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Mesoporous SiO2-derived g-C3N4@CdS core-shell heteronanostructure for efficient and stable photocatalytic H2 production
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Mesoporous SiO2-derived g-C3N4@CdS core-shell heteronanostructure for efficient and stable photocatalytic H2 production Wei Wanga,b,∗, Jiaojiao Fangb a b
School of Chemistry and Materials Science, Nanjing University of Information Science & Technology, Nanjing, 210044, China State Key Laboratory of Materials-Orient Chemical Engineering, Nanjing Tech University, Nanjing, 210009, China
ARTICLE INFO
ABSTRACT
Keywords: Heteronanostructure g-C3N4 exfoliation Core-shell photocatalyst Hydrogen production Recyclability
Semiconductor-based heteronanostructures with wide light absorption and efficient electron-hole separation properties exhibit great potential for photocatalytic applications. Here we employ the mesoporous SiO2 nanoparticles as the template to simultaneously exfoliate the nano-confined g-C3N4 and deposit the CdS nanoparticles to form a series of core-shell SiCN@CdS heteronanostructures for photocatalytic H2 production under the visible light irradiation. Based on the physicochemical characterizations, we have been able to discuss the compositions, structures, and properties of the core-shell photocatalysts. Benefiting from the optimized equilibrium state of a relative high specific surface area, light absorption, crystalline degree, and a homogeneous dispersion of the outer CdS nanoparticles, the SiCN@2CdS exhibits the highest photocatalytic activity and stability for H2 production under the visible light irradiation. Results also suggest that the existence of the SiO2 template is benefit to suppress the aggregation and improve the recyclability of the core-shell photocatalysts when they are applied in the aqueous phase. The present study suggests a new sight on the template-assisted designing and synthesizing high-reactive, low-cost, environmental-stable, and easy-recyclable photocatalysts by precisely controlling the microstructures.
1. Introduction Photocatalysis has been attracting wide interests all over the world for its promising abilities of enabling utilize the solar energy for clean fuel production and environmental remediation [1–8]. Exploring the high-reactive, environmental-stable, and recyclable photocatalysts is still a key issue to promote their potential for large-scale practical applications [9]. In the last ten years, the non-metal g-C3N4, which not only has the advantages of the star photocatast, titanium dioxide, but also has a relative narrower band gap, has been widely studied in the photocatalysis field since it was firstly reported by Wang et al. [10,11]. The bulk g-C3N4 can be easily synthesized by the thermal polymerization method with various chemicals, such as urea, melamine, and thiourea, as the precursors [12,13]. Changing the experiment parameters, annealing atmosphere, doping atoms, and coupling with other functional materials are also widely studied to optimize the physicochemical properties of the randomly synthesized g-C3N4 [14–21]. However, the low specific surface area and the fast recombination of the photogenerated electron-hole pairs are still the main shortcomings restricting the quantum efficiency of g-C3N4 applied in the natural environment.
∗
Inspired by the studies of two-dimensional graphene exfoliation, researchers have confirmed that exfoliate the bulk g-C3N4 into the fewlayered sheets is also a promising strategy to both improve the specific surface area and the separation efficiency of the photogenerated electron-hole pairs [22,23]. Various strategies, such as the acid oxidation, thermal oxidation, mechanical shearing, and ultrasonication, have been widely employed to reduce the van der Waals’ force among the g-C3N4 layers [24–28]. Moreover, various mesoporous and thermal-stable templates are employed to synthesize the g-C3N4 within the nano- or micron-pores based on the facial synthesis procedure. After removing the templates, the confined g-C3N4 are reported to have several advantages, including directional migration path of the photocarriers, improved specific surface area, and high adsorption ability etc., compared to the randomly synthesized bulk ones [29–32]. However, new issues, such as the low productivity and the poor recyclability, are emerged and needed to be solved to meet the demand of practical application [33]. As a result, it is urgent to explore a method to exfoliate the bulk g-C3N4 and simultaneously improve its productivity and recyclability. Recently, several types of g-C3N4 synthesized in the nanopores of mesoporous SiO2 templated were reported [34–36]. The quantum size
Corresponding author. School of Chemistry and Materials Science, Nanjing University of Information Science & Technology, Nanjing, 210044, China. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.ceramint.2019.09.230 Received 16 August 2019; Received in revised form 21 September 2019; Accepted 23 September 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Wei Wang and Jiaojiao Fang, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.230
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effect, specific surface area, and photocatalytic activity of the confined g-C3N4 were significantly improved when the SiO2 templates were removed. However, the light absorption, which often shows a blue-shift due to the reduced structure unit, of the confined g-C3N4 is needed to be improved. Considering that the SiO2 is an environment-stable matrix and it has been widely studied in the photocatalysis field to improve the photocatalytic activities of titanium dioxide-based photocatalysts. In the present study, we are trying to exfoliate the g-C3N4 within the nanopores of the mesoporous SiO2 nanoparticles to improve its productivity and recyclability. Moreover, CdS, an efficient visible-lightdriven photocatalyst, nanoparticles are also simultaneously deposited onto the surface to form a core-shell heteronanostructure with the exfoliated g-C3N4 to boost the photocatalytic activity [37]. Microstructures and properties of the core-shell photocatalysts are systematically discussed and optimized to meet the demands of high-efficient and stable H2 production under the visible light irradiation.
2.3. Characterizations
2. Experimental section
Morphology characterizations were conducted on a S4800 (Hitachi) scanning electron microscopy (SEM) and a JEM-2010 (JEOL) transmission electron microscopy (TEM). Powder X-ray diffraction (XRD) analysis was conducted on a SmartLab-3KW (Rigaku) X-ray diffractometer with a Cu Kα irradiation source at a scanning speed of 10°/ min. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a PHI5000 (ULVAD-PHI) Versaprobe system with monochromatic Al Kα radiation. Light absorption spectra were analyzed on the UV-3010 (Shimadzu) UV–vis–NIR spectrometer. Nitrogen adsorption-desorption analysis was conducted on the Autosorb-iQ (Quantachrome) automatic adsorption instrument. Fourier Transformed Infrared (FTIR) spectra were analyzed on a Frontier (PerkinElmer) spectrometer. Thermogravimetry (TG) analysis was conducted on a Diamond (PerkinElmer) thermos gravimetric analyzer with a temperature ramping speed of 5 °C/min in the air atmosphere. Transient photocurrent curves were obtained on the CHI660E electrochemical workstation (ChenHua, China) under the visible light irradiation.
