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Significant Enhancement of Visible-LightDriven Hydrogen Evolution by Structure Regulation of Carbon Nitrides Qing Han,* Zhihua Cheng, Bing Wang, Huimin Zhang,* and Liangti Qu* Key Laboratory of Photoelectronic/Eletrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education of China, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China S Supporting Information *
ABSTRACT: Photocatalytic water splitting for hydrogen evolution by utilizing solar energy has a great significance for high-density solar energy storage and environmental sustainability. Here, a defect-rich amorphous carbon nitride (DACN) photocatalyst has been synthesized by simply direct calcination of the rationally size-reduced urea crystals. The introduction of nitrogen vacancies combined with disordered structure cause a broad visible-light-reponsive range, countless lateral edge/exposed surface bonding sites, and quenched radiative recombination, suggesting that this DACN enhances photocatalytic activity for hydrogen production. A record high hydrogen evolution rate of 37,680 μmol h−1 g−1 under visible-light irradiation and an extraordinary apparent quantum efficiency of 34.4% at 400 nm were achieved, higher than most of the existing graphitic carbon nitride-based photocatalysts. KEYWORDS: defect-rich amorphous carbon nitride, size-reduced urea, antisolvent growth, photocatalysis, visible-light hydrogen evolution activity as reported by Niu and co-workers.14−16 They created N vacancies in GCN, which resulted in a narrowed band gap of 2.03 eV.14 In this regard, Zhang and co-workers synthesized nitrogen-deficient GCN by forming the cyano groups and losing sp2 C−NC (N2C) on the surface of GCN, leading to an enhanced visible-light photocatalytic H2-generation activity of 6900 μmol h−1 g−1 mainly because of the reduced band gap of 2.36 eV.15 These literatures have mainly focused on the creation of nitrogen vacancies in crystalline GCN as the photocatalysts, overlooking the multiformity of other kinds of carbon nitride materials with degrees of order in atomic arrangements. Recent results have shown that destruction of the long-range atomic order arrangements of crystalline GCN can enhance the photoactivity.16 Thus, the introduction of nitrogen deficiencies in amorphous carbon nitride presents great potential in photochemical catalysis. Herein, by simply direct calcination of the rationally sizereduced urea crystals from an antisolvent growth method, we have prepared a defect-rich amorphous carbon nitride
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sing solar energy and noble metal catalysts to produce hydrogen from water, splitting is the ideal technology for renewable energy storage. It has attracted intense efforts during the past 40 years but is far from being commercialized, mainly due to the low visible-light activity, the scarcity, high cost, and poor chemical stability of the current photocatalysts.1,2 Graphitic carbon nitride (GCN), a graphitelike two-dimensional layered material, is promising for hydrogen evolution because of the outstanding optical properties, reasonable cost, and good stability.3−5 Unfortunately, the photocatalytic performance is still restricted by its fast recombination of photoproduced electron−hole pairs, narrow light absorption range, and low specific surface area. To address these limitations, great efforts have been contributed to optimize the photocatalytic activity, including engineering of nanoarchitectures to increase surface area, element doping to tune optoelectronic properties, coupling with (semi)conductors, or co-catalysts to improve charge separation.6−12 While some previous attempts have been made, the photocatalytic activities of these GCN-based photocatalysts are still inadequate.13 It was recently reported that the introduction of nitrogen defects into GCN can widen light absorption range, thus absorbing more visible light for improving photocatalytic © XXXX American Chemical Society
Received: November 15, 2017 Accepted: May 4, 2018
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DOI: 10.1021/acsnano.7b08100 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Preparation process of DACN. (a) Pristine urea, (b) size-reduced urea, (c) GCN, and (d) DACN.
(DACN). Its nitrogen vacancies and disordered structure effectively result in the extended visible-light absorption, increased surface/edge active sites, and improved charge separation. As a result, the DACN shows a superior hydrogen evolution rate of 37,680 mol h−1 g−1 under visible-light irradiation, which results in a prominent turn number of 1769 after 6 h and a remarkable apparent quantum efficiency of 34.4% at 400 nm.
