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CdS hetrostructures towards high efficient photocatalytic H2 generation
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Crystallinity and phase controlling of g-C3N4 /CdS hetrostructures towards high efficient photocatalytic H2 generation Yaya Wang a, Xiao Zhang b, Yingjie Liu a, Yubin Zhao a, Cong Xie a, Yuxiang Song a, Ping Yang a,* a
School of Material Science and Engineering, University of Jinan, Jinan 250022, China Fuels and Energy Technology Institute and Department of Chemical Engineering, Curtin University, Perth WA6845, Australia
b
highlights
graphical abstract
g-C3N4 with different degrees of crystallinity was used to form a heterojunction with CdS. The
phase
transition
of
CdS
significantly improved its photocatalytic activity. The heterojunction after the phase transition is greatly different due to difference in matrix.
article info
abstract
Article history:
The heterostructures of graphitic carbon nitride (g-C3N4) and CdS were synthesized by
Received 21 July 2019
controlling the crystalline degree of g-C3N4 and the phase composition of CdS. A thermal
Received in revised form
polycondensation process of N precursors was adjusted to get amorphous and crystalline
5 September 2019
g-C3N4. A multistep adsorption method was used to deposit CdS nanoparticles on g-C3N4.
Accepted 24 September 2019
An annealed process was used to adjust the phase composition of CdS from cubic to
Available online 21 October 2019
hexagonal. The morphology of CdS was changed to rod. Amorphous g-C3N4/CdS heterostructures revealed enhanced photocatalytic activity because the amorphous g-C3N4 has a
Keywords:
lower crystallinity, it is easier to form a heterojunction with the CdS. Further, an annealing
g-C3N4
process resulted in the phase transfer and morphology change of CdS. The high stability
CdS
and rod morphology of CdS make the heterostructures with high H2 generation rate to
Heterostructure
5440 mmol h-1 g-1 which is ~5 times high compared with g-C3N4.
Photocatalytic H2
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (P. Yang). https://doi.org/10.1016/j.ijhydene.2019.09.181 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Solar energy is an inexhaustible new energy material. Hydrogen energy is a highly efficient clean energy source. Therefore, using solar energy to produce hydrogen will be one of the most ideal ways to solve energy problems. Developing photocatalytic materials with high efficient performance has attracted widespread attention [1]. A series of semiconductor photocatalysts have been explored including metal oxides [2], nitride [3], sulphide [4], carbide [5e10]. In generally, an ideal photocatalyst has suitable band gap, good reducibility, and high light stability. However, most of photocatalysts with single composition have some disadvantages, such as the ability to absorb only limited UV light (e. g. TiO2), rapidly and easily recombination of photogenerated electrons and holes (e. g. graphitic carbon nitride (g-C3N4)), which limit the wide application. Therefore, it is imperative to develop efficient, clean, and sustainable photocatalysts. The construction of semiconductor heterostructures becomes a hot topic. As a typical organic semiconductor, g-C3N4 has been developed for the degradation of organic pollutant and H2 generation via water splitting because of a unique layered structure composed of heptazine ring with a suitable band gap (2.7 eV) in visible light region. Precursor materials and preparation processes affect the composition and layer structure of g-C3N4 which governs the photocatalysis performance [11]. Bulk g-C3N4 has low conductivity, small specific surface area, wide band gap, and rapid recombination of photo-generated electrons and holes that greatly limited its application. Various methods have been developed to improve the photocatalytic performance of g-C3N4. Namely, morphology controlling increases its specific surface area, such as ultra-thin nanosheet, hollow bubble nanostructures [12], tubular nanostructures [13]. The fabrication of heterojunctions with other semiconductors decreases the quick recombination of photogenerated carriers. The heterostructures of g-C3N4 with noble metals [14], metal sulphide [15], metal oxides [16], and metal [17], non-metal [18] have been reported. The formation of the heterojunction can effectively promote the separation and transfer of electrons and holes, as well as significantly increase the specific surface area, thereby improving the photocatalytic activity. CdS, as an II-IV semiconductor, has attracted much attention in photocatalytic water splitting due to its narrow band gap (2.42 eV). But its performance needs to be improved. For example, small CdS nanoparticles are easily agglomerated into large particles, resulting in the rapid recombination of photogenerated electrons and holes [19]. In addition, photocorrosion is more serious during the photocatalytic reaction. In order to solve these problems, efforts have been developed to control the morphology including synthetic nanospheres, nanowires, nanorods, and other structures [20e26]. Another efficient way is fabricating CdS nanocomposites, for example, metal doping, constructing heterostructures (TiO2 and WO3) [27e31]. CdS heterostructures with other semiconductor are considered to be one of the best approach to promote the separation of photo-generated electrons and holes. Layered g-C3N4 as an efficient supporting has been used to form heterojunction. Because the band gaps of g-C3N4 and CdS
matching, electrons and holes can move opposite directions, effectively suppressing photo-generated electrons and holes. Constructing a hetrojunction of g-C3N4/CdS is an effective method for improving the absorption in visible light and enhancing photocatalytic hydrogen production. The formation of heterojunction of g-C3N4 and CdS has been interesting as a hot topic currently. Here, three kinds of g-C3N4 nanosheets (amorphous (A-GCN), crystalline (C-GCN) and amorphous/crystalline homojunction (A-CGCN) were prepared using the procedure in our recently published paper [32]. CdS nanoparticles were deposited on g-C3N4 nanosheets to form gC3N4/CdS heterojunctions. A phase transfer process by secondary calcination made CdS nanorods with hexagonal structure. The crystalline degree of g-C3N4 affects the properties of the heterojunctions. The CdS nanorods revealed high stability and reduced photo-corrosion. Thus, g-C3N4/CdS heterojunctions revealed enhanced photocatalytic hydrogen generation. The innovation of this paper is to show different properties of g-C3N4 nanosheet composite CdS with different crystallinity. A-GCN shows better photocatalytic hydrogen production after composite CdS, after secondary calcination Phase transitions in CdS show higher hydrogen production performance. There are the current manuscript is differ from the published one and novelty of the present manuscript [33e35]. This study provides new insights to the property improvement of g-C3N4 materials and the construction of heterostructures.
