物理化学学报  2017, Vol. 33 Issue (12): 2532-2541   (1891 KB)    
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  • Received: May 9, 2017
  • Revised: June 13, 2017
  • Published on Web: June 15, 2017
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    陈鑫, 胡绍争*, 李萍, 李薇, 马宏飞, 陆光    
    辽宁石油化工大学化学化工与环境学部, 辽宁 抚顺 113001
    摘要:工业上,双氧水的生产采用的是蒽醌法。此方法采用多步加氢和氧化过程,因此能耗很大。光催化制过氧化氢技术作为可持续和环境友好的新工艺,是传统蒽醌和电化学法的优秀替代者。本文采用熔盐辅助微波法制备了g-C3N4包覆MgO-Al2O3-Fe2O3异质结催化剂。制备的异质结催化剂在可见光下表现出优异的光催化制过氧化氢性能。熔盐的引入改变催化剂形貌的同时也影响了原料三聚氰胺的缩聚度,进而影响了其能带结构。制备的包覆结构能使两组分形成最大面积的异质结和强相互作用。这种强相互作用有利于光生电子-空穴对的分离和界面迁移,进而提高了过氧化氢的生成速率。制备的异质结催化剂的双氧水平衡浓度和生成速率分别为6.3 mmol·L-1和1.42 mmol·L-1·h-1,远高于两个单组份。不仅如此,制备的异质结催化剂还能抑制过氧化氢的分解。本文通过自由基捕获实验探讨了可能的反应机理和电子转移路径。
    关键词石墨相氮化碳    包覆结构    异质结    制过氧化氢    熔盐辅助微波法    

    1 Introduction

    Hydrogen peroxide (H2O2), as a highly efficient and green oxidant, has been widely used in bleaching and disinfectant applications, such as textile, paper pulp and medical industry. H2O2 has the highest content of active oxygen (47%, w/w) and only H2O as the by-product1, 2. Besides that, H2O2 is also an ideal energy carrier alternative to hydrogen with the low volumetric energy3, 4. The output potential of H2O2 fuel cell is 1.09 V theoretically, which is comparable with that of hydrogen fuel cell (1.23 V)5-7. In the industry, H2O2 is produced by the anthraquinone method, in which energy consumption is high because of the multistep hydrogenation and oxidation reactions. Recently, direct synthesis of H2O2 from only water, oxygen and visible light through two-electron reduction from the conduction band has been widely studied using semiconductor as photocatalyst (Reaction (1))8-10. Holes in the valence band (VB) can directly oxidize water molecules to produce O2 (Reaction (2)). But usually, hole scavenger such as alcohols are added to promote the electrons-holes separation rate (Reaction (3)). This method has many advantages such as green cleaning, mild conditions and low power consumption. However, the H2O2 can be decomposed by the reduction with e- which causes the H2O2 equilibrium concentration is not satisfactory till now (Reaction (4)).

    $ {{\rm{O}}_{\rm{2}}}{\rm{ + 2}}{{\rm{H}}^{\rm{ + }}}{\rm{ + 2}}{{\rm{e}}^-} \to {\rm{ }}{{\rm{H}}_{\rm{2}}}{{\rm{O}}_{\rm{2}}} $ (1)

    $ {{\rm{H}}_{\rm{2}}}{\rm{O + 2}}{{\rm{h}}^{\rm{ + }}} \to {\rm{1/2 }}{{\rm{O}}_{\rm{2}}}{\rm{ + 2}}{{\rm{H}}^{\rm{ + }}} $ (2)

    $ {\rm{R-OH + 2}}{{\rm{h}}^{\rm{ + }}} \to {\rm{ Oxidation product}} $ (3)

    $ {{\rm{H}}_{\rm{2}}}{{\rm{O}}_{\rm{2}}}{\rm{ + }}{{\rm{e}}^-} \to {\rm{ \bullet OH + O}}{{\rm{H}}^-} $ (4)