different sizes display different XRD peak intensity, which can be ascribed to their various morphologies with exposed face in varying degrees. This reduced precursor size apparently changes the molecular structure of the final product. As shown in Figure 2h, the GCN features two distinctive peaks at 13.0° and 27.7° arising from intralayer structural packing motif and interlayer stacking of conjugated aromatic segments, respectively.3,17−21 Compared to GCN, the 13.0° peak in DACN-100 disappears, suggesting the disruption of intralayer long-range atomic order as observed in the TEM images (Figure 2d). The high-angle peak at 27.7° (GCN) is widened and shifted to 27.3° for DACN-100, indicating the disturbance of interlayers periodic stacking and formation of fluctuant structure. Atomic force microscopy (AFM) image (Figure 2f) indicates that the thickness of the formed DACN-100 sheets is only 0.4−0.7 nm, much thinner than that of GCN (3.0−3.5 nm, Figure S3), which is supported by the increase in the Brunauer−Emmett−Teller (BET) surface area of DACN-100 (228.4 m2 g−1), 5.3 times that of GCN (43.1 m2 g−1, Figure 3a). Such destroyed structure in DACN-100 induces a 5.3-fold increase in specific surface area, which is conductive to light harvesting and utilization of active sites. Fourier transform infrared (FTIR) spectrum of DACN-100 in Figure 3b exhibits that the intensities of the breathing mode of the tri-s-triazine units at 810 cm−1, the NH/NH2 stretching peaks between 2900 and 3340 cm−1, and the broad band of hydroxyl group/adsorbed water in the region ranging from 3433 to 3570 cm−1 are lower than that of GCN.17−21 In addition, there is a strong absorption for DACN-100 in the 1200−1600 cm−1 range corresponding to the characteristic stretching modes of CN heterocycles. These results indicate the partial loss of nitrogen atoms in NH/NH2 groups from heptazine-based melon occurred during in situ annealing of the precursor SRU-100. X-ray photoelectron spectroscopy (XPS) spectrum of DACN-100 sample shows similar chemical composition to that of GCN (Figure S4), but with an increased C/N atom ratio from 3:4.6 (GCN) to 3:4.1 (DACN-100, Table S1), illustrating the loss of nitrogen atoms. High-resolution N 1s spectra of DACN-100 (Figure 3c) displays three peaks at 398.6, 399.8, and 401.2 eV corresponding to sp2-bonded nitrogen (C−NC, N2C), tertiary nitrogen N−[C]3 (N3C),
RESULTS AND DISCUSSION To effectively adjust the urea, we develop an antisolvent growth technique, in which the size-reduced urea (SRU) crystallizes from methanol-supersaturated solution by adding diethyl ether. As shown in Figure 1, the pristine urea with a grain size of 4 mm (Figures 1a and 2a) was converted into 100 μm-size mini prismoidal grains (SRU-100, Figures 1b and 2c) in a compact way. Subsequently, the direct calcination of SRU-100 resulted in the formation of DACN (notated as DACN-100, Figure 1d). Urea is specifically chosen as the precursor, which would lose hydrogen in the form of formaldehyde during the thermal polymerization, resulting in decreased protonation and increased photocatalytic activity.13 Comparing with pristine urea, the thermogravimetric-differential scanning calorimetry (TG-DSC) analysis of SRU-100 shows the decrease in the whole phase transition temperature (Figure 2g, detailed description in Figure S1) and becomes more obvious with the rise of temperature, indicating the reduced precursor size leads to the lower temperature of thermal polycondensation and evaporation. It would have an important impact on the band structures, electronic features, and performance of the final DACN products. The GCN derived from pristine urea has a large grain size of micrometers and features flat plane flake (Figures 1c and 2b; Figure S2). Interestingly enough, the DACN-100 seems like the cracked GCN, demonstrating the intralayer and interlayer structure is destroyed (Figures 1d and 2d). The high-resolution transmission electron microscopy (HRTEM) image of DACN100 displays the ideal Moiré patterns, illustrating its amorphous structure with short-range atomic order (Figure 2e). In X-ray diffraction (XRD) patterns (Figure 2h), the two urea crystals in B
DOI: 10.1021/acsnano.7b08100 ACS Nano XXXX, XXX, XXX−XXX
