Thylakoid-Inspired Multishell g‑C3N4 Nanocapsules with Enhanced Visible-Light Harvesting and Electron Transfer Properties for High-Efficiency Photocatalysis ACS Nano 2017.11:1103-1112. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/05/19. For personal use only.
Zhenwei Tong,†,∥ Dong Yang,‡,§ Zhen Li,†,∥ Yanhu Nan,†,∥ Fei Ding,†,∥ Yichun Shen,†,∥ and Zhongyi Jiang*,†,∥ †
Key Laboratory for Green Technology, School of Chemical Engineering and Technology, ‡Key Laboratory of Systems Bioengineering of Ministry of Education, School of Chemical Engineering and Technology, and §School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China ∥ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China S Supporting Information *
ABSTRACT: Inspired by the orderly stacked nanostructure and highly integrated function of thylakoids in a natural photosynthesis system, multishell g-C3N4 (MSCN) nanocapsule photocatalysts have been prepared by SiO2 hard template with different shell layers. The resultant triple-shell g-C3N4 (TSCN) nanocapsules display superior photocatalysis performance to single-shell and double-shell counterparts owing to excellent visible-light harvesting and electron transfer properties. Specially, with the increase of the shell layer number, light harvesting is greatly enhanced. There is an increase of the entire visible range absorption arising from the multiple scattering and reflection of the incident light within multishell nanoarchitectures as well as the light transmission within the porous thin shells, and an increase of absorption edge arising from the decreased quantum size effect. The electron transfer is greatly accelerated by the mesopores in the thin shells as nanoconduits and the high specific surface area of TSCN (310.7 m2 g−1). With the tailored hierarchical nanostructure features, TSCN exhibits a superior visible-light H2-generation activity of 630 μmol h−1 g−1 (λ > 420 nm), which is among one of the most efficient metal-free g-C3N4 photocatalysts. This study demonstrates a bioinspired approach to the rational design of high-performance nanostructured visible-light photocatalysts. KEYWORDS: g-C3N4, multishell nanocapsules, thylakoids, visible-light harvesting, photocatalysis
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endow g-C3N4 with specific nanostructures, such as g-C3N4 nanotubes,10 nanosheets,11 hollow nanospheres,12,13 and porous framework,14 etc. These porous and/or nanostructures could shorten the charge transfer pathways and enlarge the light-harvesting areas to drive efficient photocatalysis. Despite the great progress, the designed synthesis of an ingenious nanostructured g-C3N4 with outstanding light harvesting and electron transfer properties is still quite challenging. In nature, green plants have demonstrated a powerful system to efficiently utilize the solar energy; mimicking the exquisite nanostructure in natural photosynthesis may open a pathway to construct a
isible-light photocatalysis, as the core technology of solar energy conversion, has been explored for nearly half a century and exhibits fascinating application prospects in energy and environmentally relevant areas.1−4 Among the numerous existing photocatalysts, graphitic carbon nitride (g-C3N4), as a metal-free semiconductor, has drawn much attention owing to its desirable visible-light response (λ < 460 nm) with medium band gap (2.7 eV), chemical stability, and low cost with earth-abundant carbon and nitrogen elements.1,5−7 However, its drawbacks such as unoptimized utilization of visible light and the easy recombination of electron−hole pairs, severely hamper the photocatalytic performance of pristine bulk g-C3N4.8,9 Nanostructured photocatalysts can elevate the photocatalytic activity that could not be reached by conventional bulk counterparts, profiting by the shorter diffusion pathways and higher specific surface area. Thus, a great deal of effort has been devoted to © 2016 American Chemical Society
Received: December 8, 2016 Accepted: December 29, 2016 Published: December 29, 2016 1103
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the precursor of g-C3N4 was infiltrated into the inner lumen of the template through the pores in SiO2 shells and was adsorbed on the surface of multiple shells. After thermal polycondensation and template removal, MSCN was successfully prepared. Compared to single-shell g-C3N4, the as-prepared MSCN with triple mesoporous shells and hollow lumens can intake much more incident light via absorption and multiple light-scattering and reflection effects between shells, whereas the thin mesoporous shells can reduce the perpendicular transfer distance from the interior to the surface for charge carriers and facilitate their transport along the in-plane direction. Moreover, MSCN with a well-defined water-retention structure can hold a large amount of water for ensuring a fast surface reaction. Accordingly, the multishell g-C3N4 nanocapsules are expected to display high visible-light photocatalytic activity in H2-generation and dye degradation.
