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graphitic carbon nitride microspheres promoting photocatalytic activity
Accepted Manuscript Type II heterojunction in hierarchically porous zinc oxide/graphitic carbon nitride microspheres promoting photocatalytic activity Sijia Wu, Hong-Juan Zhao, Chao-Fan Li, Jing Liu, Wenda Dong, Heng Zhao, Chao Wang, Yang Liu, Zhi-Yi Hu, Lihua Chen, Yu Li, Bao-Lian Su PII: DOI: Reference:
S0021-9797(18)31396-1 https://doi.org/10.1016/j.jcis.2018.11.076 YJCIS 24341
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
10 September 2018 10 November 2018 19 November 2018
Please cite this article as: S. Wu, H-J. Zhao, C-F. Li, J. Liu, W. Dong, H. Zhao, C. Wang, Y. Liu, Z-Y. Hu, L. Chen, Y. Li, B-L. Su, Type II heterojunction in hierarchically porous zinc oxide/graphitic carbon nitride microspheres promoting photocatalytic activity, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/ j.jcis.2018.11.076
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Type II heterojunction in hierarchically porous zinc oxide/graphitic carbon nitride microspheres promoting photocatalytic activity
Sijia Wu1, Hong-Juan Zhao2, Chao-Fan Li1, 3, Jing Liu1,*, Wenda Dong1, Heng Zhao1, Chao Wang1, Yang Liu1, Zhi-Yi Hu1, 3, *, Lihua Chen1, Yu Li1, 3, * and Bao-Lian Su1, 4, 5
1
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of
Technology, 122 Luoshi Road, 430070 Wuhan, Hubei, China. 2
Lanzhou Petrochemical Research Center, Lanzhou, China, 730060.
3
Nanostructure Research Centre (NRC), Wuhan University of Technology, 122 Luoshi Road, 430070 Wuhan,
Hubei, China. 4
Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, 61 rue de Bruxelles, B-5000 Namur,
Belgium. 5
Clare Hall, University of Cambridge, Herschel Road, Cambridge CB3 9AL, United Kingdom.
*
Corresponding author. Email: [email protected], [email protected] and [email protected].
1
Abstract Graphitic carbon nitride (g-C3N4) is a visible light active semiconductor. However, low conductivity and high recombination rate of photogenerated electrons and holes limit its application in photocatalysis. In this work, we design and synthesize hierarchically porous zinc oxide/ graphitic carbon nitride (ZnO/g-C3N4) microspheres with type-II heterojunction to effectively degrade rhodamine B (RhB) via increasing the charge-separation efficiency. The ultraviolet-visible (UV-Vis) absorption spectra, Mott-Schottky plots and valence band X-ray photoelectron spectroscope confirm the formation of type-II heterojunction between ZnO nanocrystals and g-C3N4 nanosheets. As a result, the 1.5-ZnO/g-C3N4 composite (the mass ratio of zinc acetate dihydrate to g-C3N4 is 1.5) exhibits the highest photocatalytic activity with good stability and higher photocatalytic degradation rate comparing to pure g-C3N4 and pure ZnO. In addition, our results confirm that •O2- and h+ are the main active species for ZnO/g-C3N4 in degradation of RhB.
Key Words: Hierarchically porous structure; ZnO/g-C3N4; Heterojunction; Photocatalytic activity; Active species
2
1 Introduction In recent years, photocatalysis has attracted much attention in water depuration owing to the discovery of plentiful photocatalysts, which work in an eco-friendly way with the solar energy as a driving force. Graphitic carbon nitride (g-C3N4) stands out as an extraordinary two dimensional non-metal photocatalyst containing earth-abundant elements [1]. The basic structural unit of typical g-C3N4 is tri-s-triazine, which can be formed by the polycondensation of accessible precursors like urea and melamine [2-4]. The sp2-hybridized carbon and nitrogen establish a superior π-conjugated electronic structure of g-C3N4 [5]. As a result, g-C3N4 acts as a visible light responsive photocatalyst with the band gap of ~2.8 eV [6]. However, the poor conductivity and the easily recombination of photogenerated charges of g-C3N4 limits its further application in photocatalysis [7]. Various methods have been developed to overcome the weakness of g-C3N4, primarily including the band structure optimization and the heterojunction construction [8-11]. Elemental doping is a general method of band structure optimization, such as O-doping and P-doping, widening the light-absorbing regions and promoting charge separation of g-C3N4 by introducing defect state [12, 13]. As for heterojunction structure constructing between g-C3N4 and other suitable semiconductors, type-II heterojunction is considered as one of the most efficient heterojunction in photocatalysis [14]. In the type-II heterojunction, the band bending generated at the interface between two semiconductors brings a built-in electric field, leading to the inverse migration of photogenerated electrons and holes [15, 16]. Recently, constructing a type-II heterojunction between g-C3N4 and a wide band gap semiconductor becomes more popular due to the wide band gap semiconductor being more beneficial to charge separation [17]. For example, Boonprakob et al. constructed type-II heterojunction between g-C3N4 and titanium dioxide (TiO2), demonstrating efficiently enhanced degradation activity on methylene blue under visible light irradiation in 3
comparison with pure TiO2 [18]. Sun et al. successfully composited g-C3N4 with ZnO via a one-step method [19]. The photocatalytic performance of g-C3N4-ZnO is 3 and 6 times higher than that of pure g-C3N4 for photodegradation of methyl orange and p-nitrophenol, respectively. Therefore, constructing a type-II heterojunction between g-C3N4 and other wide band gap semiconductors is plausible to remove the organic pollutants. In this work, we report a hierarchically porous ZnO/g-C3N4 composite for effectively photodegradation of RhB and phenol. The ZnO/g-C3N4 composite was synthesized via a two-step method. The wide band gap ZnO was used to form a heterojunction structure with g-C3N4 owing to the prominent electron mobility, high quantum efficiency and diversiform morphology of hexagonal wurtzite ZnO [20-22]. Our results show that the conduction band (CB) of g-C3N4 is more negative than that of ZnO, and the valence band (VB) of ZnO is more positive than that of g-C3N4. As a result, the photogenerated electrons transfer from g-C3N4 to ZnO and the holes transfer from ZnO to g-C3N4 to enhance the photocatalytic activity via the effective charge separation. Furthermore, the photocatalytic degradation mechanism of the composite was studied. It shows that the main active species in ZnO/g-C3N4 for degradation of RhB are •O2- and h+. 2 Experimental section 2.1 Materials Melamine (2,4,6-triamino-1,3,5-triazine, AR), ethylene glycol ((CH2OH)2, AR), zinc acetate dihydrate (Zn(CH3COO)2·2H2O, AR), phenol (C₆H₆O, AR), potassium iodide (KI, AR) and tert-butyl alcohol (C4H10O, GC) were purchased from the Aladdin Reagent Company. Hydrochloric acid (HCl, AR, 36~38 wt%) was purchased from the Xinyang Reagent Factory. Ethanol (C2H6O, AR), rhodamine B (C28H31ClN2O3, AR) and benzoquinone (C6H4O2, AR) were purchased from the 4
Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received. 2.2 Preparation of g-C3N4 nanosheets In this work, g-C3N4 was prepared by thermal polymerization method. Briefly, 1.25 g of melamine was added into 10 mL of alcohol, followed by a dropwise addition of 1 mL of HCl with a drastic stir for 1 h. The white mixture was dried at 40 °C for 5 hours, followed by calcining at 500 °C for 1 h and 520 °C for 3 h with the heating rate of 5 °C/min. Finally, a fluffy yellow g-C3N4 powder was obtained. 2.3 Preparation of ZnO/g-C3N4 hierarchical porous spheres ZnO/g-C3N4 was synthesized through solvothermal method. 0.1 g of g-C3N4 was firstly dispersed in 40 mL of ethylene glycol to ultrasound exfoliate for several hours. Triplicate dispersion of g-C3N4 was prepared for subsequent use. 0.05 g, 0.15 g and 0.25 g of zinc acetate dehydrate, of which the mass was 0.5, 1.5 and 2.5 to that of g-C3N4, was added to aforementioned dispersion, respectively. After stirring for 2 hours, the obtained solution was transferred into Teflon-lined autoclave and heated at 180 °C for 90 min. The yellow precipitate was collected, washed by deionized water for several times and dried at 40 °C overnight, followed by annealing at 300 °C for 1 h with the heating rate of 2 °C/min. The prepared samples were denoted as 0.5-ZnO/g-C3N4, 1.5-ZnO/g-C3N4 and 2.5-ZnO/g-C3N4, respectively. 2.4 Preparation of ZnO nanoparticles The pure ZnO was prepared by dissolving 10 mmol zinc acetate dehydrate in 40 mL of ethylene glycol and heating at 180 °C for 90 min. The following process was the same as the aforementioned treating of ZnO/g-C3N4. 2.5 Characterizations 5
