The self-assembly chemical synthetic method is the main strategy for constructing the molecule-based magnets by a selection of proper spin source and associated structures. Usually, transition metal ions or organic radicals provide the spin source, while the associated structures are composed of coordinating ligands. The self-assembly process is dependent on the inner driving force, which is hard to adjust. As a kind of exterior driving force, the magnetic field is expected to influence the chemical reaction involved transition metal ions and organic radicals.

Our group has shown the MFEs on the pathway in a ligands reaction system.^[10] The hydrothermal synthesis reactions which involve the coordinating ligands of isonicotinic acid and sodium azide were carried out. When no magnetic field was applied, the final product contained ca. 5% red crystals with the chemical formula of [Co_1.5(N₃)(OH)(L)]_n (Fig. 2(b), antiferromagnetism) and 95% yellow crystals (Fig. 2(a), paramagnetism). However, the yield of red crystals raised to 25% under the magnetic field of 0.2 T. Obviously, the applied magnetic field favors to form the crystals with antiferromagnetism. The activation energy of the antiferromagnetic complex is expected to decrease during the chemical reaction process because of the magnetic field. Based on the final product, it is deduced that two transition states (chemical reaction pathways) which could be expressed as [Co…L… H₂O] and [Co…L…OH⁻… ] were involved during the reaction process. Without a magnetic field, the main product is yellow crystal, which indicates that the activation energy of [Co…L…H₂O] is lower than that of [Co…L…OH⁻… ]. However, the activation energy of [Co…L…OH⁻… ] can be reduced largely when a magnetic field is applied. The red crystals hold short-range antiferromagnetic coupling interaction and mainly show antiferromagnetism according to the magnetic measurement result. Under a magnetic field, the magnetic exchange can be efficiently mediated in [Co…L…OH⁻… ] activated complex which possesses bridging ligands, leading to producing a direction of magnetic phase transition between paramagnetic and antiferromagnetic states. This magnetic phase transition is attributed to the lower energy of antiferromagnetic phase compared to that of paramagnetic phase. Moreover, for the same material, compared to the paramagnetic phase, the antiferromagnetic phase is more stable under a magnetic field. Due to this transition, based on the viewpoint of thermodynamics, the self-assembly reaction will process more easily because of the decreased activated energy of the activation complex. However, under a magnetic field, no phase transition takes place for the yellow crystal with the paramagnetic property. Therefore, the activation energy of the yellow crystal is maintained (Fig. 3). Moreover, during a Ni-complex reaction process, a magnetic field of 0.3 T can totally change the final product from green into blue (Fig. 2(c)),^[11] which means that the reaction pathways can be selected by a magnetic field. As we know, organic ligands possess various coordination modes. In other words, there are several reaction pathways during the self-assembly process. The above results show that magnetic fields may be a strategy to control the reaction pathways of molecule-based magnets. A magnetic field with higher intensity is expected to bring more significant changes, and even exploring the new molecule-based magnets with high performance.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. Optical images of the products: (a) yellow crystals, (b) red crystals, and (c) 1-O: green, 1-B: bule.[10,11]

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. Scheme of the MFEs on activation energies of the self-assembly reaction.[10]

In general, it is considered that MFEs on product phase will occur in organic chemical reactions. However, many research confirmed that the product phase of some inorganic chemical reactions can be influenced by the magnetic field. Ma et al. showed that the product phase of Fe₂O₃ could be changed under a magnetic field of 12 T during the chemical reaction process at 500 °C.^[12] Without a magnetic field, the XRD pattern of the product is consistent with that of single phase α-Fe₂O₃. However, the major peaks of XRD patterns of the magnetic field-induced products can be indexed to γ-Fe₂O₃ with a cubic spinel phase, indicating that γ-Fe₂O₃ has been fabricated under 12 T (Fig. 4). It is suggested that the magnetic field, like temperature, can change the free energy during chemical reaction processes based on the thermodynamic viewpoint. Therefore, in the presence of a magnetic field, there are two aspects free-energy in the reaction system. In addition to the thermal Gibbs free energy ΔG_T(T), the magnetic field also produces the free energy which can be defined as the magnetic Gibbs free energy ΔG_M(T,H). The combination of ΔG_T and ΔG_M is the total Gibbs free energy for the phase transformation from α-Fe₂O₃ to γ-Fe₂O₃. The ΔG_M can be attributed to the difference magnetic susceptibilities of γ-Fe₂O₃ and α-Fe₂O₃. The ΔG_M can be estimated with the following formula:

