination, the adsorption of RhB pollutant on bulk g-C3N4 was low at 9%. Due to the improved BET surface area, the adsorption amounts were enhanced to 14.6% for pg-C3N4-1, 24.8% for pg-C3N4-2 and 31.5% for pg-C3N4-3. For the photocatalytic process, the degradation of RhB was hardly carried out even for 60 min irradiation without any catalyst, and the degradation efficiency was also low at 50.2% over bulk g-C3N4. Noticeably, pg-C3N4 samples exhibited significantly enhanced degradation efficiencies, and the degradation rate increased with the increment of (NH4)2S2O8 addition amounts. The pg-C3N4-3 showed the highest activity at 96% within 40 min. Additionally, in order to exclude the effect of dye self-sensitization, the photodegradation of phenol was further conducted, as shown in Fig. 8B. The adsorption amount of phenol on the pg-C3N4 samples was improved compared with bulk g-C3N4, moreover, pg-C3N4-3 exhibited the best photodegradation efficiency at 55.0%, much higher than the 20.2% efficiency over bulk g-C3N4.
The photocatalytic H2 evolution from water splitting over the as-prepared photocatalysts was also investigated (Fig. 8C). The obtained pg-C3N4 displayed a higher hydrogen evolution rate (HER = 35, 80, 100 μmol h−1) than the bulk g-C3N4 (HER = 16 μmol h−1), which showed a similar trend to those for RhB and phenol photodegradations. The photocatalytic H2 evolution rate over pg-C3N4-3 is about 6 times higher than that of bulk g-C3N4. Obviously, the introduced pores caused by (NH4)2S2O8 made pg-C3N4 exhibit the improved photocatalytic activities for degrading pollutants and hydrogen production from water splitting. The positive effects coming from the increased surface area and the formed porosity contributed to the enhanced adsorption capacity to the pollutants, provided more photocatalytic active sites, and promoted the migration and separation of charge carriers, thus, integrally leading to the significant acceleration in photocatalytic efficiency of pg-C3N4.
The stability of a photocatalyst is vital to the practical applications. The photocatalytic degradation experiments of RhB and photosplitting of H2O over pg-C3N4-3 were repeated up to four times under the same conditions and the results are shown in Fig. 9. After each run, the catalyst was centrifuged, washed by water and ethanol, dried then reused for the next run. In the case of four recycles, the RhB photodegradation activity did not change markedly and still remained at 90%, suggesting that pg-C3N4-3 was not photocorroded during the photodegradation process and kept excellent stability. Similarly, the durability of pg-C3N4-3 for H2 evolution was also investigated by four consecutive operations. The produced H2 increased steadily with irradiation time in each run without noticeable deactivation.
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Fig. 9 Recycling runs of (A) photodegradation activity of RhB and (B) H2 evolution over pg-C3N4-3 under visible-light irradiation. |
In addition, the spent catalyst structure was further measured by XRD after four runs. Fig. 10 indicates that the peaks at 13.0° and 27.6° did not change after the repeated uses. However, the reduced diffraction intensities over the spent pg-C3N4-3 resulted from the narrowed interplanar distance and/or interlayer stacking distance of g-C3N4 after the recycles.40
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