Trinh Duy Nguyen*ab,
Vinh Huu Nguyenbc,
Ai Le Hoang Phamd,
Tuyen Van Nguyenae and
Taeyoon Lee*f
aGraduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Street, Cau Giay, Ha Noi, Vietnam
bInstitute of Applied Technology and Sustainable Development, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam. E-mail: ndtrinh@ntt.edu.vn
cFaculty of Environmental and Food Engineering, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam
dFaculty of Chemical Engineering, Industrial University of Ho Chi Minh City, No. 12 Nguyen Van Bao, Ward 4, Go Vap District, Ho Chi Minh City, Vietnam
eInstitute of Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Street, Cau Giay, Ha Noi, Vietnam
fDepartment of Environmental Engineering, College of Environmental and Marine, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan, 48513, Republic of Korea. E-mail: badger74w@pknu.ac.kr
First published on 6th September 2022
In this study, g-C3N4/UU-200 heterojunction photocatalysts displaying superior photocatalytic activity for organic pollutant elimination under white LED light irradiation were fabricated via an in situ solvothermal method. The successful construction of a heterojunction between g-C3N4 and UU-200 was evidenced by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The improved photocatalytic degradation of rhodamine B (RhB) and tetracycline hydrochloride (TCH) over g-C3N4/UU-200 compared with that over the individual components can be attributed to the anchoring of the g-C3N4 layered structure on the UU-200 surface promoting the decrease of the bandgap of UU-200, as confirmed by ultraviolet–visible diffuse reflectance spectroscopy, and to the light-induced charge separation efficiency stemming from a suitable heterojunction structure, which was revealed by photoluminescence spectroscopy and electrochemical analyses. Specifically, the 40% g-C3N4/UU-200 composite showed the highest photocatalytic activity toward the degradation of RhB (97.5%) within 90 min and TCH (72.6%) within 180 min. Furthermore, this catalyst can be recycled four runs, which demonstrates the potential of the g-C3N4/UU-200 composite as an alternative visible-light-sensitive catalyst for organic pollutant elimination.
Metal–organic frameworks (MOFs) are attractive materials due to their potential applications in the fields of gas adsorption/separation,8 sensor technology,9 drug delivery,10 and especially in photocatalysis.11,12 Therefore, the development of new MOF structures with versatile properties is attracting increasing research attention. The most common metal ions used in the synthesis of MOFs include Fe(III), Zn(II), Co(II), Cu(II), and Zr(IV);13 however, bismuth-based MOFs (Bi-MOFs) remain less explored. Bismuth is nontoxic and possesses a flexible coordination geometry, which renders it a suitable metal for building MOF structures.14 Besides, bismuth compounds have catalytic activity and are often used as green catalysts. Recently, several Bi-MOFs with novel structures and interesting optical and photocatalytic properties have been synthesized.15–18 In particular, trimesate-based Bi-MOFs have been shown to possess diverse structures, such as CAU–17 (CAU: Christian-Albrechts-Universität), which bears nine Bi(III) ions, nine BTC3− anions, and nine H2O molecules in the asymmetric unit,19 UU-200 (UU: Uppsala University), which crystallizes in the space group Pnnm (No. 58) with three Bi(III) ions and four BTC3− anions in the asymmetric unit,20 and Bi-BTC having the P21/n space group, in which two Bi atoms form a {Bi2O14} dimer with six carboxyl groups from six different BTC3− anions and the dimers are interconnected with BTC3−.15 Besides their diverse structures, trimesate-based Bi-MOFs have other advantages for photocatalytic applications, including high stability and easy preparation. However, their performance as photocatalysts is poor owing to their low visible-light response and low conductivity compared with traditional inorganic semiconductors.21,22 Consequently, further modification processes are required to boost the photocatalytic activity of trimesate-based Bi-MOFs, such as doping, formation of nanostructures, crystal regulation, and surface modification.23 Among these strategies, the combination of trimesate-based Bi-MOFs with different materials to construct heterostructured composites has recently emerged as an effective method.24 However, composites of trimesate-based Bi-MOFs with metal-free semiconductors having photocatalytic application are still rare.
