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BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation
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Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation Kanlayawat Wangkawonga,b, Sukon Phanichphantb, Doldet Tantraviwatc, Burapat Inceesungvornb,* a
Graduate School, Chiang Mai University, Chiang Mai 50200 Thailand Department of Chemistry, Center of Excellence for Innovation in Chemistry (PERCH-CIC) and Centre of Excellence in Materials Science and Technology, Faculty of Science, Chiang Mai University, Chiang Mai 50200 Thailand c Department of Electrical Engineering, Faculty of Engineering, Chiang Mai University, Chiang Mai 50200, Thailand b
A R T I C L E
I N F O
Article History: Received 29 October 2019 Revised 3 January 2020 Accepted 6 January 2020 Available online xxx Keywords: CeO2 BiOI Heterojunction Z-scheme Visible light
A B S T R A C T
The Z-scheme CeO2/BiOI (C/B) was prepared and evaluated for its photocatalytic RhB degradation and selective oxidation of benzylamine to N-benzylidenebenzylamine under visible light irradiation. Close contacts and chemical interactions between CeO2 and BiOI which are important for efficient charge transfer in the heterojunction were revealed by HRTEM and XPS studies. The C/B heterostructure presented nearly 1.5 and 8.0 times higher RhB degradation activity than BiOI and CeO2, respectively. The activity of C/B in oxidative coupling of benzylamine was also firstly revealed and found to be more than 2 times higher than both BiOI and CeO2. Such enhanced photocatalytic performance of the C/B is ascribed to the combined effects of extended visible-light absorption range, increased surface area and improved charge transfer efficiency as evidenced from BET, UVvis DRS and photoelectrochemical studies. Based on XPS, UVvis DRS, MottSchottky plots and active species quenching results, a Z-scheme charge transfer where photogenerated elec tron-hole pairs can be effectively separated is proposed for the C/B and h+ and O2 are key active species responsible for RhB degradation and N-benzylidenebenzylamine formation. The present work highlights the enhancement of photocatalytic activity based on Z-scheme heterojunction formation and reveals a further application of photocatalysts in organic fine chemical syntheses. © 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction One of the most important sustainable energies is solar energy which consists of »5% of ultraviolet, »40% of visible light, and »55% of near infrared [1,2]. To exploit the solar energy in chemical reactions, photocatalysis technology has been developed and employed in many applications, for example, organic pollutant degradation [36], H2 production [7,8], and organic fine chemical syntheses [911]. Cerium dioxide (CeO2) is broadly used as electrolyte in solid oxide fuel cell [12,13], oxygen sensor [14,15], UV blocking material [16,17], photocatalyst [36] and electrocatalysts [18,19] due to its high oxygen storage capacity, strong UV absorption ability, good chemical stability and nontoxicity. For the photocatalytic application, the wide band gap energy and high charge carrier recombination of CeO2 largely limit its practical use [6,20]. To overcome this drawback, various strategies have been developed, for example, metal loading,
* Corresponding author. E-mail address: [email protected] (B. Inceesungvorn).
morphology controlling, and heterojunction formation. The heterojunction formation is widely accepted as an effective strategy to improve charge separation and transfer leading to enhanced photocatalytic performance [2125]. The coupling between a wide bandgap semiconductor and a narrow one provides an extended light absorption range and generates more photoexcited charges under visible light irradiation. To achieve a successful heterojunction, band energy levels of the two components in coupling which directly affect redox ability and charge transfer mechanism of the heterostructure have to be mainly considered. A staggered-lineup heterostructure providing either type-II or Z-scheme charge transfers is desirable as the charge transfer and separation efficiencies of photocatalysts are significantly promoted [22,25]. In comparison with the type-II mechanism, the Z-scheme pathway can maintain strong redox properties of both valence band hole and conduction band electron of the coupling materials which are essential to drive reaction kinetics, especially for sluggish reactions [26]. BiOI has a tetragonal matlockite structure in which [Bi2 O2 ] 2+ slabs are interleaved by double slabs of iodide ions. Each layer
https://doi.org/10.1016/j.jtice.2020.01.003 1876-1070/© 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: K. Wangkawong et al., Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation, Journal of the Taiwan Institute of Chemical Engineers (2020), https:// doi.org/10.1016/j.jtice.2020.01.003
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of IBiOOBiI is connected through Van der Waals interactions along c-axis, causing the formation of internal electric field in this direction which consequently promotes photoexcited charge separation [27,28]. BiOI with a narrow band gap of ca. 1.8 eV has shown to be a promising visible-light-driven photocatalyst in organic pollutant degradation and water splitting [2730]. The band energy levels of BiOI are also suitable for CeO2 in order to create a staggered band alignment, therefore coupling between BiOI and CeO2 may boost performance of the photocatalytic system. In this study, a Z-scheme CeO2/BiOI heterojunction was synthesized and evaluated for its photocatalytic performance via RhB degradation and a selective oxidation of benzylamine to N-benzylidenebenzylamine under visible light. Although previous studies have already shown the organic degradation ability of this heterostructure [29,30], its photocatalytic activity toward a selective oxidation of benzylamine to N-benzylidenebenzylamine has never been investigated before. Physicochemical properties and energy band alignment were also revealed to understand structure-activity relationship. A possible mechanism for the selective oxidation of benzylamine was also proposed. The success of this work also encourages a further development and application of heterojunction photocatalysts toward green organic fine chemical syntheses.
adsorption-desorption equilibrium. During irradiation, a sample was taken at regular intervals and centrifuged to remove the catalyst. The concentration of RhB was determined by UVvis spectrophotometer (Lambda 25, Perkin Elmer) at a maximum absorption of 554 nm. To study catalyst stability, the used catalyst was washed with DI water and dried at 80 °C for 12 h for a recycling experiment. Radical scavenging experiment was carried out by adding different trapping agents. The quenching agent concentration was 1 £ 103 M in all studies. Photocatalytic oxidative coupling of benzylamine to N-benzylidenebenzylamine was investigated by dispersing 0.4 g of catalyst in 10 ml of 0.5 M benzylamine in acetonitrile containing o-dichlorobenzene as an internal standard. The reaction was carried out under the O2 flow rate of 20 ml min1 at a temperature of 28 § 1 °C controlling by a cooling fan. The suspension was stirred under dark for 30 min, then irradiated by 50 W cool white LED which emits light in a visible range (λ > 400 nm). To identify the composition and concentration of products, a sample was taken periodically, then analyzed by GC-FID (Clarus 500/580 GC, Perkin Elmer) and further confirmed by GCMS (Agilent 7820A-5977E).
2. Experimental
Photoelectrochemical experiments were carried out on Metrohm Autolab Potentiostat/Galvanostat (PGSTAT128N) using three-electrode setup. To prepare a working electrode, 0.1 g of photocatalyst was dispersed in 2 ml of 2% nafion in ethanol, then 20 mL of this solution was dropped on FTO glass (1 £ 1 cm). The Ag/AgCl and Pt wire were employed as a reference electrode and a counter electrode, respectively. Electron impedance spectroscopy (EIS) was measured in the range of 0.1105 Hz with an AC voltage amplitude of 10 mV at an applied voltage of 1.0 V using 1.0 M Na2SO4 electrolyte solution (pH » 5.17). The flat band potentials of photocatalysts were also determined from MottSchottky plots by scanning the applied voltage from 0.8 to 1.0 V with a frequency of 100 Hz. Transient photocurrent response was recorded at a bias potential of 1.0 V using 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as an electrolyte. The same light source as that in the photocatalytic test was used in all photoelectrochemical experiments.