Experimental Synthesis of g-C3N4 nanosheets A-GCN: Typically, 1 g of melamine was added into a ceramic boat, and then the boat was put into the tube furnace. The sample was heat-treated up to 650 C using a heating rate of 5 C/min, and kept at 650 C for 2 h. The flaky sample A-GCN was ground into a powder with an agate mortar. Table 1 illustrates the synthesis conditions and properties of samples. C-GCN: 1 g of melamine was placed in the ceramic boat, and the boat was placed in the quartz tube of the tube furnace. According to the heating rate of 2 C/min, the temperature was raise to 600 C and kept for 2 h. After the precursor was ground to a powder, add 1 g into a ceramic boat, heated to 750 C in a heating rate of 5 C/min, and kept for 2 h to get crystalline phase. A-CGCN: A certain amount of crystalline g-C3N4 and 1 g of melamine were mixed in an agate mortar and fully ground. The powder sample was added in a ceramic boat. The boat is placed in a quartz tube of a tube furnace at 5 C/min heating rate to 650 C and keep it for 2 h to get A-CGCN.
Multistep deposition synthesis of g-C3N4/CdS heterostructures 90 mg of A-GCN was dissolved in 20 mL of CdCl2 solution (0.1 M) with stirring for 2 h. The solution was then centrifuged. The wet sample was placed in a bottle with stirring for 30 min and then centrifuged. 1 mL of the thioacetamide solution (TAA，0.1 M) solution was added and then heated to 90 C
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Table 1 e Synthesis conditions and property of samples. Sample Cd2þ solution S2 solution (mL) (mL) A-GCN A-GCN 1 A-GCN 2 A-GCN 3 A-GCN 4 A-GCN 5 A-GCN 6 C-GCN C-GCN 1 C-GCN 2 C-GCN 3 C-GCN 4 C-GCN 5 A-CGCN A-CGCN 1 A-CGCN 2 A-CGCN 3 A-CGCN 4
H2 generation rate (mmol/gh)
N/A 20 20 20 20 20 20 N/A 20 20 20 20 20 N/A 20
N/A 1 2 3 4 5 6 N/A 1 2 3 4 5 N/A 1
764 2273 2145 2483 2053 3097 1713 1018 1101 1016 1242 1327 993 1438 1468
20
2
1618
20
3
1764
20
4
1347
with stirring for 2 h. The sample was centrifuged and washed using water and anhydrous ethanol, naming this sample as AGCN 1. For comparison, the amount of CdS was adjusted by changing the amount of TAA solution as 2, 3, 4, and 5 mL, naming as A-GCN 2, A-GCN 3, A-GCN 4 and A-GCN 5, respectively. For other heterostructures, the preparation procedure is same, but A-GCN was replaced using C-GCN, and A-CGCN.
Co-precipitation preparation of A-GCN/CdS For comparison, CdS nanoparticles were carried out using a co-precipitation synthesis. Typically, 90 mg of A-GCN, 20 mL of CdCl2 solution (0.1 M), 1 mL of TAA (0.1 M) were mixed and put into an electric hot cap and then heated to 90 C with stirring for 2 h and cooled to room temperature. The sample was washed with deionized water and anhydrous ethanol, naming as sample SA-GCN 1.
Phase transition of CdS Typically, 90 mg of A-GCN 5 was placed in a ceramic boat, and the boat was placed in a tube furnace at a heating rate of 5 C/ min to 650 C and kept the temperature for 30 min in H2 atmosphere, and then cooled to room temperature, naming as sample A-GCN 5C. The heat-treatment of other samples is similar with sample A-GCN 5.
Characterization The powder X-ray diffraction (XRD) patterns of samples was determined by an X-ray diffractionmeter (Bruker D8, Germany), Cu Ka was used as the radiation source to record 2q at 10e80 . The morphology of samples was observed by a scanning electron microscope (SEM, QUANTA 250 FEG, FEI America) and transmission electron microscopy (TEM) (FEIG2F20).
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The energy-dispersive X-ray (EDX) analysis was used to get the composition of samples on a SEM (QUANTA 250 FEG, FEI America0. Raman spectroscopy was recorded by Horiba Jobin Yvon Lab RAMHR Evolution with 785 nm excitation laser wave length. X-ray photoelectron spectroscopy (XPS) was used to measure the composition of samples using a Thermo ESCALAB250xi instrument. The UV/Vis diffuse reflectance spectra of samples were recorded using a conventional UV/Vis spectrometer (Hitachi U-4100). The photoluminescence (PL) spectra of samples were measured by using a spectrometer (Hitachi F-4600). The photocurrent of samples was obtained on a chi660e electrochemical analyzer (Chen Hua Instruments, Shanghai, China). The Fourier transform infrared (FT-IR) spectra of samples were obtained a Fourier transform infrared spectrometer (Nicolet 380, Thermo, America).
Photocatalytic H2 production activity The photocatalytic H2 evolution experiment was carried out at room temperature in a 300 mL three-necked quartz reactor using a PLS-SXE 300 UV xenon arc lamp with UV filter (400 nm) as the light source and a three-necked flask on both necks. The mouth is connected to a closed circulation system by means of vacuum glue. 10 mg of sample was dissolved in 100 mL of an aqueous solution with 20% triethanolamine. Pt (0.2 wt%) was added as a co-catalyst. The sample was tirred and irradiated at room temperature for 30 min (300 W Xe). The reaction vessel was vigorous stirred for 30 min prior to the photocatalytic experiment to remove dissolved oxygen and ensure anaerobic conditions. The reaction vessel was vigorous stirred for 30 min prior to the photocatalytic experiment to remove dissolved oxygen and ensure anaerobic conditions. The gas was sampled intermittently through a septum, while hydrogen was analysed by gas chromatography (Shimadzu GC-14C, nitrogen as a carrier gas) and equipped with five molecular sieve columns and a thermal conductivity detector.
Results and discussion The formation of g-C3N4/CdS heterostructures and phase transfer of CdS are illustrated in Scheme 1. During reaction, the nucleation of CdS occurred on g-C3N4 nanosheets. Further growth resulted in CdS nanoparticles formed. A phase transfer process resulted in zine blend CdS nanoparticles were transferred into hexagonal CdS nanorods because of temperature of 650 C. Fig. 1a shows the XRD patterns of samples. Two XRD peaks of g-C3N4 are observed for all samples. The corresponding (100) facet at 12.81 indicates the periodic arrangement in triazine of g-C3N4. The corresponding (002) crystal plane at 27.91 indicates the accumulation of the conjugated direction system of carbon nitride. The characteristic peak at 27.9 is significantly reduced after compounding CdS. According to the Scherrer formula [36,37]: D¼
Kl bcosq
(1)
The crystal grain diameter of the tetragonal phase CdS can be calculated to be about 40 nm, which is consistent with the
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Scheme 1 e Illustration of g-C3N4/CdS heterostructure formation.
