Defects and interface states related photocatalytic properties in reduced and subsequently nitridized Fe3O4/TiO2

doi:10.1016/j.jmst.2017.09.019

Abstract

Abstract:

The Fe₃O₄@TiO₂ catalyst was reduced in a mixed H₂/N₂ atmosphere at temperatures of 400, 600, 800 and 1000 °C in order to produce the oxygen vacancies (O_v) and Ti³⁺; Simultaneously, Fe₃O₄ was reduced to Fe, a strongly magnetic material, beneficial for the magnetic separation after the photo-degradation. The optimal catalyst was obtained at the reducing temperature of 800 °C, which possesses the good photocatalytic performance and recycled activities; Moreover, its saturation magnetization M_s is highest, reaching 23.8 emu g^-1 which improves the magnetic separability. This optimal catalyst was subsequently treated in the NH₃ atmosphere at temperatures of 500, 600 and 700 °C, aiming to investigate the effects of N-doping in TiO₂. The 600 °C treated catalyst exhibited the optimal photocatalytic performance. The factors that affect the photocatalytic performance are revealed and discussed in detail, including the ratio of O_v and N dopant in TiO₂ as well as the interface states between TiO₂ and the magnetic particles.

Key words: Photocatalyst, Strong magnetic particles, Magnetic separability, Oxygen vacancy, Nitrogen doping

Chang Liu, Xiang Zhu, Peng Wang, Yisen Zhao, Yongqing Ma. Defects and interface states related photocatalytic properties in reduced and subsequently nitridized Fe₃O₄/TiO₂[J]. J. Mater. Sci. Technol., 2018, 34(6): 931-941.

Figures/Tables 18

Scheme 1. Illustration of main synthesis process.

Fig. 1. XRD patterns of standard cards in powder diffraction files (PDF) for Fe3O4 (No.19-0629) (a), samples Fe3O4 (b) and Fe3O4@TiO2 (c) and anatase TiO2 (No. 21-1272) (d).

Fig. 2. TEM (a, d), SEM (b, e) and HRTEM (c, f) images for Fe3O4 (a-c) and TiO2 (d-f); Insets in (c) and (f) show the magnified details of areas 1, 2, 3 and 4 marked with squares.

Fig. 3. Room temperature M(H) loops of Fe3O4 (a) and Fe3O4@TiO2 (b), variation of methylene blue concentration C/C0 with time in presence of Fe2O3@TiO2 catalyst with inset being the magnetic separation photo after degradation (c), linear fitting of degradation process according to pseudo first-order kinetic model (d).

Table 1 Adsorption efficiency (Ae), degradation efficiency (De), apparent reaction rate constant kapp and saturation magnetization (Ms) of all magnetic photocatalysts and commercial P25.

	A_e (%)	D_e (%)	k_app (10^-2 min^-1)	M_s (emu g^-1)
Fe₃O₄@TiO₂	72	93.4	1.29	1.4
400 °C	61	87.4	1.43	1.6
600 °C	55	82.5	1.01	19.7
800 °C	26	99.6	4.70	23.8
1000 °C	10	91.6	1.55	18.2
P25	2.3	99.0	4.20

Fig. 4. XRD patterns of samples synthesized after Fe3O4/TiO2 is reduced in H2/N2 at 400 °C (a), 600 °C (b), 800 °C (c), 1000 °C (d) and commercial P25 (e), standard PDF card of rutile TiO2 (No. 21-1276) (f).

Fig. 5. SEM (a), TEM (b) and HRTEM (c) images for reduced Fe3O4@TiO2 at 800 °C, TEM image of P25 (d); The inset in (c) shows magnified details of area marked with square.

Fig. 6. Core level XPS spectra of Fe 2p3/2 (a, d), Ti 2p3/2 (b, e) and O 1s (c, f) for Fe3O4@TiO2 (a-c), reduced Fe3O4@TiO2 at 800 °C (d-f).

Table 2 Summary of XPS details of Fe3O4@TiO2 and its reduced samples in H2/N2 at 800 °C and 1000 °C (BE: binding energy).

		Fe 2p3/2			O 1s			Ti 2p3/2
Fe₃O₄@TiO₂	BE (eV)	710.7	712.1	707.0	530.4	530.0	532.3	458.7	460.2
	CS	Fe²⁺	Fe³⁺	Fe	TiO₂	Fe-O	O_v	TiO₂	TiO_x
	Ratio (%)	15	85	0	66.0	22.6	11.4	91.5	8.5
800 °C in H₂/N₂	BE (eV)	710.7	712.1	707.0	530.3	531.1	532.3	459.0	460.2
	CS	Fe²⁺	Fe³⁺	Fe	TiO₂	TiOH	O_v	TiO₂	TiO_x
	Ratio (%)	36.0	61.6	2.4	72.1	14.8	13.1	88.1	11.9
1000 °C in H₂/N₂	BE (eV)	710.7	712.1	707.5	530.3	531.1	532.4	459.1	460.2
	CS	Fe²⁺	Fe³⁺	Fe	TiO₂	TiOH	O_v	TiO₂	TiO_x
	Ratio (%)	67.2	29.7	3.1	64.6	15.4	20.0	87.5	12.5

