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Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0·5Zn0·5S quantum dots for efficient photocatalytic hydrogen evolution
international journal of hydrogen energy xxx (xxxx) xxx
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Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0·5Zn0·5S quantum dots for efficient photocatalytic hydrogen evolution Qian Liang a,*, Chengjia Zhang a, Song Xu a, Man Zhou a, Zhongyu Li a,b,** a
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China b School of Environmental & Safety Engineering, Changzhou University, Changzhou 213164, PR China
highlights Synthesis of novel Cd0$5Zn0$5S@honeycomb-like g-C3N4 heterojunction was present. Cd0$5Zn0$5S/HeC3N4 showed the excellent photocatalytic H2 production and stability. The n-n type heterojunction improved the charge separation efficiency. The photocatalytic mechanism of Cd0$5Zn0$5S/HeC3N4 nanocomposites was proposed.
article info
abstract
Article history:
Honeycomb-like graphitic carbon nitride (HeC3N4) with unique morphology has been
Received 10 August 2019
studied as a promising polymer photocatalyst. Herein, a novel binary metal sulfide con-
Received in revised form
structed with HeC3N4 (Cd0$5Zn0$5S/HeC3N4) was prepared though the facile in situ precip-
14 September 2019
itation method. The characterization data suggest that Cd0$5Zn0$5S quantum dots (QDs) are
Accepted 24 September 2019
well dispersed on the macroporous structure of HeC3N4 (156 m2 g1), which can provide
Available online xxx
higher surface area, more catalytic active sites and larger interface contact area with accelerating the migration and separation of charge carriers. By taking advantage of 0D/3D
Keywords:
heterojunction structure, the Cd0$5Zn0$5S/HeC3N4 dramatically boosts the photocatalytic
Honeycomb-like graphitic carbon
H2 evolution rate with the visible-light illumination. The Cd0$5Zn0$5S/HeC3N4-3 yields the
nitride
highest photocatalytic activity of 5145 mmol h1 g1, which is 4.3 times as high as that of
Cd0$5Zn0$5S
pure Cd0$5Zn0.5S. Furthermore, Cd0$5Zn0$5S/HeC3N4 composite presents high stability after
Heterojunction
four recycles. The enhanced visible-light-driven photocatalytic H2 production is attributed
Photocatalytic hydrogen generation
to the construction of n-n type heterojunction as well as the large surface area, which can inhibit the agglomeration of Cd0$5Zn0$5S nanoparticles, and efficiently transfer the photoexcited electron-hole pairs in Cd0$5Zn0.5S. Therefore, this work provides a potential way for designing advanced 0D/3D heterojunction. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering,Changzhou University, Changzhou 213164, PR China. E-mail addresses: [email protected] (Q. Liang), [email protected] (Z. Li). https://doi.org/10.1016/j.ijhydene.2019.09.180 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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Introduction Photocatalytic hydrogen generation from water over an efficient photocatalyst is one of the most promising approaches to resolve the environmental problem and meet increasing energy demand [1e3]. To date, numerous semiconductors consisted of (oxy) nitrides, sulfides and oxides have been prepared and reported [4e6]. Among them, a Cd1xZnxS ternary chalcogenide solid solution has attracted plenty of research interest due to its tunable band gap through adjusting Cd/Zn ratio [7,8]. Yet the photocatalytic activity of pure Cd1-xZnxS is still very low due to its severe photocorrosion and particle aggregation. To address these bottleneck issues, many researchers have turned their attention to the assembly Cd1-xZnxS with other porous semiconductor [9,10]. For example, Qiu et al. prepared a Cd0$5Zn0$5S@ZIF-8 nanocomposite with high surface area to enhance photocatalytic performance for the reduction of Cr(VI) [11]. Therefore, it would be significant to develop the porous material with a large surface area and controllable morphology. Graphitic carbon nitride (g-C3N4), which possesses excellent chemical and thermal stability, strong visible-light absorption and low cost, has been widely studied during the past decade [12e15]. The appropriate band gap of 2.7 eV makes g-C3N4 become a promising metal-free semiconductor candidate applied in different areas such as H2 production, CO2 reduction and organocatalysis [16e18]. In spite of its suitable band gap as well as visible-light-driven photocatalytic performance, bulk g-C3N4 obtained via direct polycondensation of precursors suffers from low photocatalytic conversion efficiency owing to the small surface area and fast recombination of photoinduced charge carriers [19,20]. One strategy for improving the surface area is to form porous nanostructure. Yang et al. reported that a honeycomb-like g-C3N4 with high surface area had superior photocatalytic H2 evolution rate (459 mmol h1 g1) [21]. Other strategy for enhancing sluggish transfer of photoexcited charge carriers is to construct heterojunction [22e24]. Therefore, it would be significant to combine porous carbon nitride with Cd1-xZnxS to form heterojunction, which is helpful to improve surface areas and inhibit the electronhole recombination, thus enhancing the photocatalytic performance. However, less attention has been paid to the photocatalytic H2 evolution system of honeycomb-like gC3N4/Cd1-xZnxS.
In this study, the well-dispersed Cd0$5Zn0$5S quantum dots (QDs) are grown in situ on the surface of honeycomb-like graphitic carbon nitride (HeC3N4) to form a 0D/3D heterojunction structure though a facile precipitation method as shown in Scheme 1. The photocatalytic H2 production properties are systematically evaluated under visible-light irradiation in Na2S/Na2SO3 aqueous solution. The Cd0$5Zn0$5S/ HeC3N4 heterojunction indicates significantly enhanced H2 evolution performance compared with pristine Cd0$5Zn0$5S and HeC3N4, and meanwhile the content of Cd0$5Zn0$5S in the optimized Cd0$5Zn0$5S/HeC3N4 heterojunction is investigated. The loose and porous structure, rapid photoinduced charge separation and broadened light absorption can facilitate the photocatalytic H2 production activity and inhibit the photocorrosion of Cd0$5Zn0.5S. Besides, the photocatalytic mechanism of dramatically enhanced visible-light H2 generation activity is investigated based on electrochemical measurements including transient photocurrent, electrochemical impedance spectroscopy (EIS) as well as Mott-Schottky (M-S) curve. It is expected that the development of g-C3N4 based semiconductor may contribute to the sustainable energy applications.
Experimental section Synthesis of the honeycomb g-C3N4 2 g dicyandiamide was dispersed in 200 mL absolute ethanol. Then a saturated aqueous NaCl solution (NaCl:dicyandiamide ¼ 5:1 M ratio) was added dropwise and kept stirring for 2 h. After removing the ethanol and water, the resultant mixture was transferred to 50 mL alumina crucibles and calcined under 550 C for 2 h with a heating rate of 5 C/ min. The yellow powder was added into distilled water and kept stirring for 2 h. After that, the dispersion was gathered by centrifugation with deionized water and absolute alcohol for several times to remove impurities. Then, the yellow product was dried at 60 C and denoted as HeC3N4.
Synthesis of the Cd0·5Zn0·5S Typically, Cd(OAc)2$2H2O (0.1 mmol) and Zn(OAc)2$2H2O were added to 50 mL deionized water with stirring for 2 h, and 0.1 M Na2S (0.2 mmol, 40 mL) aqueous solution was added into the above mixture with vigorous stirring for 12 h. The orange
Scheme 1 e Schematic illustration for the formation of CZS/HeC3N4. Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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product was centrifuged, washed with deionized water several times, and dried at 60 C overnight.
