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Constructing Highly Uniform Onion-Ring-Like Graphitic Carbon Nitride for Efficient Visible-Light-Driven Photocatalytic Hydrogen Evolution Lifeng Cui, Jialing Song, Allister F. McGuire, Shifei Kang, Xueyou Fang, Junjie Wang, Chaochuang Yin, Xi Li, Yangang Wang, and Bianxiao Cui ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01271 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018
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Constructing Highly Uniform Onion-Ring-Like Graphitic Carbon Nitride for Efficient Visible-Light-Driven Photocatalytic Hydrogen Evolution Lifeng Cui1, Jialing Song1,2, Allister F. McGuire3, Shifei Kang1, Xueyou Fang1, Junjie Wang1, Chaochuang Yin1, Xi Li2, Yangang Wang*2, Bianxiao Cui*3 1
Department of Environmental Science and Engineering, University of Shanghai for
Science and Technology, Shanghai 200093, China 2
College of Biological Chemical Science and Engineering, Jiaxing University,
Jiaxing 314001, China 3
Department of Chemistry, Stanford University, Stanford, CA 94305, USA
Email:
[email protected] (Y. Wang);
[email protected] (B. Cui).
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ABSTRACT: The introduction of microstructure to the metal-free graphitic carbon nitride (g-C3N4) photocatalyst holds promise in enhancing its catalytic performance. However, producing such microstructured g-C3N4 remains technically challenging due to a complicated synthetic process and high cost. In this study, we develop a facile and in-air chemical vapor deposition (CVD) method that produces onion-ring-like g-C3N4 microstructures in a simple, reliable, and economical manner. This method involves the use of randomly packed 350 nm SiO2 microspheres as a hard template and melamine as a CVD precursor for the deposition of a thin layer of g-C3N4 in the narrow space between the SiO2 microspheres. After dissolution of the microsphere template,
the
resultant
g-C3N4 exhibits
uniquely
uniform
onion-ring-like
microstructures. Unlike previously reported g-C3N4 powder morphologies that show various degrees of agglomeration and irregularity, the onion-ring-like g-C3N4 is highly dispersed and uniform. The calculated band gap for onion-ring-like g-C3N4 is 2.58 eV which is significantly narrower than that of bulk g-C3N4 at 2.70 eV. Experimental characterization and testing suggest that, in comparison with bulk g-C3N4, onion-ring-like g-C3N4 facilitates charge separation, extends the lifetime of photo-induced carriers, exhibits five-fold higher photocatalytic hydrogen evolution, and shows great potential for photocatalytic applications.
KEYWORDS: Graphitic carbon nitride; microstructure design; chemical vapor deposition; photocatalysis; hydrogen evolution
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Semiconductor-based photocatalysis holds great promise for alleviating the world’s environmental and energy problems by mediating photo-degradation of organic pollutants in water and photocatalytic hydrogen evolution.1-6 Because of its high combustion value, renewability, and lack of pollutant byproducts, among other advantages, hydrogen is the best green energy option to address energy issues and pollution problems.1,2,5 Hydrogen can be produced from water by means of photocatalytic hydrogen evolution. Photocatalysts with excellent activity and good stability play an important role in industrial photocatalytic hydrogen evolution.7-9 Though many types of photocatalysts have been developed, problems such as a complicated synthetic process, high cost, and poor utilization of solar energy still hinder their commercial application.10,11 Multiple semiconductors that respond to visible light, such as metal nitrides and chalcogenides,12-18 (oxy)nitrides, and (oxy)sulfides,19-21 are capable of evolving hydrogen, but almost all of these have a common problem of chemical instability when exposed to sunlight. Until now, it has been a difficult task to design visible-light-responsive semiconductor photocatalysts with high energy conversion efficiency and good chemical stability. Graphitic carbon nitride (g-C3N4) is a relatively stable semiconducting photocatalyst that is composed of environmentally benign light elements and responds to visible light. It possesses a narrow band gap that is enabled by its conjugated graphitic plane consisting of sp2-hybridized nitrogen and carbon atoms.11,22,23 G-C3N4 boasts a metal-free composition, good structural stability, cheap production, a proper band gap, and potential as an efficient photocatalyst for CO2 photoreduction and hydrogen evolution.24 It is simple to synthesize bulk g-C3N4 through thermal condensation of nitrogen-enriched compounds (melamine, urea, cyanamide, and thiourea);25 however, bulk g-C3N4 shows low photoactivity because of its significant π-conjugation, small surface area, high recombination rate of charge carriers, and poor mass transfer.26-28 Optimizing the structural and photoelectric properties of a given semiconductor is a critical step in widening its response spectrum and enhancing its photocatalytic performance.1-7, 29, 30 To this end, modifying the microstructure of g-C3N4 has proven ACS Paragon Plus Environment
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to be a successful method to promote the migration and separation of photo-generated charge carriers. Micro-structured g-C3N4 such as ordered mesoporous structures,31,32 nanospheres,33,34 and hollow nanospheres,35 has attracted increasing attention.32,36 To make micro-structured g-C3N4, template-based methods often are chosen because the shape and size of the obtained g-C3N4 can be controlled by changing the geometry of the sacrificial template materials. For example, nanostructured g-C3N4, such as nanosheets and nanospheres, can be synthesized by mixing a SiO2 nanoparticle template with the cyanamide precursor through wet impregnation and high temperature reflux, and then removing the SiO2 template.31,37 However, the cyanamide precursor used in this solution phase nanocasting process is highly toxic and flammable, which limits the synthesis of micro-structured g-C3N4. In the last few years, melamine has been explored as a precursor in the synthesis of microstructured g-C3N4.35,38 Melamine is nontoxic, stable, and can be thermally polymerized to form g-C3N4 at moderate temperatures in a closed air environment.38 Previous studies have used melamine to synthesize g-C3N4 microstructures including nanosheets and mesopores,38-39 while the functional microstructures with great uniformity, dispersibility, and activity are still highly desired. In this study, we report the development of an
onion-ring-like g-C3N4
(simplified as R-CN) via a simple in-air CVD method based on a packed SiO2 microsphere template and its subsequent decoration with Pt nanoparticles. We find that the diameter of the SiO2 template is important for the microstructure and the dispersion of synthesized g-C3N4. The optimized R-CN sample exhibits greater hydrogen evolution activity and better cyclic stability in comparison with bulk g-C3N4. Experimental characterization data shows that onion-ring-like microstructure can indeed greatly enhance g-C3N4 photocatalysis.