2.1. Synthesis of CeO2, BiOI and CeO2/BiOI heterojunction For the synthesis of CeO2, an aqueous solution of 0.25 M NaOH was added dropwise into 80 ml of 0.05 M Ce(NO3)3¢6H2O aqueous solution until pH 10 was attained. The solution was kept under stirring at a constant pH for 2 h. The precipitates were collected by centrifugation and washed with DI water until neutral pH was reached. The catalyst was dried overnight, then the pale yellow CeO2 powder was finally obtained after calcination in air at 400 °C for 4 h. BiOI and 50 wt% CeO2/BiOI (denoted as C/B) were prepared by a similar procedure. Typically, 0.5 M of Bi(NO3)3¢5H2O and 3.5 ml of conc. HNO3 were dissolved in 50 ml of DI water. The obtained solution was mixed with 50 ml of KI solution with the mole ratio of Bi: I equal to 1:1. The solution pH was then adjusted to 4 by NaOH solution and kept constant under stirring for 1 h. After that, the precipitate was collected and washed with DI water several times. The catalyst was dried at 80 °C for 18 h, then a brownish orange powder was finally obtained. For the preparation of C/B, 0.2 g of calcined CeO2 was added into the BiOI precursor prior to adjusting the solution pH. 2.2. Material characterizations Structure and phase compositions were studied by powder X-ray diffraction (XRD) (Miniflex II, Rigaku) with Cu Ka radiation (λ = 1.5405 A). Morphology and microstructure were investigated by scanning electron microscope (SEM, JSM-6335F, JEOL) and transmission electron microscope (TEM, JEM-2010, JEOL). Band gap energy was determined by UVvis diffuse reflectance spectrometer (UV3101 PC, SHIMADZU). BrunauerEmmerTeller (BET) specific surface area was analyzed by N2 adsorption on an Autosorb-1, Quantachrome. The oxidation state and elemental composition of catalysts were studied by X-ray photoelectron spectroscopy (XPS) using an Axis Ultra DLD, Kratos. 2.3. Photocatalytic tests Photocatalytic performance was evaluated via a degradation of RhB and a selective oxidation of benzylamine under visible light. The photocatalytic activity for RhB degradation was studied using 50 W halogen lamp. Typically, 0.05 g of catalyst was dispersed in 100 ml of aqueous RhB solution (1 £ 105 M). Prior to light irradiation, the catalyst suspension was stirred under dark for 30 min to achieve an
2.4. Photoelectrochemical studies
3. Results and discussion 3.1. Physicochemical properties of materials The crystallographic structures of CeO2, 50 wt% CeO2/BiOI (C/B), and BiOI characterized by XRD are shown in Fig. 1(a). The diffraction peaks of CeO2 and BiOI correspond well with cubic and tetragonal phases according to JCPDS no. 340394 and 10-0445, respectively. The C/B composite exhibits the diffraction peaks of both CeO2 and BiOI which are characteristics of composite catalyst. The light absorption properties of CeO2, C/B, and BiOI were investigated as shown in Fig. 1(b). CeO2 exhibits a strong light absorption below 396 nm, while C/B and BiOI show the absorption edges at 609 and 622 nm. The band gap energies of CeO2, C/B, and BiOI are 3.13, 2.04, and 2.00 eV, respectively. Morphology and microstructure of the samples were characterized by SEM and TEM. The SEM results reveal irregular particles of CeO2 (Fig. S1(a)), while BiOI appears as microplate (Fig. S1(b)). The C/B in Fig. S1(c) presents both irregular CeO2 particles and BiOI microplates. Microstructure and particle size of the samples are revealed by TEM in Fig. 2(a)(f). The irregular shape of CeO2 with an approximate size of 15 nm is found (Fig. 2(a)). The corresponding ring pattern (Fig. 2(b)) indicates a polycrystalline character of cubic CeO2. The plate-like feature with a diameter of »0.80 mm (Fig. 2(c)) and its spotted SAED pattern (Fig. 2(d)) suggest a single crystalline nature of the BiOI microplate. The TEM image of C/B in Fig. 2(e) indicates the decoration of small irregular CeO2 particles on the microplates of BiOI. HRTEM in Fig. 2(f) shows lattice spacings of 0.31 nm
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Fig. 1. XRD patterns (a) and UVvis diffuse reflectance spectra (b) of CeO2, C/B, and BiOI.
and 0.28 nm corresponding to (111) plane of CeO2, and (110) plane of BiOI, respectively [27,29,30] and close interfacial contacts between CeO2 and BiOI which are necessary for an efficient charge transfer in heterojunction system. Surface elemental composition and chemical state of the catalysts were determined by X-ray photoelectron spectroscopy (XPS) as shown in Fig. 3. The high-resolution spectra of Ce 3d in Fig. 3(a) can be fitted into five doublet pairs of Ce3+ and Ce4+ species. The first three doublet peaks located at 881.7, 888.3, 897.8, and 900.3, 906.9, 916.1 eV are ascribed to V, V2, V3, and U, U2, U3 of Ce4+ ion [4,31]. While, the peaks at 879.9, 884.0, 898.1, and 902.6 eV are due to V0, V1, U0, and U1 of Ce3+ ion [31]. The Ce 3d peaks clearly shift to higher binding energy in the case of C/B composite. These shifts suggest chemical interactions as coupling CeO2 with BiOI, which further ensure a successful heterojunction formation. The Ce3+/(Ce3++Ce4+) ratio of the C/B sample (0.20) is lower than that of pure CeO2 (0.27), suggesting that Ce3+ is partially oxidized upon coupling with BiOI. Two peaks of Bi 4f (Fig. 3(b)) located at 159.2 and 164.5 eV are attributed to Bi 4f7/2 and Bi 4f5/2 of Bi3+ in BiOI [32,33]. In contrast to the Ce 3d peaks, these Bi 4f peaks of the C/B heterojunction shift to lower binding energy, indicating an increased electron density in the BiOI but a loss of electron density in CeO2 as forming this heterostructure. These binding energy shifts also suggest that, in the C/B heterojunction, electrons migrate from CeO2 to BiOI. This assumption is supported by a decrease of Ce3+/(Ce3++Ce4+) ratio as a result of Ce3+ oxidation after coupling CeO2 with BiOI. The I 3d peaks (Fig. 3(c)) at 619.2 and 630.7 eV correspond to I 3d5/2 and I 3d3/2 of I in BiOI [22]. These peaks in the C/B also shift to lower binding energy in correspondence with the Bi peaks. A minor doublet pair at 620.2 and 631.7 eV due to I3 ion as a result of I photooxidation [3436] is found for pure BiOI but not the C/B, thus suggesting a better photostability of the heterojunction than pure BiOI. The O 1 s spectrum of BiOI (Fig. 3(d)) presents three oxygen species at 530.0, 530.8, and 532.3 eV which are assigned to BiO in BiOI, chemisorbed oxygen, and surface hydroxyl species [22,37], respectively. The O 1 s spectrum of pure CeO2 in Fig. 3(d) can also be fitted into three components. The low binding energy peak (528529.5 eV) is attributed to lattice oxygen (OL) in CeO2 structure, and that in the range of 530531 eV originates from anion oxygen species adsorbed on oxygen vacancy sites (Ov) [20,38]. The peak centered at 531.5 eV belongs to surface hydroxyl species or adsorbed water (Oads) [20,39]. 3.2. Photocatalytic performances under visible light Photocatalytic activity for RhB removal in Fig. 4(a) shows that the C/B provides distinctly higher RhB degradation performance (95.4%) than pure CeO2 (12.0%) and BiOI (65.8%) within 3 h irradiation. The C/B also presents a superior activity than a physical mixture of BiOI and CeO2 at the same weight ratio as that of the heterojunction
catalyst. This suggests that chemical interactions and close contacts between CeO2 and BiOI are inevitable for the enhanced photocatalytic activity. The initial pseudo-first-order rate constants (k) of each catalyst in Fig. 4(b) show that the C/B heterostructure can degrade RhB ca. 2.5, 4.5 and 8.6 times faster than that of BiOI, physical mixture and CeO2, respectively. The BET surface areas of CeO2, C/B, and BiOI are 57.2, 26.7, and 6.4 m2/g, respectively. As considering the BET surface area and band gap energy of the catalysts, the enhanced photocatalytic activity of C/B heterostructure in RhB degradation may partially be ascribed to its increased surface area and visible-light absorption ability upon coupling CeO2 with BiOI. It is widely known that several active species such as h+, O2 , and OH play crucial roles in oxidizing organic pollutants, therefore radical scavenging experiment was carried out to determine the main active species and reveal a possible photocatalytic mechanism over the C/B heterojunction system. Scavenging experiments were also carried out in the presence of BiOI and CeO2 for comparison purpose. From Fig. 4(c), in the case of CeO2, a decrease of RhB activity is found only when adding h+ and OH quenchers. On the contrary, the RhB degradation activity of C/B largely decreases when adding ammonium oxalate (AO) and p-benzoquinone (p-BQ) in the C/B photocata lytic system, indicating that h+ and O2 are main active species governing the RhB degradation over this heterojunction photocatalyst. On the contrary, the introduction of isopropanol (IPA) only has a minor effect on the degradation