Results and discussion
Synthesis of the Cd0·5Zn0·5S/HeC3N4
Characterization of morphology and structure
Cd0$5Zn0$5S/HeC3N4 heterojunction was synthesized through in-situ precipitation method. A certain amount of HeC3N4 was added to 50 mL deionized water and kept stirring for 2 h. After that, Cd(OAc)2$2H2O (0.1 mmol) and Zn(OAc)2$2H2O (0.1 mmol) were dispersed in 40 mL deionized water under stirring for 1 h to form a clear solution. The above aqueous solution was added into HeC3N4 suspension drop by drop under stirring over 30 min. A Na2S aqueous solution (0.2 mmol, 40 mL) was added to the above mixture dropwise, and kept stirring at room temperature for other 24 h. The final yellow product was centrifuged with deionized water and ethanol (6000 rpm, 10 min), and dried at 60 C for 12 h. The weight ratios of HeC3N4 to Cd0$5Zn0$5S were varied (1, 3 and 5) though adjusting the HeC3N4 amount, which were named as CZS/ HeC3N4-1, CZS/HeC3N4-3 and CZS/HeC3N4-5, respectively.
The XRD patterns of Cd0$5Zn0$5S, HeC3N4 and CZS/HeC3N4 are displayed in Fig. 1a. Three characteristic peaks at 26.6 , 45.1 , 53.2 can be indexed to the (111), (220) and (311) crystal planes of hexagonal Cd0$5Zn0$5S [11]. The main diffraction peak at 27.5 of HeC3N4 agrees with the (002) crystal plane of g-C3N4, but the diffraction peak of HeC3N4 becomes broad compared with g-C3N4, showing that the reduced crystallinity could result from the incorporation of Naþ during the preparation process [21]. The XRD patterns of CZS/HeC3N4 are similar to that of pristine HeC3N4 and the diffraction peak at 27.6 are attributed to the (002) lattice plane for graphitic materials [25]. With the increase of Cd0$5Zn0$5S loading, the diffraction peak at 45.1 ascribed to Cd0$5Zn0$5S emerges, indicating that Cd0$5Zn0$5S and HeC3N4 are well maintained during the in-situ synthetic processes. The surface functional groups of asprepared samples can be confirmed by FT-IR analysis
Characterization The crystal phase of CZS/HeC3N4 samples were identified by X-ray diffraction (XRD; Shimadzu 6000) with Cu Ka (l ¼ 1.54 A) radiation. The morphology of CZS/HeC3N4 was investigated by a Quant 250FEG scanning electron microscopy and a JEOL 2100 transmission electron microscope. The nitrogen adsorption-desorption was measured on a Micromeritics ASAP 2000 analyzer. The UVeVis diffused reflectance spectroscopy experiments were analyzed by a Cary 5000 UVeVis spectrometer. Photoluminescence (PL) measurements were surveyed by a Cary Eclipse fluorescence spectrophotometer. X-ray photoelectron spectra (XPS) were conducted with a Thermo ESCALAB 250 spectrometer using Al Ka X-ray source. Fourier transform infrared spectra (FT-IR) were recorded on a Thermo Fisher FTIR6700 instrument.
Photocatalytic H2 evolution activity The photocatalytic H2 production experiments were performed in Pyrex glass cell. 10 mg of photocatalyst was dispersed in 100 mL deionized water that contained Na2S (0.25 M) and Na2SO3 (0.35 M) as the sacrificial agent. Prior to the photocatalytic reaction, the solution was degassed to remove O2 dissolved in water and irradiated under 300 W Xe-lamp with a 420 nm cutoff filter. The H2 production was determined using gas chromatograph (GC 7900) equipped with a thermal conductivity detector (TCD).
Photoelectrochemical measurements The photoelectrochemcial (PEC) performances of CZS/HeC3N4 were carried out with a three-electrode system using a Pt wire as counter electrode, Ag/AgCl as reference electrode, and FTO conductor glass as working electrode. 10 mg of as-prepared sample was added into 10 mL of DMF via sonication for 60 min to obtain the slurry. The slurry was slowly dropped onto the FTO glass, and dried naturally. Na2SO4 (0.2 M) aqueous solution was used as electrolyte.
Fig. 1 e XRD patterns of the pristine HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites (a), FTIR spectra of pristine HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites (b).
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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(Fig. 1b). For the pristine HeC3N4, the vibration band centered at ~813 cm1 is ascribed to aromatic CeN stretching, and the vibration bands in the range of 1200e1600 cm1 are assigned to typical stretching mode of CeN heterocycles. It can be seen that the broad vibration band near ~3400 cm1 is corresponding to NeH breathing modes, which suggests the presence of uncondensed amine group. The FT-IR spectra of CZS/ HeC3N4 is similar to that of HeC3N4, which is caused by the low loading of Cd0$5Zn0$5S in CZS/HeC3N4 composites. The morphology and microstructure of the HeC3N4 and CZS/HeC3N4 were observed by SEM and TEM images. As shown in Fig. 2a, the bare HeC3N4 presents the honeycomblike and porous structure after removal of NaCl templates, while the pure Cd0$5Zn0$5S is composed of aggregated spheroidal particles (Fig. S1b). Fig. 2c shows that the Cd0$5Zn0$5S
QDs (3.8 nm) are uniformly covered with porous HeC3N4 and the aggregation effect of Cd0$5Zn0$5S is not observed. Besides, the HRTEM image (inset Fig. 2f) indicates that the Cd0$5Zn0$5S QDs presents a lattice spacing of 0.32 nm, assigned to the (111) crystal plane of Cd0$5Zn0.5S. Since the excess Cd0$5Zn0$5S blocks the porous structure of HeC3N4 and the aggregation of Cd0$5Zn0$5S becomes obvious (Fig. S1d), it is noticeable that the excess loading of Cd0$5Zn0$5S leads to the decreased photocatalytic performance of CZS/HeC3N4 [26]. Besides, the chemical composition of CZS/HeC3N4 is further measured by EDS mapping. As shown in Fig. 2g, Cd, Zn and S elements are homogenously dispersed on the HeC3N4 substrates, corresponding to the results of above SEM and TEM analysis. N2-adsorption/desorption isotherms of HeC3N4, Cd0$5Zn0$5S and CZS/HeC3N4 were carried out to analyze the
Fig. 2 e SEM images of HeC3N4 (a, b) and CZS/HeC3N4-3 (c), TEM images of HeC3N4 (d, e) and CZS/HeC3N4-3 (f), elemental mapping images of CZS/HeC3N4-3 (g). Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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texture parameters. As shown in Fig. 3, the pristine Cd0$5Zn0$5S presents a low BET surface area of 20 m2 g1 as well as small pore volume of 0.06 cm3 g1, while the bare HeC3N4 presents the BET surface area of 156 m2 g1, which indicates type IV with a notable H3 hysteresis loop, representing the existence of macropores in CZS/HeC3N4 [27e29]. It can be observed that the BET surface area and pore volume of CZS/ HeC3N4 composites are both reduced in comparison with bare HeC3N4, indicating that Cd0$5Zn0$5S QDs occupy and block the macropore of HeC3N4, but CZS/HeC3N4 shows a much larger specific surface area than pure Cd0$5Zn0$5S and some other metal sulfide. The CZS/HeC3N4-5 shows an enhanced surface area of 81 m2 g1, which is approximately 4 times than that of Cd0$5Zn0.5S. It should be noted that the loose and porous structure of CZS/HeC3N4 benefits the exposure of active sites and enhancement of photocatalytic activity. XPS analysis is used to investigate the surface chemical composition and elemental valence state of HeC3N4 and CZS/HeC3N4-3. In Fig. 4a, the XPS survey spectrum of HeC3N4 is mainly composed of C and N elements, and the CZS/HeC3N4 exhibits the expected C, N, Cd, Zn and S elements. The C 1s spectrum of bare HeC3N4 is deconvoluted into three peaks located at 288.3, 286.4 and 284.9 eV (Fig. 4b). Two main peaks at 288.3 and 284.4 eV can be ascribed to the sp2-bonded C atom (NeC]N) and graphitic carbon impurities, respectively [30]. It should be noted that the peak at 286.4 eV in HeC3N4 is intensified in comparison with bulk C3N4, corresponding to the CeN bonds in the aromatic CN heterocycles [21]. The binding energies of C 1s for CZS/ HeC3N4 are shifted to the higher position at 288.6 and 286.6 eV compared to that of pristine HeC3N4, and meanwhile CZS/HeC3N4 possesses a large full width at half maximum (FWHM) because the formation of abundant nanoparticles can result in the enhanced disorder as well as chemical inhomogeneity of C atoms in HeC3N4, exhibiting that the strong interactions between honeycomb HeC3N4 and Cd0$5Zn0$5S could stabilize the Cd0$5Zn0$5S nanoparticles and inhibit aggregation [31e33]. In the N 1s spectrum as
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shown in Fig. 4c, two signals at 400.5 and 398.7 eV in pure HeC3N4 are identified as the N atom of the surface amino group and sp2-hybridized N atom, respectively [34]. For CZS/ HeC3N4, the secondary peak of N 1s spectrum at 401 eV is shift to higher binding energy compared to HeC3N4, which is similar to the shift of C 1s peaks. As displayed in Fig. 4d, the binding energies of Cd 3d for CZS/HeC3N4 at 411.9 and 405.2 eV are consistent with the Cd 3d3/2 and Cd 3d5/2, respectively, confirming the Cd2þ ions in Cd0$5Zn0$5S [35]. As reflected by the high-resolution Zn 2p spectrum of CZS/ HeC3N4 (Fig. 4e), two peaks at 1045.4 and 1022.3 eV can correspond well with Zn 2p1/2 and Zn 2p3/2, respectively [36,37]. As illustrated in Fig. 4f, the S 2p spectrum of CZS/ HeC3N4 can be fitted into three peaks. The two strong peaks at 162.6 and 161.5 eV agree with the S 2p1/2 and S 2p3/2, respectively, which can be accurately attributable to S2 existed on the composite. The S 2p peak located at 169.2 eV 2 agrees well with SO2 3 or SO4 [38,39]. Therefore, the above results again confirm the successful fabrication of impurityfree Cd0$5Zn0$5S/HeC3N4 heterojunction. The optical properties of HeC3N4, Cd0$5Zn0$5S and CZS/ HeC3N4 were studied by UVevis DRS spectra (Fig. 5a). The bare HeC3N4 presents the enhanced light absorption edge at around 450 nm compared with bulk g-C3N4 (420 nm), and the band gap of HeC3N4 is determined to be 2.54 eV according to the Tauc plot of (ahl)1/2 versus photo energy (Fig. S2), which is narrower than that of bulk g-C3N4 (2.7 eV) [40]. The pristine Cd0$5Zn0$5S has strong visible light absorption (520 nm) and the band gap is estimated at 2.33 eV. After modifying Cd0$5Zn0$5S with HeC3N4, CZS/HeC3N4 composites indicate stronger absorption edge at the visible region compared to HeC3N4, demonstrating that interfacial interaction between HeC3N4 and Cd0$5Zn0$5S can promote the light harvesting property. Photoluminescence (PL) experiments were measured to investigate the transfer behavior of photoexcited electron-hole pairs [41e44]. In Fig. 5b, the PL spectra of HeC3N4, Cd0$5Zn0$5S and CZS/ HeC3N4 were investigated at 400 nm excitation. The HeC3N4 has a strong luminescence emission at around 460 nm, which is consistent with the previous results achieved by UVevis measurements. Due to the emission of stacking defects, the pristine Cd0$5Zn0$5S exhibits a broad emission at about 500 nm [10,45]. Although the photoexcited emission of CZS/HeC3N4 is similar to that of HeC3N4, the emission peak of the CZS/HeC3N4 is efficiently inhibited because of the formation of Cd0$5Zn0$5S QDs/HeC3N4 heterojunctions. It is noted that the lower PL intensity of CZS/HeC3N4-3 suggests a lower charge recombination probability, resulting in the improved photocatalytic activity.
Photocatalytic H2 evolution
Fig. 3 e N2 absorption-desorption isotherms of HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites.
Photocatalytic H2 generation activities of HeC3N4, Cd0$5Zn0$5S and CZS/HeC3N4 were evaluated under visible-light illumination, and Na2S and Na2SO3 acted as the sacrificial agent. As shown in Fig. 6a, pristine HeC3N4 indicates negligible H2 evolution rate, which is due to the easy recombination of photogenerated charge carriers. Meanwhile, pure Cd0$5Zn0$5S also presents relatively low H2 production rate of 1206 mmol h1 g1 because of its aggregation and the low
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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Fig. 4 e XPS survey spectrum (a), high resolution XPS spectra of C 1s (b), N 1s (c), Cd 3d (d), Zn 2p (e) and S 2p (f).
charge separation efficiency. The photocatalytic H2 production rate is remarkedly enhanced after loading Cd0$5Zn0$5S QDs onto the surface of macroporous HeC3N4, demonstrating that the fabrication of heterostructured Cd0$5Zn0$5S/HeC3N4 can facilitate the proton reduction reaction. It is noted that, by
adjusting the content of Cd0$5Zn0$5S, the photocatalytic performance of CZS/HeC3N4 can be effectively tuned. The H2 evolution activity of CZS/HeC3N4 increases with a higher Cd0$5Zn0$5S amount, and the highest H2 evolution rate (5145 mmol h1 g1) is achieved over CZS/HeC3N4-3, which is
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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4.3 times as high as that of pristine Cd0$5Zn0.5S. However, when the Cd0$5Zn0$5S amount is further increased, the H2 production rate of CZS/HeC3N4 is significantly decreased because the excess Cd0$5Zn0$5S amount can affect the dispersion of Cd0$5Zn0$5S nanoparticle and speed up its aggregation (Fig. S1). Therefore, an appropriate amount of Cd0$5Zn0$5S is essential to the improvement of photocatalytic performance. As shown in Table 1, the photocatalytic activity of CZS/HeC3N4-3 is higher than many other sulfide photocatalysts, indicating that CZS/HeC3N4 is a remarkable catalyst for photocatalytic H2 production. For instance, we reported a ternary heterojunction Cd0$5Zn0$5S@UIO-66@g-C3N4, and the maximum H2 production rate can reach 1281.1 mmol h1 g1 [46]. Chu et al. presented a series of ternary CdS/Cu7S4/g-C3N4 composites, showing a high H2 production rate of 3570 mmol h1 g1 [34]. Furthermore, the stability of H2 production over CZS/HeC3N4-3 was evaluated by operating the reaction for four cycles. As shown in Fig. 6b, there is no noticeable decrease of H2 evolution rate under a prolonged visible-light illumination of 12 h, revealing that CZS/HeC3N4 possesses not only highly H2 production but also highly stability.
