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g-C3N4 nanosheets laminated homojunctions as efficient visible-light-driven photocatalysts
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Meso-g-C3N4/g-C3N4 nanosheets laminated homojunctions as efficient visible-light-driven photocatalysts Siyu Tan a, Zipeng Xing a,*, Jiaqi Zhang a, Zhenzi Li b,**, Xiaoyan Wu b, Jiayi Cui a, Junyan Kuang a, Junwei Yin a, Wei Zhou a,*** a
Department of Environmental Science, School of Chemistry and Materials Science, Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic of China, Heilongjiang University, Harbin 150080, PR China b Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin 150086, PR China
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abstract
Article history:
Mesoporous g-C3N4/g-C3N4 (Meso-g-C3N4/g-C3N4) nanosheets laminated homojunctions
Received 8 July 2017
have been fabricated via template-calcination strategy using melamine and amino cyan-
Received in revised form
amide as co-precursors. The prepared Meso-g-C3N4/g-C3N4 nanosheets laminated homo-
21 August 2017
junctions possess relative high surface area of 34 m2 g1, large pore size of 15.0 nm and
Accepted 23 August 2017
narrow band gap of 2.75 eV. The visible-light-driven photocatalytic reaction rate constant
Available online 18 September 2017
of methyl orange and hydrogen production rate (~115.6 mmol h1 g1) for Meso-g-C3N4/g-
Keywords:
the pristine g-C3N4, respectively. This may be attributed to the synergetic effect of the
g-C3N4
close-contact laminated structure contributing to the separation of photogenerated charge
Mesoporous structure
carriers and mesoporous structure facilitating the diffusion of reactants and products, and
Homojunction
offering more surface active sites. This novel laminated homojunction may open up a new
Laminated structure
avenue for designing other high-efficient photocatalysts.
C3N4 nanosheets laminated homojunctions is about 12.5 and 6.5 times higher than that of
Visible light photocatalysis
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Semiconductor photocatalysts have attracted unprecedented attention in mitigate the environmental issues and energy crisis, and mainly concentrated in photocatalytic H2 evolution and photocatalytic degradation of contaminants [1e3]. As a representative photocatalyst, titanium dioxide (TiO2) has emerged as one of the most interesting themes because of its
outstanding properties in nature [4e6]. Nevertheless, the relatively wide band gap of ~3.2 eV for anatase TiO2 seriously restricts its application [7,8]. In 2009, Wang et al. published the significative work on the polymeric graphitic carbon nitride (g-C3N4), which could split water into hydrogen [9]. Since then, g-C3N4 has drawn ever booming attention as a noble metal-free semiconductor photocatalyst owing to its low toxicity [10], easy availability
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Z. Xing), [email protected] (Z. Li), [email protected] (W. Zhou). http://dx.doi.org/10.1016/j.ijhydene.2017.08.202 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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[11], high thermal [12], chemical stability, and benign photocatalytic activity [13,14]. However, its photocatalytic efficiency also faces the invalidation because of the bulk g-C3N4 with low specific surface area (<10 m2 g1), poor quantum efficiency, and easy recombination of the photoinduced charge carriers, which restricts the photoinduced redox reaction [15e17]. Therefore, it is imperative to propose reasonable approaches to facilitate the separation efficiency of photo-generated electron-hole pairs, meanwhile, enhances the performance of photocatalysts. To date, extensive efforts have been committed to enhance the photoresponse horizon and limited the recombination of photogenerated charge carriers, such as mesostructure, the construction of heterojunction, noble metal deposition, and metal-doped/non-metal-doped semiconductor [18e21]. Among them, the methods, regulating its surface area and constructing the nanojunction structure in the composites, could enhance the photocatalytic activity of g-C3N4 by restricting the photogenerated charge carriers recombination. And it is highly appreciated, such as, fabricating advanced mesostructure and coupling with an array of materials. Wang et al. prepared the mesoporous g-C3N4 by the template method and studied on the photocatalytic hydrogen evolution, resulted in the photocatalytic H2 evolution is enhanced nearly 10-folds [22]. Besides, in some work, allotypic g-C3N4-based heterojunction such as g-C3N4/ graphene [23], TiO2/g-C3N4 [24], MoS2/g-C3N4 [25], ZnO/gC3N4 [26], WO3/g-C3N4 [27], NiMoO4/g-C3N4 [28], g-C3N4/ Ag2CrO4 [29], and g-C3N4/BiVO4 [30], have been actively studied. However, coupling with extra semiconductors will import the metal elements, causing pollution and energy penalty. Meanwhile, the reported band gap energies of g-C3N4 range from 2.4 to 2.8 eV depending on the different preparation conditions [31e33]. Different co-precursors result in different band structures, which can distinctly facilitate the charge separation and then enhance the photocatalytic activity. Subsequently, g-C3N4-based homojunctions have been found [34,35]. In consideration of that the band gap energies of gC3N4 may cause a little bit of mediation using different precursors. Coupling different components of g-C3N4 with wellmatched band structure to form a g-C3N4/g-C3N4 homojunction that exhibited the apparently photocatalytic activity [36]. Mesostructure g-C3N4 coupling with g-C3N4 to form homojunction may be good candidate for high-efficient visible light photocatalyst, in which the mesopores favor the diffusion for reactants and products, and could also provide sufficient surface active sites. Here, a novel Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction has been prepared using melamine and amino cyanamide as co-precursors via a facile template-calcination process. The prepared photocatalyst exhibits conspicuous photocatalytic activity for the degradation of methyl orange and hydrogen evolution under visible light illumination. Moreover, a possible photocatalytic mechanism of the Mesog-C3N4/g-C3N4 nanosheets laminated homojunction is also proposed. Significantly, the synthetic method will provide an efficient and environmentally friend way for the design of other homojunction structure.
Materials and methods Materials The amino cyanide aqueous solution (50 wt%), melamine, absolute ethanol (EtOH), 40% hydrofluoric acid (HF), and the silica colloidal solution (LUDOX HS-40, 40 wt% suspension in H2O) were all purchased from Sigma-Aldrich. All of the reagents used in the experiments were analytical grade and employed without further purification, and the deionized (DI) water was used throughout this study.
Synthesis of Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction The Meso-g-C3N4/g-C3N4 was prepared based on templatecalcination process. Typically, amino cyanide solution was dissolved in a dispersion of SiO2 nanoparticles of 12 nm in water with vigorous magnetic stirring in a fume hood for 9 h, then 5 g of melamine was put into the semitransparent mixture with magnetic stirring for 24 h. The well-mixed solution was placed into oven dried at 50 C until most of the water was evaporated. Then, the resulting white powder was directly heated in a ceramic combustion boat, which was annealed at 550 C for 4 h with a ramp rate of 2.3 C min1 and tempered for an additional 2.5 h. The resulting light yellow powder was treated with 200 mL of 1 M HF for 24 h, followed by centrifugation at 4000 rpm and washed with DI and ethanol for several times respectively to guarantee the complete removal of silica template. Finally, the resultant powders were dried at 80 C overnight. The ultimate yellow powder was denoted as Meso-g-C3N4/g-C3N4 (the mass ratio of Meso-gC3N4 and g-C3N4 is 2:1). For comparison, the Meso-g-C3N4 was also synthesized under the same conditions without mixing melamine. In addition, the bulk g-C3N4 has been prepared by heating melamine from the room temperature to 550 C with a ramp rate of 2.3 C min1 for 4 h in a muffle furnace, and then was kept at 550 C for 2.5 h (see Scheme 1).