activity, revealing a minor role of OH in this catalytic system. A similar behavior to that of C/B composite is observed for BiOI. Our results also agree well with those found in Ref. [29,30] where h+ and O2 but not OH play crucial role in the degradation of organic compounds in aqueous solution. Stability of the photocatalysts is also important for practical use. As shown in Fig. 4(d), the performance of C/B remains nearly the same upto four cyclic runs, whereas that of BiOI noticeably decreases from 58% to 12%. The poor stability of BiOI may be due to the photocorrosion process originating from the well-known direct oxidation of iodide by photogenerated hole [28,36]. To further extend an applicability of the developed photocatalyst, the C/B was tested for its performance in the oxidative coupling of benzylamine to N-benzylidenebenzylamine, which is an important imine intermediate for the synthesis of many pharmaceuticals and biological active nitrogen-containing compounds [23]. Traditionally, Lewis acid catalysts, harmful oxidants, and elevated temperature are required for the imine synthesis. However, due to environmental issues, the synthesis based on photocatalysis has been developed and recognized as a greener and more facile method because the toxic oxidants are substituted by air or molecular oxygen and the corrosive and harmful homogeneous catalyst is taken over by an earth abundant heterogeneous catalyst. Moreover, the conventional heating is replaced by light activation at room temperature to realize a renewable energy-based synthetic process. Fig. 5 shows conversion of
Please cite this article as: K. Wangkawong et al., Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation, Journal of the Taiwan Institute of Chemical Engineers (2020), https:// doi.org/10.1016/j.jtice.2020.01.003
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Fig. 2. TEM images and SADE patterns of CeO2 (ab) and BiOI (cd). TEM (e) and HRTEM images (f) of C/B heterostructure.
benzylamine, selectivity and yield of N-benzylidenebenzylamine product obtained from all catalysts up to 8 h irradiation. The N-benzylidenebenzylamine is found as a major product with small amounts of benzonitrile and benzaldehyde (Scheme 1). No other by-products such as benzamide and benzoic acid have been found from all catalysts. Compared with the C/B heterojunction, a physical mixture (C+B) produces two times lower imine yield, which apparently confirms that chemical interaction rather than just a physical contact between the two materials is necessary for the photocatalytic improvement. The highest yield (25.0%) is obtained from the C/B heterojunction at 8 h irradiation whereas only 8.3%, 12.1% and 11.7%
yield are found for CeO2, BiOI and the physical mixture, respectively. Although, the imine yield is still low for practical application, our results clearly show that an improved activity could be achieved from the heterojunction photocatalyst, opening up a further development and an alternative pathway toward green organic synthesis. Despite the increase of benzylamine conversion over time, the selectivity to imine is obviously decreased due to the increased amounts of benzonitrile and benzaldehyde. The highest amount of benzonitrile and benzaldehyde observed from the C/B catalyst at 8 h are 3.3% and 4.9% yield, respectively. Benzonitrile is produced from a complete oxidative dehydrogenation of benzylamine via benzylimine
Please cite this article as: K. Wangkawong et al., Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation, Journal of the Taiwan Institute of Chemical Engineers (2020), https:// doi.org/10.1016/j.jtice.2020.01.003
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Fig. 3. High resolution spectra of Ce 3d (a), Bi 4f (b), I 3d (c), and O 1 s (d).
Fig. 4. Photocatalytic RhB degradation activities (a) and corresponding pseudo-first-order kinetics (b) over CeO2, BiOI, C/B heterojunction and physical mixture under visible light irradiation. Active species quenching experiments within 3 h irradiation (c). Recycling performances of 50 C/B compared with BiOI (d).
Please cite this article as: K. Wangkawong et al., Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation, Journal of the Taiwan Institute of Chemical Engineers (2020), https:// doi.org/10.1016/j.jtice.2020.01.003
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Fig. 5. Benzylamine oxidation under visible light irradiation at different reaction times over CeO2, BiOI, C/B heterojunction and a physical mixture between CeO2 and BiOI (C+B).
intermediate whereas benzaldehyde is generated from an incomplete oxidation of benzylamine when using molecular oxygen as a green oxidant [9,40]. The benzaldehyde can also occur from the hydrolysis of N-benzylidenebenzylamine [40, 41]. As one benzonitrile molecule is formed, two water molecules are also released which can then hydrolyze N-benzylidenebenzylamine into benzaldehyde and benzylamine, resulting in a poor selectivity toward the imine. From Fig. 5, CeO2 obviously shows much lower selectivity than other photocatalysts possibly due to stronger oxidizing ability of its valence band hole as evidenced by the higher positive value of valence band potential which is shown later in Fig. 8(b). Adsorption studies indicate that BiOI adsorbs only 3% of the initial benzylamine concentration whereas CeO2 and C/B samples adsorb ca. 19.5% and 18.2%, respectively (data not shown). Although CeO2 has the highest surface area among the three catalysts (CeO2, C/B, and BiOI are 57.2, 26.7, and 6.4 m2/g, respectively), the benzylamine adsorption over CeO2 is nearly the same as that over the C/B, which suggests that the surface area is not the dominant factor governing benzylamine adsorption, hence its conversion. This rather indicates different surface properties of the three catalysts. Different conversion and selectivity values observed among all catalysts used may be ascribed to different amount and strength of acidic sites and the redox nature of solid
catalysts [40,42]. These sites are believed to help activate benzylamine to benzylimine intermediate which subsequently reacts with another benzylamine to finally form imine product [10]. Further investigations into catalyst surface acidity may help explain the different activities observed. Photocatalytic stability in benzylamine oxidation was also evaluated as shown in Fig. 6(a). Only a slight decrease in imine yield (< 4%) is observed for all catalysts within four consecutive cycles, indicating excellent stability of these catalysts under the conditions applied. Active radical quenching study (Fig. 6(b)) was also carried out to reveal the active species involving in the benzylamine oxidation. It is clear that, in all photocatalytic systems, the addition of t-BuOH which is a OH scavenger does not affect the product yield, suggesting that the OH is not engaged in the imine formation. In CeO2 system, the introduction of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and K2Cr2O7 as O2 and e quenchers as well as replacing O2 with N2 does not reduce the imine yield either. However, a significant decrease of imine amount is found when adding triethanolamine (TEA), a h+ quencher. These observations suggest an essential role of h+ in the oxidative cou pling of benzylamine in the CeO2 system. On the contrary, O2, O2 , + e and h are all involved in the imine formation in both BiOI and C/B heterojunction systems, although the role of each of these species in both photocatalytic systems possibly differs to some extent. Based on our results from quenching experiments and previous literatures [9, 40], a possible mechanism for the benzylamine oxidation in the C/B heterostructure system is proposed in Scheme 2. Benzylamine molecule is firstly adsorbed on the photocatalyst surface and then oxidized by h+ at the valence band (VB) of photocatalyst to produce aminium cation radical. At the same time, O2 reacts with e at the conduction band (CB) to generate O2 which further reacts with the formed cation radical to produce benzylimine by releasing H2O2. Then, another benzylamine molecule will react with the formed benzylimine to finally obtain N-benzylidenebenzylamine with NH3 as a by-product [43,44]. In our study, benzonitrile and benzaldehyde are also found. It is possible that the benzylimine intermediate is further oxidized to benzonitrile and water molecules. The benzylimine may also react with water to produce benzaldehyde which subsequently couples with free benzylamine to deliver the imine product [23,41]. The imine product is susceptible to water and can be hydrolyzed back into benzaldehyde and benzylamine. From the results above, it is obvious that the C/B heterojunction provides superior photocatalytic RhB degradation and selective
Scheme 1. Oxidation products of benzylamine under visible light irradiation.