A bruker diffractometer with Cu Kα radiation (λ = 1.540596 Å) was used to measure powder X-ray diffraction (XRD) patterns, operated at 40 kV and 40 mA. The morphologies of samples were observed by a Hitachi S-4800 field emission scanning electron microscope (SEM) at an acceleration voltage of 5.0 kV. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and energy dispersive spectroscopy (EDS) were performed on FEI Talos F200S microscope fitted with Super-X EDX system, operated at 200kV. Brunauer–Emmett–Teller (BET) method was carried out to detect the adsorption branch of the isotherm based on the Barett–Joyner–Halenda (BJH) and determine the specific surface area and pore size distribution of the samples. The surface chemical composition and valence states of products were studied using X-ray photoelectron spectroscope (XPS, Thermo Scientific Escalab 250Xi) with monochromatic Al Kα radiation. The binding energies were calibrated versus the C 1 s peak (284.8 eV). Room-temperature UV–vis absorption spectra of samples were recorded using a UV-2550 Shimadzu spectrophotometer. Photoluminescence emission studies (PL) were recorded by a PerkinElmer Lambda LS 55 fluorescence spectrometer at room temperature with a Xe lamp excitation at 320 nm. The degradation products were detected by gas chromatography-mass spectrometer (GC-MS, Agilent 7890B-5977B). Fourier transform infrared spectroscopy (FTIR) was detected by Nicolet FTIR 6700. The Mott-Schottky plots were measured by Autolab 86005 to investigate the Fermi level of samples. The zeta potential of sample was determined by Nano-ZS ZEN3600 zetasizer. The photocurrent of the samples was detected by electrochemical workstation (CH1660D) with 0.5 mol/L of NaSO4 aqueous solution as electrolyte under UV-Vis light. 2.6 Photocatalytic activity 6
Rhodamine B (RhB) and phenol were used as targeted pollutants to evaluate the photocatalytic activity of the synthesized materials. Typically, 100 mg of photocatalyst was added into RhB aqueous solution (100 mL, 10-5 mol/L) in a 250 mL beaker under stirring condition at room temperature. Before illumination, the reactants were stirring in dark for 1 h to reach an adsorption-desorption equilibrium of RhB molecular on photocatalyst surface. The reactor was irradiated under simulated solar light (250nm-780nm, PLS-SXE-300UV with a visible light intensity of 158 mW cm-2, and an ultraviolet (UV) light intensity of 34 mW cm-2, Beijing).
3 Results and discussion
Fig. 1. (a) Schematic illustration of the synthetic process of hierarchical porous ZnO/g-C3N4 microspherical heterostructure (the grey spheres are ZnO and the dark yellow sheets are g-C3N4). SEM images of (b) pure g-C3N4 prepared by sintering, (c) pure ZnO and (d) 1.5-ZnO/g-C3N4.
Fig. 1a displays the schematic synthetic process of hierarchically porous ZnO/g-C3N4 microspheres, which were formed via gathering ZnO nanoparticles with g-C3N4 nanosheets to reduce surface energy. The morphologies of pure ZnO, pure g-C3N4 and 1.5-ZnO/g-C3N4 samples 7
were observed by FESEM. Fig. 1b shows that the pure g-C3N4 is in morphology of nanosheets. The pure ZnO presents a spherical shape (Fig. 1c), formed by 20 nm ZnO nanoparticles (Fig. 1c inset). Fig. 1d presents the typical SEM image of 1.5-ZnO/g-C3N4, showing the porous microsphere with a size of ~700 nm. The morphologies of 0.5-ZnO/g-C3N4 and 2.5-ZnO/g-C3N4 are also respectively shown in Fig. S1. It can be seen that the diameter (~300 nm for 0.5-ZnO/g-C3N4, and 1~2 μm for 2.5-ZnO/g-C3N4) and the compactness of ZnO/g-C3N4 microspheres increase with the ZnO content rising. According to the previous report, this hierarchically porous microstructure is beneficial for light harvesting and dye absorption [23].
Fig. 2. (a) HAADF-STEM image, (b) HRTEM image (ZnO nanoparticles are indicated by red dashed circles), (c) the corresponding SAED pattern of the whole area in (a), the indicated diffraction rings as below, 1: C 3N4 0002; 2: 8
ZnO
; 3: ZnO
; 4: ZnO
; 5: ZnO
; 6: ZnO
; 7: ZnO
, (d-f) the corresponding
EDX elemental maps: C (blue), N (green) and Zn (red).
TEM is further conducted to investigate the microstructure of the ZnO/g-C3N4 nanocomposites. The HAADF-STEM image (Fig. 2a) shows a typical hierarchically porous spherical structure of 1.5-ZnO/g-C3N4. The corresponding selected area electron diffraction (SAED) pattern (Fig. 2c) presents a series of ZnO crystal diffraction rings (hexagonal ZnO ,
,
,
,
,
respectively, indicated by arrows) and g-C3N4-0002 diffraction ring, suggesting the
hybrid structure. In order to understand the details of heterojunction, HRTEM is used to investigate the interface between ZnO and g-C3N4. Fig. 2b reveals that the ZnO nanoparticles (indicated by red dashed circles) deposit on g-C3N4 nanosheets. This ensures the formation of heterojunction between ZnO and g-C3N4 for the efficient charge separation. As g-C3N4 is easily damaged under the electron beam irradiation, the crystal structure of g-C3N4 cannot be observed in HRTEM image, whereas the diffraction ring of g-C3N4 can be observed due to the extremely low electron dose in electron diffraction mode. In addition, the EDX spectrum (Fig. S2) and STEM-EDX elemental maps (Fig. 2d-f) demonstrate that ZnO and g-C3N4 are homogenously distributed in the hierarchically porous spherical structure. This gives further evidence that the ZnO nanoparticles have successfully decorated on g-C3N4 to form a heterostructure. The specific surface area and pore diameter of the samples were measured by N2 adsorption/desorption isotherms (Fig. 3). The plot of the pore size distribution was determined by BJH analysis of the desorption branch of the isotherm (Fig. 3 insets). The shape of hysteresis loop of ZnO/g-C3N4 with different ZnO content indicates the hierarchically porous structure deriving from the stacking of ZnO nanoparticles and g-C3N4 nanosheets. The larger hysteresis loop of ZnO/g-C3N4 with different ZnO content at relative high pressure suggests the richer mesoporosity 9
comparing to pure g-C3N4. The specific surface area of 0.5-ZnO/g-C3N4, 1.5-ZnO/g-C3N4 and 2.5-ZnO/g-C3N4 is 32.7, 29.8 and 32.5 cm2/g respectively, about three times higher than pure g-C3N4 (9.9 cm2/g).
Fig. 3. N2 adsorption/desorption isotherms of (a) g-C3N4, (b) 0.5-ZnO/g-C3N4, (c) 1.5-ZnO/g-C3N4, (d) 2.5-ZnO/g-C3N4 (Inset: corresponding BJH pore distribution curve).
The crystalline phases of the ZnO, g-C3N4 and ZnO/g-C3N4 samples were further investigated by XRD (Fig. 4a). The pure ZnO with three strong diffraction peaks at 31.7º, 34.5º and 36.2º defines a hexagonal wurtzite structure (JCPDS No. 36-1451) of
, (0002) and
crystal planes, respectively. For pure g-C3N4, the strong diffraction peak at 2θ = 27.5º represents the typical (0002) plane of aromatic system [24]. The ZnO/g-C3N4 composites demonstrate the characteristic peaks of both g-C3N4 and ZnO, in accordance with the above TEM results. Fig. 4b shows the FTIR spectra of the ZnO, g-C3N4 and ZnO/g-C3N4 samples. For g-C3N4, the peaks at 1637, 1242 and 808 cm-1 symbolize the C=N, C–N stretching vibrations and s-triazine ring 10
vibrations, respectively [25, 26]. The peaks in the region from 400 to 600 cm-1 are the intrinsic absorption of ZnO, originating from the lattice vibration [27]. Again, the FTIR spectra of ZnO/g-C3N4 demonstrate the main characteristic peaks of ZnO and g-C3N4, indicating the successful formation of the ZnO/g-C3N4 heterostructure.