Characterization The crystallinity of the Meso-g-C3N4/g-C3N4 was characterized by X-ray Diffraction (XRD) Patterns with a Bruker D8 advance under Cu Ka radiation (l ¼ 1.5406 A). Fourier transformed infrared (FT-IR) spectra were recorded on a PerkinElmer spectrum one system using KBr as diluents. Scanning electron microscopy (SEM) was performed with a Philips XL-30-ESEMFEG operated at an accelerating voltage of 20 kV. The structures and morphologies of all samples were characterized by transmission electron microscopy (JEM-2100 TEM). X-ray photoelectron spectroscopy (XPS) was measured by a PHI-5700 ESCA system. The specific surface area and pore size distribution of all samples were performed on Brunauer-EmmettTeller (BET) method on nitrogen adsorption apparatus with an AUTOSORB-1 instrument. UVevis diffuse reflectance spectroscopy (UV-DRS) was recorded with a UV-2550, Shimadzu UVevis spectrophotometer, in which BaSO4 was employed as the background. (The wavelength of UV lamp
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Scheme 1 e The formation procedure of Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction.
ranging from 200 to 400 nm, and the main wavelength is 365 nm) The electrochemical impedance spectroscopy (EIS) was carried out on an electrochemical system (IM6e Impedance measurement, Germany).
Photocatalytic degradation The photocatalytic activities of all samples were examined by the degradation of MO under visible light illumination. Visible-light irradiation was provided by a 300 W Xe light with a 420 nm cutoff filter at reaction temperature (20 ± 2 C). The degradation tests were carried out with 50 mL of MO solution (10.0 mg L1) containing 15 mg of as-synthesized photocatalyst, which was placed in a glass beaker. Before irradiation, the suspensions were magnetically stirred in the dark for 0.5 h to ensure an adsorption-desorption equilibrium. At intervals of every 15 min, 5 mL of the suspensions were withdrawn centrifuged and filtrated. The concentration of MO was analyzed by using a T6 UVevis spectrophotometer at l ¼ 464 nm.
Photoelectrochemical test To investigate the photoelectrochemical performance of the samples, an electrochemical workstation (CHI760E, Shanghai) was employed, with a standard three-electrode cell with the working electrode, the counter electrode (Pt sheet) and the reference electrode (Ag/AgCl). H2SO4 was utilized as the electrolyte solution. The photoelectrode were prepared as follows: the photocatalyst (50 mg) was suspended in 35 mL of terpineol under grinding and ultrasonication, which was then dipcoated onto a FTO-glass electrode and heated at 200 C for 2 h. Visible light irradiation was provided by a 300 W xenon lamp equipped with a 420 nm cutoff filter.
Photocatalytic hydrogen evolution The photocatalytic hydrogen production was carried out an online photocatalytic H2 generation system (Au Light, Beijing, CEL-SPH2N) at room temperature. Typically, 50 mg of the photocatalyst were suspended by constant stirring in 100 mL aqueous solution containing 20 mL of methanol and 80 mL of deionized water. Before irradiation, the suspension was purged with N2 for several times to remove air. A 300 W Xeonlamp with an AM 1.5 G filter (Oriel, USA) was utilized as a light source. Then, the hydrogen content was analyzed by on-line gas chromatograph (SP7800, TCD, molecular sieve 5 A, N2 carrier, Beijing Keruida, Ltd).
Results and discussion The crystal structures and crystalline phases of the assynthesized samples are probed by X-ray diffraction, which significantly affects the photocatalytic properties. Fig. 1a depicts the XRD patterns for the bare g-C3N4, Meso-g-C3N4, and Meso-g-C3N4/g-C3N4 homojunctions, respectively. The bare gC3N4 reveals a major contribution around 2q ¼ 27.3 , corresponding to the (002) plane, which is originated from the stacked conjugated aromatic systems. And a weak diffraction situates at 2q ¼ 13.1 stemmed from the crystal plane of tri-striazine units, which is well-indexed to the (100) crystal plane [37]. Moreover, other shoulder diffractions appears at about 17.8 and 21.4 can be ascribed to (600) and (650) planes of Mesog-C3N4, respectively, originating from the denser packing or a distortion of the melon structure in which every second melon sheet is replaced [38]. A minor observation shows that the characteristic (002) diffraction peak position of Meso-g-C3N4 shifts from 27.3 to 27.5 , which discloses the successful
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Fig. 1 e XRD patterns (a), FT-IR spectra (b), N2-adsorption/desorption isotherm curves (c), BJH pore size distribution plots (d), UVevis diffuse reflectance absorption spectra (e) and the corresponding calculated band gap (f) of g-C3N4, Meso-g-C3N4, and Meso-g-C3N4/g-C3N4, respectively. The inset of (a) is the magnified patterns between 23.0 and 33.9 .
delamination of the bulk g-C3N4. Interestingly, the diffraction angle of (002) for Meso-g-C3N4/g-C3N4 is located between gC3N4 and Meso-g-C3N4, corroborating the formation of Meso-gC3N4/g-C3N4 homojunction. Remarkably, the overall peaks weaken intensities are examined for Meso-g-C3N4/g-C3N4, which may reflect the crystallinity of the sample decrease when g-C3N4 combines into the Meso-g-C3N4 network, correlating well with the SEM and the TEM observation. From Fig. 1b, the molecular structure information is confirmed by the typical FT-IR spectra of the g-C3N4, Meso-gC3N4 and Meso-g-C3N4/g-C3N4 homojunction the results present almost the same characteristics, indicating that the basic structure and chemical composition have slightly changes. In the FT-IR spectra of all the samples, several strong bands in
the range of 1240e1580 cm1 can be ascribed to the presence of two main bonds in the materials, wherein the peaks at 1408 and 1571 cm1 are typical stretching vibration modes of C]N [39,40], respectively, and the peaks at 1240 and 1318 cm1 are corresponding to the CeH groups [14]. Moreover, the peak at 1640 cm1 is caused by the in-of-plane ]CeH stretching vibration modes and the peak near 2175 cm1 is assigned to the stretch vibration of C^N [41,42]. In addition, the sharper peak at 808 cm1 is attributed to the typical characteristic bending modes of the triazine ring system, which is in good agreement with literature [43]. Several wide peaks ranging from 3100 to 3300 cm1 can be contributed to the eNH2 or eNH groups and OeH stretching vibration due to the adsorption of physically water in all the samples [44,45].