Fig. 6. Imine yield within 8 h irradiation for 4 consecutive runs (a) and the effect of different scavengers on the imine product at 8 h (b).
Please cite this article as: K. Wangkawong et al., Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation, Journal of the Taiwan Institute of Chemical Engineers (2020), https:// doi.org/10.1016/j.jtice.2020.01.003
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Scheme 2. Possible reaction pathway of oxidative coupling of benzylamine in the C/B heterojunction photocatalytic system.
Fig. 7. EIS Nyquist plots (a) and transient photocurrent response (b) of CeO2, C/B heterojunction, and BiOI.
oxidation of benzylamine compared with its individual constituents. A lower interfacial resistance and a better charge transfer of the C/B revealed by a smaller arc radius in EIS Nyquist plots than others (Fig. 7(a)) could be responsible for such excellent photocatalytic performances of this heterostructure. Additionally, the higher transient photocurrent response of the C/B in Fig. 7(b) also suggests the better light harvesting and lower charge recombination efficiencies in the C/B heterojunction [45,46]. Likewise, a superior charge transfer and separation efficiency achieved upon forming a composite between BiOI and CeO2 was also mainly responsible for the enhanced photocatalytic degradation of organic pollutants in Ref. [29,30].
3.3. Energy band alignment and electron transfer mechanism of CeO2/ BiOI heterojunction To illustrate energy band alignment, flat band potentials of CeO2 and BiOI were determined by MottSchottky model as shown in Fig. 8(a). The Mott Schottky curves of CeO2 and BiOI present positive slopes, indicating that both samples are n-type semiconductors. From this figure, the flat band potentials of CeO2 and BiOI are 0.27 and 0.82 V vs. Ag/AgCl. Since the CB is ca. 0.1 eV more negative than the flat band potential of n-type semiconductors [47], the CB of CeO2 and BiOI are found as 0.37 and 0.92 V vs. Ag/AgCl. The CB in NHE scale can then be calculated using the following equation [48]: o ENHE ¼ EAg =AgCl þ EAg=AgCl
E°Ag/AgCl = 0.21 V
ð1Þ
Where (3.0 M KCl) [48]. Thus, the CB levels of CeO2 and BiOI are 0.16 and 0.71 V vs. NHE, respectively. Using the band
gaps obtained from UVvis DRS results in Fig. 1(b), the VB of CeO2 and BiOI are 2.97 V and 1.29 V vs. NHE, respectively. Accordingly, the band energy diagram of CeO2/BiOI heterjunction is illustrated in Fig. 8(b). The C/B heterojunction presents a staggered-gap band alignment in which an efficient charge transfer and separation between CeO2 and BiOI can be accomplished, therefore promoting its photocatalytic activ ity. From Fig. 8(b), the VB of CeO2 is more positive than OH /OH (1.99 V vs NHE) and OH /H2O (2.70 V vs NHE) [49], thus OH radical can be produced. However, the VB of BiOI is more negative than OH / OH , therefore OH is not able to generate from the BiOI. This may be the reason why there is no decrease in RhB degradation activity or loss of imine yield when adding OH scavenger in the quenching studies. Since the CB of BiOI is more negative than O2/O2 (0.33 V vs. NHE) [5,50], electrons on the CB can react with O2 to produce O2 . However, this is not the case for CeO2 as the CB of CeO2 is more positive than O2/O2 , thus O2 could not be generated in the CeO2 system. This supports the results in Fig. 4(c) and 6(b) where O2 clearly involves in the photocatalytic activity of BiOI but not that of CeO2. Based on the results from quenching experiments where O2 is found as a main reactive oxygen species in C/B heterostructure system, it is likely that photogenerated electrons on the CB of BiOI do not transfer to the CB of CeO2 but rather stay and react with O2 to produce O2 . The scavenging results together with the positive shift of Ce 3d peak and the negative shift of Bi 4f peak from the XPS study suggest that a charge transfer mechanism in the CeO2/BiOI heterojunction is a Zscheme type where photogenerated electrons from the CB of CeO2 migrate to the VB of BiOI, while keeping the high-energy electrons at the CB of BiOI and holes at the VB of CeO2 available for reactions (Fig. 8 (b)). Our proposed Z-scheme mechanism also agrees well with that
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Fig. 8. Mott-Schottky curves of CeO2 and BiOI (a) and a proposed Z-scheme mechanism of CeO2/BiOI heterojunction system (b).
reported by Sultana et al. [51] According to the results above and a proposed band alignment, it is clear that the synergistic effects derived from the heterojunction formation such as better charge transfer and separation efficiencies, extended visible-light harvesting ability, improved photocatalytic stability, and increased surface area are essential factors influencing the superior photocatalytic performance of the C/B heterojunction in this study. 4. Conclusions The CeO2/BiOI heterojunction was successfully synthesized as confirmed by HRTEM and XPS results. Visible-light-driven photocatalytic tests of the C/B heterostructure in RhB degradation and oxidative coupling of benzylamine clearly demonstrate the superior performance of this heterojunction over CeO2 and BiOI. Such excellent activities could be ascribed to better charge separation and transfer efficiency and enhanced visible-light absorption upon coupling between CeO2 and BiOI as revealed by UVvis DRS, EIS and transient photocurrent response. Based on the results from UVvis DRS, Mott-Schottky, XPS and active species scavenging studies, a staggered band lineup with Zscheme charge transfer mechanism is proposed for the C/B heterojunc tion system with h+ and O2 being main active species in RhB degradation and oxidative coupling of benzylamine reaction. Declaration of Competing Interest 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. Acknowledgments The authors would like to thank Science Achievement Scholarship of Thailand (SAST); Graduate School; Center of Excellence in Materials Science and Technology; Chiang Mai University; Office of Higher Education Commission (OHEC) and Thailand Research Fund (RSA6280014) for financial supports. Supplementary materials Supplementary material associated with this article can be found in the online version at doi:10.1016/j.jtice.2020.01.003. References [1] Fagan R, McCormack DE, Dionysiou DD, Pillai SC. A review of solar and visible light active TiO2 photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern. Mat Sci Semicon Proc 2016;42:2–14.