    Recently, graphitic carbon nitride (g-C3N4), a metal-free visible light photocatalyst, has received increasing attention due to its suitable electric band-gap, good chemical stability, and unique electronic structure. Besides that, its conduction band potential is ~ -1.3 V, more negative than the reduction potential of O2/H2O2 (0.695 V). Thus g-C3N4 can reduce O2 to H2O2 thermodynamically10. However, the low separation efficiency of photogenerated electron-hole pairs and the poor visible light utilization limits its practical applications. In order to improve the above situation, researchers have developed many strategies, including tailoring microstructures 11, 12, forming surface defects13, 14, doping15, 16 and building heterojunctions17, 18. These strategies mainly focus on shortening the distance of transfer paths, forming unique transmission channels, offering more active sites for trapping carriers, and facilitating the separation and transmission of photogenerated electron-hole pairs. Metal oxides and sulphides are widely used to build the heterojunction with g-C3N4. In addition to single metal sulfide, some multi-metal sulfides coupled g-C3N4 composites are also reported19-21. However, few studies concerning multi-metal oxide (MMO) coupled g-C3N4 composites are reported22, 23. With the tunable composition, MMO possesses the special optical properties and electronic structure, leading to the formation of tunable band structure. This is beneficial to the energy level matching of two semiconductors, which is significant important to form the heterojunction.

    Molten salt method is widely used in the materials synthesis field in recent years because it can accelerate diffusion of constituent ions, control crystal growth and easily separate from the solid product by dissolving in water24-26. In general, molten salt can serve as a reaction medium for reactant dissolution and precipitation, as soft template for tailoring micro and mesoporosity of the materials, and as structure-directing agent in the polycondensation and deamination reaction to obtain graphitic materials. The features of this synthesis method are related to the surface and interface energies between the constituents and the salt, resulting in a tendency to minimize the energies by forming a specific morphology. Though the electric-resistance heating molten-salt method can be used to synthesize g-C3N4 photocatalysts, the problems of long time consuming and high energy consumption in synthesis, large emission of harmful gas and low catalyst yield are still difficult to overcome27.

    Recently, microwave-assisted heating synthesis has also been widely used to prepare nanomaterials28-30. The microwave treatment can transfer energy from microwave to the microwave-absorber material which induces strong heating in minutes. When the microwave energy is absorbed by the raw material, the molecules are orderly arrangement in the electromagnetic field of the microwave. Then the high frequency reciprocating motion occurs inside the molecules of raw materials, causes the frequent collisions between molecules, leading to the generation of a lot of frictional heat. Under this heating method, the raw material is rapidly heated without the presence of temperature gradient. It is reported that g-C3N4 can be synthesized by the microwave-assisted heating method in a few minutes, suggesting this is a potential way in rapid synthesis of carbon nitride based materials31. Besides that, the catalyst yield is high and the emission of harmful gas is also low with this method. It is known that the solid-state reaction system needs a uniform reaction medium to achieve a stable and reliable condition. The molten-salt process can offer a unique liquid condition that is stable and convenient for the solid-state reaction system. Thus, we hypothesize that combination two methods mentioned above should be an effective strategy to synthesize g-C3N4 based materials with high performance. In this work, the g-C3N4 coated MgO-Al2O3-Fe2O3 (MgAlFeO) heterojunction catalysts were synthesized via a novel molten salt-assisted microwave process. The photocatalytic activities were evaluated in the photocatalytic H2O2 production under visible light. The possible mechanism is proposed.

    2 Experimental
    2.1 Preparation and characterization

    All the chemicals used in this experiment were reagent gradeand without further treatment. Polymetallic oxide MgAlFeO was prepared as follow. Mixed salt solutions of Mg(NO3)2·6H2O, Al(NO3)3·6H2O and Fe(NO3)3·9H2O (molar ratio of Mg:Al:Fe is 5:2:1) were added into 80 mL deionized water under stirring. Then, desired amount of urea (molar ratio urea/total metal = 2) was added. The obtained solution was placed in a stainless autoclave, which has a 100 ml Teflon inner liner. The autoclave was sealed, placed in an oven and maintained at 120 ℃ for 8 h. The solid was collected by centrifugation, washed with deionized water fore three times, dried at 70 ℃ overnight and denoted as MAFO.