Fig. 5 e UVevis DRS of pristine HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites (a), PL spectra of pristine HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites (b).
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Photocatalytic mechanism Photoelectrochemical experiments including the transient photocurrent and electrochemical impedance spectroscopy (EIS) are used to elucidate the separation efficiency of photoinduced charge carriers over HeC3N4, Cd0$5Zn0$5S and CZS/ HeC3N4 composites. It can be seen that the fast and uniform photocurrent responses of studied samples at light-on and light-off can be clearly observed in Fig. 7a. The CZS/HeC3N4-3 possesses the highest photocurrent density of 7.4 mA cm2, which is approximately 12.3 and 3.1 times higher than those of bare Cd0$5Zn0$5S (0.6 mA cm2) and HeC3N4 (2.4 mA cm2), respectively, indicating improved charge transfer efficiency between HeC3N4 and Cd0$5Zn0$5S [51]. Besides, the average photocurrent intensity of as-prepared samples is in the order of CZS/HeC3N4-3 > CZS/HeC3N4-1 > CZS/HeC3N4-5 > Cd0$5Zn0$5S > HeC3N4, which is consistent with the results of H2 evolution rate. Moreover, the charge mobility of HeC3N4, Cd0$5Zn0$5S and CZS/HeC3N4 was evaluated by EIS under visible light [52e54]. As displayed in Fig. 7b, compared with bare HeC3N4 and Cd0$5Zn0$5S, the CZS/HeC3N4-x samples have smaller semicircle, particularly CZS/HeC3N4-3,
Fig. 6 e H2 evolution rate of HeC3N4, Cd0·5Zn0·5S and CZS/ HeC3N4 composites under visible light irradiation (a); recycling test of photocatalytic H2 evolution over the optimized CZS/HeC3N4-3 (b).
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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Table 1 e Comparison of the H2 evolution rates between the work reported and our work. Photocatalyst Cd0$5Zn0$5S-geC3N4 eMoS2 Cd0$5Zn0$5S/BiVO4 Cd0$5Zn0$5S/OLC CdS/ZnS TiO2-x/g-C3N4/CdS Cd0$5Zn0$5S/HeC3N4
H2 production (mmol h1 g1)
Ref.
4.57
[10]
2.35 2.02 0.11 2.55 5.14
[47] [48] [49] [50] This work
suggesting that CZS/HeC3N4-3 has faster interfacial electron transfer [55]. The above results suggest that CZS/HeC3N4 composite can promote the interface charge transfer, further improving the photocatalytic activity. Mott-Schottky (M-S) plots of HeC3N4 as well as Cd0$5Zn0$5S were employed to clarify the band structures as shown in Fig. 7c. Due to the positive slopes of the M-S curves, HeC3N4 and Cd0$5Zn0$5S represent n-type semiconductors. Based on the intercept of a tangent to the curve, the EFB of HeC3N4 and Cd0$5Zn0$5S is 1.24 and 0.81 V versus the Ag/AgCl, respectively. The CB position (ECB) is approximately determined as the EFB. The ECB versus the Ag/AgCl is converted to the ENHE (normal hydrogen electrode) according to the following equation: ENHE ¼ EAg/ AgCl þ 0.197 V, and thus, the CB levels of HeC3N4 and Cd0$5Zn0$5S are reckoned up to be 1.04 and 0.61 V versus NHE, respectively. By combining the band gap energies from optical absorption spectrum and the ECB values calculated from M-S plots, the corresponding band structures can be determined [56,57]. As a result, the EVB values of HeC3N4 and Cd0$5Zn0$5S are 1.50 and 1.72 V, respectively. In order to confirm the injection direction of photogenerated electrons and holes, the transient open-circuit potential (OCP) were measured in Fig. S3. The OCP spectra of CZS/HeC3N4-3 shows that the electrons are generated from the n-type semiconductor and transferred to the cathode for reduction reaction. The photogenerated holes will take part in the oxidation reaction [58,59]. A schematic diagram of the proposed photocatalytic mechanism over CZS/HeC3N4 composite is displayed in Scheme 2. Owing to the intimate heterojunction structure between HeC3N4 and Cd0$5Zn0$5S, the photoinduced electron on the CB level of HeC3N4 are rapidly injected into the CB of Cd0$5Zn0$5S under visible light illumination, and meanwhile the holes on the VB of Cd0$5Zn0$5S are fast transferred to the CB of HeC3N4, effectively promoting the electron-hole pairs separation. The CB of Cd0$5Zn0$5S is more negative than the redox potential of Hþ/H2, and therefore the electron left on the CB of Cd0$5Zn0$5S can quickly participate in the reduction reaction to produce H2. Furthermore, the holes on the VB of HeC3N4 can suppress photocorrosion of Cd0$5Zn0.5S. Based on the above experimental results, the prominent photocatalytic H2 production activity and stability over formed CZS/HeC3N4 heterojunction can be ascribed to the following four excellent features. First of all, Cd0$5Zn0$5S nanoparticles are uniformly distributed on the honeycomb-like C3N4 to construct n-n heterojunction, which can promote carrier mobility and extend the light absorption range. Secondly, owing to the 3D structure of honeycomb-like C3N4, the large and regular shapes of the pores have a positive effect on activation and
Fig. 7 e Transient photocurrent responses (a) and EIS Nyquist plots (b) of HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites, Mott-Schottky plots (c) of HeC3N4 and Cd0·5Zn0·5S.
adsorption of H2O molecules. Thirdly, HeC3N4 with a loose and porous structure not only contributes to the increased active sites for reaction, but also provides an intimate contact, which can boost the charge separation ability and protect the Cd0$5Zn0$5S from photocorrosion [60]. Thus, the construction of the CZS/HeC3N4 heterojunction photocatalyst with a large surface area is of great importance for the improvement of photocatalytic H2-production performance.
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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Scheme 2 e Schematic diagram of photocatalytic H2 production over the CZS/HeC3N4 under visible light irradiation.
Conclusion In summary, NaCl as the template was introduced and removed by a facile calcination approach to fabricate the honeycomb-like structured graphitic carbon nitride (HeC3N4), and the novel Cd0$5Zn0$5S/HeC3N4 nanocomposites were rationally prepared using in situ precipitation method. Although the pristine HeC3N4 showed negligible H2 evolution rate, the modified Cd0$5Zn0$5S/HeC3N4-3 without the existence of noble metal had the highest photocatalytic H2 evolution rate of 5145 mmol h1 g1, which was 4.3 times as high as that of pure Cd0$5Zn0.5S. Besides, the photocatalytic performance of Cd0$5Zn0$5S/HeC3N4 composite was influenced by Cd0$5Zn0$5S content, and meanwhile the as-synthesized Cd0$5Zn0$5S/HeC3N4-3 exhibited the excellent stability by maintaining visible-light irradiation of 12 h. The enhanced photocatalytic H2 performance of 0D/3D Cd0$5Zn0$5S/HeC3N4 is assigned to the high surface areas, improved light harvesting, fabrication of n-n type heterojunction and enhanced spatial separation of photogenerated charges. Furthermore, the honeycomb-like structure in the composite can provide abundant interfacial catalytic active-site and boost charge transfer capacity. The present study may provide a new insight into the exploration and fabrication of 0D/3D heterojunction for energy and environmental applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21703019, 51702025).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.180.