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that comparing with that of GCN (458 nm, yellow), DACN100 (orange) exhibits a strong red shift of the intrinsic absorption edge band to 504 nm and an increased band tails to 700 nm, mainly due to the existence of large numbers of defect sites arising from the nitrogen vacancies and the structure destruction.14,15 The obtained band gaps are 1.78 eV for DACN-100 and 2.61 eV for GCN (inset in Figure 4a). XPS valence band spectrum (Figure 4b) shows that the valence band of DACN-100 rises by 0.24 eV. Based on the experimentally measured bandgap for DACN-100 with the decrease of 0.83 eV relative to GCN, the CB of DACN-100 downshifted by 0.59 eV (Figure 4c). This narrowed band gap of DACN-100 is beneficial for visible-light harvesting. Meanwhile, the strong photoluminescence (PL) quenching for DACN-100 implies that the recombination rate of electron−hole pairs is substantially suppressed by reducing the long-range order of atomic arrangements and introducing N defects (Figure 4d), thus improving charge separation.15,16 The time-resolved PL spectra (inset in Figure 4d) show that the average PL lifetime (τ) is 37.87 ns for DACN-100, much shorter than that of GCN (56.17 ns). It can be attributed to the emergence of the band tail states, for the excitation of electron−hole pairs, in accordance with the optical spectra results.22 Electron paramagnetic resonance (EPR) spectra of DACN-100 and GCN show one single Lorentzian line with a g value of 2.0032, which is induced by the unpaired electrons on the carbon atoms of the heptazine rings (Figure 4e).23 The EPR intensity of DACN-100 greatly strengthens as compared to GCN, indicating a higher concentration of lone pair electrons, which is conducive to the photocatalytic reaction. As a result, the DACN-100 exhibits the notably enhanced photocatalytic activity relative to GCN, as shown in Figure 4f. Note that the DACN-100 with the excellent hydrogen evolution rate (HER) of 37,680 μmol h−1 g−1 is 8.6 times higher than that of GCN (4365 μmol h−1 g−1), as reflected in the turnover number (TON): DACN-100 has a TON of 1769 in 6 h, an order of magnitude greater than that of GCN (156). The observed 8.6-fold increase in HER of DACN100 is manifestly more significant than that 5.3-fold improvement in the specific surface area, illustrating the combination of disorderly structure and loss of nitrogen atoms induced narrowed bandgap also results in higher photocatalytic activities. The precursor size has a significant effect on the content of nitrogen vacancies, degree of disorder in atomic arrangements, and performance of DACN products. For comparison, various DACN samples were synthesized by calcination of the precursors of 500 μm-size and 10 μm-size SRU, which were designated as DACN-500 and DACN-10, respectively. The SEM, TEM, XRD, FTIR, and XPS spectra and ζ potentials of DACN samples (Figures S5−8, Tables S3 and S4) imply: with decrease in precursor grain sizes (form pristine urea to SRU100), the destruction of in-plane and interlayer periodic structure magnifies, and the loss of nitrogen atoms increases. However, a further decrease of precursor size (SRU-10) causes the reduced nitrogen vacancies (DACN-10), possibly because of the cleavage of the C−N bonds during the thermal treatment. The excessive disrupt DACN-10 shows a blueshift in absorption edge as well as the PL spectra, which should be attributed to the quantum confinement effect. 24−36 In consequence, the HER of DACN samples is obviously higher than that of GCN (4365 μmol h−1 g−1), which increases first (19,280 μmol h−1 g−1 for DACN-500) and achieves a maximum of 37,480 μmol h−1 g−1 for DACN-100, then decreases to
Figure 2. (a) SEM image of pristine urea. (b) TEM image of GCN. (c) SEM image SRU-100. (d) TEM image of DACN-100. (e) The HRTEM image of the as-prepared DACN-100. (f) AFM image and height profile along the line in (f) of DACN-100. (g) Thermal polymerization process of GCN and DACN-100 derived from the TG-DSC thermograms. (h) XRD patterns of pristine urea, SRU100, GCN, and DACN-100.
and amino groups (N−Hx, x = 1, 2), respectively.6−12,17−21 A reduced content of N−Hx from 10.9 (GCN) to 7.8 (DACN100, Table S2) implies the reduction of amino groups. Simultaneously, the ratio of N2C/(N3C + N−Hx) increases from 1.7 (GCN) to 2.5 (DACN-100). Accordingly, the intensity of 288.1 eV peak corresponding to (N2)C(NH2) in the C 1s spectra of DACN-100 (Figure 3d) increases relative to GCN further confirms the loss of one nitrogen (Table S2). Meanwhile, the ζ potential measurements in distilled water (pH = 6.8) for DACN-100 (−13.9 mV) are smaller than that of GCN (−32.8 mV), suggesting the lower level of surface protonation, which can lead to the break of intralayer hydrogen bonds and generate long-range atomic disorder structure. Therefore, it can be concluded that the effectively controlled precursor sizes alters the chemical composition and the order in atomic arrangement of the final crystalline GCN, producing nitrogen-deficient amorphous DACN with short-range atomic order. The nitrogen defects and structure distortion greatly extended the optical properties of DACN-100 with respect to GCN. In Figure 4a, UV−vis diffuse reflectance spectra reveal C
DOI: 10.1021/acsnano.7b08100 ACS Nano XXXX, XXX, XXX−XXX
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Figure 3. (a) N2 adsorption isotherms of GCN and DACN-100. (b) FT-IR spectra of GCN and DACN-100. (c) N 1s and (d) C 1s XPS spectra of GCN and DACN-100.