Fig. 7. UV-vis diffuse reflectance spectra (DRS) of Fe3O4, Fe3O4@TiO2 and reduced Fe3O4@TiO2 at 400, 600, 800 and 1000 °C and UV-vis DRS of P25 (inset) (a), fitting curves of UV-vis DRS for Fe3O4@TiO2 (b), and reduced Fe3O4@TiO2 at 400 °C (c), 600 °C (d), 800 °C (e) and 1000 °C (f): The black solid line is the experimental curve; The colorful solid lines are the fitting curves. Empty circles are the sum of the fitting curves.

Table 3 Peak positions in wavelength region of 400-500 nm obtained by fitting the UV-vis spectra.

Peak	Fe₃O₄@TiO₂	Reduction in H₂/N₂
		400 °C	600 °C	800 °C	1000 °C
P1 (nm)	423	420	429	432	432
P2 (nm)	492	491	492	493	490

Fig. 8. Room temperature M(H) loops of reduced Fe3O4@TiO2 at 400, 600, 800 and 1000 °C (a); Variation of methylene blue concentration C/Ce (empty circles) with time in presence of reduced Fe2O3@TiO2 at 400, 600, 800 and 1000 °C as well as commercial P25 (b), in which the solid lines are the fitting curves according to the pseudo first-order kinetic model; The inset shows the magnetic separation photo after degradation for the 800 °C reduced sample. Variation of methylene blue concentration C/Ce with time in presence of 800 °C reduced Fe2O3@TiO2 during four runs (c). Concentration variation of phenol, C/Ce, with time in the presence of 600 and 800 °C reduced Fe2O3@TiO2 as well as the commercial P25 (d).

Fig. 9. XRD patterns of treated samples in NH3 at 500 °C (a), 600 °C (b) and 700 °C (c).

Fig. 10. Core level XPS spectra of Fe 2p3/2 (a, d, g), N 1s (b, e, h) and Ti 2p3/2 (c, f, i) for nitridized samples at 500 °C (a-c), 600 °C (d-f) and 700 °C (g-i).

Table 4 Summary of XPS details of nitridized samples in NH3 at 500 °C, 600 °C and 700 °C.

Treated in NH₃		N 1s				Fe 2p3/2			Ti 2p3/2
500 °C	BE (eV)	396.7	400.8	398.3	397.3	710.8	707.8	712.1	459.1	460.2	456.7
	CS	TiN	TiO_2-_xN_x		Fe₂N	Fe²⁺	Fe₂N	Fe³⁺	TiO₂	TiO_x	TiN
	Ratio (%)	4.5	71.8	23.5	0.2	67.0	1.3	31.7	84.1	15.9	0.0
600 °C	BE (eV)	396.5	400.9	398.4	397.3	710.8	707.8	712.1	459.2	460.2	456.7
	CS	TiN	TiO_2-_xN_x		Fe₂N	Fe²⁺	Fe₂N	Fe³⁺	TiO₂	TiO_x	TiN
	Ratio (%)	34.2	48.2	15.2	2.4	70.0	1.4	28.6	99.8	0.0	0.2
700 °C	BE (eV)	396.6	400.9	398.4	397.3	710.7	708.0	712.1	458.7	460.2	456.7
	CS	TiN	TiO_2-_xN_x		Fe₂N	Fe²⁺	Fe₂N	Fe³⁺	TiO₂	TiO_x	TiN
	Ratio (%)	60.2	22.8	14.3	2.7	94.8	5.2	0.0	61.0	0.0	39.0

Fig. 11. UV-vis DRS of samples nitridized at 500, 600 and 700 °C (a), variation of methylene blue concentration C/Ce (empty symbols) with time in the presence of nitridized samples at 500, 600 and 700 °C (b), in which the solid lines are the fitting curves according to the pseudo first-order kinetic model, room temperature M(H) loops of nitridized samples at 500, 600 and 700 °C (c).

Table 5 Adsorption efficiency (Ae), degradation efficiency (De) and apparent reaction rate constant kapp of all nitridized photocatalysts.

Nitridized in NH₃	A_e (%)	D_e (%)	k_app (10^-2 min^-1)
500 °C	12.7	99.7	2.5
600 °C	16.1	99.8	3.5
700 °C	18.2	75.8	0.8

Scheme 2. Chemical states of 800 °C reduced Fe3O4@TiO2.

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Defects and interface states related photocatalytic properties in reduced and subsequently nitridized Fe₃O₄/TiO₂

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