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Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation Kanlayawat Wangkawonga,b, Sukon Phanichphantb, Doldet Tantraviwatc, Burapat Inceesungvornb,* a
Graduate School, Chiang Mai University, Chiang Mai 50200 Thailand Department of Chemistry, Center of Excellence for Innovation in Chemistry (PERCH-CIC) and Centre of Excellence in Materials Science and Technology, Faculty of Science, Chiang Mai University, Chiang Mai 50200 Thailand c Department of Electrical Engineering, Faculty of Engineering, Chiang Mai University, Chiang Mai 50200, Thailand b
A R T I C L E
I N F O
Article History: Received 29 October 2019 Revised 3 January 2020 Accepted 6 January 2020 Available online xxx Keywords: CeO2 BiOI Heterojunction Z-scheme Visible light
A B S T R A C T
The Z-scheme CeO2/BiOI (C/B) was prepared and evaluated for its photocatalytic RhB degradation and selective oxidation of benzylamine to N-benzylidenebenzylamine under visible light irradiation. Close contacts and chemical interactions between CeO2 and BiOI which are important for efficient charge transfer in the heterojunction were revealed by HRTEM and XPS studies. The C/B heterostructure presented nearly 1.5 and 8.0 times higher RhB degradation activity than BiOI and CeO2, respectively. The activity of C/B in oxidative coupling of benzylamine was also firstly revealed and found to be more than 2 times higher than both BiOI and CeO2. Such enhanced photocatalytic performance of the C/B is ascribed to the combined effects of extended visible-light absorption range, increased surface area and improved charge transfer efficiency as evidenced from BET, UVvis DRS and photoelectrochemical studies. Based on XPS, UVvis DRS, MottSchottky plots and active species quenching results, a Z-scheme charge transfer where photogenerated elec tron-hole pairs can be effectively separated is proposed for the C/B and h+ and O2 are key active species responsible for RhB degradation and N-benzylidenebenzylamine formation. The present work highlights the enhancement of photocatalytic activity based on Z-scheme heterojunction formation and reveals a further application of photocatalysts in organic fine chemical syntheses. © 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction One of the most important sustainable energies is solar energy which consists of »5% of ultraviolet, »40% of visible light, and »55% of near infrared [1,2]. To exploit the solar energy in chemical reactions, photocatalysis technology has been developed and employed in many applications, for example, organic pollutant degradation [36], H2 production [7,8], and organic fine chemical syntheses [911]. Cerium dioxide (CeO2) is broadly used as electrolyte in solid oxide fuel cell [12,13], oxygen sensor [14,15], UV blocking material [16,17], photocatalyst [36] and electrocatalysts [18,19] due to its high oxygen storage capacity, strong UV absorption ability, good chemical stability and nontoxicity. For the photocatalytic application, the wide band gap energy and high charge carrier recombination of CeO2 largely limit its practical use [6,20]. To overcome this drawback, various strategies have been developed, for example, metal loading,
* Corresponding author. E-mail address: [email protected] (B. Inceesungvorn).
morphology controlling, and heterojunction formation. The heterojunction formation is widely accepted as an effective strategy to improve charge separation and transfer leading to enhanced photocatalytic performance [2125]. The coupling between a wide bandgap semiconductor and a narrow one provides an extended light absorption range and generates more photoexcited charges under visible light irradiation. To achieve a successful heterojunction, band energy levels of the two components in coupling which directly affect redox ability and charge transfer mechanism of the heterostructure have to be mainly considered. A staggered-lineup heterostructure providing either type-II or Z-scheme charge transfers is desirable as the charge transfer and separation efficiencies of photocatalysts are significantly promoted [22,25]. In comparison with the type-II mechanism, the Z-scheme pathway can maintain strong redox properties of both valence band hole and conduction band electron of the coupling materials which are essential to drive reaction kinetics, especially for sluggish reactions [26]. BiOI has a tetragonal matlockite structure in which [Bi2 O2 ] 2+ slabs are interleaved by double slabs of iodide ions. Each layer
https://doi.org/10.1016/j.jtice.2020.01.003 1876-1070/© 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: K. Wangkawong et al., Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation, Journal of the Taiwan Institute of Chemical Engineers (2020), https:// doi.org/10.1016/j.jtice.2020.01.003
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of IBiOOBiI is connected through Van der Waals interactions along c-axis, causing the formation of internal electric field in this direction which consequently promotes photoexcited charge separation [27,28]. BiOI with a narrow band gap of ca. 1.8 eV has shown to be a promising visible-light-driven photocatalyst in organic pollutant degradation and water splitting [2730]. The band energy levels of BiOI are also suitable for CeO2 in order to create a staggered band alignment, therefore coupling between BiOI and CeO2 may boost performance of the photocatalytic system. In this study, a Z-scheme CeO2/BiOI heterojunction was synthesized and evaluated for its photocatalytic performance via RhB degradation and a selective oxidation of benzylamine to N-benzylidenebenzylamine under visible light. Although previous studies have already shown the organic degradation ability of this heterostructure [29,30], its photocatalytic activity toward a selective oxidation of benzylamine to N-benzylidenebenzylamine has never been investigated before. Physicochemical properties and energy band alignment were also revealed to understand structure-activity relationship. A possible mechanism for the selective oxidation of benzylamine was also proposed. The success of this work also encourages a further development and application of heterojunction photocatalysts toward green organic fine chemical syntheses.
adsorption-desorption equilibrium. During irradiation, a sample was taken at regular intervals and centrifuged to remove the catalyst. The concentration of RhB was determined by UVvis spectrophotometer (Lambda 25, Perkin Elmer) at a maximum absorption of 554 nm. To study catalyst stability, the used catalyst was washed with DI water and dried at 80 °C for 12 h for a recycling experiment. Radical scavenging experiment was carried out by adding different trapping agents. The quenching agent concentration was 1 £ 103 M in all studies. Photocatalytic oxidative coupling of benzylamine to N-benzylidenebenzylamine was investigated by dispersing 0.4 g of catalyst in 10 ml of 0.5 M benzylamine in acetonitrile containing o-dichlorobenzene as an internal standard. The reaction was carried out under the O2 flow rate of 20 ml min1 at a temperature of 28 § 1 °C controlling by a cooling fan. The suspension was stirred under dark for 30 min, then irradiated by 50 W cool white LED which emits light in a visible range (λ > 400 nm). To identify the composition and concentration of products, a sample was taken periodically, then analyzed by GC-FID (Clarus 500/580 GC, Perkin Elmer) and further confirmed by GCMS (Agilent 7820A-5977E).
2. Experimental
Photoelectrochemical experiments were carried out on Metrohm Autolab Potentiostat/Galvanostat (PGSTAT128N) using three-electrode setup. To prepare a working electrode, 0.1 g of photocatalyst was dispersed in 2 ml of 2% nafion in ethanol, then 20 mL of this solution was dropped on FTO glass (1 £ 1 cm). The Ag/AgCl and Pt wire were employed as a reference electrode and a counter electrode, respectively. Electron impedance spectroscopy (EIS) was measured in the range of 0.1105 Hz with an AC voltage amplitude of 10 mV at an applied voltage of 1.0 V using 1.0 M Na2SO4 electrolyte solution (pH » 5.17). The flat band potentials of photocatalysts were also determined from MottSchottky plots by scanning the applied voltage from 0.8 to 1.0 V with a frequency of 100 Hz. Transient photocurrent response was recorded at a bias potential of 1.0 V using 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as an electrolyte. The same light source as that in the photocatalytic test was used in all photoelectrochemical experiments.
