Photocatalytic reforming of biomass for hydrogen production over ZnS nanoparticles modified carbon nitride nanosheets

Journal of Colloid and Interface Science 555 (2019) 22–30

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Photocatalytic reforming of biomass for hydrogen production over ZnS nanoparticles modified carbon nitride nanosheets Xinyuan Xu a, Jinqiang Zhang a, Shuaijun Wang a, Zhengxin Yao b, Hong Wu a, Lei Shi a, Yu Yin a, Shaobin Wang c, Hongqi Sun a,⇑ a b c

School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 14 May 2019 Revised 23 July 2019 Accepted 24 July 2019 Available online 25 July 2019 Keywords: ZnS Carbon nitride Photoreforming Biomass Hydrogen

a b s t r a c t Hydrogen generation from biomass reforming via solar energy utilisation has become a fascinating strategy toward future energy sustainability. In this study, ZnS nanoparticles with an average size around 10–15 nm were synthesised by a facile hydrothermal method, and then hybridised with g-C3N4 (MCN, DCN, and UCN) derived from melamine, dicyandiamide and urea, producing the heterojunctions denoted as ZMCN, ZDCN and ZUCN, respectively. Advanced characterisations were employed to investigate the physiochemical properties of the materials. ZMCN and ZDCN showed a slight red shift and better light absorbance ability. Their catalytic performances were evaluated by photocatalytic biomass reforming for hydrogen generation. The hydrogen generation rate on ZMCN, the best photocatalyst among MCN, DCN, UCN, ZDCN and ZUCN, was around 2.5 times higher than the pristine MCN. However, the photocatalytic efficiency of ZUCN experienced decrease of 36.6% compared to pure UCN. The mechanism of the photocatalytic reforming process was discussed. The photoluminescence spectra of ZMCN suggested that the introduction of ZnS for ZMCN would reduce the recombination of photoinduced carriers. It was also found that both microstructure and band structure would influence the photocatalytic reforming efficiency. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding author. E-mail address: [email protected] (H. Sun). https://doi.org/10.1016/j.jcis.2019.07.066 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

The depletion of fossil fuels gradually becomes a serious issue to both energy and environmental sustainability. A clean, renewable and sustainable energy source is then highly in demand. Hydrogen,