    In a typical process, eutectic mixture of KCl-LiCl (1:1 weight ratio) was selected as a solvent with the melting point of 350 ℃. The mixture of eutectic salts, melamine and MAFO (with a weight ratio of 50:2:1) were finely ground in a mortar, then transferred to an alumina crucible and treated by microwave for 20 min in a normal microwave oven (G70D20CN1P-D2, Galanz). The input power of microwave oven is 1.0 kW·h-1. The obtained catalyst was denoted as MV-MS-CN/MAFO, in which MV and MS stand for microwave treatment and molten salt assistant respectively. For comparison, MV-CN, MV-MS-CN and MV-CN/MAFO were prepared following the same procedure as in the synthesis of MV-MS-CN/MAFO but in the absence of eutectic salts and MAFO, MAFO, and eutectic salts, respectively. Bulk g-C3N4 was prepared by heating melamine at 550 ℃ for 4 h at the rate of 5 ℃·min-1. The product was denoted as B-CN.

    The XRD patterns of the prepared samples were recorded on a Rigaku D/max-2400 instrument (Shimadzu, Japan) using Cu-Kα radiation (λ = 0.154 nm). The scan rate, step size, voltage and current were 0.05 (°)·min-1, 0.01°, 40 kV and 30 mA, respectively. UV-Vis spectroscopy was carried out on a V-550 model UV-Vis spectrophotometer (JASCO Japan) using BaSO4 as the reflectance sample. The morphologies of prepared catalyst were observed by using a scanning electron microscope (SEM, JSM 5600LV, JEOL Ltd. Japan). TEM images were taken on a Philips Tecnai G220 model microscope (Holand). Nitrogen adsorption was measured at -196 ℃ on a Micromeritics 2010 analyser (USA). All the samples were degassed at 393 K prior to the measurement. The BET surface area (SBET) was calculated based on the adsorption isotherm. The pore-size distributions were obtained from the desorption branches using the Barrett-Joyner-Halenda (BJH) method. ICP was performed on a Perkin-Elmer Optima 3300DV apparatus (USA). The photoluminescence (PL) spectra were measured at room temperature with a fluorospectrophotometer (JASCO FP-6300, Japan) using a Xe lamp as the excitation source. The electrochemical impedance spectra (EIS) were recorded using an EIS spectrometer (EC-Lab SP-150, BioLogic Science Instruments, USA) in a three electrode cell by applying a 10 mV alternative signal versus the reference electrode (SCE) over a frequency range of 1 MHz to 100 mHz. The cyclic voltammograms were measured in a 0.1 mol∙L-1 KCl solution containing 2.5 mmol∙L-1 K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) as a redox probe at a scanning rate of 20 mV·s-1 in the same three electrode cell as the EIS measurement.

    2.2 Photocatalytic reaction

    The photocatalytic H2O2 production ability of the samples were evaluated by the reduction of molecular oxygen. For these experiments, 0.2 g of photocatalyst was added to 200 mL deionized water. The suspension was dispersed using an ultrasonicator for 10 min. During the photoreaction under visible light irradiation, the suspension was exposed to a 250 W high-pressure sodium lamp with main emission in the range of 400 to 800 nm, and O2 was bubbled at 80 mL∙min-1 through the solution. The UV light portion of the sodium lamp was filtered by a 0.5 mol∙L-1 NaNO2 solution. All runs were conducted at ambient pressure and 30 ℃. At given time intervals, 5 mL aliquots of the suspension were collected and immediately centrifuged to separate the liquid samples from the solid catalyst. The concentration of H2O2 was analyzed by normal iodometric method32, 33.

