Controllable Synthesis of Mn3O4 Nanowires and Application in the Treatment of Phenol at Room Temperature

Nanosized Mn3O4 nanowires are prepared with KMnO4 and ethanol in mild conditions by facile hydrothermal method. Hydrothermal reaction temperature is optimized to get uniform nanowires. The prepared Mn3O4 nanowires exhibit high activity in the treatment of phenol at acid condition and room temperature. The 20 mg Mn3O4 nanowires can efficiently dispose of 50 mL phenol solution (0.2 g·L−1) at pH 2 and 25 °C. The nanowires before and after phenol treatment are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and the reaction mechanism is discussed.


Introduction
Manganese is one of the most earth-abundant elements and manganese oxides are generally non-toxic. Manganese oxide nanomaterials also show great potential in sustainable nanotechnology. They are widely utilized in catalytic reactions, sensors and batteries because of their low cost and high activity [1,2]. Mn 3 O 4 , also known as hausmannite, is a mixed valence oxide and a promising candidate for catalysts, microwave absorption materials, sensors and anode materials [3]. It has been used in the catalytic oxidation of methane and reduction of nitrobenzene [4]. Lu et al. prepared the amorphous MnO 2 thin film first then hydrothermally transformed it into Mn 3 O 4 nanowires under room temperature in a solution bath of 0.01 M manganese acetate and 0.01 M sodium sulfate mixture [5]. One-dimensional (1D) nanostructures, especially nanowires, are of great importance because of their specific shape and potential applications. Nanowires feature superior functional properties and mechanical strengths which have been used in micro/nanoelectromechanical systems and photovoltaic applications [6]. Veeramani [8].
Tremendous discharge of toxic organic compounds has contributed to serious pollution to our eco-environment and human beings. Phenolic compounds play an important role in the production of pesticides, resins and antioxidants [9]. However, the phenolic waste waters are highly toxic and persistent, and are difficult to treat with traditional biodegradation. Strong oxidants such as H 2 O 2 and O 3 , without secondary pollution, have been utilized in the treatment of organic compound waste waters. However, the treatment efficiency is relatively low [10,11]. To achieve high mineralization, Nanomaterials 2020, 10, 461 2 of 9 advanced oxidation processes (AOPs) were proposed to degrade organic pollutants through the generation of highly reactive hydroxyl radicals [12,13]. The Fenton process combines H 2 O 2 and iron as a catalyst to generate hydroxyl radicals, which have strong oxidation capability on the phenol removal [14]. However, the homogeneous Fenton process is limited by its narrow pH range and iron sludge generation [15][16][17].
In this work, uniform Mn 3 O 4 nanowires are fabricated with facile hydrothermal method and common raw materials in mild conditions. They are utilized to deal with phenol solution without extra H 2 O 2 or O 3 at room temperature and air condition. The mechanism of the Mn 3 O 4 nanowires on the treatment of phenol is also studied with the characterization of scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).

Preparation and Characterization of Mn 3 O 4
The nanosized Mn 3 O 4 was provided by US research Nanomaterials, Inc (Houston, TX, USA) for comparison. Analytically pure KMnO 4 and ethanol was used as raw materials. First, 0.2 g KMnO 4 and 20 mL ethanol/water solution were added in a 45 mL Teflon-lined steel autoclave. The ratio of ethanol and reaction temperature in the furnace are investigated to get uniform nanowire and improved performance in phenol treatment. All products were cleaned with deionized water several times and dried in air at 80 • C for 12 h. Then the Mn 3 O 4 powder was ground before utilization. The 3Flex Surface Characterization Analyzer (Micromeritics Corporation, Norcross, GA, USA) is able to determine the adsorption-desorption isotherm of nitrogen at 77 K, then, as a function of the kind of isotherm, according to international union of pure and applied chemistry (IUPAC) classification, a mathematical model to find specific surface areas (e.g., Brunauer-Emmett-Teller (BET) adsorption). The prepared Mn 3 O 4 and used nanoparticles were tested with SEM (FE-SEM, Hitachi S-4800, Tokyo, Japan), XPS (ThermoESCALAB250Xi, Waltham, MA, USA) and XRD (XRD-7000S, Shimadzu, Tokyo, Japan) to determine the morphology and chemical composition of the material.