Among the numerous metal-free semiconductor materials, graphitic carbon nitrides (g-C3N4) are considered as effective catalysts for the photodegradation of organic pollutants because of their raw material abundance, narrow bandgap (ca. 2.7 eV), facile fabrication methods, chemical stability, low economic cost, and versatility for structural modifications.25 Unfortunately, their photocatalytic performance is rather low owing to the poor separation efficiency of photogenerated charges. Nevertheless, the separation of photogenerated electron/hole (e−/h+) pairs and, in turn, the photocatalytic activity, can be improved by combining g-C3N4 with other semiconductor photocatalysts to form heterostructured composite structures.26 Moreover, g-C3N4 possesses a stacked two-dimensional structure with triazine or tri-s-triazine building units that facilitates the construction of their composites with MOFs due to the formation of π–π stacking interactions between the aromatic rings of the organic ligands in the MOFs and the triazine rings of g-C3N4.27 Furthermore, g-C3N4 interacts and can be easily encapsulated on the MOF surfaces due to large surface electrostatic interactions,28 promoting the separation of photoinduced charges. A series of MOF/g-C3N4 heterostructured composite structures have been constructed by combining g-C3N4 semiconductors with various MOFs, such as MIL–101(Fe),29 NH2–MIL–101(Fe),30 UiO–66,19 MIL–53(Al),31,32 MIL–100(Fe),22 MIL–125(Ti),33 MIL–88B(Fe),34 and NH2–MIL–88B(Fe),35 which exhibit enhanced photocatalytic activity as a result of the improvement in pore volume, high-efficiency separation and transfer of photoinduced charge carriers, and visible-light responsiveness. However, despite these advances, the construction of binary g-C3N4/UU-200 composites for the removal of organic pollutants has not been reported to date.
Herein, g-C3N4/UU-200 composites were fabricated via an in situ solvothermal method and applied to the removal of rhodamine B (RhB) and tetracycline hydrochloride (TCH) for the first time. UU-200, a trimesate-based Bi-MOF, was selected as the representative MOF because it is nontoxic, relatively inexpensive, and photostable and has great potential in catalysis.36 The g-C3N4 component was prepared according to a calcination method using a mixture of melamine and thiourea as nitrogen-rich organic precursors and NH4Cl as a gas bubble template. The obtained g-C3N4 was then mixed with the UU-200 precursors for the in situ synthesis of g-C3N4/UU-200 composites. The obtained composites were characterized systematically and tested for RhB and TCH elimination under white LED light irradiation to evaluate the effect of the heterojunction structure. According to the experimental results, the g-C3N4/UU-200 composites show better performance in the photocatalytic degradation of RhB and TCH than the individual components. A plausible photocatalytic reaction mechanism is discussed in detail.
The interactions between UU-200 and g-C3N4 were further studied using Fourier transform infrared (FT-IR) spectroscopy (Fig. 2). In the spectrum of the UU-200 catalyst, the stretching vibrations of the carboxylate groups were observed at 1360 and 1549 cm−1, and the Bi–O bond vibration produced a band at 522 cm−1.39 Compared with H3BTC, the stretching vibrations of the carboxylate groups were right-shifted due to the interaction between the Bi(III) ions and the carboxylate groups of the BTC3− linker.22 The FT-IR spectrum of pure g-C3N4 exhibited peaks at 810, 1200 − 1700, and 3100 − 3300 cm−1, which can be ascribed to the characteristic breathing modes of the s-triazine units and sp2 CN,40 the stretching vibrational modes of CN heterocycles,41 and the N–H stretching vibration,42–44 respectively. The spectra of the g-C3N4/UU-200 composites showed the typical absorption bands of UU-200 and g-C3N4, although a shift in the vibration peaks at 810 and 1240 cm−1 was observed compared with pure g-C3N4, which can be attributed to the interactions between g-C3N4 and UU-200.
The morphologies of the binary g-C3N4/UU-200 composites and their individual components were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques (Fig. 3). It was found that the UU-200 catalyst exhibited condensed bundles with a rod-like shape having a rod thickness of around 200 − 400 nm and smooth surfaces (Fig. 3 A and D), which is in agreement with previously reported experimental data for UUU-200.12 A layered structure containing irregular pieces was observed for g-C3N4 (Fig. 3C). Meanwhile, the TEM images of g-C3N4 showed relatively thin and transparent features with layered wrinkles (Fig. 3 F). The surface of the g-C3N4/UU-200 samples was rougher than that of pure UU-200 (Fig. 3B and S1†), indicating that the UU-200 surface was covered by g-C3N4. The binary composite structure could be observed clearly in the TEM images of the g-C3N4/UU-200 samples (Fig. 3E and S2†). According to these observations, the g-C3N4/UU-200 heterostructure could be expected to be suitable for photocatalysis under visible-light irradiation.