RESULTS AND DISCUSSION R-CN samples were prepared by in-air CVD using three different diameters of SiO2 microspheres at 200 nm, 350 nm and 500 nm, which are denoted as R-CN-200,
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R-CN-350, and R-CN-500, respectively (Figure 1). We also prepared another sample without CVD by thoroughly mixing precursor melamine and 350 nm SiO2 microspheres in the heating crucible (Mix-CN). Bulk g-C3N4 (B-CN) was prepared as a reference sample, where no SiO2 microspheres were added to the heating crucible. Detailed synthetic procedures are described in the experimental section. X-ray diffraction (XRD) was utilized to obtain information about the structure of the synthesized materials (Figure 2). Two characteristic peaks were identified in the XRD patterns, which can be assigned to the typical diffraction peaks of g-C3N4 with PDF number of 87-1526. The peak at around 13.1° is the (100) diffraction peak with an interplanar distance of 0.68 nm. Another intense peak around 27.3° is attributed to the interlayer stacking of aromatic units of CN with a distance of 0.33 nm and corresponds the (002) diffraction peak of g-C3N4.35,40 Comparing different synthesized materials, the (002) peak intensities of Mix-CN and all R-CNs are lower than that of B-CN. This is likely due to the imperfect g-C3N4 crystal structure cast from a SiO2 microsphere template. In particular, the (002) reflection peak of R-CN-350 is less intense than those of R-CN-200, R-CN-500, and Mix-CN, suggesting that the size of the template affects the resultant g-C3N4 crystal structure. To further investigate the synthesized materials, we used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study their microscopic morphologies. The SEM image of the 350 nm SiO2 microspheres before CVD deposition clearly shows a uniform size distribution (Figure 3a). Melamine deposits and polymerizes in the narrow space between SiO2 microspheres, yielding various microstructured g-C3N4 geometries after removal of the SiO2 template. SEM reveals that the onion-ring-like structures of R-CN-350 are individually separated and well dispersed (Figure 3b). In contrast, R-CN-200 (Figure 3c) shows hole-like or honeycomb-like structures with a significant degree of agglomeration and irregularity. R-CN-500 shows many broken ring structures (Figure 3d), possibly because the 500 nm diameter SiO2 microspheres generate unstable rings which break or collapse during sonication and centrifugation. The Mix-CN shows some honeycomb-like
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structures with severe agglomeration (Figure 3e) and the B-CN shows no microstructures and only large particles with irregular morphology (Figure 3f). In TEM, R-CN-350 appears to have a thin ring morphology and the average inner diameter of this ring is approximately 350 nm (Figure 4a). Interestingly, TEM of R-CN-350 after 4 h of ultrasonic processing shows that a complete onion-ring-like structure is preserved, supporting the high stability of R-CN-350 (Figure 4b). In addition, the ring structure of R-CN-350 neither collapses nor breaks after decoration with Pt nanoparticles and three cycles of photocatalysis (Figure 4c, d). Thus, onion-ring-like g-C3N4 can be used as a uniform and stable support on which other nano-scale catalysts can be loaded uniformly. UV–vis diffuse reflectance spectroscopy (DRS) was applied to evaluate the optical absorption properties of the as-prepared photocatalysts. All measured materials show absorption at blue light wavelengths as is typical of semiconductor absorption caused by band transitions from the valence band to the conduction band (Figure 5a). In the visible wavelength range of 390420 nm). Photocatalytic stability of Pt/R-CN-350 was examined over multiple reaction cycles. After each cycle, the light source was turned off and the closed system was thoroughly flushed with nitrogen for 30 minutes before the next cycle started.
Visible light
Quartz Reactor
O2 Oxidation e-
H2 ·OH Reduction
Catalysts
Pt/R-CN
h+ h+ h+
e-
OH-
Figure 7. Schematic illustration of the photocatalytic reaction on Pt/R-CN.
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·O2 e-
TEOA