2.1. Chemicals and materials
2.4. Photocatalytic H2 production
All chemical reagents were commercially available and used without further purification. Cyanamide (≥98%), cadmium acetate dehydrate Cd(Ac)2.2H2O, (AR), tetraethyl orthosilicate (TEOS, AR, 98%), ethanol (AR, moisture < 0.3%), ammonia aqueous solution (AR, 25–28%), trimethoxy(octadecyl)silane (C18TMOS, 90%), dimethyl sulfoxide (DMSO, AR, > 99%), acetone (AR), methanol (AR), H2PtCl6·6H2O (AR), and triethanolamine (AR) were purchased from Aladdin.
20 mg of photocatalyst was dispersed in 40 mL of aqueous solution containing 100 μL of methanol and 320 μL of H2PtCl6 aqueous solution (3.2 g/L), followed by irradiating under a 365 nm LED (40 W) for 2 h to deposit the Pt nanoparticles, the cocatalyst, onto the photocatalyst surface. The suspension was then washed with deionized water twice and redispersed into 40 mL of triethanolamine aqueous solution (10 wt %), followed by deoxygenating with argon and irradiating with a 300 W xenon lamp (CEAULIGHT) fixed with an UV-cut-off filter to conduct the photocatalytic reaction. The produced H2 was analyzed on a gas chromatography system (Agilent Technologies 7890B).
2.2. Synthesis of the photocatalysts
3. Results and discussion
(1) Core-shell SiO2 nanoparticles. Mesoporous SiO2 nanoparticles with a core-shell structure were prepared based on the reported procedure with minor modification [36]. Typically, 73 mL of ethanol and 10 mL of deionized water were mixed, followed by the addition of 3.44 mL of ammonium hydroxide at room temperature. Then 5.6 mL of TEOS was quickly poured into the above solution and stirred for 1 h to form a homogeneous sol. Then a solution containing 4.5 mL of TEOS and 2 mL of C18TMOS was slowly added in to the above sol with a speed of 325 μL/min 3 h later, the product was washed with deionized water for 3 times, followed by drying at 80 °C overnight and further annealed at 550 °C for 6 h with a temperature ramping speed of 5 °C/min in the air atmosphere. (2) Exfoliation of the confined g-C3N4. 1 g of SiO2 was dispersed in 10 mL of deionized water containing 10 g of cyanamide in a vacuum tube at 55 °C under ultrasonication so as to infuse the mesoporous shell with the cyanamide. 30 min later, the suspension was centrifuged to remove the redundant cyanamide and dried at 40 °C. The solid product was transferred into a 5 mL covered crucible and annealed at 550 °C for 4 h with a temperature ramping speed of 5 °C/min in the N2 atmosphere. The sample, named SiCN10, was grinded, washed with deionized water, and dried. Then 0.1 g of SiCN10 was dispersed in 80 mL of DMSO under ultrasonication for 3 h at room temperature, followed by transferring to a 100 mL Teflon-lined stainless steel vessel and heating at 180 °C for 12 h. The product, named SiCN(H), was washed with acetone and ethanol, followed by drying at 60 °C overnight. (3) CdS deposition. The core-shell heteronanostructures were prepared with the same hydrothermal procedure except a certain amount of Cd(Ac)2.2H2O was added at the right beginning [38]. The weight ratio of SiCN10 to Cd(Ac)2.2H2O was 1:1, 1:2, 1:4, and 1:8, and the final sample was named SiCN@CdS, SiCN@2CdS, SiCN@4CdS, and SiCN@8CdS, respectively. Pure CdS nanoparticles were also synthesized with the same procedure without the presence of SiCN10.
The as-prepared SiO2 nanoparticle with a solid core and a mesoporous shell has a size of ~200 nm (Fig. S1). With the vacuum-assisted ultrasonication, the cyanamide can be injected into the nanopores of the mesoporous shell. Then the SiCN10, which has a size close to that of the SiO2 nanoparticle, has a homogeneous dispersion state (Fig. S2a). The g-C3N4 confined in the mesoporous shell can be confirmed by the exposed edges covered on the SiO2 surface (Fig. S2b). The homogeneous dispersion state is maintained for SiCN(H) after the hydrothermal treatment, and the SEM elemental mapping analysis confirms the co-existence of C, N, O, and Si elements (Fig. 1a). The SiCN(H) with confined g-C3N4 can be confirmed by the TEM image which also shows the characteristic sharp edges of the few-layered g-C3N4 (Fig. 1b and Fig. S3), but the fragmentary g-C3N4 covered on the outer surface is disappeared, which is due to the hydrothermal exfoliation and further dispersion into the aqueous solution [39,40]. This result also suggests that the mesoporous shell is helpful to protect the confined g-C3N4 from external environmental damage. XRD spectrum of SiCN10 shows two peaks centered at 27.3° and 22.3°, respectively, ascribing to the amorphous SiO2 and the confined g-C3N4 (Fig. S4). Similar crystal structure is also observed for SiCN(H), but the relative peak intensity of the gC3N4 to that of the amorphous SiO2 is reduced (Fig. 1c). This variation may be due to the disappearance of the surface covered g-C3N4 and the exfoliation of the confined g-C3N4. TG-DSC curves show that the SiCN10 undergoes a continuous weight loss of 31.17 wt% with the increased temperature (Fig. 1d). A sharp exothermal peak centered at 680 °C in the range of 656–720 °C was observed, suggesting the oxidation of the confined g-C3N4 [41]. Similarly, the total weight loss of SiCN(H) is 30.49 wt%, suggesting most of the g-C3N4 confined in the mesoporous shell of SiCN10 are remained after the hydrothermal treatment (Fig. 1e). This result further confirms the superiority of the SiO2 template in protecting the confined g-C3N4. However, the 2