desirable nanostructured g-C3N4 photocatalyst for highperformance visible-light photocatalysis. Natural photosynthesis is a platform in which light harvesting, photoinduced charge separation, and catalytic reaction are coupled to implement the processes of water splitting and CO2 reduction. Light harvesting is the first step. A quantum efficiency greater than 90% can be realized in the light-harvesting process, which is primarily attributed to the unparalleled structure of chloroplast.15 The chloroplast is primarily composed of grana and stroma and the grana contains cylindrical stacks of ∼10 to 20 tightly attached and interconnected thylakoids with a diameter from 300 to 600 nm. Thylakoids, as a kind of flattened membranous vesicles, distribute in the stroma and parallelly align along the long axis of chloroplast (Scheme 1a).16 Both the light-harvesting and Scheme 1. Overall Synthetic Procedure of MSCNa
RESULTS AND DISCUSSION Scheme 1b shows the overall synthetic procedure of MSCN nanocapsules by using multishell SiO2 nanospheres as templates. At first, monodisperse multishell hollow SiO2 nanospheres were prepared using a well-documented sol−gel process.23 The shell layer number of SiO2 nanospheres can be manipulated by altering the adding times of tetraethoxysilane (TEOS) and 1,2-bis(triethoxysilyl)ethane (BTSE) to the reaction system. Typically, triple-shell SiO2 nanospheres are obtained by adding the mixture of TEOS and BTSE to the reaction solution three times in turn. These hollow SiO2 nanospheres were employed as a sacrificial template to prepare MSCN nanocapsules. Their excellent thermal and mechanical stabilities contribute to the construction of well-developed gC3N4 nanoarchitectures at high temperature. Moreover, mesopores in the multiple shells are beneficial for the rapid diffusion of the g-C3N4 precursor, cyanamide (CY) molecules, into the lumens.24 As shown in Scheme 1b, the overall synthetic procedure of MSCN contains three steps. First, some CY molecules penetrated into the mesopores of the shells and the remaining CY molecules adsorbed onto the shells of SiO2 nanospheres (Figure S1a) to obtain the CY-bearing silica nanospheres. Then, these nanospheres were converted to the silica/g-C3N4 nanospheres through thermal polycondensation of CY at 550 °C. Finally, MSCN were obtained after the silica template removal by Na2CO3 (see the Supporting Information in Figure S1b,c). The morphology and nanostructure of MSCN nanocapsules were detected by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and high angle annular dark field (HADDF) analysis. As revealed in Figure 1a, the SiO2 template has a spherical morphology about 360 nm in size, and its tripleshell structure can be confirmed via the contrast in their interiors and intermediate layers. Three shells are about 7, 10, and 30 nm in thickness from outside to inner, respectively. By using the templating method, the resultant g-C3N4 materials (Figure 1b−e) inherit the majority of the structural features from the parent SiO2 nanospheres. SEM and TEM images (Figure 1b,c) show that the g-C3N4 possess uniformly spherical morphology, monodisperse distribution, and smooth surface. No cracked nanocapsules are observed in the g-C3N4 products, indicating the high mechanical strength of MSCN. HRTEM and HADDF images (Figure 1c,d) reveal that a g-C3N4 nanocapsule has three distinct rings and is composed of inner, intermediate, and outer small cavities with triple shells.
(a) Natural photosystem with green leafs, and the enlarged figure (right) depicts the light conversion in the stacked thylakoids; (b) schematic illustration for the preparation of MSCN nanocapsules. a
light-reaction occur in the thylakoid membranes as described below.17,18 First, solar energy is captured and transferred by the natural light-harvesting complexes located on the thylakoid membranes.17 The stacked and interconnected thylakoids acquire an increased surface area, which can uptake a large proportion of incident light via direct absorption, multiple reflection, and scattering in thylakoid lumen.19−21 After light is captured, electrons are excited and then transferred to the thylakoid membrane. Then, water stored in the lumen accepts the photoexcited electrons to generate O2 and H+ in the light reaction. Thylakoids have a well-defined water-retention structure and can serve as a “water reservoir” for ensuring a fast reaction rate.20,22 Consequently, thylakoids endow the efficient light reaction. It can be thus conjectured that by mimicking the thylakoids, a hierarchical visible-light photocatalyst could be designed with greatly elevated light utilization, charge transfer, and thus photocatalytic performance. In this study, inspired by the hierarchical structure and excellent performance of thylakoids, multishell g-C 3 N 4 (MSCN) nanocapsules were synthesized by using multishell SiO2 nanospheres as a hard template, and utilized for visiblelight H2-generation and degradation. MSCN possesses multiple mesoporous shells and hollow lumens. These shells divide the lumens into spatially confined nanosized reactors, which are similar to the structure of stacked and interconnected thylakoids to enhance the light harvesting and the charge carrier transfer from bulk to surface. For preparing MSCN, the multishell SiO2 nanospheres were first synthesized, and then 1104
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Figure 1. (a) TEM images of triple-shell hollow SiO2 nanospheres; (b) SEM, (c) TEM, (d) HRTEM, and (e) HADDF images of MSCN nanocapsules; (f) N2 adsorption−desorption isotherm and Barret−Joyner−Halenda (BJH) pore size distribution plot (inset) of MSCN nanocapsules; (g) EELS mapping of MSCN nanocapsules.
calculated as 3.98 nm and 310.7 m2 g−1, respectively. It is found that the pore size of MSCN is slightly greater than the pore size of the triple-shell SiO2 template (2.81 nm) (Figure S1d), while the BET surface area is smaller than that of the triple-shell SiO2 template with 917.8 m2 g−1. Thus, it is deduced that mesopores on the shells of MSCN do not inherit from the mesopores of SiO2 templates. According to the TEM images of MSCN, the pores presumably arise from the thermal polycondensation process and the collapse of the carbon nitride upon template removal.25 In addition, the EELS images reveal the uniform distribution of C and N elements on the inner and outer shells (Figure 1g), indicating the well-defined structure of MSCN nanocapsules. The g-C3N4 nanocapsules with single- and double-shell structure were also prepared and characterized by TEM (Figure S2) and the N2-absorption isotherm (Figure S3a). As for the single-shell g-C3N4 (SSCN) nanocapsules, the diameter and shell thickness are tested to be 480 and 60 nm, respectively; while for double-shell g-C3N4 (DSCN) nanocapsules, the diameter and shell thickness of the outer shell are 460 and 46 nm, and the diameter and shell thickness of the inner shell are 300 and 20 nm. These results confirm that the gC3N4 nanocapsules with tunable layers can be facilely synthesized via an adjustment of the shell layer number (1− 3) of SiO2 templates. Direct evidence for the g-C3N4 formation is obtained by XRD, FTIR spectroscopy, and XPS. From Figure 2a, the XRD
The internal shell is mainly located in the central position of the nanocapsules, hinting that a weak connection may exist between shells. The triple-shell g-C3N4 nanocapsules are abbreviated as TSCN. The diameters of outer, intermediate, and inner shells of TSCN are about 400, 360, and 320 nm, while the thickness are respectively 20, 20, and 40 nm. It is noted that the thickness of the outside shell is smaller than the inside shell, which can be attributed to the thick inner shell of SiO2 templates. The size of TSCN nanocapsules and the thickness of shells are all slightly larger than that of the silica template, because the simultaneous adsorption of CY molecules on shells and within mesopores increases the shell thickness of the CY-bearing silica nanospheres (Figure S1a). Furthermore, the obvious light and dark areas in shells and cavities can be also observed from HADDF and HRTEM images of TSCN nanocapsules, suggesting the presence of radially directed, mesoporous channels having openings on the surface. Interestingly, solid g-C3N4 nanospheres and single- and double-shell g-C3N4 nanocapsules are not observed in these TSCN samples, which suggest that the hard templating strategy is suited to create a tailored hierarchical structure. The textural information on the MSCN nanocapsules was analyzed from the N2-absorption isotherm. A type-IV isotherm bearing a H1-type hysteresis loop is observed in Figure 1f, indicating the presence of mesopores in MSCN frameworks. In addition, the pore size and specific surface area (BET) are 1105
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Figure 2. (a) XRD spectra of SiO2 template, bulk CN, and TSCN; XPS spectra of bulk CN and TSCN: (b) full-scan spectra, high-resolution (c) C 1s and (d) N 1s spectra.
