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g-C3N4 heterojunction with enhanced photocatalytic performance under simulated solar irradiation
Materials Science in Semiconductor Processing 113 (2020) 105056
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A novel type-II Bi2W2O9/g-C3N4 heterojunction with enhanced photocatalytic performance under simulated solar irradiation �n a, *, M.A. Ruíz-Go �mez b, V. Rodríguez-Gonza �lez c, A. V�azquez d, e, D. S. Obrego a B. Hern� andez-Uresti a
Universidad Aut� onoma de Nuevo Le� on, Centro de Investigaci� on en Ciencias Físico Matem� aticas, Facultad de Ciencias Físico Matem� aticas, Av. Universidad S/N, San Nicol� as de los Garza, 66455, Nuevo Le� on, Mexico CONACYT-Departamento de Física Aplicada, CINVESTAV-IPN, Antigua Carretera a Progreso km 6, M�erida, Yucat� an, 97310, Mexico c Divisi� on de Materiales Avanzados, IPICYT (Instituto Potosino de Investigaci� on Científica y Tecnol� ogica), Camino a la Presa San Jos�e 2055, Col. Lomas 4a, secci� on C.P, 78216, San Luis Potosí, S.L.P., Mexico d Universidad Aut� onoma de Nuevo Le� on, Facultad de Ciencias Químicas, Av. Universidad S/N, San Nicol� as de los Garza, 66455, Nuevo Le� on, Mexico e Centro de Investigaci� on en Biotecnología y Nanotecnología, Facultad de Ciencias Químicas, Universidad Aut� onoma de Nuevo Le� on, Parque de Investigaci� on e Innovaci� on Tecnol� ogica, Km. 10 Autopista al Aeropuerto Internacional Mariano Escobedo, Apodaca, 66629, Nuevo Le� on, Mexico b
A R T I C L E I N F O
A B S T R A C T
Keywords: Semiconductors Heterojunction Bi2W2O9 g-C3N4 Photocatalysis
In this work, the Bi2W2O9/g-C3N4 type-II heterojunction system has been prepared for the first time by an incipient impregnation method. From the results, we have stated that the addition of low amounts of Bi2W2O9 (�5 wt%) does not provide changes in the structural and textural properties of g-C3N4, but it improves sub stantially the photocatalytic performance under simulated solar irradiation. The best photoactive behavior was attained in the sample with 2 wt% of Bi2W2O9, providing a removal degree of the tetracycline antibiotic about 2.6 times higher than the pristine g-C3N4. This result can be explained due to the difference in the edge positions of the valence and conduction bands of both semiconductors, which provides an efficient separation of the photogenerated charge carriers. From the study of the photocatalytic activity under several conditions, it can be inferred that the Bi2W2O9 works as a sink for the photoexcited electrons of the g-C3N4 and provides the formation of hydroxyl radicals through sequential reactions involving the superoxide ions. Furthermore, a possible pho tocatalytic mechanism of tetracycline removal is discussed.
1. Introduction The graphitic carbon nitride (g-C3N4) is a semiconductor that has attracted a wide attention in the last years due to its interesting prop erties such as absorption in the visible-light region, graphene-like layered structure, high thermal stability and outstanding photo catalytic performance [1]. The metal-free nature and low cost of prep aration are essential parameters for its promising use on an industrial scale. Some photocatalytic applications have been reported, including the degradation of organic pollutants, water splitting, CO2 reduction, NOx oxidation and so on [2–5]. However, the main disadvantage of the graphitic carbon nitride is the high recombination rate of its photo generated charge carriers, which limits the photoactive behavior. Several strategies have been performed to reduce these recombination processes, such as the exfoliation of the bulk layered material, the
addition of doping agents, the surface modification with noble metal nanoparticles and the formation of heterojunction systems with other semiconductors [6]. In this sense, several hybrid systems based on g-C3N4 have been reported [7–10]. For instance, Liu and co-workers have reported the preparation of the Ni2P/g-C3N4 system through an in-situ hydrothermal process [11]. The samples showed outstanding photocatalytic performance in the H2 production, up to 68 times higher than that obtained with pure g-C3N4. Chen et al., have also adopted this strategy using the graphitic carbon nitride [12]. The authors carried out a microwave heating procedure to obtain the AgNbO3/g-C3N4 system using AgNbO3 and melamine as precursors. Such hybrid photocatalysts also have demonstrated enhanced photoactive features compared to bare g-C3N4. Previous studies as described above demonstrate the high efficiency of the heterojunction systems based on g-C3N4, so the search for new hybrid photocatalysts remains a latent challenge.
* Corresponding author. E-mail address: [email protected] (S. Obreg� on). https://doi.org/10.1016/j.mssp.2020.105056 Received 3 December 2019; Received in revised form 10 February 2020; Accepted 6 March 2020 Available online 12 March 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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The Bi2WO6 is a semiconductor oxide with narrow band gap energy and excellent visible-light-driven photocatalytic performance [13]. This material belongs to the Aurivillius-type layered oxides with a corner-sharing structure of WO6 octahedron interleaved by (Bi2O2)2þ layers. Furthermore, it has been used in the preparation of the Bi2WO6/g-C3N4 system which exhibits enhanced photoactive properties in the degradation of several organic compounds [14,15]. Other mate rial of interest is the Bi2W2O9 oxide, which is a member of the same family of the cation deficient Aurivillius phases [16]. To date, few studies have been reported using the Bi2W2O9 in the formation of het erojunction systems and, to the best of our knowledge, the coupling of this oxide with g-C3N4 has not been studied so far [17–19]. In this work, we report the preparation of the Bi2W2O9/g-C3N4 system by an incipient impregnation method using 2-propanol as dispersing agent. The tetra cycline antibiotic was chosen as the model pollutant to evaluate the photoactive behavior of the coupled system under simulated solar irradiation. In this sense, the enhanced photocatalytic activity was investigated systematically through the modification of several vari ables of the photocatalytic reaction such as the pH of the aqueous dispersion and the use of several scavengers to elucidate the role of the photogenerated reactive species.
The powders were dispersed directly on carbon tape and placed in an aluminum sample-holder. The surface properties were studied by N2 adsorption-desorption isotherms using a Bel-Japan Minisorp II Surface Area & Pore Size analyzer. The physisorption measurements were per formed at -196 � C after a pretreatment at 150 � C during 12 h. The surface area was estimated by the BET (Brunauer-Emmett-Teller) method. The photoluminescence (PL) emission spectra were analyzed using a Hitachi F-7000 spectrophotometer. The measurements were performed with an excitation light source of 350 nm at room temperature. 2.3. Photocatalytic tests The photocatalytic performance of the hybrid system was examined through the degradation of the tetracycline antibiotic in aqueous solu tion. First, 0.2 g of each sample were added to 200 mL of an aqueous solution of tetracycline hydrochloride (Sigma-Aldrich �95%) with an initial concentration of 10 mg/L. The pH value of the resulting sus pension was estimated at 4.89. The suspension was left under dark conditions for 30 min to reach the adsorption-desorption equilibrium of the antibiotic molecules on the surface of the photocatalyst. Then, the system was irradiated with a 35 W Xenon lamp used as source of simulated solar light. The concentration of the antibiotic was followed through the absorbance of its characteristic band at 357 nm using an aliquot ca. 3 mL of the suspension filtered with a nylon syringe filter (0.2 μm) from Thermo Scientific. Several tests were performed by varying the initial pH value of the suspension, in a range of 2.8–10.5, using HNO3 or NH4OH. Besides, several radical scavengers were used to elucidate the role of the reactive oxygen species involved in the photo catalytic process [21].
2. Experimental 2.1. Synthesis of the Bi2W2O9/g-C3N4 system The graphitic carbon nitride (g-C3N4) was prepared by means of the polycondensation of the melamine precursor. Thereby, 10 g of melamine were placed in a covered crucible and submitted to thermal treatment at 500 � C for 4 h. The synthesis of the Bi2W2O9 oxide was carried out through a coprecipitation route, as reported in our previous work [16]. Briefly, two aqueous solutions containing the precursors Bi(NO3)3⋅5H2O (Sig ma-Aldrich, 99.9%) and (NH4)10(W12O41)⋅H2O (Aldrich, 99.9%) were mixed to obtain a milky suspension, which was adjusted to a pH ~ 5.0 by using concentrated ammonia. Then, the suspension was evaporated slowly at 70 � C until a white powder was obtained. This precursor was calcined at 700 � C for 24 h to yield the Bi2W2O9 semiconductor. For the preparation of the coupled system, the Bi2W2O9 and g-C3N4 semiconductors were assembled by an incipient impregnation proced ure. In detail, appropriate amounts of the powdered materials were dispersed in 100 mL of 2-propanol and submitted to ultrasound radia tion at 37 kHz for 1 h. Then, each hybrid photocatalyst was obtained by evaporating the alcohol at 90 � C. Several samples were prepared with different loadings of the Bi2W2O9 oxide (0.5, 1, 2, 3 and 5% wt.).
3. Results and discussions 3.1. Synthesis and characterization of the samples The Bi2W2O9/g-C3N4 heterojunction system was characterized through powder X-ray diffraction, as shown in Fig. 1. The pure Bi2W2O9 oxide can be assigned to an orthorhombic crystal structure with space group Pbn21 according to the PDF 33–0221. It is worth noting that the small peak at 28.3� (2θ) is associated with the diffraction plane (131) of the Bi2WO6 oxide, as it has been discussed previously [16]. For pristine g-C3N4, the diffraction peaks observed at 13.2� and 27.5� (2θ) are related to the periodic arrangement of the condensed tris-s-triazine units and to the interlayer stacking of the conjugated aromatic system in the
2.2. Characterization of the samples The Bi2W2O9/g-C3N4 system was structurally characterized by X-ray powder diffraction using a Bruker D-8 Advance diffractometer with Cu Kα radiation (0.15406 nm). The measurements were recorded between 10� and 90� (2θ) with a scanning rate of 0.02� s-1. Fourier-transform infrared spectroscopy (FTIR) was carried out using a JASCO FT/IR4000 spectrometer and the spectra were recorded in the range of 500–4000 cm-1. Diffuse reflectance measurements were performed using an Agilent Technologies UV–Vis–NIR spectrophotometer model Cary 5000 series equipped with an integrating sphere. The band gap energy values were estimated from the absorption spectra of the samples as previously described [20]. XPS spectra were recorded through a Thermo Scientific K-Alpha XPS spectrometer equipped with a hemi spherical electron analyzer and working with Al Kα radiation (hν ¼ 1486.6 eV). The C 1s signal (284.6 eV) was used as an internal energy reference in all the experiments. The photocatalysts were also analyzed by field emission scanning electron microscopy (FE-SEM) using a JEOL JSM-7600 F microscope and by transmission electron microscopy (TEM) through a FEI Tecnai F30 Super-win equipped with a LaB6 emission gun.