Fig. 4. (a) XRD patterns and (b) FTIR spectra of a: g-C3N4, b: 0.5-ZnO/g-C3N4, c: 1.5-ZnO/g-C3N4, d: 2.5-ZnO/g-C3N4 and e: ZnO.
The XPS was performed to further illucidate the chemical composition and the surface properties of the as-prepared samples. Fig. S3 presents the survey scan spectrum of 1.5-ZnO/g-C3N4, showing the peaks of Zn, O, C and N. Fig. 5 displays the high resolution spectra of C 1s, N 1s, O 1s and Zn 2p of g-C3N4, ZnO and 1.5-ZnO/g-C3N4 samples. The C 1s spectrum of g-C3N4 can be deconvolved to three peaks at 288.8, 288.3 and 284.8 eV (Fig. 5a), corresponding to the carbon in O-C=O [13], sp2-hybridized carbon in N-containing aromatic ring (N=C-N) and sp2 C-C bonds, respectively [28, 29]. The other two emerged C 1s peaks in the spectrum of 1.5-ZnO/g-C3N4 can be identified as C-O (286.9 eV) and Zn-C (283.3 eV), respectively [30, 31]. The decreased binding energy of C atom in N=C-N indicates a weaker band strengths of C=N and C-N in 1.5-ZnO/g-C3N4, resulting from a wider conjugated system containing ZnO and g-C3N4 with Zn-C bond, which is beneficial for the charge transfer between ZnO and g-C3N4. This result confirms the formation of heterojunction between ZnO nanoparticles and g-C3N4 nanosheets. 11
Fig. 5. High resolution XPS spectra of (A) g-C3N4, (B) 1.5-ZnO/g-C3N4 and (C) ZnO: (a) C 1s; (b) N 1s; (c) O 1s; and (d) Zn 2p.
Fig. 5b presents the N 1s spectra of g-C3N4 and 1.5-ZnO/g-C3N4. The peaks of g-C3N4 at 398.5, 400.2 and 401.0 eV are attributed to sp2-hybridized nitrogen existing in triazine rings (C-N=C), the tertiary nitrogen N-(C)3 groups and the free amino groups (C-N-H), respectively [28, 32]. Comparing to pure g-C3N4, the C-N=C and C-N-H in 1.5-ZnO/g-C3N4 shifts to a higher value of 398.9 eV and 401.5 eV, owing to the aforementioned conjugated system containing ZnO and g-C3N4, decreasing the electron density around nitrogen. Fig. 5c displays the O 1s spectra of ZnO and 1.5-ZnO/g-C3N4. The peaks of ZnO at 530.2 eV and 531.7 eV are respectively assigned to O2and Ox- (O- and O2 -) generated from the oxygen vacancy [33, 34]. These two peaks shift to a higher binding energies resulted from the strong interaction between ZnO and g-C3N4. Fig. 5d presents the Zn 2p spectra of ZnO and 1.5-ZnO/g-C3N4. The peaks at 1044.1 eV and 1021.2 eV are attributed to 12
the Zn 2p1/2 and 2p3/2, respectively [34]. Compared to the pure ZnO, the binding energy of Zn 2p1/2 (1044.5 eV) and 2p3/2 (1021.4 eV) increases a little, indicating the strong interaction between ZnO and g-C3N4. Again, another two peaks at 1046.0 eV and 1022.9 eV appear in the Zn 2p spectrum of 1.5-ZnO/g-C3N4, which could be assigned to the Zn 2p1/2 and 2p3/2 of Zn-C according to the above results from C 1s spectra. As mentioned above, the formed heterostructure between ZnO and g-C3N4 could bring a built-in electric field to effectively separate the photogenerated electrons and holes.
Fig. 6. (a) UV–vis absorption spectra of ZnO, g-C3N4 and ZnO/g-C3N4 samples with different ZnO content. The insert spectrum is the Tauc plots of ZnO and g-C3N4. (b) PL spectra of ZnO/g-C3N4 and g-C3N4 samples.
Fig. 6a presents the UV–visible absorption spectra of the ZnO, g-C3N4 and ZnO/g-C3N4 samples. It distinctly shows that the absorption edges of ZnO sample and g-C3N4 sample present at ~390 nm and ~448 nm, respectively. The absorption edges of the ZnO/g-C3N4 samples are nearly the same as the g-C3N4 sample, indicating no obvious change of the bandgap of ZnO and g-C3N4. This further confirms the formation of ZnO/g-C3N4 heterostructure. Fig. 6a inset displays the Tauc plots of g-C3N4 and ZnO, showing the electronic bandgap of 2.86 eV and 3.23 eV, respectively. Fig. 6b displays the PL spectra of g-C3N4 and ZnO/g-C3N4 to reveal the recombination rate of the photogenerated charges. It shows that the ZnO/g-C3N4 samples exhibit higher charge separation efficiency than the g-C3N4 sample. This means that the formation of heterostructure in the 13
ZnO/g-C3N4 composite is helpful to effectively improve the charge separation in g-C3N4. The photocatalytic activity of the ZnO, pure g-C3N4 and ZnO/g-C3N4 samples was evaluated by the photodegradation of RhB under simulated solar light irradiation. The pH value of RhB solution in this work was measured to be 7.3. The 1.5-ZnO/g-C3N4 sample was selected as a representative to detect the surface potential of samples under this pH value in water. The result shows that the surface potential of 1.5-ZnO/g-C3N4 is -20.4 mV (Fig. S4). This means that RhB as a cationic dye has the ability to attract negatively charged nanoparticles at this pH value [35]. Therefore, RhB molecules tend to be adsorbed on the surface of 1.5-ZnO/g-C3N4, which is beneficial for the photodegradation of RhB. Fig. 7a presents the relationship between C/Ci and illumination time (Ci is the initial concentration of RhB before dark reaction and C is the concentration of RhB at sampling time). It shows that 9.5%, 13.7%, 18.5%, 19.7% and 22.1% of RhB is respectively adsorbed by ZnO, 2.5-ZnO/g-C3N4, 1.5-ZnO/g-C3N4, 0.5-ZnO/g-C3N4 and g-C3N4 in dark after 60 min, respectively. This result indicates that g-C3N4 is plausible for RhB adsorption. After 30 min in UV-visible light irradiation, 73.7%, 97.6%, 100%, 97.7% and 93.8% of RhB was removed by ZnO, 2.5-ZnO/g-C3N4, 1.5-ZnO/g-C3N4, 0.5-ZnO/g-C3N4 and g-C3N4 for photodegradation, respectively. It can be seen that the 1.5-ZnO/g-C3N4 sample exhibits the best photodegradation activity, where RhB can be photodegrade in 30 min. Fig. 7b illustrates the fitted experimental data according to the pseudo first-order kinetic equation ln(C/C0) = -kt (C0 is the concentration of RhB at the end of dark reaction, k is the reaction rate constant). The 1.5-ZnO/g-C3N4 sample has the highest reaction rate of 0.102 min-1, compares to that of the pure g-C3N4 (0.081 min-1), 0.5-ZnO/g-C3N4 (0.092 min-1), 2.5-ZnO/g-C3N4 (0.091 min-1) and pure ZnO (0.047 min-1). The photocatalytic stability of 1.5-ZnO/g-C3N4 under simulated solar light was also investigated. After every 30 min for degradation of RhB, the sample was collected and dried to repeat the degradation next time. Fig. 7c 14
shows that after 5 cycles, the degradation rate is still as high as 95.2%. In addition, after 5 cycles, there is no change on the structure of 1.5-ZnO/g-C3N4 according to the XRD pattern shown in Fig. S5, suggesting the very good stability under simulated solar light irradiation.