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blunt edges, which may be due to the deposition of very small CdS nanoparticles, are also exposed to the outer surface. High resolution TEM image shows the crystalline nanoparticle deposited on the SiCN@CdS surface has a d spacing of 0.335 nm, consisting with the (111) lattice plane of CdS (Fig. 3c). In contrast, the CdS nanoparticles are seriously aggregated and almost covered on the whole surfaces of SiCN@4CdS (Fig. 3e) and SiCN@8CdS (Fig. 3f). The aggregation state changes are resulted from the more CdS deposition and the crystal growth, which is also proved by the XRD analysis (Fig. 2e). Besides the diffraction peaks of amorphous SiO2 (22.3°) and g-C3N4 (27.3°), all the samples deposited with the CdS nanoparticles show peaks centered at 2θ values of 25.0°, 26.6°, 28.3°, 43.9°, 52.1°, and 72.8° match with the (100), (002), (101), (110), (112), and (114) crystalline planes of the CdS hexagonal structure (ICDD PDF 80-0006) [44]. The diffraction peaks intensity of CdS is gradually improved for SiCN@CdS, SiCN@ 2CdS, SiCN@4CdS, and SiCN@8CdS, suggesting the increased particle size and the crystalline degree. The light absorption of the as-prepared photocatalysts is gradually increased in both the UV and visible light regions with more deposited CdS nanoparticles, making them efficient candidates as visible lightdriven photocatalysts (Fig. 4a). All the core-shell heteronanostructures show weaker light absorption than the pure CdS nanoparticles, which is due to that the SiCN(H) can only absorb the photo energy before ~430 nm. The reduced light absorption of SiCN(H) than the reported bulk g-C3N4 suggests the exfoliated structure with improved quantum size effect [35]. The different light absorption properties of the photocatalysts can also be proved by their photographs (Fig. S6). The band gaps of the photocatalysts are calculated based on the light absorption spectra (Fig. 4b). Typically, the SiCN(H), SiCN@2CdS, and CdS has a band gap of 2.87 eV, 2.44 eV, and 2.37 eV, respectively. The transient photocurrent intensity of SiCN(H) under the visible light irradiation is reduced compared to that of SiCN10, which may be due to the disappearance of the surface-adsorbed g-C3N4 and the reduced crystalline degree of the confined g-C3N4 (Fig. S7). The photocurrent intensity of SiCN@CdS and SiCN@2CdS is gradually increased, and then gradually decreased for SiCN@4CdS and SiCN@8CdS (Fig. 5a). Moreover, the photocurrent intensities of SiCN@2CdS and SiCN@4CdS are much higher than that of CdS. The different photocurrent shapes between pure CdS nanoparticles and the hybrid photocatalysts is resulted from the SiO2-confined g-C3N4. Then the deposited CdS nanoparticles help to synergistically amply this effect and a proper ratio between them makes SiCN@2CdS the one to have the highest photocurrent intensity. These results suggest that depositing the CdS nanoparticles onto the SiCN(H) surface is helpful to generate more electron-hole pairs, and an electron transfer path which can facilitate the separation of the electron-hole pairs can be established [45,46]. Considering the intensity variations, the crystalline degree and the aggregation state of the CdS nanoparticles can also change the efficiencies of generating and separating the electrons and holes, and an optimal state is established on SiCN@2CdS which has a specific surface area of 49.06 m2/g, lower than that of SiCN(H) (Fig. 1f). This reduced specific surface area of SiCN@2CdS is due to the deposition of CdS nanoparticles which contact tightly with the expoliated g-C3N4, thus some of the nanopores are blocked. The photocatalytic H2 production performances of the photocatalysts under the visible light irradiation are shown in Fig. 5b. The H2 produced by SiCN10 is 6.10 times higher than that by the SiCN(H), however, the H2 production efficiency of SiCN(H) is increased when CdS is deposited onto the surface. The SiCN@CdS, SiCN@2CdS, SiCN@4CdS, and SiCN@8CdS have a photocatalytic activity of 1.12, 2.43, 2.15, and 1.24 times higher than the SiCN10, respectively. Moreover, only the SiCN@2CdS and SiCN@4CdS have a photocatalytic activity of 1.35 and 1.19 times, respectively, higher than that of the CdS. These results suggest that not only the
Fig. 1. (a) SEM and elemental mapping images, (b) TEM image, and (c) XRD spectrum of SiCN(H). TG-DSC curves of (d) SiCN10 and (e) SiCN(H). (f) N2 adsorption-desorption curves and the specific surface areas of SiCN10, SiCN(H), and SiCN@2CdS.
exothermal peak ascribing to the oxidation of SiCN(H) is greatly blueshifted to ~589 °C, which is due to the changes of the structures and chemistry environments of the confined g-C3N4 [42,43]. The confined g-C3N4 exfoliation induced changes can also be proved by the N2 adsorption-desorption analysis. Both SiCN10 and SiCN(H) show typical type IV isotherms, and the specific area is 61.35 m2/g and 79.47 m2/g, respectively (Fig. 1f). Considering the stable chemical property of the SiO2 nanoparticles, the improved surface area must be derived from the exfoliated g-C3N4. However, the exfoliation degree of the confined gC3N4 is constrained by the volume of the nanopores. Thus we can infer that the mesoporous shell of SiCN(H) is fully filled with the exfoliated gC3N4 with a much loose structure. The CdS nanoparticles with a uniform particle size are seriously aggregated (Fig. 3a and Fig. S5), but they can be homogeneously deposited onto the SiCN(H) surface to form a core-shell structure. It can be observed that the CdS aggregation state of the SiCN@CdS (Fig. 2a), SiCN@2CdS (Fig. 2b), SiCN@4CdS (Fig. 2c), and SiCN@8CdS (Fig. 2d) is gradually improved. TEM images also indicate that the CdS nanoparticles can be homogeneously dispersed on the surfaces of SiCN@CdS (Fig. 3b) and SiCN@2CdS (Fig. 3d), and the exfoliated g-C3N4 with a
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Fig. 2. SEM images of (a) SiCN@CdS, (b) SiCN@2CdS, (c) SiCN@4CdS, and (d) SiCN@8CdS. (e) XRD spectra of CdS, SiCN@CdS, SiCN@2CdS, SiCN@4CdS, and SiCN@8CdS.
