Effect of the Synthesis Parameters on the Physicochemical Properties and Photocatalytic Performance of Nb₂O₅ to Water Treatment and H₂ Production

Abstract

Nb₂O₅ is a promising photocatalyst for degradation of organic pollutants and production of H₂ (renewable and green fuel) from water is an alternative to overcome environmental and energy concerns. Therefore, the effect of Nb precursor and synthesis method on the Nb₂O₅ physical-chemical properties and photocatalytic performance were investigated in this work. X-ray diffraction and Raman spectroscopy results showed that synthesis parameters had a significant impact on the structural properties of the materials obtained. The synthesis parameters were able to tune Nb₂O₅ crystalline phase, morphology and specific surface area. Nb₂O₅ sample obtained by hydrothermal method resulted in materials with smaller size and homogeneous particles compared to the other samples, which resulted in a higher specific surface area (200 m² g^-1) and surface hydroxylation. Nb₂O₅ samples were active to promote both the degradation of Rhodamine B and amiloride, the specific surface area and surface hydroxylation play a key role in the performance of the materials. The most active sample for organic pollutant degradation was also active in H₂ production, additionally, the decoration with Pt nanoparticles increased its performance. This work describes the effect of synthesis parameters of Nb₂O₅ on its photocatalytic properties and demonstrates the potential application of this material in heterogeneous photocatalysis.

Keywords:
niobium pentoxide; hydrothermal method; organic pollutant degradation; water splitting

Introduction

Water is one of the most valuable and crucial natural resources for sustaining life on Earth.¹ While approximately 70% of the Earth’s surface is covered with water, only 2.5% of it is considered safe for consumption. The contamination due to improper disposal of domestic, industrial, and agricultural effluents is exacerbating this situation,²,³ highlighting the need for strategies to treat and reuse water.⁴ Additionally, the increasing demand for energy from non-renewable sources and its contribution to air pollution, global warming and climate changes are a major environmental and energy concern worldwide. Then, the development of methods to obtain energy in a clean and sustainable way becomes imperative. Therefore, solar light as a primary energy resource stands out for its advantages in terms of spread availability and accessibility in an inexhaustible and inexpensive way around the world.⁵ The possibility to convert light into valuable solar fuels from water is a promising approach to provide clean energy and, at the same time, contribute to mitigate the global warming effect.⁶

Heterogeneous photocatalysis is a promising method for treating water effluents (organic pollutant degradation) as well as for renewable energy production, e.g., hydrogen production.⁷,⁸ This method stands out due to its low cost and high efficiency, which can be conducted under solar radiation. A wide range of metal oxide semiconductors have been applied as photocatalysts because they have interesting optical and electrical properties as well as high surface areas, which enhance their performance in photocatalytic applications.⁹ TiO₂ and ZnO are the benchmark photocatalysts to promote organic pollutant degradation and H₂ production.¹⁰-¹⁵ However, these semiconductors face several challenges for practical applications, such as fast charge recombination and low stability. For this reason, many other semiconductor classes have been studied, for example, the perovskite materials based on Ti (BaTiO₃, SrTiO₃, CaTiO₃).¹⁶,¹⁷ Among a variety of photocatalysts, niobium pentoxide (Nb₂O₅) has been investigated owing to its suitable electronic and morphological properties for light-driving reactions and due to its characteristics very similar to TiO₂.¹⁸-²⁰ Nb₂O₅ exhibits more than 15 different structural configurations, and its oxides exhibit different properties depending on the crystalline phase, preparation and desired application.²¹ Thus, Nb₂O₅ is a very complex material to evaluate, and its understanding is still noticeably insufficient.²²-²⁴