    3 Results and discussion

    Fig. 1(a) shows the XRD patterns of as-prepared catalysts. Two typical diffraction peaks of g-C3N4 are present in the B-CN, MV-CN and MV-MS-CN. The peak at 13.1° corresponds to in-plane structural packing motif of tri-s-triazine units, which is indexed as (100) peak. The peak at 27.5° corresponds to interlayer stacking of aromatic segments with distance of 0.324 nm, which is indexed as (002) peak. For MAFO, six diffraction peaks are observed, which are assigned to MgO, Al2O3 and Fe2O3, respectively34-36. In the case of as-prepared heterojunction catalysts, both diffraction peaks for g-C3N4and MAFO are observed. Neither diffraction peak of other species nor the diffraction peak shift is observed indicating that no doping occurs. To characterize the specific surface area of as-prepared catalysts, the nitrogen adsorption and desorption isotherms were measured (Fig. 1(b)). The isotherms of all the catalysts are of classical type IV, suggesting the presence of mesopores. The BET specific surface areas (SBET) of B-CN, MV-CN, MV-MS-CN, MAFO, MV-CN/MAFO and MV-MS-CN/ MAFO are calculated to be 6.5, 7.2, 14.9, 10.6, 8.8 and 13.7 m2·g-1. This hints that the morphology of MV-MS-CN could be changed by addition of molten salt in the synthesis process, leading to the significantly promoted SBET. The large SBET can promote adsorption, desorption and diffusion of reactants and products, which is favorable to the photocatalytic performance.

    Fig. 1 XRD patterns (a), N2 adsorption-desorption isotherms (b), UV-Vis spectra (c) and plots of the transformed Kubelka-Munk function versus the energy of light (d) of as-prepared catalyst.

    The UV-Vis spectra of the as-prepared photocatalysts are shown in Fig. 1(c). The band gaps are estimated from the tangent lines in the plots of the square root of the Kubelka-Munk function as a function of the photon energy (Fig. 1(d))37. B-CN displays an absorption edge at approximately 450 nm, corresponding to a band gap of 2.75 eV. The absorption edge for MV-CN and MV-MS-CN are 480 and 491 nm, and the corresponding band gap is estimated to be 2.58 and 2.52 eV. This reveals that both microwave and molten salt can influence the polycondensation degree of melamine, leading to the change of band gap energy. MAFO shows obviously improved visible light absorption ability compared with as-prepared g-C3N4 catalysts. Its absorption edge is observed at 647 nm, and the corresponding band gap is estimated to be 1.92 eV. For the as-prepared heterojunction photocatalysts, the typical two absorption edges for g-C3N4 and MgAlFeO are observed, hinting the heterojunction photocatalysts are composed of these two components. No intrinsical difference between MV-CN/MAFO and MV-MS-CN/ MAFO can be observed.

    The components of as-prepared catalysts were obtained by ICP (Table 1). The C and N contents for as-prepared g-C3N4 catalysts are approximately 39% and 57% (w, mass fraction), which is close to the theoretical values. For MAFO, the Mg, Al, Fe and O contents are 31.5%, 14.1%, 14.7% and 39.7% (w), respectively. Both the as-prepared heterojunction photocatalysts show the similar element percentages, indicating that the addition of molten salt do not influence the component of catalyst. According to this element percentage, the mass ratio of g-C3N4to MgAlFeO is approximately 6:4 for the as-prepared heterojunction photocatalysts. In addition, no K, Li and Cl are detected in the as-prepared catalysts, confirming that no ion-doping occurs.

    Table 1 The components of as-prepared catalysts obtained by ICP.

    The morphologies of the representative samples were examined by using SEM analysis. Fig. 2a indicates that as-prepared B-CN is composed of a large number of irregular particles. Those particles exhibit layer structure that is similar to its analogue graphite. In Fig. 2b, the nanorod-like MAFO with ~2 µm long is observed. Besides that, it is noted that the surface of MAFO is smooth. For MV-CN, the catalyst with layered structure is still observed (Fig. 2c). In the case of MV-MS-CN, the catalyst morphology changes to nanoparticles (Fig. 2d). This confirms that the addition of molten salt significantly influence the morphology of as-prepared catalyst. Those nanoparticles could form more intra-aggregated pores, leading to the increased SBET (Fig. 1b). Fig. 2e clearly displays that the as-prepared MV-CN/MAFO is composed of layer structural MV-CN and nanorod-like MAFO. MAFO nanorods seem to stick to the MV-CN surface. This interaction between MV-CN and MAFO is poor. For MV-MS-CN/MAFO (Fig. 2f), it can be seen that the nanorod-like MAFO is coated by the MV-MS-CN nanoparticles. The MAFO surface is not as smooth as shown in Fig. 2b but very rough, confirming the MV-MS-CN coating (Fig. 2g). High-resolution transmission electron microscopy (HRTEM) analysis was performed to get information on the microstructure of as-prepared MV-MS-CN/ MAFO (Fig. 2h). The observed lattice fringe spacing of 0.324 nm corresponds to the (002) crystal plane of g-C3N4 (JCPDS 87-1526). The lattice fringes of 0.373, 0.265 and 0.208 nm in the HRTEM image should be assigned to the (012), (006) and (200) planes of Al2O3, Fe2O3 and MgO, respectively. Smooth and intimate interfaces are clearly observed between MV-MS-CN and MAFO, which confirms the formation of g-C3N4/MgAlFeO heterojunction. Such strong interaction can result in the higher interfacial charge transfer rate and H2O2 production ability.