content and crystalline degree but also the dispersion state of the CdS nanoparticles are key factors in determining the overall photocatalytic activities of the core-shell photocatalysts. The average H2 production rate of each photocatalyst is calculated and shown in Fig. 5c. In fact, the SiO2 template has no activity in driving the H2 production under the visible light irradiation. Thus, based on the TG-DSC discussion, the normalized H2 production rate driven only by the CdS and the confined g-C3N4 of each photocatalyst is shown in Fig. 5d. Even though the SiCN (H) has the lowest efficiency, the SiCN@2CdS is still the most active photocatalyt which has a H2 production rate of 1.13 times higher than that of SiCN10. Moreover, the H2 production rate of SiCN10 is 1.79 times higher than that of the CdS. Our previous study indicated that the g-C3N4 exfoliated in the DMSO has a H2 production rate of 0.68 mmol g−1h−1, but the confined g-C3N4 of SiCN10 has a H2 production rate of 2.73 mmol g−1h−1 [38]. All these results confirm that: (1) the confined g-C3N4 of SiCN10 is much active than the traditional CdS and bulk g-C3N4 in driving the H2 production, and (2) the synthesized core-shell heteronanostructure can further optimize the H2 production efficiency because of the optimized microstructures. Moreover, SiCN(H) and SiCN@2CdS were also chosen as the typical photocatalysts to degradate RhB (10 mg/L) under the visible light irradiation (Fig. S8). Results also show that the photocatalytic activity of SiCN@
2CdS is higher than that of SiCN(H) and the photocatalysts with similar structures [47]. The SiCN@2CdS is taken as a typical photocatalyst to analyze the surface chemical environment by the XPS technique. Compared to SiCN (H), SiCN@2CdS shows additional S 2p and Cd 3d signals (Fig. 6a). The C 1s peaks of SiCN(H) (284.6 eV, 286.0 eV, and 288.1 eV) and SiCN@ 2CdS (284.6 eV, 285.8 eV, and 288.2 eV) are slightly different to each other, which is induced by the core-shell g-C3N4@CdS heteronanojunction resulted charge transfer between CdS and the confined gC3N4 (Fig. 6b) [48]. This charge transfer behavior can also be confirmed by the different N 1s signals of SiCN(H) (399.5 eV and 400.9 eV) and SiCN@2CdS (398.6 eV and 399.4 eV) (Fig. 6c). The slightly weakened N 1s signals is due to the exfoliation of g-C3N4 and the bridging effect of the deposited CdS which has a strong Cd 3d spectrum with peaks centered at 404.7 eV and 411.5 eV (Fig. S8a) and a S 2p spectrum with peak centered at 161.57 eV (Fig. S8b). This variation is also reflected in the FTIR spectra (Fig. 6d and Fig. S9). Both SiCN(H) and SiCN@2CdS show the typical vibration bands of SiO2, the breathing vibration at 810 cm−1 of triazine units, and the skeletal vibration at 1200–1600 cm−1 of aromatic CN heterocycles [49]. However, different from that of SiCN10, the characteristic absorption bands around 2180 cm−1 for C≡N and N=C=N groups are weakened for SiCN(H)
Fig. 3. TEM images of (a) CdS nanoparticles, (b) SiCN@CdS, (d) SiNC@2CdS, (e) SiCN@4CdS, and (f) SiCN@8CdS. (c) High resolution TEM image of the CdS nanoparticles deposited on the surface of SiCN@CdS. The dashed curves highlight the deposited CdS nanoparticles with different particle size and aggregation state.
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Fig. 4. (a) The UV–vis light absorption spectra and (b) the estimated band gaps of CdS, SiCN(H), SiCN@CdS, SiNC@2CdS, SiCN@4CdS, and SiCN@8CdS.
The VBXPS spectra of SiCN(H), SiCN10, and SiCN@2CdS are shown in Fig. 7a. The SiCN(H) shows a slight red-shift (from 1.63 eV of SiCN10 to 1.69 eV of SiCN(H)) of the VB edge compared to that of SiCN10, which is due to the exfoliation improved quantum size effect. The VB
and SiCN@2CdS, suggesting the slight microstructure changes of the confined and exfoliated g-C3N4. The SiCN@2CdS also shows a weak stretching vibration band at 620 cm−1, confirms the deposition of CdS onto the surface [50].
Fig. 5. (a) Transient photocurrent curves, (b) photocatalytic H2 production analysis, (c) calculated average H2 production rate, and (d) normalized H2 production rate of the photocatalysts under the visible light irradiation.
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Fig. 6. (a) XPS full spectra, (b) C 1s, and (c) N 1s XPS spectra of SiCN(H) and SiCN@2CdS. (d) FTIR spectra of the SiCN(H), SiCN@2CdS, and CdS.
edge, which is a combination of SiCN(H) and CdS, of SiCN@2CdS is located at ~1.12 eV. Based on the these results, the band gap structures with calculated VB and CB potentials of SiCN(H) and SiCN@2Cds are shown in Fig. 7c and d, respectively. The SiCN(H) has the ability to produce H2 under the visible light irradiation, and the reduction potential of SiCN@2CdS is further improved. Moreover, as discussed in the TEM images, the SiCN@2CdS surface is partially covered with the well-dispersed CdS nanoparticles with a relative high crystalline degree, thus both the CdS and the confined g-C3N4 can be exposed to the outer surface to be excited by the visible light. In contrast, the SiCN@CdS has less deposited CdS nanoparticles with a relative lower crystalline degree, and the surfaces of the SiCN@4CdS and SiCN@8CdS are almost wholly covered with the CdS nanoparticles, reducing the separation efficiency of the photocarriers between g-C3N4 and CdS. That is why the SiCN@2CdS also has the highest photocurrent intensity. In Fig. 7e, benefitted from the relative VB and CB potentials of g-C3N4 and CdS, the photogenerated electrons and holes can be transferred to the CdS and g-C3N4, respectively [38]. Then the anti-photocorrosion ability of CdS is greatly improved, thus the SiCN@2CdS shows a stable ability for visible light-driven photocatalytic H2 production (Fig. 7b).
4. Conclusions This work provides a mesoporous template-assisted method for the synthesis of high-reactive, low-cost, environmental-stable, and easyrecyclable photocatalyst by precisely controlling the microstructures. Mesoporous SiO2 nanoparticles supported core-shell g-C3N4@CdS heteronanostructures are synthesized by simultaneously exfoliating the nano-confined g-C3N4 and depositing the CdS nanoparticles with the help of the DMSO by the hydrothermal method. Precisely adjusting the crystalline degree, specific surface area, light absorption, and dispersion state of the confined g-C3N4 and the CdS nanoparticles enables us to optimize the structures and photocatalytic activities of the core-shell heteronanostructures for H2 production under the visible light irradiation. The distribution state of the CdS nanoparticles is believed to play a vital role on establishing an efficient path for separating the photogenerated electrons and holes, and then improving the anti-photocorrsion ability of the core-shell photocatalysts. The results gained from this work provide key design and synthesis principles for the mesoporous template-assisted semiconductor photocatalysts that can be used efficiently for clean fuel production and environmental remediation.
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Fig. 7. (a) VBXPS spectra of SiCN(H), CdS, and SiCN@2CdS. (b) Repeated photocatalytic H2 production of SiCN@2CdS under the visible light irradiation. Proposed band gap structures of (c) SiCN(H) and (d) SiCN@2CdS. (e) H2 production model of SiCN@2CdS with improved anti-photocorrosion abilityby transferring the electrons and holes to the CdS and confined g-C3N4, respectively.