Fig. 3 SEM images (A–C) and TEM images (D–F) of UU-200 (A and D), g-C3N4/UU-200 composite (B and E) and g-C3N4 (C and F). |
The surface chemical composition and the chemical states of the surface elements present in UU-200, 40% g-C3N4/UU-200, and g-C3N4 were analyzed using X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum of pristine UU-200 showed the presence of Bi, O, and C elements, while that of bare g-C3N4 revealed C, N, and O elements (Fig. S3†). The presence of C, N, O, and Bi elements was detected in the XPS survey spectrum of 40% g-C3N4/UU-200 (Fig. S3†).
The binding energy (BE) values of the Bi 4f spectra of UU-200 and 40% g-C3N4/UU-200 (Fig. 4A) were located at around 159.5 (Bi 4f7/2) and 164.8 eV (Bi 4f5/2) with a ΔE (Bi 4f5/2–Bi 4f7/2) of 5.3 eV, indicating that Bi was mainly in the form of Bi(III).45 The C 1s peak of 40% g-C3N4/UU-200 can be divided into five main peaks at 284.8, 286.3, 287.9, 288.9, 289.5, and 291.9 eV (Fig. 4C). The peak at 284.8 eV can be attributed to the C–C/CC bonds of UU-200 and g-C3N4.40,46 The BEs at 286.3 and 288.9 eV correspond to the C–O and CO bonds, respectively, of UU-200 and g-C3N4.22,47 The peak at 287.9 eV is characteristic of the C–N/CN bonds of g-C3N4.48 The peak at 289.5 eV can be assigned to the O–CO bonds of UU-200, and that at 291.9 eV is ascribable to π–π* satellite bands in g-C3N4.49 Fig. 4D shows the O 1s spectra of UU-200, 40% g-C3N4/UU-200, and g-C3N4. For g-C3N4, the deconvoluted peak at 532.0 eV can be ascribed to the surface-adsorbed H2O in the three samples.50,51 The O 1s spectra of UU-200 and 40% g-C3N4/UU-200 exhibited three peaks at 530.9, 531.4, and 532.5 eV, which correspond to Bi–O bands, the carboxylate groups of the BTC linkers, and surface hydroxyl groups, respectively.52,53 The N 1s peak of the 40% g-C3N4/UU-200 composite can be separated into four peaks at 398.7, 399.7, 400.8, and 403.6 eV (Fig. 4 B). These peaks stem from the C–NC bonds of the triazine rings, the tertiary nitrogen N–(C)3 groups, the C–N–H bonds of the free amino groups, and π-excitations, respectively.46 Meanwhile, pristine g-C3N4 gave rise to a C–NC peak at 398.5 eV. Compared with pristine g-C3N4, the BE of the C–NC bonds of the 40% g-C3N4/UU-200 composite shifted to higher binding energy by 0.2 eV, revealing an interaction between the Bi and N atoms in the g-C3N4/UU-200 composite. Therefore, the XPS spectra further demonstrated the formation of the binary g-C3N4/UU-200 composites and the interaction between UU-200 and g-C3N4 components. The BEs at 286.3 and 288.9 eV correspond to the C–O and CO bonds, respectively, of UU-200 and g-C3N4.22,47 The peak at 287.9 eV is characteristic of the C–N/CN bonds of g-C3N4.48 The peak at 289.5 eV can be assigned to the O–CO bonds of UU-200, and that at 291.9 eV is ascribable to π–π* satellite bands in g-C3N4.49 Fig. 4D shows the O 1s spectra of UU-200, 40% g-C3N4/UU-200, and g-C3N4. For g-C3N4, the deconvoluted peak at 532.0 eV can be ascribed to the surface-adsorbed H2O in the three samples.50,51 The O 1s spectra of UU-200 and 40% g-C3N4/UU-200 exhibited three peaks at 530.9, 531.4, and 532.5 eV, which correspond to Bi–O bands, the carboxylate groups of the BTC linkers, and surface hydroxyl groups, respectively.52,53 The N 1s peak of the 40% g-C3N4/UU-200 composite can be separated into four peaks at 398.7, 399.7, 400.8, and 403.6 eV (Fig. 4 B). These peaks stem from the C–NC bonds of the triazine rings, the tertiary nitrogen N–(C)3 groups, the C–N–H bonds of the free amino groups, and π-excitations, respectively.54 Meanwhile, pristine g-C3N4 gave rise to a C–NC peak at 398.5 eV. Compared with pristine g-C3N4, the BE of the C–NC bonds of the 40% g-C3N4/UU-200 composite shifted to higher binding energy by 0.2 eV, revealing an interaction between the Bi and N atoms in the g-C3N4/UU-200 composite. Therefore, the XPS spectra further demonstrated the formation of the binary g-C3N4/UU-200 composites and the interaction between UU-200 and g-C3N4 components.