    Fig. 2 SEM images of as-prepared B-CN (a), MAFO (b), MV-CN (c), MV-MS-CN (d), MV-CN/MAFO (e), MV-MS-CN/MAFO (f and g) and HRTEM of MV-MS-CN/MAFO (h).

    XP spectra are used to investigate the structure of the as-prepared heterojunction catalyst. In Mg 2p, Al 2p and Fe 2p regions (Fig. 3(a-c)), the binding energies for MAFO located at 49.7, 73.8 and 711.4 eV are assigned to the Mg2+, Al3+ and Fe3+ respectively38-40. For MV-MS-CN/MAFO, the binding energies in Mg 2p, Al 2p and Fe 2p regions exhibit obvious blue-shifts compared with that of MAFO. This is probably due to the electron transfer from electron-rich g-C3N4 to MAFO, leading to the electron density change. In O 1s region (Fig. 3d), the MAFO displays a single peak at 531.5 eV, which is assigned to the O2- bond to the metal ions. For MV-MS-CN/MAFO, another peak located at 532.6 eV is observed. As reported by previous literatures, this peak should be assigned to the adsorbed oxygen species41-43.In Fig. 3e, the spectra of both two catalystsin C 1s regions can be fitted with two contributions which located at 284.6, and 287.8 eV. The sharp peak around 284.6 eV is attributed to the pure graphitic species in the CN matrix. The peak with binding energy of 287.8 eV indicates the presence of sp2 C atoms bonded to aliphatic amine (-NH2 or -NH-) in the aromatic rings44.In Fig. 3(f), the main N 1s peak of MV-MS-CN located at 398.3 eV can be assigned to sp2-hybridized nitrogen (C=N-C), thus confirming the presence of sp2-bonded graphitic carbon nitride. The peak at a higher binding energy of 400.1 eV is attributed to tertiary nitrogen (N-(C)3) groups45. For MV-MS-CN/MAFO, the 0.2 eV shift to higher binding energy is observed, indicating the decreased electron density of nitrogen atoms. Combined with the phenomenon of binding energy shift in Mg 2p, Al 2p and Fe 2p regions, it is deduced that the strong electronic interaction between the MV-MS-CN and MAFO is formed in MV-MS-CN/ MAFO.

    Fig. 3 XPS of as prepared catalysts in the region of Mg 2p (a), Al 2p (b), Fe 2p (c), O 1s (d), C 1s (e) and N 1s (f).

    The energy level positions of MV-MS-CN and MAFO are confirmed by VB XPS. In Fig. 4a, the VB positions of MV-MS-CN and MAFO are +1.29 and +2.08 V. It is obtained from the UV-Vis results that the band gaps for CN and MgAlFeO are 2.52 and 1.92 eV. Thus the ECB for MV-MS-CN and MAFO is -1.23 and +0.16 V, respectively. Once MV-MS-CN and MAFO are electronically coupled together, the band alignment between the two components results in the formation of heterojunction with well-matched band structure. The potential difference is the main driving force for efficient charge separation and transfer. These two charge transfer processes are beneficial for overcoming the high dissociation barrier of the Frenkel exciton and stabilizing electrons and holes. As the photogenerated electrons and holes are spatially separated into two components, the charge recombination is drastically inhibited, which is of great benefit for enhancing the photocatalytic activity. In addition, with effective separation of electron/hole pairs, the lifetime of photogenerated charge carriers is expected to be prolonged. The prolonged lifetime allows the fast charge transfer to the reactive substrates on the photocatalyst surface, promoting the photocatalysis reaction.