Some studies have shown that Nb₂O₅ samples exhibit a remarkable photocatalytic performance. For example, Su et al.²⁵ developed a modified sol-gel with a calcination step at 550 ºC to obtain Pt over 2D Nb₂O₅ nanoparticles. This material presented a high photoactivity for the reaction of phenylacetylene hydrogenation. Guo and co-workers²⁶ developed a two-step synthesis route (hydrothermal and calcination) to obtain N-doped Nb₂O₅ microsphere. They observed that N doping increased the charge carrier lifetime of Nb₂O₅ and resulted in higher photocatalytic performance for pesticide degradation.²⁷ Ücker et al.²⁸ evaluated the photocatalytic performance of Fe inserted in Nb₂O₅ obtained by microwave-assisted hydrothermal synthesis. They stated that Fe insertion in the Nb₂O₅ increased the charge carrier lifetime and resulted in the improvement of photocatalytic performance for Rhodamine B degradation. Cui et al.²⁹ reported a strategy to improve the photocatalytic performance of the commercial TiO₂ (Degussa P25) by addition of Nb₂O₅. The composite was obtained by annealing a mixture of TiO₂ and Nb₂O₅ at 550 ºC. The samples were applied in the photodegradation of 1,4-dichlorobenzene; it was necessary 10 h to promote the degradation of only 60% of the organic pollutant. Peng et al.³⁰ proposed the preparation of a ternary Ag/Nb₂O₅@Nb₂CT_x nanohybrid material through a hydrothermal method followed by an Ag photodeposition. The composite materials were applied as photocatalyst for hydrogen evolution reaction. Despite the high Nb₂O₅’s photoactivity for organic pollutants degradation and hydrogen evolution reaction, Nb₂O₅ samples are generally obtained by a calcination step in a high temperature (> 500 ºC) that results in material with a low specific surface area and density of active sites, which is a key property for application in heterogenous process.³¹ Additionally, Nb₂O₅ is a very complex material due to its many phases and polymorphs, and many works reported contradictory or inconsistent information.²²,²³ Therefore, obtaining Nb₂O₅ nanoparticles with controlled particle size, morphology, specific surface area and crystalline phase is essential to advance the application of this material.²⁴

Therefore, the main objective of this work is to investigate the effect of synthesis parameters (different synthesis methods and niobium precursor) on the physical-chemical properties of Nb₂O₅ and its photocatalytic performance in the degradation of organic pollutants and H₂ production. The photocatalytic performance of the Nb₂O₅ samples and the degradation mechanisms involved reaction were evaluated using Rhodamine B (dye) and amiloride (drug) as probes. Nb₂O₅ modified with Pt nanoparticles was also applied as a photocatalyst for H₂ production.

Experimental

Nb₂O₅ synthesis

In order to investigate the precursor effect on the Nb₂O₅ synthesis by oxidant peroxo method with crystallization by hydrothermal conditions, two different precursors were employed: niobium ammonium oxalate (NH₄NbO(C₂O₄)₂(H₂O)₂]nH₂O) and commercial niobium oxide (Nb₂O₅, optical grade, > 99.8%) both provided by Companhia Brasileira de Metalurgia e Mineração (CBMM), Brazil. In both cases, it was added 5 mmol of the Nb precursor in 100 mL of distillated water. To that suspension, it was added 6.7 mL of hydrogen peroxide (H₂O₂) at the molar ratio of 1:10 of Nb:H₂O₂, according to the peroxide oxidation synthesis.²⁰,³² The addition of H₂O₂ to Nb₂O₅ generated the formation of the niobium peroxocomplex, which was confirmed by the formation of a yellow solution. This solution was added to teflon jar and hydrothermally treated at 120 °C for 18 h. After the reaction, the material was washed twice with distillated water and once with isopropanol. The obtained material was dried at 50 °C for 5 h. The sample obtained from the niobium ammonium oxalate precursor was named Nb₂O₅ A.H. and the sample synthetized using commercial Nb₂O₅ was labeled as Nb₂O₅ C.H. To investigate the effect of the crystallinity degree of Nb₂O₅, the niobium ammonium oxalate precursor was annealed to obtain a material with high crystallinity and orthorhombic crystalline phase.³³ To achieve this aim, 2 g of this precursor were annealed at 600 °C for 2 h, at 5 °C min^-1. This material was named Nb₂O₅ A.C. The samples were labeled according to the synthesis method and precursor employed as can be seen in the Table 1.