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Table 1 e Parameters obtained from N2 desorption isotherm measurements. Samples g-C3N4 Meso-g-C3N4 Meso-g-C3N4/g-C3N4
SBET (m2 g1)
Average pore size (nm)
Pore volume (cm3 g1)
9 86 34
2.5 13.1 14.3
0.006 0.288 0.128
The nitrogen adsorption-desorption isotherms is measured to determine BET surface area and pore diameters of the as prepared samples. It can be seen from Fig. 1c that all samples are characteristic of type IV with a hysteresis loop, of Type H3 according to IUPAC classification, signifying the existence of mesoporous structure [46]. It is noticed that the isotherms of the Meso-g-C3N4 shifts up compared with bare g-C3N4 and Meso-g-C3N4/g-C3N4, implying Meso-g-C3N4 with higher BET surface area and pore volume. The pore size distribution in accordance with nonlocal density functional theory method in Fig. 1d shows that the sharp peak at 14.34 nm of Meso-g-C3N4/g-C3N4 is larger than the pure g-C3N4, that can be attributed to composite materials are stacked to form mesoporous structure, corroborating the formation of Meso-g-C3N4/gC3N4 homojunction.
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The multi-point Brunauer-Emmett-Teller (BET) surface area and cumulative pore volume distribution obtain for Meso-g-C3N4/g-C3N4 homojunction are 34 m2 g1 and 0.128 cm3 g1, respectively. These values are lower than that of the Meso-g-C3N4, 86 m2 g1 and 0.288 cm3 g1, which are summarized in Table 1. Obviously, the reduced specific surface area of Meso-g-C3N4/g-C3N4 may be caused by the excessive g-C3N4 that self-aggregating in the strong coupling between g-C3N4 and Meso-g-C3N4 wafer, thereby leading to the decrease of repulsive forces and the interlayer distance, which is consistent with the XRD. What's more, the surface area of bare g-C3N4 is merely 9 m2 g1 because of the higher thermal condensation process, the microstructure shows large patches, which is in close agreement with the SEM observations. As further elucidate the optical properties of g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/g-C3N4, UVevis diffuse reflectance spectra is performed. Fig. 1e shows that the gC3N4 exhibits an intense absorption peak centered at 471 nm. Comparing to the g-C3N4, the photo absorption edge of Mesog-C3N4 achieves a blue shift from 471 to 460 nm concomitantly. This phenomenon unfolds that the existence of abundantly defect sites correlates to the mesoporous structures. The absorption edge of Meso-g-C3N4/g-C3N4 is located between that of the g-C3N4 and the Meso-g-C3N4, which
Fig. 2 e TEM images of g-C3N4 (a), Meso-g-C3N4 (b) and Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction (c and d), respectively. The inset of (b) is the corresponding SEM image.
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confirms the efficient construction of homojunction between g-C3N4 and Meso-g-C3N4. Furthermore, the band gaps of the samples are estimated from the (ahn) 1/2 versus photon energy [47]. Accordingly, the band gaps of g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/g-C3N4 homojunction are calculated to be 2.69, 2.75, and 2.85 eV, respectively. The distinct divergence in band gap energy of gC3N4 specimens from melamine and cyanamide are liable to construct g-C3N4-based homojunction with well-matched band structure. The band gaps are accordance with the previous results [48]. The TEM images of g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/ g-C3N4 are presented in Fig. 2. From Fig. 2a, we can clearly see that the bare g-C3N4 shows large and smooth paper-fold layer sheets structure. However, the surface of Meso-g-C3N4 (Fig. 2b) becomes coarse compared with the bare g-C3N4, which unfolds thinner sheets with mesoporous morphology. In addition, the inset of Fig. 2b clearly shows the representative SEM image of the Meso-g-C3N4/g-C3N4 consists of some stacking flaky-like porous layer, which discriminates from that of bulk agglomerates. This discloses that such structure has the merits of providing larger surface area. The structure of Mesog-C3N4/g-C3N4 (Fig. 2c and d) looks layer-by-layer petals. The dense Meso-g-C3N4 sheet is found cladding tightly onto surface of g-C3N4 layer sheets, both of which congruously form a homojunction, inducing the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction with intimate contacts. Apparently,
a
Table 2 e The determined VB position, CB position, and band gap energy for g-C3N4, Meso-g-C3N4 and Meso-gC3N4/g-C3N4, respectively. Sample
CB position (eV)
Band gap Eg (eV)
1.96 2.31 1.76
0.73 0.54 0.99
2.69 2.85 2.75
284.7 eV
b
C 1s
Intensity (a.u.)
Intensity (a.u.)
C-C
O 1s
700
600
500
400
300
200
100
aromatic N
N 1s
397.2 eV Pyridine N
400.5 eV Pyrrol N
292
290
288
Binding energy (eV)
286
284
282
Binding energy (eV)
280
278
d
2.31
Meso-g-C3N4
1.96
g-C3N4
406 404 402 400 398 396 394 392 390 388
286.3 eV C-N
N=C-N2
Intensity (a.u.)
395.3 eV
c
290.5 eV
294
Binding energy (eV)
Intensity (a.u.)
VB position (eV)
g-C3N4 Meso-g-C3N4 Meso-g-C3N4/g-C3N4
N 1s
C 1s
800
the lamellar structure and the mesoporous structure are distinguished in the sample with an interface obviously. The specific morphology can provide more active sites to enhance the photocatalytic performance. XPS is elucidated to the chemical structure and the elements of samples. Fig. 3a displays the XPS survey spectra, which corroborate entity C 1s, N 1s, and O 1s peaks, the weak O 1s peak is attested to the adsorbed H2O, OH, O2, etc. which is commonly found in literature. The XPS spectrum in both C 1s (Fig. 3b) and N 1s (Fig. 3c) region can be fitted with three peaks, respectively. The C 1s spectrum signal of the Meso-gC3N4/g-C3N4 can be split into three components, containing the graphitic carbon (CeC) at 284.7 eV and the peak locate at 286.3 eV which can be accredited to carbon atoms bond to nitrogen atoms (CeN) [49]. The other peak at 290.5 eV is assigned to defect-containing sp2-bonded carbon in N]CeN2 [50]. The N 1s spectrum signal of the Meso-g-C3N4/g-C3N4 can
-2
0
2
4
6
Binding energy (eV)
8
10
Fig. 3 e XPS spectra of Meso-g-C3N4/g-C3N4 (a), C 1s (b), N 1s (c), and VB XPS spectra (d) for g-C3N4 and Meso-g-C3N4, respectively.