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as a renewable energy, can be obtained by converting solar energy via photocatalysis, was firstly discovered in photoelectrochemical water splitting process on a TiO2 electrode in 1972 [1]. Nonetheless, there are two shortages in the hydrogen evolution reaction (HER) process. For instance, i) hydrogen generation process always relies on the sacrificial agents, and ii) it lacks of efficient semiconductor materials. As an alternative, photoreforming of biomass emerged because biomass can be oxidised and reformed to produce fine, value-added chemicals, meanwhile the electrons from this process would reduce the proton of water simultaneously [2,3]. It is expected that the structures of biomass and the properties of photocatalyst will be two key points determining the efficiency of reforming process. The conversion of glucose, a kind of simple structure sugar aldoses, is currently rather attractive in this field to be proceeded for preliminarily understanding the mechanism of reforming process [2,4–6]. Photocatalytic reforming usually undergoes two pathways, e.g., a) direct hole transfer to oxidise biomass, and b) indirect mechanism to produce radicals. Efficient photocatalysts are essential to facilitate the processes to produce hydrogen and intermediate chemicals in the photocatalytic reforming process. To this end, a variety of semiconductors with specific morphologies have been proposed. Recently, two-dimensional (2D) semiconductors have been employed as efficient photocatalysts because of their high electronic conductivity and other unique physiochemical properties [7]. As an excellent candidate, graphitic carbon nitride (g-C3N4) has drawn considerable attention as a 2D, polymeric, metal-free photocatalyst, which is of the extension of packed p conjugated planes, narrow bandgap (2.7 eV), nontoxicity, and high absorbance of visible light [7–11]. The g-C3N4 photocatalyst can be feasibly synthesised from a variety of different precursors, for example, urea, melamine and dicyanamide, making it hold a great potential for large scale applications [12]. However, 2D g-C3N4 also has its own shortcomings such as low separation rate of electron-hole pairs and electronic conductivity, leading to a low photocatalytic activity. The introduction of a cocatalyst to modify the bulk g-C3N4 is defined to be one of the effective ways to overcome these disadvantages based on the generation of high quality heterojunctions. More recently, transitional metal sulfides such as MoS2, CdS and ZnS [7,13,14] have been used as the cocatalysts for photocatalytic process. For instance, zinc sulfide nanoparticles (ZnS) were employed as an excellent cocatalyst and photocatalyst to improve the hydrogen generation [15–18]. Many materials, for example, ZnS/g-C3N4, ZnS/CdS and ZnS/ZnO, can show the increase in optical properties after including ZnS nanoparticles for forming the hybrids [15,19,20]. All of these modified composites can work well in photocatalytic hydrogen generation and degradation process under sunlight owing to the high efficiency of ZnS nanoparticles. However, as a kind of sulfide ZnS still has its own shortcomings such as the photo-corrosion property and the toxicity as compared to g-C3N4 [21]. Accordingly, it is urgent to address the challenges of both ZnS and g-C3N4 to develop new photocatalysts which are more stable and environmentally benign for the further practical applications. Herein, this work reports the synthesis of three pristine g-C3N4 (MCN, DCN and UCN) and their ZnS modified composites (ZMCN, ZDCN and ZUCN), and their applications in the photocatalytic reforming process of glucose for hydrogen evolution. The as prepared ZMCN shows remarkable enhancements in the absorption of visible light and the photoinduced carriers’ separation compared with other unmodified g-C3N4. As a result, these novel photocatalysts achieved enhanced photocatalytic reforming of glucose for hydrogen generation towards solar energy utilisation.

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2. Materials and methods 2.1. Chemical reagents Melamine, urea, dicyanamide, anhydrous sodium sulfide (Na2S), zinc acetate dehydrate (Zn(Ac)2
2H2O), chloroplatinic acid hexahydrate (37.5% Pt basis), D-+-glucose, Nafion, sodium sulfate and anhydrous ethanol were supplied by Sigma-Aldrich, Australia.

2.2. Preparation of g-C3N4 Three different precursors, e.g. melamine, dicyanamide and urea were used to synthesise 2D g-C3N4 using a same calcination process. Their final products were accordingly denoted as MCN, DCN and UCN. In a typical synthesis procedure, the precursor was placed in a crucible and heated to 550 at the rate of 5 °C min1 and kept at this temperature for 4 h in air within a muffle furnace. The yellowish powders were then obtained.

2.3. Preparation of ZnS ZnS nanoparticles were synthesised using a facile one-pot hydrothermal method [22]. Typically, 2.2 g Zn(Ac)2
2H2O and 0.63 g Na2S were dispersed in 30 mL ultra-pure water with stirring to form a homogeneous suspension. The white slurry mixture was sealed into a 50 mL Teflon-lined autoclave and kept at 160 °C for 12 h after 60 min stirring. Finally, the white ZnS powders were then collected after washing with ultra-pure water and ethanol. After that, the white powders were dried in a vacuum oven at 60 °C for overnight. 2.4. Preparation of ZnS modified g-C3N4 The as-prepared 0.4 g MCN, DCN and UCN were separately dispersed with 80 mg ZnS in 30 mL anhydrous ethanol and ultrasonicated for 30 min to ensure that the mixture was dispread well. Then, the light-yellow mixtures were stirred under 800 rpm in a fume cupboard until the full evaporation of the solvent. After that, the yellowish powders were collected and dried in a vacuum oven again at 60 °C for 12 h. Finally, the calcination was proceeded in a tube furnace under N2 atmosphere at 300 °C for 1 h with the rate of 5 °C min1. These g-C3N4 based materials were denoted as ZMCN, ZDCN and ZUCN according to the name of g-C3N4. 2.5. Characterisation Transition electron microscopy (TEM) images were performed and collected on FEI Titan TEM microscope. X-ray diffraction (XRD) patterns were measured on Panalytical Empyrean multipurpose research diffractometer utilizing a Cu Ka radiation (k = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was operated on the Thermo Escalab 250 spectrometer (Al Ka X-ray). High resolution transmission electron microscopy (HRTEM) was collected with TITAN. Fourier transform infrared (FT-IR) spectra were recorded in the range of 500–4000 cm1 by PerkinElmer UATR Two. UV-Visible diffuse reflectance spectra (DRS) were conducted on Agilent Cary 300 UV-Vis spectrophotometer with an integrated sphere. Agilent Cary Eclipse Fluorescence Spectrophotometer was used to collect the data for photoluminescence (PL) of samples. Brunauer-Emmett-Teller (BET) was tested on Micromeritics TriStar II (Surface Area and Porosity).

