Enhanced Photocatalytic Performance through Magnetic Field Boosting Carrier

Transcription

1 Supporting Information for Enhanced Photocatalytic Performance through Magnetic Field Boosting Carrier Transport Jun Li,, Qi Pei, Ruyi Wang,, Yong Zhou,, Zhengming Zhang,, Qingqi Cao,, Dunhui Wang,*,, Wenbo Mi,*, and Youwei Du, National Laboratory of Solid State Microstructures and Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing , China Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing , China Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, School of Science, Tianjin University, Tianjin , China *To whom correspondence should be addressed. 1

2 Figure S1. Schematic illustration of the synthesis procedure of α-fe 2 O 3 /rgo hybrid nanostructures through a facile hydrothermal method. 2

3 Figure S2. (a) and (b) SEM of α-fe 2 O 3 and α-fe 2 O 3 /rgo nanocomposites respectively. (c) and (d) EDX of α-fe 2 O 3 and α-fe 2 O 3 /rgo nanocomposites respectively. The scanning electron microscope (SEM) images of α-fe 2 O 3 and α-fe 2 O 3 /rgo are displayed in Figure S2. Energy-dispersive X-ray analysis (EDX) performed on these two samples suggests that two elements Fe and O are in the α-fe 2 O 3 sample with 2:3 atomic ratio; while 7.25 wt% of carbon is introduced into the α-fe 2 O 3 /rgo nanostructure. 3

4 Figure S3. (a) Wide scan of XPS spectra of α-fe 2 O 3 /rgo nanocomposites. (b) Valence-band XPS spectra of α-fe 2 O 3 /rgo nanocomposites. 4

5 Figure S4. (a) Representative tapping mode AFM topographical image (2 2 µm) of α-fe 2 O 3 /rgo nanoparticles deposited on a silicon wafer. (b) The 3D view of AFM image (a). (c) The cross-sectional view corresponds to the line drawn in the image (a). 5

6 Figure S5. FTIR spectra of α-fe 2 O 3 /rgo. The surface state of α-fe 2 O 3 /rgo was studied by Fourier-transform infrared (FTIR) spectroscopy. As shown in Figure S5, absorption bands at around 3158 and 2614 cm -1 are assigned to the asymmetrical stretching vibrations, symmetrical stretching vibrations of physically adsorbed H 2 O molecules in the sample. The absorption from 1667 to 1100 cm -1 can be assigned to carboxylic groups. The absorption at 1085 cm -1 is attributed to the asymmetric stretching vibration of C O C. The absorption bands located below 600 cm -1 are attributed to characteristic lattice vibrations of α-fe 2 O 3. 6

7 Figure S6. (a) UV Vis NIR diffuse reflectance absorption (DRS) spectrum of α-fe 2 O 3 and α-fe 2 O 3 /rgo nanocomposites. The inset shows the corresponding digital photos of α-fe 2 O 3 and α-fe 2 O 3 /rgo nanoparticles. (b) The energy band gap of α-fe 2 O 3 and α-fe 2 O 3 /rgo calculated according to the Kubelka-Munk function. Figure S6a shows the UV Vis NIR diffuse reflectance absorption (DRS) spectra of α-fe 2 O 3 and α-fe 2 O 3 /rgo, in which absorption peaks corresponding to the 2( 6 A 1 ) ( 4 T 1 ) ligand field transition of Fe 3+ are observed around 530 nm. It is clear that the incorporation of rgo into α-fe 2 O 3 leads to an increase in the absorption over the entire wavelength range, which is due to the presence of blackbody properties of rgo sheets. 1 The optical band gap of a semiconductor can be estimated from the Tauc plot based on the Kubelka-Munk model (Equation S1): 2 αhν A hν E (S1) where α, ν, A, and E g are the absorption coefficient, light frequency, proportionality constant, and band gap, respectively. In the case of α-fe 2 O 3 nanoparticle, we choose n=1/2 for direct transition. 1 According to the Tauc plot analysis, the direct transition 7

8 band gap of α-fe 2 O 3 is ca ev, which is consistent with the previously reported results, 3 while with the appearance of rgo, the band gap of α-fe 2 O 3 /rgo has a negligible variation (Figure S6b). 8