Fig. 1 e (a) XRD patterns and (b) FT-IR spectra of samples.
transmission electron microscope results. Where D is the nanocrystalline grain size, K is the Scherrer constant, l is the wavelength of the x-ray, b is the full width at half maximum of the diffraction peak, and q is the Bragg diffraction angle. Fig. 1b shows the FT-IR spectra of samples A-GCN, A-GCN 5, A-GCN 5C, C-GCN and C-GCN 4. The characteristic stretching peaks were in three regions at 3296, 1200e1700 and 809 cm1. The peak at 3296 cm1 is the stretching vibration peak of eNH2 or NeH. The bands appeared at 1200-1700 cm1 corresponding to the stretching vibration peak of CeN, and the peak at 809 cm1 correspond to the characteristic vibration of the triazine. For CdS, the peak concentrated at 3460 and 1646 cm1 correspond to the surface-adsorbed water molecules, and the 1385 and 1128 cm1 are correspond to the stretching vibration peak of CdeS. The stretching vibration peaks at 2928 and 2856 cm1 are attributable to the bending vibration of eCH2 and eCH3 [38,39]. The stretching vibration peaks of g-C3N4 and CdS in the FT-IR spectra are observed. This confirms that the formation of g-C3N4/CdS heterostructures. Fig. 2 shows the SEM images and element analysis of samples. Fig. 2a shows that CdS nanoparticles are uniformly dispersed on A-GCN nanosheets, and Fig. 2b shows that CdS nanoparticles are dispersed on C-GCN nanosheets. Fig. 2c and d shows that after heat-treatment at 650 C in H2 atmosphere
phase transformation, the surface CdS of A-GCN and C-GCN have changed from particles to rods. For C-GCN, the deposition of CdS nanoparticles is different from A-GCN because of the difference of microstructure and surface state [40]. Fig. 2e is a SEM of Fig. 2g, The EDX analysis in Fig. 2f shows that the amount of C, N elements is relatively high, and the amount of Cd and S are relatively low. Fig. 2g shows the mapping of elements. The signal of C, N. The EDX analysis in Fig. 2f shows that the amount of C, N elements is relatively high, and the amount of Cd and S are relatively low. The atomic percentage of C is 37.65%, and N is 62.26%, and Cd is 0.04%, and S is 0.06%. Fig. 2g shows the mapping of elements. The signal of C, N elements is much powerful. It can be seen from the map that the C, N element is evenly distributed in the sample, but due to the low content of Cd, S the distribution is not clearly seen in the elemental map. Fig. 3 shows the TEM and high resolution TEM (HRTEM) images of samples. Both of amorphous and crystalline g-C3N4 exhibited superior thin nanosheet shape. The average size of CdS nanoparticles is 60 nm as shown in Fig. 3a. The nanoparticles attached closed with g-C3N4 support to form heterostructures. The HRTEM image in Fig. 3d shows that the lattice fringes width of CdS are 0.333 nm, corresponding to (002) facet. The nanoparticle embedded in the nanosheet. After
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Fig. 2 e SEM images of (a), A-GCN 5, (b), C-GCN 4, (c), A-GCN 5C, (d), C-GCN 4C, (e), A-GCN 5 for EDX analysis. (f) EDX analysis of A-GCN 5. (g) Element mapping of A-GCN 5.
Fig. 3 e TEM and HRTEM images of samples. (a), (d), A-GCN 5. (b), (e), A-GCN 5C. (c), (f), C-GCN 4C. phase transfer treatment at 650 C, The shape of g-C3N4 nanosheets was remained (Fig. 3c). However, the morphology of CdS changed into nanorods. Fig. 3c show CdS nanorods distributed homogeneously in on the nanosheets. This means a re-crystallinity and growth process occurred.
The average length of CdS nanorods is about 60 nm while the diameter is about 5 nm. Thus, a diffusion transmission controls the formation of CdS nanorods. The lattice fringes of CdS are at 0.333 nm in Fig. 3e indicates the growth of the nanorod is along (002) facet. This also confirms the in-situ
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phase transition of CdS. Fig. 3c and f the HRTEM of sample CGCN 4C. For sample C-GCN, the shape of CdS is also changed into nanorods. The average length of the nanorods is about 80 nm as shown in Fig. 3f. The lattice fringes of CdS are 0.333 nm, corresponding to the (002) plane. Crystalline degree of g-C3N4 did not affect the phase transfer of CdS. However, the size of the nanorods revealed a little difference. Fig. 4 (a) shows the Raman spectra of A-GCN and C-GCN, for A-GCN, the peak at 1231.7 cm1 corresponds to the D peak of C, showing the ring vibration of the C atom ring, indicating that there is a lattice defect in C in A-GCN, while the C-GCN has no peak indicating the defects in the material are reduced and the crystallinity is improved. The surface chemical states were investigated by XPS. Fig. 4b shows the XPS survey of samples, indicating the successful composite of A-GCN 5. Fig. 4c shows that C 1s spectrum has two peaks at 284.3 and
288 eV corresponding to the standard reference carbon (C]C) and sp2 hybridization at C in NeC]N. Fig. 4d shows that N 1s spectrum has three peaks corresponding to CeN]C (398.4eV, pyridinic-C), Ne(C)3 (400.1eV, pyrrole-C), and p-excitation (404.3 eV) [41]. Fig. 4e shows that Cd 3d spectrum has two peaks located at 404.7 and 411.6 eV attributed to the Cd 3d5/2 and Cd 3d3/2 of CdS [42]. For the S 2p in Fig. 4f, the two peaks at 161 and 162.6 eV correspond to the S 2p3/2 and S 2p1/2 of CdS [43], respectively. Compared to A-GCN 5, the main peaks of AGCN 5C are all shifted to high energy, which is due to the error caused by the charge correction, here is no change in the chemical composition of the surface of A-GCN5C and sample A-GCN 5, and it is also proved that the improvement of its properties is due to the change of CdS itself. The result from XPS survey confirms the formation of g-C3N4/CdS heterostructures.
Fig. 4 e (a), Raman spectra of A-GCN and C-GAN, XPS spectra of sample A-GCN 5 and A-GCN 5C, (b) XPS survey, (c) C 1s, (d) N 1s, (e) Cd 3d, (f) S 2p.