Fig. 1. XRD patterns of the Bi2W2O9/g-C3N4 photocatalysts. 2
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g-C3N4, respectively [22]. For the Bi2W2O9/g-C3N4 composites, the peak at 27.5� related to the (002) diffraction plane of the g-C3N4 became weaker as the Bi2W2O9 amount is increased in the system. From the 1% Bi2W2O9 sample, the samples began to exhibit diffraction peaks associ ated with the bismuth tungstate, confirming the presence of the semi conductor in the coupled system. FTIR spectra of the samples are displayed in Fig. 2a. As shown, the pure Bi2W2O9 exhibits infrared bands at 949, 855 and 801 cm-1 asso ciated with the symmetric and asymmetric vibrations of the W-O bond in the WO6 octahedron [18]. The broad band in the 830-650 cm-1 region can be attributed to the Bi-O stretching vibration in the material. For pristine g-C3N4, a broad peak is observed between 3000 and 3450 cm-1 which can be associated to the N-H stretching vibration modes in the material, indicating the presence of uncondensed amino functional groups [23,24]. In addition, the peaks located at 1636, 1566, 1401, 1326 and 1230 cm-1 are related to the stretching modes of the C-N heterocycles, while the sharp band at approximately 806 cm-1 corre sponds to the characteristic breathing mode of the triazine units [25,26]. For the Bi2W2O9/g-C3N4 samples, only the peaks associated to the graphitic carbon nitride were observed due to the high amount of this material in the samples (�95% wt.). It worth noting that these charac teristic bands did not show any shift after combination with the Bi2W2O9, indicating that there are no covalent bonds between the above oxide and the g-C3N4. The optical properties of the hybrid system were investigated using UV–vis absorption spectroscopic studies. Diffuse reflectance measurements are shown in Fig. 2b, where pure Bi2W2O9 displays an absorption edge near 425 nm while bare g-C3N4 exhibits a significant absorption in the visible region with an edge at 460 nm. As seen in FTIR analysis, the Bi2W2O9/g-C3N4 samples show similar profiles to pure graphitic carbon nitride due to the high amount of the semi conductor in the samples (�95 wt %). The band gap value for Bi2W2O9 was estimated at 2.91 eV while the values for the g-C3N4 and Bi2W2O9/g-C3N4 samples were calculated at around 2.70 eV, which is the value reported for pristine g-C3N4 prepared by direct calcination of the melamine precursor [27]. X-ray photoelectron spectroscopy (XPS) was conducted to study the surface chemical composition and chemical state of the samples. Fig. 3a displays the XPS survey spectra of the g-C3N4, 2%Bi2W2O9 and Bi2W2O9 photocatalysts, chosen as representative samples of the Bi2W2O9/g-C3N4 system. From the survey spectra, it can be noted that pristine g-C3N4 exhibits only peaks attributed to C, N and O elements while the Bi2W2O9 shows peaks associated to the Bi, W and O elements. For the 2%Bi2W2O9 sample, the peaks attributed to the C, N, Bi, W and O elements were observed. Fig. 3b–f shows the high resolution XPS spectra of bare g-C3N4
and 2%Bi2W2O9 material. The high-resolution C 1s spectrum in g-C3N4 can be divided into three peaks centered at 284.6, 286.1 and 287.9 eV. The energy peak at 284.6 eV is assigned to adventitious carbon which is determined as standard carbon [28]. The peaks at 286.1 and 287.9 eV are assigned to the C-(N)3 and N¼C-N bonds in the tris-s-triazine units of g-C3N4, respectively [23]. Similar peaks were observed in the high-resolution C 1s spectrum of the 2%Bi2W2O9 sample, where the binding energies were barely shifted. The high-resolution N 1s spectra of both samples are deconvoluted into three peaks at 398.4, 400.2 and 401.1 eV, see Fig. 3c. The main peak at 398.4 eV is related to the sp2-hybridized aromatic nitrogen bonded to carbon atoms (C¼N-C). The peak at 400.2 eV is ascribed to ternary nitrogen in the form of N-(C)3 and the peak at 401.1 eV is assigned to the nitrogen from amino groups (C-N-H) [29]. Fig. 3d and e displays the high-resolution spectra of the Bi 4f and W 4f regions, where obviously, the bare g-C3N4 does not show any detectable signal for the Bi and W elements. For the 2%Bi2W2O9 sample, the high-resolution Bi 4f spectrum can be divided into 4 peaks. The peaks located at 159.1 and 164.2 eV are assigned to the Bi 4f7/2 and Bi 4f5/2 spin states of the Bi3þ species [30]. The peaks at 156.7 and 161.9 eV are attributed to the Bi 4f7/2 and Bi 4f5/2 of metallic Bi species as it was previously reported [16]. In addition, the peaks at 34.9 and 36.8 eV are attributed to the W 4f7/2 and W 4f5/2 spin-orbital splitting photo electrons of the W6þ species, see Fig. 3e. The g-C3N4 and 2%Bi2W2O9 samples also exhibit signal in the O 1s region, where the hybrid pho tocatalyst displays a slight asymmetric peak that can be resolved into two peaks at 530.6 and 532.2 eV, related to the crystal lattice oxygen in the Bi2W2O9 oxide and the adsorbed oxygen in the sample, respectively. On the other hand, the g-C3N4 shows a well-defined symmetric peak centered at 532.1 eV assigned only to oxygen adsorbed on the surface, see Fig. 3f. The morphology of the Bi2W2O9/g-C3N4 samples was analyzed by scanning electron microscopy, see Fig. 4. For Bi2W2O9, the morphology consisted of particles with irregular rectangular shapes and micrometric sizes, while bare g-C3N4 exhibited a morphology based on aggregates of particles with irregular shapes. Moreover, the Bi2W2O9/g-C3N4 com posites show the micrometric particles of Bi2W2O9 surrounded by the particles of g-C3N4. For instance, elemental mapping and EDX analysis confirm the presence of the Bi2W2O9 oxide in the 2%Bi2W2O9 sample, as seen in Figs. S1–S2 and Table S1. TEM images also clearly reveal the intimate junction between the g-C3N4 and Bi2W2O9 semiconductors, see Fig. 5. In fact, the high-resolution TEM image (Fig. 5b) shows the g-C3N4 at the interfacial junction with a particle displaying a lattice fringe of approximate 0.298 nm, corresponding to the (115) plane of the Bi2W2O9 oxide. The interfacial connection between both semiconductors can
Fig. 2. (a) FTIR and (b) UV–vis absorption spectra of the Bi2W2O9/g-C3N4 samples. 3
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Fig. 3. (a) XPS survey spectra of Bi2W2O9, 2%Bi2W2O9 and g-C3N4 samples. XPS high resolution spectra of (b) C 1s, (c) N 1s, (d) Bi 4f, (e) W 4f and (f) O 1s of 2% Bi2W2O9 and g-C3N4 samples.
Fig. 4. SEM images of (a) pure Bi2W2O9, (b) 1% Bi2W2O9, (c) 2% Bi2W2O9, (d) 3% Bi2W2O9, (e) 5% Bi2W2O9 and (f) pure g-C3N4.
improve the spatial separation of the photogenerated charge carriers, which increases the photocatalytic features of the hybrid system. The surface area of the samples was estimated by the BET (BrunauerEmmett-Teller) method using nitrogen physisorption measurements. The specific surface area of bare g-C3N4 and Bi2W2O9 were estimated at 4.8 m2/g and 1.5 m2/g, respectively. For the hybrid photocatalysts, the values were quite similar (4.1–4.6 m2/g) to the estimated value for the
pristine g-C3N4, probably due to the high amount of the material in the composites, see Table 1. 3.2. Photocatalytic degradation of tetracycline antibiotic The photocatalytic features of the Bi2W2O9/g-C3N4 system were examined by the degradation of the antibiotic tetracycline under 4
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Fig. 5. TEM images of the 2%Bi2W2O9 hybrid photocatalyst.
