Facile synthesis of a high purity α-MnO2 nanorod for rapid degradation of Rhodamine B


 Manganese dioxide (α-MnO2) nanorods with diameters of about 5-15 nm and lengths of 100-150 nm were synthesized by a simple co-precipitation method. XRD, TEM, HRTEM, SAED and XPS were used to analyze the crystallographic information, microstructure and chemical bonding of the as-prepared sample. The α-MnO2 nanorod exhibited a high efficiency and rapid removal rate of rhodamine B (RhB), which reached about 97.5% within 10 min when pH=4 (and pH=6.6) and 97.7% within 50 min when pH = 9 in the presence of H2O2. The results also indicated that a lower pH value is conducive to the movement of the characteristic peak and the attenuation of the intensity of the characteristic peak of RhB dye. Then a possible catalytic mechanism was revealed. Moreover, the α-MnO2 nanorod exhibits an excellent recyclability and catalytic stability. This research indicates that α-MnO2 nanorods have a potential application in practical dye pollutant treatment.


Introduction
Random discharge of dye wastewater from the textile industries will cause toxicological and aesthetic problems, reduce the photosynthetic activity of aquatic plants, and bring about adverse effects on aquatic organisms, human health and the environment due to high chemical oxygen demand, high intensity of color, hard to degrade, high toxicity and even potential carcinogenic risk [1][2][3][4][5]. Rhodamine B (RhB) is a water-soluble xanthene organic dye, which contains four N-ethyl groups at either side of the xanthene ring, and it is widely used in textile, printing, photography and other industries. However, its harm to human and animals cannot be ignored, such as carcinogenicity, reproductive and developmental toxicity, chronic toxicity and so on. Therefore, it is of great concern to decompose RhB into non-toxic and pollution-free substances.
Recently, much effort has been focused on the degradation of dyes by semiconductor photocatalyst. In pursuit of low cost, non-toxicity and chemical stability, titanium dioxide (TiO 2 ) shows good application as a photocatalyst [6,7]. However, TiO 2 can only absorb ultraviolet (UV) light in the solar spectrum, which greatly limits the application of TiO 2 in photocatalytic degradation. Meanwhile, the dye with high concentration tends to show a dense color, which will reduce the photocatalytic efficiency due to poor light transmittance. Therefore, it is urgent to find a more effective method to achieve the degradation of organic dyes. Some reports have demonstrated that MnO 2 is expected to degrade organic dyes in different surroundings because of its superior catalytic activities and adsorption capabilities [8][9][10][11][12][13][14][15][16][17]. And the polymorphic structure of MnO 2 materials determines their catalytic performance [10][11][12][18][19][20][21]. The α-MnO 2 possesses higher oxidative property than β-MnO 2 and γ-MnO 2 due to the more exposure of [MnO 6 ] edges in α-MnO 2 . For instance, Saputra et al. have synthesized different crystallographic phases of MnO 2 materials by hydrothermal method, and they 3 reported that α-MnO 2 presented the highest activity in activation of Oxone® for phenol degradation than γ-MnO 2 and β-MnO 2 due to the high surface area, oxygen loss and double tunneled structure [19]. Meng et al. have reported that α-MnO 2 has higher catalytic activity in oxygen evolution reaction than β-，δ-MnO 2 and amorphous Mn oxide because of its abundant di-μ-oxo bridges, low charge transfer resistance, and strongest O 2 adsorption ability [20].
Herein, uniform-sized α-MnO 2 nanorods were synthesized by a simple method using potassium permanganate (KMnO 4 ) and manganese chloride tetrahydrate (MnCl 2 · 4H 2 O) as manganese source. In order to reveal the catalytic mechanism of α-MnO 2 on RhB dye, the efficient oxidative degradation and adsorption ability of the α-MnO 2 nanorods toward RhB at different H 2 O 2 dosage (1-14 ml) and pH values (pH = 4-12) was investigated. The obtained α-MnO 2 nanorods present excellent performance in degradation of complex organic dyes.

Synthesis of α-MnO 2 nanorod
All the reagents were analytical grade and used without any further purification. The α-MnO 2 nanorods were prepared by co-precipitation method with refluxing process as follows: 180 mg of MnCl 2 · 4H 2 O was dissolved in 50 mL isopropanol under magnetic stirring at 85 °C and the reflux reaction was carried out simultaneously. Then KMnO 4 solution (127 mM, 5 mL) was added dropwise into the above-mentioned solution at a uniform rate within 5 min at 25 °C . The mixed solution was stirred constantly at 85°C for 90 min with refluxing. The as-obtained dark brown precipitates were washed with deionized water and ethanol for several times. Finally, the powder of α-MnO 2 nanorods was collected after drying in an oven at 60 °C.

Characterization
The crystallographic information, morphology, microstructure and chemical bonding of as-prepared product were analyzed by X-ray diffraction (

Degradation of RhB
HCl or KOH solutions were used to adjust the pH values of initial RhB solutions (20 mg L −1 ) and the degradation process of RhB was as followed: 10 mg of as-prepared α-MnO 2 nanorods was dispersed in RhB solution (50 mL) at 25 °C . Before H 2 O 2 addition, the suspension was stirred uniformly for 30 min to achieve absorption/desorption equilibrium. Then, 5 mL of the mixture solution was pipetted, centrifuged and named as 0 min. Subsequently, a certain amount of 30 % H 2 O 2 was added to above suspension. At certain intervals, 5 mL of the suspension was taken out and centrifuged to remove solid nanoparticles. The clear upper layer solution was analyzed by a UV-Vis spectrophotometer.