Fig. 7. (a) C/Ci and (b) First order rate constant k (min-1) of ZnO, ZnO/g-C3N4 and g-C3N4 samples for RhB under UV-vis light. (c) Stability test of 1.5-ZnO/g-C3N4 over five consecutive cycles under UV-vis light. (d) C/Ci and (e) First order rate constant k (min-1) of ZnO, ZnO/g-C3N4 and g-C3N4 samples for RhB under visible light. (f) Transient photocurrent response of different samples. C/C0 of (g) 1.5-ZnO/g-C3N4, (h) pure g-C3N4 and (i) pure ZnO for RhB after adding different scavengers.
In addition, the photocatalytic activity of pure g-C3N4, ZnO and ZnO/g-C3N4 under visible light (λ > 420 nm) was further investigated (Fig. 6d). It displays that ZnO has little ability for degradation under visible light. Again, 1.5-ZnO/g-C3N4 presents a higher photocatalytic activity (k=0.062 min-1) than that of pure g-C3N4 (k=0.055 min-1), 0.5-ZnO/g-C3N4 (k=0.060 min-1) and 15
2.5-ZnO/g-C3N4 (k=0.230 min-1) (Fig. 7e). This confirms that the formation of heterojunction between g-C3N4 and ZnO is beneficial to the enhancement of photocatalytic activity. The transient photocurrent response experiment was further studied to convince the charge transfer properties of g-C3N4 and 1.5-ZnO/g-C3N4 (Fig. 7f). It shows that the photocurrent of ZnO/g-C3N4 increased with the rising content of ZnO, demonstrating an easier migration and separation of photogenerated charges in ZnO/g-C3N4, in accordance with the PL results. This is because the high electronic transmission efficiency of ZnO is in favor of the electron transport from ZnO/g-C3N4 to FTO (fluorine-doped tin oxide) film [36], leading to the superior transient photocurrent response of ZnO/g-C3N4 with higher ZnO content. Generally, the possible active species for photodegradation of RhB include hydroxyl radicals (•OH), superoxide anion radicals (•O2 -) and photogenerated holes (h+) [37]. Therefore, different scavengers are used to investigate the photocatalytic mechanism of ZnO, g-C3N4 and 1.5-ZnO/g-C3N4. When the h+ scavenger (potassium iodide, KI) or •O2- scavenger (benzoquinone, BQ) is added into reaction system, the degradation rate obviously decreases (Fig. 7g), indicating that both h+ and •O2- are evidently conductive to the degradation reaction. Specifically, RhB scarcely be degraded at the presence of BQ, demonstrating that •O2- is the primary active specie in this reaction. Whereas, the •OH scavenger (tert-butanol, t-BuOH) initiates little change in degradation process, suggesting that •OH is a neutral specie. The active species of pure g-C3N4 and ZnO are investigated to explain why •O2- and h+ play dominant roles but •OH is inessential in degradation of RhB, as displayed in Fig. 7h~i. It turns out that •O2- and h+ act as the main active species for g-C3N4. However, the role of •O2 -, h+, and •OH cannot be all neglected for ZnO. Furthermore, the potential energy (1.58 V vs RHE) of holes in g-C3N4 is higher than that of OH-/•OH (1.99 V) and H2O/•OH (2.34 V) couples [38]. This means that the holes in g-C3N4 cannot 16
oxidize OH- or H2O into •OH. However, the potential energy of holes in ZnO is low enough to oxidize OH- or H2O into •OH. This explains the role of •OH for pure ZnO. Therefore, the effect of •OH scavenger on RhB degradation for ZnO, g-C3N4 and 1.5-ZnO/g-C3N4 indicates that the holes in the VB of ZnO can transfer to VB of g-C3N4, suggesting the existence of a type-II heterojunction. The degraded products of RhB were further detected by GC-MS (Fig. S7). Apart from the same peaks as H2O, four exclusive peaks are observed on the curve of degraded RhB solution sample. The peaks at 4.51, 21.30 and 24.53 min are probably 4-methoxy-alpha-methyl benzeneethanamine, benzaldehyde and dl-phenylephrine, respectively (Table S1). These three products still contain one benzene ring in their molecular. However, the peak at 20.34 min is identified as propanamide, a micromolecule with no benzene rings. Therefore, RhB is indeed degraded by the photocatalyst. To extend the application of our photocatalysts, we also compared the photodegradation performance of colorless phenol on ZnO, pure g-C3N4 and 1.5-ZnO/g-C3N4. The result is show in Fig. S6. It can be seen that the 1.5-ZnO/g-C3N4 sample exhibits a higher photodegradation activity than that of pure g-C3N4 and ZnO. This confirms the superiority of hierarchically porous ZnO/g-C3N4 microspheres with type II heterojunction. The Fermi levels of g-C3N4, ZnO and 1.5-ZnO/g-C3N4 are then investigated by Mott-Schottky plots measured at 500 Hz and 1000 Hz (Fig. 8a-c). The calculated flat band potential (V fb) for g-C3N4 and ZnO is -0.80 V and -0.26 V, respectively. This means that the Fermi level of ZnO is lower than that of g-C3N4. XPS are further carried out to determine the distance between the VB maximum and the Fermi level for g-C3N4 and ZnO, respectively (Fig. 8d). It shows that the VB maximum of g-C3N4 and ZnO is respectively 2.38 eV and 2.57 eV lower than the corresponding
17
Fermi level. Therefore, the VB of pure g-C3N4 and ZnO can be respectively calculated to be 1.58 eV and 2.31 eV. Moreover, the CB of pure g-C3N4 and ZnO is deduced to -1.28 eV and -0.92 eV, respectively. Accordingly, in the ZnO/g-C3N4 system, the energy band of ZnO bends upward and that of g-C3N4 bends downward toward the interface, respectively, forming the typical type-II composite (Fig. 8e). As a result, the flat band potential of 1.5-ZnO/g-C3N4 is -0.56 V, between the ones of g-C3N4 and ZnO. Since the VB maximum of 1.5-ZnO/g-C3N4 is 2.61 eV lower than its Fermi level, and its band gap is the same as g-C3N4 (2.86 eV), the VB and CB of 1.5-ZnO/g-C3N4 can be calculated to be 2.05 eV and -0.81 eV. The change of the band position of 1.5-ZnO/g-C3N4 may results from the formation of Zn-C bond.
Fig. 8. Mott-Schottky plots of (a) g-C3N4, (b) ZnO, and (c) 1.5-ZnO/g-C3N4. (d) Valence band spectra of the ZnO, g-C3N4 and 1.5-ZnO/g-C3N4 samples. (e) Possible photocatalytic mechanism diagram of ZnO/g-C3N4. RHE: reversible hydrogen electrode.
4 Conclusion In summary, the hierarchically porous ZnO/g-C3N4 spherical composites are successfully fabricated to form the type-II heterojunction between g-C3N4 and ZnO. The type-II heterojunction plays an important role in charge separation through the energy band of ZnO bending upward and 18
that of g-C3N4 bending downward toward the interface to increase the photocatalytic performance. Consequently, the 1.5-ZnO/g-C3N4 sample shows the best photocatalytic performance in RhB degradation (k=0.102 min-1) with a very good stability, superior to previous g-C3N4 based materials [13, 39]. Our study also shows that the •O2 - and h+ are the main active species for 1.5-ZnO/g-C3N4 in RhB photodegradation. Furthermore, the 1.5-ZnO/g-C3N4 sample demonstrates higher photodegradation activity on phenol than that of ZnO and pure g-C3N4. Therefore, our work here confirms that combining electronically heterostructure with spatially hierarchical structure is an efficient strategy for enhanced photodegradation of organic pollutants. In addition, our strategy may advance the formation of g-C3N4 based heterostructure for highly enhanced photocatalytic hydrogen production in the future. Acknowledgements Y. Li acknowledges Hubei Provincial Department of Education for the “Chutian Scholar” program. B. L. Su acknowledges the Chinese Central Government for an “Expert of the State” position in the Program of the “Thousand Talents”. This work is supported by National Key R&D Program of China (2016YFA0202602), National Natural Science Foundation of China (U1663225, 21671155, 21805220), Hubei Provincial Natural Science Foundation (2018CFB242), Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R52) and the Fundamental Research Funds for the Central Universities (WUT: 2017III001, 2017III055, 2018III039GX, 2018IVA095).