9 Figure S7. Photoluminescence spectra of α-fe 2 O 3 and α-fe 2 O 3 /rgo nanocomposite. The fluorescence spectra of the samples were taken by the photoluminescence (PL) spectrofluorometer. The emission peak of α-fe 2 O 3 is located at 740 nm, while the fluorescence of α-fe 2 O 3 /rgo nanocomposite is quenched by rgo. 9

10 Figure S8. (a) The magnetic hysteresis loop for α-fe 2 O 3 (red) and α-fe 2 O 3 /rgo (blue) at room temperature. (b) The magnetic hysteresis loop for pure rgo at room temperature. 10

11 Figure S9. Time-dependent absorbance changes at 554 nm for RhB of α-fe 2 O 3 and α-fe 2 O 3 /rgo composites under dark condition with 0 koe. 11

12 Figure S10. Photocatalytic degradation of RhB in the presence of P25, α-fe 2 O 3 and α-fe 2 O 3 /rgo hybrid nanostructures under Xe light irradiation. 12

13 Figure S11. The adsorption of RhB on α-fe 2 O 3 /rgo and rgo under dark condition with 0 koe, 6 koe, and 8 koe. (a)(b) α-fe 2 O 3 /rgo without settling and with 24 h settling, respectivly. (c)(d) pure rgo without settling and with 24 h settling, respectivly. In order to diminish the impact of Lorentz force under external magnetic field, the adsorption of RhB by both α-fe 2 O 3 /rgo and pure rgo under different fields have been studied, as shown in Figure S11. Without settling, adsorption performances of both α-fe 2 O 3 /rgo and pure rgo are evidently promoted by external magnetic fields and show no sign of saturation (Figure S11a and c). However, after 24 h settling, which means that the samples have been stood still for 24 hours before light 13

14 irradiation, the magnetic field effects have been reduced (Figure S11b and d). This is due to the fact that for short settling time periods, the formation of the molecular oxygen complex is less complete, and an appropriate amount of dissolved oxygen and photogenerated-electrons may efficiently react to produce the superoxide radicals to decompose the rather bare organic dye, resulting in the higher photodegradation rate. While, for the longer settling time, the key bonding of the MB molecule for decoloration is blocked by the existence of oxygen molecules, possibly due to formation of molecular oxygen complex. The reaction on the photocatalyst surface would then be one between photogenerated-electrons or superoxide radicals and the molecular oxygen complex. Although many superoxide radicals would be generated on the powder surface, they could not easily decompose the molecular oxygen complex due to the blocked bonding. 4 In our work, before photocatalytic test, all the samples were settled 24 hours under corresponding magnetic field to exclude the impact of Lorentz force on adsorption. 14

15 Figure S12. (a) Photocatalytic degradation of RhB under 0 koe, 6 koe and 8 koe fields in the presence of pure rgo under Xe light irradiation. 15

16 Figure S13. (a) Photocatalytic degradation of RhB under 6 and 8 koe fields in the presence of α-fe 2 O 3 /rgo under Xe light irradiation. (b) Photocatalytic degradation of RhB at various magnetic fields in the presence of α-fe 2 O 3 under Xe light irradiation. 16

17 Figure S14. Schematic diagram of the photoelectrochemical apparatus with a NdFeB magnet. 17

18 Figure S15. Time dependence of open circuit potential of α-fe 2 O 3 /rgo with or without magnetic field. When the illumination at open circuit is interrupted, the excess electrons are removed due to recombination, with the OCPD rate directly related to the electron lifetime by the following expression (Equation S2): 5 τ (S2) Here, κ B is Boltzmann s constant, T is the temperature, and e is the elementary charge. 18

19 Computational Details and Model Figure S16. (a) Top view of α-fe 2 O 3 (001)-plane surface. (b) a TEM image of rehombus structure of α-fe 2 O 3. The white scale bar depicts 20 nm. Figure S17. Schematic drawing of various terminations of α-fe 2 O 3 (001) surface and the side view of charge difference density for monolayer and bilayer graphene on the top of α-fe 2 O 3 (001) surfaces. The isosurface value is e Å -3. Yellow and blue regions represent net charge gain and loss, respectively. The dark brown, red and 19