Conflicts of interest
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There are no conflicts to declare.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgemetns This work was supported by the National Natural Science Foundation of China (No.51502143), Natural Science Foundation of Jiangsu Province, China (No.BK20150919), Postdoctoral Science Foundation of China (2016M601791), and Jiangsu Provincial Government Scholarship Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.09.230. 7
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Mesoporous SiO2-derived g-C3N4@CdS core-shell heteronanostructure for efficient and stable photocatalytic H2 production Wei Wanga,b,∗, Jiaojiao Fangb a b
School of Chemistry and Materials Science, Nanjing University of Information Science & Technology, Nanjing, 210044, China State Key Laboratory of Materials-Orient Chemical Engineering, Nanjing Tech University, Nanjing, 210009, China
ARTICLE INFO
ABSTRACT
Keywords: Heteronanostructure g-C3N4 exfoliation Core-shell photocatalyst Hydrogen production Recyclability
Semiconductor-based heteronanostructures with wide light absorption and efficient electron-hole separation properties exhibit great potential for photocatalytic applications. Here we employ the mesoporous SiO2 nanoparticles as the template to simultaneously exfoliate the nano-confined g-C3N4 and deposit the CdS nanoparticles to form a series of core-shell SiCN@CdS heteronanostructures for photocatalytic H2 production under the visible light irradiation. Based on the physicochemical characterizations, we have been able to discuss the compositions, structures, and properties of the core-shell photocatalysts. Benefiting from the optimized equilibrium state of a relative high specific surface area, light absorption, crystalline degree, and a homogeneous dispersion of the outer CdS nanoparticles, the SiCN@2CdS exhibits the highest photocatalytic activity and stability for H2 production under the visible light irradiation. Results also suggest that the existence of the SiO2 template is benefit to suppress the aggregation and improve the recyclability of the core-shell photocatalysts when they are applied in the aqueous phase. The present study suggests a new sight on the template-assisted designing and synthesizing high-reactive, low-cost, environmental-stable, and easy-recyclable photocatalysts by precisely controlling the microstructures.
1. Introduction Photocatalysis has been attracting wide interests all over the world for its promising abilities of enabling utilize the solar energy for clean fuel production and environmental remediation [1–8]. Exploring the high-reactive, environmental-stable, and recyclable photocatalysts is still a key issue to promote their potential for large-scale practical applications [9]. In the last ten years, the non-metal g-C3N4, which not only has the advantages of the star photocatast, titanium dioxide, but also has a relative narrower band gap, has been widely studied in the photocatalysis field since it was firstly reported by Wang et al. [10,11]. The bulk g-C3N4 can be easily synthesized by the thermal polymerization method with various chemicals, such as urea, melamine, and thiourea, as the precursors [12,13]. Changing the experiment parameters, annealing atmosphere, doping atoms, and coupling with other functional materials are also widely studied to optimize the physicochemical properties of the randomly synthesized g-C3N4 [14–21]. However, the low specific surface area and the fast recombination of the photogenerated electron-hole pairs are still the main shortcomings restricting the quantum efficiency of g-C3N4 applied in the natural environment.
∗
Inspired by the studies of two-dimensional graphene exfoliation, researchers have confirmed that exfoliate the bulk g-C3N4 into the fewlayered sheets is also a promising strategy to both improve the specific surface area and the separation efficiency of the photogenerated electron-hole pairs [22,23]. Various strategies, such as the acid oxidation, thermal oxidation, mechanical shearing, and ultrasonication, have been widely employed to reduce the van der Waals’ force among the g-C3N4 layers [24–28]. Moreover, various mesoporous and thermal-stable templates are employed to synthesize the g-C3N4 within the nano- or micron-pores based on the facial synthesis procedure. After removing the templates, the confined g-C3N4 are reported to have several advantages, including directional migration path of the photocarriers, improved specific surface area, and high adsorption ability etc., compared to the randomly synthesized bulk ones [29–32]. However, new issues, such as the low productivity and the poor recyclability, are emerged and needed to be solved to meet the demand of practical application [33]. As a result, it is urgent to explore a method to exfoliate the bulk g-C3N4 and simultaneously improve its productivity and recyclability. Recently, several types of g-C3N4 synthesized in the nanopores of mesoporous SiO2 templated were reported [34–36]. The quantum size
Corresponding author. School of Chemistry and Materials Science, Nanjing University of Information Science & Technology, Nanjing, 210044, China. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.ceramint.2019.09.230 Received 16 August 2019; Received in revised form 21 September 2019; Accepted 23 September 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Wei Wang and Jiaojiao Fang, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.230
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effect, specific surface area, and photocatalytic activity of the confined g-C3N4 were significantly improved when the SiO2 templates were removed. However, the light absorption, which often shows a blue-shift due to the reduced structure unit, of the confined g-C3N4 is needed to be improved. Considering that the SiO2 is an environment-stable matrix and it has been widely studied in the photocatalysis field to improve the photocatalytic activities of titanium dioxide-based photocatalysts. In the present study, we are trying to exfoliate the g-C3N4 within the nanopores of the mesoporous SiO2 nanoparticles to improve its productivity and recyclability. Moreover, CdS, an efficient visible-lightdriven photocatalyst, nanoparticles are also simultaneously deposited onto the surface to form a core-shell heteronanostructure with the exfoliated g-C3N4 to boost the photocatalytic activity [37]. Microstructures and properties of the core-shell photocatalysts are systematically discussed and optimized to meet the demands of high-efficient and stable H2 production under the visible light irradiation.
2.3. Characterizations
2. Experimental section
Morphology characterizations were conducted on a S4800 (Hitachi) scanning electron microscopy (SEM) and a JEM-2010 (JEOL) transmission electron microscopy (TEM). Powder X-ray diffraction (XRD) analysis was conducted on a SmartLab-3KW (Rigaku) X-ray diffractometer with a Cu Kα irradiation source at a scanning speed of 10°/ min. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a PHI5000 (ULVAD-PHI) Versaprobe system with monochromatic Al Kα radiation. Light absorption spectra were analyzed on the UV-3010 (Shimadzu) UV–vis–NIR spectrometer. Nitrogen adsorption-desorption analysis was conducted on the Autosorb-iQ (Quantachrome) automatic adsorption instrument. Fourier Transformed Infrared (FTIR) spectra were analyzed on a Frontier (PerkinElmer) spectrometer. Thermogravimetry (TG) analysis was conducted on a Diamond (PerkinElmer) thermos gravimetric analyzer with a temperature ramping speed of 5 °C/min in the air atmosphere. Transient photocurrent curves were obtained on the CHI660E electrochemical workstation (ChenHua, China) under the visible light irradiation.