of the tetracycline molecules. In this sense, the 2%Bi2W2O9 sample ex hibits the highest photocatalytic performance reaching about 95% decrease in the initial concentration of the antibiotic, compared to the 75% of degradation observed for the pristine g-C3N4. However, at higher concentrations of Bi2W2O9, the samples showed a marked decrease in the photoactive behavior. This phenomenon could be ascribed to an excessive amount of the Bi2W2O9 semiconductor which can act as a trap of the photogenerated holes and electrons, as it been reported in other heterojunction systems [31]. In a previous study, Zhu and co-workers have reported the formation of several intermediates during the pho tocatalytic degradation of the tetracycline using TiO2 as photocatalyst [32]. The analysis was carried out using an HPLC-MS system demon strating that the fragmentation of the molecules is highly complex and yields to the formation of a large number of by-products. Likewise, similar studies on the photocatalytic degradation of the tetracycline have confirmed that the molecules are mainly transformed into in termediates but not completely mineralized [33,34]. However, such by-products exhibit a lower biotoxicity than the non-degraded antibiotic [35,36]. The photodegradation rates were estimated according to Langmuir-Hinshelwood model and assuming a first-order kinetic given by the equation ln (C/C0) ¼ -kt, where C is the concentration of the antibiotic at a given time (t), C0 is the initial concentration of the tetracycline solution and the slope k is the apparent photodegradation rate constant. Table 1 shows the photodegradation rates using the hybrid photocatalysts, where the 2%Bi2W2O9 sample exhibited the best photoactive behavior with a reaction rate of 7.14 � 10-4 s-1, around 2.6 times higher than the rate estimated for pristine g-C3N4 (2.69 � 10-4 s-1). This enhanced photocatalytic performance could be explained accord ing to a possible decrease in the recombination of the photogenerated electron-hole pairs [37]. In this sense, the separation efficiency of the charge carriers was demonstrated by photoluminescence (PL) analysis [38]. Fig. 7 displays the PL spectra, where the pristine g-C3N4 exhibits a strong emission peak at approximately 460 nm, corresponding to a significant recombination of the electron-hole pairs in the material [39]. Conversely, the 2%Bi2W2O9 sample exhibits a lower intensity peak related to a decrease in the recombination rate of its charge carriers and, therefore, an improved photocatalytic performance. The pH value is an essential parameter in the behavior of a photo catalyst in aqueous dispersion. In order to examine the influence of the initial pH on the photodegradation of the tetracycline, several tests were performed over a wide range of pH values. Fig. 8 shows the degradation profiles of the antibiotic in the presence of the 2%Bi2W2O9 sample, where the highest photodegradation was reached at alkaline conditions of the aqueous suspension (pH~10.54). As reported by Safari et al., the effect of the pH value on the photocatalytic degradation of the tetra cycline is complicated to interpret due to multiple phenomena that
Table 1 Electronic, surface and photocatalytic properties of the Bi2W2O9/g-C3N4 system. Sample
Band gap (eV)
Surface area (m2/g)
Photodegradation of tetracycline (10-4 s-1)
g-C3N4 0.5% Bi2W2O9 1% Bi2W2O9 2% Bi2W2O9 3% Bi2W2O9 5% Bi2W2O9 Bi2W2O9
2.70 2.70
4.8 4.2
2.69 4.24
2.69
4.3
4.90
2.70
4.2
7.14
2.69
4.2
3.63
2.70
4.1
2.69
2.91
1.5
1.89
simulated solar irradiation. Fig. 6 shows the evolution of the tetracycline concentration in the presence of the photocatalysts. Under dark condi tions, all samples exhibited negligible adsorption of the antibiotic (less than 3% of the initial concentration). It is worth noting that the presence of Bi2W2O9 in the g-C3N4 semiconductor yields a high photodegradation
Fig. 6. Photocatalytic degradation of tetracycline antibiotic (C0 ¼ 10 mg/L, pH ¼ 4.89) as a function of irradiation time over Bi2W2O9, g-C3N4 and Bi2W2O9/gC3N4 samples. 5
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alkaline conditions, the photocatalyst particles and tetracycline mole cules exhibit negative charges and the surface interaction becomes negligible. The high photodegradation observed at these pH values may be since the basic conditions could favor the formation of superoxide ions which would react with the molecules of the antibiotic causing its cleavage [40]. To elucidate the effect of the radicals generated during the photo catalytic process, the 2%Bi2W2O9 sample was examined in the degra dation of the tetracycline in presence of several scavenger agents added to the reaction medium [43,44]. In this regard, 2-propanol, benzoqui none, potassium iodide (KI) and catalase were used to determine the role of the hydroxyl radicals (�OH), superoxide ions (O-2), photogenerated holes and hydrogen peroxide (H2O2), respectively [21,45]. As seen in Fig. 9, the presence of 2-propanol provided a decrease in the degrada tion of the antibiotic by about 30%, reaching a reaction rate of 5.03 � 10-4 s-1 compared to the reaction rate of 7.14 � 10-4 s-1 of the control test (without scavengers). An almost similar behavior was obtained in the presence of the benzoquinone, scavenger agent of the superoxide anion radicals, yielding a reaction rate of 4.78 � 10-4 s-1. However, the entrapment of H2O2 and photogenerated holes through catalase and KI scavengers, provided the most drastic decrease in the photodegradation, with reaction rates of 3.58 � 10-4 s-1 and 3.3 � 10-4 s-1, respectively. These data suggest that, although all the reactive species are involved in the photocatalytic process, the H2O2 and photogenerated holes play the major role in the degradation of the antibiotic tetracycline under our experimental conditions (pH ¼ 4.89). For a heterojunction system, the improvement in the separation of the charge carriers can be associated to the charge transfer at the interfacial junction between both materials. Based on it, the enhanced photocatalytic performance of the Bi2W2O9/g-C3N4 system can be explained according to the positions of the valence and conduction bands of each semiconductor. For the Bi2W2O9 prepared by coprecipitation, the bottom edge of the conduction band has been previ ously estimated at -0.90 eV [16]. The above value differs significantly from that reported by Bhoi and Mishra (þ0.325 eV) for the Bi2W2O9 prepared by a combustion route [18]. However, the empirical value estimated by these authors is not sufficiently negative to provide the formation of superoxide ions according to the reduction potential via one-electron transfer to singlet oxygen (-0.33 eV) [46]. Moreover, pre vious works have shown that Bi2W2O9 has the ability to evolve hydrogen gas from the water splitting process (0 eV) and to yield superoxide ions [47]. According to these reports, the conduction band edge must be
Fig. 7. Photoluminescence spectra of g-C3N4 and 2%Bi2W2O9 samples.
Fig. 8. Photocatalytic degradation of tetracycline (C0 ¼ 10 mg/L) at several initial pH values using the 2%Bi2W2O9 sample.
occur simultaneously such as adsorption, surface charge distribution and oxidation potential of the valence band of the photocatalyst [40]. According to literature, the point of zero charge (pHPZC) of the g-C3N4 prepared from melamine is around 5.2 [21,23]. If we consider that our sample has an almost similar value, then at pH values below the pHPZC the surface of the catalyst must exhibit a positive surface charge while at values above than the point of zero charge the surface charge of the catalyst is negative. On the other hand, the tetracycline molecule has pKa values of 3.3, 7.7, 9.7 and 12 predominantly existing as species H4TCþ, H3TC, H2TC- and HTC2 at pH values of 3, 7, 9 and 12, respectively [41,42]. Under strongly acidic conditions, the tetracycline and our sample must have positive surface charges and therefore, the adsorption of the antibiotic molecules on the surface of the photo catalyst can be considered negligible and the degradation must occur in the aqueous medium. At slightly acidic values and close to neutral pH, the surface of the photocatalyst has a negative charge and the tetracy cline exists as H4TCþ and H3TC, so the adsorption is feasible. Under
Fig. 9. Photocatalytic degradation of tetracycline (C0 ¼ 10 mg/L, pH ¼ 4.89) using the 2%Bi2W2O9 sample in presence of several radical scavengers. 6
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located at more negative values than -0.33 eV. In order to elucidate the redox properties of Bi2W2O9 and g-C3N4, the top edge of the valence band (VB) of each semiconductor was estimated by X-ray photoelectron spectroscopy, see Fig. 10a. From the above figure, the valence band maximum was estimated at þ2.02 eV and þ1.65 eV for the Bi2W2O9 and g-C3N4, respectively. The VB value for g-C3N4 is almost similar to that estimated through calculations of the Density Functional Theory (DFT) [48]. By linking these values and the band gap energies obtained by DRS, the bottom of the conduction band (CB) is estimated at -0.90 eV and -1.05 eV for the Bi2W2O9 and g-C3N4, respectively. According to the results of the photodegradation tests and the band edges of the involved semiconductors, a possible mechanism is described as follows: Bi2W2O9/g-C3N4 þ hν → Bi2W2O9 (e- þ hþ)/g-C3N4 (e- þ hþ)
(6)). The above mechanism is displayed in Fig. 10b. According to the results, the scavenging of H2O2 and the photogenerated holes lead to the most drastic decrease of the photodegradation rate, indicating that are the main species in the oxidation of the model pollutant. The Bi2W2O9 plays a crucial role in the hybrid photocatalyst since it works as a sink for the photoexcited electrons of the g-C3N4, thus promoting the spatial separation of the charge carriers generated in the graphitic-like carbon nitride. Furthermore, the Bi2W2O9 oxide provides the formation of �OH radicals through sequential reactions involving the superoxide ions. 4. Conclusions In summary, a novel type-II heterojunction photocatalytic system based on the Bi2W2O9 and g-C3N4 semiconductors has been prepared using an incipient impregnation method. From the characterization re sults, an interfacial connection between the particles of both semi conductors has been observed. Besides, the addition of Bi2W2O9 to gC3N4 did not affect the structural and textural properties of the graphitic carbon nitride. The heterojunction system exhibited an enhanced pho tocatalytic behavior in the degradation of the tetracycline antibiotic under simulated solar irradiation. In this way, the sample with 2% wt. of Bi2W2O9 showed the highest photocatalytic performance and the removal degree of the antibiotic was about 2.6 and 3.8 times as high as that of the pure g-C3N4 and Bi2W2O9 semiconductors, respectively. The improved photoactivity can be attributed to the different positions of the valence and conduction bands of Bi2W2O9 and g-C3N4, which favors an adequate spatial separation of the charge carriers through a type-II heterojunction. The energy band levels of the pristine semiconductors were determined experimentally by diffuse reflectance and X-ray photoelectron spectroscopies. From these results, a possible degradation mechanism of the tetracycline antibiotic has been explained. Since the gC3N4 does not have the redox properties to produce ∙OH radicals, the Bi2W2O9 plays an essential role by providing the formation of the hy droxyl radicals through sequential reactions involving the superoxide ions. The effect of the pH of the reaction medium has also been studied, demonstrating that under alkaline conditions the degradation process of the tetracycline is favored. Furthermore, by trapping tests was eluci dated that the H2O2 and photogenerated holes play the main role in the photodegradation of the antibiotic. This work demonstrates that the Bi2W2O9 oxide is a promising candidate for the assembly of highly photoactive heterojunction systems for energy and environmental applications.
(1)
Bi2W2O9 (e- þ hþ)/g-C3N4 (e- þ hþ) → Bi2W2O9 (e-CB) þ g-C3N4 (hþVB)(2) Bi2W2O9 (e-CB) þ O2 (adsorbed) → Bi2W2O9 þ O-2
(3)
O-2 þ 2Hþ þ Bi2W2O9 (e-CB) → Bi2W2O9 (e-CB) þ H2O2
(4)
H2O2 þ Bi2W2O9 (e-CB) → Bi2W2O9 þ �OH þ OH-
(5)
O-2/�OH/g-C3N4
(hþVB)
þ tetracycline molecules → degradation products (6)
Under UV–vis irradiation, the photogenerated charge carriers in Bi2W2O9 and g-C3N4 are formed (Eq. (1)). Due to the VB and CB edge positions of the semiconductors, the photoexcited electrons in the gC3N4 can migrate to the conduction band of the Bi2W2O9, while the photogenerated holes in this semiconductor can migrate to the VB of gC3N4 and thus the charge carriers can be effectively separated (Eq. (2)). This configuration in the edge potentials between both semiconductors can be attributed to a type-II heterojunction system [49]. The photo excited electrons in the Bi2W2O9 can react with the adsorbed molecular oxygen in the surface of the photocatalyst to yield superoxide ions (Eq. (3)) because its CB potential is more negative than the reduction po tential of the O2/O-2 redox couple (-0.33 eV). Besides, the photo generated holes accumulated in the g-C3N4 cannot directly oxidize the adsorbed hydroxyl groups to generate �OH radicals, since the VB po tential of the g-C3N4 (þ1.57 eV) has a lower positive value than the oxidation potential of OH-/�OH (þ2.40 eV). Nevertheless, the �OH radicals can also be formed from the transformation of the hydrogen peroxide (Eq. (5)) generated from the superoxide ions and the photo excited electrons in the Bi2W2O9 (Eq. (4)) [50]. In fact, our trapping tests have confirmed that the H2O2 plays an important role in the photo catalytic process of this heterojunction system. In this regard, the �OH radicals, the O-2 ions and the photogenerated holes in the g-C3N4 can oxidize the tetracycline molecules to yield oxidized by-products (Eq.