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. The XRD patterns for the products obtained at the temperature of 500 °C under no magnetic field (a) and 12 T: (b) B⊥ the sample and (c) B ∥ the sample.[12]

Moreover, Yu et al. demonstrated the fabrication of ferrosulfide minerals assisted by the magnetic field, which showed that the MFEs could effectively change the phase of the product.^[14] At 180 °C, without a magnetic field, a product composed of Fe₃S₄, and FeS₂ was obtained, while a metastable phase of pure cubic Fe₃S₄ could be formed under a magnetic field of 0.045 T. The free energy produced by the magnetic field is the main factor to determine the formation of cubic Fe₃S₄ (ferromagnetic phase). Interestingly, pure cubic Fe₃S₄ was also formed under no magnetic field at a relative low temperature of 160 °C, which further proves the similar effect of the magnetic field and temperature.

The semiconducting phase (2H) of MoS₂ is constructed by two S–Mo–S layers while the metallic phase (1T) is built by a single S–Mo–S layer. The 1T phase has emerged as an attractive candidate for wide applications and is expected to exhibit better performance than its semiconducting counterpart due to its high electrical conductivity, exotic ferromagnetic behavior, and superconductivity. Therefore, obtaining the MoS₂ nanostructure with a high concentration of 1T phase is a significant strategy to realize the enhanced properties. A new strategy, named as high magnetic field-induced method for synthesizing stable 1T phase MoS₂ nanosheets, has been demonstrated. When the magnetic field intensity reaches 9 T in the reaction system, the stable metallic phase concentration as high as ∼ 100% can be successfully achieved in MoS₂ nanosheets. Interestingly, the 1T phase concentration is related with the magnetic field intensity. The 1T phase percentage of the MoS₂ nanosheets synthesized under 0 T, 5 T, 8 T, and 9 T is estimated to be ∼ 25%, 50%, 75%, and 100%, respectively. As we know, the Mo⁴⁺ in 2H phase shows the trigonal prismatic local coordination, which leads to the zero net magnetic moment because of two spin-antiparallel 4d electrons. As a result, the 2H phase of MoS₂ is macroscopically nonmagnetic. In contrast, the magnetic moment of 1T-MoS₂ is 2 μ_B/Mo ion. Therefore, 1T phase incorporation in 2H phase will display ferromagnetism at room temperature. At 300 K, the measured saturation magnetizations (M_s) of the as-prepared MoS₂ nanosheets obtained under 0 T, 5 T, 8 T, and 9 T are 0.005 emu/g, 0.013 emu/g, 0.017 emu/g, and 0.022 emu/g, respectively. It should be noted that the value of M_s agrees with the ratio of 1T phase and 2H phase. These results indicate that the applied magnetic fields can promote the fabrication of magnetic materials during the reaction process.