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2.6. Photocatalytic activity evaluation These photocatalytic reforming processes were evaluated by the hydrogen generation within in a quartz vessel sealed with a quartz window under the top-irradiation light source. A Newport 300 W Xeon lamp was employed as a simulated solar energy source with one solar energy irradiation intensity. The hydrogen evolution based on photocatalytic reforming was performed by mixing 50 mg sample powder with 50 mL glucose solution (50 ppm), following by ultrasonication for 30 min. 1.0 wt% Pt as the cocatalyst was loaded on ZnS, ZnS modified three different types of g-C3N4 and pure g-C3N4 by the photo-deposition approach under the simulated solar light irradiations. Then, the mixture was purged with ultra-pure N2 to remove air in the quartz reactor for another 30 min. Moreover, the evaluated hydrogens were tested by a gas chromatography (Shimadzu GC-2014 offline, Argon carrier gas) with a thermal conductivity detector. 2.7. Electrochemical measurements The working electrodes of ZMCN, ZDCN and ZUCN were prepared via a dip-coating approach to fabricate films on Fluorinedoped tin oxide (FTO) glasses. Typically, FTO was cut to 1.5 cm  1 cm size and washed by acetone and ethanol followed by drying under 60 °C for 12 h. Specifically, 5 mg g-C3N4 and ZnS modified g-C3N4 were weighted and dispersed in 250 mL anhydrous ethanol solution under ultrasonication for 30 min. Afterwards, 12.5 mL Nafion was added into above suspension and ultrasonicated for another 5 min. The homogenous suspension was dropped on the surface area of FTO, which was controlled at 1 cm2. The prepared working electrodes were kept and dried in air for 12 h. Electrochemical impedance spectroscopy (EIS) was evaluated and performed on a Zahner Zennium electrochemical workstation with a standard three-electrode system, in which 0.2 M Na2SO4 (pH = 6.8) was used as the electrolyte solution. Pt wire worked as the counter electrode and a saturated Ag/AgCl was adopted as a reference electrode. 3. Results and discussion 3.1. Crystal and micro structures, compositions, and optical properties The crystal structures of ZnS, g-C3N4 and modified g-C3N4 were investigated by X-ray diffraction (XRD) as shown in Fig. 1a. Two characteristic diffraction peaks at 13.1° and 27.7° were observed on the three pure g-C3N4, representing the characteristic diffraction peaks at (1 0 0) and (0 0 2) respectively [23,24]. The peak at 13.1° can be assigned to the interplanar ordering of tri-s-triazine units. Furthermore, graphitic layer stacking controlled by the Van der Walls force locates at 27.7°. The three main characteristic diffraction peaks of pure ZnS at 28.6, 47.7 and 56.6° can be observed in Fig. 1a [25]. These peaks respectively matched well with (0 0 8), (1 1 0) and (1 1 8) planes, which can be indexed to a cubic phase (JCPDS#39-1363). All characteristic diffraction peaks of ZnS/g-C3N4 samples can be indexed to the cubic Wurtzite-8H ZnS and g-C3N4 without any other peaks, confirming the formation of pure ZnS and g-C3N4 in the modified samples without any other impurities. The main characteristic peaks at 27.7° of MCN, DCN and UCN (0 0 2) overlap with (0 0 8) plane of ZnS due to the hybridisation between ZnS and pristine g-C3N4. In addition, the peak of ZnS (0 0 8) plane slightly moves to a higher angle after the impregnation process, indicating a smaller distance of lattice spacing than pristine ZnS. FT-IR spectra of UCN, DCN, MCN, ZMCN, ZDCN and ZUCN are displayed in Fig. 1b. All ZnS modified nanomaterials have a same