Table 1
Nb₂O₅ samples label according to the preparation conditions

Sample	Precursor	Synthesis method
Nb2O5 A.H.	ammonium niobium oxalate	hydrothermal
Nb2O5 A.C.	ammonium niobium oxalate	annealing
Nb2O5 C.H.	commercial Nb2O5	hydrothermal
Nb2O5 C	commercial Nb2O5	no treatment

Characterization

The Nb₂O₅ powder samples synthesized were characterized by X-ray diffraction (XRD) on a Shimadzu XRD 6000 diffractometer (Kyoto, Japan) using Ni-filtered Cu Kα radiation (λ = 1.5405 Å). The XRD patterns were taken over the 2θ range of 10-70 degrees with a scan speed of 1 degree min^-1; Fourier transform infrared (FTIR) analyses were performed on a PerkinElmer Frontier spectrometer (Massachusetts, United States) using spectral resolution of 4 cm^-1 in the range of 4000 400 cm^-1. Raman spectroscopy analysis was performed at Horiba LabRAM HR Evolution (Kyoto, Japan), with laser excitation of 532 nm, potential filter 3.2%, 600 lines mm^-1 and 6 scans accumulation. The specific surface area of the samples was estimated by N₂ physisorption analysis at 77 K in the Micrometrics ASAP 2000 equipment (Norcross, Georgia, USA) applying BET (Brunauer-Emmett-Teller) model. Prior to the analyses, the samples were pre-treated (degasification) by heating at 80 °C under vacuum until reaching a degassing pressure lower than 10 μmHg.

Scanning electron microscopy (SEM) was conducted on a TESCAN microscope (Brno, Czech Republic) running at 5 kV. X-ray dispersive energy spectroscopy (EDX) was used to determine qualitatively the elemental composition of the Nb compounds. Transmission electronic microscopy (TEM) was conducted on Hitachi HT7700 microscope (Kyoto, Japan). Diffuse reflectance spectra (DRS) in the ultraviolet-visible region were recorded between 200 and 800 nm, at room temperature, using a UV-Vis Shimadzu UV-2600 (Kyoto, Japan) with sphere of integration (ISR-2600 Plus) operated in diffuse reflectance mode to determine the band gap of the materials.

Evaluation of Nb₂O₅ photoactivity

The photocatalytic performance of the Nb₂O₅ samples was evaluated using two different organic contaminants as probes, Rhodamine B dye (RhB, Sigma-Aldrich, Saint Louis, Missouri, USA) and amiloride drug (Sigma-Aldrich, Saint Louis, Missouri, USA). The photoactivities were studied under UV radiation using Lamps Phillips (Eindhoven, Netherlands) TUV, 15 W and maximum intensity at 254 nm, with medium intensity of irradiance of 40 mW m^-2 in a photoreactor at 18 °C. To perform the photocatalytic tests, 25 mg of each photocatalyst were dispersed in 50 mL of aqueous RhB (5 mg L^-1, pH = 6.3) or amiloride (10 mg L^-1, pH = 5.5). At regular time intervals, the samples were collected and analyzed in a UV-Vis spectrophotometer Evolution 201/220, ThermoFisher Scientific (Massachusetts, USA). RhB and amiloride were monitored by its maximum absorption peak at 532 and 286 nm, respectively. The samples in dye or in drug solution were kept in the dark for 12 h before the photocatalytic experiment to reach the adsorption-desorption equilibrium.