Based on the experimental results, the possible mechanism is deduced as follows. Without applied magnetic fields, the as-prepared MoS₂ nanosheets contain ∼ 25% 1T phase, which are similar to the MoS₂ nanosheets obtained by the two-step hydrothermal synthesis.^[15] The 1T phase tends to appear near the edge of the 2H phase MoS₂ nanosheets as previous reports.^[16] Under a high magnetic field, a strain can be produced because of the nonzero magnetic moment of 1T phase. As a result, the original MoS₂ lattice inevitably undergoes a structural distortion due to the strain, which will lead to the MoS₂ nanosheets become thinned, resulting in the transformation of the 2H phase into 1T phase. The driving force of this process mainly arises from the magnetic fields, which may be defined as magnetic exfoliation. With the increase of the magnetic field intensities, the strength of the strain will be increased, resulting in the enhanced concentration of 1T phase in MoS₂ nanosheets. Also, another explanation is considered. The previous report showed that pure 1T MoS₂ was obtained by a hydrothermal synthesis process at 200 °C while 2H MoS₂ was formed at 240 °C.^[17] This is because the octahedral structure of 1T MoS₂ cannot be maintained under the growth conditions used, such as temperature at 240 °C. Similarly, without high magnetic fields, the 1T MoS₂ formed during synthesis process may easily transform to 2H MoS₂. However, the octahedral coordination of 1T MoS₂ may be maintained under the assistant of high magnetic fields, resulting in the high concentration of 1T MoS₂ in the product.

In electrochemical reactions, the MFEs on electrogenerated chemiluminescence (MFE_ECL) intensity, which can be generated through density and conversion channels, have been widely employed to study spin-dependent reaction routes (Fig. 5(a)). In the density channel, it is shown that the density of light-emitting states can be changed under a magnetic field due to the existence of magnetizing and Lorentz forces on the reactant radicals. Under a magnetic field, in the conversion channel, the spin mixing may modify the conversion between singlets and triplets in both light emitting states and intermediate charge-transfer complexes. In the electrochemical reaction, a considerable MFE_CEL intensity with the amplitude less than 30% has been achieved in early experimental studies. Using a conveniently controllable electrochemical co-reaction, Hu et al. reported a giant MFECEL with a magnitude larger than 400% based on Lorentz force effects in liquid states^[9] (Fig. 5(c)). During the electrochemical oxidation catalytic process, the tripropylamine (TPrA) acts as an co-reactant for . In the Ru(bpy)₃-based electrochemical reaction, a giant MFE_CEL can be generated at different electrical biases under a two-electrode electrochemical configuration in which an external magnetic field is applied. Under the field intensity of 0.07 T, the MFE_CEL can reach 400% at 3.3 V. This result is much larger than the MFE_CEL reported previously. The observed giant MFE_CEL is mainly attributed to the existence of the Lorentz force effects which can cause the convection for the reactive species in the liquid solution by the momentum transfer between the solvent molecule and reactive ion. The convection generated by the Lorentz force will promote the ion penetration through diffusion layer in the electrochemical reaction. Moreover, the decrease of the diffusion layer thickness is achieved. The electrochemical reaction can be enhanced due to the increase of ion penetration, which increases the electroluminescence intensity. As a result, a positive MFE_CEL is observed. Moreover, in liquid states, it is suggested that regulating the Lorentz force effects to a proper level will present a novel strategy to gain giant MFEs based on the electrochemical reaction.

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. (a) MFEECL is generated by density and conversion channels. (b) Experimental setup with a two-electrode configuration or an electrochemical cell placed in a magnetic field. (c) MFECEL at different voltages. (d) Normal and abnormal MFEECL under fast field-sweeping rate of 32 mT/s. (e) Normal and abnormal MFEECL under fast field-sweeping rate of 3.2 mT/s.[8]

In early researches, it was found that the normal MFE_ECL appeared in the electrochemical system with the existence of magnetic fields (Fig. 5(b)). Based on the electrochemical system of Ru(bpy)₃Cl₂-TPrA, for the first time, Hu et al. observed the abnormal MFE_ECL after removing the applied magnetic field,^[8] which may be attributed to the magnetized state of the charge-transfer [ ] complex and experience the magnetic relaxation with long time. The magnetized state of the charge-transfer complex is confirmed by the distinct behavior between magnetocurrent and MFE_ECL, which is responsible for the abnormal MFE_ECL from an electrochemical system. The magnetic coupling between the charge-transfer complexes with the magnetic relaxation may be revealed by the experimental results on abnormal MFEs in solution.