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Fig. 5 shows the diffuse reflectance spectra, photocurrent, and PL spectra of samples. Fig. 5a shows the Mott-Schottky curve for sample A-GCN 5C. The flat band potential of the sample can be from the Mott-Schottky equation [44]. 1 2 KB T Þ ¼ ðV Vfb C2 εε0 A2 eND e
(2)
where C is the interface capacitance; e is the dielectric constant of the semiconductor; e0 is the dielectric constant of free space; A is the interface area; e is the charge; ND is the number of donors, that is, the electron concentration; V is the applied voltage, KB is BoltzMann’s constant, T is the absolute temperature, and Vfb is the flat band potential. The flat band potential of the sample can be obtained from the tangent of the Mott-Schottaky curve to the position of the x axis. The flat band potential of sample A-GCN 5C is approximately 1.1 V. Thus, the band gaps of samples were adjusted as shown in Fig. 5b according to the relationship between the absorption coefficient near the absorption edge and the incident photon energy [45,46]. Sample A-GCN shows a strong absorbance with an absorption edge around 450 nm. The band gaps of samples A-GCN, A-GCN 5 and A-GCN 5C are 2.67, 2.76, and 2.99 eV. Fig. 5c shows the photocurrent of samples with and without UV light irradiation. The photocurrent density of heterostructure samples increased. The enhancement of photocurrent further indicates the effective separation of photogenerated electrons and holes [47e50]. Fig. 5d shows the PL spectra of samples. Due to the recombination of photogenerated electrons and hole, sample A-GCN exhibits a strong emission peak at around 470 nm. However, the PL intensity of the heterostructure sample decreased significantly. The PL peak intensity of sample A-GCN 5C is the lowest. The result indicates that the heterostructure consisted of
Fig. 6 e Schematic diagram of electron and hole separation under visible light irradiation.
amorphous g-C3N4 and CdS nanorods revealed the highest photogenerated carrier separation efficiency, in which the recombination of electrons and holes is suppressed. Fig. 6 shows that schematic illustration of electron and hole separation under visible light. According to the semiconductor band theory, when the conduction band potential is negative to the hydrogen electrode, the H2O is reduced to H2 by photogenerated electrons. 1 H2 O þ e /OH þ H2 2
(2)
When the semiconductor is subjected to illumination with energy equal to or higher than the band gap of the
Fig. 5 e (a), Mott-Schottky curves of A-GCN 5C. (b), plots of (ahv)2 photon energy of samples. (c), Photocurrent of samples. (d), PL spectra of samples.
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Fig. 7 e (a, b), Hydrogen generation of samples A-GCN 1 and SA-GCN 1. (c, d), Hydrogen generation of samples A-GCN, AGCN5, A-GCN5C, C-GCN, C-GCN4.
semiconductor, electrons within the semiconductor are excited to transition from the valence band to the conduction band. The appropriate band potential of the semiconductor is beneficial to transfer electrons generated on the surface of gC3N4 to CdS, and holes generated on the surface of CdS can be transferred to g-C3N4. Electrons and holes generated by light move toward opposite phases, which effectively reduces the probability of recombination and improves the efficiency of charge separation. In particular, the movement of holes toward g-C3N4 under illumination reduces the surface holes; there are not enough holes on the surface of CdS to cause photo-corrosion. Fig. 7 shows the photocatalytic hydrogen generation rate of samples. As illustrated in Table 1, A-GCN reveal the lowest H2 generation (764mmol/gh). The H2 generation of sample ACGCN is almost 2 times of that of amorphous. The value of CGCN is between A-GCN and the A-CGCN. This is ascribed the homojunction enhanced the separation of photogenerated carriers. For the heterostructures prepared using amorphous g-C3N4 and CdS nanoparticles via a multistep adsorption synthesis, sample A-GCN 5 revealed the best H2 generation (3097 mmol/gh). Fig. 7a shows that all samples revealed a linear increase with time for H2 generation. Sample A-GCN revealed low photocatalytic activity, probably due to rapid recombination of photo-generated electrons and holes. A-GCN 1 revealed high photocatalytic activity compared. This confirms the hetero-structure enhanced photocatalysis performance. To indicate the role of CdS nanoparticles process, sample SACCN was prepared using a normal co-precipitation method. Obviously, the H2 generation rate of sample SA-GCN 1 is low compared with sample A-GCN. This is ascribed that a multistep adsorption synthesis approach resulted in the effective adsorption of Cd2þ ions which decreased the interface defects of the heterostructure. For quantitative comparison, Fig. 7b
shows the average hydrogen generation rate of samples. Thinking of the effect of g-C3N4 support, C-GCN and A-CGCN were also used to create g-C3N4/CdS heterostructures. Table 1 illustrates the photocatalytic H2 generation rate of all samples. Although A-CGCN revealed the best photocatalysis activity for water splitting, the heterostructures with CdS and the homojunctions exhibited the worst H2 generation. This phenomenon is related that CdS addition did not increased the separation of photogenerated carriers. As for C-GCN/CdS, the photocatalysis performance is also not satisfactory compared with amorphous although CdS nanorods were also obtained after phase transfer as shown in Fig. 3f. This may be ascribed to the band gap matching. Fig. 7c shows the photocatalytic activity of the sample after CdS phase transition, A-GCN-5C exhibits a higher H2 production slope. Fig. 7d shows that AGCN 5C exhibits higher photocatalytic activity than that of sample A-GCN 5 due to more stable and light corrosive reduction after CdS phase transition.
Conclusions g-C3N4/CdS heterostructures were prepared using three kinds of g-C3N4 supports. The photocatalytic activity of g-C3N4/CdS heterostructures prepared by multistep adsorption synthesis is much higher than that of pure g-C3N4 catalyst under visible light. Hydrogen production performance is also greatly improved. The super-high hydrogen production is obtained by the in-situ phase transition of CdS whose morphology was changed from irregular nanoparticle to nanorods. Although ACGCN exhibited a high H2 generation rate compared with crystalline and amorphous ones, A-GCN/CdS heterostructures revealed the best H2 generation rate. This may be ascribed to the band gap is matching well with CdS nanorods. The
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construction of heterostructures and ideal photocatalysis activity give a new sight for the synthesis of g-C3N4 based nanomaterials and their applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant no. 51772130, 51572109).
