Supporting Information (SI)

Energy-efficient fabrication of a novel multivalence Mn3O4-

MnO2 heterojunction for dye degradation under visible light

irradiation

Jianhui Zhaoa, Jun Nana,*, Zhiwei Zhaoa, b,*, Ning Lia, Jie Liua, Fuyi Cuia

a State Key Laboratory of Urban Water Resource and Environment, School of Municipal and

Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China

b Department of National Defense Construction Planning and Environmental Engineering,

Logistical Engineering University, Chongqing 401311, China

Corresponding Author

*Tel.: + 86 (451) 8628 2298, e-mail: hitnanjun@163.com (Jun Nan)

*Tel.: + 86 (451) 8628 3001, e-mail: hit_zzw@163.com (Zhiwei Zhao).

Supplementary Information: 9 pages, 8 figures.

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1. The effct of Mn3O4/MnO2 molar ratios on methylene blue (MB) degradation

The Mn3O4-MnO2 composites with mole ratios of 1:5, 2:5 and 3:5 were fabricated

by adjusting the MnCl2/KMnO4 molar ratios of 3:1, 4.5:1 and 6:1, respectively. The X-

ray diffraction (XRD) technique was used to characterize the three photocatalysts and

the results were shown in Fig. S1. As can be seen from Fig. S1, the diffraction peaks

of tetragonal Mn3O4 (JCPDS NO. 24-0734) were observed. Besides, a broad peak

located at 2θ = 19.2° appeared, which was identified as amorphous MnO2 [1]. The

composite photocatalysts fabricated in this study consisted of crystalline Mn3O4 and

amorphous MnO2. Obviously, the peak intensity of amorphous MnO2 increased with

its increased percentage in the composite.

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Fig. S1. XRD patterns of Mn3O4-MnO2 composites with Mn3O4/MnO2 molar ratios of (a) 3:5, (b)

2:5 and (c) 1:5.

The Mn3O4-MnO2 composite photocatalysts (0.5 g/L) with molar ratios of 3:5, 2:5

and 1:5 were used for adsorption and degradation of MB solution (10 mg/L, 100 mL).

The results were shown in Fig. S2. The adsorption efficiency of Mn3O4-MnO2 with

molar ratios of 3:5, 2:5 and 1:5 for MB were 24.9%, 31.0% and 35.2%, respectively.

The Mn3O4-MnO2 composite (Mn3O4/MnO2 = 1:5) presented the highest adsorption

rate, which may attribute to its higher surface area and more adsorption sites.

However, in the degradation process, the Mn3O4-MnO2 composite (Mn3O4/MnO2 =

2:5) exhibited excellent photocatalytic activity among the three photocatalysts. The

molar ratio of Mn2+, Mn3+ and Mn4+ in Mn3O4-MnO2 composite (Mn3O4/MnO2 = 2:5)

was 2:4:5, which was the optimum proportion for enhancing the photocatalytic

activity. Therefore, the Mn3O4/MnO2 composite with ratio of 2:5 was mainly

investigated and reported in the paper.

Fig. S2. Photodegradation of MB by Mn3O4-MnO2 composites with molar ratios of 3:5, 2:5 and

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1:5.

2. The effect of the Mn3O4-MnO2 dosage on the MB degradation

In this study, the MB concentration used for photodegradation was 10 mg/L. When

little photocatalyst was added into the reaction solution, the active sites for adsorption

and photocatalysis were not enough for the MB removal, resulting in lower removal

efficiency. The removal efficiency of MB increased with the dosage of photocatalyst

within certain limits due to the higher active sites availability. However, when

excessive photocatalyst was added to the MB solution, the aggregation between

Mn3O4-MnO2 nanosheets may result in the decline of the active sites [2]. Besides, the

increased turbidity of the reaction solution could shield the visible light, declining the

photo-degradation efficiency [3, 4]. Thus, the dosage should have an optimal value. In

the preliminary experiments, we have studied the effect of photocatalyst dosage on

MB removal, shown in Fig. S3. The removal efficiency of MB by Mn3O4-MnO2 was

low with the dosage of 0.2 g/L due to the insufficient active sites. The removal

efficiency was significantly improved with the dosage increasing up to 0.5 g/L.

However, the removal efficiency was declined with further increased dosage. It may

result from a shielding effect of excess particles and the aggregation between Mn3O4-

MnO2 nanosheets. Therefore, the optimal dosage of Mn3O4-MnO2 photocatalyst was

set at 0.5 g/L.

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Fig. S3. The effect of the Mn3O4-MnO2 dosage on the MB degradation under visible irradiation.

3. Adsorption of MB on Mn3O4, Mn3O4-MnO2 and MnO2 in dark

In the first 30 min, the suspensions (MB solution and photocatalysts) were kept

stirring in dark to reach adsorption-desorption equilibrium before visible light

irradiation.

Fig. S4. Adsorption of MB on Mn3O4, Mn3O4-MnO2 and MnO2 in dark.