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be divided into three peaks. The high content of peak at 395.3 eV is ascribed to the amino N, and the peak at 398.6 eV is corresponding to the pyridinic-like N [51]. The weaker peak at 400.2 eV is attributed to the pyrrolic N. These assignments of C 1s and N 1s are in good agreement with the literature [52]. The band structure of g-C3N4 and Meso-g-C3N4 are researched by valence band (VB) XPS (Fig. 3d), indicating that the VB positions for bulk g-C3N4 and Meso-g-C3N4 are 2.31 and 1.96 eV, respectively. Therefore, the conduction band (CB) potential can be calculated by combining the band gap energy (Fig. 3d) and the VB XPS spectra, as shown in Table 2. The VB potential of g-C3N4 (1.96 eV) is more negative than that of Meso-g-C3N4 (2.31 eV), and the CB position of g-C3N4 (0.73 eV) is more negative than that of the Meso-g-C3N4 (0.54 eV). By coupling different composite precursors, the Meso-gC3N4/g-C3N4 nanosheets laminated homojunction can be formed effectively. As the mechanisms given in Fig. 4, mesoporous structure with larger specific surface area is prone to provide reaction active sites to advance the photocatalytic activity. Since the CB position of g-C3N4 is higher than that of Meso-g-C3N4, the photo-induced electrons tend to transfer from g-C3N4 to Meso-g-C3N4 driven by an offset of 0.19 eV, whereas photogenerated holes can be transported from Mesog-C3N4 to g-C3N4 driven by an offset at 0.35 eV, which facilitate photogenerated charge carriers separation and reducing the recombination rates, along with improving the photocatalytic property of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction. Considering the adsorption equilibrium between g-C3N4 and MO, the reaction mixture is magnetically stirred in dark for 15 min. Furthermore, considering the possibility that MO can self-degrade by irradiation without catalyst. Therefore, a blank test is also carried out as a norm. As shown in Fig. 5a and Fig. S2, the result reveals that no degradation is detected in the 90 min, and thus it can be considered that the self-degradation can be neglected. The different specific surface areas for different samples cause the disparity of the adsorption capacity. The g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/g-C3N4 are used as references. The inset of Fig. 5a, in the presence of the bare g-C3N4 only degrades nearly 15.1% MO within 90 min
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illumination. However, both Meso-g-C3N4 and Meso-g-C3N4/gC3N4 exhibit much better MO degradation capacity than that of g-C3N4. Evidently, the Meso-g-C3N4/g-C3N4 shows relatively significant degradation ability of photocatalytic for MO, and it can reach 91.0%. Furthermore, the photocatalytic activity of the Meso-g-C3N4/g-C3N4 is higher than that of individual Meso-g-C3N4. This may be attributed to the double type morphology, which can effectively facilitate electrons and holes separated. The inaccurate results of photocatalytic degradation of methyl orange are mainly caused by the incomplete precipitation during centrifugation, which leads to modest suspending g-C3N4 extracted. The tested results show that the error is less than 3%. In addition, the first-order rate constants of g-C3N4, Meso-g-C3N4, and Meso-g-C3N4/g-C3N4 show in Fig. 5b are 0.002, 0.010 and 0.025 min1, respectively. Furthermore, the reaction constant of the Meso-g-C3N4/gC3N4 is about 12.5 times higher than that of the g-C3N4. It is pronounced that Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction shows a strengthened photocatalytic activity. To further testify the excellent photocatalytic performance of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction, all samples are corroborated in an assay of photocatalytic water splitting using methanol as sacrificial reagent. As shown in Fig. S3, Meso-g-C3N4/g-C3N4 shows poor photocatalytic hydrogen evolution activity, which does not respond in dark. However, all photocatalysts can be excited under visible light, the results are shown in Fig. 5c, a significantly enhanced H2 production rate is achieved on the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction, which is approximately 6.5 times higher than that of the bare g-C3N4, and 1.7 times in relevant to that of the Meso-gC3N4. The lower H2 production rate of two single component photocatalysts are due to the fast recombination of photoinduced electron-holes and the reduced light-harvesting efficiency. Despite the Meso-g-C3N4 possesses larger specific surface areas, the hydrogen generation rate is still inferior in contrast to the Meso-g-C3N4/g-C3N4, revealing that the formation of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction structure can extremely favorable for facilitating the mobility of charge carriers as a function of
Fig. 4 e Transfer path mechanism of photogenerated electron-hole in Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction.
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Fig. 5 e Photodegradation of MO by using different samples under visible-light irradiation (a), variations of -ln(C/C0) versus visible-light irradiation time with different samples (b) (C is the corresponding degradative concentration of MO and C0 is concentration of MO after adsorption), the photocatalytic H2 evolution of different samples (c), the recyclability tests of the Meso-g-C3N4/g-C3N4 under AM 1.5 (d), the electrochemical impedance spectra of g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/gC3N4, respectively (e), and the Mott-Schottky plots of g-C3N4 and Meso-g-C3N4/g-C3N4 (f). interfacial charge transfer. Stability is another essential standard in practical applications. The stability of the Mesog-C3N4/g-C3N4 photocatalyst was tested under irradiation of 25 h, which is probed by Fig. 5d, no obvious loss in H2
evolution activity after five cycling experiments, elucidating the high stability. Electrochemical impedance spectroscopy (EIS) is an efficient electrochemical polarization technique to illustrate the
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efficiency of the charge mobility. Fig. 5e renders similar arc Nyquist plots for g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction. However, the impedance arc radius of the Meso-g-C3N4/g-C3N4 is smaller than that of bare g-C3N4 and Meso-g-C3N4, which indicates that the Mesog-C3N4/g-C3N4 has a lower resistance compared with that of the bare g-C3N4 and Meso-g-C3N4. This further discloses that the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction can facilitate separation and transfer of photogenerated electronhole pairs and enhance the photocatalytic property. As displayed in Fig. 5f, the positive slopes have been observed in the MotteSchottky (MeS) plots, elucidating that samples are n-type semiconductors. The result indicates that the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction shows a smaller slope in M-S plot than those of the bare g-C3N4, suggesting a higher charge carrier density and a better conductivity. Carrier density can be calculated from the corresponding slope according to Eq. (1) [53]. Nd ¼
2=e0 εε0 dð1=C2 Þ=dV
(1)
where Nd is carrier density, e0 is the electron charge, ε is the dielectric constant of the C3N4, and ε0 is the permittivity of vacuum. Taking the dielectric coefficient as 4.6 for the C3N4, the electron densities of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction and the g-C3N4 are 2.9 1020 cm3 and 2.4 1020 cm3, respectively. The richer donor density in the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction is due to the unique lamellar and mesoporous structure of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction, which is liable to facilitate the charge carrier mobility and then enhance photocatalytic and photoelectrochemical property.
Conclusions In summary, we have developed direct template-calcination method to synthesize the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction photocatalyst using melamine and amino cyanamide as co-precursors and SiO2 as the template. The mesoporous structure possesses larger surface area, and the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction performs higher visible light photocatalytic activity for MO degradation (degrades 91.0% MO within 90 min illumination) and photocatalytic hydrogen evolution (115.6 mmol h1 g1) in comparison to that of the bare g-C3N4, due to the synergetic effects of the g-C3N4 and Meso-g-C3N4. During photocatalytic reaction, the photo-induced electrons tend to transfer from the CB of the g-C3N4 to Meso-g-C3N4, facilitating photogenerated charge carriers separation and reducing the recombination rates. Thus, this ideally versatile homojunction may open up a new avenue for application of organic photosynthesis, photovoltaic, water purification and environmental remediation in future.
Acknowledgments We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21376065,
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51672073, and 81573134), the Heilongjiang Postdoctoral Startup Fund (LBH-Q14135), the Postdoctoral Science Foundation of China (2017M611399), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2015014 and UNPYSCT-2016018), the Postdoctoral Science Foundation of Heilongjiang Province (LBH-Z16150), and the Innovative Science Research Project of Harbin Medical University (2016JCZX13).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.08.202.