skeleton which is similar to their basic pure g-C3N4 without notable peak changes and deviations. Also, there are no considerable changes on distinctive stretch modes of ZMCN, ZDCN and ZUCN when compared to their g-C3N4 bases after the modification. These results can demonstrate that the well-organised molecular structures of MCN, DCN and UCN were remained. As shown in Fig. 1b, there is a broad peak from 3100 to 3600 cm1 because of the stretching of N-H [23]. Afterwards, a considerable amount of characteristic bands appear in the range from 1200 to 1650 cm1, which are 1230, 1321, 1427, 1568 and 1633 cm1 and can be ascribed to the stretching vibration modes of g-C3N4 heterocyclic compounds such as sp2 C@N and sp3 CAN [15,26]. Typically, CAN can be verified by the peaks of 1568 and 1633 cm1, when the stretching vibration modes locating at 1230, 1321 and 1427 cm1 were detected and defined as CANHAC [27]. Another characteristic peak at 807 cm1 shows the vibration mode of tris-triazine unit and a slight blue shift can be observed after the ZnS modification [28]. These minor changes might indicate the formation of NAZn, verifying the perfect interfacial interaction between ZnS and pure g-C3N4. The textural properties of pure g-C3N4 and ZnS modified g-C3N4, N2 adsorption-desorption isotherms were investigated and the results are displayed in Fig. 1c and d. ZMCN and ZMDN have similar specific Brunauer-Emmett-Teller (BET) surface areas after the modification, which are 24.01 and 25.51 m2/g, respectively (Table S1). Besides, their specific BET surface areas were increased after the modifications with ZnS nanoparticles as compared to their bare g-C3N4. Additionally, the specific BET surface area of ZMUN also increased after loading ZnS nanoparticles. Fig. 1d shows that the pore size distributions of all modified g-C3N4 and pure gC3N4 within the range from 2 to 100 nm. These results indicate that ZMCN, ZDCN and ZUCN obtained the mesoporous structure after the modification process as compared to their pure MCN, DCN and UCN. The dispersion of ZnS nanoparticles can also offer large surface areas and more active sites for pure g-C3N4. The surface chemical states of all samples were investigated by X-ray photoelectron spectroscopy (XPS). The XPS spectra of MCN, ZMCN and ZnS are shown in Fig. 2, which are mainly composed of C, N, Zn, and S, suggesting the successful formation of immobilised heterojunction between ZnS and g-C3N4. The XPS spectrum of C1s for g-C3N4 can be deconvoluted into two peaks at 284.8 and 289.3 eV in Fig. 2a [29]. Specifically, 284.8 eV can work for the calibration of the binding energies as the reference. The peak at 284.8 eV can be ascribed to the CAC and C@C bonds due to the adsorption of the amorphous carbon. Another peak at 287.6 eV can be assigned to NAC@N aromatic rings in the structure of pure g-C3N4. There are three peaks at 398.7, 399.5 and 401.1 eV, which can be ascribed to the division of N 1s XPS spectrum in Fig. 2b. These peaks correspond to sp2-hybrideized nitrogen CAN@C, tertiary nitrogen groups CAN(3) nitrogen atoms and CANAH as the amino functional groups, respectively [7]. On the basis of Fig. 2c and d, two peaks at 1021.4 and 1044.4 eV can be divided from Zn 2p XPS spectrum of pure ZnS [30]. These peaks can be fitted to Zn 2p3/2 and Zn 2p1/2, respectively. Meanwhile, the S 2p XPS spectrum can be classified into two peaks at 162.4 and 163.4 eV, which can be ascribed to S 2p3/2 and S 2p1/2 [17,31]. The shift peaks can indicate that the presence of the ZnS and MCN has the electron coupling. All the results of S 2p XPS spectra can show and explain the existence of the unsaturated edge S atoms on ZnS [15]. The morphologies of ZnS and three types of ZnS modified g-C3N4 are shown by TEM and HRTEM images. According to Fig. 3a–c, the crystallized ZnS displays as the spherical nanoparticles with the mean diameter around 10–15 nm. ZnS has a high dispersed ability and a smaller diameter, compared to ZnS in a previous report [15].

