Facile Synthesis of Uniform Mesoporous Nb2O5 Micro-Flowers for Enhancing Photodegradation of Methyl Orange

The removal of organic pollutants using green environmental photocatalytic degradation techniques urgently need high-performance catalysts. In this work, a facile one-step hydrothermal technique has been successfully applied to synthesize a Nb2O5 photocatalyst with uniform micro-flower structure for the degradation of methyl orange (MO) under UV irradiation. These nanocatalysts are characterized by transmission and scanning electron microscopies (TEM and SEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) method, and UV-Vis diffuse reflectance spectroscopy (DRS). It is found that the prepared Nb2O5 micro-flowers presents a good crystal phases and consist of 3D hierarchical nanosheets with 400–500 nm in diameter. The surface area is as large as 48.6 m2 g−1. Importantly, the Nb2O5 micro-flowers exhibit superior catalytic activity up to 99.9% for the photodegradation of MO within 20 mins, which is about 60-fold and 4-fold larger than that of without catalysts (W/O) and commercial TiO2 (P25) sample, respectively. This excellent performance may be attributed to 3D porous structure with abundant catalytic active sites.


Introduction
In recent years, the ever-increasing organic pollutants have caused serious threat to global environment [1][2][3]. Among different kinds of organic pollutants, azo dyes have been suspected to be human carcinogens as they can produce toxic aromatic amines. Methyl orange (MO), as one of the most used water-soluble azo dyes, have been extensively employed in painting, chemistry and papermaking, Unfortunately, the dye lost in the dyeing process are discharged into the water bodies, which can cause serious environmental issues and is harmful to human health. Therefore, ongoing interest in both the scientific and engineering communities is to find a pollution remediation strategy for degradation of organic pollutants into clean sources by selecting MO as a model organic pollutant [4][5][6][7].
Semiconductor photocatalysis technique has the outstanding characteristics of low cost, simple operation, efficient degradation of organic pollutants, and less secondary pollution. It has become an emerging environmental governance method in recent years [8][9][10]. Especially, the nanostructured semiconductor catalysts with high photocatalytic oxidation activity show a great potential application in organic pollutant degradation. Among the semiconductor-based photocatalysts, Niobium materials are promising materials as photocatalysts and solid acid catalysts, thus has been widely applied in different kinds of fields [11][12][13]. Because of the excellent properties like non-toxicity, corrosion resistance, high stability, and lots of surface acid sites, Niobium oxide (Nb 2 O 5 ) has a wide range of catalytic activities for various reactions including dehydration, hydration, dehydrogenation, and many types of oxidation reactions [11,[14][15][16][17][18].
Previous studies have indicated that morphologies of catalysts have a significant impact on its performance. Up to now, various interesting morphologies of Nb 2 O 5 nanostructures, such as nanowires [11], nanobelts [19], nanorods [20], nanotubes [21], hollow nanospheres [22], nanoplates [23], have been synthesized. For instance, Zhao et al. fabricated Nb 2 O 5 nanorods which have higher activity (99%) in the photodegradation of methylene blue than nanospheres (40%) attribute to the higher intensity of the 001 crystal plane after 90 min [24]. Du and coworkers found a rod-like Nb 2 O 5 showed the best activity (95%) in the photocatalytic degradation of methylene blue after 150 min irradiation [25]. Qi et al. prepared Nb 2 O 5 nanofibers via electrospun approach and prove the hexagonal-Nb 2 O 5 nanofibers have a superior activity than orthorhombic -Nb 2 O 5 during decomposing MO [26]. Therefore, many approaches have been proposed to control the morphology of Nb 2 O 5 , such as ionic liquid derived technology [25], sol-gel, precipitation, solvent thermal and hydrothermal methods. Among them, Hydrothermal method has advantages in crystallizing with a mild condition (<200 • C), and keeping the hydroxyl groups at the surfaces with a larger number of acid sites [27]. Thus, the hydrothermal route shows a high potential to produce Nb 2 O 5 nanostructures with high photocatalytic activity, but has not been properly investigated In this work, we have applied a facile hydrothermal approach for the fabrication of Nb 2 O 5 with micro-flower. The microstructure and morphology, surface area, crystalline phase and optical property of as-obtained Nb 2 O 5 catalyst has been systematically charactered by SEM and TEM, BET, XRD and UV/Vis spectrophotometer. Furthermore, its photocatalytic performance of degradation of MO under UV irradiation is investigated, and compared to commercial TiO 2 (P25) and reported Nb 2 O 5 catalyst. The mechanism for the superior performance of as-prepared Nb 2 O 5 micro-flowers is discussed.