Fig. 10. (a) Valence-band XPS of Bi2W2O9 and g-C3N4 semiconductors; (b) proposed photocatalytic mechanism for the Bi2W2O9/g-C3N4 system under solar-like irradiation. 7
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Declaration of competing interest
[12] P. Chen, P. Xing, Z. Chen, X. Hu, H. Lin, L. Zhao, Y. He, J. Colloid Interface Sci. 534 (2019) 163–171. [13] N. Zhang, R. Ciriminna, M. Pagliaro, Y.J. Xu, Chem. Soc. Rev. 43 (2014) 5276–5287. [14] G. Long, J. Ding, L. Xie, R. Sun, M. Chen, Y. Zhou, X. Huang, G. Han, Y. Li, W. Zhao, Appl. Surf. Sci. 455 (2018) 1010–1018. [15] Y. Zhao, X. Liang, Y. Wang, H. Shi, E. Liu, J. Fan, X. Hu, J. Colloid Interface Sci. 523 (2018) 7–17. [16] S. Obreg� on, M.A. Ruíz-G� omez, D.B. Hern� andez-Uresti, J. Colloid Interface Sci. 506 (2017) 111–119. [17] Y.P. Bhoi, C. Behera, D. Majhi, Sk.Md Equeenuddin, B.G. Mishra, New J. Chem. 42 (2018) 281–292. [18] Y.P. Bhoi, B.G. Mishra, Eur. J. Inorg. Chem. (2018) 2648–2658, 2018. [19] Y.P. Bhoi, D. Majhi, K. Das, B.G. Mishra, ChemistrySelect 4 (2019) 3423–3431. [20] S.O. Alfaro, A. Martínez-de la Cruz, Appl. Catal. Gen. 383 (2010) 128–133. [21] D.B. Hern� andez-Uresti, A. V� azquez, D. Sanchez-Martinez, S. Obreg� on, J. Photochem. Photobiol. Chem. 324 (2016) 47–52. [22] K. Katsumata, R. Motoyoshi, N. Matsushita, K. Okada, J. Hazard Mater. 260 (2013) 475–482. [23] B. Zhu, P. Xia, W. Ho, J. Yu, Appl. Surf. Sci. 344 (2015) 188–195. [24] X. Chen, D.H. Kuo, D. Lu, RSC Adv. 6 (2016) 66814–66821. [25] F. Dong, Y. Li, Z. Wang, W.K. Ho, Appl. Surf. Sci. 358 (2015) 393–403. [26] Y. Ma, E. Liu, X. Hu, C. Tang, J. Wan, J. Li, J. Fan, Appl. Surf. Sci. 358 (2015) 246–251. [27] I. Aslam, M.H. Farooq, U. Ghani, M. Rizwan, G. Nabi, W. Shahzad, R. Boddula, Mater. Sci. Energy Technol. 2 (2019) 401–407. [28] F. Wei, Y. Liu, H. Zhao, X. Ren, J. Liu, T. Hasan, L. Chen, Y. Li, B.L. Su, Nanoscale 10 (2018) 4515–4522. [29] S. Vadivel, D. Maruthamani, A. Habibi-Yangjeh, B. Paul, S.S. Dhar, K. Selvam, J. Colloid Interface Sci. 480 (2016) 126–136. [30] A.P. Jakhade, M.V. Biware, R.C. Chikate, ACS Omega 2 (2017) 7219–7229. [31] K.C. Christoforidis, T. Montini, E. Bontempi, S. Zafeiratos, J.J.D. Ja� en, P. Fornasiero, Appl. Catal. B Environ. 187 (2016) 171–180. [32] X.D. Zhu, Y.J. Wang, R.J. Sun, D.M. Zhou, Chemosphere 92 (2013) 925–932. [33] X. Chu, G. Shan, C. Chang, Y. Fu, L. Yue, L. Zhu, Front. Environ. Sci. Eng. 10 (2016) 211–218. [34] X. Hu, Z. Sun, J. Song, G. Zhang, C. Li, S. Zheng, J. Colloid Interface Sci. 533 (2019) 238–250. [35] K. Zhou, X.D. Xie, C.T. Chang, Appl. Surf. Sci. 416 (2017) 248–258. [36] J. Niu, S. Ding, L. Zhang, J. Zhao, C. Feng, Chemosphere 93 (2013) 1–8. [37] L. Zeng, F. Zhe, Y. Wang, Q. Zhang, X. Zhao, X. Hu, Y. Wu, Y. He, J. Colloid Interface Sci. 539 (2019) 563–574. [38] J. Yu, Z. Chen, L. Zeng, Y. Ma, Z. Feng, Y. Wu, H. Lin, L. Zhao, Y. He, Sol. Energy Mater. Sol. Cells 179 (2018) 45–56. [39] E. Liu, J. Chen, Y. Ma, J. Feng, J. Jia, J. Fan, X. Hu, J. Colloid Interface Sci. 524 (2018) 313–324. [40] G.H. Safari, M. Hoseini, M. Seyedsalehi, H. Kamani, J. Jaafari, A.H. Mahvi, Int. J. Environ. Sci. Technol. 12 (2015) 603–616. [41] S. Jiao, S. Zheng, D. Yin, L. Wang, L. Chen, Chemosphere 73 (2008) 377–382. [42] P. Wang, P.S. Yap, T.T. Lim, Appl. Catal. Gen. 399 (2011) 252–261. [43] S. Meng, D. Li, M. Sun, W. Li, J. Wang, J. Chen, X. Fu, G. Xiao, Catal. Commun. 12 (2011) 972–975. [44] L.S. Zhang, K.H. Wong, H.Y. Yip, C. Hu, J.C. Yu, C.Y. Chan, P.K. Wong, Environ. Sci. Technol. 44 (2010) 1392–1398. [45] M. Yin, Z. Li, J. Kou, Z. Zou, Environ. Sci. Technol. 43 (2009) 8361–8366. [46] B. Li, C. Lai, G. Zeng, L. Qin, H. Yi, D. Huang, C. Zhou, X. Liu, M. Cheng, P. Xu, C. Zhang, F. Huang, S. Liu, ACS Appl. Mater. Interfaces 10 (2018) 18824–18836. [47] J. Tang, J. Ye, J. Mater. Chem. 15 (2005) 4246–4251. [48] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–80. [49] S. Obreg� on, G. Amor, A. V� azquez, Adv. Colloid Interface Sci. 269 (2019) 236–255. [50] Y. He, J. Cai, T. Li, Y. Wu, Y. Yi, M. Luo, L. Zhao, Ind. Eng. Chem. Res. 51 (2012) 14729–14737.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement � n: Conceptualization, Funding acquisition, Formal anal S. Obrego ysis, Writing - original draft, Writing - review & editing. M.A. Ruíz� mez: Funding acquisition, Formal analysis, Writing - review & edit Go � lez: Funding acquisition, Formal analysis, ing. V. Rodríguez-Gonza �zquez: Formal analysis, Writing - re Writing - review & editing. A. Va �ndez-Uresti: Funding acquisition, Writing view & editing. D.B. Herna review & editing. Acknowledgements S. Obreg� on thanks CONACYT for the project approved by the sectorial research fund for education CB 2017–2018 No. A1-S-9529. We also thank to Emmanuel Ramos-Moreno for valuable technical assis tance in the experimental tests during the program “XX Verano de la Investigaci� on Científica y Tecnol� ogica UANL PROVERICYT 2018”. Au thors are grateful for the use of the facilities of LANNBIO CinvestavM�erida supported principally by the grants FOMIX Yucat� an 2008–108160 and CONACYT LAB-2009-01-123913. Technical assis tance of D. Aguilar, D. Huerta and W. Cauich is appreciated. Also, we gratefully acknowledge to H. Silva Pereyra from LINAN-IPICYT for TEM analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2020.105056. References [1] J. Liu, H. Wang, M. Antonietti, Chem. Soc. Rev. 45 (2016) 2308–2326. [2] Y. Lan, Z. Li, D. Li, G. Yan, Z. Yang, S. Guo, Appl. Surf. Sci. 467–468 (2019) 411–422. [3] B. Lin, J. Li, B. Xu, X. Yan, B. Yang, J. Wei, G. Yang, Appl. Catal. B Environ. 243 (2019) 94–105. [4] Z. Sun, H. Wang, Z. Wu, L. Wang, Catal. Today 300 (2018) 160–172. [5] J. Ma, C. Wang, H. He, Appl. Catal. B Environ. 184 (2016) 28–34. [6] J. Wen, J. Xie, X. Chen, X. Li, Appl. Surf. Sci. 391 (2017) 72–123. [7] W. Chang, W. Xue, E. Liu, F. Fan, B. Zhao, Chem. Eng. J. 362 (2019) 392–401. [8] Z. Chen, P. Chen, P. Xing, X. Hu, H. Lin, L. Zhao, Y. Wu, Y. He, Fuel 241 (2019) 1–11. [9] Z. Feng, L. Zeng, Q. Zhang, S. Ge, X. Zhao, H. Lin, Y. He, J. Environ. Sci. 87 (2020) 149–162. [10] C. Zhao, Y. Chen, C. Li, Q. Zhang, P. Chen, K. Shi, Y. Wu, Y. He, J. Phys. Chem. Solid. 136 (2020), 109122. [11] E. Liu, C. Jin, C. Xu, J. Fan, X. Hu, Int. J. Hydrogen Energy 43 (2018) 21355–21364.