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Meso-g-C3N4/g-C3N4 nanosheets laminated homojunctions as efficient visible-light-driven photocatalysts Siyu Tan a, Zipeng Xing a,*, Jiaqi Zhang a, Zhenzi Li b,**, Xiaoyan Wu b, Jiayi Cui a, Junyan Kuang a, Junwei Yin a, Wei Zhou a,*** a
Department of Environmental Science, School of Chemistry and Materials Science, Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic of China, Heilongjiang University, Harbin 150080, PR China b Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin 150086, PR China
article info
abstract
Article history:
Mesoporous g-C3N4/g-C3N4 (Meso-g-C3N4/g-C3N4) nanosheets laminated homojunctions
Received 8 July 2017
have been fabricated via template-calcination strategy using melamine and amino cyan-
Received in revised form
amide as co-precursors. The prepared Meso-g-C3N4/g-C3N4 nanosheets laminated homo-
21 August 2017
junctions possess relative high surface area of 34 m2 g1, large pore size of 15.0 nm and
Accepted 23 August 2017
narrow band gap of 2.75 eV. The visible-light-driven photocatalytic reaction rate constant
Available online 18 September 2017
of methyl orange and hydrogen production rate (~115.6 mmol h1 g1) for Meso-g-C3N4/g-
Keywords:
the pristine g-C3N4, respectively. This may be attributed to the synergetic effect of the
g-C3N4
close-contact laminated structure contributing to the separation of photogenerated charge
Mesoporous structure
carriers and mesoporous structure facilitating the diffusion of reactants and products, and
Homojunction
offering more surface active sites. This novel laminated homojunction may open up a new
Laminated structure
avenue for designing other high-efficient photocatalysts.
C3N4 nanosheets laminated homojunctions is about 12.5 and 6.5 times higher than that of
Visible light photocatalysis
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Semiconductor photocatalysts have attracted unprecedented attention in mitigate the environmental issues and energy crisis, and mainly concentrated in photocatalytic H2 evolution and photocatalytic degradation of contaminants [1e3]. As a representative photocatalyst, titanium dioxide (TiO2) has emerged as one of the most interesting themes because of its
outstanding properties in nature [4e6]. Nevertheless, the relatively wide band gap of ~3.2 eV for anatase TiO2 seriously restricts its application [7,8]. In 2009, Wang et al. published the significative work on the polymeric graphitic carbon nitride (g-C3N4), which could split water into hydrogen [9]. Since then, g-C3N4 has drawn ever booming attention as a noble metal-free semiconductor photocatalyst owing to its low toxicity [10], easy availability
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Z. Xing), [email protected] (Z. Li), [email protected] (W. Zhou). http://dx.doi.org/10.1016/j.ijhydene.2017.08.202 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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[11], high thermal [12], chemical stability, and benign photocatalytic activity [13,14]. However, its photocatalytic efficiency also faces the invalidation because of the bulk g-C3N4 with low specific surface area (<10 m2 g1), poor quantum efficiency, and easy recombination of the photoinduced charge carriers, which restricts the photoinduced redox reaction [15e17]. Therefore, it is imperative to propose reasonable approaches to facilitate the separation efficiency of photo-generated electron-hole pairs, meanwhile, enhances the performance of photocatalysts. To date, extensive efforts have been committed to enhance the photoresponse horizon and limited the recombination of photogenerated charge carriers, such as mesostructure, the construction of heterojunction, noble metal deposition, and metal-doped/non-metal-doped semiconductor [18e21]. Among them, the methods, regulating its surface area and constructing the nanojunction structure in the composites, could enhance the photocatalytic activity of g-C3N4 by restricting the photogenerated charge carriers recombination. And it is highly appreciated, such as, fabricating advanced mesostructure and coupling with an array of materials. Wang et al. prepared the mesoporous g-C3N4 by the template method and studied on the photocatalytic hydrogen evolution, resulted in the photocatalytic H2 evolution is enhanced nearly 10-folds [22]. Besides, in some work, allotypic g-C3N4-based heterojunction such as g-C3N4/ graphene [23], TiO2/g-C3N4 [24], MoS2/g-C3N4 [25], ZnO/gC3N4 [26], WO3/g-C3N4 [27], NiMoO4/g-C3N4 [28], g-C3N4/ Ag2CrO4 [29], and g-C3N4/BiVO4 [30], have been actively studied. However, coupling with extra semiconductors will import the metal elements, causing pollution and energy penalty. Meanwhile, the reported band gap energies of g-C3N4 range from 2.4 to 2.8 eV depending on the different preparation conditions [31e33]. Different co-precursors result in different band structures, which can distinctly facilitate the charge separation and then enhance the photocatalytic activity. Subsequently, g-C3N4-based homojunctions have been found [34,35]. In consideration of that the band gap energies of gC3N4 may cause a little bit of mediation using different precursors. Coupling different components of g-C3N4 with wellmatched band structure to form a g-C3N4/g-C3N4 homojunction that exhibited the apparently photocatalytic activity [36]. Mesostructure g-C3N4 coupling with g-C3N4 to form homojunction may be good candidate for high-efficient visible light photocatalyst, in which the mesopores favor the diffusion for reactants and products, and could also provide sufficient surface active sites. Here, a novel Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction has been prepared using melamine and amino cyanamide as co-precursors via a facile template-calcination process. The prepared photocatalyst exhibits conspicuous photocatalytic activity for the degradation of methyl orange and hydrogen evolution under visible light illumination. Moreover, a possible photocatalytic mechanism of the Mesog-C3N4/g-C3N4 nanosheets laminated homojunction is also proposed. Significantly, the synthetic method will provide an efficient and environmentally friend way for the design of other homojunction structure.
Materials and methods Materials The amino cyanide aqueous solution (50 wt%), melamine, absolute ethanol (EtOH), 40% hydrofluoric acid (HF), and the silica colloidal solution (LUDOX HS-40, 40 wt% suspension in H2O) were all purchased from Sigma-Aldrich. All of the reagents used in the experiments were analytical grade and employed without further purification, and the deionized (DI) water was used throughout this study.
Synthesis of Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction The Meso-g-C3N4/g-C3N4 was prepared based on templatecalcination process. Typically, amino cyanide solution was dissolved in a dispersion of SiO2 nanoparticles of 12 nm in water with vigorous magnetic stirring in a fume hood for 9 h, then 5 g of melamine was put into the semitransparent mixture with magnetic stirring for 24 h. The well-mixed solution was placed into oven dried at 50 C until most of the water was evaporated. Then, the resulting white powder was directly heated in a ceramic combustion boat, which was annealed at 550 C for 4 h with a ramp rate of 2.3 C min1 and tempered for an additional 2.5 h. The resulting light yellow powder was treated with 200 mL of 1 M HF for 24 h, followed by centrifugation at 4000 rpm and washed with DI and ethanol for several times respectively to guarantee the complete removal of silica template. Finally, the resultant powders were dried at 80 C overnight. The ultimate yellow powder was denoted as Meso-g-C3N4/g-C3N4 (the mass ratio of Meso-gC3N4 and g-C3N4 is 2:1). For comparison, the Meso-g-C3N4 was also synthesized under the same conditions without mixing melamine. In addition, the bulk g-C3N4 has been prepared by heating melamine from the room temperature to 550 C with a ramp rate of 2.3 C min1 for 4 h in a muffle furnace, and then was kept at 550 C for 2.5 h (see Scheme 1).