The organic pollutant photodegradation mechanism driven by the Nb₂O₅ photocatalysts was investigated by using scavengers of active species. This experiment consisted of adding reactive species scavengers directly to the samples containing RhB dye. In this procedure, tert-butanol (t-BuOH, 100 μmol L^-1), ethylenediaminetetraacetic acid (EDTA, 50 μmol L^-1), or benzoquinone (BQ, 50 μmol L^-1) were added as scavengers for radical ^•OH radicals, valence band (VB) photogenerated holes (h⁺), and conduction band (CB) photogenerated electrons (e^-), respectively.³⁴,³⁵

Photocatalytic production of H₂

The sample Nb₂O₅ A.H. presented the highest photoacatalytic performance for organic pollutant degradation, so it was chosen to be used as catalyst for H₂ production. To improve the sample’s photocatalytic performance, it was modified by the deposition of Pt nanoparticles on the surface using a photodeposition method, largely employed in the literature.³⁶-³⁸ Pt modification was carried out by the addition of 1 mL of 2.09 × 10^-3 mol L^-1 H₂PtCl₆ aqueous to 40 mg of the Nb₂O₅ A.H. suspended in 20% v/v methanol/water. The mixture was stirred for 24 h under UV irradiation (315 380 nm, 24 W), and afterward, the solid was filtered, washed with deionized water, and dried at 80 °C. The final metal weight percentage in relation to the sample was 1% wt.

The photocatalytic performance of the Nb₂O₅ to produce H₂ were carried out at 25 °C in a septum-sealed 18 mL borosilicate double-walled reactor connected to a thermostatic bath. The photocatalysts (5 mg) were added to methanol/water solution (20% v/v) and kept under magnetic stirring. The reactor was purged with argon for 10 min and then exposed to irradiation of xenon lamp (300 W, λ > 320 nm) placed in a 67011 Newport lamp house connected to a 69911 source, with 120 mW cm^-2 of irradiance during 8 h. The irradiance was measured using Powermeter Newport 1916-R (Andover, Massachusetts, US). At regular time interval, 0.5 mL gas samples were collected from the headspace of the reactor and analyzed by gas chromatography using a Clarus 580, PerkinElmer (Massachusetts, US) equipped with two columns (Porapak N 2 mm and molecular sieve) and one detector of thermal conductivity. The measurements were performed at 120 °C on the injector and 150 °C on the detector, with thermal ramp of 35-120 °C. Ar was used as carrier gas at 30 mL min^-1.³⁷

Results and Discussion

Microstructural characterization

The structural and phase formation analysis in the crystalline lattice of the Nb₂O₅ samples synthetized were evaluated by XRD, as can be seen in Figures 1a-1b. It can be verified that the Nb₂O₅ samples are crystalline. Nb₂O₅ A.H. showed characteristic peaks of the pseudohexagonal (TT) crystalline phase of Nb₂O₅ (JCPDS: 28-317).²⁰,³²,³⁹ The TT-Nb₂O₅ is a metastable phase and it can be stabilized due to the presence of impurities at the crystal lattice.³² This sample exhibits also two large diffraction peaks at 12 and 26 degrees, related to niobium oxide hydrated (Nb₂O₅.nH₂O).²⁰,⁴⁰ On the other hand, the sample Nb₂O₅ A.C., obtained by the annealing of the ammoniacal complex precursor, exhibits characteristic peaks related to the orthorhombic (T) crystalline phase of Nb₂O₅ (JCPDS No. 00-027-1313).³³ The sample Nb₂O₅ A.C. showed peaks with a higher intensity and well-shaped in comparison to Nb₂O₅ A.H., because the calcination in high temperatures provides enough thermal energy for the material’s crystallization. Therefore, when niobium ammonium oxalate was used as precursor, the synthesis method (hydrothermal or calcination) had significant impact on the structural materials properties.

Figure 1
XRD patterns of Nb₂O₅ samples obtained by different methods and using as precursor: (a) ammonium oxalate complex of niobium, where * is referent to Nb₂O₅.nH₂O and the indexed peaks are referent to the TT- Nb₂O₅, JCPDS: 28-317 phase; (b) commercial Nb₂O₅, where X and ° represents the crystalline phases, T-Nb₂O₅ and H-Nb₂O₅, respectively.