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Crystallinity and phase controlling of g-C3N4 /CdS hetrostructures towards high efficient photocatalytic H2 generation Yaya Wang a, Xiao Zhang b, Yingjie Liu a, Yubin Zhao a, Cong Xie a, Yuxiang Song a, Ping Yang a,* a
School of Material Science and Engineering, University of Jinan, Jinan 250022, China Fuels and Energy Technology Institute and Department of Chemical Engineering, Curtin University, Perth WA6845, Australia
b
highlights
graphical abstract
g-C3N4 with different degrees of crystallinity was used to form a heterojunction with CdS. The
phase
transition
of
CdS
significantly improved its photocatalytic activity. The heterojunction after the phase transition is greatly different due to difference in matrix.
article info
abstract
Article history:
The heterostructures of graphitic carbon nitride (g-C3N4) and CdS were synthesized by
Received 21 July 2019
controlling the crystalline degree of g-C3N4 and the phase composition of CdS. A thermal
Received in revised form
polycondensation process of N precursors was adjusted to get amorphous and crystalline
5 September 2019
g-C3N4. A multistep adsorption method was used to deposit CdS nanoparticles on g-C3N4.
Accepted 24 September 2019
An annealed process was used to adjust the phase composition of CdS from cubic to
Available online 21 October 2019
hexagonal. The morphology of CdS was changed to rod. Amorphous g-C3N4/CdS heterostructures revealed enhanced photocatalytic activity because the amorphous g-C3N4 has a
Keywords:
lower crystallinity, it is easier to form a heterojunction with the CdS. Further, an annealing
g-C3N4
process resulted in the phase transfer and morphology change of CdS. The high stability
CdS
and rod morphology of CdS make the heterostructures with high H2 generation rate to
Heterostructure
5440 mmol h-1 g-1 which is ~5 times high compared with g-C3N4.
Photocatalytic H2
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (P. Yang). https://doi.org/10.1016/j.ijhydene.2019.09.181 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Solar energy is an inexhaustible new energy material. Hydrogen energy is a highly efficient clean energy source. Therefore, using solar energy to produce hydrogen will be one of the most ideal ways to solve energy problems. Developing photocatalytic materials with high efficient performance has attracted widespread attention [1]. A series of semiconductor photocatalysts have been explored including metal oxides [2], nitride [3], sulphide [4], carbide [5e10]. In generally, an ideal photocatalyst has suitable band gap, good reducibility, and high light stability. However, most of photocatalysts with single composition have some disadvantages, such as the ability to absorb only limited UV light (e. g. TiO2), rapidly and easily recombination of photogenerated electrons and holes (e. g. graphitic carbon nitride (g-C3N4)), which limit the wide application. Therefore, it is imperative to develop efficient, clean, and sustainable photocatalysts. The construction of semiconductor heterostructures becomes a hot topic. As a typical organic semiconductor, g-C3N4 has been developed for the degradation of organic pollutant and H2 generation via water splitting because of a unique layered structure composed of heptazine ring with a suitable band gap (2.7 eV) in visible light region. Precursor materials and preparation processes affect the composition and layer structure of g-C3N4 which governs the photocatalysis performance [11]. Bulk g-C3N4 has low conductivity, small specific surface area, wide band gap, and rapid recombination of photo-generated electrons and holes that greatly limited its application. Various methods have been developed to improve the photocatalytic performance of g-C3N4. Namely, morphology controlling increases its specific surface area, such as ultra-thin nanosheet, hollow bubble nanostructures [12], tubular nanostructures [13]. The fabrication of heterojunctions with other semiconductors decreases the quick recombination of photogenerated carriers. The heterostructures of g-C3N4 with noble metals [14], metal sulphide [15], metal oxides [16], and metal [17], non-metal [18] have been reported. The formation of the heterojunction can effectively promote the separation and transfer of electrons and holes, as well as significantly increase the specific surface area, thereby improving the photocatalytic activity. CdS, as an II-IV semiconductor, has attracted much attention in photocatalytic water splitting due to its narrow band gap (2.42 eV). But its performance needs to be improved. For example, small CdS nanoparticles are easily agglomerated into large particles, resulting in the rapid recombination of photogenerated electrons and holes [19]. In addition, photocorrosion is more serious during the photocatalytic reaction. In order to solve these problems, efforts have been developed to control the morphology including synthetic nanospheres, nanowires, nanorods, and other structures [20e26]. Another efficient way is fabricating CdS nanocomposites, for example, metal doping, constructing heterostructures (TiO2 and WO3) [27e31]. CdS heterostructures with other semiconductor are considered to be one of the best approach to promote the separation of photo-generated electrons and holes. Layered g-C3N4 as an efficient supporting has been used to form heterojunction. Because the band gaps of g-C3N4 and CdS
matching, electrons and holes can move opposite directions, effectively suppressing photo-generated electrons and holes. Constructing a hetrojunction of g-C3N4/CdS is an effective method for improving the absorption in visible light and enhancing photocatalytic hydrogen production. The formation of heterojunction of g-C3N4 and CdS has been interesting as a hot topic currently. Here, three kinds of g-C3N4 nanosheets (amorphous (A-GCN), crystalline (C-GCN) and amorphous/crystalline homojunction (A-CGCN) were prepared using the procedure in our recently published paper [32]. CdS nanoparticles were deposited on g-C3N4 nanosheets to form gC3N4/CdS heterojunctions. A phase transfer process by secondary calcination made CdS nanorods with hexagonal structure. The crystalline degree of g-C3N4 affects the properties of the heterojunctions. The CdS nanorods revealed high stability and reduced photo-corrosion. Thus, g-C3N4/CdS heterojunctions revealed enhanced photocatalytic hydrogen generation. The innovation of this paper is to show different properties of g-C3N4 nanosheet composite CdS with different crystallinity. A-GCN shows better photocatalytic hydrogen production after composite CdS, after secondary calcination Phase transitions in CdS show higher hydrogen production performance. There are the current manuscript is differ from the published one and novelty of the present manuscript [33e35]. This study provides new insights to the property improvement of g-C3N4 materials and the construction of heterostructures.