Treatment of Phenol Waste Water at Room Temperature
The phenol was dissolved in de-ionized water with H 2 SO 4 to adjust the pH. In a typical procedure, 20 mg Mn 3 O 4 was added into a phenol solution (50 mL, 0.2 g·L −1 and pH = 2) at room temperature in an air flow. The concentration of phenol is tested with UV-Vis spectrum at 270 nm every 15 min after the reaction solution was filtrated with a 0.1 µm poly tetra fluoroethylene (PTFE) syringe filter (Whatman TM , Shanghai, China).

Composition and Morphologies of the Prepared Mn 3 O 4
The effect of hydrothermal temperature on the diffraction patterns are shown in Figure 1. It is obvious that the prepared nanoparticles are all Mn 3 O 4 which are similar with the commercial Mn 3 O 4 nanoparticles. What is more, they are all in accordance with hausmannite type Mn 3 O 4 PDF card (JCPDS no. 24-0734) even though the intensity of some peaks are different which may be induced by the formation of nanowires. When the reaction temperature decreases to 110 • C, the intensity of (211) plane is very high while some other planes such as (101), (220) and (400) planes almost disappeared. The effect of hydrothermal temperature on the diffraction patterns are shown in Figure 1. It is obvious that the prepared nanoparticles are all Mn3O4 which are similar with the commercial Mn3O4 nanoparticles. What is more, they are all in accordance with hausmannite type Mn3O4 PDF card (JCPDS no. 24-0734) even though the intensity of some peaks are different which may be induced by the formation of nanowires. When the reaction temperature decreases to 110 °C, the intensity of (211) plane is very high while some other planes such as (101), (220) and (400) planes almost disappeared.  Table 1. Along with this, the adsorption average pore diameter of Mn 3 O 4 (110 • C) is about 10.6 nm which is positive to the adsorption and reaction activity of the nanowires. The effect of hydrothermal temperature on the diffraction patterns are shown in Figure 1. It is obvious that the prepared nanoparticles are all Mn3O4 which are similar with the commercial Mn3O4 nanoparticles. What is more, they are all in accordance with hausmannite type Mn3O4 PDF card (JCPDS no. 24-0734) even though the intensity of some peaks are different which may be induced by the formation of nanowires. When the reaction temperature decreases to 110 °C, the intensity of (211) plane is very high while some other planes such as (101), (220) and (400) planes almost disappeared.  Table 1. Along with this, the adsorption average pore diameter of Mn3O4 (110 °C) is about 10.6 nm which is positive to the adsorption and reaction activity of the nanowires.

Treatment of Phenol with Mn3O4 at Room Temperature
Phenol is widely used in the manufacturing of plastics, pesticides and pharmaceuticals. Because of its high toxicity, a lot of method is recommended but still not efficient. In this work, nanosized Mn3O4 (20 mg) was used to deal with the phenol waste water at room temperature in the air condition. As shown in Figure 3, the phenol concentration decreases obviously in 60 min. Especially when the preparation temperature is 110 °C, the performance of the Mn3O4 is the best because the Mn3O4 prepared at 110 °C shows the uniform nanowires and highest specific surface area. As shown in Table  2, the Mn3O4 nanowires can efficiently deal with phenol waste water with high concentration at low temperature.

Treatment of Phenol with Mn 3 O 4 at Room Temperature
Phenol is widely used in the manufacturing of plastics, pesticides and pharmaceuticals. Because of its high toxicity, a lot of method is recommended but still not efficient. In this work, nanosized Mn 3 O 4 (20 mg) was used to deal with the phenol waste water at room temperature in the air condition. As shown in Figure 3, the phenol concentration decreases obviously in 60 min. Especially when the preparation temperature is 110 • C, the performance of the Mn 3 O 4 is the best because the Mn 3 O 4 prepared at 110 • C shows the uniform nanowires and highest specific surface area. As shown in Table 2, the Mn 3 O 4 nanowires can efficiently deal with phenol waste water with high concentration at low temperature. Effect of pH on the performance of Mn3O4 is studied as shown in Figure 4. At neutral condition, almost no phenol will be consumed at all, which means that Mn3O4 will not react with phenol or adsorb it. With the decrease of pH, the reactivity of Mn3O4 increases obviously. When the pH is below 2, the Mn3O4 has good performance on treatment of phenol. However, lower pH will cause serious leaching of Mn3O4 because the nanosized Mn3O4 can be dissolved in concentrated sulfuric acid and hydrochloric acid.  Effect of pH on the performance of Mn 3 O 4 is studied as shown in Figure 4. At neutral condition, almost no phenol will be consumed at all, which means that Mn 3 O 4 will not react with phenol or Nanomaterials 2020, 10, 461 5 of 9 adsorb it. With the decrease of pH, the reactivity of Mn 3 O 4 increases obviously. When the pH is below 2, the Mn 3 O 4 has good performance on treatment of phenol. However, lower pH will cause serious leaching of Mn 3 O 4 because the nanosized Mn 3 O 4 can be dissolved in concentrated sulfuric acid and hydrochloric acid.  Effect of pH on the performance of Mn3O4 is studied as shown in Figure 4. At neutral condition, almost no phenol will be consumed at all, which means that Mn3O4 will not react with phenol or adsorb it. With the decrease of pH, the reactivity of Mn3O4 increases obviously. When the pH is below 2, the Mn3O4 has good performance on treatment of phenol. However, lower pH will cause serious leaching of Mn3O4 because the nanosized Mn3O4 can be dissolved in concentrated sulfuric acid and hydrochloric acid.