Because the properties of materials or performances of devices mainly depend on structures, it is significant to control the size, shape, and microstructure of nanomaterials in modern science and engineering. The recent research shows that the growth behavior of both the ferro/ferromagnetic and paramagnetic/diamagnetic nanomaterials can be tailored by magnetic fields during the chemical synthesis process, which leads to great change of the as-prepared product.

For magnetic materials, the oriented growth can be induced by a magnetic field. As a result, the material will grow along the easy magnetic axis to form one-dimensional nanostructure. In 2004, for the first time, Chen et al. showed the growth of Fe₃O₄ nanowires with single crystalline feature under a magnetic field, which was carried out in a self-made autoclave with cylindrical NdFeB magnets.^[18] Without an applied magnetic field, Fe₃O₄ particles with hexagonal and square shapes were observed. Interestingly, under a field intensity of 0.35 T, a fibrous nanostructure and nanowires were obtained at a large scale. Meanwhile, the aquare particles disappeared (Figs. 6(a) and 6(b)). The electron diffraction (ED) pattern combined with the HRTEM results indicated that the Fe₃O₄ nanowire was single crystalline and grew along the easy magnetic axes of [110] (Figs. 6(c) and 6(d)). It is suggested that the Lorentz force produced by the magnetic field can influence the behavior of individual ions, leading to the preferential nucleation of particles during the growth process. Moreover, under magnetic fields, metallic nickel wires with width diameter fluctuation were obtained by chemical reduction of [Ni(N₂H₄)_x]²⁺ complexes. The magnetic attraction generated from the magnetic field can lead to the alignment of [Ni(N₂H₄)_x]²⁺ complexes along the lines of magnetic force. As a result, the chemical reduction of [Ni(N₂H₄)_x]²⁺ complexes will occur along the lines of magnetic force to form metallic nickel wires. It is worth mentioning that the magnetic field-induced method could be recommended as one of the six ways to form nanowires.

Fig. 6.

	Figure Option ViewDownloadNew Window
	Fig. 6. TEM images of the samples obtained in zero magnetic field (a) and 0.35 T (b), (c) TEM image of a typical Fe3O4 nanowire, (d) HRTEM image of the nanowire obtained under a magnetic field.[18]

The magnetic field with a low intensity may generate negligible effects on the growth of paramagnetic or diamagnetic structures during the chemical synthesis process. However, the MFEs are related with the magnetic field intensity according to the formulas. Therefore, for the nanostructures of weak magnetic materials, a high magnetic field needs to be applied in the chemical reaction systems for tailoring the growth behavior. Carbon filaments were prepared by the scientists in Japan and Israel under the field intensity of 10 T.^[19] The paramagnetism and large diamagnetic anisotropy of graphitic clusters were caused by the localized spins which appeared in large numbers at the boundaries of graphitic plane edges, which must be related to the growth induced by a magnetic field. Moreover, a magnetic field-assisted approach to fabricate Bi nanowires has been demonstrated.^[20] The Bi microspheres with different diameters were obtained under the zero magnetic field (Fig. 7(a)). Evidently, the morphology remarkably changed when the magnetic fields were applied during the synthesis process. A small amount of nanowires were formed under the magnetic field of 1 T (Fig. 7(b)). When the strength reached 4 T, many nanowires combined with nanobelts were fabricated (Fig. 7(c)). Under the magnetic field of 8 T, a large number of Bi nanowires could be observed (Fig. 7(d)), accompanied by the disappearance of other morphologies. The representative HRTEM image showed that the as-prepared nanowire is single crystalline (Fig. 7(e)). To some extent, the MFEs could change the surface energy of Bi nucleus when the synthesis process was progressed under a magnetic field with high intensity. However, the different axes of Bi possess distinct magnetic susceptibilities. As a result, along each axis, the MFEs induced surface energy changes are different. Based on the magnetic susceptibilities of each axis, it is deduced that the c-axis possesses the higher increase of surface energy compared to other axes. Moreover, because the energy of the (0001) plane is highest, the growth perpendicular to this high energy plane can be induced to decrease the system energy (Fig. 7(f)).