Characterization The crystallinity of the Meso-g-C3N4/g-C3N4 was characterized by X-ray Diffraction (XRD) Patterns with a Bruker D8 advance under Cu Ka radiation (l ¼ 1.5406 A). Fourier transformed infrared (FT-IR) spectra were recorded on a PerkinElmer spectrum one system using KBr as diluents. Scanning electron microscopy (SEM) was performed with a Philips XL-30-ESEMFEG operated at an accelerating voltage of 20 kV. The structures and morphologies of all samples were characterized by transmission electron microscopy (JEM-2100 TEM). X-ray photoelectron spectroscopy (XPS) was measured by a PHI-5700 ESCA system. The specific surface area and pore size distribution of all samples were performed on Brunauer-EmmettTeller (BET) method on nitrogen adsorption apparatus with an AUTOSORB-1 instrument. UVevis diffuse reflectance spectroscopy (UV-DRS) was recorded with a UV-2550, Shimadzu UVevis spectrophotometer, in which BaSO4 was employed as the background. (The wavelength of UV lamp
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Scheme 1 e The formation procedure of Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction.
ranging from 200 to 400 nm, and the main wavelength is 365 nm) The electrochemical impedance spectroscopy (EIS) was carried out on an electrochemical system (IM6e Impedance measurement, Germany).
Photocatalytic degradation The photocatalytic activities of all samples were examined by the degradation of MO under visible light illumination. Visible-light irradiation was provided by a 300 W Xe light with a 420 nm cutoff filter at reaction temperature (20 ± 2 C). The degradation tests were carried out with 50 mL of MO solution (10.0 mg L1) containing 15 mg of as-synthesized photocatalyst, which was placed in a glass beaker. Before irradiation, the suspensions were magnetically stirred in the dark for 0.5 h to ensure an adsorption-desorption equilibrium. At intervals of every 15 min, 5 mL of the suspensions were withdrawn centrifuged and filtrated. The concentration of MO was analyzed by using a T6 UVevis spectrophotometer at l ¼ 464 nm.
Photoelectrochemical test To investigate the photoelectrochemical performance of the samples, an electrochemical workstation (CHI760E, Shanghai) was employed, with a standard three-electrode cell with the working electrode, the counter electrode (Pt sheet) and the reference electrode (Ag/AgCl). H2SO4 was utilized as the electrolyte solution. The photoelectrode were prepared as follows: the photocatalyst (50 mg) was suspended in 35 mL of terpineol under grinding and ultrasonication, which was then dipcoated onto a FTO-glass electrode and heated at 200 C for 2 h. Visible light irradiation was provided by a 300 W xenon lamp equipped with a 420 nm cutoff filter.
Photocatalytic hydrogen evolution The photocatalytic hydrogen production was carried out an online photocatalytic H2 generation system (Au Light, Beijing, CEL-SPH2N) at room temperature. Typically, 50 mg of the photocatalyst were suspended by constant stirring in 100 mL aqueous solution containing 20 mL of methanol and 80 mL of deionized water. Before irradiation, the suspension was purged with N2 for several times to remove air. A 300 W Xeonlamp with an AM 1.5 G filter (Oriel, USA) was utilized as a light source. Then, the hydrogen content was analyzed by on-line gas chromatograph (SP7800, TCD, molecular sieve 5 A, N2 carrier, Beijing Keruida, Ltd).
Results and discussion The crystal structures and crystalline phases of the assynthesized samples are probed by X-ray diffraction, which significantly affects the photocatalytic properties. Fig. 1a depicts the XRD patterns for the bare g-C3N4, Meso-g-C3N4, and Meso-g-C3N4/g-C3N4 homojunctions, respectively. The bare gC3N4 reveals a major contribution around 2q ¼ 27.3 , corresponding to the (002) plane, which is originated from the stacked conjugated aromatic systems. And a weak diffraction situates at 2q ¼ 13.1 stemmed from the crystal plane of tri-striazine units, which is well-indexed to the (100) crystal plane [37]. Moreover, other shoulder diffractions appears at about 17.8 and 21.4 can be ascribed to (600) and (650) planes of Mesog-C3N4, respectively, originating from the denser packing or a distortion of the melon structure in which every second melon sheet is replaced [38]. A minor observation shows that the characteristic (002) diffraction peak position of Meso-g-C3N4 shifts from 27.3 to 27.5 , which discloses the successful
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Fig. 1 e XRD patterns (a), FT-IR spectra (b), N2-adsorption/desorption isotherm curves (c), BJH pore size distribution plots (d), UVevis diffuse reflectance absorption spectra (e) and the corresponding calculated band gap (f) of g-C3N4, Meso-g-C3N4, and Meso-g-C3N4/g-C3N4, respectively. The inset of (a) is the magnified patterns between 23.0 and 33.9 .
delamination of the bulk g-C3N4. Interestingly, the diffraction angle of (002) for Meso-g-C3N4/g-C3N4 is located between gC3N4 and Meso-g-C3N4, corroborating the formation of Meso-gC3N4/g-C3N4 homojunction. Remarkably, the overall peaks weaken intensities are examined for Meso-g-C3N4/g-C3N4, which may reflect the crystallinity of the sample decrease when g-C3N4 combines into the Meso-g-C3N4 network, correlating well with the SEM and the TEM observation. From Fig. 1b, the molecular structure information is confirmed by the typical FT-IR spectra of the g-C3N4, Meso-gC3N4 and Meso-g-C3N4/g-C3N4 homojunction the results present almost the same characteristics, indicating that the basic structure and chemical composition have slightly changes. In the FT-IR spectra of all the samples, several strong bands in
the range of 1240e1580 cm1 can be ascribed to the presence of two main bonds in the materials, wherein the peaks at 1408 and 1571 cm1 are typical stretching vibration modes of C]N [39,40], respectively, and the peaks at 1240 and 1318 cm1 are corresponding to the CeH groups [14]. Moreover, the peak at 1640 cm1 is caused by the in-of-plane ]CeH stretching vibration modes and the peak near 2175 cm1 is assigned to the stretch vibration of C^N [41,42]. In addition, the sharper peak at 808 cm1 is attributed to the typical characteristic bending modes of the triazine ring system, which is in good agreement with literature [43]. Several wide peaks ranging from 3100 to 3300 cm1 can be contributed to the eNH2 or eNH groups and OeH stretching vibration due to the adsorption of physically water in all the samples [44,45].