Experimental Synthesis of g-C3N4 nanosheets A-GCN: Typically, 1 g of melamine was added into a ceramic boat, and then the boat was put into the tube furnace. The sample was heat-treated up to 650 C using a heating rate of 5 C/min, and kept at 650 C for 2 h. The flaky sample A-GCN was ground into a powder with an agate mortar. Table 1 illustrates the synthesis conditions and properties of samples. C-GCN: 1 g of melamine was placed in the ceramic boat, and the boat was placed in the quartz tube of the tube furnace. According to the heating rate of 2 C/min, the temperature was raise to 600 C and kept for 2 h. After the precursor was ground to a powder, add 1 g into a ceramic boat, heated to 750 C in a heating rate of 5 C/min, and kept for 2 h to get crystalline phase. A-CGCN: A certain amount of crystalline g-C3N4 and 1 g of melamine were mixed in an agate mortar and fully ground. The powder sample was added in a ceramic boat. The boat is placed in a quartz tube of a tube furnace at 5 C/min heating rate to 650 C and keep it for 2 h to get A-CGCN.
Multistep deposition synthesis of g-C3N4/CdS heterostructures 90 mg of A-GCN was dissolved in 20 mL of CdCl2 solution (0.1 M) with stirring for 2 h. The solution was then centrifuged. The wet sample was placed in a bottle with stirring for 30 min and then centrifuged. 1 mL of the thioacetamide solution (TAA，0.1 M) solution was added and then heated to 90 C
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Table 1 e Synthesis conditions and property of samples. Sample Cd2þ solution S2 solution (mL) (mL) A-GCN A-GCN 1 A-GCN 2 A-GCN 3 A-GCN 4 A-GCN 5 A-GCN 6 C-GCN C-GCN 1 C-GCN 2 C-GCN 3 C-GCN 4 C-GCN 5 A-CGCN A-CGCN 1 A-CGCN 2 A-CGCN 3 A-CGCN 4
H2 generation rate (mmol/gh)
N/A 20 20 20 20 20 20 N/A 20 20 20 20 20 N/A 20
N/A 1 2 3 4 5 6 N/A 1 2 3 4 5 N/A 1
764 2273 2145 2483 2053 3097 1713 1018 1101 1016 1242 1327 993 1438 1468
20
2
1618
20
3
1764
20
4
1347
with stirring for 2 h. The sample was centrifuged and washed using water and anhydrous ethanol, naming this sample as AGCN 1. For comparison, the amount of CdS was adjusted by changing the amount of TAA solution as 2, 3, 4, and 5 mL, naming as A-GCN 2, A-GCN 3, A-GCN 4 and A-GCN 5, respectively. For other heterostructures, the preparation procedure is same, but A-GCN was replaced using C-GCN, and A-CGCN.
Co-precipitation preparation of A-GCN/CdS For comparison, CdS nanoparticles were carried out using a co-precipitation synthesis. Typically, 90 mg of A-GCN, 20 mL of CdCl2 solution (0.1 M), 1 mL of TAA (0.1 M) were mixed and put into an electric hot cap and then heated to 90 C with stirring for 2 h and cooled to room temperature. The sample was washed with deionized water and anhydrous ethanol, naming as sample SA-GCN 1.
Phase transition of CdS Typically, 90 mg of A-GCN 5 was placed in a ceramic boat, and the boat was placed in a tube furnace at a heating rate of 5 C/ min to 650 C and kept the temperature for 30 min in H2 atmosphere, and then cooled to room temperature, naming as sample A-GCN 5C. The heat-treatment of other samples is similar with sample A-GCN 5.
Characterization The powder X-ray diffraction (XRD) patterns of samples was determined by an X-ray diffractionmeter (Bruker D8, Germany), Cu Ka was used as the radiation source to record 2q at 10e80 . The morphology of samples was observed by a scanning electron microscope (SEM, QUANTA 250 FEG, FEI America) and transmission electron microscopy (TEM) (FEIG2F20).
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The energy-dispersive X-ray (EDX) analysis was used to get the composition of samples on a SEM (QUANTA 250 FEG, FEI America0. Raman spectroscopy was recorded by Horiba Jobin Yvon Lab RAMHR Evolution with 785 nm excitation laser wave length. X-ray photoelectron spectroscopy (XPS) was used to measure the composition of samples using a Thermo ESCALAB250xi instrument. The UV/Vis diffuse reflectance spectra of samples were recorded using a conventional UV/Vis spectrometer (Hitachi U-4100). The photoluminescence (PL) spectra of samples were measured by using a spectrometer (Hitachi F-4600). The photocurrent of samples was obtained on a chi660e electrochemical analyzer (Chen Hua Instruments, Shanghai, China). The Fourier transform infrared (FT-IR) spectra of samples were obtained a Fourier transform infrared spectrometer (Nicolet 380, Thermo, America).
Photocatalytic H2 production activity The photocatalytic H2 evolution experiment was carried out at room temperature in a 300 mL three-necked quartz reactor using a PLS-SXE 300 UV xenon arc lamp with UV filter (400 nm) as the light source and a three-necked flask on both necks. The mouth is connected to a closed circulation system by means of vacuum glue. 10 mg of sample was dissolved in 100 mL of an aqueous solution with 20% triethanolamine. Pt (0.2 wt%) was added as a co-catalyst. The sample was tirred and irradiated at room temperature for 30 min (300 W Xe). The reaction vessel was vigorous stirred for 30 min prior to the photocatalytic experiment to remove dissolved oxygen and ensure anaerobic conditions. The reaction vessel was vigorous stirred for 30 min prior to the photocatalytic experiment to remove dissolved oxygen and ensure anaerobic conditions. The gas was sampled intermittently through a septum, while hydrogen was analysed by gas chromatography (Shimadzu GC-14C, nitrogen as a carrier gas) and equipped with five molecular sieve columns and a thermal conductivity detector.
Results and discussion The formation of g-C3N4/CdS heterostructures and phase transfer of CdS are illustrated in Scheme 1. During reaction, the nucleation of CdS occurred on g-C3N4 nanosheets. Further growth resulted in CdS nanoparticles formed. A phase transfer process resulted in zine blend CdS nanoparticles were transferred into hexagonal CdS nanorods because of temperature of 650 C. Fig. 1a shows the XRD patterns of samples. Two XRD peaks of g-C3N4 are observed for all samples. The corresponding (100) facet at 12.81 indicates the periodic arrangement in triazine of g-C3N4. The corresponding (002) crystal plane at 27.91 indicates the accumulation of the conjugated direction system of carbon nitride. The characteristic peak at 27.9 is significantly reduced after compounding CdS. According to the Scherrer formula [36,37]: D¼
Kl bcosq
(1)
The crystal grain diameter of the tetragonal phase CdS can be calculated to be about 40 nm, which is consistent with the
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Scheme 1 e Illustration of g-C3N4/CdS heterostructure formation.