Fig. 7.

	Figure Option ViewDownloadNew Window
	Fig. 7. The representative SEM images of Bi nanomaterials formed under diverse field strengths with scale bar of 5 μm: (a) 0 T, (b) 1 T, (c) 4 T, (d) 8 T. (e) The representative TEM image of the nanowire formed under 8 T, the inset is the SAED pattern of the nanowire. (f) The HRTEM image of a nanowire formed under 8 T.[20]

Moreover, the experimental and theoretical researches have been carried out to investigate the MFEs on the surface energy during the process of nanoparticle growth.^[21] Experimental results showed that the morphology of Co₃O₄ nanoparticles could be changed from irregular spheres into nanocubes when the chemical synthesis occurs under a magnetic field (Fig. 8). As we know, for the spinel structure, the morphology of Co₃O₄ nanoparticles is depended on the surface energies which are related with the competitive growth between different faces. The energies of {111} and {100} faces with and without applied magnetic fields were calculated. Without the field, the energy of the {100} faces (1.849 J·m⁻²) is higher than that of the {111}#1 faces (1.645 J·m⁻²) and {111}#2 (1.638 J·m⁻²). However, under a magnetic field of 0.4 T, the energy of the {100} faces (1.326 J·m⁻²) becomes lower than that of the {111}#1 faces (1.429 J·m⁻²) and {111}#2 (1.447 J·m⁻²). Obviously, under the applied magnetic field, the energies of both faces decrease, however, the reduction extent of {100} faces is larger than that of {111} faces. As a result, due to the relatively lower face energy, the {100} faces turn to be a growth limited form, leading to the change of the final morphology.

Fig. 8.

	Figure Option ViewDownloadNew Window
	Fig. 8. (a) Structures of bulk Co3O4 and surface models for Co3O4 {100} and {111}. (b) and (c) The TEM images of Co3O4 particles obtained in the absence and presence of a magnetic field.[21]

A lot of attention has been paid to the magnetic domain structure of one-dimensional materials due to the promising properties and applications as high density magnetic recording media. The magnetic force microscopy (MFM) has been employed to investigate the domain of one-dimensional nickel nanostructure with different diameters obtained under 0.25 T.^[22,23] A single domain state was observed on the wire with the diameter of 250 nm, which is proved by the uniform dark contrast shown in Figs. 9(a) and 9(b). Moreover, the domain structure strongly associated with the diameter of the wires. In the wire with the diameter of 2 μm, the cylindrical domain with a unique core–shell structure was clearly observed, which is proved by the dark middle part and bright shell, as shown in Figs. 9(c) and 9(d). These results suggested that the unique multidomain can be aroused under an applied field during the reaction process. As mentioned above, under the magnetic field, nickel crystallites will grow along the lines of magnetic force to form the nickel wires in which the magnetic moments are lined in the same direction, leading to the formation of a single domain in the thin wire. In contrast, the exchange energy is increased in multi-domain configurations due to the unparallel spins of the inside domain walls. Moreover, the magnetization of the inside domain wall shifts away from the easy axis, leading to the increase of the magnetic anisotropy energy. In the wires with large diameters, the increase of anisotropy and exchange energy will be smaller than the decrease of magnetostatic energy, which results in the formation of multi-domains. For nickel wires with large diameters, a cylindrical core–shell multidomain in which the magnetic moments in the core possess the same magnetization and in the shell have the opposite magnetization will be formed to minimize the energy of the nickel wires. As shown in Fig. 9(e), the core possesses the magnetic moments with a downward direction, while an upward direction is observed for the moments of the shell.