The physicochemical properties of the samples were investigated via N2 adsorption–desorption isotherms. Fig. 5 displays the N2 adsorption–desorption isotherms and the pore size distribution curves of UU-200, g-C3N4, and the g-C3N4/UU-200 composites. As shown in Fig. 5, all samples exhibited type IV sorption isotherms according to IUPAC classification, indicating their mesoporous characteristics.55 The Brunauer–Emmett–Teller (BET) specific surface area (SBET) values of the 20% g-C3N4/UU-200, 40% g-C3N4/UU-200, and 60% g-C3N4/UU-200 composites were 44.6, 42.8 and 40.1 m2 g−1, respectively, which were higher than that of UU-200 (37.6 m2 g−1) (Table 1). As shown in the SEM results, the presence of g-C3N4 during the synthesis of UU-200 may affect the formation of the condensed bundles of the rod-like crystals of UU-200, resulting in a decrease in their size. The SBET of the composites decreased with the increase of the g-C3N4 loading. Although the SBET values of the g-C3N4/UU-200 composites were lower than that of g-C3N4, they were much higher than that of pristine UU-200, demonstrating that the g-C3N4/UU-200 composites could provide abundant surface active sites and accelerate the transfer of photogenerated charges.56 In addition, the large pore size of 40% g-C3N4/UU-200 can be expected to enhance the adsorption capacity for organic molecules on the surface active sites of the composite, thereby accelerating the photocatalytic activity.
Fig. 5 (A) N2 adsorption–desorption isotherms and (B) pore size distributions of g-C3N4. UU-200 and 40% g-C3N4/UU-200 composites. |
Samples | SBET | V | D | Eg | RhB | TCH | ||
---|---|---|---|---|---|---|---|---|
(m2 g−1) | (10−3 cm3 g−1) | (nm) | (eV) | k (10−3 min−1) | R2 | k (10−3 min−1) | R2 | |
UU-200 | 37.6 | 95.9 | 2.45 | 3.77 | 17.4 | 0.987 | 4.33 | 0.983 |
20% g-C3N4/UU-200 | 44.6 | 164 | 1.92 | 2.68 | 31.3 | 0.993 | 5.00 | 0.958 |
40% g-C3N4/UU-200 | 42.8 | 311 | 4.53 | 2.66 | 38.2 | 0.993 | 5.87 | 0.973 |
60% g-C3N4/UU-200 | 40.1 | 178 | 4.15 | 2.64 | 23.6 | 0.983 | 5.63 | 0.974 |
gC3N4 | 47.1 | 399 | 3.24 | 2.54 | 16.1 | 0.993 | 5.11 | 0.998 |
The optical adsorption properties of the g-C3N4/UU-200 composites and their individual components were studied by UV–vis diffuse reflectance spectroscopy (UV–vis DRS) (Fig. 6 A). The photoabsorption edges of UU-200 and g-C3N4 were observed at 330 and 465 nm, respectively. Meanwhile, the UV-vis DRS spectra of the g-C3N4/UU-200 photocatalysts displayed a red shift compared with that of UU-200 because of the distribution of g-C3N4 layers on the UU-200 surface, which suggests that g-C3N4 can extend the absorption of UU-200 into the visible-light region. The energy gaps (Eg) of UU-200, g-C3N4, 20% g-C3N4/UU-200, 40% g-C3N4/UU-200, and 60% g-C3N4/UU-200 were obtained from the plots of (αhν)1/2 versus photon energy (Fig. 6 B), and the estimated values are shown in Table 1.57,58
The recombination of photogenerated charges can create photons, which can be detected with high sensitivity by photoluminescence (PL) emission spectroscopy. Specifically, a lower PL intensity indicates less recombination of photoexcited charge carriers, which is related to higher photocatalytic activity.59 The PL spectra of UU-200, g-C3N4, and 40% g-C3N4/UU-200 composites are presented in Fig. 6C. The highest PL emission peak intensity was observed for g-C3N4 at around 459 nm, indicating its highest recombination of photoexcited charge carriers. However, after the incorporation of UU-200 to construct the g-C3N4/UU-200 composites, the intensity of the PL emission peak decreased, which means that the recombination of charge carriers was comparatively suppressed in the composites. The PL emission analyses revealed that the combination of UU-200 and g-C3N4 efficiently enhanced the separation of charge carriers, thereby improving the photocatalytic activity. Especially, UU-200 exhibited the lowest intensity in the emission peak at around 400 nm, implying the lowest recombination rate of charge carriers. However, the photocatalytic activity of UU-200 was much lower than that of the 40% g-C3N4/UU-200 composite. The highest photocatalytic efficiency of the 40% g-C3N4/UU-200 composite could be ascribed to its low recombination rate of charge carriers. In fact, the strong migration and separation of photoexcited charge carriers was confirmed by analyzing the electrochemical impedance spectra (EIS). Two common methods have applied for treating the impedance data, the Nyquist and the Bode plots.60 Fig. 6D shows the Nyquist impedance plots of the UU-200, g-C3N4, and 40% g-C3N4/UU-200 samples. The corresponding Bodes' plots used to determine the equivalent electrochemical circuit are shown in Fig. S4 (ESI)†. The arc radius on the EIS Nyquist plot shows a clear trend of charge transfer behavior in the three different photocatalysts, which correlates with interfacial charge transport resistance; generally, a small arc radius is indicative of a fast charge transfer rate on the catalyst surface.60,61 As expected, the EIS Nyquist plot of the 40% g-C3N4/UU-200 composite showed a smaller arc radius than that of UU-200, which means that the separation and migration of charge carriers was enhanced in the 40% g-C3N4/UU-200 composite.