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Graphic Abstract
Hierarchically porous ZnO/g-C3N4 microspheres with type-II heterojunction exhibit an enhanced photocatalytic degradation on rhodamine B and phenol.
25
S0021-9797(18)31396-1 https://doi.org/10.1016/j.jcis.2018.11.076 YJCIS 24341
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
10 September 2018 10 November 2018 19 November 2018
Please cite this article as: S. Wu, H-J. Zhao, C-F. Li, J. Liu, W. Dong, H. Zhao, C. Wang, Y. Liu, Z-Y. Hu, L. Chen, Y. Li, B-L. Su, Type II heterojunction in hierarchically porous zinc oxide/graphitic carbon nitride microspheres promoting photocatalytic activity, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/ j.jcis.2018.11.076
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Type II heterojunction in hierarchically porous zinc oxide/graphitic carbon nitride microspheres promoting photocatalytic activity
Sijia Wu1, Hong-Juan Zhao2, Chao-Fan Li1, 3, Jing Liu1,*, Wenda Dong1, Heng Zhao1, Chao Wang1, Yang Liu1, Zhi-Yi Hu1, 3, *, Lihua Chen1, Yu Li1, 3, * and Bao-Lian Su1, 4, 5
1
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of
Technology, 122 Luoshi Road, 430070 Wuhan, Hubei, China. 2
Lanzhou Petrochemical Research Center, Lanzhou, China, 730060.
3
Nanostructure Research Centre (NRC), Wuhan University of Technology, 122 Luoshi Road, 430070 Wuhan,
Hubei, China. 4
Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, 61 rue de Bruxelles, B-5000 Namur,
Belgium. 5
Clare Hall, University of Cambridge, Herschel Road, Cambridge CB3 9AL, United Kingdom.
*
Corresponding author. Email: [email protected], [email protected] and [email protected].
1
Abstract Graphitic carbon nitride (g-C3N4) is a visible light active semiconductor. However, low conductivity and high recombination rate of photogenerated electrons and holes limit its application in photocatalysis. In this work, we design and synthesize hierarchically porous zinc oxide/ graphitic carbon nitride (ZnO/g-C3N4) microspheres with type-II heterojunction to effectively degrade rhodamine B (RhB) via increasing the charge-separation efficiency. The ultraviolet-visible (UV-Vis) absorption spectra, Mott-Schottky plots and valence band X-ray photoelectron spectroscope confirm the formation of type-II heterojunction between ZnO nanocrystals and g-C3N4 nanosheets. As a result, the 1.5-ZnO/g-C3N4 composite (the mass ratio of zinc acetate dihydrate to g-C3N4 is 1.5) exhibits the highest photocatalytic activity with good stability and higher photocatalytic degradation rate comparing to pure g-C3N4 and pure ZnO. In addition, our results confirm that •O2- and h+ are the main active species for ZnO/g-C3N4 in degradation of RhB.
Key Words: Hierarchically porous structure; ZnO/g-C3N4; Heterojunction; Photocatalytic activity; Active species
2
1 Introduction In recent years, photocatalysis has attracted much attention in water depuration owing to the discovery of plentiful photocatalysts, which work in an eco-friendly way with the solar energy as a driving force. Graphitic carbon nitride (g-C3N4) stands out as an extraordinary two dimensional non-metal photocatalyst containing earth-abundant elements [1]. The basic structural unit of typical g-C3N4 is tri-s-triazine, which can be formed by the polycondensation of accessible precursors like urea and melamine [2-4]. The sp2-hybridized carbon and nitrogen establish a superior π-conjugated electronic structure of g-C3N4 [5]. As a result, g-C3N4 acts as a visible light responsive photocatalyst with the band gap of ~2.8 eV [6]. However, the poor conductivity and the easily recombination of photogenerated charges of g-C3N4 limits its further application in photocatalysis [7]. Various methods have been developed to overcome the weakness of g-C3N4, primarily including the band structure optimization and the heterojunction construction [8-11]. Elemental doping is a general method of band structure optimization, such as O-doping and P-doping, widening the light-absorbing regions and promoting charge separation of g-C3N4 by introducing defect state [12, 13]. As for heterojunction structure constructing between g-C3N4 and other suitable semiconductors, type-II heterojunction is considered as one of the most efficient heterojunction in photocatalysis [14]. In the type-II heterojunction, the band bending generated at the interface between two semiconductors brings a built-in electric field, leading to the inverse migration of photogenerated electrons and holes [15, 16]. Recently, constructing a type-II heterojunction between g-C3N4 and a wide band gap semiconductor becomes more popular due to the wide band gap semiconductor being more beneficial to charge separation [17]. For example, Boonprakob et al. constructed type-II heterojunction between g-C3N4 and titanium dioxide (TiO2), demonstrating efficiently enhanced degradation activity on methylene blue under visible light irradiation in 3
comparison with pure TiO2 [18]. Sun et al. successfully composited g-C3N4 with ZnO via a one-step method [19]. The photocatalytic performance of g-C3N4-ZnO is 3 and 6 times higher than that of pure g-C3N4 for photodegradation of methyl orange and p-nitrophenol, respectively. Therefore, constructing a type-II heterojunction between g-C3N4 and other wide band gap semiconductors is plausible to remove the organic pollutants. In this work, we report a hierarchically porous ZnO/g-C3N4 composite for effectively photodegradation of RhB and phenol. The ZnO/g-C3N4 composite was synthesized via a two-step method. The wide band gap ZnO was used to form a heterojunction structure with g-C3N4 owing to the prominent electron mobility, high quantum efficiency and diversiform morphology of hexagonal wurtzite ZnO [20-22]. Our results show that the conduction band (CB) of g-C3N4 is more negative than that of ZnO, and the valence band (VB) of ZnO is more positive than that of g-C3N4. As a result, the photogenerated electrons transfer from g-C3N4 to ZnO and the holes transfer from ZnO to g-C3N4 to enhance the photocatalytic activity via the effective charge separation. Furthermore, the photocatalytic degradation mechanism of the composite was studied. It shows that the main active species in ZnO/g-C3N4 for degradation of RhB are •O2- and h+. 2 Experimental section 2.1 Materials Melamine (2,4,6-triamino-1,3,5-triazine, AR), ethylene glycol ((CH2OH)2, AR), zinc acetate dihydrate (Zn(CH3COO)2·2H2O, AR), phenol (C₆H₆O, AR), potassium iodide (KI, AR) and tert-butyl alcohol (C4H10O, GC) were purchased from the Aladdin Reagent Company. Hydrochloric acid (HCl, AR, 36~38 wt%) was purchased from the Xinyang Reagent Factory. Ethanol (C2H6O, AR), rhodamine B (C28H31ClN2O3, AR) and benzoquinone (C6H4O2, AR) were purchased from the 4
Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received. 2.2 Preparation of g-C3N4 nanosheets In this work, g-C3N4 was prepared by thermal polymerization method. Briefly, 1.25 g of melamine was added into 10 mL of alcohol, followed by a dropwise addition of 1 mL of HCl with a drastic stir for 1 h. The white mixture was dried at 40 °C for 5 hours, followed by calcining at 500 °C for 1 h and 520 °C for 3 h with the heating rate of 5 °C/min. Finally, a fluffy yellow g-C3N4 powder was obtained. 2.3 Preparation of ZnO/g-C3N4 hierarchical porous spheres ZnO/g-C3N4 was synthesized through solvothermal method. 0.1 g of g-C3N4 was firstly dispersed in 40 mL of ethylene glycol to ultrasound exfoliate for several hours. Triplicate dispersion of g-C3N4 was prepared for subsequent use. 0.05 g, 0.15 g and 0.25 g of zinc acetate dehydrate, of which the mass was 0.5, 1.5 and 2.5 to that of g-C3N4, was added to aforementioned dispersion, respectively. After stirring for 2 hours, the obtained solution was transferred into Teflon-lined autoclave and heated at 180 °C for 90 min. The yellow precipitate was collected, washed by deionized water for several times and dried at 40 °C overnight, followed by annealing at 300 °C for 1 h with the heating rate of 2 °C/min. The prepared samples were denoted as 0.5-ZnO/g-C3N4, 1.5-ZnO/g-C3N4 and 2.5-ZnO/g-C3N4, respectively. 2.4 Preparation of ZnO nanoparticles The pure ZnO was prepared by dissolving 10 mmol zinc acetate dehydrate in 40 mL of ethylene glycol and heating at 180 °C for 90 min. The following process was the same as the aforementioned treating of ZnO/g-C3N4. 2.5 Characterizations 5