Fig. 9.

	Figure Option ViewDownloadNew Window
	Fig. 9. The AFM and MFM images of the as-prepared nickel wires with different diameters under 0.25 T: (a), (b) 250 nm, (c), (d) 2 μm, (e) the graphical representation of the core–shell cylindrical domain layout.[22]

The Verwey transition (T_V), which is defined as the sharp and first order transition in magnetite on cooling below 120 K due to the charge ordering of the Fe³⁺ and Fe²⁺ in alternating layers on octahedral sites, was discovered in 1939. As we know, Fe₃O₄ nanoparticles experience a Verwey transition at approximately 120 K. Interestingly, this Verwey transition vanishes in Fe₃O₄ nanoparticles obtained under a field intensity of 0.25 T.^[24] Figure 10 is the EPR spectra at low temperatures of the samples obtained with and without magnetic fields. The transition is clearly showed for the sample obtained without a magnetic field, which is proved by the reduced signal with the lowering of temperature and a sudden decrease near 120 K, as shown in Fig. 10(a). In contrast, the Verwey transition vanishes in the product obtained by a similar way under 0.25 T (Fig. 10(b)). The Mössbauer spectra further indicated that the oxidation of the Fe²⁺ ions to Fe³⁺ ions under a magnetic field leaded to the disappear of long-range charge ordering on octahedral sites below T_V because of the absence of Fe²⁺ ions in layers. Fe²⁺ ions could be easily oxidized to Fe³⁺ ions in order to achieve a higher spin state under a magnetic field in an aqueous solution. The black brown color of Fe²⁺ ions solution experiences a quicker transition to red (Fe³⁺ ions) under 0.25 T compare to the transition without a magnetic field. Under a field intensity of 0.25 T, it is deduced that an enhanced oxidation process could also occur during the chemical synthesis of Fe₃O₄ nanoparticles, leading to the oxidation of Fe²⁺ ions to Fe³⁺ ions on octahedron sites. Therefore, on octahedral sites, the numbers of Fe²⁺ ions and Fe³⁺ ions are different below T_V, resulting in the vanish of charge ordering. This result reveals that the magnetic fields can alter the structure at atomic scales during chemical reaction processes.

Fig. 10.

	Figure Option ViewDownloadNew Window
	Fig. 10. The EPR results measured at various temperatures for the samples obtained under 0 T (a) and 0.25 T (b). The corresponding integral curves exhibit the same variation tendency, as shown in the insets.[24]

The MFEs on catalytic reactions have been widely investigated, which shows that the catalytic rate can be controlled by altering the magnetic field intensity. However, the experimental and theoretical work about the MFEs on the activity of nanocatalysts is rarely seen. Pd nanocatalysts have been selected as the model to study the MFEs on the catalytic activity of nanocatalysts. It is suggested that Pd nanocatalysts with the diameter of 2.5 nm are paramagnetic based on their g value of 2.0, and this means that these Pd nanocatalysts may be affected by applied magnetic fields. The catalytic activity of nanocatalysts consisted of Pd nanoparticles and MIL-100(Cr) in the reduction of 4-nitrophenol has been systematically investigated^[25] (Fig. 11), which shows that the reduction time is decreased from 2.6 min to 1.4 min under a magnetic field of 0.5 T, indicating the increase of the reaction rate. Moreover, the time of completion for the reaction, t_c, is related with the magnetic field intensities. The t_c gradually decreases from 2.6 min, 2.0 min, 1.8 min, 1.6 min to 1.4 min when the magnetic field intensities increase from about 0 T, 0.2 T, 0.3 T, 0.4 T to 0.5 T. Based on the measurement of ESR spectrum, the Pd nanocatalysts are paramagnetic due to uncanceled surface spins. When a magnetic field is applied, the spin configuration on Pd surface will change from paramagnetic to ferromagnetic. Interestingly, the adsorption of 4-nitrophenol is dependent on the spin arrangement on Pd surface. The magnetic field-induced ferromagnetic configuration will inevitably increase the adsorption number of 4-nitrophenol molecules on the catalyst surface, and then accelerate the reaction rate. The theoretical results further reveal that the change of spin configurations on nanocatalyst surface (paramagnetic convert into ferromagnetic configuration) can reduce adsorption energies in the reaction system. As a result, more 4-nitrophenol molecules could be adsorbed on the nanocatalyst surface (Fig. 12). This shows that the magnetic field can regulate the surface spins configuration which plays significant roles on the catalytic activity. The similar research is worth carrying out under high magnetic fields.