Preparation of Nb 2 O 5
In the typical preparation method [15], niobium oxalate (1.5 mmol; Aladdin, 98%) and ammonium carbonate (7.5 mmol; Aladdin, 99.99%) were added to deionized water (30 mL). After stirring for 15 min, the mixture was transferred to a 50 mL polytetrafluoroethylene lined autoclave, sealed in an oven, heated to 200 • C and kept for 12 h. The white precipitate was collected, washed with ethanol and water. Finally, the samples were kept at 60 • C for 3 h in in a vacuum oven, and the Nb 2 O 5 micro flower were obtained.

Characterization Methods
An X-ray diffraction (XRD) analysis of the catalysts was carried out at the XPert Pro MPD (PANalytical B.V., Almelo, The Netherlands) with Cu-Kα radiation generated at 45 kV and 40 mA. The morphologies of the catalysts were charactered by means of field emission scanning electron microscopy (FESEM, S-4800; Hitachi, Tokyo, Japan), which was operated at an accelerating voltage of 4 kV. The transmission electron microscopy (TEM) investigation was performed at the Tecnai G2F30 S-Twin TEM (FEI, Amsterdam, The Netherlands). The X-ray photoelectron spectroscopy (XPS) experiments were performed with a Thermo Scientific ESCA-Lab-200i-XL spectrometer (Waltham, MA, USA), which utilized monochromatic Al Alα radiation (1486.6 eV), and the C 1s and N 1s peak spectra were analyzed by using XPS Peak 4.1 software. To quantify the specific surface areas of examined catalysts, Brunauer-Emmett-Teller analysis was performed (BET, ASAT 2020M+C, Micromeritics, Micromeritics Instrument Co., Norcross, GA, USA), using the nitrogen adsorption and desorption isotherms at liquid nitrogen temperature (77 K). UV-Vis diffuse reflectance spectroscopy was recorded on a UV/Vis spectrophotometer (UV/Vis DRS, UV-2550, Shimadzu, Tokyo, Japan) over the wavelength range between 200 and 800 nm.

Photocatalytic Activity Tests
The photocatalytic degradation of MO was carried out in a cylindrical reactor under UV irradiation. A thermostat (THD-2015, Ningbo Tianheng Instrument Factory, Ningbo, China) was employed to maintain the temperature of the reactant solution at 25 • C. For each test, 5 mg of ground photocatalyst powder was dispersed in 100 mL of an aqueous solution containing 20 mg L −1 MO. the samples were collected and analyzed at 463 nm of the UV/Vis spectrophotometer (Shimad-zu-1601pc) within the preset time interval. Before the photocatalytic degradation experiment, the dye samples were continuously stirred in the dark for about 15 min to eliminate the adsorption effect.

Characterization of Nb 2 O 5 Micro-flowers
The microstructure analysis of the as-obtained Nb 2 O 5 micro-flower sample was carried out by using powder X-ray diffraction (XRD), as shown in the  [21][22][23]. Moreover, the sharp peak indicates the high crystallinity of as-obtained Nb 2 O 5 sample, which could enhance the photocatalytic degradation of organic molecules according to previous reports [15].

Photocatalytic Activity Tests
The photocatalytic degradation of MO was carried out in a cylindrical reactor under UV irradiation. A thermostat (THD-2015, Ningbo Tianheng Instrument Factory, Ningbo, China) was employed to maintain the temperature of the reactant solution at 25 °C. For each test, 5 mg of ground photocatalyst powder was dispersed in 100 mL of an aqueous solution containing 20 mg L −1 MO. the samples were collected and analyzed at 463 nm of the UV/Vis spectrophotometer (Shimad-zu-1601pc) within the preset time interval. Before the photocatalytic degradation experiment, the dye samples were continuously stirred in the dark for about 15 min to eliminate the adsorption effect.