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A novel type-II Bi2W2O9/g-C3N4 heterojunction with enhanced photocatalytic performance under simulated solar irradiation �n a, *, M.A. Ruíz-Go �mez b, V. Rodríguez-Gonza �lez c, A. V�azquez d, e, D. S. Obrego a B. Hern� andez-Uresti a
Universidad Aut� onoma de Nuevo Le� on, Centro de Investigaci� on en Ciencias Físico Matem� aticas, Facultad de Ciencias Físico Matem� aticas, Av. Universidad S/N, San Nicol� as de los Garza, 66455, Nuevo Le� on, Mexico CONACYT-Departamento de Física Aplicada, CINVESTAV-IPN, Antigua Carretera a Progreso km 6, M�erida, Yucat� an, 97310, Mexico c Divisi� on de Materiales Avanzados, IPICYT (Instituto Potosino de Investigaci� on Científica y Tecnol� ogica), Camino a la Presa San Jos�e 2055, Col. Lomas 4a, secci� on C.P, 78216, San Luis Potosí, S.L.P., Mexico d Universidad Aut� onoma de Nuevo Le� on, Facultad de Ciencias Químicas, Av. Universidad S/N, San Nicol� as de los Garza, 66455, Nuevo Le� on, Mexico e Centro de Investigaci� on en Biotecnología y Nanotecnología, Facultad de Ciencias Químicas, Universidad Aut� onoma de Nuevo Le� on, Parque de Investigaci� on e Innovaci� on Tecnol� ogica, Km. 10 Autopista al Aeropuerto Internacional Mariano Escobedo, Apodaca, 66629, Nuevo Le� on, Mexico b
A R T I C L E I N F O
A B S T R A C T
Keywords: Semiconductors Heterojunction Bi2W2O9 g-C3N4 Photocatalysis
In this work, the Bi2W2O9/g-C3N4 type-II heterojunction system has been prepared for the first time by an incipient impregnation method. From the results, we have stated that the addition of low amounts of Bi2W2O9 (�5 wt%) does not provide changes in the structural and textural properties of g-C3N4, but it improves sub stantially the photocatalytic performance under simulated solar irradiation. The best photoactive behavior was attained in the sample with 2 wt% of Bi2W2O9, providing a removal degree of the tetracycline antibiotic about 2.6 times higher than the pristine g-C3N4. This result can be explained due to the difference in the edge positions of the valence and conduction bands of both semiconductors, which provides an efficient separation of the photogenerated charge carriers. From the study of the photocatalytic activity under several conditions, it can be inferred that the Bi2W2O9 works as a sink for the photoexcited electrons of the g-C3N4 and provides the formation of hydroxyl radicals through sequential reactions involving the superoxide ions. Furthermore, a possible pho tocatalytic mechanism of tetracycline removal is discussed.
1. Introduction The graphitic carbon nitride (g-C3N4) is a semiconductor that has attracted a wide attention in the last years due to its interesting prop erties such as absorption in the visible-light region, graphene-like layered structure, high thermal stability and outstanding photo catalytic performance [1]. The metal-free nature and low cost of prep aration are essential parameters for its promising use on an industrial scale. Some photocatalytic applications have been reported, including the degradation of organic pollutants, water splitting, CO2 reduction, NOx oxidation and so on [2–5]. However, the main disadvantage of the graphitic carbon nitride is the high recombination rate of its photo generated charge carriers, which limits the photoactive behavior. Several strategies have been performed to reduce these recombination processes, such as the exfoliation of the bulk layered material, the
addition of doping agents, the surface modification with noble metal nanoparticles and the formation of heterojunction systems with other semiconductors [6]. In this sense, several hybrid systems based on g-C3N4 have been reported [7–10]. For instance, Liu and co-workers have reported the preparation of the Ni2P/g-C3N4 system through an in-situ hydrothermal process [11]. The samples showed outstanding photocatalytic performance in the H2 production, up to 68 times higher than that obtained with pure g-C3N4. Chen et al., have also adopted this strategy using the graphitic carbon nitride [12]. The authors carried out a microwave heating procedure to obtain the AgNbO3/g-C3N4 system using AgNbO3 and melamine as precursors. Such hybrid photocatalysts also have demonstrated enhanced photoactive features compared to bare g-C3N4. Previous studies as described above demonstrate the high efficiency of the heterojunction systems based on g-C3N4, so the search for new hybrid photocatalysts remains a latent challenge.
* Corresponding author. E-mail address: [email protected] (S. Obreg� on). https://doi.org/10.1016/j.mssp.2020.105056 Received 3 December 2019; Received in revised form 10 February 2020; Accepted 6 March 2020 Available online 12 March 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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The Bi2WO6 is a semiconductor oxide with narrow band gap energy and excellent visible-light-driven photocatalytic performance [13]. This material belongs to the Aurivillius-type layered oxides with a corner-sharing structure of WO6 octahedron interleaved by (Bi2O2)2þ layers. Furthermore, it has been used in the preparation of the Bi2WO6/g-C3N4 system which exhibits enhanced photoactive properties in the degradation of several organic compounds [14,15]. Other mate rial of interest is the Bi2W2O9 oxide, which is a member of the same family of the cation deficient Aurivillius phases [16]. To date, few studies have been reported using the Bi2W2O9 in the formation of het erojunction systems and, to the best of our knowledge, the coupling of this oxide with g-C3N4 has not been studied so far [17–19]. In this work, we report the preparation of the Bi2W2O9/g-C3N4 system by an incipient impregnation method using 2-propanol as dispersing agent. The tetra cycline antibiotic was chosen as the model pollutant to evaluate the photoactive behavior of the coupled system under simulated solar irradiation. In this sense, the enhanced photocatalytic activity was investigated systematically through the modification of several vari ables of the photocatalytic reaction such as the pH of the aqueous dispersion and the use of several scavengers to elucidate the role of the photogenerated reactive species.
The powders were dispersed directly on carbon tape and placed in an aluminum sample-holder. The surface properties were studied by N2 adsorption-desorption isotherms using a Bel-Japan Minisorp II Surface Area & Pore Size analyzer. The physisorption measurements were per formed at -196 � C after a pretreatment at 150 � C during 12 h. The surface area was estimated by the BET (Brunauer-Emmett-Teller) method. The photoluminescence (PL) emission spectra were analyzed using a Hitachi F-7000 spectrophotometer. The measurements were performed with an excitation light source of 350 nm at room temperature. 2.3. Photocatalytic tests The photocatalytic performance of the hybrid system was examined through the degradation of the tetracycline antibiotic in aqueous solu tion. First, 0.2 g of each sample were added to 200 mL of an aqueous solution of tetracycline hydrochloride (Sigma-Aldrich �95%) with an initial concentration of 10 mg/L. The pH value of the resulting sus pension was estimated at 4.89. The suspension was left under dark conditions for 30 min to reach the adsorption-desorption equilibrium of the antibiotic molecules on the surface of the photocatalyst. Then, the system was irradiated with a 35 W Xenon lamp used as source of simulated solar light. The concentration of the antibiotic was followed through the absorbance of its characteristic band at 357 nm using an aliquot ca. 3 mL of the suspension filtered with a nylon syringe filter (0.2 μm) from Thermo Scientific. Several tests were performed by varying the initial pH value of the suspension, in a range of 2.8–10.5, using HNO3 or NH4OH. Besides, several radical scavengers were used to elucidate the role of the reactive oxygen species involved in the photo catalytic process [21].
2. Experimental 2.1. Synthesis of the Bi2W2O9/g-C3N4 system The graphitic carbon nitride (g-C3N4) was prepared by means of the polycondensation of the melamine precursor. Thereby, 10 g of melamine were placed in a covered crucible and submitted to thermal treatment at 500 � C for 4 h. The synthesis of the Bi2W2O9 oxide was carried out through a coprecipitation route, as reported in our previous work [16]. Briefly, two aqueous solutions containing the precursors Bi(NO3)3⋅5H2O (Sig ma-Aldrich, 99.9%) and (NH4)10(W12O41)⋅H2O (Aldrich, 99.9%) were mixed to obtain a milky suspension, which was adjusted to a pH ~ 5.0 by using concentrated ammonia. Then, the suspension was evaporated slowly at 70 � C until a white powder was obtained. This precursor was calcined at 700 � C for 24 h to yield the Bi2W2O9 semiconductor. For the preparation of the coupled system, the Bi2W2O9 and g-C3N4 semiconductors were assembled by an incipient impregnation proced ure. In detail, appropriate amounts of the powdered materials were dispersed in 100 mL of 2-propanol and submitted to ultrasound radia tion at 37 kHz for 1 h. Then, each hybrid photocatalyst was obtained by evaporating the alcohol at 90 � C. Several samples were prepared with different loadings of the Bi2W2O9 oxide (0.5, 1, 2, 3 and 5% wt.).
3. Results and discussions 3.1. Synthesis and characterization of the samples The Bi2W2O9/g-C3N4 heterojunction system was characterized through powder X-ray diffraction, as shown in Fig. 1. The pure Bi2W2O9 oxide can be assigned to an orthorhombic crystal structure with space group Pbn21 according to the PDF 33–0221. It is worth noting that the small peak at 28.3� (2θ) is associated with the diffraction plane (131) of the Bi2WO6 oxide, as it has been discussed previously [16]. For pristine g-C3N4, the diffraction peaks observed at 13.2� and 27.5� (2θ) are related to the periodic arrangement of the condensed tris-s-triazine units and to the interlayer stacking of the conjugated aromatic system in the
2.2. Characterization of the samples The Bi2W2O9/g-C3N4 system was structurally characterized by X-ray powder diffraction using a Bruker D-8 Advance diffractometer with Cu Kα radiation (0.15406 nm). The measurements were recorded between 10� and 90� (2θ) with a scanning rate of 0.02� s-1. Fourier-transform infrared spectroscopy (FTIR) was carried out using a JASCO FT/IR4000 spectrometer and the spectra were recorded in the range of 500–4000 cm-1. Diffuse reflectance measurements were performed using an Agilent Technologies UV–Vis–NIR spectrophotometer model Cary 5000 series equipped with an integrating sphere. The band gap energy values were estimated from the absorption spectra of the samples as previously described [20]. XPS spectra were recorded through a Thermo Scientific K-Alpha XPS spectrometer equipped with a hemi spherical electron analyzer and working with Al Kα radiation (hν ¼ 1486.6 eV). The C 1s signal (284.6 eV) was used as an internal energy reference in all the experiments. The photocatalysts were also analyzed by field emission scanning electron microscopy (FE-SEM) using a JEOL JSM-7600 F microscope and by transmission electron microscopy (TEM) through a FEI Tecnai F30 Super-win equipped with a LaB6 emission gun.