Figure 3. (a) UV−vis diffuse reflectance spectra of g-C3N4 samples and (b) their corresponding plots of (αhv)1/2 vs photon energy; (c) Mott− Schottky for MSCN electrodes measured in 0.1 mol L−1 Na2SO4 solution at 1 kHz in the dark; (d) valence band XPS spectra of bulk CN and TSCN.
significant reduction in the intensity is observed for TSCN nanocapsules, which was also observed in some reports of nanostructured g-C3N4.13,27 Meanwhile, the disappearance of the characteristic peak of SiO2 at 22.3° in the XRD pattern of TSCN, along with the energy dispersion spectrometry (EDS)
patterns of TSCN and bulk g-C3N4 (bulk CN) have a characteristic peak at 27.6° (002), which is ascribed to the periodic stacking of layers with a 0.323 nm d-value.26 Notably, owing to the reduced correlation length of tri-s-triazine building blocks with interlayer periodicity, a broader XRD peak with a 1106
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Figure 4. (a) PL spectra of bulk CN, SSCN, DSCN, and TSCN samples (inset is their magnified PL peaks); (b) photocurrent responses for bulk CN, SSCN, DSCN, and TSCN electrodes measured in 0.1 mol L−1 Na2SO4 solution at 1 kHz.
hollow structures, multishell hollow structures with outer and inner shells possess a more favorable architecture for sunlight multireflection; meanwhile, their surface area is larger compared to the single-shell structure with identical size, which facilitates light absorption and redox reactions.36−38 Besides, it was reported that the multilayer structure with a light-reflecting cavity could confine the large majority of incident light within cavities,39−41 and the cavity with wavelength-scale size could maximize the light harvesting via a scattering effect.38 In the TSCN, the mesochannels work as a light-transfer pathway to guide the incident photon flux into the inner cavity of g-C3N4 nanocapsules. This ensures light waves penetrate the deeper interior of photocatalyst without inappreciable loss, thus forming a high-efficient light harvester. It has been reported that 90% of the original intensity of a 420 nm wavelength is reduced after passing through an 8.5 μm distance on agglomerated TiO2.42 The presence of mesochannels in the thin shell and the inner cavity, however, makes it possible to illuminate the outer and inner surface of MSCN. In view of the absorption, scattering, and reflection within such a highly porous multishell system, the valid surface area for light harvesting could be distinctly elevated, which is in favor of the photocatalytic performance. The corresponding bandgap energies of SSCN, DSCN, and TSCN nanocapsules are 2.72, 2.67, and 2.58 eV, respectively (Figure 3b), which exhibits a blue shift compared with that of bulk CN (2.57 eV) and the blue shift becomes less pronounced with increasing shell number. It is deduced that this blue-shift phenomenon can be explained by the quantum size effect of nanosized g-C3N4 shells, which is also observed in other nanostructured g-C3N4 photocatalysts (CN nanosheet, CN nanosphere, etc.).24 However, the light absorption enhances gradually with the increase of the shell number because of the large surface area and the light multireflection, thereby the blue shift is not clearly observed for TSCN nanocapsules. Thus, the superiority of MSCN to SSCN is reflected by the inhibited quantum size effect and the increased absorption in the entire visible range, leading to an excellent visible-light harvesting property. It can be speculated that the MSCN nanocapsules with four or more shells may possess better light response, but they are not prepared in this study after weighing the tedious synthetic procedure and the likely limited performance improvement. To verify whether the resulting MSCN samples are suitable for H2 production, the electronic band structure was explored via Mott−Schottky plots. As shown in Figure 3c, both MSCN and bulk CN exhibit positive slopes, suggesting the n-type
and thermogravimetric analysis (TGA) results (Figure S1b,c), jointly indicate the nearly complete removal of SiO2 template.23 For FTIR analysis of the TSCN sample (Figure S3b), the characteristic peaks arise at 807 cm−1 for aromatic CN heterocycles and 1200−1600 cm−1 for triazine units, in agreement with those of bulk CN.28 In addition, the XRD and FTIR analysis for SSCN and DSCN were also tested (Figure S3b,c), which exhibit similar characteristic peaks to those of the TSCN sample. The chemical states for C and N backbone elements of TSCN and bulk CN were probed by XPS analysis. The survey XPS spectra verify the presence of C and N elements, and no other heteroelements are detected (Figure 2b). In high resolution C 1s spectrum of TSCN, the two binding energies of 284.6 and 288.0 eV belong to the sp2 C−C bonds and the sp2 hybridized carbon (N−CN bonds), respectively (Figure 2c).29 The N 1s spectrum was divided into two main peaks, containing 398.3 eV for triazine rings (C−N C) and 399.8 eV for tertiary nitrogen N−(C)3 groups (Figure 2d).30 In short, these characterization results demonstrate that pure g-C3N4 materials are successfully prepared, and the SiO2 template can be easily removed without altering the multishell structure. To elucidate the optical properties of MSCN nanocapsules, UV−vis diffuse reflectance spectroscopy and resonant light scattering (RLS) were performed. In Figure 3a, bulk CN exhibits an obvious absorption edge around 478 nm; while a strong light absorbance over the range of 450−800 nm is observed for TSCN nanocapsules, which is about 25% higher than that of bulk CN. With the increase of the shell layer number, the light harvesting capacity in the visible-light range increases. Meanwhile, the RLS spectra of SSCN, DSCN, and MSCN are shown in Figure S4. It can be seen that the RLS intensity of H2O/Triton X-100 solvent (blank) is very weak over the whole scanning region. When the photocatalyst is added in the solvent, an enhanced RLS intensity appears. The bulk CN only exhibits a little higher intensity than the blank, while three MSCN samples have much higher intensities. Moreover, the RLS intensity increases with the increase of the shell number of MSCN samples, indicating that this kind of multishell structure facilitates light trapping considerably. The RLS characteristic peaks of SSCN, DSCN, and TSCN are distributed at about 450 nm.31−33 Thus, the strong diffuse reflection capability can be primarily attributed to the multiple scattering and reflection of the incident light within multishell nanoarchitectures and the porous shells, which would increase the optical pathway for the incident light.34,35 Previous investigations have reported that compared to the single-shell 1107
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Figure 5. (a) Photocatalytic H2 production and (b) photocatalytic degradation of RhB (C0 = 10 mg mL−1) over bulk CN, SSCN, DSCN, and TSCN nanocapsules under visible light irradiation and (c) cycle runs of TSCN photocatalyst for RhB degradation.