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Fig. 1. (a) XRD pattern of ZnS and modified g-C3N4 samples; (b) FT-IR spectra of all ZnS, pure g-C3N4 and their composites; (c) spectra of N2 adsorption-desorption isotherms, and (d) pore size distributions.

The size, morphology, and uniform distribution of ZnS can make sure that the surface of ZnS exposes more active sites, leading to better photocatalytic performances. The lattice fringe spacing of ZnS nanoparticles was observed by high-resolution TEM (HRTEM), which is around 0.32 nm in the amplified image of Fig. 3c, matching the (0 0 8) plane of wurtzite ZnS. This result is in agreement to the XRD pattern in Fig. 1a. As can be seen in Fig. 3d–f, the mapping results can prove that ZnS nanoparticles were successfully synthesised with a high dispersed degree. Meanwhile, TEM images of ZMCN, ZDCN and ZUCN exhibit that the spherical ZnS nanoparticles were randomly distributed on the thin surface of sheet-like 2D MCN, DCN and UCN, forming intimate heterojunctions via the processes of impregnation and annealing in Fig. 4a–c. According to Fig. 4e and f, the lattice fringe spacing of loaded ZnS can be found on the surface of MCN after the impregnation approach. Thus, the photogenerated electrons and holes can be efficiently separated owing to the high-quality heterojunctions on ZMCN and ZDCN. Moreover, the mapping results of all the ZnS modified g-C3N4 were tested and are shown in Fig. S1, S2 and S3, proving the intimate contact of ZnS with g-C3N4. UV-Vis spectra of ZnS modified g-C3N4 are shown in Fig. 5. Three pure g-C3N4 prepared by melamine, dicyanamide and urea show their photo absorption edges at around 480 nm. Fig. 5a shows the slight 1red-shift of g-C3N4 based composites compared to their own pristine g-C3N4, because of the introduction of ZnS nanoparticles. Fig. 5b shows that the visible light absorption 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.

properties of ZMCN and ZUCN are better than ZDCN, while there is no an obvious difference between ZDCN and DCN in terms of optical properties. All the bandgap energies for pristine g-C3N4 and ZnS were n calculated by Tauc’s equation: ðahvÞ ¼ kðhv  Eg Þ, where n ¼ 1=2and2 were respectively interpolated into the function to calculate the bandgap energies for pure g-C3N4 (direct semiconductor) and ZnS (indirect semiconductor) in Fig. 5c and d. [32,33]. These results are listed in Table 1. 3.2. Photocatalytic hydrogen production In this study, D-+-glucose was used as a simplified biomass for the photocatalytic reforming reactions. In the bench scale tests, 50 mL glucose (50 ppm) solution in a reactor was employed for the hydrogen generation to provide fundamental understanding of the feasibility. H2 generation performances on the three different pristine g-C3N4 and their modified samples (ZMCN, ZDCN and ZUCN) were evaluated by the photocatalytic reforming at 30 °C under 3 h irradiations. In addition, the performance of ZnS was tested and the results are shown in Fig. S4. The cocatalyst of ZnS is a significant factor to modify two pure g-C3N4 samples of MCN and DCN to achieve higher H2 generation. As shown in Fig. 6a, pristine MCN had the lowest H2 evolution of 83.4 lmol.g1 while UCN showed the highest H2 generation in 3-h reaction among all of bare g-C3N4 samples. In contrast, modified MCN achieved the best H2 evolution performance among all the tested photocatalysts, including ZUCN, ZDCN, UCN, DCN and MCN. This might be because of the well formation of