The XRD patterns for commercial Nb₂O₅ (Nb₂O₅ C) exhibit characteristic peaks of the orthorhombic (T) and monoclinic (H) crystalline phases (JCPDS No. 01 72 1121), presenting a phase mixture (Figure 1b) that agrees with the literature.⁴¹ The hydrothermal treatment using commercial Nb₂O₅ as precursor does not result in significant modifications of the XRD profile of the sample Nb₂O₅ C.H. Therefore, the hydrothermal treatment did not show a significant effect at the crystalline structure of the sample Nb₂O₅ C.H. This could be related to the high stability and insolubility of the commercial Nb₂O₅, for example, it was required a thermal treatment higher than 900 ºC to modify the structure of the Nb₂O₅ C sample.⁴²,⁴³

Figure 2a shows the FTIR spectra of the Nb₂O₅ samples. All the samples present bands in the region of 1000 500 cm^-1 which are characteristics of Nb-O bonding. The bands around 718 and 500 cm^-1 are related to Nb=O stretch and angular vibrations of Nb-O-Nb.⁴⁴ The sample Nb₂O₅ A.H. shows bands around 1720-1410 cm^-1 attributed to C-O and C=O that can be related to oxalate and carbonyl groups from the synthesis residues. The presence of these residues may be responsible for the stabilization of the pseudohexagonal metastable phase of Nb₂O₅. Weissman et al.⁴⁵ proposed that niobium atoms at TT-Nb₂O₅ phase are in more symmetric Wyckoff positions, which can produce an open unit cell, with a more evenly spaced position to accommodate impurities and defects.²⁰,⁴⁵-⁴⁷ This sample also presents centered bands around 3357 cm^-1 that can be attributed to hydroxyl groups adsorbed on the surface of the sample. It is worth pointing out that the presence of hydroxyl groups at the surface of the semiconductors may affect positively the photocatalytic performance of the sample, because the photogenerated holes may oxide the hydroxyl groups and generate hydroxyl radicals (^•OH) that promote the degradation of organic pollutants.⁴⁸-⁵⁰ This happens because the adsorption of the hydroxyl group can decrease the reduction potential to convert it in hydroxyl radicals. The Nb₂O₅ A.C. sample did not exhibit the presence of IR bands related to the carboxyl and hydroxyl groups; it means that the calcination step at high temperature was efficient to remove the synthesis residues. Additionally, the hydrothermal treatment of commercial Nb₂O₅ was not enough to cause significant modification in its functional groups on the samples surface.

Figure 2
(a) FTIR-ATR and (b) Raman spectra for Nb₂O₅ samples obtained by different synthesis methods and different precursors.

The Nb₂O₅ samples were characterized by Raman spectroscopy (Figure 2b). The Nb₂O₅ A.H. sample shows large and less defined peaks in comparison to the other samples, which can be related to TT-Nb₂O₅ phase. The peaks around 685 and 228 cm¹ are attributed to octahedral coordination of NbO₆ and to vibrational modes of Nb O Nb bonding, respectively. NbO₆ of low distortion is characteristic of samples with low crystallinity degree.⁵¹,⁵² The sample Nb₂O₅ A.C. showed sharper peaks and a displacement of the main peak from 685 to 700 cm^-1, related to the crystalline phase changes. The small peak at 890 cm^-1 may be attributed to low concentrations of the surface groups Nb=O. These observed modifications reinforce the conclusion that the calcination promoted the formation of Nb₂O₅ on the orthorhombic phase, as observed by XRD results. The commercial Nb₂O₅ exhibits defined peaks and additional peaks around 900 and 997 cm^-1, attributed to the symmetric elongation mode and antisymmetric mode of Nb=O terminal bonds, respectively.⁵¹,⁵² These peaks may be related to the monoclinic phase of Nb₂O₅, in agreement with XRD results. The hydrothermal treatment on commercial Nb₂O₅ resulted in peaks more sharp and intense, which indicates the increase of structural ordering of the sample, caused by the hydrothermal treatment.