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Table 1 e Parameters obtained from N2 desorption isotherm measurements. Samples g-C3N4 Meso-g-C3N4 Meso-g-C3N4/g-C3N4
SBET (m2 g1)
Average pore size (nm)
Pore volume (cm3 g1)
9 86 34
2.5 13.1 14.3
0.006 0.288 0.128
The nitrogen adsorption-desorption isotherms is measured to determine BET surface area and pore diameters of the as prepared samples. It can be seen from Fig. 1c that all samples are characteristic of type IV with a hysteresis loop, of Type H3 according to IUPAC classification, signifying the existence of mesoporous structure [46]. It is noticed that the isotherms of the Meso-g-C3N4 shifts up compared with bare g-C3N4 and Meso-g-C3N4/g-C3N4, implying Meso-g-C3N4 with higher BET surface area and pore volume. The pore size distribution in accordance with nonlocal density functional theory method in Fig. 1d shows that the sharp peak at 14.34 nm of Meso-g-C3N4/g-C3N4 is larger than the pure g-C3N4, that can be attributed to composite materials are stacked to form mesoporous structure, corroborating the formation of Meso-g-C3N4/gC3N4 homojunction.
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The multi-point Brunauer-Emmett-Teller (BET) surface area and cumulative pore volume distribution obtain for Meso-g-C3N4/g-C3N4 homojunction are 34 m2 g1 and 0.128 cm3 g1, respectively. These values are lower than that of the Meso-g-C3N4, 86 m2 g1 and 0.288 cm3 g1, which are summarized in Table 1. Obviously, the reduced specific surface area of Meso-g-C3N4/g-C3N4 may be caused by the excessive g-C3N4 that self-aggregating in the strong coupling between g-C3N4 and Meso-g-C3N4 wafer, thereby leading to the decrease of repulsive forces and the interlayer distance, which is consistent with the XRD. What's more, the surface area of bare g-C3N4 is merely 9 m2 g1 because of the higher thermal condensation process, the microstructure shows large patches, which is in close agreement with the SEM observations. As further elucidate the optical properties of g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/g-C3N4, UVevis diffuse reflectance spectra is performed. Fig. 1e shows that the gC3N4 exhibits an intense absorption peak centered at 471 nm. Comparing to the g-C3N4, the photo absorption edge of Mesog-C3N4 achieves a blue shift from 471 to 460 nm concomitantly. This phenomenon unfolds that the existence of abundantly defect sites correlates to the mesoporous structures. The absorption edge of Meso-g-C3N4/g-C3N4 is located between that of the g-C3N4 and the Meso-g-C3N4, which
Fig. 2 e TEM images of g-C3N4 (a), Meso-g-C3N4 (b) and Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction (c and d), respectively. The inset of (b) is the corresponding SEM image.
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confirms the efficient construction of homojunction between g-C3N4 and Meso-g-C3N4. Furthermore, the band gaps of the samples are estimated from the (ahn) 1/2 versus photon energy [47]. Accordingly, the band gaps of g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/g-C3N4 homojunction are calculated to be 2.69, 2.75, and 2.85 eV, respectively. The distinct divergence in band gap energy of gC3N4 specimens from melamine and cyanamide are liable to construct g-C3N4-based homojunction with well-matched band structure. The band gaps are accordance with the previous results [48]. The TEM images of g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/ g-C3N4 are presented in Fig. 2. From Fig. 2a, we can clearly see that the bare g-C3N4 shows large and smooth paper-fold layer sheets structure. However, the surface of Meso-g-C3N4 (Fig. 2b) becomes coarse compared with the bare g-C3N4, which unfolds thinner sheets with mesoporous morphology. In addition, the inset of Fig. 2b clearly shows the representative SEM image of the Meso-g-C3N4/g-C3N4 consists of some stacking flaky-like porous layer, which discriminates from that of bulk agglomerates. This discloses that such structure has the merits of providing larger surface area. The structure of Mesog-C3N4/g-C3N4 (Fig. 2c and d) looks layer-by-layer petals. The dense Meso-g-C3N4 sheet is found cladding tightly onto surface of g-C3N4 layer sheets, both of which congruously form a homojunction, inducing the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction with intimate contacts. Apparently,
a
Table 2 e The determined VB position, CB position, and band gap energy for g-C3N4, Meso-g-C3N4 and Meso-gC3N4/g-C3N4, respectively. Sample
CB position (eV)
Band gap Eg (eV)
1.96 2.31 1.76
0.73 0.54 0.99
2.69 2.85 2.75
284.7 eV
b
C 1s
Intensity (a.u.)
Intensity (a.u.)
C-C
O 1s
700
600
500
400
300
200
100
aromatic N
N 1s
397.2 eV Pyridine N
400.5 eV Pyrrol N
292
290
288
Binding energy (eV)
286
284
282
Binding energy (eV)
280
278
d
2.31
Meso-g-C3N4
1.96
g-C3N4
406 404 402 400 398 396 394 392 390 388
286.3 eV C-N
N=C-N2
Intensity (a.u.)
395.3 eV
c
290.5 eV
294
Binding energy (eV)
Intensity (a.u.)
VB position (eV)
g-C3N4 Meso-g-C3N4 Meso-g-C3N4/g-C3N4
N 1s
C 1s
800
the lamellar structure and the mesoporous structure are distinguished in the sample with an interface obviously. The specific morphology can provide more active sites to enhance the photocatalytic performance. XPS is elucidated to the chemical structure and the elements of samples. Fig. 3a displays the XPS survey spectra, which corroborate entity C 1s, N 1s, and O 1s peaks, the weak O 1s peak is attested to the adsorbed H2O, OH, O2, etc. which is commonly found in literature. The XPS spectrum in both C 1s (Fig. 3b) and N 1s (Fig. 3c) region can be fitted with three peaks, respectively. The C 1s spectrum signal of the Meso-gC3N4/g-C3N4 can be split into three components, containing the graphitic carbon (CeC) at 284.7 eV and the peak locate at 286.3 eV which can be accredited to carbon atoms bond to nitrogen atoms (CeN) [49]. The other peak at 290.5 eV is assigned to defect-containing sp2-bonded carbon in N]CeN2 [50]. The N 1s spectrum signal of the Meso-g-C3N4/g-C3N4 can
-2
0
2
4
6
Binding energy (eV)
8
10
Fig. 3 e XPS spectra of Meso-g-C3N4/g-C3N4 (a), C 1s (b), N 1s (c), and VB XPS spectra (d) for g-C3N4 and Meso-g-C3N4, respectively.