references
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Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
Available online at www.sciencedirect.com
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Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0·5Zn0·5S quantum dots for efficient photocatalytic hydrogen evolution Qian Liang a,*, Chengjia Zhang a, Song Xu a, Man Zhou a, Zhongyu Li a,b,** a
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China b School of Environmental & Safety Engineering, Changzhou University, Changzhou 213164, PR China
highlights Synthesis of novel Cd0$5Zn0$5S@honeycomb-like g-C3N4 heterojunction was present. Cd0$5Zn0$5S/HeC3N4 showed the excellent photocatalytic H2 production and stability. The n-n type heterojunction improved the charge separation efficiency. The photocatalytic mechanism of Cd0$5Zn0$5S/HeC3N4 nanocomposites was proposed.
article info
abstract
Article history:
Honeycomb-like graphitic carbon nitride (HeC3N4) with unique morphology has been
Received 10 August 2019
studied as a promising polymer photocatalyst. Herein, a novel binary metal sulfide con-
Received in revised form
structed with HeC3N4 (Cd0$5Zn0$5S/HeC3N4) was prepared though the facile in situ precip-
14 September 2019
itation method. The characterization data suggest that Cd0$5Zn0$5S quantum dots (QDs) are
Accepted 24 September 2019
well dispersed on the macroporous structure of HeC3N4 (156 m2 g1), which can provide
Available online xxx
higher surface area, more catalytic active sites and larger interface contact area with accelerating the migration and separation of charge carriers. By taking advantage of 0D/3D
Keywords:
heterojunction structure, the Cd0$5Zn0$5S/HeC3N4 dramatically boosts the photocatalytic
Honeycomb-like graphitic carbon
H2 evolution rate with the visible-light illumination. The Cd0$5Zn0$5S/HeC3N4-3 yields the
nitride
highest photocatalytic activity of 5145 mmol h1 g1, which is 4.3 times as high as that of
Cd0$5Zn0$5S
pure Cd0$5Zn0.5S. Furthermore, Cd0$5Zn0$5S/HeC3N4 composite presents high stability after
Heterojunction
four recycles. The enhanced visible-light-driven photocatalytic H2 production is attributed
Photocatalytic hydrogen generation
to the construction of n-n type heterojunction as well as the large surface area, which can inhibit the agglomeration of Cd0$5Zn0$5S nanoparticles, and efficiently transfer the photoexcited electron-hole pairs in Cd0$5Zn0.5S. Therefore, this work provides a potential way for designing advanced 0D/3D heterojunction. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering,Changzhou University, Changzhou 213164, PR China. E-mail addresses: [email protected] (Q. Liang), [email protected] (Z. Li). https://doi.org/10.1016/j.ijhydene.2019.09.180 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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Introduction Photocatalytic hydrogen generation from water over an efficient photocatalyst is one of the most promising approaches to resolve the environmental problem and meet increasing energy demand [1e3]. To date, numerous semiconductors consisted of (oxy) nitrides, sulfides and oxides have been prepared and reported [4e6]. Among them, a Cd1xZnxS ternary chalcogenide solid solution has attracted plenty of research interest due to its tunable band gap through adjusting Cd/Zn ratio [7,8]. Yet the photocatalytic activity of pure Cd1-xZnxS is still very low due to its severe photocorrosion and particle aggregation. To address these bottleneck issues, many researchers have turned their attention to the assembly Cd1-xZnxS with other porous semiconductor [9,10]. For example, Qiu et al. prepared a Cd0$5Zn0$5S@ZIF-8 nanocomposite with high surface area to enhance photocatalytic performance for the reduction of Cr(VI) [11]. Therefore, it would be significant to develop the porous material with a large surface area and controllable morphology. Graphitic carbon nitride (g-C3N4), which possesses excellent chemical and thermal stability, strong visible-light absorption and low cost, has been widely studied during the past decade [12e15]. The appropriate band gap of 2.7 eV makes g-C3N4 become a promising metal-free semiconductor candidate applied in different areas such as H2 production, CO2 reduction and organocatalysis [16e18]. In spite of its suitable band gap as well as visible-light-driven photocatalytic performance, bulk g-C3N4 obtained via direct polycondensation of precursors suffers from low photocatalytic conversion efficiency owing to the small surface area and fast recombination of photoinduced charge carriers [19,20]. One strategy for improving the surface area is to form porous nanostructure. Yang et al. reported that a honeycomb-like g-C3N4 with high surface area had superior photocatalytic H2 evolution rate (459 mmol h1 g1) [21]. Other strategy for enhancing sluggish transfer of photoexcited charge carriers is to construct heterojunction [22e24]. Therefore, it would be significant to combine porous carbon nitride with Cd1-xZnxS to form heterojunction, which is helpful to improve surface areas and inhibit the electronhole recombination, thus enhancing the photocatalytic performance. However, less attention has been paid to the photocatalytic H2 evolution system of honeycomb-like gC3N4/Cd1-xZnxS.
In this study, the well-dispersed Cd0$5Zn0$5S quantum dots (QDs) are grown in situ on the surface of honeycomb-like graphitic carbon nitride (HeC3N4) to form a 0D/3D heterojunction structure though a facile precipitation method as shown in Scheme 1. The photocatalytic H2 production properties are systematically evaluated under visible-light irradiation in Na2S/Na2SO3 aqueous solution. The Cd0$5Zn0$5S/ HeC3N4 heterojunction indicates significantly enhanced H2 evolution performance compared with pristine Cd0$5Zn0$5S and HeC3N4, and meanwhile the content of Cd0$5Zn0$5S in the optimized Cd0$5Zn0$5S/HeC3N4 heterojunction is investigated. The loose and porous structure, rapid photoinduced charge separation and broadened light absorption can facilitate the photocatalytic H2 production activity and inhibit the photocorrosion of Cd0$5Zn0.5S. Besides, the photocatalytic mechanism of dramatically enhanced visible-light H2 generation activity is investigated based on electrochemical measurements including transient photocurrent, electrochemical impedance spectroscopy (EIS) as well as Mott-Schottky (M-S) curve. It is expected that the development of g-C3N4 based semiconductor may contribute to the sustainable energy applications.
Experimental section Synthesis of the honeycomb g-C3N4 2 g dicyandiamide was dispersed in 200 mL absolute ethanol. Then a saturated aqueous NaCl solution (NaCl:dicyandiamide ¼ 5:1 M ratio) was added dropwise and kept stirring for 2 h. After removing the ethanol and water, the resultant mixture was transferred to 50 mL alumina crucibles and calcined under 550 C for 2 h with a heating rate of 5 C/ min. The yellow powder was added into distilled water and kept stirring for 2 h. After that, the dispersion was gathered by centrifugation with deionized water and absolute alcohol for several times to remove impurities. Then, the yellow product was dried at 60 C and denoted as HeC3N4.
Synthesis of the Cd0·5Zn0·5S Typically, Cd(OAc)2$2H2O (0.1 mmol) and Zn(OAc)2$2H2O were added to 50 mL deionized water with stirring for 2 h, and 0.1 M Na2S (0.2 mmol, 40 mL) aqueous solution was added into the above mixture with vigorous stirring for 12 h. The orange
Scheme 1 e Schematic illustration for the formation of CZS/HeC3N4. Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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product was centrifuged, washed with deionized water several times, and dried at 60 C overnight.