The microstructure and morphology of sample
XRD was used to characterize the crystal structure and phase composition of the product, and the corresponding results are presented in Fig. 1 (110) ions in the α-MnO 2 [9,[22][23][24]. The Mn 3s spectrum of as-synthesized α-MnO 2 with a peak separation of about 4.7 eV was observed, which indicates that the Mn in the sample has an oxidation state of 4 [13,25]. In the case of oxygen (Fig. 3d), three different peaks centered at 529.3, 531.2 and 532.6 eV, which correspond to the lattice oxygen (in the form of O 2-), the surface adsorbed oxygen (such as OH) and H-O-H bond for residual water, respectively [24,25]. Fig. 4a shows the UV-vis absorption spectra of RhB aqueous solution. It can be seen that only a small amount of RhB was physically adsorbed by α-MnO 2 nanorods at t=0. However, the intensity of RhB absorption peak was quickly decreased after the addition of 6 mL H 2 O 2 , which confirms that H 2 O 2 has a certain degradation effect on RhB. The absorption peak of RhB dye at λ=553 nm (λ max ) shifted to 546 nm after 10 min, attributing to the formation of N-deethylated intermediates of RhB [3,26].  It can be seen that when 1 mL H 2 O 2 is added, the degradation rate of RhB reaches 60% in a short time of 5 min, and a large number of pink bubbles are observed in the solution, which indicates that the reaction is violent. After that, the degradation rate remained unchanged, indicating that the content of H 2 O 2 in the solution system was insufficient, and the radical species reaction was completed. When the amount of H 2 O 2 increased from 1 mL to 14 mL, the degradation rate of the RhB solution increased from 60% to nearly 100% after 5 min. Comparing the degradation rates of all reaction systems within 20 min, we found that the degradation rate was 91.8%, 97.4% and 98.8% when the H 2 O 2 dosage was 2 , 4 and 6 mL, respectively, which indicated that the increase of H 2 O 2 dosage in a certain range would increase the content of O 2 •− and OH • radical species in the system, and then make more RhB degraded, that is, the degradation rate of RhB increased. When the amount of H 2 O 2 is 8 mL and 14 mL, the degradation rate is faster, but the color change of the whole reaction system is not as good as that of the 6 mL and the color of the 6 mL group shows a ladder shape from deep to shallow, which is easy to observe.

Degradation of RhB
The stability and reusability of catalyst is indispensable for its practical application. Therefore, the recycling reactions of the α-MnO 2 nanorods in degradation of RhB were carried out (Fig. 5b). The α-MnO 2 nanorods exhibited almost the same catalytic activity with the increase of recycling times,

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which indicated that the obtained α-MnO 2 nanorods possess high stability in RhB degradation. In order to grasp the changes of α-MnO 2 nanorods after catalytic reaction, the XRD and TEM were used to analyze the recycled α-MnO 2 catalyst. As shown in Fig. 6a-b, there is no significant difference between the reacted and fresh sample, suggesting that α-MnO 2 nanorods have high stability and potential application prospect in degradation of organic dyes.
As shown in Fig. 7, pH value plays a very important role in the degradation of RhB solution. The characteristic peak (λ max =553 nm) of RhB decreased significantly as well as a blue shift from 553 to 544 nm after 10 min when pH=4 (Fig. 7a), similar with the case of pH=6.6 ( Fig. 4a). For pH=9 and pH=12, there is nearly no shifted of characteristic peak with increasing interaction time ( Fig. 7b and 7c), suggesting no generation of N-deethylated intermediates of RhB. It is well known that the surface charge of catalyst depends largely on the pH value of the solution [1,[30][31][32]. The pH PZC of α-MnO 2 nanorods is determined to be 3.08 (Fig. 7e), above which the surface charge was negative because of the deprotonation reaction, and this would facilitate the adsorption of cationic RhB on α-MnO 2 nanorods. In addition, elevating solution pH would increase the amount of surface negative charges.
Therefore, the removal of RhB dye could be attributed to the oxidative degradation and adsorption decolorization when pH< 9. Then, the oxidative degradation was gradually weakened and adsorption decolorization played a leading role in the degradation mechanism with the increase of pH values.
According to the above discussion, the possible degradation mechanism is illustrated in Fig. 8. Fig. 7d shows the removal percentage of RhB by α-MnO 2 nanorods at different pH values. The removal efficiency of RhB at pH=4 and pH=6.6 is much higher than that at pH=9 within 10 min (97.5% removed at pH=4 and pH=6.6, while 87.7% removed at pH=9), but eventually reached similar degradation efficiency within 50 min. However, the removal efficiency is just less than 80% after 150 min in the case of pH=12. Apparently, low pH value is beneficial to degrade RhB dye.

Conclusions
A high purity α-MnO 2 nanorod has been synthesized by a simple co-precipitation method using potassium permanganate and manganese chloride tetrahydrate as manganese source. The α-MnO 2 nanorod exhibited a high efficiency and rapid removal of RhB. Furthermore, a lower pH value is conducive to the movement of the characteristic peak and the attenuation of the intensity of the characteristic peak. The RhB removal rate reached as high as 97.5% within 10 min (pH = 4 and 6.6) and 97.7% within 50 min (pH = 9), indicating that α-MnO 2 nanorod has more extensive application for RhB removal both in acid and alkaline conditions. Subsequently, the possible decolorization mechanism was proposed. The removal of RhB is attributed to the combination of oxidation and adsorption. It is believed that α-MnO 2 nanorods synthesized by such a simple and convenient approach holds great promise for the degradation of dye wastewater in practical application.