Fig. 1 e (a) XRD patterns and (b) FT-IR spectra of samples.
transmission electron microscope results. Where D is the nanocrystalline grain size, K is the Scherrer constant, l is the wavelength of the x-ray, b is the full width at half maximum of the diffraction peak, and q is the Bragg diffraction angle. Fig. 1b shows the FT-IR spectra of samples A-GCN, A-GCN 5, A-GCN 5C, C-GCN and C-GCN 4. The characteristic stretching peaks were in three regions at 3296, 1200e1700 and 809 cm1. The peak at 3296 cm1 is the stretching vibration peak of eNH2 or NeH. The bands appeared at 1200-1700 cm1 corresponding to the stretching vibration peak of CeN, and the peak at 809 cm1 correspond to the characteristic vibration of the triazine. For CdS, the peak concentrated at 3460 and 1646 cm1 correspond to the surface-adsorbed water molecules, and the 1385 and 1128 cm1 are correspond to the stretching vibration peak of CdeS. The stretching vibration peaks at 2928 and 2856 cm1 are attributable to the bending vibration of eCH2 and eCH3 [38,39]. The stretching vibration peaks of g-C3N4 and CdS in the FT-IR spectra are observed. This confirms that the formation of g-C3N4/CdS heterostructures. Fig. 2 shows the SEM images and element analysis of samples. Fig. 2a shows that CdS nanoparticles are uniformly dispersed on A-GCN nanosheets, and Fig. 2b shows that CdS nanoparticles are dispersed on C-GCN nanosheets. Fig. 2c and d shows that after heat-treatment at 650 C in H2 atmosphere
phase transformation, the surface CdS of A-GCN and C-GCN have changed from particles to rods. For C-GCN, the deposition of CdS nanoparticles is different from A-GCN because of the difference of microstructure and surface state [40]. Fig. 2e is a SEM of Fig. 2g, The EDX analysis in Fig. 2f shows that the amount of C, N elements is relatively high, and the amount of Cd and S are relatively low. Fig. 2g shows the mapping of elements. The signal of C, N. The EDX analysis in Fig. 2f shows that the amount of C, N elements is relatively high, and the amount of Cd and S are relatively low. The atomic percentage of C is 37.65%, and N is 62.26%, and Cd is 0.04%, and S is 0.06%. Fig. 2g shows the mapping of elements. The signal of C, N elements is much powerful. It can be seen from the map that the C, N element is evenly distributed in the sample, but due to the low content of Cd, S the distribution is not clearly seen in the elemental map. Fig. 3 shows the TEM and high resolution TEM (HRTEM) images of samples. Both of amorphous and crystalline g-C3N4 exhibited superior thin nanosheet shape. The average size of CdS nanoparticles is 60 nm as shown in Fig. 3a. The nanoparticles attached closed with g-C3N4 support to form heterostructures. The HRTEM image in Fig. 3d shows that the lattice fringes width of CdS are 0.333 nm, corresponding to (002) facet. The nanoparticle embedded in the nanosheet. After
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Fig. 2 e SEM images of (a), A-GCN 5, (b), C-GCN 4, (c), A-GCN 5C, (d), C-GCN 4C, (e), A-GCN 5 for EDX analysis. (f) EDX analysis of A-GCN 5. (g) Element mapping of A-GCN 5.
Fig. 3 e TEM and HRTEM images of samples. (a), (d), A-GCN 5. (b), (e), A-GCN 5C. (c), (f), C-GCN 4C. phase transfer treatment at 650 C, The shape of g-C3N4 nanosheets was remained (Fig. 3c). However, the morphology of CdS changed into nanorods. Fig. 3c show CdS nanorods distributed homogeneously in on the nanosheets. This means a re-crystallinity and growth process occurred.
The average length of CdS nanorods is about 60 nm while the diameter is about 5 nm. Thus, a diffusion transmission controls the formation of CdS nanorods. The lattice fringes of CdS are at 0.333 nm in Fig. 3e indicates the growth of the nanorod is along (002) facet. This also confirms the in-situ
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phase transition of CdS. Fig. 3c and f the HRTEM of sample CGCN 4C. For sample C-GCN, the shape of CdS is also changed into nanorods. The average length of the nanorods is about 80 nm as shown in Fig. 3f. The lattice fringes of CdS are 0.333 nm, corresponding to the (002) plane. Crystalline degree of g-C3N4 did not affect the phase transfer of CdS. However, the size of the nanorods revealed a little difference. Fig. 4 (a) shows the Raman spectra of A-GCN and C-GCN, for A-GCN, the peak at 1231.7 cm1 corresponds to the D peak of C, showing the ring vibration of the C atom ring, indicating that there is a lattice defect in C in A-GCN, while the C-GCN has no peak indicating the defects in the material are reduced and the crystallinity is improved. The surface chemical states were investigated by XPS. Fig. 4b shows the XPS survey of samples, indicating the successful composite of A-GCN 5. Fig. 4c shows that C 1s spectrum has two peaks at 284.3 and
288 eV corresponding to the standard reference carbon (C]C) and sp2 hybridization at C in NeC]N. Fig. 4d shows that N 1s spectrum has three peaks corresponding to CeN]C (398.4eV, pyridinic-C), Ne(C)3 (400.1eV, pyrrole-C), and p-excitation (404.3 eV) [41]. Fig. 4e shows that Cd 3d spectrum has two peaks located at 404.7 and 411.6 eV attributed to the Cd 3d5/2 and Cd 3d3/2 of CdS [42]. For the S 2p in Fig. 4f, the two peaks at 161 and 162.6 eV correspond to the S 2p3/2 and S 2p1/2 of CdS [43], respectively. Compared to A-GCN 5, the main peaks of AGCN 5C are all shifted to high energy, which is due to the error caused by the charge correction, here is no change in the chemical composition of the surface of A-GCN5C and sample A-GCN 5, and it is also proved that the improvement of its properties is due to the change of CdS itself. The result from XPS survey confirms the formation of g-C3N4/CdS heterostructures.
Fig. 4 e (a), Raman spectra of A-GCN and C-GAN, XPS spectra of sample A-GCN 5 and A-GCN 5C, (b) XPS survey, (c) C 1s, (d) N 1s, (e) Cd 3d, (f) S 2p.