2.1. Synthesis of CeO2, BiOI and CeO2/BiOI heterojunction For the synthesis of CeO2, an aqueous solution of 0.25 M NaOH was added dropwise into 80 ml of 0.05 M Ce(NO3)3¢6H2O aqueous solution until pH 10 was attained. The solution was kept under stirring at a constant pH for 2 h. The precipitates were collected by centrifugation and washed with DI water until neutral pH was reached. The catalyst was dried overnight, then the pale yellow CeO2 powder was finally obtained after calcination in air at 400 °C for 4 h. BiOI and 50 wt% CeO2/BiOI (denoted as C/B) were prepared by a similar procedure. Typically, 0.5 M of Bi(NO3)3¢5H2O and 3.5 ml of conc. HNO3 were dissolved in 50 ml of DI water. The obtained solution was mixed with 50 ml of KI solution with the mole ratio of Bi: I equal to 1:1. The solution pH was then adjusted to 4 by NaOH solution and kept constant under stirring for 1 h. After that, the precipitate was collected and washed with DI water several times. The catalyst was dried at 80 °C for 18 h, then a brownish orange powder was finally obtained. For the preparation of C/B, 0.2 g of calcined CeO2 was added into the BiOI precursor prior to adjusting the solution pH. 2.2. Material characterizations Structure and phase compositions were studied by powder X-ray diffraction (XRD) (Miniflex II, Rigaku) with Cu Ka radiation (λ = 1.5405 A). Morphology and microstructure were investigated by scanning electron microscope (SEM, JSM-6335F, JEOL) and transmission electron microscope (TEM, JEM-2010, JEOL). Band gap energy was determined by UVvis diffuse reflectance spectrometer (UV3101 PC, SHIMADZU). BrunauerEmmerTeller (BET) specific surface area was analyzed by N2 adsorption on an Autosorb-1, Quantachrome. The oxidation state and elemental composition of catalysts were studied by X-ray photoelectron spectroscopy (XPS) using an Axis Ultra DLD, Kratos. 2.3. Photocatalytic tests Photocatalytic performance was evaluated via a degradation of RhB and a selective oxidation of benzylamine under visible light. The photocatalytic activity for RhB degradation was studied using 50 W halogen lamp. Typically, 0.05 g of catalyst was dispersed in 100 ml of aqueous RhB solution (1 £ 105 M). Prior to light irradiation, the catalyst suspension was stirred under dark for 30 min to achieve an
2.4. Photoelectrochemical studies
3. Results and discussion 3.1. Physicochemical properties of materials The crystallographic structures of CeO2, 50 wt% CeO2/BiOI (C/B), and BiOI characterized by XRD are shown in Fig. 1(a). The diffraction peaks of CeO2 and BiOI correspond well with cubic and tetragonal phases according to JCPDS no. 340394 and 10-0445, respectively. The C/B composite exhibits the diffraction peaks of both CeO2 and BiOI which are characteristics of composite catalyst. The light absorption properties of CeO2, C/B, and BiOI were investigated as shown in Fig. 1(b). CeO2 exhibits a strong light absorption below 396 nm, while C/B and BiOI show the absorption edges at 609 and 622 nm. The band gap energies of CeO2, C/B, and BiOI are 3.13, 2.04, and 2.00 eV, respectively. Morphology and microstructure of the samples were characterized by SEM and TEM. The SEM results reveal irregular particles of CeO2 (Fig. S1(a)), while BiOI appears as microplate (Fig. S1(b)). The C/B in Fig. S1(c) presents both irregular CeO2 particles and BiOI microplates. Microstructure and particle size of the samples are revealed by TEM in Fig. 2(a)(f). The irregular shape of CeO2 with an approximate size of 15 nm is found (Fig. 2(a)). The corresponding ring pattern (Fig. 2(b)) indicates a polycrystalline character of cubic CeO2. The plate-like feature with a diameter of »0.80 mm (Fig. 2(c)) and its spotted SAED pattern (Fig. 2(d)) suggest a single crystalline nature of the BiOI microplate. The TEM image of C/B in Fig. 2(e) indicates the decoration of small irregular CeO2 particles on the microplates of BiOI. HRTEM in Fig. 2(f) shows lattice spacings of 0.31 nm
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3
Fig. 1. XRD patterns (a) and UVvis diffuse reflectance spectra (b) of CeO2, C/B, and BiOI.
and 0.28 nm corresponding to (111) plane of CeO2, and (110) plane of BiOI, respectively [27,29,30] and close interfacial contacts between CeO2 and BiOI which are necessary for an efficient charge transfer in heterojunction system. Surface elemental composition and chemical state of the catalysts were determined by X-ray photoelectron spectroscopy (XPS) as shown in Fig. 3. The high-resolution spectra of Ce 3d in Fig. 3(a) can be fitted into five doublet pairs of Ce3+ and Ce4+ species. The first three doublet peaks located at 881.7, 888.3, 897.8, and 900.3, 906.9, 916.1 eV are ascribed to V, V2, V3, and U, U2, U3 of Ce4+ ion [4,31]. While, the peaks at 879.9, 884.0, 898.1, and 902.6 eV are due to V0, V1, U0, and U1 of Ce3+ ion [31]. The Ce 3d peaks clearly shift to higher binding energy in the case of C/B composite. These shifts suggest chemical interactions as coupling CeO2 with BiOI, which further ensure a successful heterojunction formation. The Ce3+/(Ce3++Ce4+) ratio of the C/B sample (0.20) is lower than that of pure CeO2 (0.27), suggesting that Ce3+ is partially oxidized upon coupling with BiOI. Two peaks of Bi 4f (Fig. 3(b)) located at 159.2 and 164.5 eV are attributed to Bi 4f7/2 and Bi 4f5/2 of Bi3+ in BiOI [32,33]. In contrast to the Ce 3d peaks, these Bi 4f peaks of the C/B heterojunction shift to lower binding energy, indicating an increased electron density in the BiOI but a loss of electron density in CeO2 as forming this heterostructure. These binding energy shifts also suggest that, in the C/B heterojunction, electrons migrate from CeO2 to BiOI. This assumption is supported by a decrease of Ce3+/(Ce3++Ce4+) ratio as a result of Ce3+ oxidation after coupling CeO2 with BiOI. The I 3d peaks (Fig. 3(c)) at 619.2 and 630.7 eV correspond to I 3d5/2 and I 3d3/2 of I in BiOI [22]. These peaks in the C/B also shift to lower binding energy in correspondence with the Bi peaks. A minor doublet pair at 620.2 and 631.7 eV due to I3 ion as a result of I photooxidation [3436] is found for pure BiOI but not the C/B, thus suggesting a better photostability of the heterojunction than pure BiOI. The O 1 s spectrum of BiOI (Fig. 3(d)) presents three oxygen species at 530.0, 530.8, and 532.3 eV which are assigned to BiO in BiOI, chemisorbed oxygen, and surface hydroxyl species [22,37], respectively. The O 1 s spectrum of pure CeO2 in Fig. 3(d) can also be fitted into three components. The low binding energy peak (528529.5 eV) is attributed to lattice oxygen (OL) in CeO2 structure, and that in the range of 530531 eV originates from anion oxygen species adsorbed on oxygen vacancy sites (Ov) [20,38]. The peak centered at 531.5 eV belongs to surface hydroxyl species or adsorbed water (Oads) [20,39]. 3.2. Photocatalytic performances under visible light Photocatalytic activity for RhB removal in Fig. 4(a) shows that the C/B provides distinctly higher RhB degradation performance (95.4%) than pure CeO2 (12.0%) and BiOI (65.8%) within 3 h irradiation. The C/B also presents a superior activity than a physical mixture of BiOI and CeO2 at the same weight ratio as that of the heterojunction
catalyst. This suggests that chemical interactions and close contacts between CeO2 and BiOI are inevitable for the enhanced photocatalytic activity. The initial pseudo-first-order rate constants (k) of each catalyst in Fig. 4(b) show that the C/B heterostructure can degrade RhB ca. 2.5, 4.5 and 8.6 times faster than that of BiOI, physical mixture and CeO2, respectively. The BET surface areas of CeO2, C/B, and BiOI are 57.2, 26.7, and 6.4 m2/g, respectively. As considering the BET surface area and band gap energy of the catalysts, the enhanced photocatalytic activity of C/B heterostructure in RhB degradation may partially be ascribed to its increased surface area and visible-light absorption ability upon coupling CeO2 with BiOI. It is widely known that several active species such as h+, O2 , and OH play crucial roles in oxidizing organic pollutants, therefore radical scavenging experiment was carried out to determine the main active species and reveal a possible photocatalytic mechanism over the C/B heterojunction system. Scavenging experiments were also carried out in the presence of BiOI and CeO2 for comparison purpose. From Fig. 4(c), in the case of CeO2, a decrease of RhB activity is found only when adding h+ and OH quenchers. On the contrary, the RhB degradation activity of C/B largely decreases when adding ammonium oxalate (AO) and p-benzoquinone (p-BQ) in the C/B photocata lytic system, indicating