Hydrothermal Synthesis of Boron-Doped MnO 2 and Its Decolorization Performance

To functionalize MnO 2 with foreign ions is one of the commonly used methods to improve the adsorption/oxidation properties of MnO 2 . Boron-doped MnO 2 was prepared by the reaction of MnSO 4 , KMnO 4 , and boric acid by a facile hydrothermal method. Boron-MnO 2 was characterized by X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction pattern (SAED), and X-ray photo-electron spectroscopy (XPS) techniques. The characterization of XPS and EDX confirms that boron has been doped into MnO 2 , but the boron dopant has no obvious effect on the crystallization of MnO 2 as shown by the results of XRD and Raman characterization. The boron-doped MnO 2 nanorods display high performance in the methyl orange degradation with a decolorization degree of 90% in 2min (5% B-MnO 2 dosage, 5mg; methyl orange concentration, 20mg L).


Introduction
Manganese oxides have diverse structures with many derivative compounds [1].Generally, the valence of Mn in manganese oxides can be +2, +3 or +4, and there may be two or three kinds of valence coexisting in the same crystalline structure.The characteristics in structure make manganese oxides possess unique chemical and physical properties.Thus, research on manganese oxides has received considerable attention in various fields like energy storage/conversion material [2,3], catalysis material [4], ion-exchange material, and so forth [5].Of all the applications, the elimination of organic pollutants or heavy metal in water has always been the focus of research from the aspect of environment protection.Manganese oxides are widely used to remove/oxide heavy metal [6,7], acetaminophen [8], phenol [9], nonylphenol [10], naproxen [11], ciprofloxacin [12], Congo red [13], rhodamine B [14], and so forth.
The adsorption/oxidation properties of manganese oxides are generally influenced by their microstructure, shape, size, and/or composition.Therefore, long-term attention has been paid to the preparation and modification of MnO 2 .Generally, there are two ways to improve the adsorption/oxidation properties of MnO 2 .One way is to fabricate large-surfacearea/mesoporous MnO 2 [15] or loaded MnO 2 with a highsurface-area carrier [16][17][18], and the other is to functionalize MnO 2 with foreign ions [19,20].Compared with the commonly used metal ions, nonmetal elements for MnO 2 are rarely reported [21,22].It is notable that nonmetal elements such as B, N, or S have been widely used as dopants for TiO 2 and proved to be very effective for enhancing the photoactivity [23].The structure of TiO 2 and MnO 2 has somewhat similarity.Inspired by such fact, we tried to synthesis borondoped MnO 2 via reactions among MnSO 4 , KMnO 4 , and boric acid under hydrothermal conditions.During the review process of our paper, very recently, Chi et al. [24] reported that boron-doped manganese dioxide showed superior electrochemical performance as supercapacitors and the boron dopant was an effective way to improve and modify the characteristics of manganese oxide.Herein, we systematically characterized the physicochemical properties of the prepared manganese oxide using various techniques.The application in the decolorization of industrial acid dye, methyl orange, was studied as probe reaction to evaluate its activity.

Experimental
A group of boron-doped MnO 2 were prepared by mixing MnSO 4 (22.1 mmol), KMnO 4 (16.8 mmol), and different amounts of boric acid with 55 mL H 2 O under hydrothermal conditions maintained at 160 ∘ C for 24 h.The solid precipitate was collected and washed by centrifugation and then dried at 60 ∘ C. The products are marked as % B-MnO 2 , where  means the theoretical molar ratio of boron to Mn in the raw materials.
X-ray powder diffraction (XRD) was carried out using ULTIMA-III X-ray diffractometer (40 kV, 40 mA, Cu K radiation).Scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDX) were performed on a Digital Scanning Microscope S-3400N operated at 15 kV.The transmission electron microscopy (TEM) image, selected area electron diffraction pattern (SAED) and high-resolution transmission electron microscopy (HRTEM) image were obtained on a JEOL JEM-2100HR using an acceleration voltage of 200 kV.Raman spectroscopy was recorded on a dispersive Horiva Jobin Yvon LabRam HR800 Microscope, with a 24 mW He-Ne green laser (633 nm).X-ray photoelectron spectroscopy (XPS) was obtained by a Thermo ESCALAB 250 instrument equipped with a monochromatic Al K (1486.6 eV) X-ray source.N 2 adsorption-desorption isotherms were measured using a Micromeritics ASAP 2020 Analyzer.
The decolorization experiment was performed in a round bottom flask at room temperature.B-MnO 2 of 5 mg was added into a solution of methyl orange (20 mg L −1 , 100 mL) with a pH of 1.7 adjusted by diluted H 2 SO 4 .A small quantity of mixture was withdrawn at definite intervals and then centrifuged to remove the sedimentation before UV analysis.The decolorization performance was calculated by UV-visible spectrum (T-245, Shimadzu) by monitoring its characteristic peak at 507 nm.