Fig. 11.

	Figure Option ViewDownloadNew Window
	Fig. 11. Schematic illustration of catalytic reaction of 4-nitrophenol under magnetic fields.[25]

Fig. 12.

	Figure Option ViewDownloadNew Window
	Fig. 12. Schemes of (a) 4-nitrophenol molecules adsorbed on the MIL-100(Cr) surface and (b) 4-nitrophenol reduction on the Pd surface without (i) and with (ii) a magnetic field.[25]

At room temperature, the Suzuki cross-coupling reaction rate is slow even with Pd nanoparticles as catalysts. The MFEs on the Suzuki cross-coupling reaction rate were also investigated in the presence of Pd@Co₃[Co(CN)₆]₂ catalysts, which showed that the yield of production can be remarkably influenced at 30 °C (Table 1 and Fig. 13).^[26] Moreover, the reaction rate is dependent on the magnetic field intensity. However, the MFEs on the reaction rate become weaker when the temperature increases, and almost disappear at 60 °C. The paramagnetism of Pd nanoparticles is related to the temperature, and the increased temperature will cause the decrease of magnetic susceptibility, which leads to weakening MFEs on the spin at Pd surface. As a result, the advantage produced by MFEs will gradually diminish with the increase of temperature. This result indicates that the magnetic field can act as an external condition to enhance the reaction rates of Suzuki cross-coupling at the lower temperature.

Fig. 13.

	Figure Option ViewDownloadNew Window
	Fig. 13. The curves of the Suzuki reaction yield at 30 °C with and without magnetic fields for different time, respectively.[26]

Table 1

Table 1

Table 1 At temperature of 30 °C, the yield of the Suzuki reaction in the presence of Pd@Co3[Co(CN)6]2 nanocatalyst with and without magnetic fields.[26] . Time/h 2 4 6 8 10 12 24 0 T 13% 19% 25% 29% 32% 35% 44% 0.1 T 15% 23% 31% 35% 38% 40% 49% 0.25 T 16% 31% 37% 40% 42% 45% 54% 0.5 T 17% 32% 40% 45% 50% 54% 65%

Table 1 At temperature of 30 °C, the yield of the Suzuki reaction in the presence of Pd@Co3[Co(CN)6]2 nanocatalyst with and without magnetic fields.[26] .

The density functional theory was employed to gain further insights into the MFEs on the catalytic activity. As we know, in most cases, the adsorption processes are considered as the initial stages for various catalytic reactions. Therefore, the adsorption energies of the reactant molecules on the catalyst surface were calculated by choosing the Pd (111) surface and bri30 conformation as the optimizing model (Fig. 14). Table 2 is the calculated energies of the reactant molecules adsorbed on the above mentioned conformation with and without magnetic fields. The adsorption energy under a magnetic field is lower (−1.12 eV) than that (−1.07 eV) without a field, which results in the change of adsorption behaviors between reactant molecule and catalyst surface. Moreover, due to the decrease of adsorption energy in the reaction process, the stronger chemisorption of bromobenzene molecules on the Pd (111) surface can occur. Therefore, the catalytic activity will be improved because the number of reactant molecules adsorbed on the catalyst surface is increased. Moreover, the analysis of partial density of states (PDOS) was carried out (Fig. 15). Without the magnetic field, one spinous peak has been observed for the 2p orbit in the C atom. In contrast, a more close overlap has been shown under a magnetic field condition, implying the enhanced hybridization between C atoms and Pd atoms. The enhanced hybridization will favor to transfer the electrons from the catalyst substrate to bromobenzene molecules, leading to the increase of the reaction rates of Suzuki cross-coupling. From these studies, one can gain a profound understanding of the MFEs on the surface spin arrangement of nanocatalysts and open a new avenue to control the activities of nanocatalysts by magnetic fields.