Fig. 1. XRD patterns of the Bi2W2O9/g-C3N4 photocatalysts. 2
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Materials Science in Semiconductor Processing 113 (2020) 105056
g-C3N4, respectively [22]. For the Bi2W2O9/g-C3N4 composites, the peak at 27.5� related to the (002) diffraction plane of the g-C3N4 became weaker as the Bi2W2O9 amount is increased in the system. From the 1% Bi2W2O9 sample, the samples began to exhibit diffraction peaks associ ated with the bismuth tungstate, confirming the presence of the semi conductor in the coupled system. FTIR spectra of the samples are displayed in Fig. 2a. As shown, the pure Bi2W2O9 exhibits infrared bands at 949, 855 and 801 cm-1 asso ciated with the symmetric and asymmetric vibrations of the W-O bond in the WO6 octahedron [18]. The broad band in the 830-650 cm-1 region can be attributed to the Bi-O stretching vibration in the material. For pristine g-C3N4, a broad peak is observed between 3000 and 3450 cm-1 which can be associated to the N-H stretching vibration modes in the material, indicating the presence of uncondensed amino functional groups [23,24]. In addition, the peaks located at 1636, 1566, 1401, 1326 and 1230 cm-1 are related to the stretching modes of the C-N heterocycles, while the sharp band at approximately 806 cm-1 corre sponds to the characteristic breathing mode of the triazine units [25,26]. For the Bi2W2O9/g-C3N4 samples, only the peaks associated to the graphitic carbon nitride were observed due to the high amount of this material in the samples (�95% wt.). It worth noting that these charac teristic bands did not show any shift after combination with the Bi2W2O9, indicating that there are no covalent bonds between the above oxide and the g-C3N4. The optical properties of the hybrid system were investigated using UV–vis absorption spectroscopic studies. Diffuse reflectance measurements are shown in Fig. 2b, where pure Bi2W2O9 displays an absorption edge near 425 nm while bare g-C3N4 exhibits a significant absorption in the visible region with an edge at 460 nm. As seen in FTIR analysis, the Bi2W2O9/g-C3N4 samples show similar profiles to pure graphitic carbon nitride due to the high amount of the semi conductor in the samples (�95 wt %). The band gap value for Bi2W2O9 was estimated at 2.91 eV while the values for the g-C3N4 and Bi2W2O9/g-C3N4 samples were calculated at around 2.70 eV, which is the value reported for pristine g-C3N4 prepared by direct calcination of the melamine precursor [27]. X-ray photoelectron spectroscopy (XPS) was conducted to study the surface chemical composition and chemical state of the samples. Fig. 3a displays the XPS survey spectra of the g-C3N4, 2%Bi2W2O9 and Bi2W2O9 photocatalysts, chosen as representative samples of the Bi2W2O9/g-C3N4 system. From the survey spectra, it can be noted that pristine g-C3N4 exhibits only peaks attributed to C, N and O elements while the Bi2W2O9 shows peaks associated to the Bi, W and O elements. For the 2%Bi2W2O9 sample, the peaks attributed to the C, N, Bi, W and O elements were observed. Fig. 3b–f shows the high resolution XPS spectra of bare g-C3N4
and 2%Bi2W2O9 material. The high-resolution C 1s spectrum in g-C3N4 can be divided into three peaks centered at 284.6, 286.1 and 287.9 eV. The energy peak at 284.6 eV is assigned to adventitious carbon which is determined as standard carbon [28]. The peaks at 286.1 and 287.9 eV are assigned to the C-(N)3 and N¼C-N bonds in the tris-s-triazine units of g-C3N4, respectively [23]. Similar peaks were observed in the high-resolution C 1s spectrum of the 2%Bi2W2O9 sample, where the binding energies were barely shifted. The high-resolution N 1s spectra of both samples are deconvoluted into three peaks at 398.4, 400.2 and 401.1 eV, see Fig. 3c. The main peak at 398.4 eV is related to the sp2-hybridized aromatic nitrogen bonded to carbon atoms (C¼N-C). The peak at 400.2 eV is ascribed to ternary nitrogen in the form of N-(C)3 and the peak at 401.1 eV is assigned to the nitrogen from amino groups (C-N-H) [29]. Fig. 3d and e displays the high-resolution spectra of the Bi 4f and W 4f regions, where obviously, the bare g-C3N4 does not show any detectable signal for the Bi and W elements. For the 2%Bi2W2O9 sample, the high-resolution Bi 4f spectrum can be divided into 4 peaks. The peaks located at 159.1 and 164.2 eV are assigned to the Bi 4f7/2 and Bi 4f5/2 spin states of the Bi3þ species [30]. The peaks at 156.7 and 161.9 eV are attributed to the Bi 4f7/2 and Bi 4f5/2 of metallic Bi species as it was previously reported [16]. In addition, the peaks at 34.9 and 36.8 eV are attributed to the W 4f7/2 and W 4f5/2 spin-orbital splitting photo electrons of the W6þ species, see Fig. 3e. The g-C3N4 and 2%Bi2W2O9 samples also exhibit signal in the O 1s region, where the hybrid pho tocatalyst displays a slight asymmetric peak that can be resolved into two peaks at 530.6 and 532.2 eV, related to the crystal lattice oxygen in the Bi2W2O9 oxide and the adsorbed oxygen in the sample, respectively. On the other hand, the g-C3N4 shows a well-defined symmetric peak centered at 532.1 eV assigned only to oxygen adsorbed on the surface, see Fig. 3f. The morphology of the Bi2W2O9/g-C3N4 samples was analyzed by scanning electron microscopy, see Fig. 4. For Bi2W2O9, the morphology consisted of particles with irregular rectangular shapes and micrometric sizes, while bare g-C3N4 exhibited a morphology based on aggregates of particles with irregular shapes. Moreover, the Bi2W2O9/g-C3N4 com posites show the micrometric particles of Bi2W2O9 surrounded by the particles of g-C3N4. For instance, elemental mapping and EDX analysis confirm the presence of the Bi2W2O9 oxide in the 2%Bi2W2O9 sample, as seen in Figs. S1–S2 and Table S1. TEM images also clearly reveal the intimate junction between the g-C3N4 and Bi2W2O9 semiconductors, see Fig. 5. In fact, the high-resolution TEM image (Fig. 5b) shows the g-C3N4 at the interfacial junction with a particle displaying a lattice fringe of approximate 0.298 nm, corresponding to the (115) plane of the Bi2W2O9 oxide. The interfacial connection between both semiconductors can
Fig. 2. (a) FTIR and (b) UV–vis absorption spectra of the Bi2W2O9/g-C3N4 samples. 3
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Fig. 3. (a) XPS survey spectra of Bi2W2O9, 2%Bi2W2O9 and g-C3N4 samples. XPS high resolution spectra of (b) C 1s, (c) N 1s, (d) Bi 4f, (e) W 4f and (f) O 1s of 2% Bi2W2O9 and g-C3N4 samples.
Fig. 4. SEM images of (a) pure Bi2W2O9, (b) 1% Bi2W2O9, (c) 2% Bi2W2O9, (d) 3% Bi2W2O9, (e) 5% Bi2W2O9 and (f) pure g-C3N4.
improve the spatial separation of the photogenerated charge carriers, which increases the photocatalytic features of the hybrid system. The surface area of the samples was estimated by the BET (BrunauerEmmett-Teller) method using nitrogen physisorption measurements. The specific surface area of bare g-C3N4 and Bi2W2O9 were estimated at 4.8 m2/g and 1.5 m2/g, respectively. For the hybrid photocatalysts, the values were quite similar (4.1–4.6 m2/g) to the estimated value for the
pristine g-C3N4, probably due to the high amount of the material in the composites, see Table 1. 3.2. Photocatalytic degradation of tetracycline antibiotic The photocatalytic features of the Bi2W2O9/g-C3N4 system were examined by the degradation of the antibiotic tetracycline under 4
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Fig. 5. TEM images of the 2%Bi2W2O9 hybrid photocatalyst.