2.1. Chemicals and materials
2.4. Photocatalytic H2 production
All chemical reagents were commercially available and used without further purification. Cyanamide (≥98%), cadmium acetate dehydrate Cd(Ac)2.2H2O, (AR), tetraethyl orthosilicate (TEOS, AR, 98%), ethanol (AR, moisture < 0.3%), ammonia aqueous solution (AR, 25–28%), trimethoxy(octadecyl)silane (C18TMOS, 90%), dimethyl sulfoxide (DMSO, AR, > 99%), acetone (AR), methanol (AR), H2PtCl6·6H2O (AR), and triethanolamine (AR) were purchased from Aladdin.
20 mg of photocatalyst was dispersed in 40 mL of aqueous solution containing 100 μL of methanol and 320 μL of H2PtCl6 aqueous solution (3.2 g/L), followed by irradiating under a 365 nm LED (40 W) for 2 h to deposit the Pt nanoparticles, the cocatalyst, onto the photocatalyst surface. The suspension was then washed with deionized water twice and redispersed into 40 mL of triethanolamine aqueous solution (10 wt %), followed by deoxygenating with argon and irradiating with a 300 W xenon lamp (CEAULIGHT) fixed with an UV-cut-off filter to conduct the photocatalytic reaction. The produced H2 was analyzed on a gas chromatography system (Agilent Technologies 7890B).
2.2. Synthesis of the photocatalysts
3. Results and discussion
(1) Core-shell SiO2 nanoparticles. Mesoporous SiO2 nanoparticles with a core-shell structure were prepared based on the reported procedure with minor modification [36]. Typically, 73 mL of ethanol and 10 mL of deionized water were mixed, followed by the addition of 3.44 mL of ammonium hydroxide at room temperature. Then 5.6 mL of TEOS was quickly poured into the above solution and stirred for 1 h to form a homogeneous sol. Then a solution containing 4.5 mL of TEOS and 2 mL of C18TMOS was slowly added in to the above sol with a speed of 325 μL/min 3 h later, the product was washed with deionized water for 3 times, followed by drying at 80 °C overnight and further annealed at 550 °C for 6 h with a temperature ramping speed of 5 °C/min in the air atmosphere. (2) Exfoliation of the confined g-C3N4. 1 g of SiO2 was dispersed in 10 mL of deionized water containing 10 g of cyanamide in a vacuum tube at 55 °C under ultrasonication so as to infuse the mesoporous shell with the cyanamide. 30 min later, the suspension was centrifuged to remove the redundant cyanamide and dried at 40 °C. The solid product was transferred into a 5 mL covered crucible and annealed at 550 °C for 4 h with a temperature ramping speed of 5 °C/min in the N2 atmosphere. The sample, named SiCN10, was grinded, washed with deionized water, and dried. Then 0.1 g of SiCN10 was dispersed in 80 mL of DMSO under ultrasonication for 3 h at room temperature, followed by transferring to a 100 mL Teflon-lined stainless steel vessel and heating at 180 °C for 12 h. The product, named SiCN(H), was washed with acetone and ethanol, followed by drying at 60 °C overnight. (3) CdS deposition. The core-shell heteronanostructures were prepared with the same hydrothermal procedure except a certain amount of Cd(Ac)2.2H2O was added at the right beginning [38]. The weight ratio of SiCN10 to Cd(Ac)2.2H2O was 1:1, 1:2, 1:4, and 1:8, and the final sample was named SiCN@CdS, SiCN@2CdS, SiCN@4CdS, and SiCN@8CdS, respectively. Pure CdS nanoparticles were also synthesized with the same procedure without the presence of SiCN10.
The as-prepared SiO2 nanoparticle with a solid core and a mesoporous shell has a size of ~200 nm (Fig. S1). With the vacuum-assisted ultrasonication, the cyanamide can be injected into the nanopores of the mesoporous shell. Then the SiCN10, which has a size close to that of the SiO2 nanoparticle, has a homogeneous dispersion state (Fig. S2a). The g-C3N4 confined in the mesoporous shell can be confirmed by the exposed edges covered on the SiO2 surface (Fig. S2b). The homogeneous dispersion state is maintained for SiCN(H) after the hydrothermal treatment, and the SEM elemental mapping analysis confirms the co-existence of C, N, O, and Si elements (Fig. 1a). The SiCN(H) with confined g-C3N4 can be confirmed by the TEM image which also shows the characteristic sharp edges of the few-layered g-C3N4 (Fig. 1b and Fig. S3), but the fragmentary g-C3N4 covered on the outer surface is disappeared, which is due to the hydrothermal exfoliation and further dispersion into the aqueous solution [39,40]. This result also suggests that the mesoporous shell is helpful to protect the confined g-C3N4 from external environmental damage. XRD spectrum of SiCN10 shows two peaks centered at 27.3° and 22.3°, respectively, ascribing to the amorphous SiO2 and the confined g-C3N4 (Fig. S4). Similar crystal structure is also observed for SiCN(H), but the relative peak intensity of the gC3N4 to that of the amorphous SiO2 is reduced (Fig. 1c). This variation may be due to the disappearance of the surface covered g-C3N4 and the exfoliation of the confined g-C3N4. TG-DSC curves show that the SiCN10 undergoes a continuous weight loss of 31.17 wt% with the increased temperature (Fig. 1d). A sharp exothermal peak centered at 680 °C in the range of 656–720 °C was observed, suggesting the oxidation of the confined g-C3N4 [41]. Similarly, the total weight loss of SiCN(H) is 30.49 wt%, suggesting most of the g-C3N4 confined in the mesoporous shell of SiCN10 are remained after the hydrothermal treatment (Fig. 1e). This result further confirms the superiority of the SiO2 template in protecting the confined g-C3N4. However, the 2