property of semiconductor.29 Compared to bulk CN, TSCN exhibits a smaller slope, revealing a greater density of electron donor. The flat band potentials of bulk CN and TSCN are around −1.08 and −0.91 eV, respectively, using saturated calomel electrode as reference. The 0.17 eV decline in the conduction band (CB) probably arises from the nanoshell structure of TSCN.13 From valence band (VB) XPS (Figure 3d), the positions of the VB edge maxima of bulk CN and TSCN nanospheres are located at ∼1.55 and ∼1.72 eV. Meanwhile, the potentials of H2/H+ and O2/H2O on the basis of SCE are exhibited in Figure S3d. Thus, MSCN meets the thermodynamic requirement for the photocatalytic hydrogen evolution by water splitting. Photoluminescence (PL) was carried out to investigate the separation efficiency of photogenerated charge carriers of semiconductor photocatalyst. As exhibited in Figure 4a, the peak intensity of these g-C3N4 nanocapsules is far weaker than that of bulk CN and decreases with the increase of shell layers. This result suggests that the multishell nanocapsule structure is in favor of inhibiting the radiative recombination of photoexcited electrons and holes. According to the recent report, the electronic transport in the g-C3N4 nanosheet is predominantly perpendicular to the nanosheet, from bulk to surface.43 Therefore, thin shell with abundantly embedded mesopores may allow highly efficient charge transport across the surface, further inhibiting their recombination. Besides, the photoelectronic properties of the g-C3N4 nanocapsules with different shell layer number were also investigated. Figure 4b reveals the photocurrent density-potential data of bulk CN and these gC3N4 nanocapsules under AM 1.5 G illumination. The dark current densities in all samples are negligible. TSCN possesses the highest photocurrent density of 1.5 μA cm−2 (all potential data are determined using a reversible hydrogen electrode (RHE) as reference), which is about 3.7, 2.1, and 1.3 times
greater than that of bulk CN, SSCN, and DSCN nanocapsules, respectively. The high photocurrent density can be ascribed to both the higher light utilization and more efficient electron transfer. Additionally, the mesopore would serve as an efficient nanoconduit that makes the electron transfer along pore wall to surface more easily.13 The photocatalytic performance of MSCN nanocapsules was evaluated by hydrogen production and dye degradation under visible light irradiation. In Figure 5a, H2 production activities were compared between bulk CN and the g-C3N4 nanocapsules with different shell layer number. Bulk CN exhibits a rather low H2-generation rate of 5.1 μmol h−1 (0.04 g, λ > 420 nm), arising from its limited visible-light absorption, poor charge transport capability and low specific surface area; while these gC3N4 nanocapsules exhibit an apparent enhancement in the H2generation rate (13.7, 20.1, and 25.2 μmol h−1 for SSCN, DSCN, and TSCN, respectively), primarily due to the increased visible-light utilization, low electron−hole recombination and high specific surface area. The excellent H2generation performance of TSCN reveals one of the highest H2 production rate in the reported g-C3N4 photocatalysts.11,44,45 In addition, the photocatalytic degradation of dye was also measured under visible light irradiation (λ > 420 nm). As presented in Figure 5b, nearly no RhB was photodegraded in the absence of photocatalyst. Bulk CN only degrades nearly 35% RhB within 80 min irradiation; while SSCN, DSCN, and TSCN nanocapsules all exhibit much higher RhB degradation capacity than bulk CN. TSCN is able to completely degrade RhB within 80 min irradiation, which is much higher than SSCN and DSCN, even higher than the reported mesoporous g-C3N4 materials and g-C3N4 nanosheets.46,47 The RhB degradation over g-C3N4 samples obeys the first-order kinetics, that is, ln(C0/C) = kt, where C0 and C are the concentration at the time zero and the concentration at time t for RhB, k is the 1108
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ACS Nano degradation reaction rate constant, respectively. The k value of TSCN is 1.459 h−1, which is about 2.5, 1.5, and 6.0 times higher than that of SSCN, DSCN, and bulk CN, respectively (Figure S5a). This greatly elevated degradation capability of MSCN nanocapsules can be maintained within five-cycle photocatalytic reactions (Figure 5c). The absorption peak gradually shifts from 553 to 498 nm, and its intensity gets weaker over time, further indicating the degradation of RhB (Figure S5b).48 The trapping experiments shows that h+ is the major reactive species involved in the RhB degradation over TSCN (inset of Figure S5b). Meanwhile, MSCN preserves the whole nanocapsule structure without collapse after the long-time reaction (Figure S1e), indicating its promising practical application. The mesoporous structure of MSCN affects the reaction kinetics. The adsorption capacity of TSCN could reach 38% after adsorption equilibrium, while only 17% and 8% for the SSCN and bulk CN, respectively. Accordingly, it was found that the multiple-shell structure does not inhibit the mass transfer (compared with SSCN) but increase the kinetic response due to its large specific surface area (115.2, 209.5, and 310.7 m2 g−1 for SSCN, DSCN, and TSCN, respectively) and porous channels. To further verify the positive effects of a multishell structure in light harvesting, we destroyed the three shells of MSCN by mechanical milling and strong sonication, and evaluated its photocatalytic performance. Upon shattering, the photocatalyst still preserved mesopores but the multishell morphology was broken. Without light penetration and reflection, a 36% decrease appeared in degradation activity (Figure S5c) just as expected. In addition, TGA in the moisture loss stage suggests that the mass loss of MSCN is more pronounced than that of bulk CN under the identical thermal treatment condition and uniform pretreatment (see the Supporting Information in Figure S5d), which demonstrated that MSCN has better water-retention capacity due to its nanocapsule structure. On the basis of the above analysis, the photocatalytic mechanism of MSCN is proposed, as shown in Scheme 2. It is