that h+ and O2 are main active species governing the RhB degradation over this heterojunction photocatalyst. On the contrary, the introduction of isopropanol (IPA) only has a minor effect on the degradation activity, revealing a minor role of OH in this catalytic system. A similar behavior to that of C/B composite is observed for BiOI. Our results also agree well with those found in Ref. [29,30] where h+ and O2 but not OH play crucial role in the degradation of organic compounds in aqueous solution. Stability of the photocatalysts is also important for practical use. As shown in Fig. 4(d), the performance of C/B remains nearly the same upto four cyclic runs, whereas that of BiOI noticeably decreases from 58% to 12%. The poor stability of BiOI may be due to the photocorrosion process originating from the well-known direct oxidation of iodide by photogenerated hole [28,36]. To further extend an applicability of the developed photocatalyst, the C/B was tested for its performance in the oxidative coupling of benzylamine to N-benzylidenebenzylamine, which is an important imine intermediate for the synthesis of many pharmaceuticals and biological active nitrogen-containing compounds [23]. Traditionally, Lewis acid catalysts, harmful oxidants, and elevated temperature are required for the imine synthesis. However, due to environmental issues, the synthesis based on photocatalysis has been developed and recognized as a greener and more facile method because the toxic oxidants are substituted by air or molecular oxygen and the corrosive and harmful homogeneous catalyst is taken over by an earth abundant heterogeneous catalyst. Moreover, the conventional heating is replaced by light activation at room temperature to realize a renewable energy-based synthetic process. Fig. 5 shows conversion of
Please cite this article as: K. Wangkawong et al., Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation, Journal of the Taiwan Institute of Chemical Engineers (2020), https:// doi.org/10.1016/j.jtice.2020.01.003
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Fig. 2. TEM images and SADE patterns of CeO2 (ab) and BiOI (cd). TEM (e) and HRTEM images (f) of C/B heterostructure.
benzylamine, selectivity and yield of N-benzylidenebenzylamine product obtained from all catalysts up to 8 h irradiation. The N-benzylidenebenzylamine is found as a major product with small amounts of benzonitrile and benzaldehyde (Scheme 1). No other by-products such as benzamide and benzoic acid have been found from all catalysts. Compared with the C/B heterojunction, a physical mixture (C+B) produces two times lower imine yield, which apparently confirms that chemical interaction rather than just a physical contact between the two materials is necessary for the photocatalytic improvement. The highest yield (25.0%) is obtained from the C/B heterojunction at 8 h irradiation whereas only 8.3%, 12.1% and 11.7%
yield are found for CeO2, BiOI and the physical mixture, respectively. Although, the imine yield is still low for practical application, our results clearly show that an improved activity could be achieved from the heterojunction photocatalyst, opening up a further development and an alternative pathway toward green organic synthesis. Despite the increase of benzylamine conversion over time, the selectivity to imine is obviously decreased due to the increased amounts of benzonitrile and benzaldehyde. The highest amount of benzonitrile and benzaldehyde observed from the C/B catalyst at 8 h are 3.3% and 4.9% yield, respectively. Benzonitrile is produced from a complete oxidative dehydrogenation of benzylamine via benzylimine
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Fig. 3. High resolution spectra of Ce 3d (a), Bi 4f (b), I 3d (c), and O 1 s (d).
Fig. 4. Photocatalytic RhB degradation activities (a) and corresponding pseudo-first-order kinetics (b) over CeO2, BiOI, C/B heterojunction and physical mixture under visible light irradiation. Active species quenching experiments within 3 h irradiation (c). Recycling performances of 50 C/B compared with BiOI (d).
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Fig. 5. Benzylamine oxidation under visible light irradiation at different reaction times over CeO2, BiOI, C/B heterojunction and a physical mixture between CeO2 and BiOI (C+B).
intermediate whereas benzaldehyde is generated from an incomplete oxidation of benzylamine when using molecular oxygen as a green oxidant [9,40]. The benzaldehyde can also occur from the hydrolysis of N-benzylidenebenzylamine [40, 41]. As one benzonitrile molecule is formed, two water molecules are also released which can then hydrolyze N-benzylidenebenzylamine into benzaldehyde and benzylamine, resulting in a poor selectivity toward the imine. From Fig. 5, CeO2 obviously shows much lower selectivity than other photocatalysts possibly due to stronger oxidizing ability of its valence band hole as evidenced by the higher positive value of valence band potential which is shown later in Fig. 8(b). Adsorption studies indicate that BiOI adsorbs only 3% of the initial benzylamine concentration whereas CeO2 and C/B samples adsorb ca. 19.5% and 18.2%, respectively (data not shown). Although CeO2 has the highest surface area among the three catalysts (CeO2, C/B, and BiOI are 57.2, 26.7, and 6.4 m2/g, respectively), the benzylamine adsorption over CeO2 is nearly the same as that over the C/B, which suggests that the surface area is not the dominant factor governing benzylamine adsorption, hence its conversion. This rather indicates different surface properties of the three catalysts. Different conversion and selectivity values observed among all catalysts used may be ascribed to different amount and strength of acidic sites and the redox nature of solid
catalysts [40,42]. These sites are believed to help activate benzylamine to benzylimine intermediate which subsequently reacts with another benzylamine to finally form imine product [10]. Further investigations into catalyst surface acidity may help explain the different activities observed. Photocatalytic stability in benzylamine oxidation was also evaluated as shown in Fig. 6(a). Only a slight decrease in imine yield (< 4%) is observed for all catalysts within four consecutive cycles, indicating excellent stability of these catalysts under the conditions applied. Active radical quenching study (Fig. 6(b)) was also carried out to reveal the active species involving in the benzylamine oxidation. It is clear that, in all photocatalytic systems, the addition of t-BuOH which is a OH scavenger does not affect the product yield, suggesting that the OH is not engaged in the imine formation. In CeO2 system, the introduction of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and K2Cr2O7 as O2 and e quenchers as well as replacing O2 with N2 does not reduce the imine yield either. However, a significant decrease of imine amount is found when adding triethanolamine (TEA), a h+ quencher. These observations suggest an essential role of h+ in the oxidative cou pling of benzylamine in the CeO2 system. On the contrary, O2, O2 , + e and h are all involved in the imine formation in both BiOI and C/B heterojunction systems, although the role of each of these species in both photocatalytic systems possibly differs to some extent. Based on our results from quenching experiments and previous literatures [9, 40], a possible mechanism for the benzylamine oxidation in the C/B heterostructure system is proposed in Scheme 2. Benzylamine molecule is firstly adsorbed on the photocatalyst surface and then oxidized by h+ at the valence band (VB) of photocatalyst to produce aminium cation radical. At the same time, O2 reacts with e at the conduction band (CB) to generate O2 which further reacts with the formed cation radical to produce benzylimine by releasing H2O2. Then, another benzylamine molecule will react with the formed benzylimine to finally obtain N-benzylidenebenzylamine with NH3 as a by-product [43,44]. In our study, benzonitrile and benzaldehyde are also found. It is possible that the benzylimine intermediate is further oxidized to benzonitrile and water molecules. The benzylimine may also react with water to produce benzaldehyde which subsequently couples with free benzylamine to deliver the imine product [23,41]. The imine product is susceptible to water and can be hydrolyzed back into benzaldehyde and benzylamine. From the results above, it is obvious that the C/B heterojunction provides superior photocatalytic RhB degradation and selective
Scheme 1. Oxidation products of benzylamine under visible light irradiation.