of the tetracycline molecules. In this sense, the 2%Bi2W2O9 sample ex hibits the highest photocatalytic performance reaching about 95% decrease in the initial concentration of the antibiotic, compared to the 75% of degradation observed for the pristine g-C3N4. However, at higher concentrations of Bi2W2O9, the samples showed a marked decrease in the photoactive behavior. This phenomenon could be ascribed to an excessive amount of the Bi2W2O9 semiconductor which can act as a trap of the photogenerated holes and electrons, as it been reported in other heterojunction systems [31]. In a previous study, Zhu and co-workers have reported the formation of several intermediates during the pho tocatalytic degradation of the tetracycline using TiO2 as photocatalyst [32]. The analysis was carried out using an HPLC-MS system demon strating that the fragmentation of the molecules is highly complex and yields to the formation of a large number of by-products. Likewise, similar studies on the photocatalytic degradation of the tetracycline have confirmed that the molecules are mainly transformed into in termediates but not completely mineralized [33,34]. However, such by-products exhibit a lower biotoxicity than the non-degraded antibiotic [35,36]. The photodegradation rates were estimated according to Langmuir-Hinshelwood model and assuming a first-order kinetic given by the equation ln (C/C0) ¼ -kt, where C is the concentration of the antibiotic at a given time (t), C0 is the initial concentration of the tetracycline solution and the slope k is the apparent photodegradation rate constant. Table 1 shows the photodegradation rates using the hybrid photocatalysts, where the 2%Bi2W2O9 sample exhibited the best photoactive behavior with a reaction rate of 7.14 � 10-4 s-1, around 2.6 times higher than the rate estimated for pristine g-C3N4 (2.69 � 10-4 s-1). This enhanced photocatalytic performance could be explained accord ing to a possible decrease in the recombination of the photogenerated electron-hole pairs [37]. In this sense, the separation efficiency of the charge carriers was demonstrated by photoluminescence (PL) analysis [38]. Fig. 7 displays the PL spectra, where the pristine g-C3N4 exhibits a strong emission peak at approximately 460 nm, corresponding to a significant recombination of the electron-hole pairs in the material [39]. Conversely, the 2%Bi2W2O9 sample exhibits a lower intensity peak related to a decrease in the recombination rate of its charge carriers and, therefore, an improved photocatalytic performance. The pH value is an essential parameter in the behavior of a photo catalyst in aqueous dispersion. In order to examine the influence of the initial pH on the photodegradation of the tetracycline, several tests were performed over a wide range of pH values. Fig. 8 shows the degradation profiles of the antibiotic in the presence of the 2%Bi2W2O9 sample, where the highest photodegradation was reached at alkaline conditions of the aqueous suspension (pH~10.54). As reported by Safari et al., the effect of the pH value on the photocatalytic degradation of the tetra cycline is complicated to interpret due to multiple phenomena that
Table 1 Electronic, surface and photocatalytic properties of the Bi2W2O9/g-C3N4 system. Sample
Band gap (eV)
Surface area (m2/g)
Photodegradation of tetracycline (10-4 s-1)
g-C3N4 0.5% Bi2W2O9 1% Bi2W2O9 2% Bi2W2O9 3% Bi2W2O9 5% Bi2W2O9 Bi2W2O9
2.70 2.70
4.8 4.2
2.69 4.24
2.69
4.3
4.90
2.70
4.2
7.14
2.69
4.2
3.63
2.70
4.1
2.69
2.91
1.5
1.89
simulated solar irradiation. Fig. 6 shows the evolution of the tetracycline concentration in the presence of the photocatalysts. Under dark condi tions, all samples exhibited negligible adsorption of the antibiotic (less than 3% of the initial concentration). It is worth noting that the presence of Bi2W2O9 in the g-C3N4 semiconductor yields a high photodegradation
Fig. 6. Photocatalytic degradation of tetracycline antibiotic (C0 ¼ 10 mg/L, pH ¼ 4.89) as a function of irradiation time over Bi2W2O9, g-C3N4 and Bi2W2O9/gC3N4 samples. 5
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alkaline conditions, the photocatalyst particles and tetracycline mole cules exhibit negative charges and the surface interaction becomes negligible. The high photodegradation observed at these pH values may be since the basic conditions could favor the formation of superoxide ions which would react with the molecules of the antibiotic causing its cleavage [40]. To elucidate the effect of the radicals generated during the photo catalytic process, the 2%Bi2W2O9 sample was examined in the degra dation of the tetracycline in presence of several scavenger agents added to the reaction medium [43,44]. In this regard, 2-propanol, benzoqui none, potassium iodide (KI) and catalase were used to determine the role of the hydroxyl radicals (�OH), superoxide ions (O-2), photogenerated holes and hydrogen peroxide (H2O2), respectively [21,45]. As seen in Fig. 9, the presence of 2-propanol provided a decrease in the degrada tion of the antibiotic by about 30%, reaching a reaction rate of 5.03 � 10-4 s-1 compared to the reaction rate of 7.14 � 10-4 s-1 of the control test (without scavengers). An almost similar behavior was obtained in the presence of the benzoquinone, scavenger agent of the superoxide anion radicals, yielding a reaction rate of 4.78 � 10-4 s-1. However, the entrapment of H2O2 and photogenerated holes through catalase and KI scavengers, provided the most drastic decrease in the photodegradation, with reaction rates of 3.58 � 10-4 s-1 and 3.3 � 10-4 s-1, respectively. These data suggest that, although all the reactive species are involved in the photocatalytic process, the H2O2 and photogenerated holes play the major role in the degradation of the antibiotic tetracycline under our experimental conditions (pH ¼ 4.89). For a heterojunction system, the improvement in the separation of the charge carriers can be associated to the charge transfer at the interfacial junction between both materials. Based on it, the enhanced photocatalytic performance of the Bi2W2O9/g-C3N4 system can be explained according to the positions of the valence and conduction bands of each semiconductor. For the Bi2W2O9 prepared by coprecipitation, the bottom edge of the conduction band has been previ ously estimated at -0.90 eV [16]. The above value differs significantly from that reported by Bhoi and Mishra (þ0.325 eV) for the Bi2W2O9 prepared by a combustion route [18]. However, the empirical value estimated by these authors is not sufficiently negative to provide the formation of superoxide ions according to the reduction potential via one-electron transfer to singlet oxygen (-0.33 eV) [46]. Moreover, pre vious works have shown that Bi2W2O9 has the ability to evolve hydrogen gas from the water splitting process (0 eV) and to yield superoxide ions [47]. According to these reports, the conduction band edge must be
Fig. 7. Photoluminescence spectra of g-C3N4 and 2%Bi2W2O9 samples.
Fig. 8. Photocatalytic degradation of tetracycline (C0 ¼ 10 mg/L) at several initial pH values using the 2%Bi2W2O9 sample.
occur simultaneously such as adsorption, surface charge distribution and oxidation potential of the valence band of the photocatalyst [40]. According to literature, the point of zero charge (pHPZC) of the g-C3N4 prepared from melamine is around 5.2 [21,23]. If we consider that our sample has an almost similar value, then at pH values below the pHPZC the surface of the catalyst must exhibit a positive surface charge while at values above than the point of zero charge the surface charge of the catalyst is negative. On the other hand, the tetracycline molecule has pKa values of 3.3, 7.7, 9.7 and 12 predominantly existing as species H4TCþ, H3TC, H2TC- and HTC2 at pH values of 3, 7, 9 and 12, respectively [41,42]. Under strongly acidic conditions, the tetracycline and our sample must have positive surface charges and therefore, the adsorption of the antibiotic molecules on the surface of the photo catalyst can be considered negligible and the degradation must occur in the aqueous medium. At slightly acidic values and close to neutral pH, the surface of the photocatalyst has a negative charge and the tetracy cline exists as H4TCþ and H3TC, so the adsorption is feasible. Under
Fig. 9. Photocatalytic degradation of tetracycline (C0 ¼ 10 mg/L, pH ¼ 4.89) using the 2%Bi2W2O9 sample in presence of several radical scavengers. 6
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located at more negative values than -0.33 eV. In order to elucidate the redox properties of Bi2W2O9 and g-C3N4, the top edge of the valence band (VB) of each semiconductor was estimated by X-ray photoelectron spectroscopy, see Fig. 10a. From the above figure, the valence band maximum was estimated at þ2.02 eV and þ1.65 eV for the Bi2W2O9 and g-C3N4, respectively. The VB value for g-C3N4 is almost similar to that estimated through calculations of the Density Functional Theory (DFT) [48]. By linking these values and the band gap energies obtained by DRS, the bottom of the conduction band (CB) is estimated at -0.90 eV and -1.05 eV for the Bi2W2O9 and g-C3N4, respectively. According to the results of the photodegradation tests and the band edges of the involved semiconductors, a possible mechanism is described as follows: Bi2W2O9/g-C3N4 þ hν → Bi2W2O9 (e- þ hþ)/g-C3N4 (e- þ hþ)
(6)). The above mechanism is displayed in Fig. 10b. According to the results, the scavenging of H2O2 and the photogenerated holes lead to the most drastic decrease of the photodegradation rate, indicating that are the main species in the oxidation of the model pollutant. The Bi2W2O9 plays a crucial role in the hybrid photocatalyst since it works as a sink for the photoexcited electrons of the g-C3N4, thus promoting the spatial separation of the charge carriers generated in the graphitic-like carbon nitride. Furthermore, the Bi2W2O9 oxide provides the formation of �OH radicals through sequential reactions involving the superoxide ions. 4. Conclusions In summary, a novel type-II heterojunction photocatalytic system based on the Bi2W2O9 and g-C3N4 semiconductors has been prepared using an incipient impregnation method. From the characterization re sults, an interfacial connection between the particles of both semi conductors has been observed. Besides, the addition of Bi2W2O9 to gC3N4 did not affect the structural and textural properties of the graphitic carbon nitride. The heterojunction system exhibited an enhanced pho tocatalytic behavior in the degradation of the tetracycline antibiotic under simulated solar irradiation. In this way, the sample with 2% wt. of Bi2W2O9 showed the highest photocatalytic performance and the removal degree of the antibiotic was about 2.6 and 3.8 times as high as that of the pure g-C3N4 and Bi2W2O9 semiconductors, respectively. The improved photoactivity can be attributed to the different positions of the valence and conduction bands of Bi2W2O9 and g-C3N4, which favors an adequate spatial separation of the charge carriers through a type-II heterojunction. The energy band levels of the pristine semiconductors were determined experimentally by diffuse reflectance and X-ray photoelectron spectroscopies. From these results, a possible degradation mechanism of the tetracycline antibiotic has been explained. Since the gC3N4 does not have the redox properties to produce ∙OH radicals, the Bi2W2O9 plays an essential role by providing the formation of the hy droxyl radicals through sequential reactions involving the superoxide ions. The effect of the pH of the reaction medium has also been studied, demonstrating that under alkaline conditions the degradation process of the tetracycline is favored. Furthermore, by trapping tests was eluci dated that the H2O2 and photogenerated holes play the main role in the photodegradation of the antibiotic. This work demonstrates that the Bi2W2O9 oxide is a promising candidate for the assembly of highly photoactive heterojunction systems for energy and environmental applications.
(1)
Bi2W2O9 (e- þ hþ)/g-C3N4 (e- þ hþ) → Bi2W2O9 (e-CB) þ g-C3N4 (hþVB)(2) Bi2W2O9 (e-CB) þ O2 (adsorbed) → Bi2W2O9 þ O-2
(3)
O-2 þ 2Hþ þ Bi2W2O9 (e-CB) → Bi2W2O9 (e-CB) þ H2O2
(4)
H2O2 þ Bi2W2O9 (e-CB) → Bi2W2O9 þ �OH þ OH-
(5)
O-2/�OH/g-C3N4
(hþVB)
þ tetracycline molecules → degradation products (6)
Under UV–vis irradiation, the photogenerated charge carriers in Bi2W2O9 and g-C3N4 are formed (Eq. (1)). Due to the VB and CB edge positions of the semiconductors, the photoexcited electrons in the gC3N4 can migrate to the conduction band of the Bi2W2O9, while the photogenerated holes in this semiconductor can migrate to the VB of gC3N4 and thus the charge carriers can be effectively separated (Eq. (2)). This configuration in the edge potentials between both semiconductors can be attributed to a type-II heterojunction system [49]. The photo excited electrons in the Bi2W2O9 can react with the adsorbed molecular oxygen in the surface of the photocatalyst to yield superoxide ions (Eq. (3)) because its CB potential is more negative than the reduction po tential of the O2/O-2 redox couple (-0.33 eV). Besides, the photo generated holes accumulated in the g-C3N4 cannot directly oxidize the adsorbed hydroxyl groups to generate �OH radicals, since the VB po tential of the g-C3N4 (þ1.57 eV) has a lower positive value than the oxidation potential of OH-/�OH (þ2.40 eV). Nevertheless, the �OH radicals can also be formed from the transformation of the hydrogen peroxide (Eq. (5)) generated from the superoxide ions and the photo excited electrons in the Bi2W2O9 (Eq. (4)) [50]. In fact, our trapping tests have confirmed that the H2O2 plays an important role in the photo catalytic process of this heterojunction system. In this regard, the �OH radicals, the O-2 ions and the photogenerated holes in the g-C3N4 can oxidize the tetracycline molecules to yield oxidized by-products (Eq.