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blunt edges, which may be due to the deposition of very small CdS nanoparticles, are also exposed to the outer surface. High resolution TEM image shows the crystalline nanoparticle deposited on the SiCN@CdS surface has a d spacing of 0.335 nm, consisting with the (111) lattice plane of CdS (Fig. 3c). In contrast, the CdS nanoparticles are seriously aggregated and almost covered on the whole surfaces of SiCN@4CdS (Fig. 3e) and SiCN@8CdS (Fig. 3f). The aggregation state changes are resulted from the more CdS deposition and the crystal growth, which is also proved by the XRD analysis (Fig. 2e). Besides the diffraction peaks of amorphous SiO2 (22.3°) and g-C3N4 (27.3°), all the samples deposited with the CdS nanoparticles show peaks centered at 2θ values of 25.0°, 26.6°, 28.3°, 43.9°, 52.1°, and 72.8° match with the (100), (002), (101), (110), (112), and (114) crystalline planes of the CdS hexagonal structure (ICDD PDF 80-0006) [44]. The diffraction peaks intensity of CdS is gradually improved for SiCN@CdS, SiCN@ 2CdS, SiCN@4CdS, and SiCN@8CdS, suggesting the increased particle size and the crystalline degree. The light absorption of the as-prepared photocatalysts is gradually increased in both the UV and visible light regions with more deposited CdS nanoparticles, making them efficient candidates as visible lightdriven photocatalysts (Fig. 4a). All the core-shell heteronanostructures show weaker light absorption than the pure CdS nanoparticles, which is due to that the SiCN(H) can only absorb the photo energy before ~430 nm. The reduced light absorption of SiCN(H) than the reported bulk g-C3N4 suggests the exfoliated structure with improved quantum size effect [35]. The different light absorption properties of the photocatalysts can also be proved by their photographs (Fig. S6). The band gaps of the photocatalysts are calculated based on the light absorption spectra (Fig. 4b). Typically, the SiCN(H), SiCN@2CdS, and CdS has a band gap of 2.87 eV, 2.44 eV, and 2.37 eV, respectively. The transient photocurrent intensity of SiCN(H) under the visible light irradiation is reduced compared to that of SiCN10, which may be due to the disappearance of the surface-adsorbed g-C3N4 and the reduced crystalline degree of the confined g-C3N4 (Fig. S7). The photocurrent intensity of SiCN@CdS and SiCN@2CdS is gradually increased, and then gradually decreased for SiCN@4CdS and SiCN@8CdS (Fig. 5a). Moreover, the photocurrent intensities of SiCN@2CdS and SiCN@4CdS are much higher than that of CdS. The different photocurrent shapes between pure CdS nanoparticles and the hybrid photocatalysts is resulted from the SiO2-confined g-C3N4. Then the deposited CdS nanoparticles help to synergistically amply this effect and a proper ratio between them makes SiCN@2CdS the one to have the highest photocurrent intensity. These results suggest that depositing the CdS nanoparticles onto the SiCN(H) surface is helpful to generate more electron-hole pairs, and an electron transfer path which can facilitate the separation of the electron-hole pairs can be established [45,46]. Considering the intensity variations, the crystalline degree and the aggregation state of the CdS nanoparticles can also change the efficiencies of generating and separating the electrons and holes, and an optimal state is established on SiCN@2CdS which has a specific surface area of 49.06 m2/g, lower than that of SiCN(H) (Fig. 1f). This reduced specific surface area of SiCN@2CdS is due to the deposition of CdS nanoparticles which contact tightly with the expoliated g-C3N4, thus some of the nanopores are blocked. The photocatalytic H2 production performances of the photocatalysts under the visible light irradiation are shown in Fig. 5b. The H2 produced by SiCN10 is 6.10 times higher than that by the SiCN(H), however, the H2 production efficiency of SiCN(H) is increased when CdS is deposited onto the surface. The SiCN@CdS, SiCN@2CdS, SiCN@4CdS, and SiCN@8CdS have a photocatalytic activity of 1.12, 2.43, 2.15, and 1.24 times higher than the SiCN10, respectively. Moreover, only the SiCN@2CdS and SiCN@4CdS have a photocatalytic activity of 1.35 and 1.19 times, respectively, higher than that of the CdS. These results suggest that not only the
Fig. 1. (a) SEM and elemental mapping images, (b) TEM image, and (c) XRD spectrum of SiCN(H). TG-DSC curves of (d) SiCN10 and (e) SiCN(H). (f) N2 adsorption-desorption curves and the specific surface areas of SiCN10, SiCN(H), and SiCN@2CdS.
exothermal peak ascribing to the oxidation of SiCN(H) is greatly blueshifted to ~589 °C, which is due to the changes of the structures and chemistry environments of the confined g-C3N4 [42,43]. The confined g-C3N4 exfoliation induced changes can also be proved by the N2 adsorption-desorption analysis. Both SiCN10 and SiCN(H) show typical type IV isotherms, and the specific area is 61.35 m2/g and 79.47 m2/g, respectively (Fig. 1f). Considering the stable chemical property of the SiO2 nanoparticles, the improved surface area must be derived from the exfoliated g-C3N4. However, the exfoliation degree of the confined gC3N4 is constrained by the volume of the nanopores. Thus we can infer that the mesoporous shell of SiCN(H) is fully filled with the exfoliated gC3N4 with a much loose structure. The CdS nanoparticles with a uniform particle size are seriously aggregated (Fig. 3a and Fig. S5), but they can be homogeneously deposited onto the SiCN(H) surface to form a core-shell structure. It can be observed that the CdS aggregation state of the SiCN@CdS (Fig. 2a), SiCN@2CdS (Fig. 2b), SiCN@4CdS (Fig. 2c), and SiCN@8CdS (Fig. 2d) is gradually improved. TEM images also indicate that the CdS nanoparticles can be homogeneously dispersed on the surfaces of SiCN@CdS (Fig. 3b) and SiCN@2CdS (Fig. 3d), and the exfoliated g-C3N4 with a
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Fig. 2. SEM images of (a) SiCN@CdS, (b) SiCN@2CdS, (c) SiCN@4CdS, and (d) SiCN@8CdS. (e) XRD spectra of CdS, SiCN@CdS, SiCN@2CdS, SiCN@4CdS, and SiCN@8CdS.