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Fig. 5 shows the diffuse reflectance spectra, photocurrent, and PL spectra of samples. Fig. 5a shows the Mott-Schottky curve for sample A-GCN 5C. The flat band potential of the sample can be from the Mott-Schottky equation [44]. 1 2 KB T Þ ¼ ðV Vfb C2 εε0 A2 eND e
(2)
where C is the interface capacitance; e is the dielectric constant of the semiconductor; e0 is the dielectric constant of free space; A is the interface area; e is the charge; ND is the number of donors, that is, the electron concentration; V is the applied voltage, KB is BoltzMann’s constant, T is the absolute temperature, and Vfb is the flat band potential. The flat band potential of the sample can be obtained from the tangent of the Mott-Schottaky curve to the position of the x axis. The flat band potential of sample A-GCN 5C is approximately 1.1 V. Thus, the band gaps of samples were adjusted as shown in Fig. 5b according to the relationship between the absorption coefficient near the absorption edge and the incident photon energy [45,46]. Sample A-GCN shows a strong absorbance with an absorption edge around 450 nm. The band gaps of samples A-GCN, A-GCN 5 and A-GCN 5C are 2.67, 2.76, and 2.99 eV. Fig. 5c shows the photocurrent of samples with and without UV light irradiation. The photocurrent density of heterostructure samples increased. The enhancement of photocurrent further indicates the effective separation of photogenerated electrons and holes [47e50]. Fig. 5d shows the PL spectra of samples. Due to the recombination of photogenerated electrons and hole, sample A-GCN exhibits a strong emission peak at around 470 nm. However, the PL intensity of the heterostructure sample decreased significantly. The PL peak intensity of sample A-GCN 5C is the lowest. The result indicates that the heterostructure consisted of
Fig. 6 e Schematic diagram of electron and hole separation under visible light irradiation.
amorphous g-C3N4 and CdS nanorods revealed the highest photogenerated carrier separation efficiency, in which the recombination of electrons and holes is suppressed. Fig. 6 shows that schematic illustration of electron and hole separation under visible light. According to the semiconductor band theory, when the conduction band potential is negative to the hydrogen electrode, the H2O is reduced to H2 by photogenerated electrons. 1 H2 O þ e /OH þ H2 2
(2)
When the semiconductor is subjected to illumination with energy equal to or higher than the band gap of the
Fig. 5 e (a), Mott-Schottky curves of A-GCN 5C. (b), plots of (ahv)2 photon energy of samples. (c), Photocurrent of samples. (d), PL spectra of samples.
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Fig. 7 e (a, b), Hydrogen generation of samples A-GCN 1 and SA-GCN 1. (c, d), Hydrogen generation of samples A-GCN, AGCN5, A-GCN5C, C-GCN, C-GCN4.
semiconductor, electrons within the semiconductor are excited to transition from the valence band to the conduction band. The appropriate band potential of the semiconductor is beneficial to transfer electrons generated on the surface of gC3N4 to CdS, and holes generated on the surface of CdS can be transferred to g-C3N4. Electrons and holes generated by light move toward opposite phases, which effectively reduces the probability of recombination and improves the efficiency of charge separation. In particular, the movement of holes toward g-C3N4 under illumination reduces the surface holes; there are not enough holes on the surface of CdS to cause photo-corrosion. Fig. 7 shows the photocatalytic hydrogen generation rate of samples. As illustrated in Table 1, A-GCN reveal the lowest H2 generation (764mmol/gh). The H2 generation of sample ACGCN is almost 2 times of that of amorphous. The value of CGCN is between A-GCN and the A-CGCN. This is ascribed the homojunction enhanced the separation of photogenerated carriers. For the heterostructures prepared using amorphous g-C3N4 and CdS nanoparticles via a multistep adsorption synthesis, sample A-GCN 5 revealed the best H2 generation (3097 mmol/gh). Fig. 7a shows that all samples revealed a linear increase with time for H2 generation. Sample A-GCN revealed low photocatalytic activity, probably due to rapid recombination of photo-generated electrons and holes. A-GCN 1 revealed high photocatalytic activity compared. This confirms the hetero-structure enhanced photocatalysis performance. To indicate the role of CdS nanoparticles process, sample SACCN was prepared using a normal co-precipitation method. Obviously, the H2 generation rate of sample SA-GCN 1 is low compared with sample A-GCN. This is ascribed that a multistep adsorption synthesis approach resulted in the effective adsorption of Cd2þ ions which decreased the interface defects of the heterostructure. For quantitative comparison, Fig. 7b
shows the average hydrogen generation rate of samples. Thinking of the effect of g-C3N4 support, C-GCN and A-CGCN were also used to create g-C3N4/CdS heterostructures. Table 1 illustrates the photocatalytic H2 generation rate of all samples. Although A-CGCN revealed the best photocatalysis activity for water splitting, the heterostructures with CdS and the homojunctions exhibited the worst H2 generation. This phenomenon is related that CdS addition did not increased the separation of photogenerated carriers. As for C-GCN/CdS, the photocatalysis performance is also not satisfactory compared with amorphous although CdS nanorods were also obtained after phase transfer as shown in Fig. 3f. This may be ascribed to the band gap matching. Fig. 7c shows the photocatalytic activity of the sample after CdS phase transition, A-GCN-5C exhibits a higher H2 production slope. Fig. 7d shows that AGCN 5C exhibits higher photocatalytic activity than that of sample A-GCN 5 due to more stable and light corrosive reduction after CdS phase transition.
Conclusions g-C3N4/CdS heterostructures were prepared using three kinds of g-C3N4 supports. The photocatalytic activity of g-C3N4/CdS heterostructures prepared by multistep adsorption synthesis is much higher than that of pure g-C3N4 catalyst under visible light. Hydrogen production performance is also greatly improved. The super-high hydrogen production is obtained by the in-situ phase transition of CdS whose morphology was changed from irregular nanoparticle to nanorods. Although ACGCN exhibited a high H2 generation rate compared with crystalline and amorphous ones, A-GCN/CdS heterostructures revealed the best H2 generation rate. This may be ascribed to the band gap is matching well with CdS nanorods. The
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 1 5 1 e3 0 1 5 9
construction of heterostructures and ideal photocatalysis activity give a new sight for the synthesis of g-C3N4 based nanomaterials and their applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant no. 51772130, 51572109).
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