Analyse of the Function of Mn3O4 in the Treatment of Phenol
Different reaction atmosphere is utilized to investigate the mechanism of phenol treatment. As

Analyse of the Function of Mn 3 O 4 in the Treatment of Phenol
Different reaction atmosphere is utilized to investigate the mechanism of phenol treatment. As shown in Figure 5, in all the reaction condition, the Mn 3 O 4 has the similar performance which means oxygen is not necessary for the treatment of phenol waste water. In the product of the reaction, benzoquinone was detected which means Mn 3 O 4 nanowires can oxidize phenol at room temperature. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 10 large specific area has stronger oxidation reactivity compared with H2O2 because it can oxidize H2O2. After reaction with H2O2, oxygen is formed while Mn3O4 is dissolved in the solution. KMnO4 which is the raw material of Mn3O4 nanowires also have strong oxidation ability. In order to completely oxidize phenol, excess dosage of KMnO4 is needed which will induce secondary pollution to the waste water. Excess Mn3O4 nanowires can be easily filtrated and leaching Mn ions can also be removed by neutralization of the acidic waste water.    [20]. However, H 2 O 2 (0.1 mol·L −1 ) cannot oxidize phenol at the same pH in air condition. In contrast, Mn 3 O 4 nanowires prepared in this work with large specific area has stronger oxidation reactivity compared with H 2 O 2 because it can oxidize H 2 O 2 . After reaction with H 2 O 2 , oxygen is formed while Mn 3 O 4 is dissolved in the solution. KMnO 4 which is the raw material of Mn 3 O 4 nanowires also have strong oxidation ability. In order to completely oxidize phenol, excess dosage of KMnO 4 is needed which will induce secondary pollution to the waste water. Excess Mn 3 O 4 nanowires can be easily filtrated and leaching Mn ions can also be removed by neutralization of the acidic waste water.
Benzoquinone and hydroquinone are the common oxidation products of phenol. In this work, pure benzoquinone and hydroquinone solutions are used as the waste water separately, while 20 mg Mn 3 O 4 is added in the 50 mL solution (pH = 2, adjusted with H 2 SO 4 ). After 5 min, about 60% hydroquinone is converted to benzoquinone as shown in Figure 6. It was also found that the solution is changed into yellow (the color of benzoquinone) while all Mn 3 O 4 nanowires are dissolved which demonstrates that hydroquinone can react with the nanoparticles efficiently. For the benzoquinone solution, the absorbance does not change which means that the Mn 3 O 4 nanowires cannot react with stable benzoquinone.
is the raw material of Mn3O4 nanowires also have strong oxidation ability. In order to completely oxidize phenol, excess dosage of KMnO4 is needed which will induce secondary pollution to the waste water. Excess Mn3O4 nanowires can be easily filtrated and leaching Mn ions can also be removed by neutralization of the acidic waste water.  Benzoquinone and hydroquinone are the common oxidation products of phenol. In this work, pure benzoquinone and hydroquinone solutions are used as the waste water separately, while 20 mg Mn3O4 is added in the 50 mL solution (pH = 2, adjusted with H2SO4). After 5 min, about 60% hydroquinone is converted to benzoquinone as shown in Figure 6. It was also found that the solution is changed into yellow (the color of benzoquinone) while all Mn3O4 nanowires are dissolved which demonstrates that hydroquinone can react with the nanoparticles efficiently. For the benzoquinone After reaction with phenol, the morphology of Mn 3 O 4 changes obviously as shown in Figure 7. The uniform nanowires disappear while a lot of block-shaped particles are formed. After reaction, the reactivity of Mn 3 O 4 also disappears because the reaction will not continue after adding extra phenol.
Although the morphology of the Mn 3 O 4 particles changes obviously after reaction with phenol, the XRD patterns of Mn 3 O 4 after reaction as shown in Figure 8 does not change very much and the characteristic peaks keep similar intensity which means only partial active surface area reacts with phenol. The reacted Mn 3 O 4 still has the similar crystal structure with hausmannite.
The chemical composition and element valence of the materials before and after phenol removal are further examined by XPS (Figure 9). The Mn 2p3/2 peak of the prepared Mn 3 O 4 and reacted Mn 3 O 4 consisted of three separate peaks at 642.8, 641.6 and 640.6 eV (Figure 9b) which are in accordance with Mn 4+ , Mn 3+ and Mn 2+ , respectively. Before reaction, the relative atomic percentages of Mn 4+ , Mn 3+ and Mn 2+ are 41.9%, 53.5% and 4.6%, respectively. After reaction with phenol, the relative percentages change into 68.1%, 25.5% and 6.4%. Although the average valence is improved according to the data, the possible reason is the dissolution of Mn with low valence in acid condition because the XPS spectrum of reacted Mn 3 O 4 shows obvious noise which indicates pollution of the sample and low content of Mn. Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 10 solution, the absorbance does not change which means that the Mn3O4 nanowires cannot react with stable benzoquinone. After reaction with phenol, the morphology of Mn3O4 changes obviously as shown in Figure 7. The uniform nanowires disappear while a lot of block-shaped particles are formed. After reaction, the reactivity of Mn3O4 also disappears because the reaction will not continue after adding extra phenol. Although the morphology of the Mn3O4 particles changes obviously after reaction with phenol, the XRD patterns of Mn3O4 after reaction as shown in Figure 8 does not change very much and the characteristic peaks keep similar intensity which means only partial active surface area reacts with phenol. The reacted Mn3O4 still has the similar crystal structure with hausmannite. The chemical composition and element valence of the materials before and after phenol removal are further examined by XPS (Figure 9). The Mn 2p3/2 peak of the prepared Mn3O4 and reacted Mn3O4 consisted of three separate peaks at 642.8, 641.6 and 640.6 eV (Figure 5b) which are in accordance with Mn 4+ , Mn 3+ and Mn 2+ , respectively. Before reaction, the relative atomic percentages of Mn 4+ , Mn 3+ and Mn 2+ are 41.9%, 53.5% and 4.6%, respectively. After reaction with phenol, the relative percentages with Mn 4+ , Mn 3+ and Mn 2+ , respectively. Before reaction, the relative atomic percentages of Mn 4+ , Mn 3+ and Mn 2+ are 41.9%, 53.5% and 4.6%, respectively. After reaction with phenol, the relative percentages change into 68.1%, 25.5% and 6.4%. Although the average valence is improved according to the data, the possible reason is the dissolution of Mn with low valence in acid condition because the XPS spectrum of reacted Mn3O4 shows obvious noise which indicates pollution of the sample and low content of Mn.

