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

Abstract

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.

1. 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 (Nb₂O₅) 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 Nb₂O₅ 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 Nb₂O₅ 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 Nb₂O₅ showed the best activity (95%) in the photocatalytic degradation of methylene blue after 150 min irradiation [25]. Qi et al. prepared Nb₂O₅ nanofibers via electrospun approach and prove the hexagonal-Nb₂O₅ nanofibers have a superior activity than orthorhombic -Nb₂O₅ during decomposing MO [26]. Therefore, many approaches have been proposed to control the morphology of Nb₂O₅, 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 Nb₂O₅ 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 Nb₂O₅ with micro-flower. The microstructure and morphology, surface area, crystalline phase and optical property of as-obtained Nb₂O₅ 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 TiO₂ (P25) and reported Nb₂O₅ catalyst. The mechanism for the superior performance of as-prepared Nb₂O₅ micro-flowers is discussed.

2. Materials and Methods

2.1. Preparation of Nb₂O₅

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 Nb₂O₅ micro flower were obtained.

2.2. 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.

2.3. 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.

3. Results and Discussion

3.1. 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 Figure 1. Similar to reported literatures, the main diffraction peak located at 24.8°, 29.9°, 32°, 38.9°, 44.3°, 51°, 52.6°, 53°, 64°, can be indexed to (121), (324), (314), (518), (809), (1010), (1119), (936), (154) plane of Nb₂O₅ (JCPDS 19-0862#) [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). Figure 2e,f present the HRTEM image of the sample, the lattice fringes in the magnified HRTEM image (Figure 2f) are found to be ca.0.358 and 0.173 nm, in accordance with the planes of the (121) and (1119) d spacings of Nb₂O₅ [15], respectively.

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 (H³ 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 m² g⁻¹, which is larger than the reported Nb₂O₅ hollow fibers (32.8 m² g⁻¹) [28], hexagonal-like Nb₂O₅ (31.7 m² g⁻¹) [23] and Nb₂O₅ (N-100) (2.45 m² 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 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]:

$$\mathsf{\alpha }\mathrm{h}\mathsf{\nu } = \mathrm{A}{\left(\mathrm{h}\mathsf{\nu } - \mathrm{Eg}\right)}^{\frac{n}{2}}$$

(1)

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 micro-flowers is estimated to be 3.34 eV similar to the reported values of 3.40 eV for Nb₂O₅ NC [32].

3.2. 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 Nb₂O₅ 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 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.

Table 1. A comparison of the photocatalytic performance.

Catalysts	Surface Area (m2 g−1)	Dye	Reaction Time (min)	Efficiency	Reference
Rod-like Nb2O5	/	Methylene Blue	150	95%	[25]
Nb2O5 (N-100)	2.445	Methylene Blue	270	55%	[29]
H-Nb2O5 hollow fibers	32.8	Methylene orange	50	93%	[28]
Sphere-like Nb2O5	104.3	Methylene blue	50	87%	[22]
		Rose bengal	180	62%
Hexagonal-like Nb2O5 nanoplates	31.7	Methylene blue	60	92%	[23]
		Rhdamine B	60	98%
Micro-flowers Nb2O5 (this work)	48.6	Methylene orange	20	99.9%	This work

Table 1. A comparison of the photocatalytic performance.

Catalysts	Surface Area (m2 g−1)	Dye	Reaction Time (min)	Efficiency	Reference
Rod-like Nb2O5	/	Methylene Blue	150	95%	[25]
Nb2O5 (N-100)	2.445	Methylene Blue	270	55%	[29]
H-Nb2O5 hollow fibers	32.8	Methylene orange	50	93%	[28]
Sphere-like Nb2O5	104.3	Methylene blue	50	87%	[22]
		Rose bengal	180	62%
Hexagonal-like Nb2O5 nanoplates	31.7	Methylene blue	60	92%	[23]
		Rhdamine B	60	98%
Micro-flowers Nb2O5 (this work)	48.6	Methylene orange	20	99.9%	This work

Next, we also performance the reuse tests of the obtained Nb₂O₅ 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 Nb₂O₅ micro-flowers. All the above results demonstrate that Nb₂O₅ 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 Nb₂O₅ is illustrated in Figure 5d. As evidenced above, Nb₂O₅ 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 O₂, to further produce a superoxide radical anion (O₂⁻), 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 Nb₂O₅ to work as a high-performance photocatalyst.

4. Conclusions

In conclusion, we have developed a facile hydrothermal process to fabricate uniform micro-flower Nb₂O₅ photocatalysts for degrading MO under UV irradiation. The morphology and structures characterizations show that the prepared Nb₂O₅ 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 m² 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 TiO₂ (P25) sample, respectively. The photocatalytic reuse tests also show that the performance of Nb₂O₅ micro-flowers has a long life and high stability. This present work insights into the design and synthesis of highly active and stable Nb₂O₅ photocatalysts for the degradation of dyes.