    Fig. 4 VB XPS (a), EIS (b) and PL (c) of as-prepared catalysts.

    EIS was used to characterize charge-carrier migration and confirm the interfacial charge transfer effect of the as-prepared heterojunction catalysts. As shown in Fig. 4b, the as-prepared heterojunction catalysts exhibit a decreased arc radius compared to that of MV-MS-CN and MAFO. In general, the radius of the arc in the EIS spectra reflects the reaction rate on the surface of the electrode46. The reduced arc radius indicates a diminished resistance of the working electrodes, suggesting a decrease in the solid-state interface layer resistance and the charge transfer resistance across the solid-liquid junction on the surface between g-C3N4 and MgAlFeO47, 48. MV-MS-CN/ MAFO displays the smallest arc radius, confirming that a more effective separation of photogenerated electron-hole pairs and a faster interfacial charge transfer occur on the MV-MS-CN/ MAFO surface compared with MV-CN/MAFO. PL spectra are shown in Fig. 4c. In general, the higher PL intensity, the lower separation rate of electrons-holes. Obviously, the PL intensity follows the order: MAFO > MV-MS-CN > MV-CN/MAFO > MV-MS-CN/MAFO, which is consistent with the order of arc radius of EIS spectra. MV-MS-CN/MAFO exhibits a significant fluorescence quenching phenomenon, indicating its significantly improved separation efficiency. This confirms the existence of strong interaction between MV-MS-CN and MAFO.

    The influence of molar ratio of Mg:Al:Fe and mass ratio of melamine to MAFO on the H2O2 production performance is present in Table 2. The result shows that the optimal molar ratio of Mg:Al:Fe is 5:2:1 and mass ratio of melamine to MAFO is 2: 1. Fig. 5a shows the H2O2 production ability of MV-MS-CN/MAFO under different reaction conditions. The addition of AgNO3 as electron scavenger sharply suppresses the H2O2 production ability, indicating the main active species is the photogenerated electron. The H2O2 generation rate can be ignored in the absence of O2 or photocatalyst, indicating that H2O2 is produced via a photocatalytic O2 reduction process. No H2O2 is generated when using DMF (aprotic solvents) instead of water, confirming that H2O as the proton source is necessary for the H2O2 production. Fig. 5b shows the H2O2 production ability over as-prepared catalysts under visible light. It is clearly seen that the H2O2 concentration of as-prepared catalyst increases with time for about 6 h when it reaches a constant level. This level corresponds to the steady-state where the rate of H2O2 production is equal to the rate of decomposition49, 50. According to the previous work, the rates for formation and decomposition of H2O2 follow the zero-and first-order kinetics toward H2O2 concentration, respectively 49. The kinetic data are calculated by the equation: [H2O2] = (kf/kd){1 -exp(-kdt)}, where kf and kd are the rate constants for formation and decomposition of H2O2, respectively. The rate constants as well as the equilibrium concentration of H2O2 are summarized in Table 3. MAFO shows the lowest H2O2 concentration and kf value, probably due to the worst charge separation rate (Fig. 4b). B-CN and MV-CN show the similar H2O2 concentration, kf and kd values, which are larger than that of MAFO. For MV-MS-CN, the kd value shows no obvious change whereas the kf value further increases compared with MV-CN, which is probably due to that the larger SBET of MV-MS-CN can provide more active sites for H2O2 production simultaneously. In the case of as-prepared heterojunction catalysts, the kf (kd) values significantly increase (decrease) compared with those of as-prepared g-C3N4 catalysts, causing the much higher H2O2 concentration. This indicates that the formation of heterojunction could promote the formation and restrain the decomposition of H2O2 simultaneously. The promoted H2O2 formation rate is attributed to the higher separation rate of photogenerated electrons and holes for heterojunction catalysts. The depressed H2O2 decomposition rate is probably due to the formation of •OH. The UV-Vis and XPS results show that the ECB and EVB of MAFO are +0.16 and +2.08 V. The redox potential of ·OH/OH- is +1.99 V 51. The VB holes of MAFO are positive enough to generate •OH which is the H2O2 decomposition product according to Reaction (4). Thus, the H2O2 decomposition is depressed. In addition, MV-MS-CN/ MAFO displays much larger kf value than that of MV-CN/ MAFO, confirming this molten salt-assisted microwave process is an effective method to prepare heterojunction catalyst with high photocatalytic performance.