Conclusions
Hydrothermal temperature and ethanol content have a strong effect on the morphology and structure of nanoparticles. Uniform Mn3O4 nanowires with high specific surface area (BET surface area, 248.8 m 2 •g −1 ) are formed when the hydrothermal temperature is 110 °C and the ethanol content is 10 mL. The Mn3O4 nanowires show high efficiency in phenol removal at room temperature and air condition because it has strong oxidation ability compared with H2O2. The 20 mg Mn3O4 nanowires

Conclusions
Hydrothermal temperature and ethanol content have a strong effect on the morphology and structure of nanoparticles. Uniform Mn 3 O 4 nanowires with high specific surface area (BET surface area, 248.8 m 2 ·g −1 ) are formed when the hydrothermal temperature is 110 • C and the ethanol content is 10 mL. The Mn 3 O 4 nanowires show high efficiency in phenol removal at room temperature and air condition because it has strong oxidation ability compared with H 2 O 2 . The 20 mg Mn 3 O 4 nanowires can efficiently dispose of 50 mL phenol solution (0.2 g·L −1 ) at pH 2 and 25 • C. According to the SEM, XRD and XPS characterization, the Mn 3 O 4 shows strong oxidation capability and reacts with phenol. After reaction, partial Mn with low valence is leaching into the acid solution. Therefore, it is very efficient in the quick removal of phenol in water treatment without special operation parameter. Excess Mn 3 O 4 nanowires can be easily filtrated and leaching Mn ions also can be removed by neutralization of the acidic waste water.