A bruker diffractometer with Cu Kα radiation (λ = 1.540596 Å) was used to measure powder X-ray diffraction (XRD) patterns, operated at 40 kV and 40 mA. The morphologies of samples were observed by a Hitachi S-4800 field emission scanning electron microscope (SEM) at an acceleration voltage of 5.0 kV. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and energy dispersive spectroscopy (EDS) were performed on FEI Talos F200S microscope fitted with Super-X EDX system, operated at 200kV. Brunauer–Emmett–Teller (BET) method was carried out to detect the adsorption branch of the isotherm based on the Barett–Joyner–Halenda (BJH) and determine the specific surface area and pore size distribution of the samples. The surface chemical composition and valence states of products were studied using X-ray photoelectron spectroscope (XPS, Thermo Scientific Escalab 250Xi) with monochromatic Al Kα radiation. The binding energies were calibrated versus the C 1 s peak (284.8 eV). Room-temperature UV–vis absorption spectra of samples were recorded using a UV-2550 Shimadzu spectrophotometer. Photoluminescence emission studies (PL) were recorded by a PerkinElmer Lambda LS 55 fluorescence spectrometer at room temperature with a Xe lamp excitation at 320 nm. The degradation products were detected by gas chromatography-mass spectrometer (GC-MS, Agilent 7890B-5977B). Fourier transform infrared spectroscopy (FTIR) was detected by Nicolet FTIR 6700. The Mott-Schottky plots were measured by Autolab 86005 to investigate the Fermi level of samples. The zeta potential of sample was determined by Nano-ZS ZEN3600 zetasizer. The photocurrent of the samples was detected by electrochemical workstation (CH1660D) with 0.5 mol/L of NaSO4 aqueous solution as electrolyte under UV-Vis light. 2.6 Photocatalytic activity 6
Rhodamine B (RhB) and phenol were used as targeted pollutants to evaluate the photocatalytic activity of the synthesized materials. Typically, 100 mg of photocatalyst was added into RhB aqueous solution (100 mL, 10-5 mol/L) in a 250 mL beaker under stirring condition at room temperature. Before illumination, the reactants were stirring in dark for 1 h to reach an adsorption-desorption equilibrium of RhB molecular on photocatalyst surface. The reactor was irradiated under simulated solar light (250nm-780nm, PLS-SXE-300UV with a visible light intensity of 158 mW cm-2, and an ultraviolet (UV) light intensity of 34 mW cm-2, Beijing).
3 Results and discussion
Fig. 1. (a) Schematic illustration of the synthetic process of hierarchical porous ZnO/g-C3N4 microspherical heterostructure (the grey spheres are ZnO and the dark yellow sheets are g-C3N4). SEM images of (b) pure g-C3N4 prepared by sintering, (c) pure ZnO and (d) 1.5-ZnO/g-C3N4.
Fig. 1a displays the schematic synthetic process of hierarchically porous ZnO/g-C3N4 microspheres, which were formed via gathering ZnO nanoparticles with g-C3N4 nanosheets to reduce surface energy. The morphologies of pure ZnO, pure g-C3N4 and 1.5-ZnO/g-C3N4 samples 7
were observed by FESEM. Fig. 1b shows that the pure g-C3N4 is in morphology of nanosheets. The pure ZnO presents a spherical shape (Fig. 1c), formed by 20 nm ZnO nanoparticles (Fig. 1c inset). Fig. 1d presents the typical SEM image of 1.5-ZnO/g-C3N4, showing the porous microsphere with a size of ~700 nm. The morphologies of 0.5-ZnO/g-C3N4 and 2.5-ZnO/g-C3N4 are also respectively shown in Fig. S1. It can be seen that the diameter (~300 nm for 0.5-ZnO/g-C3N4, and 1~2 μm for 2.5-ZnO/g-C3N4) and the compactness of ZnO/g-C3N4 microspheres increase with the ZnO content rising. According to the previous report, this hierarchically porous microstructure is beneficial for light harvesting and dye absorption [23].
Fig. 2. (a) HAADF-STEM image, (b) HRTEM image (ZnO nanoparticles are indicated by red dashed circles), (c) the corresponding SAED pattern of the whole area in (a), the indicated diffraction rings as below, 1: C 3N4 0002; 2: 8
ZnO
; 3: ZnO
; 4: ZnO
; 5: ZnO
; 6: ZnO
; 7: ZnO
, (d-f) the corresponding
EDX elemental maps: C (blue), N (green) and Zn (red).
TEM is further conducted to investigate the microstructure of the ZnO/g-C3N4 nanocomposites. The HAADF-STEM image (Fig. 2a) shows a typical hierarchically porous spherical structure of 1.5-ZnO/g-C3N4. The corresponding selected area electron diffraction (SAED) pattern (Fig. 2c) presents a series of ZnO crystal diffraction rings (hexagonal ZnO ,
,
,
,
,
respectively, indicated by arrows) and g-C3N4-0002 diffraction ring, suggesting the
hybrid structure. In order to understand the details of heterojunction, HRTEM is used to investigate the interface between ZnO and g-C3N4. Fig. 2b reveals that the ZnO nanoparticles (indicated by red dashed circles) deposit on g-C3N4 nanosheets. This ensures the formation of heterojunction between ZnO and g-C3N4 for the efficient charge separation. As g-C3N4 is easily damaged under the electron beam irradiation, the crystal structure of g-C3N4 cannot be observed in HRTEM image, whereas the diffraction ring of g-C3N4 can be observed due to the extremely low electron dose in electron diffraction mode. In addition, the EDX spectrum (Fig. S2) and STEM-EDX elemental maps (Fig. 2d-f) demonstrate that ZnO and g-C3N4 are homogenously distributed in the hierarchically porous spherical structure. This gives further evidence that the ZnO nanoparticles have successfully decorated on g-C3N4 to form a heterostructure. The specific surface area and pore diameter of the samples were measured by N2 adsorption/desorption isotherms (Fig. 3). The plot of the pore size distribution was determined by BJH analysis of the desorption branch of the isotherm (Fig. 3 insets). The shape of hysteresis loop of ZnO/g-C3N4 with different ZnO content indicates the hierarchically porous structure deriving from the stacking of ZnO nanoparticles and g-C3N4 nanosheets. The larger hysteresis loop of ZnO/g-C3N4 with different ZnO content at relative high pressure suggests the richer mesoporosity 9
comparing to pure g-C3N4. The specific surface area of 0.5-ZnO/g-C3N4, 1.5-ZnO/g-C3N4 and 2.5-ZnO/g-C3N4 is 32.7, 29.8 and 32.5 cm2/g respectively, about three times higher than pure g-C3N4 (9.9 cm2/g).
Fig. 3. N2 adsorption/desorption isotherms of (a) g-C3N4, (b) 0.5-ZnO/g-C3N4, (c) 1.5-ZnO/g-C3N4, (d) 2.5-ZnO/g-C3N4 (Inset: corresponding BJH pore distribution curve).
The crystalline phases of the ZnO, g-C3N4 and ZnO/g-C3N4 samples were further investigated by XRD (Fig. 4a). The pure ZnO with three strong diffraction peaks at 31.7º, 34.5º and 36.2º defines a hexagonal wurtzite structure (JCPDS No. 36-1451) of
, (0002) and
crystal planes, respectively. For pure g-C3N4, the strong diffraction peak at 2θ = 27.5º represents the typical (0002) plane of aromatic system [24]. The ZnO/g-C3N4 composites demonstrate the characteristic peaks of both g-C3N4 and ZnO, in accordance with the above TEM results. Fig. 4b shows the FTIR spectra of the ZnO, g-C3N4 and ZnO/g-C3N4 samples. For g-C3N4, the peaks at 1637, 1242 and 808 cm-1 symbolize the C=N, C–N stretching vibrations and s-triazine ring 10
vibrations, respectively [25, 26]. The peaks in the region from 400 to 600 cm-1 are the intrinsic absorption of ZnO, originating from the lattice vibration [27]. Again, the FTIR spectra of ZnO/g-C3N4 demonstrate the main characteristic peaks of ZnO and g-C3N4, indicating the successful formation of the ZnO/g-C3N4 heterostructure.