To reveal band position of g-C3N4/UU-200, UPS and VB-XPS were investigated. Fig. 6E and F show the XPS-VB spectra and UP spectra of UU-200, g-C3N4, and 40% g-C3N4/UU-200, respectively. The energy difference between the Fermi level and valence band maximum (EF − EVBM) of +0.60, +1.12, and +0.70 eV for UU-200, g-C3N4, and 40% g-C3N4/UU-200, respectively. The work function (φ) of UU-200 (−4.14 eV vs. vacuurn), g-C3N4 (−2.92 eV vs. vacuurn), and 40% g-C3N4/UU-200 (−3.28 eV vs. vacuurn) was calculated from eqn (1). ΔE is the cut-off energy of secondary electron emission spectra (Fig. 6F). Using the above bandgap energies and the eqn (2),62,63 the conduction band (CB) potential of UU-200, g-C3N4, and 40% g-C3N4/UU-200 were estimated, showing that the CB level of UU-200 was more negative than that of g-C3N4. The bandgap structures and charge migration of 40% g-C3N4/UU-200 are described in Fig. S5.†
φ = 21.22 − ΔE | (1) |
EVB = Eg + ECB | (2) |
On the basis of these results, the photocatalytic mechanism depicted in Fig. 8C can be proposed for the photocatalytic degradation of TCH and RhB over 40% g-C3N4/UU-200. Upon exposure to LED light, g-C3N4 in the heterostructure is excited to produce photogenerated e−/h+ pairs. Then, the e− in the CB of g-C3N4 can diffuse to the VB of UU-200 driven by the internal electric field, thus promoting the separation of photogenerated charge carriers. The e− on g-C3N4 can capture surface-absorbed O2 to generate ˙O−2 because the CB level of g-C3N4 is higher than the redox potential of O2/˙O−2 (−0.33 eV vs. NHE),67,68 which is consistent with the results of the trapping test. Meanwhile, the transferred h+ on UU-200 and the remained h+ in the CB of g-C3N4 cannot oxidize the adsorbed H2O to form ˙OH because the EVB of g-C3N4 and UU-200 are lower than the ˙OH/H2O potential (+2.68 eV vs. NHE).69,70 Then, these reactive species (h+, e−, and ˙O−2) can attack the CC bond of RhB and TCH to generate degradation intermediates. The main byproducts of the visible-light catalytic degradation of TCH by the 40% g-C3N4/UU-200 composite were identified by performing a liquid chromatography–mass spectrometry analysis (Fig. S6†). According to the results, Fig. 9 depicts the possible pathways for the photocatalytic TCH degradation. The characteristic ion signal observed at m/z 445 corresponds to TCH, and its intensity decreased gradually with increasing the irradiation time. Various intermediates were also detected, namely, P1 (m/z 427), P2 (m/z 410), and P3 (m/z 288), which were generated via dihydroxylation, deamination, and ring–opening reactions, respectively.71,72 These intermediates further decomposed into smaller fragments, including P4 (m/z 85), P5 (m/z 74), and P6 (m/z 57). Upon prolonging the irradiation time, all intermediates were converted into H2O and CO2.
Footnote |
† Electronic supplementary information (ESI) available: Materials, synthesis of UU-200 and g-C3N4, trapping test, recycling test, electrochemical test, SEM and TEM images of g-C3N4/UU200 composites, XPS spectra, LC-mass spectra of TCH, photo-stability of g-C3N4/UU200 composite, and XRD, IR spectra and SEM images of g-C3N4/UU200 composite before and after photocatalytic reaction. See https://doi.org/10.1039/d2ra04222c |
This journal is © The Royal Society of Chemistry 2022 |