4. Electron energy loss spectroscopy (EELS) analysis

In this study, the EELS was performed by a field-emission transmission electron

microscopy (FETEM, Tecnai G2 F30, US) with an energy resolution of 0.7 eV. The

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high-angle annular dark field (HAADF) image of the Mn3O4-MnO2 nanosheet was

shown in Fig. S5 (a). The EEL spectra were recorded at every 5 nm along the

randomly selected line from A to B (AB = 225 nm) in Fig. S5 (a). It has been reported

that the Mn oxidation states could be identified by the Mn L3 energy values [5-7].

Thus, the Mn L3 peak values of the selected spectra were analyzed after background

subtraction. The distance between point A and the detection point was defined as “X”.

The variation of Mn L3 energy with the distance (X) was shown in Fig. S5 (b). As

shown in Fig. S5 (b), the Mn L3 values (around 644.0 eV) at the edge of the Mn3O4-

MnO2 nanosheet were obviously higher than that obtained in the central areas, which

were consist with the reported Mn L3 value of Mn4+ [5-7]. These results indicated that

MnO2 was formed at the edge of the as-prepared photocatalyst (about 15-25 nm). The

Mn L3 values in the central areas ranged from 642.21 to 643.10 eV, which were

recognized as Mn3O4 [5-7]. It was worth mentioning that the Mn L3 values in the

central areas were positively shifted compared to the previous report [6, 7]. It was

attributed to electron migration in the presence of the MnO2 thin layer.

Fig. S5. (a) High-angle annular dark field (HAADF) image of Mn3O4-MnO2 nanosheet; (b)

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Energies of Mn-L3 peak along the line AB in HAADF image.

5. The results of radical-trapping experiment

Fig. S6. Photo-degradation of MB by Mn3O4-MnO2 with different scavengers under visible

irradiation for 1 h. (The Ca represents the MB concentration after stirring for 30 min in dark. The

initial MB concentration was 10 mg/L and the catalyst dosage was 0.5 g/L. The molar

concentration of the scavengers was 1mM.)

6. The effect of atmospheric oxygen on MB photo-degradation by Mn3O4-MnO2

We have investigated the effect of atmospheric oxygen on MB photo-degradation

by Mn3O4-MnO2. The schematic diagram of the photocatalytic system with bubbling

nitrogen or air was shown in Fig. S7. The MB solution was kept inletting nitrogen for

1 h before the adsorption process in order to ensure the solution free of oxygen. As

shown in Fig. S8A, the removal rate of MB with Mn3O4-MnO2 photocatalyst

decreased by 34.2% in nitrogen atmosphere. The MB solution showed a definite

degradation in the first 15 min under visible light irradiation. The reason should be

that the absorbed oxygen and bulk oxygen of amorphous MnO2 formed active oxygen

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species with electrons at the early stage [8, 9]. Next, however, the degradation rate

became very slow without forming the superoxide radicals (played the major role in

degradation) due to absence of oxygen.

In order to demonstrate that the oxygen anion vacancies can be filled by

atmospheric oxygen, a contrast experiment was carried out and the results showed in

Fig. S8B. The reaction solution placed in a dark environment after visible light

irradiation for 30 min. Then the MB solution was pumped with air for 30 min. After

that, the solution was pumped with nitrogen for 1 h to remove the dissolved oxygen.

The lamp was turned on and the photo-degradation process proceeded. Observably,

the degradation rate (120-135 min in Fig. S8B) was higher than that of the whole

degradation process in nitrogen (30-45 min in Fig. S8A). Thus, it can be inferred that

the oxygen vacancies on Mn3O4-MnO2 surface could be replenished by dissolved

oxygen, regaining the photocatalytic activity.

Fig. S7. Schematic diagram of the photocatalytic system with bubbling nitrogen or air.

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Fig. S8. Effect of oxygen on MB photo-degradation process by Mn3O4-MnO2 (A: photo-

degradation of MB in nitrogen and air, respectively; B: photo-degradation of MB in nitrogen or

bubbling air).

References

[1] Z. Zhao, J. Liu, F. Cui, H. Feng, L. Zhang, J. Mater. Chem. 22 (2012) 9052-9057.

[2] P. Wongkalasin, S. Chavadej, T. Sreethawong, Colloid. Surface. A. 384 (2011) 519-528.

[3] H.Y. Zhu, L. Xiao, R. Jiang, G.M. Zeng, L. Liu, Chem. Eng. J. 172 (2011) 746-753.

[4] H.H. Huang, D.H. Tseng, L.C. Juang, J. Hazard. Mater. 156 (2008) 186-193.

[5] S. Estradé, L. Yedra, A. López-Ortega, M. Estrader, G. Salazar-Alvarez, Bar, oacute, M.D. , J.

Nogués, F. Peiró, Micron 43 (2012) 30-36.

[6] L. Laffont, P. Gibot, Mater. Charact. 61 (2010) 1268-1273.

[7] H.K. Schmid, W. Mader, Micron 37 (2006) 426-432.

[8] H. Cao, S.L. Suib, J. Am. Chem. Soc. 116 (1994) 5334-5342.

[9] J.C. Lin, J. Chen, S.L. Suib, M.B. Cutlip, J.D. Freihaut, J. Catal. 161 (1996) 659-666.

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