Fig. 4. (a) XRD patterns and (b) FTIR spectra of a: g-C3N4, b: 0.5-ZnO/g-C3N4, c: 1.5-ZnO/g-C3N4, d: 2.5-ZnO/g-C3N4 and e: ZnO.
The XPS was performed to further illucidate the chemical composition and the surface properties of the as-prepared samples. Fig. S3 presents the survey scan spectrum of 1.5-ZnO/g-C3N4, showing the peaks of Zn, O, C and N. Fig. 5 displays the high resolution spectra of C 1s, N 1s, O 1s and Zn 2p of g-C3N4, ZnO and 1.5-ZnO/g-C3N4 samples. The C 1s spectrum of g-C3N4 can be deconvolved to three peaks at 288.8, 288.3 and 284.8 eV (Fig. 5a), corresponding to the carbon in O-C=O [13], sp2-hybridized carbon in N-containing aromatic ring (N=C-N) and sp2 C-C bonds, respectively [28, 29]. The other two emerged C 1s peaks in the spectrum of 1.5-ZnO/g-C3N4 can be identified as C-O (286.9 eV) and Zn-C (283.3 eV), respectively [30, 31]. The decreased binding energy of C atom in N=C-N indicates a weaker band strengths of C=N and C-N in 1.5-ZnO/g-C3N4, resulting from a wider conjugated system containing ZnO and g-C3N4 with Zn-C bond, which is beneficial for the charge transfer between ZnO and g-C3N4. This result confirms the formation of heterojunction between ZnO nanoparticles and g-C3N4 nanosheets. 11
Fig. 5. High resolution XPS spectra of (A) g-C3N4, (B) 1.5-ZnO/g-C3N4 and (C) ZnO: (a) C 1s; (b) N 1s; (c) O 1s; and (d) Zn 2p.
Fig. 5b presents the N 1s spectra of g-C3N4 and 1.5-ZnO/g-C3N4. The peaks of g-C3N4 at 398.5, 400.2 and 401.0 eV are attributed to sp2-hybridized nitrogen existing in triazine rings (C-N=C), the tertiary nitrogen N-(C)3 groups and the free amino groups (C-N-H), respectively [28, 32]. Comparing to pure g-C3N4, the C-N=C and C-N-H in 1.5-ZnO/g-C3N4 shifts to a higher value of 398.9 eV and 401.5 eV, owing to the aforementioned conjugated system containing ZnO and g-C3N4, decreasing the electron density around nitrogen. Fig. 5c displays the O 1s spectra of ZnO and 1.5-ZnO/g-C3N4. The peaks of ZnO at 530.2 eV and 531.7 eV are respectively assigned to O2and Ox- (O- and O2 -) generated from the oxygen vacancy [33, 34]. These two peaks shift to a higher binding energies resulted from the strong interaction between ZnO and g-C3N4. Fig. 5d presents the Zn 2p spectra of ZnO and 1.5-ZnO/g-C3N4. The peaks at 1044.1 eV and 1021.2 eV are attributed to 12
the Zn 2p1/2 and 2p3/2, respectively [34]. Compared to the pure ZnO, the binding energy of Zn 2p1/2 (1044.5 eV) and 2p3/2 (1021.4 eV) increases a little, indicating the strong interaction between ZnO and g-C3N4. Again, another two peaks at 1046.0 eV and 1022.9 eV appear in the Zn 2p spectrum of 1.5-ZnO/g-C3N4, which could be assigned to the Zn 2p1/2 and 2p3/2 of Zn-C according to the above results from C 1s spectra. As mentioned above, the formed heterostructure between ZnO and g-C3N4 could bring a built-in electric field to effectively separate the photogenerated electrons and holes.
Fig. 6. (a) UV–vis absorption spectra of ZnO, g-C3N4 and ZnO/g-C3N4 samples with different ZnO content. The insert spectrum is the Tauc plots of ZnO and g-C3N4. (b) PL spectra of ZnO/g-C3N4 and g-C3N4 samples.
Fig. 6a presents the UV–visible absorption spectra of the ZnO, g-C3N4 and ZnO/g-C3N4 samples. It distinctly shows that the absorption edges of ZnO sample and g-C3N4 sample present at ~390 nm and ~448 nm, respectively. The absorption edges of the ZnO/g-C3N4 samples are nearly the same as the g-C3N4 sample, indicating no obvious change of the bandgap of ZnO and g-C3N4. This further confirms the formation of ZnO/g-C3N4 heterostructure. Fig. 6a inset displays the Tauc plots of g-C3N4 and ZnO, showing the electronic bandgap of 2.86 eV and 3.23 eV, respectively. Fig. 6b displays the PL spectra of g-C3N4 and ZnO/g-C3N4 to reveal the recombination rate of the photogenerated charges. It shows that the ZnO/g-C3N4 samples exhibit higher charge separation efficiency than the g-C3N4 sample. This means that the formation of heterostructure in the 13
ZnO/g-C3N4 composite is helpful to effectively improve the charge separation in g-C3N4. The photocatalytic activity of the ZnO, pure g-C3N4 and ZnO/g-C3N4 samples was evaluated by the photodegradation of RhB under simulated solar light irradiation. The pH value of RhB solution in this work was measured to be 7.3. The 1.5-ZnO/g-C3N4 sample was selected as a representative to detect the surface potential of samples under this pH value in water. The result shows that the surface potential of 1.5-ZnO/g-C3N4 is -20.4 mV (Fig. S4). This means that RhB as a cationic dye has the ability to attract negatively charged nanoparticles at this pH value [35]. Therefore, RhB molecules tend to be adsorbed on the surface of 1.5-ZnO/g-C3N4, which is beneficial for the photodegradation of RhB. Fig. 7a presents the relationship between C/Ci and illumination time (Ci is the initial concentration of RhB before dark reaction and C is the concentration of RhB at sampling time). It shows that 9.5%, 13.7%, 18.5%, 19.7% and 22.1% of RhB is respectively adsorbed by ZnO, 2.5-ZnO/g-C3N4, 1.5-ZnO/g-C3N4, 0.5-ZnO/g-C3N4 and g-C3N4 in dark after 60 min, respectively. This result indicates that g-C3N4 is plausible for RhB adsorption. After 30 min in UV-visible light irradiation, 73.7%, 97.6%, 100%, 97.7% and 93.8% of RhB was removed by ZnO, 2.5-ZnO/g-C3N4, 1.5-ZnO/g-C3N4, 0.5-ZnO/g-C3N4 and g-C3N4 for photodegradation, respectively. It can be seen that the 1.5-ZnO/g-C3N4 sample exhibits the best photodegradation activity, where RhB can be photodegrade in 30 min. Fig. 7b illustrates the fitted experimental data according to the pseudo first-order kinetic equation ln(C/C0) = -kt (C0 is the concentration of RhB at the end of dark reaction, k is the reaction rate constant). The 1.5-ZnO/g-C3N4 sample has the highest reaction rate of 0.102 min-1, compares to that of the pure g-C3N4 (0.081 min-1), 0.5-ZnO/g-C3N4 (0.092 min-1), 2.5-ZnO/g-C3N4 (0.091 min-1) and pure ZnO (0.047 min-1). The photocatalytic stability of 1.5-ZnO/g-C3N4 under simulated solar light was also investigated. After every 30 min for degradation of RhB, the sample was collected and dried to repeat the degradation next time. Fig. 7c 14
shows that after 5 cycles, the degradation rate is still as high as 95.2%. In addition, after 5 cycles, there is no change on the structure of 1.5-ZnO/g-C3N4 according to the XRD pattern shown in Fig. S5, suggesting the very good stability under simulated solar light irradiation.