20 golden brown spheres represent C, Fe and O atoms, respectively. Figure S18. (a) Band structure obtained for the fully relaxed model V3. The red and blue lines represent spin-up and spin-down bands of the graphene component, respectively. (b) Total DOS of V3 heterostructure and partial DOS of interfacial atoms. (c) Partial DOS of O and Fe atoms in bulk and in different layers. Fermi level is indicated by the vertical line and set to zero. 20

21 Figure S19. Band structure, total and partial DOS obtained for the fully relaxed models V1 (a) and V2 (b). The red and blue lines represent spin-up and spin-down bands of the graphene component, respectively. Fermi level is indicated by the vertical line and set to zero. The α-fe 2 O 3 /graphene heterostructure is constructed based on supercell model, where graphene is stacked on the top of α-fe 2 O 3 (001) surface (Figure S16a). In the TEM images, one can find that most of the α-fe 2 O 3 grains have grown with the 001 orientation (Figure S16b) since the shape of the grain is the same as the (001) plane. Meanwhile, the graphene lattice constant is Å so that the lattice mismatch for supercell is only 2.7% (compare to α-fe 2 O 3 lattice of Å). Hence we set the model by stacking the graphene on the (001) plane of α-fe 2 O 3. We consider three different supercell models in terms of three different terminations of α-fe 2 O 3 (001) surface, as show in Figure S17. By employing the periodic boundary conditions, each model contains an interface: model V1 contains the O 3 -Fe-Fe-R interface; model V2 contains the Fe-O 3 -Fe-R interface; and model V3 contains the Fe-Fe-O 3 -R interface, where the R denotes to the remaining atomic layers with the bulk stacking sequence. Two stoichiometric Fe 2 O 3 atomic layers at the bottom are fixed at bulk positions, while the remaining atoms are allowed to relax. Electronic and atomic structures calculations for α-fe 2 O 3 (001)/graphene heterostructures are performed by using a k-point mesh in Brillouin zone. A 20-Å vacuum slab is inserted perpendicularly on the top of graphene for minimizing the interaction between 21

22 periodic images. The van der Waals correction DFT-D2 functional is used during the calculations. 6 After fully relaxed the geometric structures of α-fe 2 O 3 (001)/graphene heterostructures, we obtain the equilibrium separation d between graphene and α-fe 2 O 3 (001) surface, which can be used to characterize the interfacial hybridization to some extent. The calculated values are 2.84, 2.80 and 2.13 Å for models V1, V2 and V3, respectively. It is notable that the d for V3 is much smaller than V1 and V2, indicating a stronger interfacial hybridization between Fe-Fe-O 3 -R substrate and graphene. In order to reflect the charge redistribution at the interface, the charge density difference for V1-V3 has been further calculated, defined by (Equation S3): (S3) where the, and represent the charge densities of heterostructure, isolated α-fe 2 O 3 (001) substrate and graphene, respectively. The isosurface value is e Å -3. The yellow and blue regions represent the charge accumulation and depletion. In Figure S17, different charge redistribution characters appear in three heterostructures. For V1, the charge depletes around C atoms and accumulates around interfacial O atoms. The relatively less charge redistribution in V2 exactly reflects that graphene has a weaker interaction with Fe-O 3 -Fe-R termination, leading to the inconspicuous charge transfer and weak bonding effect between C and single Fe layer. One can see that the charge redistribution is much more significant for V3 than the other two models, with the opposite charge transfer 22