Characterization of Nb2O5 Micro-flowers
The microstructure analysis of the as-obtained Nb2O5 micro-flower sample was carried out by using powder X-ray diffraction (XRD), as shown in the  [21][22][23]. Moreover, the sharp peak indicates the high crystallinity of as-obtained Nb2O5 sample, which could enhance the photocatalytic degradation of organic molecules according to previous reports [15]. The morphology of as-obtained Nb2O5 sample was detected by using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), as well as high-resolution transmission electron microscope (HRTEM). Figure 2a reveals that the material has a structure consisting of uniform micro-flowers with a diameter of 2-3 micrometers. Each flower possesses a 3D structure composed of nanosheets as displayed in More structure details are further confirmed by TEM images as presented in Figure 2c. Indeed, the lateral sizes of these nanosheets are typically in the ranges of 400-500 nm in diameter (Figure 2d).  The morphology of as-obtained Nb 2 O 5 sample was detected by using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), as well as high-resolution transmission electron microscope (HRTEM). Figure 2a reveals that the material has a structure consisting of uniform micro-flowers with a diameter of 2-3 micrometers. Each flower possesses a 3D structure composed of nanosheets as displayed in More structure details are further confirmed by TEM images as presented in Figure 2c. Indeed, the lateral sizes of these nanosheets are typically in the ranges of 400-500 nm in diameter (Figure 2d). To investigate the porosity and specific surface area of the as-obtained materials, the N 2 adsorption-desorption measurement is conducted. As displayed in Figure 3, the Nb 2 O 5 has type IV isotherm with a discrete hysteresis loop (H 3 typedepecit) in the higher relative pressure (P/P 0 , 0.51-1.0) range, indicating the presence of slit-like pores and capillary condensation in the mesoporous. This mesopore feature could be further confirmed by the relevant pore size distributions as implied in the inset of Figure 3. From the plot, the surface area of Nb 2 O 5 micro-flowers is estimated at 48.6 m 2 g −1 , which is larger than the reported Nb 2 O 5 hollow fibers (32.8 m 2 g −1 ) [28], hexagonal-like Nb 2 O 5 (31.7 m 2 g −1 ) [23] and Nb 2 O 5 (N-100) (2.45 m 2 g −1 ) [29]. This suggests that the hierarchical structure of To investigate the porosity and specific surface area of the as-obtained materials, the N2 adsorption-desorption measurement is conducted. As displayed in Figure 3, the Nb2O5 has type IV isotherm with a discrete hysteresis loop (H 3 typedepecit) in the higher relative pressure (P/P0, 0.51-1.0) range, indicating the presence of slit-like pores and capillary condensation in the mesoporous. This mesopore feature could be further confirmed by the relevant pore size distributions as implied in the inset of Figure 3. From the plot, the surface area of Nb2O5 micro-flowers is estimated at 48.6 m 2 g −1 , which is larger than the reported Nb2O5 hollow fibers (32.8 m 2 g −1 ) [28], hexagonal-like Nb2O5 (31.7 m 2 g −1 ) [23] and Nb2O5 (N-100) (2.45 m 2 g −1 ) [29]. This suggests that the hierarchical structure of Nb2O5 micro-flowers has a large surface area and accordingly provide more catalytically active sites during the process of molecular adsorption and reaction. The optical absorption of Nb2O5 micro-flowers is investigated by UV/Vis diffuse reflectance spectroscopy (DRS) spectrum. As illustrated in Figure 4, the Nb2O5 shows a strong absorption bands in the range of 230-400 nm, and the absorption edge value of Nb2O5 is observed at 384 nm. This enhanced optical absorption would make the Nb2O5 The optical absorption of Nb 2 O 5 micro-flowers is investigated by UV/Vis diffuse reflectance spectroscopy (DRS) spectrum. As illustrated in Figure 4, the Nb 2 O 5 shows a strong absorption bands in the range of 230-400 nm, and the absorption edge value of Nb 2 O 5 is observed at 384 nm. This enhanced optical absorption would make the Nb 2 O 5 effectively interact and employ incident light to generate abundant active species for driving catalytic reactions. Based on previously published literatures, the band gap of the as-prepared materials can be measured by the following equation [30,31]: where A is a constant, Eg is the band gap, and n is a number that varies with the transition of the semiconductor which takes value 4 for indirectly transition [30,31]. Therefore, as can be seen from the inset of Figure 4, the corresponding band gap value of Nb 2 O 5 microflowers is estimated to be 3.34 eV similar to the reported values of 3.40 eV for Nb 2 O 5 NC [32]. The optical absorption of Nb2O5 micro-flowers is investigated by UV/Vis diffuse reflectance spectroscopy (DRS) spectrum. As illustrated in Figure 4, the Nb2O5 shows a strong absorption bands in the range of 230-400 nm, and the absorption edge value of Nb2O5 is observed at 384 nm. This enhanced optical absorption would make the Nb2O5 effectively interact and employ incident light to generate abundant active species for driving catalytic reactions. Based on previously published literatures, the band gap of the asprepared materials can be measured by the following equation [30,31]: where A is a constant, Eg is the band gap, and n is a number that varies with the transition of the semiconductor which takes value 4 for indirectly transition [30,31]. Therefore, as can be seen from the inset of Figure 4, the corresponding band gap value of Nb2O5 microflowers is estimated to be 3.34 eV similar to the reported values of 3.40 eV for Nb2O5 NC [32].