Fig. 10. (a) Valence-band XPS of Bi2W2O9 and g-C3N4 semiconductors; (b) proposed photocatalytic mechanism for the Bi2W2O9/g-C3N4 system under solar-like irradiation. 7
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Declaration of competing interest
[12] P. Chen, P. Xing, Z. Chen, X. Hu, H. Lin, L. Zhao, Y. He, J. Colloid Interface Sci. 534 (2019) 163–171. [13] N. Zhang, R. Ciriminna, M. Pagliaro, Y.J. Xu, Chem. Soc. Rev. 43 (2014) 5276–5287. [14] G. Long, J. Ding, L. Xie, R. Sun, M. Chen, Y. Zhou, X. Huang, G. Han, Y. Li, W. Zhao, Appl. Surf. Sci. 455 (2018) 1010–1018. [15] Y. Zhao, X. Liang, Y. Wang, H. Shi, E. Liu, J. Fan, X. Hu, J. Colloid Interface Sci. 523 (2018) 7–17. [16] S. Obreg� on, M.A. Ruíz-G� omez, D.B. Hern� andez-Uresti, J. Colloid Interface Sci. 506 (2017) 111–119. [17] Y.P. Bhoi, C. Behera, D. Majhi, Sk.Md Equeenuddin, B.G. Mishra, New J. Chem. 42 (2018) 281–292. [18] Y.P. Bhoi, B.G. Mishra, Eur. J. Inorg. Chem. (2018) 2648–2658, 2018. [19] Y.P. Bhoi, D. Majhi, K. Das, B.G. Mishra, ChemistrySelect 4 (2019) 3423–3431. [20] S.O. Alfaro, A. Martínez-de la Cruz, Appl. Catal. Gen. 383 (2010) 128–133. [21] D.B. Hern� andez-Uresti, A. V� azquez, D. Sanchez-Martinez, S. Obreg� on, J. Photochem. Photobiol. Chem. 324 (2016) 47–52. [22] K. Katsumata, R. Motoyoshi, N. Matsushita, K. Okada, J. Hazard Mater. 260 (2013) 475–482. [23] B. Zhu, P. Xia, W. Ho, J. Yu, Appl. Surf. Sci. 344 (2015) 188–195. [24] X. Chen, D.H. Kuo, D. Lu, RSC Adv. 6 (2016) 66814–66821. [25] F. Dong, Y. Li, Z. Wang, W.K. Ho, Appl. Surf. Sci. 358 (2015) 393–403. [26] Y. Ma, E. Liu, X. Hu, C. Tang, J. Wan, J. Li, J. Fan, Appl. Surf. Sci. 358 (2015) 246–251. [27] I. Aslam, M.H. Farooq, U. Ghani, M. Rizwan, G. Nabi, W. Shahzad, R. Boddula, Mater. Sci. Energy Technol. 2 (2019) 401–407. [28] F. Wei, Y. Liu, H. Zhao, X. Ren, J. Liu, T. Hasan, L. Chen, Y. Li, B.L. Su, Nanoscale 10 (2018) 4515–4522. [29] S. Vadivel, D. Maruthamani, A. Habibi-Yangjeh, B. Paul, S.S. Dhar, K. Selvam, J. Colloid Interface Sci. 480 (2016) 126–136. [30] A.P. Jakhade, M.V. Biware, R.C. Chikate, ACS Omega 2 (2017) 7219–7229. [31] K.C. Christoforidis, T. Montini, E. Bontempi, S. Zafeiratos, J.J.D. Ja� en, P. Fornasiero, Appl. Catal. B Environ. 187 (2016) 171–180. [32] X.D. Zhu, Y.J. Wang, R.J. Sun, D.M. Zhou, Chemosphere 92 (2013) 925–932. [33] X. Chu, G. Shan, C. Chang, Y. Fu, L. Yue, L. Zhu, Front. Environ. Sci. Eng. 10 (2016) 211–218. [34] X. Hu, Z. Sun, J. Song, G. Zhang, C. Li, S. Zheng, J. Colloid Interface Sci. 533 (2019) 238–250. [35] K. Zhou, X.D. Xie, C.T. Chang, Appl. Surf. Sci. 416 (2017) 248–258. [36] J. Niu, S. Ding, L. Zhang, J. Zhao, C. Feng, Chemosphere 93 (2013) 1–8. [37] L. Zeng, F. Zhe, Y. Wang, Q. Zhang, X. Zhao, X. Hu, Y. Wu, Y. He, J. Colloid Interface Sci. 539 (2019) 563–574. [38] J. Yu, Z. Chen, L. Zeng, Y. Ma, Z. Feng, Y. Wu, H. Lin, L. Zhao, Y. He, Sol. Energy Mater. Sol. Cells 179 (2018) 45–56. [39] E. Liu, J. Chen, Y. Ma, J. Feng, J. Jia, J. Fan, X. Hu, J. Colloid Interface Sci. 524 (2018) 313–324. [40] G.H. Safari, M. Hoseini, M. Seyedsalehi, H. Kamani, J. Jaafari, A.H. Mahvi, Int. J. Environ. Sci. Technol. 12 (2015) 603–616. [41] S. Jiao, S. Zheng, D. Yin, L. Wang, L. Chen, Chemosphere 73 (2008) 377–382. [42] P. Wang, P.S. Yap, T.T. Lim, Appl. Catal. Gen. 399 (2011) 252–261. [43] S. Meng, D. Li, M. Sun, W. Li, J. Wang, J. Chen, X. Fu, G. Xiao, Catal. Commun. 12 (2011) 972–975. [44] L.S. Zhang, K.H. Wong, H.Y. Yip, C. Hu, J.C. Yu, C.Y. Chan, P.K. Wong, Environ. Sci. Technol. 44 (2010) 1392–1398. [45] M. Yin, Z. Li, J. Kou, Z. Zou, Environ. Sci. Technol. 43 (2009) 8361–8366. [46] B. Li, C. Lai, G. Zeng, L. Qin, H. Yi, D. Huang, C. Zhou, X. Liu, M. Cheng, P. Xu, C. Zhang, F. Huang, S. Liu, ACS Appl. Mater. Interfaces 10 (2018) 18824–18836. [47] J. Tang, J. Ye, J. Mater. Chem. 15 (2005) 4246–4251. [48] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–80. [49] S. Obreg� on, G. Amor, A. V� azquez, Adv. Colloid Interface Sci. 269 (2019) 236–255. [50] Y. He, J. Cai, T. Li, Y. Wu, Y. Yi, M. Luo, L. Zhao, Ind. Eng. Chem. Res. 51 (2012) 14729–14737.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement � n: Conceptualization, Funding acquisition, Formal anal S. Obrego ysis, Writing - original draft, Writing - review & editing. M.A. Ruíz� mez: Funding acquisition, Formal analysis, Writing - review & edit Go � lez: Funding acquisition, Formal analysis, ing. V. Rodríguez-Gonza �zquez: Formal analysis, Writing - re Writing - review & editing. A. Va �ndez-Uresti: Funding acquisition, Writing view & editing. D.B. Herna review & editing. Acknowledgements S. Obreg� on thanks CONACYT for the project approved by the sectorial research fund for education CB 2017–2018 No. A1-S-9529. We also thank to Emmanuel Ramos-Moreno for valuable technical assis tance in the experimental tests during the program “XX Verano de la Investigaci� on Científica y Tecnol� ogica UANL PROVERICYT 2018”. Au thors are grateful for the use of the facilities of LANNBIO CinvestavM�erida supported principally by the grants FOMIX Yucat� an 2008–108160 and CONACYT LAB-2009-01-123913. Technical assis tance of D. Aguilar, D. Huerta and W. Cauich is appreciated. Also, we gratefully acknowledge to H. Silva Pereyra from LINAN-IPICYT for TEM analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2020.105056. References [1] J. Liu, H. Wang, M. Antonietti, Chem. Soc. Rev. 45 (2016) 2308–2326. [2] Y. Lan, Z. Li, D. Li, G. Yan, Z. Yang, S. Guo, Appl. Surf. Sci. 467–468 (2019) 411–422. [3] B. Lin, J. Li, B. Xu, X. Yan, B. Yang, J. Wei, G. Yang, Appl. Catal. B Environ. 243 (2019) 94–105. [4] Z. Sun, H. Wang, Z. Wu, L. Wang, Catal. Today 300 (2018) 160–172. [5] J. Ma, C. Wang, H. He, Appl. Catal. B Environ. 184 (2016) 28–34. [6] J. Wen, J. Xie, X. Chen, X. Li, Appl. Surf. Sci. 391 (2017) 72–123. [7] W. Chang, W. Xue, E. Liu, F. Fan, B. Zhao, Chem. Eng. J. 362 (2019) 392–401. [8] Z. Chen, P. Chen, P. Xing, X. Hu, H. Lin, L. Zhao, Y. Wu, Y. He, Fuel 241 (2019) 1–11. [9] Z. Feng, L. Zeng, Q. Zhang, S. Ge, X. Zhao, H. Lin, Y. He, J. Environ. Sci. 87 (2020) 149–162. [10] C. Zhao, Y. Chen, C. Li, Q. Zhang, P. Chen, K. Shi, Y. Wu, Y. He, J. Phys. Chem. Solid. 136 (2020), 109122. [11] E. Liu, C. Jin, C. Xu, J. Fan, X. Hu, Int. J. Hydrogen Energy 43 (2018) 21355–21364.
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