ment of catalytic activity. MSCN synthesized here presents such a successful example. First, both the morphology and electronic structure of MSCN have been manipulated to utilize more incident light. The electrons in VB of g-C3N4 can be migrated to the CB by visible light excitation, keeping the photogenerated holes in VB. The multiple mesoporous shells and cavities of MSCN allow the multiple reflections and scattering of light within the interiors of cavities as well as inside their pore channels, thus enhancing light harvesting and offering more photogenerated electrons and holes. Second, the hierarchical morphology of MSCN can facilitate the photogenerated electrons and holes transfer. In detail, photogenerated charge carriers in bulk of MSCN can quickly migrate to its surface along the interplanar direction, because of the small thickness of shells, or transfer to the wall of mesopores in the shell; meanwhile the charge carriers on the surface can readily migrate to the reaction sites along in-plane direction. Third, features including large accessible surface area, good permeability, and high water-retention capacity can not only supply plentiful catalytic active centers, but also improve the adsorption and diffusion process during the reaction. At last, H2O or dye molecules will experience the redox reactions around the catalytic center, generating H2 or being degraded.
CONCLUSIONS In summary, inspired by the orderly stacked nanostructure and the highly integrated functionalities of thylakoids in natural photosynthesis system, multishell g-C3N4 nanocapsules were synthesized by a facile and controllable templating method for high-efficiency visible-light photocatalysis. The tailored nanostructure of resultant MSCN, which encompasses the multiple shells, hollow cavities, and mesopores in thin shells, is highly favorable for visible-light harvesting and electron transfer with long lifetimes. Accordingly, the triple-shell g-C3N4 nanocapsules exhibit superior photocatalytic performance to single-shell, double-shell g-C3N4 nanocapsules, which can produce H2 as high as 630 μmol h−1 g−1 and completely degrade dye within 80 min under visible light. Our bioinspired strategy may represent a significant advance in developing highefficiency and cost-effective visible-light photocatalysts for solar energy conversion.
Scheme 2. Schematic Diagram Showing the Photocatalytic Reaction over MSCN Photocatalyst under Visible Light Irradiation
EXPERIMENT SECTION Materials. Cyanamide (98%), tetraethyl orthosilicate (TEOS), Triton X-100, and 1,2-bis(triethoxysilyl)ethane (BTSE) were bought from Tianjin Xiensi Ltd. Cetyltrimethylammonium bromide (CTAB), HCl (37%), ethanol, and ammonia aqueous solution (30%) were purchased from Tianjin Kewei Ltd. All chemical regents were of analytical grade. Synthesis of Multishell SiO2 Nanospheres. Multishell SiO2 nanospheres were obtained by a sol−gel method according to a modified previously described procedure.23 Typically, CTAB (0.08 g) was added in a mixture (53 mL) of concentrated ammonia (30 wt %), water, and ethanol (at a volume ratio of 1:75:30). After stirring for 1 h at 35 °C, BTSE (0.125 mL) and TEOS (0.125 mL) were mixed at first and then quickly dissolved in the solution under drastic stirring (1100 rpm) for 24 h. Then a mixed solution of BTSE and TEOS was quickly added again under the same condition, and the mixture was stirred for another 24 h. Next, the mixed solution was quickly added into the solution the third time, and allowed to stir for another 24 h. Afterward, the procured ethane-bridge
well-known that common heterogeneous photocatalysis often contains the following four steps: (i) light harvesting; (ii) photoexcitation to generate charge carriers; (iii) charge separation and transfer to surface of catalyst and recombination thereby; (iv) surface adsorption as well as redox reaction.9 These four steps are significantly influenced by the intrinsic features of photocatalysts, including bandgap, composition, and morphology, an integration of these characteristics would optimize the above four steps, acquiring a maximum enhance1109
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recorded with AM 1.5 (100 W cm−2) simulated sunlight irradiation. Photocatalytic Activity. The experiments on hydrogen evolution from the water splitting were carried out under visible light at room temperature. Specifically, 40 mg of photocatalyst was suspended in a mixture of 10 mL of triethanolamine (TEOA, as sacrificial reagent) and 80 mL of water under stirring. Pt at 3 wt % was in situ photoreduced onto the catalyst surface during the reaction. The suspension was bubbled with argon gas for 30 mins to remove air and then sealed, which has been described in detail in our recent work.49 A Xe lamp (300 W) with a cutoff filter (λ > 420 nm) was employed to irradiate the sealed tube. In the time of reaction, 0.2 mL of gas evolution was sampled every hour and analyzed with a Shimadzu GC gas chromatography. To get an accurate amount of the generated H2, an average value from three times’ measurement was adopted. Photodegradation of RhB (10 mg L−1) was carried out using a Xe lamp with a 420 nm cutoff filter (500 W, 10 mW cm−2). The reaction suspension consists of 30 mL of RhB solution and 30 mg of photocatalyst. Before illumination, this suspension was stirred in the dark to guarantee that the adsorption− desorption equilibrium was reached. A 2.0 mL portion of the suspension was sampled in every irradiation interval. Then, a UV−vis spectrophotometer (Hitachi, U-3010) was used to analyze the RhB concentration at its maximal absorption wavelength (553 nm).