Figure 4. (a) UV−vis diffuse reflectance spectra (inset: bandgap energies and photographs). (b) XPS valence band spectra (CB: conduction band; VB: valence band). (c) Electronic band structure. (d) PL spectra (with the excitation wavelength of 400 nm; inset: time-resolved transient PL decay spectra). (e) EPR spectra and (f) photocatalytic activities of GCN and DACN-100 under irradiation with visible light.
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DOI: 10.1021/acsnano.7b08100 ACS Nano XXXX, XXX, XXX−XXX
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Figure 5. (a) Photocatalytic activities of GCN, DACN-500, DACN-100, and DACN-10 (λ ≥ 420 nm). (b) Photocatalytic activities of various DACN samples derived from different solvents: ethanol, water, methanol-1, methanol-4, methanol-24 (1, 4, and 24 represent the mass of urea crystals), and acetonitrile, respectively. (c) Stable hydrogen evolution by DACN-100 (λ ≥ 420 nm). (d) TEM images of DACN-100 and GCN after photocatalytic reaction. (e) Wavelength dependence of H2 evolution rate on DACN-100. (f) Hydrogen-evolution rate for DACN-100 in comparison with other GCN-based photocatalysts. The concrete test conditions are described in Table S5.
17,545 μmol h−1 g−1 for DACN-10 (Figure 5a) with reducing precursor size, in accordance with the trend in bonding ratio (Table S3). This result demonstrates that precursor size reduction results in the increase of N defects and disorder in product DACN, which can enhance visible-light absorption and quench radiative recombination (Figure S6d). It is particularly worth mentioning that the BET surface area of DACN-500 (113.5 m2 g−1) is larger than that of GCN (39.7 m2 g−1), but DACN-10 is 34.6 m2 g−1 (Figure S9). Thus, the outperformed photocatalytic activity of DACN-10 relative to GCN can be owed to the N vacancies and amorphous structure. Considering alternative explanations (i.e., impurity or effect of organic solvent) for the dramatically enhanced photocatalytic efficiency, various urea crystals with different morphologies, and particle sizes obtained from different solvents (ethanol, water, methanol and acetonitrile) were designated as U-ethanol-derived DACN, U-water-derived DACN, U-methanol-1-derived DACN, Umethanol-4-derived DACN, U-methanol-24-derived DACN, and U-acetonitrile-derived DACN, respectively (detailed discussion in Figure S10). As shown in Figure 5b, the photocatalytic hydrogen evolution activities increase with the decreasing of the urea crystals size obtained from different solvents in the following order: U-ethanol-derived DACN (3920 μmol h−1 g−1) < GCN (4365 μmol h−1 g−1) < U-waterderived DACN (6407 μmol h−1 g−1) < U-methanol-4-derived DACN (16,071 μmol h−1 g−1) < U-acetonitrile-derived DACN (22,577 μmol h−1 g−1). This effect could also be found for the precursors in the same solvent, the order of HER is Umethanol-1-derived DACN (1864 μmol h−1 g−1) < Umethanol-4-derived DACN (16,071 μmol h−1 g−1) < U-
methanol-24-derived DACN (29,790 μmol h−1 g−1). It is well to be reminded that although the grain size of SRU-100 is larger than recrystallized urea obtained from acetonitrile (designed as U-acetonitrile), the HER of DACN-100 (37,480 μmol h−1 g−1) is better than that of U-acetonitrile-derived DACN (22,577 μmol h−1 g−1), further demonstrating the precursor size plays a key role in the catalytic properties of the final DACN products (other comparison results in Figure S10). From the results above, it can be concluded that other factors for the enhanced photocatalytic activities are ruled out, and the precursor size is the real reason for the significantly improved catalytic properties. The special 100 μm-size urea derived optimal DACN-100 displays sustained H2 production without noticeable deactivation over 24 h, suggesting it has good catalysis stability (Figure 5c). After photocatalytic reaction, the co-catalyst Pt particles with a uniform particle size of