Fig. 6. Imine yield within 8 h irradiation for 4 consecutive runs (a) and the effect of different scavengers on the imine product at 8 h (b).
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Scheme 2. Possible reaction pathway of oxidative coupling of benzylamine in the C/B heterojunction photocatalytic system.
Fig. 7. EIS Nyquist plots (a) and transient photocurrent response (b) of CeO2, C/B heterojunction, and BiOI.
oxidation of benzylamine compared with its individual constituents. A lower interfacial resistance and a better charge transfer of the C/B revealed by a smaller arc radius in EIS Nyquist plots than others (Fig. 7(a)) could be responsible for such excellent photocatalytic performances of this heterostructure. Additionally, the higher transient photocurrent response of the C/B in Fig. 7(b) also suggests the better light harvesting and lower charge recombination efficiencies in the C/B heterojunction [45,46]. Likewise, a superior charge transfer and separation efficiency achieved upon forming a composite between BiOI and CeO2 was also mainly responsible for the enhanced photocatalytic degradation of organic pollutants in Ref. [29,30].
3.3. Energy band alignment and electron transfer mechanism of CeO2/ BiOI heterojunction To illustrate energy band alignment, flat band potentials of CeO2 and BiOI were determined by MottSchottky model as shown in Fig. 8(a). The Mott Schottky curves of CeO2 and BiOI present positive slopes, indicating that both samples are n-type semiconductors. From this figure, the flat band potentials of CeO2 and BiOI are 0.27 and 0.82 V vs. Ag/AgCl. Since the CB is ca. 0.1 eV more negative than the flat band potential of n-type semiconductors [47], the CB of CeO2 and BiOI are found as 0.37 and 0.92 V vs. Ag/AgCl. The CB in NHE scale can then be calculated using the following equation [48]: o ENHE ¼ EAg =AgCl þ EAg=AgCl
E°Ag/AgCl = 0.21 V
ð1Þ
Where (3.0 M KCl) [48]. Thus, the CB levels of CeO2 and BiOI are 0.16 and 0.71 V vs. NHE, respectively. Using the band
gaps obtained from UVvis DRS results in Fig. 1(b), the VB of CeO2 and BiOI are 2.97 V and 1.29 V vs. NHE, respectively. Accordingly, the band energy diagram of CeO2/BiOI heterjunction is illustrated in Fig. 8(b). The C/B heterojunction presents a staggered-gap band alignment in which an efficient charge transfer and separation between CeO2 and BiOI can be accomplished, therefore promoting its photocatalytic activ ity. From Fig. 8(b), the VB of CeO2 is more positive than OH /OH (1.99 V vs NHE) and OH /H2O (2.70 V vs NHE) [49], thus OH radical can be produced. However, the VB of BiOI is more negative than OH / OH , therefore OH is not able to generate from the BiOI. This may be the reason why there is no decrease in RhB degradation activity or loss of imine yield when adding OH scavenger in the quenching studies. Since the CB of BiOI is more negative than O2/O2 (0.33 V vs. NHE) [5,50], electrons on the CB can react with O2 to produce O2 . However, this is not the case for CeO2 as the CB of CeO2 is more positive than O2/O2 , thus O2 could not be generated in the CeO2 system. This supports the results in Fig. 4(c) and 6(b) where O2 clearly involves in the photocatalytic activity of BiOI but not that of CeO2. Based on the results from quenching experiments where O2 is found as a main reactive oxygen species in C/B heterostructure system, it is likely that photogenerated electrons on the CB of BiOI do not transfer to the CB of CeO2 but rather stay and react with O2 to produce O2 . The scavenging results together with the positive shift of Ce 3d peak and the negative shift of Bi 4f peak from the XPS study suggest that a charge transfer mechanism in the CeO2/BiOI heterojunction is a Zscheme type where photogenerated electrons from the CB of CeO2 migrate to the VB of BiOI, while keeping the high-energy electrons at the CB of BiOI and holes at the VB of CeO2 available for reactions (Fig. 8 (b)). Our proposed Z-scheme mechanism also agrees well with that
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Fig. 8. Mott-Schottky curves of CeO2 and BiOI (a) and a proposed Z-scheme mechanism of CeO2/BiOI heterojunction system (b).
reported by Sultana et al. [51] According to the results above and a proposed band alignment, it is clear that the synergistic effects derived from the heterojunction formation such as better charge transfer and separation efficiencies, extended visible-light harvesting ability, improved photocatalytic stability, and increased surface area are essential factors influencing the superior photocatalytic performance of the C/B heterojunction in this study. 4. Conclusions The CeO2/BiOI heterojunction was successfully synthesized as confirmed by HRTEM and XPS results. Visible-light-driven photocatalytic tests of the C/B heterostructure in RhB degradation and oxidative coupling of benzylamine clearly demonstrate the superior performance of this heterojunction over CeO2 and BiOI. Such excellent activities could be ascribed to better charge separation and transfer efficiency and enhanced visible-light absorption upon coupling between CeO2 and BiOI as revealed by UVvis DRS, EIS and transient photocurrent response. Based on the results from UVvis DRS, Mott-Schottky, XPS and active species scavenging studies, a staggered band lineup with Zscheme charge transfer mechanism is proposed for the C/B heterojunc tion system with h+ and O2 being main active species in RhB degradation and oxidative coupling of benzylamine reaction. Declaration of Competing Interest 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. Acknowledgments The authors would like to thank Science Achievement Scholarship of Thailand (SAST); Graduate School; Center of Excellence in Materials Science and Technology; Chiang Mai University; Office of Higher Education Commission (OHEC) and Thailand Research Fund (RSA6280014) for financial supports. Supplementary materials Supplementary material associated with this article can be found in the online version at doi:10.1016/j.jtice.2020.01.003. References [1] Fagan R, McCormack DE, Dionysiou DD, Pillai SC. A review of solar and visible light active TiO2 photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern. Mat Sci Semicon Proc 2016;42:2–14.
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Please cite this article as: K. Wangkawong et al., Photocatalytic efficiency improvement of Z-scheme CeO2/BiOI heterostructure for RHB degradation and benzylamine oxidation under visible light irradiation, Journal of the Taiwan Institute of Chemical Engineers (2020), https:// doi.org/10.1016/j.jtice.2020.01.003
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