organosilica spheres were gathered, redispersed in 720 mL of water and heated for 5 h at 120 °C to accomplish the multiinterface conversion. The product was collected and subsequently CTAB in the product was removed by the solventextraction process. Finally, triple-shell hollow SiO2 nanospheres were acquired after ethanol washing and vacuum drying. The single- and double-shell SiO2 nanospheres were also synthesized via one-batch and two-batch addition of the mixture of BTSE and TEOS following the above procedure, respectively. Synthesis of MSCN Nanocapsules. After being annealed at 600 °C for 2 h in air atmosphere, the obtained multishell SiO2 nanosphere (0.1 g) was mixed with 2 g of cyanamide and together poured into deionized water (5 mL), being stirred at 40 °C for 8 h. Next, the mixed solution was centrifuged, freezing-dried, and heated to 550 °C for 3 h under N2 protective flow of 50 mL min−1. The resulting powder was treated with Na2CO3 solution (0.3 mol L−1, 20 mL) at 60 °C for 24 h to remove SiO2 template. Finally, the multishell gC3N4 nanocapsules were obtained. Characterization. Most of instruments used in this manuscript are identical to that in our recent work, and the detailed information has been described in the related reference.49 High resolution transmission (HRTEM) and high angle annular dark field (HADDF) were conducted at 90 K using a JEOL-2011. Scanning electron microscopy (SEM) was performed on an FEI Nova XL430 instrument. X-ray diffractometer was operated on a Rigaku D/max 2500 V/PC instrument. Fourier transform infrared spectroscopies (FTIR) were recorded by a Nicolet-560 spectrometer. X-ray photoelectron spectroscopy (XPS) was performed on a PerkinElmer PHI 1600 ESCA instrument. The diffuse reflectance spectra were conducted using a UV−vis spectrophotometer (U-3010, Hitachi). Photocurrent performance was performed on a BAS Epsilon Electrochemical System. The characterization of resonant light scattering (RLS) spectra: 5 mL 0.05% (v/v) Triton X-100 was first mixed with 50 mL of absolute ethanol, and then 20 mg of MSCN samples were added into the solution. After an ultrasound treatment for 30 min, the obtained mixture was tested by a spectrofluorimeter (Hitachi, F-4600). After excitation and emission was synchronized, the RLS spectra were conducted with the wavelength range of 200−800 and 2.5 nm slit width. The RLS intensity is calculated as ΔIRLS = IRLS − I0RLS, in which IRLS and I0RLS are the magnitudes of the solution with and without photocatalyst. Photoelectrochemical Measurements. The photoelectrochemical performance of the samples was tested using a standard three-electrode system with Pt filament as a counter electrode, Ag/AgCl as a reference electrode, and Na2SO4 (0.5 mol L−1) as the electrolyte solution. For preparing a working electrode, an F-doped SnO2 (FTO) glass (0.8 × 0.8 cm2) was coated by multishell g-C3N4 nanocapsules slurry. Specifically, the slurry was dipped (3 drops) and spin-coated (2000 r min−1, 30 s) onto an FTO glass, and then dried at 40 °C for the next coating. This coating process was repeated five times. Subsequently, the coated FTO glass was calcined at 300 °C for 1 h. Total working electrodes of MSCN nanocapsules were prepared using the above process with the same concentration, coating times, and coating area, thus leading to a similar loading weight of catalyst (1.151, 1.118, 1.130, and 1.125 mg for bulk CN, SSCN, DSCN, and TSCN electrode, respectively). The photocurrent and Mott−Schottky spectroscopy were measured by using a VERASTA2273 analyzer. The photocurrent was
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08251. Additional experimental data including the TEM images, XRD, and FTIR spectra of SSCN and DSCN, the kinetic curves of RhB degradation, the TG curves of TSCN and bulk CN, the RLS spectra (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Zhongyi Jiang: 0000-0002-2492-4094 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors thank the financial support from the National Science Fund for Distinguished Young Scholars (21125627), National Natural Science Fund of China (21406163, 91534126, 21621004), Tianjin Research Program of Application Foundation and Advanced Technology (15JCQNJC10000), Program of Introducing Talents of Discipline to Universities (B06006). REFERENCES (1) Ong, W. J.; Tan, L. L.; Ng, Y. H.; Yong, S. T.; Chai, S. P. Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer to Achieving Sustainability? Chem. Rev. 2016, 116, 7159−7329. (2) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43, 7520−7535. 1110