23 direction from substrate to graphene compared to V1. The charge redistribution characters are fully consistent with the trend of the equilibrium separation d, as revealed in Figure S17. In order to clarify that the hybridization between graphene and α-fe 2 O 3 mainly concentrates on the interfacial atoms, we take V3 as an example and recalculate the charge density difference for graphene/fe-fe-o 3 -R interface with bilayer graphene. For convenience, we define the label O I as the first O layer located in α-fe 2 O 3 regions, C I as the first C layer lied in graphene regions, Fe I and Fe I refer to the first Fe layers where the Fe ions possess positive and negative moments respectively. The isosurface value is also set to e Å -3. It is worth noting that there is hardly any charge accumulation and depletion at the second C layer. The charge redistribution still aggregates in the interface area, between Fe I and C I atoms. Hence, the influence of graphene thickness will not be taken into consideration in the subsequent calculations. Since the interaction between graphene and Fe-Fe-O 3 -R substrate is quite stronger than that in V1 and V2 (as evidenced by the smaller equilibrium distance and more significant charge redistribution), it would suggest that the properties of graphene are susceptible to the magnetic substrate. In model V3, the surface Fe atom of Fe-Fe-O 3 -R substrate behaves positive magnetic moment (represented by Fe I ). After contact with graphene, the calculated magnetic moment is found to be much decreased, about µ B, compared to the bulk value µ B. Besides, the C atom which lies directly above the Fe I is spin polarized with a magnetic moment of

24 µ B, while the magnetic moments of other C atoms are different due to the difference of surrounding circumstance. Hence, we can draw the conclusion that the interaction with magnetic substrate remarkably affects the magnetic properties of graphene. We can also reach the similar conclusions in the band structure and spin DOS of model V3. The red and blue lines in the band structure represent the spin-up and spin-down bands of graphene component in heterostructures. From the band structure, Fermi level shifts into the conduction bands compared to the equilibrium situation of isolate graphene. This n-type doping can be ascribed to the charge transfer from Fe I to graphene, consistent with the result of charge density difference. Simultaneously, the graphene bands become renormalized and the spins cease to be degenerate, indicating the presence of magnetism. The interaction mechanism of V3 system can be further manifested by an analysis of the DOS. Figure S18b shows the calculated total and orbital-resolved DOS of C, Fe and O atoms at the interface in V3. For the interfacial Fe I atom, it is observed that only the spin-down channels pass through Fermi level to form 100% spin polarization, resulting in the decrease of magnetic moment as compared to bulk. Synchronously, O I-p state has a strong spin splitting, resulting in the corresponding magnetic moment increasing. It should be noted that the partial DOS of C atom at the interface no longer remains symmetrical and passes through Fermi level mainly in the spin-down channel. The strong peak of polarization Fe I -d state get hybridized with C I-p orbital, hence the induced magnetism in graphene. Figure S18c shows the contrast of O and Fe atoms in bulk and different layers. Due to the limited interaction and hybridization effects, most atoms have 24

25 similar behavior as the bulk except for the interfacial and a few sublayer atoms. Figure S19a shows the calculated band structure, total and partial DOS obtained for the fully relaxed α-fe 2 O 3 /graphene model V1. Although Fermi level shifts into the valance bands compared to the equilibrium situation of isolate graphene, the spin-up and spin-down channels of C atom at the interface still remain symmetrical. Hence, the hybridization between graphene and interfacial O atoms only induced p-type doping instead of magnetism in graphene system. This p-type doping illustrates that the charge transfer from graphene to substrate, accordance with the result of charge density difference depicted in Figure S17. After contact with Fe-Fe-O 3 -R substrate, the spin-up and spin-down branches of graphene are almost degenerated, as depicted in Figure S19b, and the Dirac point crosses Fermi level, approaching the typical band structure characters of isolate graphene. These results may be the consequence of the weak hybridization at the contact interface in V2 system. References (1) Pradhan, G. K.; Padhi, D. K.; Parida, K. Fabrication of α-fe 2 O 3 Nanorod/RGO Composite: A Novel Hybrid Photocatalyst for Phenol Degradation. ACS Appl. Mater. Interfaces 2013, 5, (2) Butler, M. A. Photoelectrolysis and Physical-Properties of Semiconducting Electrode WO 3. J. Appl. Phys. 1977, 48, (3) Zhang, Z.; Hossain, M. F.; Takahashi, T. Self-Assembled Hematite (α-fe 2 O 3 ) Nanotube Arrays for Photoelectrocatalytic Degradation of Azo Dye under 25

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