Fig. 14.

	Figure Option ViewDownloadNew Window
	Fig. 14. The different adsorption geometries of bromobenzene molecules absorbed on catalyst surface after geometry optimizations: (a) top site,(b) bridge site, and (c) parallel geometry.

Fig. 15.

	Figure Option ViewDownloadNew Window
	Fig. 15. The partial density states for brombenzene molecules adsorbed on Bri30 site at catalyst surface (a) with and (b) without the magnetic field.[26]

Table 2

Table 2

Table 2 Adsorption energies (eV) for brombenzene molecules adsorbed on Bri30 site at catalyst surface with and without magnetic fields.[26] . Energy Emolecular Eslab Eslab+molecular Eads No-MFs –74.11 eV –176.83 eV –252.01 eV –1.07 eV MFs –74.11 eV –176.84 eV –252.07 eV –1.12 eV

Table 2 Adsorption energies (eV) for brombenzene molecules adsorbed on Bri30 site at catalyst surface with and without magnetic fields.[26] .

The MFEs research on heterogeneous reaction systems, photocatalysis in particular, is rather interesting and important.^[27] For the first time, Fu et al. investigated the MFEs on the heterogeneous photocatalytic reaction by degradation of benzene using the Pt/TiO₂ as the catalyst.^[28] As shown in Fig. 16, without a magnetic field, the benzene conversion and CO₂ production were 15.5% and 52 ppm at the steady state. Interestingly, when a magnetic field with the intensity of 59.42 mT was employed during the photocatalytic reaction process, the benzene conversion and the CO₂ production could be increased to 18% and 175 ppm, respectively. At room temperature, He et al. demonstrated the synergistic effect between nanosized TiO₂ and applied magnetic fields by investigating the degradation of phenol in a photocatalytic reaction.^[29] The phenol degradation rate (C/C₀) will be enhanced under magnetic fields with the intensity higher than 0.082 T. In contrast, a low-intensity magnetic field (< 0.044 T) played a negative role on C/C₀. With the raising of magnetic field strength, the yield of hydroxyl radicals increases initially. However, the yield reaches the maximum concentration when the strength is up to 0.082 T. The carriers induced by photo initially decrease when the magnetic field strength is lower than 0.024 T, and then enhance with the increase of magnetic field strength. The MFEs on carriers induced by photo can be interpreted based on the Δg mechanism combined with the hyperfine coupling mechanism. The recombination of holes and electrons can be accelerated and the generation of carriers induced by photo can be suppressed under a magnetic field with low strength, which further limits the phenol degradation. However, the presence of high-strength magnetic field can hinder the hydroxyl radical recombination and thus enhance the hydroxyl radical generation which is the main factor in determining phenol degradation process in the high-strength magnetic field region (Fig. 17).

Fig. 16.

	Figure Option ViewDownloadNew Window
	Fig. 16. (a) The MFEs on the photocatalytic conversion of benzene. (b) The MFEs on CO2 yield for photocatalytic degradation of benzene.[28]

Fig. 17.

	Figure Option ViewDownloadNew Window
	Fig. 17. Effects of applied magnetic fields on the degradation process of phenol over TiO2 photocatalytic system.[29]