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ACS Nano (3) Liu, J.; Liu, Y.; Liu, N. Y.; Han, Y. Z.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. H. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-electron Pathway. Science 2015, 347, 6225−6230. (4) Chang, K.; Mei, Z. W.; Wang, T.; Kang, Q.; Ouyang, S. X.; Ye, J. H. MoS2/Graphene Cocatalyst for Efficient Photocatalytic H2 Evolution under Visible Light Irradiation. ACS Nano 2014, 8, 7078− 7087. (5) Wang, Y.; Wang, X. C.; Antonietti, M. Polymeric Graphitic Carbon Nitride As a Heterogeneous Organocatalyst: from Photochemistry to Multipurpose Catalysis to Sustainable Chemistry. Angew. Chem., Int. Ed. 2012, 51, 68−89. (6) Cao, S. W.; Yu, J. G. g-C3N4-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2014, 5, 2101−2107. (7) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Porous C3N4 Nanolayers@N-Graphene Films As Catalyst Electrodes for Highly Efficient Hydrogen Evolution. ACS Nano 2015, 9, 931−940. (8) Cao, S. W.; Low, J. X.; Yu, J. G.; Jaroniec, M. Polymeric Photocatalysts based on Graphitic Carbon Nitride. Adv. Mater. 2015, 27, 2150−2176. (9) Ran, J. R.; Ma, T. Y.; Gao, G. P.; Du, X. W.; Qiao, S. Z. Porous Pdoped Graphitic Carbon Nitride Nanosheets for Synergistically Enhanced Visible-Light Photocatalytic H2 Production. Energy Environ. Sci. 2015, 8, 3708−3717. (10) Wang, S. P.; Li, C. J.; Wang, T.; Zhang, P.; Li, A.; Gong, J. L. Controllable Synthesis of Nanotube-Type Graphitic C3N4 and Their Visible-Light Photocatalytic and Fluorescent Properties. J. Mater. Chem. A 2014, 2, 2885−2890. (11) Niu, P.; Zhang, L. L.; Liu, G.; Cheng, H. M. Graphene-like Carbon Nitride Nanosheets for Improved Photocatalytic Activities. Adv. Funct. Mater. 2012, 22, 4763−4770. (12) Sun, J. H.; Zhang, J. S.; Zhang, M. W.; Antonietti, M.; Fu, X. Z.; Wang, X. C. Bioinspired Hollow Semiconductor Nanospheres As Photosynthetic Nanoparticles. Nat. Commun. 2012, 3, 1139−1145. (13) Jun, Y. S.; Lee, E. Z.; Wang, X. C.; Hong, W. H.; Stucky, G. D.; Thomas, A. From Melamine-Cyanuric Acid Supramolecular Aggregates to Carbon Nitride Hollow Spheres. Adv. Funct. Mater. 2013, 23, 3661−3667. (14) Zhang, J. S.; Zhang, M. W.; Yang, C.; Wang, X. C. Nanospherical Carbon Nitride Frameworks with Sharp Edges Accelerating Charge Collection and Separation at a Soft Photocatalytic Interface. Adv. Mater. 2014, 26, 4121−4126. (15) Zou, Q. L.; Liu, K.; Abbas, M.; Yan, X. H. Peptide-Modulated Self-Assembly of Chromophores toward Biomimetic Light-Harvesting Nanoarchitectonics. Adv. Mater. 2016, 28, 1031−1043. (16) Liu, Z. F.; Yan, H. C.; Wang, K. B.; Kuang, T. Y.; Zhang, J. P.; Gui, L. L.; An, X. M.; Chang, W. R. Crystal Structure of Spinach Major Light-Harvesting Complex at 2.72A° Resolution. Nature 2004, 428, 287−292. (17) Jarvi, S.; Gollan, P. J.; Aro, E. M. Understanding the Roles of the Thylakoid Lumen in Photosynthesis Regulation. Front. Plant Sci. 2013, 4, 434−447. (18) Lu, Y. Identification and Roles of Photosystem II Assembly, Stability and Repair Factors in Arabidopsis. Front. Plant Sci. 2016, 7, 168−194. (19) Goltsev, V.; Doltchinkova, V.; Yordanov, I. Photo-Induced Changes of the Thylakoid Membrane Light-Scattering. I. Effect of the Surface Charge. Photobiochem. Photobiophys. 1986, 73−77. (20) Unnep, R.; Zsiros, O.; Solymosi, K.; Kovacs, L.; Lambrev, P. H.; Toth, T.; Schweins, R.; Posselt, D.; Szekely, N. K.; Rosta, L.; et al. The Ultrastructure and Flexibility of Thylakoid Membranes in Leaves and Isolated Chloroplasts As Revealed by Small-Angle Neutron Scattering. Biochim. Biophys. Acta, Bioenerg. 2014, 1837, 1572−1580. (21) Daum, B.; Kühlbrandt, W. Electron Tomography of Plant Thylakoid Membranes. J. Exp. Bot. 2011, 62, 2393−2402. (22) Mustárdy, L.; Buttle, K.; Steinbach, G.; Garab, G. The ThreeDimensional Network of the Thylakoid Membranes in Plants: Quasihelical Model of the Granum-Stroma Assembly. Plant Cell 2008, 20, 2552−2557.
(23) Teng, Z. G.; Su, X. D.; Zheng, Y. Y.; Zhang, J. J.; Liu, Y.; Wang, S. J.; Wu, J.; Chen, G. T.; Wang, J. D.; Zhao, D. Y.; et al. A Facile Multi-Interface Transformation Approach to Monodisperse MultipleShelled Periodic Mesoporous Organosilica Hollow Spheres. J. Am. Chem. Soc. 2015, 137, 7935−7944. (24) Chen, X. F.; Jun, Y. S.; Takanabe, K.; Maeda, K.; Domen, K.; Fu, X. Z.; Antonietti, M.; Wang, X. C. Ordered Mesoporous SBA-15 Type Graphitic Carbon Nitride: A Semiconductor Host Structure for Photocatalytic Hydrogen Evolution with Visible Light. Chem. Mater. 2009, 21, 4093−4095. (25) Wang, J. Y.; Tang, H. J.; Ren, H.; Yu, R. B.; Qi, J.; Mao, D.; Zhao, H. J.; Wang, D. pH-Regulated Synthesis of Multi-Shelled Manganese Oxide Hollow Microspheres As Supercapacitor Electrodes Using Carbonaceous Microspheres As Templates. Adv. Sci. 2014, 1, 1400011−1400016. (26) Shalom, M.; Inal, S.; Fettkenhauer, C.; Neher, D.; Antonietti, M. Improving Carbon Nitride Photocatalysis by Supramolecular Preorganization of Monomers. J. Am. Chem. Soc. 2013, 135, 7118−7121. (27) Tahir, M.; Cao, C.; Butt, F. K.; Idrees, F.; Mahmood, N.; Ali, Z.; Aslam, I.; Tanveer, M.; Rizwan, M.; Mahmood, T. Tubular GraphiticC3N4: A Prospective Material for Energy Storage and Green Photocatalyst. J. Mater. Chem. A 2013, 1, 13949−13955. (28) Han, Q.; Wang, B.; Gao, J.; Cheng, Z. H.; Zhao, Y.; Zhang, Z. P.; Qu, L. T. Atomically Thin Mesoporous Nanomesh of GraphiticC3N4 for High-Efficiency Photocatalytic Hydrogen Evolution. ACS Nano 2016, 10, 2745−2751. (29) Liang, Q. H.; Li, Z.; Huang, Z. H.; Kang, F. Y.; Yang, Q. H. Holey Graphitic Carbon Nitride Nanosheets with Carbon Vacancies for Highly Improved Photocatalytic Hydrogen Production. Adv. Funct. Mater. 