Results and Discussion
The structure of the B-doped MnO 2 was characterized by XRD and Raman spectroscopy as displayed in Figures 1 and 2, respectively.Figure 1 shows the XRD patterns with peaks located at 2 = 12.6 ∘ , 17.8 ∘ , 28.5 ∘ , 37.3 ∘ , 41.8 ∘ , 49.7 ∘ , and 60.0 ∘ , which match well with the standard patterns of -MnO 2 (JCPDS 44-0141).With boron dopant increasing, no vital difference for the XRD peaks is observed.Besides, the peaks belonging to borate impurities are not detected.
The Raman spectra (Figure 2) are almost identical for all the samples, indicating that the amount of boron dopant had no effect on the structure of MnO 2 .The Raman spectra feature four main bands at 187, 392, 582, and 647 cm −1 .The two Raman bands at 582 and 647 cm −1 are indicative of the vibration modes of MnO 6 octahedron [25].No peaks at around 770 cm −1 and 805 cm −1 , corresponding to [BO 4 ] tetrahedron and B 2 O 3 , are found [26], and this observation proves that no isolate boron exists.
Figure 3 shows one-dimensional stacked rod-like morphologies of B-MnO 2 with the length of hundreds of nanometers, and this result is in agreement with that observed by Ma et al. [13].No vital difference can be found with the rise of boron doping.The TEM image in Figure 4(a) clearly shows 5% B-MnO 2 is in a typical nanorod shape with various lengths and a diameter of about 35 nm.The representative HRTEM is given in Figure 4(b).The separated spacing of 0.49 nm corresponds to (200) plane in the -MnO 2 crystal structure.The single crystal feature of B-MnO 2 is proved by the inset electron diffraction image, which shows the B-MnO 2 nanorod grows along the [001] crystal direction.The SAED pattern also confirms the single crystal feature of the B-MnO 2 .The TEM and HRTEM results are consistent with the XRD and Raman data, verifying the good crystallinity of the B-MnO 2 .Elementary composition analysis by EDX and XPS confirms the presence of boron (Figures 4(c) and 4(d)).The EDX spectrum demonstrates peaks of O, K, Mn, and B. The binding energy located at around 198 eV for B 1s is different from the B 1s of H 3 BO 3 or B 2 O 3 located at 192∼193 eV [27].
Figure 5 shows the nitrogen adsorption-desorption isotherms.The B-MnO 2 nanorod possesses a typical type II nanorods show remarkable decolorization performance for MO, and the decolorization degree reaches 97% in 50 min.
The ability of the nanostructured B-MnO 2 follows the order as 5% B > 10% B > 3% B > 0% B. The activity order shows that (1) the boron dopant on MnO 2 can enhance the activity and that (2) the amount of boron dopant has optimum value.Compared with the 3% B sample, the 5% B and 10% B samples have relatively larger surface area, which will benefit the MO degradation; thus they have relatively good performance.We tentatively deduced that too much boron present on the 15% B-MnO 2 sample and excessive boron may block the tunnel of the MnO 2 , which will hamper its activity.The inset figure (Figure 6) displays the UV-Vis absorption spectra of MO under the reaction with 5% B-MnO 2 .The characteristic absorption peak at 507 nm decreases sharply with prolonged time.Within 2 min, the value of peak is reduced at least an order of magnitude, and the decolorization degree reaches nearly 90%.After 50 min, the color of the solution changes from bright red to colorless.Usually, as an acid dye, the MO contained in wastewater is discharged under acid conditions; this is why we perform the experiment under acid conditions.Under pH lower than the isoelectric point (4.7) [28], the surface of MnO 2 is positively charged by protonation, and the electrostatic attraction between the surface and the anion group (R-SO 3 − ) of MO contributes a lot to the absorption.This is also true in our case for B-MnO 2 with an isoelectric point of about 2.0.It is interesting that even when the concentration of MO is increased from 20 mg L −1 to 40 mg L −1 the used B-MnO 2 is still capable of decolorizing MO efficiently.As for such a high concentration of MO, apparently, pure adsorption is not enough to explain the decolorizing ability.Kuan and Chan [28] pointed out that methyl blue could be adsorbed or oxidized by tunneled manganese oxide.Similarly, MO can also be oxidized by B-MnO 2 .Figure 5 shows that a new peak at around 250 nm appeared after 2 min, inferring an intermediate produced by the oxidation of MO.The oxidation ability may come from the excess surface oxygen of B-MnO 2 [19,28].Based on the above analysis, we can say that both adsorption and oxidation degradation play a role in the decolorizing of MO.Zhang et al. [29] also reported that the mechanism for decoloration of methyl blue can be attributed to oxidation degradation as well as adsorption.However, one thing to point out is that, under the acid conditions, the B-MnO 2 can dissolve slowly into the solvent.After the decolorization test over 5% B-MnO 2 , we have detected about 20 ppm Mn 2+ using atomic absorption spectrometry.Hydrogen peroxide or tert-butyl hydroperoxide was used in some reported decolorization reactions [14,19,30].However, in this study, the addition of H 2 O 2 caused large amount of gas bubble, and the color of the solution did not change at all after 50 min.This is because MnO 2 is consumed preferentially by the decomposition of H 2 O 2 , thus leading to the failure of decolorization.Therefore, H 2 O 2 is not necessary in our case.

Conclusions
In summary, different amounts of boron-doped -MnO 2 nanostructures have been prepared by hydrothermal route.The dopant boron has no effect on the structure of MnO 2 .The B-MnO 2 showed nanorods morphology with length of few hundred nanometers.The 5% B-MnO 2 exhibited the highest efficiency in the MO decolorization without the assistance of H 2 O 2 .The prepared B-MnO 2 is promising to be used in degradation of other organic pollutants.