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Fig. 2. XPS spectra for ZnS and modified MCN samples. (a) C 1s, (b) N 1s, (c) Zn 2p, and (d) S 2p.

heterojunctions between ZnS and MCN, promoting the separation of photo-generated electrons and holes. The hydrogen generation of ZMCN increases around 2.51 times than bare MCN, suggesting that the heterojunction can efficiently hinder the recombination of electrons and holes. Besides, the H2 generation performance of ZDCN was also improved for approximate 1.72 times than the bare DCN. Another interesting result is that the efficiency of ZUCN has a sharp decrease around 36.6% than the pristine UCN, becoming the sample with the lowest hydrogen generation performance. ZUCN has a larger BET (77.14 m2/g) than UCN (56.20 m2/g), so the declined H2 evolution on ZUCN as compared to UCN cannot be ascribed to the microstructure. Therefore, the wider band gap of UCN (2.81 eV) and the associated CB position change, which is closer to CB of ZnS than those of MCN and DCN, might be the reason for the decreased H2 evolution on ZUCN. Three times successive runs were proceeded based on ZMCN under the same condition as each hydrogen generation test cycle. Each time-circle hydrogen generation was performed after 30 min purge with ultra-pure N2, reducing the effect from dissolved oxygen. The obvious decrease of hydrogen amount of ZMCN in three times reactions was not significant, suggesting that a robust heterojunction was formed on ZMCN as shown in Fig. 6b. Moreover, this heterojunction can prolong the lifetime of ZMCN in the photocatalytic process. After that, different concentrations of glucose with ZMCN were tested and investigated for optimizing the amount of hydrogen generation during the photocatalytic reforming process. For instance, 0, 5, 10 and 30 ppm glucose solutions can promote the

photocatalytic hydrogen evolution. Interestingly, 10 ppm glucose solution can support and boost the hydrogen generation performance, giving the highest efficiency during this process. These results in Fig. S5 can firmly prove that the hydrogen generation process is supported by the engagement of glucose. 3.3. Photocatalytic mechanism The efficiency of charge transfers on the interface between ZnS and pristine g-C3N4 was evaluated by photoluminescence (PL) and electrochemical impedance spectroscopy (EIS). Fig. 7a shows the PL spectra of pure g-C3N4 and ZnS modified samples. These modified nanomaterials exhibited the same tendency at the 320 nm excitation wavelength. The main peak raised at 460 nm was attributed to the transition and recombination of photogenerated electron-hole pairs [34]. Pristine UCN has the highest charge carrier separation efficiency than pure MCN and DCN. This result was consistent well with the H2 evolution results of three different pure g-C3N4 (Fig. 6a.). ZMCN and ZDCN had lower intensities than ZUCN and their pure g-C3N4 counterparts, indicating that their heterojunctions can efficiently impede the recombination of electron-hole pairs after the excitation. According to Fig. 7a, the intensities of PL of pure g-C3N4 and ZnS modified g-C3N4 exhibit the same tendency as the hydrogen generations. Meanwhile, it can be found that the intensity of ZUCN almost overlaps UCN, exhibiting the low efficiency and negative effect of ZnS modification on ZUCN. On the contrary, the intensity of ZMCN was higher enough than pure MCN. It is interesting to note that the modified

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Fig. 3. TEM images of ZnS nanoparticles (a-c) and STEM-EDX element mapping of ZnS (d-f).

Fig. 4. TEM images of ZMCN, ZDCN and ZUCN (a-c) and HRTEM of ZMCN (e-f).