Photoactivity Evaluation of As-Prepared Materials
The photocatalytic activity is evaluated by the degradation of MO under UV irradiation. Figure 5a shows the photocatalytic performance of as-prepared catalysts. Obviously, the Nb 2 O 5 micro-flowers completely bleach MO in only 20 min, at which the blank solution without catalysts and P25 sample show a photooxidation efficiency of 6.3% and 81.4%, respectively. Furthermore, the photocatalytic experimental data are found to fit pseudofirst-order kinetics as shown in Figure 5b, and the degradation constants for W/O sample, P25 catalyst and Nb 2 O 5 micro-flowers are 0.003, 0.058, and 0.208 min −1 , respectively. The apparent rate constant of Nb 2 O 5 micro-flowers is almost 4 times higher than that of P25, revealing superior performance of Nb 2 O 5 micro-flowers.
We also compare the photocatalytic performance of the present Nb 2 O 5 micro-flowers with those of previously reported Nb 2 O 5 nanostructures, the data are summarized in Table 1. Obviously, the Nb 2 O 5 micro-flowers in the present work with highest efficiency up to 99.9% in the shortest time, is superior to other reported Nb 2 O 5 catalysts for the photodegradation of dye wastewater. This might arise from that the large surface area of 3D porous structure, and greater number of acid sites generated by the hydrothermal method for photooxidation. the Nb2O5 micro-flowers completely bleach MO in only 20 min, at which the blank solu-tion without catalysts and P25 sample show a photooxidation efficiency of 6.3% and 81.4%, respectively. Furthermore, the photocatalytic experimental data are found to fit pseudo-first-order kinetics as shown in Figure 5b, and the degradation constants for W/O sample, P25 catalyst and Nb2O5 micro-flowers are 0.003, 0.058, and 0.208 min −1 , respectively. The apparent rate constant of Nb2O5 micro-flowers is almost 4 times higher than that of P25, revealing superior performance of Nb2O5 micro-flowers. We also compare the photocatalytic performance of the present Nb2O5 micro-flowers with those of previously reported Nb2O5 nanostructures, the data are summarized in Table  1. Obviously, the Nb2O5 micro-flowers in the present work with highest efficiency up to 99.9% in the shortest time, is superior to other reported Nb2O5 catalysts for the photodegradation of dye wastewater. This might arise from that the large surface area of 3D porous structure, and greater number of acid sites generated by the hydrothermal method for photooxidation.  Next, we also performance the reuse tests of the obtained Nb 2 O 5 micro-flowers. Notably, after 5 cycles, the photo-catalytic performance only slightly decreased under UV light as shown in Figure 5c. This confirms the high stability of Nb 2 O 5 micro-flowers. All the above results demonstrate that Nb 2 O 5 micro-flowers is a good photocatalyst for degradation of MO.
To reveal the enhanced photocatalytic activity, a mechanism proposed for the photodegradation of methyl orange by Nb 2 O 5 is illustrated in Figure 5d. As evidenced above, Nb 2 O 5 micro-flowers had a well-developed mesoporous structure with a large surface area and adsorption capacitance, which provides a great number of active adsorption sites. An important step during the catalytic process is the adsorption of reacting substances onto the surface of the catalyst [33,34]. Then the photoelectron can be easily trapped by electronic acceptors like adsorbed O 2 , to further produce a superoxide radical anion (O 2 − ), whereas the photo-induced holes can be easily trapped by organic pollutants, to further oxidize organic pollutants [35,36]. Furthermore, the two sides of the pore wall individually provide a place for the oxidation and reduction reaction, which facilitate separation of the electrons and holes. This might be the most beneficial character of mesoporous Nb 2 O 5 to work as a highperformance photocatalyst.

Conclusions
In conclusion, we have developed a facile hydrothermal process to fabricate uniform micro-flower Nb 2 O 5 photocatalysts for degrading MO under UV irradiation. The morphology and structures characterizations show that the prepared Nb 2 O 5 micro-flowers consisted of 3D hierarchical nanosheets with 400-500 nm in diameter have a good crystal phases and a surface area up to 48.6 m 2 g −1 . Such 3D porous structure provides abundant catalytic active sites and lead to a superior catalytic activity with degradation rate of 0.208 min −1 , which is about 60-fold and 4-fold larger than that without the catalysts (W/O) and commercial TiO 2 (P25) sample, respectively. The photocatalytic reuse tests also show that the performance of Nb 2 O 5 micro-flowers has a long life and high stability. This present work insights into the design and synthesis of highly active and stable Nb 2 O 5 photocatalysts for the degradation of dyes.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.