Fig. 7. (a) C/Ci and (b) First order rate constant k (min-1) of ZnO, ZnO/g-C3N4 and g-C3N4 samples for RhB under UV-vis light. (c) Stability test of 1.5-ZnO/g-C3N4 over five consecutive cycles under UV-vis light. (d) C/Ci and (e) First order rate constant k (min-1) of ZnO, ZnO/g-C3N4 and g-C3N4 samples for RhB under visible light. (f) Transient photocurrent response of different samples. C/C0 of (g) 1.5-ZnO/g-C3N4, (h) pure g-C3N4 and (i) pure ZnO for RhB after adding different scavengers.
In addition, the photocatalytic activity of pure g-C3N4, ZnO and ZnO/g-C3N4 under visible light (λ > 420 nm) was further investigated (Fig. 6d). It displays that ZnO has little ability for degradation under visible light. Again, 1.5-ZnO/g-C3N4 presents a higher photocatalytic activity (k=0.062 min-1) than that of pure g-C3N4 (k=0.055 min-1), 0.5-ZnO/g-C3N4 (k=0.060 min-1) and 15
2.5-ZnO/g-C3N4 (k=0.230 min-1) (Fig. 7e). This confirms that the formation of heterojunction between g-C3N4 and ZnO is beneficial to the enhancement of photocatalytic activity. The transient photocurrent response experiment was further studied to convince the charge transfer properties of g-C3N4 and 1.5-ZnO/g-C3N4 (Fig. 7f). It shows that the photocurrent of ZnO/g-C3N4 increased with the rising content of ZnO, demonstrating an easier migration and separation of photogenerated charges in ZnO/g-C3N4, in accordance with the PL results. This is because the high electronic transmission efficiency of ZnO is in favor of the electron transport from ZnO/g-C3N4 to FTO (fluorine-doped tin oxide) film [36], leading to the superior transient photocurrent response of ZnO/g-C3N4 with higher ZnO content. Generally, the possible active species for photodegradation of RhB include hydroxyl radicals (•OH), superoxide anion radicals (•O2 -) and photogenerated holes (h+) [37]. Therefore, different scavengers are used to investigate the photocatalytic mechanism of ZnO, g-C3N4 and 1.5-ZnO/g-C3N4. When the h+ scavenger (potassium iodide, KI) or •O2- scavenger (benzoquinone, BQ) is added into reaction system, the degradation rate obviously decreases (Fig. 7g), indicating that both h+ and •O2- are evidently conductive to the degradation reaction. Specifically, RhB scarcely be degraded at the presence of BQ, demonstrating that •O2- is the primary active specie in this reaction. Whereas, the •OH scavenger (tert-butanol, t-BuOH) initiates little change in degradation process, suggesting that •OH is a neutral specie. The active species of pure g-C3N4 and ZnO are investigated to explain why •O2- and h+ play dominant roles but •OH is inessential in degradation of RhB, as displayed in Fig. 7h~i. It turns out that •O2- and h+ act as the main active species for g-C3N4. However, the role of •O2 -, h+, and •OH cannot be all neglected for ZnO. Furthermore, the potential energy (1.58 V vs RHE) of holes in g-C3N4 is higher than that of OH-/•OH (1.99 V) and H2O/•OH (2.34 V) couples [38]. This means that the holes in g-C3N4 cannot 16
oxidize OH- or H2O into •OH. However, the potential energy of holes in ZnO is low enough to oxidize OH- or H2O into •OH. This explains the role of •OH for pure ZnO. Therefore, the effect of •OH scavenger on RhB degradation for ZnO, g-C3N4 and 1.5-ZnO/g-C3N4 indicates that the holes in the VB of ZnO can transfer to VB of g-C3N4, suggesting the existence of a type-II heterojunction. The degraded products of RhB were further detected by GC-MS (Fig. S7). Apart from the same peaks as H2O, four exclusive peaks are observed on the curve of degraded RhB solution sample. The peaks at 4.51, 21.30 and 24.53 min are probably 4-methoxy-alpha-methyl benzeneethanamine, benzaldehyde and dl-phenylephrine, respectively (Table S1). These three products still contain one benzene ring in their molecular. However, the peak at 20.34 min is identified as propanamide, a micromolecule with no benzene rings. Therefore, RhB is indeed degraded by the photocatalyst. To extend the application of our photocatalysts, we also compared the photodegradation performance of colorless phenol on ZnO, pure g-C3N4 and 1.5-ZnO/g-C3N4. The result is show in Fig. S6. It can be seen that the 1.5-ZnO/g-C3N4 sample exhibits a higher photodegradation activity than that of pure g-C3N4 and ZnO. This confirms the superiority of hierarchically porous ZnO/g-C3N4 microspheres with type II heterojunction. The Fermi levels of g-C3N4, ZnO and 1.5-ZnO/g-C3N4 are then investigated by Mott-Schottky plots measured at 500 Hz and 1000 Hz (Fig. 8a-c). The calculated flat band potential (V fb) for g-C3N4 and ZnO is -0.80 V and -0.26 V, respectively. This means that the Fermi level of ZnO is lower than that of g-C3N4. XPS are further carried out to determine the distance between the VB maximum and the Fermi level for g-C3N4 and ZnO, respectively (Fig. 8d). It shows that the VB maximum of g-C3N4 and ZnO is respectively 2.38 eV and 2.57 eV lower than the corresponding
17
Fermi level. Therefore, the VB of pure g-C3N4 and ZnO can be respectively calculated to be 1.58 eV and 2.31 eV. Moreover, the CB of pure g-C3N4 and ZnO is deduced to -1.28 eV and -0.92 eV, respectively. Accordingly, in the ZnO/g-C3N4 system, the energy band of ZnO bends upward and that of g-C3N4 bends downward toward the interface, respectively, forming the typical type-II composite (Fig. 8e). As a result, the flat band potential of 1.5-ZnO/g-C3N4 is -0.56 V, between the ones of g-C3N4 and ZnO. Since the VB maximum of 1.5-ZnO/g-C3N4 is 2.61 eV lower than its Fermi level, and its band gap is the same as g-C3N4 (2.86 eV), the VB and CB of 1.5-ZnO/g-C3N4 can be calculated to be 2.05 eV and -0.81 eV. The change of the band position of 1.5-ZnO/g-C3N4 may results from the formation of Zn-C bond.
Fig. 8. Mott-Schottky plots of (a) g-C3N4, (b) ZnO, and (c) 1.5-ZnO/g-C3N4. (d) Valence band spectra of the ZnO, g-C3N4 and 1.5-ZnO/g-C3N4 samples. (e) Possible photocatalytic mechanism diagram of ZnO/g-C3N4. RHE: reversible hydrogen electrode.
4 Conclusion In summary, the hierarchically porous ZnO/g-C3N4 spherical composites are successfully fabricated to form the type-II heterojunction between g-C3N4 and ZnO. The type-II heterojunction plays an important role in charge separation through the energy band of ZnO bending upward and 18
that of g-C3N4 bending downward toward the interface to increase the photocatalytic performance. Consequently, the 1.5-ZnO/g-C3N4 sample shows the best photocatalytic performance in RhB degradation (k=0.102 min-1) with a very good stability, superior to previous g-C3N4 based materials [13, 39]. Our study also shows that the •O2 - and h+ are the main active species for 1.5-ZnO/g-C3N4 in RhB photodegradation. Furthermore, the 1.5-ZnO/g-C3N4 sample demonstrates higher photodegradation activity on phenol than that of ZnO and pure g-C3N4. Therefore, our work here confirms that combining electronically heterostructure with spatially hierarchical structure is an efficient strategy for enhanced photodegradation of organic pollutants. In addition, our strategy may advance the formation of g-C3N4 based heterostructure for highly enhanced photocatalytic hydrogen production in the future. Acknowledgements Y. Li acknowledges Hubei Provincial Department of Education for the “Chutian Scholar” program. B. L. Su acknowledges the Chinese Central Government for an “Expert of the State” position in the Program of the “Thousand Talents”. This work is supported by National Key R&D Program of China (2016YFA0202602), National Natural Science Foundation of China (U1663225, 21671155, 21805220), Hubei Provincial Natural Science Foundation (2018CFB242), Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R52) and the Fundamental Research Funds for the Central Universities (WUT: 2017III001, 2017III055, 2018III039GX, 2018IVA095).
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Graphic Abstract
Hierarchically porous ZnO/g-C3N4 microspheres with type-II heterojunction exhibit an enhanced photocatalytic degradation on rhodamine B and phenol.
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