2015, 25, 6885−6892. (30) Muñoz-Batista, M. J.; Kubacka, A.; Fernández-García, M. Effective Enhancement of TiO2 Photocatalysis by Synergistic Interaction of Surface Species: from Promoters to Co-catalysts. ACS Catal. 2014, 4, 4277−4288. (31) Ye, J.; Zhou, R. H.; Zheng, C. B.; Sun, Q.; Lv, Y.; Li, C. Y.; Hou, X. D. Size-Controllable Synthesis of Spherical ZnO Nanoparticles: Size- and Concentration-Dependent Resonant Light Scattering. Microchem. J. 2012, 100, 61−65. (32) Wang, S.; Ding, Y.; Xu, S.; Zhang, Y.; Li, G.; Hu, L.; Dai, S. TiO2 Nanospheres: A Facile Size-Tunable Synthesis and Effective LightHarvesting Layer for Dye-Sensitized Solar Cells. Chem. - Eur. J. 2014, 20, 4916−4920. (33) Osterloh, F. E. Inorganic Nanostructures for Photoelectrochemical and Photocatalytic Water Splitting. Chem. Soc. Rev. 2013, 42, 2294−2320. (34) Qian, J.; Liu, P.; Xiao, Y.; Jiang, Y.; Cao, Y.; Ai, X.; Yang, H. TiO2-Coated Multilayered SnO2 Hollow Microspheres for DyeSensitized Solar Cells. Adv. Mater. 2009, 21, 3663−3667. (35) Tian, Z. P.; Tian, H. M.; Wang, X. Y.; Yuan, S. K.; Zhang, J. Y.; Zhang, X. B.; Yu, T.; Zou, Z. G. Multilayer Structure with Gradual Increasing Porosity for Dye-Sensitized Solar Cells. Appl. Phys. Lett. 2009, 94, 031905−031908. (36) Qi, J.; Lai, X. Y.; Wang, J. Y.; Tang, H. J.; Ren, H.; Yang, Y.; Jin, Q.; Zhang, L. J.; Yu, R. B.; Ma, G. H.; Su, Z. G.; Zhao, H. J.; Wang, D. Multi-Shelled Hollow Micro-/Nanostructures. Chem. Soc. Rev. 2015, 44, 6749−6774. (37) Ferber, J.; Luther, J. Computer Simulations of Light Scattering and Absorption in Dye-Sensitized Solar Cells. Sol. Energy Mater. Sol. Cells 1998, 54, 265−275. (38) Zhao, Y.; Jiang, L. Hollow Micro/Nanomaterials with Multilevel Interior Structures. Adv. Mater. 2009, 21, 3621−3638. (39) Dong, Z. H.; Lai, X. Y.; Halpert, J. E.; Yang, N. L.; Yi, L. X.; Zhai, J.; Wang, D.; Tang, Z. Y.; Jiang, L. Accurate Control of MultiShelled ZnO Hollow Microspheres for Dye-Sensitized Solar Cells with High Efficiency. Adv. Mater. 2012, 24, 1046−1049. (40) Hwang, S. H.; Yun, J.; Jang, J. Multi-Shell Porous TiO2 Hollow Nanoparticles for Enhanced Light Harvesting in Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2014, 24, 7619−7626. 1111
DOI: 10.1021/acsnano.6b08251 ACS Nano 2017, 11, 1103−1112
Article
ACS Nano (41) Caruso, F.; Caruso, R. A.; Mohwald, H. Nanoengineering of Inorganic and Hybrid Hollow Spheres by Colloidal Templating. Science 1998, 282, 1111−1114. (42) Hagfeldt, A.; Graetzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev. 1995, 95, 49−68. (43) Merschjann, C.; Tschierlei, S.; Tyborski, T.; Kailasam, K.; Orthmann, S.; Hollmann, D.; Schedel-Niedrig, T.; Thomas, A.; Lochbrunner, S. Complementing Graphenes: 1D Interplanar Charge Transport in Polymeric Graphitic Carbon Nitrides. Adv. Mater. 2015, 27, 7993−7999. (44) Li, K. X.; Zeng, Z. X.; Yan, L. S.; Luo, S. L.; Luo, X. B.; Huo, M. X.; Guo, Y. H. Fabrication of Platinum-Deposited Carbon Nitride Nanotubes by a One-Step Solvothermal Treatment Strategy and Their Efficient Visible-Light Photocatalytic Activity. Appl. Catal., B 2015, 165, 428−437. (45) Guo, S. E.; Deng, Z. P.; Li, M. X.; Jiang, B. J.; Tian, C. G.; Pan, Q. J.; Fu, H. G. Phosphorus-Doped Carbon Nitride Tubes with a Layered Micro-Nanostructure for Enhanced Visible-Light Photocatalytic Hydrogen Evolution. Angew. Chem., Int. Ed. 2016, 55, 1830−1834. (46) Xu, J.; Wang, Y. J.; Zhu, Y. F. Nanoporous Graphitic Carbon Nitride with Enhanced Photocatalytic Performance. Langmuir 2013, 29, 10566−10572. (47) Dong, F.; Wu, L. W.; Sun, Y. J.; Fu, M.; Wu, Z. B.; Lee, S. C. Efficient Synthesis of Polymeric g-C3N4 Layered Materials As Novel Efficient Visible Light Driven Photocatalysts. J. Mater. Chem. 2011, 21, 15171−15174. (48) Liu, C.; Yang, D.; Jiao, Y.; Tian, Y.; Wang, Y. G.; Jiang, Z. Y. Biomimetic Synthesis of TiO 2-SiO 2-Ag Nanocomposites with Enhanced Visible-Light Photocatalytic Activity. ACS Appl. Mater. Interfaces 2013, 5, 3824−3432. (49) Tong, Z. W.; Yang, D.; Sun, Y. Y.; Nan, Y. H.; Jiang, Z. Y. Tubular g-C3N4 Isotype Heterojunction: Enhanced Visible-Light Photocatalytic Activity through Cooperative Manipulation of Oriented Electron and Hole Transfer. Small 2016, 12, 4093−4101.
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