content and crystalline degree but also the dispersion state of the CdS nanoparticles are key factors in determining the overall photocatalytic activities of the core-shell photocatalysts. The average H2 production rate of each photocatalyst is calculated and shown in Fig. 5c. In fact, the SiO2 template has no activity in driving the H2 production under the visible light irradiation. Thus, based on the TG-DSC discussion, the normalized H2 production rate driven only by the CdS and the confined g-C3N4 of each photocatalyst is shown in Fig. 5d. Even though the SiCN (H) has the lowest efficiency, the SiCN@2CdS is still the most active photocatalyt which has a H2 production rate of 1.13 times higher than that of SiCN10. Moreover, the H2 production rate of SiCN10 is 1.79 times higher than that of the CdS. Our previous study indicated that the g-C3N4 exfoliated in the DMSO has a H2 production rate of 0.68 mmol g−1h−1, but the confined g-C3N4 of SiCN10 has a H2 production rate of 2.73 mmol g−1h−1 [38]. All these results confirm that: (1) the confined g-C3N4 of SiCN10 is much active than the traditional CdS and bulk g-C3N4 in driving the H2 production, and (2) the synthesized core-shell heteronanostructure can further optimize the H2 production efficiency because of the optimized microstructures. Moreover, SiCN(H) and SiCN@2CdS were also chosen as the typical photocatalysts to degradate RhB (10 mg/L) under the visible light irradiation (Fig. S8). Results also show that the photocatalytic activity of SiCN@
2CdS is higher than that of SiCN(H) and the photocatalysts with similar structures [47]. The SiCN@2CdS is taken as a typical photocatalyst to analyze the surface chemical environment by the XPS technique. Compared to SiCN (H), SiCN@2CdS shows additional S 2p and Cd 3d signals (Fig. 6a). The C 1s peaks of SiCN(H) (284.6 eV, 286.0 eV, and 288.1 eV) and SiCN@ 2CdS (284.6 eV, 285.8 eV, and 288.2 eV) are slightly different to each other, which is induced by the core-shell g-C3N4@CdS heteronanojunction resulted charge transfer between CdS and the confined gC3N4 (Fig. 6b) [48]. This charge transfer behavior can also be confirmed by the different N 1s signals of SiCN(H) (399.5 eV and 400.9 eV) and SiCN@2CdS (398.6 eV and 399.4 eV) (Fig. 6c). The slightly weakened N 1s signals is due to the exfoliation of g-C3N4 and the bridging effect of the deposited CdS which has a strong Cd 3d spectrum with peaks centered at 404.7 eV and 411.5 eV (Fig. S8a) and a S 2p spectrum with peak centered at 161.57 eV (Fig. S8b). This variation is also reflected in the FTIR spectra (Fig. 6d and Fig. S9). Both SiCN(H) and SiCN@2CdS show the typical vibration bands of SiO2, the breathing vibration at 810 cm−1 of triazine units, and the skeletal vibration at 1200–1600 cm−1 of aromatic CN heterocycles [49]. However, different from that of SiCN10, the characteristic absorption bands around 2180 cm−1 for C≡N and N=C=N groups are weakened for SiCN(H)
Fig. 3. TEM images of (a) CdS nanoparticles, (b) SiCN@CdS, (d) SiNC@2CdS, (e) SiCN@4CdS, and (f) SiCN@8CdS. (c) High resolution TEM image of the CdS nanoparticles deposited on the surface of SiCN@CdS. The dashed curves highlight the deposited CdS nanoparticles with different particle size and aggregation state.
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Fig. 4. (a) The UV–vis light absorption spectra and (b) the estimated band gaps of CdS, SiCN(H), SiCN@CdS, SiNC@2CdS, SiCN@4CdS, and SiCN@8CdS.
The VBXPS spectra of SiCN(H), SiCN10, and SiCN@2CdS are shown in Fig. 7a. The SiCN(H) shows a slight red-shift (from 1.63 eV of SiCN10 to 1.69 eV of SiCN(H)) of the VB edge compared to that of SiCN10, which is due to the exfoliation improved quantum size effect. The VB
and SiCN@2CdS, suggesting the slight microstructure changes of the confined and exfoliated g-C3N4. The SiCN@2CdS also shows a weak stretching vibration band at 620 cm−1, confirms the deposition of CdS onto the surface [50].
Fig. 5. (a) Transient photocurrent curves, (b) photocatalytic H2 production analysis, (c) calculated average H2 production rate, and (d) normalized H2 production rate of the photocatalysts under the visible light irradiation.
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Fig. 6. (a) XPS full spectra, (b) C 1s, and (c) N 1s XPS spectra of SiCN(H) and SiCN@2CdS. (d) FTIR spectra of the SiCN(H), SiCN@2CdS, and CdS.
edge, which is a combination of SiCN(H) and CdS, of SiCN@2CdS is located at ~1.12 eV. Based on the these results, the band gap structures with calculated VB and CB potentials of SiCN(H) and SiCN@2Cds are shown in Fig. 7c and d, respectively. The SiCN(H) has the ability to produce H2 under the visible light irradiation, and the reduction potential of SiCN@2CdS is further improved. Moreover, as discussed in the TEM images, the SiCN@2CdS surface is partially covered with the well-dispersed CdS nanoparticles with a relative high crystalline degree, thus both the CdS and the confined g-C3N4 can be exposed to the outer surface to be excited by the visible light. In contrast, the SiCN@CdS has less deposited CdS nanoparticles with a relative lower crystalline degree, and the surfaces of the SiCN@4CdS and SiCN@8CdS are almost wholly covered with the CdS nanoparticles, reducing the separation efficiency of the photocarriers between g-C3N4 and CdS. That is why the SiCN@2CdS also has the highest photocurrent intensity. In Fig. 7e, benefitted from the relative VB and CB potentials of g-C3N4 and CdS, the photogenerated electrons and holes can be transferred to the CdS and g-C3N4, respectively [38]. Then the anti-photocorrosion ability of CdS is greatly improved, thus the SiCN@2CdS shows a stable ability for visible light-driven photocatalytic H2 production (Fig. 7b).
4. Conclusions This work provides a mesoporous template-assisted method for the synthesis of high-reactive, low-cost, environmental-stable, and easyrecyclable photocatalyst by precisely controlling the microstructures. Mesoporous SiO2 nanoparticles supported core-shell g-C3N4@CdS heteronanostructures are synthesized by simultaneously exfoliating the nano-confined g-C3N4 and depositing the CdS nanoparticles with the help of the DMSO by the hydrothermal method. Precisely adjusting the crystalline degree, specific surface area, light absorption, and dispersion state of the confined g-C3N4 and the CdS nanoparticles enables us to optimize the structures and photocatalytic activities of the core-shell heteronanostructures for H2 production under the visible light irradiation. The distribution state of the CdS nanoparticles is believed to play a vital role on establishing an efficient path for separating the photogenerated electrons and holes, and then improving the anti-photocorrsion ability of the core-shell photocatalysts. The results gained from this work provide key design and synthesis principles for the mesoporous template-assisted semiconductor photocatalysts that can be used efficiently for clean fuel production and environmental remediation.
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Fig. 7. (a) VBXPS spectra of SiCN(H), CdS, and SiCN@2CdS. (b) Repeated photocatalytic H2 production of SiCN@2CdS under the visible light irradiation. Proposed band gap structures of (c) SiCN(H) and (d) SiCN@2CdS. (e) H2 production model of SiCN@2CdS with improved anti-photocorrosion abilityby transferring the electrons and holes to the CdS and confined g-C3N4, respectively.
Conflicts of interest
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There are no conflicts to declare.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgemetns This work was supported by the National Natural Science Foundation of China (No.51502143), Natural Science Foundation of Jiangsu Province, China (No.BK20150919), Postdoctoral Science Foundation of China (2016M601791), and Jiangsu Provincial Government Scholarship Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.09.230. 7
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