Results and discussion
Synthesis of the Cd0·5Zn0·5S/HeC3N4
Characterization of morphology and structure
Cd0$5Zn0$5S/HeC3N4 heterojunction was synthesized through in-situ precipitation method. A certain amount of HeC3N4 was added to 50 mL deionized water and kept stirring for 2 h. After that, Cd(OAc)2$2H2O (0.1 mmol) and Zn(OAc)2$2H2O (0.1 mmol) were dispersed in 40 mL deionized water under stirring for 1 h to form a clear solution. The above aqueous solution was added into HeC3N4 suspension drop by drop under stirring over 30 min. A Na2S aqueous solution (0.2 mmol, 40 mL) was added to the above mixture dropwise, and kept stirring at room temperature for other 24 h. The final yellow product was centrifuged with deionized water and ethanol (6000 rpm, 10 min), and dried at 60 C for 12 h. The weight ratios of HeC3N4 to Cd0$5Zn0$5S were varied (1, 3 and 5) though adjusting the HeC3N4 amount, which were named as CZS/ HeC3N4-1, CZS/HeC3N4-3 and CZS/HeC3N4-5, respectively.
The XRD patterns of Cd0$5Zn0$5S, HeC3N4 and CZS/HeC3N4 are displayed in Fig. 1a. Three characteristic peaks at 26.6 , 45.1 , 53.2 can be indexed to the (111), (220) and (311) crystal planes of hexagonal Cd0$5Zn0$5S [11]. The main diffraction peak at 27.5 of HeC3N4 agrees with the (002) crystal plane of g-C3N4, but the diffraction peak of HeC3N4 becomes broad compared with g-C3N4, showing that the reduced crystallinity could result from the incorporation of Naþ during the preparation process [21]. The XRD patterns of CZS/HeC3N4 are similar to that of pristine HeC3N4 and the diffraction peak at 27.6 are attributed to the (002) lattice plane for graphitic materials [25]. With the increase of Cd0$5Zn0$5S loading, the diffraction peak at 45.1 ascribed to Cd0$5Zn0$5S emerges, indicating that Cd0$5Zn0$5S and HeC3N4 are well maintained during the in-situ synthetic processes. The surface functional groups of asprepared samples can be confirmed by FT-IR analysis
Characterization The crystal phase of CZS/HeC3N4 samples were identified by X-ray diffraction (XRD; Shimadzu 6000) with Cu Ka (l ¼ 1.54 A) radiation. The morphology of CZS/HeC3N4 was investigated by a Quant 250FEG scanning electron microscopy and a JEOL 2100 transmission electron microscope. The nitrogen adsorption-desorption was measured on a Micromeritics ASAP 2000 analyzer. The UVeVis diffused reflectance spectroscopy experiments were analyzed by a Cary 5000 UVeVis spectrometer. Photoluminescence (PL) measurements were surveyed by a Cary Eclipse fluorescence spectrophotometer. X-ray photoelectron spectra (XPS) were conducted with a Thermo ESCALAB 250 spectrometer using Al Ka X-ray source. Fourier transform infrared spectra (FT-IR) were recorded on a Thermo Fisher FTIR6700 instrument.
Photocatalytic H2 evolution activity The photocatalytic H2 production experiments were performed in Pyrex glass cell. 10 mg of photocatalyst was dispersed in 100 mL deionized water that contained Na2S (0.25 M) and Na2SO3 (0.35 M) as the sacrificial agent. Prior to the photocatalytic reaction, the solution was degassed to remove O2 dissolved in water and irradiated under 300 W Xe-lamp with a 420 nm cutoff filter. The H2 production was determined using gas chromatograph (GC 7900) equipped with a thermal conductivity detector (TCD).
Photoelectrochemical measurements The photoelectrochemcial (PEC) performances of CZS/HeC3N4 were carried out with a three-electrode system using a Pt wire as counter electrode, Ag/AgCl as reference electrode, and FTO conductor glass as working electrode. 10 mg of as-prepared sample was added into 10 mL of DMF via sonication for 60 min to obtain the slurry. The slurry was slowly dropped onto the FTO glass, and dried naturally. Na2SO4 (0.2 M) aqueous solution was used as electrolyte.
Fig. 1 e XRD patterns of the pristine HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites (a), FTIR spectra of pristine HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites (b).
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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(Fig. 1b). For the pristine HeC3N4, the vibration band centered at ~813 cm1 is ascribed to aromatic CeN stretching, and the vibration bands in the range of 1200e1600 cm1 are assigned to typical stretching mode of CeN heterocycles. It can be seen that the broad vibration band near ~3400 cm1 is corresponding to NeH breathing modes, which suggests the presence of uncondensed amine group. The FT-IR spectra of CZS/ HeC3N4 is similar to that of HeC3N4, which is caused by the low loading of Cd0$5Zn0$5S in CZS/HeC3N4 composites. The morphology and microstructure of the HeC3N4 and CZS/HeC3N4 were observed by SEM and TEM images. As shown in Fig. 2a, the bare HeC3N4 presents the honeycomblike and porous structure after removal of NaCl templates, while the pure Cd0$5Zn0$5S is composed of aggregated spheroidal particles (Fig. S1b). Fig. 2c shows that the Cd0$5Zn0$5S
QDs (3.8 nm) are uniformly covered with porous HeC3N4 and the aggregation effect of Cd0$5Zn0$5S is not observed. Besides, the HRTEM image (inset Fig. 2f) indicates that the Cd0$5Zn0$5S QDs presents a lattice spacing of 0.32 nm, assigned to the (111) crystal plane of Cd0$5Zn0.5S. Since the excess Cd0$5Zn0$5S blocks the porous structure of HeC3N4 and the aggregation of Cd0$5Zn0$5S becomes obvious (Fig. S1d), it is noticeable that the excess loading of Cd0$5Zn0$5S leads to the decreased photocatalytic performance of CZS/HeC3N4 [26]. Besides, the chemical composition of CZS/HeC3N4 is further measured by EDS mapping. As shown in Fig. 2g, Cd, Zn and S elements are homogenously dispersed on the HeC3N4 substrates, corresponding to the results of above SEM and TEM analysis. N2-adsorption/desorption isotherms of HeC3N4, Cd0$5Zn0$5S and CZS/HeC3N4 were carried out to analyze the
Fig. 2 e SEM images of HeC3N4 (a, b) and CZS/HeC3N4-3 (c), TEM images of HeC3N4 (d, e) and CZS/HeC3N4-3 (f), elemental mapping images of CZS/HeC3N4-3 (g). Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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texture parameters. As shown in Fig. 3, the pristine Cd0$5Zn0$5S presents a low BET surface area of 20 m2 g1 as well as small pore volume of 0.06 cm3 g1, while the bare HeC3N4 presents the BET surface area of 156 m2 g1, which indicates type IV with a notable H3 hysteresis loop, representing the existence of macropores in CZS/HeC3N4 [27e29]. It can be observed that the BET surface area and pore volume of CZS/ HeC3N4 composites are both reduced in comparison with bare HeC3N4, indicating that Cd0$5Zn0$5S QDs occupy and block the macropore of HeC3N4, but CZS/HeC3N4 shows a much larger specific surface area than pure Cd0$5Zn0$5S and some other metal sulfide. The CZS/HeC3N4-5 shows an enhanced surface area of 81 m2 g1, which is approximately 4 times than that of Cd0$5Zn0.5S. It should be noted that the loose and porous structure of CZS/HeC3N4 benefits the exposure of active sites and enhancement of photocatalytic activity. XPS analysis is used to investigate the surface chemical composition and elemental valence state of HeC3N4 and CZS/HeC3N4-3. In Fig. 4a, the XPS survey spectrum of HeC3N4 is mainly composed of C and N elements, and the CZS/HeC3N4 exhibits the expected C, N, Cd, Zn and S elements. The C 1s spectrum of bare HeC3N4 is deconvoluted into three peaks located at 288.3, 286.4 and 284.9 eV (Fig. 4b). Two main peaks at 288.3 and 284.4 eV can be ascribed to the sp2-bonded C atom (NeC]N) and graphitic carbon impurities, respectively [30]. It should be noted that the peak at 286.4 eV in HeC3N4 is intensified in comparison with bulk C3N4, corresponding to the CeN bonds in the aromatic CN heterocycles [21]. The binding energies of C 1s for CZS/ HeC3N4 are shifted to the higher position at 288.6 and 286.6 eV compared to that of pristine HeC3N4, and meanwhile CZS/HeC3N4 possesses a large full width at half maximum (FWHM) because the formation of abundant nanoparticles can result in the enhanced disorder as well as chemical inhomogeneity of C atoms in HeC3N4, exhibiting that the strong interactions between honeycomb HeC3N4 and Cd0$5Zn0$5S could stabilize the Cd0$5Zn0$5S nanoparticles and inhibit aggregation [31e33]. In the N 1s spectrum as
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shown in Fig. 4c, two signals at 400.5 and 398.7 eV in pure HeC3N4 are identified as the N atom of the surface amino group and sp2-hybridized N atom, respectively [34]. For CZS/ HeC3N4, the secondary peak of N 1s spectrum at 401 eV is shift to higher binding energy compared to HeC3N4, which is similar to the shift of C 1s peaks. As displayed in Fig. 4d, the binding energies of Cd 3d for CZS/HeC3N4 at 411.9 and 405.2 eV are consistent with the Cd 3d3/2 and Cd 3d5/2, respectively, confirming the Cd2þ ions in Cd0$5Zn0$5S [35]. As reflected by the high-resolution Zn 2p spectrum of CZS/ HeC3N4 (Fig. 4e), two peaks at 1045.4 and 1022.3 eV can correspond well with Zn 2p1/2 and Zn 2p3/2, respectively [36,37]. As illustrated in Fig. 4f, the S 2p spectrum of CZS/ HeC3N4 can be fitted into three peaks. The two strong peaks at 162.6 and 161.5 eV agree with the S 2p1/2 and S 2p3/2, respectively, which can be accurately attributable to S2 existed on the composite. The S 2p peak located at 169.2 eV 2 agrees well with SO2 3 or SO4 [38,39]. Therefore, the above results again confirm the successful fabrication of impurityfree Cd0$5Zn0$5S/HeC3N4 heterojunction. The optical properties of HeC3N4, Cd0$5Zn0$5S and CZS/ HeC3N4 were studied by UVevis DRS spectra (Fig. 5a). The bare HeC3N4 presents the enhanced light absorption edge at around 450 nm compared with bulk g-C3N4 (420 nm), and the band gap of HeC3N4 is determined to be 2.54 eV according to the Tauc plot of (ahl)1/2 versus photo energy (Fig. S2), which is narrower than that of bulk g-C3N4 (2.7 eV) [40]. The pristine Cd0$5Zn0$5S has strong visible light absorption (520 nm) and the band gap is estimated at 2.33 eV. After modifying Cd0$5Zn0$5S with HeC3N4, CZS/HeC3N4 composites indicate stronger absorption edge at the visible region compared to HeC3N4, demonstrating that interfacial interaction between HeC3N4 and Cd0$5Zn0$5S can promote the light harvesting property. Photoluminescence (PL) experiments were measured to investigate the transfer behavior of photoexcited electron-hole pairs [41e44]. In Fig. 5b, the PL spectra of HeC3N4, Cd0$5Zn0$5S and CZS/ HeC3N4 were investigated at 400 nm excitation. The HeC3N4 has a strong luminescence emission at around 460 nm, which is consistent with the previous results achieved by UVevis measurements. Due to the emission of stacking defects, the pristine Cd0$5Zn0$5S exhibits a broad emission at about 500 nm [10,45]. Although the photoexcited emission of CZS/HeC3N4 is similar to that of HeC3N4, the emission peak of the CZS/HeC3N4 is efficiently inhibited because of the formation of Cd0$5Zn0$5S QDs/HeC3N4 heterojunctions. It is noted that the lower PL intensity of CZS/HeC3N4-3 suggests a lower charge recombination probability, resulting in the improved photocatalytic activity.
Photocatalytic H2 evolution
Fig. 3 e N2 absorption-desorption isotherms of HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites.
Photocatalytic H2 generation activities of HeC3N4, Cd0$5Zn0$5S and CZS/HeC3N4 were evaluated under visible-light illumination, and Na2S and Na2SO3 acted as the sacrificial agent. As shown in Fig. 6a, pristine HeC3N4 indicates negligible H2 evolution rate, which is due to the easy recombination of photogenerated charge carriers. Meanwhile, pure Cd0$5Zn0$5S also presents relatively low H2 production rate of 1206 mmol h1 g1 because of its aggregation and the low
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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Fig. 4 e XPS survey spectrum (a), high resolution XPS spectra of C 1s (b), N 1s (c), Cd 3d (d), Zn 2p (e) and S 2p (f).
charge separation efficiency. The photocatalytic H2 production rate is remarkedly enhanced after loading Cd0$5Zn0$5S QDs onto the surface of macroporous HeC3N4, demonstrating that the fabrication of heterostructured Cd0$5Zn0$5S/HeC3N4 can facilitate the proton reduction reaction. It is noted that, by
adjusting the content of Cd0$5Zn0$5S, the photocatalytic performance of CZS/HeC3N4 can be effectively tuned. The H2 evolution activity of CZS/HeC3N4 increases with a higher Cd0$5Zn0$5S amount, and the highest H2 evolution rate (5145 mmol h1 g1) is achieved over CZS/HeC3N4-3, which is
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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4.3 times as high as that of pristine Cd0$5Zn0.5S. However, when the Cd0$5Zn0$5S amount is further increased, the H2 production rate of CZS/HeC3N4 is significantly decreased because the excess Cd0$5Zn0$5S amount can affect the dispersion of Cd0$5Zn0$5S nanoparticle and speed up its aggregation (Fig. S1). Therefore, an appropriate amount of Cd0$5Zn0$5S is essential to the improvement of photocatalytic performance. As shown in Table 1, the photocatalytic activity of CZS/HeC3N4-3 is higher than many other sulfide photocatalysts, indicating that CZS/HeC3N4 is a remarkable catalyst for photocatalytic H2 production. For instance, we reported a ternary heterojunction Cd0$5Zn0$5S@UIO-66@g-C3N4, and the maximum H2 production rate can reach 1281.1 mmol h1 g1 [46]. Chu et al. presented a series of ternary CdS/Cu7S4/g-C3N4 composites, showing a high H2 production rate of 3570 mmol h1 g1 [34]. Furthermore, the stability of H2 production over CZS/HeC3N4-3 was evaluated by operating the reaction for four cycles. As shown in Fig. 6b, there is no noticeable decrease of H2 evolution rate under a prolonged visible-light illumination of 12 h, revealing that CZS/HeC3N4 possesses not only highly H2 production but also highly stability.
Fig. 5 e UVevis DRS of pristine HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites (a), PL spectra of pristine HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites (b).