MCN obtained an efficient heterojunction after modification but the modified UCN did not. High quality heterojunction of ZMCN can also be proven by EIS which is an electrochemical method to

estimate the resistances of interfacial charge transport. According to the spectra of Fig. 7b, the arc radius of ZMCN is smaller than those of ZUCN and ZDCN, showing the low charge transfer

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Fig. 5. UV-vis DRS spectra of all samples (a), ZnS modified DCN (b), Tauc’s plots for direct pure g-C3N4 (c), and pure ZnS nanoparticles (d).

Table 1 The bandgap energy of pure g-C3N4 and ZnS. Samples

Bandgap (eV)

Valence Band (eV)

Conduction Band (eV)

MCN DCN UCN ZnS

2.67 2.69 2.81 3.51

1.81 1.90 1.91 1.45

0.86 0.79 0.90 2.06

resistance and high conductivity owing to the formation of high efficient heterostructure. Also, EIS of pure g-C3N4 was also tested and are exhibited in Fig. S6 to show the differences among these pristine materials. The band structures of these ZnS modified composites were illustrated and are compared in Fig. 7c–e. The data for all band structures were calculated on the basis of XPS valence test (Fig. S7) and as shown in Table 1.

Fig. 6. (a) H2 evolution on pure g-C3N4 and ZnS modified samples and (b) stability tests of ZMCN under 300 W Xe lamp for three times.

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Fig. 7. Comparison of (a) PL spectra of all pure g-C3N4 and ZnS modified samples, (b) EIS of all ZnS modified g-C3N4. Schematic band structures for (c) ZMCN, (d) ZDCN and (e) ZUCN.

The mechanism of photocatalytic reforming of glucose aqueous solution on ZMCN is proposed and shown in Fig. 8. The photogenerated electrons move to CB from VB of ZnS after the excitation by solar energy and these electrons might move to the CB of MCN from the loaded ZnS because of the higher potential of CB in MCN. Besides, the solar energy can also excite the electrons from VB of MCN. The water reduction reaction can be proceeded during this process which can also produce hydrogen. The reforming process are conducting with the engagement of holes, which are left on the VB of ZnS and MCN. The holes of ZnS would immigrate to the VB of MCN due to the lower potential of VB in MCN. These holes with positive charges have an important role to move further for the reformation of glucose because they can directly oxidise glucose to generate the electrons during the oxidisation process of glucose aqueous solution [5]. Moreover, the electrons will be donated by the reformation of glucose due to the oxidisation of glucose. Meanwhile, arabinose, erythrose, gluconic and formic acid can be obtained by the oxidization of glucose [35,36]. These electrons will immigrate to the surface of ZnS and MCN to drive the reduction reaction. Therefore, photocatalytic reforming process can work successfully with the mechanism as above. 4. Conclusions In conclusion, ZnS nanoparticles with a size around 10–15 nm can efficiently enhance the photocatalytic properties of carbon nitride nanosheet derived from melamine and dicyanamide after the loading. ZnS nanoparticles might also introduce some negative effects to the optical property of ZUCN, which decrease the

Fig. 8. The mechanism of photocatalytic reforming of glucose on ZMCN.

efficiency of photocatalytic reforming. The characterisation results suggested that the introduction of ZnS nanoparticles leads to the red shift and better light absorption abilities compared to pristine MCN and DCN. The highest quality heterojunction can work well to hinder the recombination of charge carriers after the introduction of ZnS on MCN and as a result, enhanced hydrogen generation during photocatalytic reforming process was confirmed. This study provides some promises for better utilisation of solar energy and

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biomass, both of which are abundant and renewable, toward a sustainable future.

[16]

Acknowledgement [17]

The work was partially supported by Australian Research Council (DP170104264). All the authors in this work acknowledge for the support from Centre for Microscopy, Characterization and Analysis, University of Western Australia; Electron Microscope Facility, Curtin University.

[19]

Appendix A. Supplementary material

[20]

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.07.066. [21]

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