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be divided into three peaks. The high content of peak at 395.3 eV is ascribed to the amino N, and the peak at 398.6 eV is corresponding to the pyridinic-like N [51]. The weaker peak at 400.2 eV is attributed to the pyrrolic N. These assignments of C 1s and N 1s are in good agreement with the literature [52]. The band structure of g-C3N4 and Meso-g-C3N4 are researched by valence band (VB) XPS (Fig. 3d), indicating that the VB positions for bulk g-C3N4 and Meso-g-C3N4 are 2.31 and 1.96 eV, respectively. Therefore, the conduction band (CB) potential can be calculated by combining the band gap energy (Fig. 3d) and the VB XPS spectra, as shown in Table 2. The VB potential of g-C3N4 (1.96 eV) is more negative than that of Meso-g-C3N4 (2.31 eV), and the CB position of g-C3N4 (0.73 eV) is more negative than that of the Meso-g-C3N4 (0.54 eV). By coupling different composite precursors, the Meso-gC3N4/g-C3N4 nanosheets laminated homojunction can be formed effectively. As the mechanisms given in Fig. 4, mesoporous structure with larger specific surface area is prone to provide reaction active sites to advance the photocatalytic activity. Since the CB position of g-C3N4 is higher than that of Meso-g-C3N4, the photo-induced electrons tend to transfer from g-C3N4 to Meso-g-C3N4 driven by an offset of 0.19 eV, whereas photogenerated holes can be transported from Mesog-C3N4 to g-C3N4 driven by an offset at 0.35 eV, which facilitate photogenerated charge carriers separation and reducing the recombination rates, along with improving the photocatalytic property of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction. Considering the adsorption equilibrium between g-C3N4 and MO, the reaction mixture is magnetically stirred in dark for 15 min. Furthermore, considering the possibility that MO can self-degrade by irradiation without catalyst. Therefore, a blank test is also carried out as a norm. As shown in Fig. 5a and Fig. S2, the result reveals that no degradation is detected in the 90 min, and thus it can be considered that the self-degradation can be neglected. The different specific surface areas for different samples cause the disparity of the adsorption capacity. The g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/g-C3N4 are used as references. The inset of Fig. 5a, in the presence of the bare g-C3N4 only degrades nearly 15.1% MO within 90 min
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illumination. However, both Meso-g-C3N4 and Meso-g-C3N4/gC3N4 exhibit much better MO degradation capacity than that of g-C3N4. Evidently, the Meso-g-C3N4/g-C3N4 shows relatively significant degradation ability of photocatalytic for MO, and it can reach 91.0%. Furthermore, the photocatalytic activity of the Meso-g-C3N4/g-C3N4 is higher than that of individual Meso-g-C3N4. This may be attributed to the double type morphology, which can effectively facilitate electrons and holes separated. The inaccurate results of photocatalytic degradation of methyl orange are mainly caused by the incomplete precipitation during centrifugation, which leads to modest suspending g-C3N4 extracted. The tested results show that the error is less than 3%. In addition, the first-order rate constants of g-C3N4, Meso-g-C3N4, and Meso-g-C3N4/g-C3N4 show in Fig. 5b are 0.002, 0.010 and 0.025 min1, respectively. Furthermore, the reaction constant of the Meso-g-C3N4/gC3N4 is about 12.5 times higher than that of the g-C3N4. It is pronounced that Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction shows a strengthened photocatalytic activity. To further testify the excellent photocatalytic performance of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction, all samples are corroborated in an assay of photocatalytic water splitting using methanol as sacrificial reagent. As shown in Fig. S3, Meso-g-C3N4/g-C3N4 shows poor photocatalytic hydrogen evolution activity, which does not respond in dark. However, all photocatalysts can be excited under visible light, the results are shown in Fig. 5c, a significantly enhanced H2 production rate is achieved on the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction, which is approximately 6.5 times higher than that of the bare g-C3N4, and 1.7 times in relevant to that of the Meso-gC3N4. The lower H2 production rate of two single component photocatalysts are due to the fast recombination of photoinduced electron-holes and the reduced light-harvesting efficiency. Despite the Meso-g-C3N4 possesses larger specific surface areas, the hydrogen generation rate is still inferior in contrast to the Meso-g-C3N4/g-C3N4, revealing that the formation of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction structure can extremely favorable for facilitating the mobility of charge carriers as a function of
Fig. 4 e Transfer path mechanism of photogenerated electron-hole in Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction.
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Fig. 5 e Photodegradation of MO by using different samples under visible-light irradiation (a), variations of -ln(C/C0) versus visible-light irradiation time with different samples (b) (C is the corresponding degradative concentration of MO and C0 is concentration of MO after adsorption), the photocatalytic H2 evolution of different samples (c), the recyclability tests of the Meso-g-C3N4/g-C3N4 under AM 1.5 (d), the electrochemical impedance spectra of g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/gC3N4, respectively (e), and the Mott-Schottky plots of g-C3N4 and Meso-g-C3N4/g-C3N4 (f). interfacial charge transfer. Stability is another essential standard in practical applications. The stability of the Mesog-C3N4/g-C3N4 photocatalyst was tested under irradiation of 25 h, which is probed by Fig. 5d, no obvious loss in H2
evolution activity after five cycling experiments, elucidating the high stability. Electrochemical impedance spectroscopy (EIS) is an efficient electrochemical polarization technique to illustrate the
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efficiency of the charge mobility. Fig. 5e renders similar arc Nyquist plots for g-C3N4, Meso-g-C3N4 and Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction. However, the impedance arc radius of the Meso-g-C3N4/g-C3N4 is smaller than that of bare g-C3N4 and Meso-g-C3N4, which indicates that the Mesog-C3N4/g-C3N4 has a lower resistance compared with that of the bare g-C3N4 and Meso-g-C3N4. This further discloses that the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction can facilitate separation and transfer of photogenerated electronhole pairs and enhance the photocatalytic property. As displayed in Fig. 5f, the positive slopes have been observed in the MotteSchottky (MeS) plots, elucidating that samples are n-type semiconductors. The result indicates that the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction shows a smaller slope in M-S plot than those of the bare g-C3N4, suggesting a higher charge carrier density and a better conductivity. Carrier density can be calculated from the corresponding slope according to Eq. (1) [53]. Nd ¼
2=e0 εε0 dð1=C2 Þ=dV
(1)
where Nd is carrier density, e0 is the electron charge, ε is the dielectric constant of the C3N4, and ε0 is the permittivity of vacuum. Taking the dielectric coefficient as 4.6 for the C3N4, the electron densities of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction and the g-C3N4 are 2.9 1020 cm3 and 2.4 1020 cm3, respectively. The richer donor density in the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction is due to the unique lamellar and mesoporous structure of the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction, which is liable to facilitate the charge carrier mobility and then enhance photocatalytic and photoelectrochemical property.
Conclusions In summary, we have developed direct template-calcination method to synthesize the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction photocatalyst using melamine and amino cyanamide as co-precursors and SiO2 as the template. The mesoporous structure possesses larger surface area, and the Meso-g-C3N4/g-C3N4 nanosheets laminated homojunction performs higher visible light photocatalytic activity for MO degradation (degrades 91.0% MO within 90 min illumination) and photocatalytic hydrogen evolution (115.6 mmol h1 g1) in comparison to that of the bare g-C3N4, due to the synergetic effects of the g-C3N4 and Meso-g-C3N4. During photocatalytic reaction, the photo-induced electrons tend to transfer from the CB of the g-C3N4 to Meso-g-C3N4, facilitating photogenerated charge carriers separation and reducing the recombination rates. Thus, this ideally versatile homojunction may open up a new avenue for application of organic photosynthesis, photovoltaic, water purification and environmental remediation in future.
Acknowledgments We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21376065,
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51672073, and 81573134), the Heilongjiang Postdoctoral Startup Fund (LBH-Q14135), the Postdoctoral Science Foundation of China (2017M611399), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2015014 and UNPYSCT-2016018), the Postdoctoral Science Foundation of Heilongjiang Province (LBH-Z16150), and the Innovative Science Research Project of Harbin Medical University (2016JCZX13).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.08.202.
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