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Photocatalytic mechanism Photoelectrochemical experiments including the transient photocurrent and electrochemical impedance spectroscopy (EIS) are used to elucidate the separation efficiency of photoinduced charge carriers over HeC3N4, Cd0$5Zn0$5S and CZS/ HeC3N4 composites. It can be seen that the fast and uniform photocurrent responses of studied samples at light-on and light-off can be clearly observed in Fig. 7a. The CZS/HeC3N4-3 possesses the highest photocurrent density of 7.4 mA cm2, which is approximately 12.3 and 3.1 times higher than those of bare Cd0$5Zn0$5S (0.6 mA cm2) and HeC3N4 (2.4 mA cm2), respectively, indicating improved charge transfer efficiency between HeC3N4 and Cd0$5Zn0$5S [51]. Besides, the average photocurrent intensity of as-prepared samples is in the order of CZS/HeC3N4-3 > CZS/HeC3N4-1 > CZS/HeC3N4-5 > Cd0$5Zn0$5S > HeC3N4, which is consistent with the results of H2 evolution rate. Moreover, the charge mobility of HeC3N4, Cd0$5Zn0$5S and CZS/HeC3N4 was evaluated by EIS under visible light [52e54]. As displayed in Fig. 7b, compared with bare HeC3N4 and Cd0$5Zn0$5S, the CZS/HeC3N4-x samples have smaller semicircle, particularly CZS/HeC3N4-3,
Fig. 6 e H2 evolution rate of HeC3N4, Cd0·5Zn0·5S and CZS/ HeC3N4 composites under visible light irradiation (a); recycling test of photocatalytic H2 evolution over the optimized CZS/HeC3N4-3 (b).
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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Table 1 e Comparison of the H2 evolution rates between the work reported and our work. Photocatalyst Cd0$5Zn0$5S-geC3N4 eMoS2 Cd0$5Zn0$5S/BiVO4 Cd0$5Zn0$5S/OLC CdS/ZnS TiO2-x/g-C3N4/CdS Cd0$5Zn0$5S/HeC3N4
H2 production (mmol h1 g1)
Ref.
4.57
[10]
2.35 2.02 0.11 2.55 5.14
[47] [48] [49] [50] This work
suggesting that CZS/HeC3N4-3 has faster interfacial electron transfer [55]. The above results suggest that CZS/HeC3N4 composite can promote the interface charge transfer, further improving the photocatalytic activity. Mott-Schottky (M-S) plots of HeC3N4 as well as Cd0$5Zn0$5S were employed to clarify the band structures as shown in Fig. 7c. Due to the positive slopes of the M-S curves, HeC3N4 and Cd0$5Zn0$5S represent n-type semiconductors. Based on the intercept of a tangent to the curve, the EFB of HeC3N4 and Cd0$5Zn0$5S is 1.24 and 0.81 V versus the Ag/AgCl, respectively. The CB position (ECB) is approximately determined as the EFB. The ECB versus the Ag/AgCl is converted to the ENHE (normal hydrogen electrode) according to the following equation: ENHE ¼ EAg/ AgCl þ 0.197 V, and thus, the CB levels of HeC3N4 and Cd0$5Zn0$5S are reckoned up to be 1.04 and 0.61 V versus NHE, respectively. By combining the band gap energies from optical absorption spectrum and the ECB values calculated from M-S plots, the corresponding band structures can be determined [56,57]. As a result, the EVB values of HeC3N4 and Cd0$5Zn0$5S are 1.50 and 1.72 V, respectively. In order to confirm the injection direction of photogenerated electrons and holes, the transient open-circuit potential (OCP) were measured in Fig. S3. The OCP spectra of CZS/HeC3N4-3 shows that the electrons are generated from the n-type semiconductor and transferred to the cathode for reduction reaction. The photogenerated holes will take part in the oxidation reaction [58,59]. A schematic diagram of the proposed photocatalytic mechanism over CZS/HeC3N4 composite is displayed in Scheme 2. Owing to the intimate heterojunction structure between HeC3N4 and Cd0$5Zn0$5S, the photoinduced electron on the CB level of HeC3N4 are rapidly injected into the CB of Cd0$5Zn0$5S under visible light illumination, and meanwhile the holes on the VB of Cd0$5Zn0$5S are fast transferred to the CB of HeC3N4, effectively promoting the electron-hole pairs separation. The CB of Cd0$5Zn0$5S is more negative than the redox potential of Hþ/H2, and therefore the electron left on the CB of Cd0$5Zn0$5S can quickly participate in the reduction reaction to produce H2. Furthermore, the holes on the VB of HeC3N4 can suppress photocorrosion of Cd0$5Zn0.5S. Based on the above experimental results, the prominent photocatalytic H2 production activity and stability over formed CZS/HeC3N4 heterojunction can be ascribed to the following four excellent features. First of all, Cd0$5Zn0$5S nanoparticles are uniformly distributed on the honeycomb-like C3N4 to construct n-n heterojunction, which can promote carrier mobility and extend the light absorption range. Secondly, owing to the 3D structure of honeycomb-like C3N4, the large and regular shapes of the pores have a positive effect on activation and
Fig. 7 e Transient photocurrent responses (a) and EIS Nyquist plots (b) of HeC3N4, Cd0·5Zn0·5S and CZS/HeC3N4 composites, Mott-Schottky plots (c) of HeC3N4 and Cd0·5Zn0·5S.
adsorption of H2O molecules. Thirdly, HeC3N4 with a loose and porous structure not only contributes to the increased active sites for reaction, but also provides an intimate contact, which can boost the charge separation ability and protect the Cd0$5Zn0$5S from photocorrosion [60]. Thus, the construction of the CZS/HeC3N4 heterojunction photocatalyst with a large surface area is of great importance for the improvement of photocatalytic H2-production performance.
Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180
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Scheme 2 e Schematic diagram of photocatalytic H2 production over the CZS/HeC3N4 under visible light irradiation.
Conclusion In summary, NaCl as the template was introduced and removed by a facile calcination approach to fabricate the honeycomb-like structured graphitic carbon nitride (HeC3N4), and the novel Cd0$5Zn0$5S/HeC3N4 nanocomposites were rationally prepared using in situ precipitation method. Although the pristine HeC3N4 showed negligible H2 evolution rate, the modified Cd0$5Zn0$5S/HeC3N4-3 without the existence of noble metal had the highest photocatalytic H2 evolution rate of 5145 mmol h1 g1, which was 4.3 times as high as that of pure Cd0$5Zn0.5S. Besides, the photocatalytic performance of Cd0$5Zn0$5S/HeC3N4 composite was influenced by Cd0$5Zn0$5S content, and meanwhile the as-synthesized Cd0$5Zn0$5S/HeC3N4-3 exhibited the excellent stability by maintaining visible-light irradiation of 12 h. The enhanced photocatalytic H2 performance of 0D/3D Cd0$5Zn0$5S/HeC3N4 is assigned to the high surface areas, improved light harvesting, fabrication of n-n type heterojunction and enhanced spatial separation of photogenerated charges. Furthermore, the honeycomb-like structure in the composite can provide abundant interfacial catalytic active-site and boost charge transfer capacity. The present study may provide a new insight into the exploration and fabrication of 0D/3D heterojunction for energy and environmental applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21703019, 51702025).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.180.
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Please cite this article as: Liang Q et al., Nanocomposites based on 3D honeycomb-like carbon nitride with Cd0$5Zn0$5S quantum dots for efficient photocatalytic hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.180