    Table 2 Influences of molar ratio of Mg:Al:Fe and mass ratio of melamine to MAFO on the H2O2 production performance.

    Table 3 H2O2 concentration and the kinetic parameters of as-prepared catalysts.

    Fig. 5 H2O2 production ability of MV-MS-CN/MAFO under different reaction conditions (a) and the H2O2 production ability over as-prepared catalysts (b).

    The catalytic stability of MV-MS-CN/MAFO is shown in Fig. 6. It is clearly seen that the H2O2 concentration reaches a constant level at 6 h. After 48 h, the equilibrium concentration of H2O2 is almost unchanged, hinting its excellent catalytic stability. Fig. 6 inset shows the XRD results of fresh and reused MV-MS-CN/MAFO. No obvious difference between them is shown, confirming its outstanding structure stability.

    Fig. 6 Catalytic stability of MV-MS-CN/MAFO.

    The catalytic stability of MV-MS-CN/MAFO is shown in Fig. 6. It is clearly seen that the H2O2 concentration reaches a constant level at 6 h. After 48 h, the equilibrium concentration of H2O2 is almost unchanged, hinting its excellent catalytic stability. Fig. 6 inset shows the XRD results of fresh and reused MV-MS-CN/MAFO. No obvious difference between them is shown, confirming its outstanding structure stability.

    In order to clarify the type of heterojunction, photocatalytic RhB degradation performance over MV-MS-CN/MAFO is investigated (Fig. 7(a)). t-BuOH was used as hydroxyl radical (•OH) scavenger. The results indicate that the photocatalytic degradation rate of RhB is over 80% within 4 h. When t-BuOH is added, the degradation rate of RhB sharply decreases to approximately 50%. This hints hydroxyl radical is one of the main oxidative species for RhB degradation. It is known that the redox potentials for •OH/OH- is +1.99 V 51. The EVB of MV-MS-CN and MAFO is +1.29 and +2.08 V, respectively. Thus, the holes in valence band of MAFO are positive enough to generate •OH but MV-MS-CN cannot. It is deduced that the holes are in the valence band of MAFO but not MV-MS-CN. Accordingly, the heterojunction should be "Z" type. The possible electron transfer mechanism is shown in Fig. 7(b). Under visible light irradiation, the electrons-holes are formed. The electrons in the conduction band of MAFO transfer to the valence band of MV-MS-CN to combine with the holes. Thus, the electrons in the conduction band of MV-MS-CN will reduce oxygen to form H2O2: O2 + 2H+ + 2e- → H2O2. Holes in the VB of MAFO can oxidize water to form oxygen as follow: H2O + 2h+ → 1/2 O2 + 2H+52.

    Fig. 7 Photocatalytic RhB degradation performance (a) and the possible electrons transfer route (b) over MV-MS-CN/MAFO.

    4 Conclusions

    In this work, the g-C3N4 coated MgAlFeO heterojunction catalyst was synthesized via a novel molten salt-assisted microwave process. The addition of molten salt into synthesis process not only changes the morphology of as-prepared catalysts but also influence the polycondensation degree of melamine, leading to the change of band gap energy. SEM and HRTEM results show that the MgAlFeO nanorods are coated by the g-C3N4 nanoparticles, leading to the strong electronic coupling between two components. This strong electronic coupling results in more effective separation of photogenerated electron-hole pairs and faster interfacial charge transfer, causing the higher H2O2 formation rate (kf). In addition, the VB holes of MAFO are positive enough to generate •OH which is the H2O2 decomposition product, leading to the lower H2O2 decomposition rate (kd) over as-prepared heterojunction catalysts. This work provides a novel method to prepare heterojunction catalyst with high electron-hole separation rate and photocatalytic performance.

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