The electronic properties of the Nb₂O₅ samples were characterized by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) (Figure S1, ^{Supplementary Information (SI)} section). The band gap energies of the synthetized samples were determined applying the Tauc equation to the DRS data:

(1) $$(\alpha hv{)}^{\frac{1}{n}}=A(hf-E_g)$$

where, α, h, f, E_g, A are the absorption coefficient, Planck’s constant, radiation frequency, band gap energy and a constant, respectively. The n constant depends on the nature of the electron transition and is equal to 1/2 or 2 for the direct and indirect transition band gaps, respectively. Nb₂O₅ exhibits an indirect transition band gap. Tauc plots with the respective value of band gap energy for all Nb₂O₅ samples can be observed in Figure 3. Nb₂O₅ A.H. sample exhibited the higher value for band gap energy in comparison to the other samples, around 3.3 eV. The other samples exhibited values of band gap around 3.1 eV, which indicates that the orthorhombic and monoclinic phases of Nb₂O₅ presented a band gap value slightly lower than the pseudohexagonal phase. The values of the band gap and the observed tendency agree with other studies.⁵³,⁵⁴ Because the Nb₂O₅ A.H. sample presented a band gap energy higher than the other samples, it can result in band edge levels with higher redox potential to promote the reactions of organic pollutant degradation and hydrogen evolution.⁵⁴-⁵⁶

Figure 3
Tauc plot obtained from UV-Vis diffuse reflectance spectroscopy data of the samples: (a) Nb₂O₅ A.H., (b) Nb₂O₅ A.C., (c) Nb₂O₅ C and (d) Nb₂O₅ C.H.

Specific surface area, pore size and pore volume average were determined from nitrogen adsorption/desorption isotherms, Table 2. Figure 4 shows the N₂ adsorption-desorption isotherms of Nb₂O₅ samples. The N₂ adsorption-desorption isotherms of the three samples exhibit typical type IV with hysteresis loop type H2 of International Union of Pure and Applied Chemistry (IUPAC) classification, which is associated with capillary condensation taking place in mesopores materials. Table 2 shows that Nb₂O₅ A.H. sample exhibits a specific surface area (SSA) value around 9 and 20 times higher than Nb₂O₅ A.C. and Nb₂O₅ C samples, respectively. The same trend is observed when evaluating the total pore volume. All samples exhibit average pore size in the range of 2.6 to 6.7 nm, which can be classified as mesoporous materials. Additionally, the samples do not exhibit a plateau at high-pressure values, suggesting the presence of a wider pore size distribution.

Table 2
Band gap values obtained by Tauc equation and N₂ physisorption data of Nb₂O₅ samples

Sample	Band gap / eV	SSA / (m2 g-1)	Pore volume / (cm3 g-1)	Pore size / nm
Nb2O5 A.H.	3.3	267	0.55	4.7
Nb2O5 A.C.	3.1	29	0.09	6.7
Nb2O5 C.H.	3.1	-	-	-
Nb2O5 C	3.1	13	0.02	2.6

SSA: specific surface area.

Figure 4
N₂ adsorption-desorption isotherms of Nb₂O₅ samples.

The morphology of the synthetized samples was characterized by SEM, as can be seen in Figure 5. All the synthetized samples exhibit particles with spherical morphology. Nb₂O₅ A.H. exhibited nanometric particles with homogenous size and morphology, which indicates that hydrothermal method was efficient (Figure 5a). However, the calcination of ammonium oxalate complex of niobium resulted on the formation of micrometrics particles agglomerated, caused by the particles calcination at high temperatures (Figure 5b). It agrees with the specific surface area of Nb₂O₅ A.H. and Nb₂O₅ A.C. samples, where the precursor calcination resulted in a material with specific surface area around 9 times smaller than the samples obtained by the hydrothermal route.³³ On the other hand, the comparison between Nb₂O₅ C sample (Figure 5c) with Nb₂O₅ C.H (Figure 5d) did not show any significant modification caused by the hydrothermal treatment.⁵⁷

Figure 5
SEM images of the samples: (a) Nb₂O₅ A.H., (b) Nb₂O₅ A.C., (c) Nb₂O₅ commercial and (d) Nb₂O₅ C.H.

Nb₂O₅ A.H. sample was characterized by transmission electron microscopy and compared with the sample annealed (Nb₂O₅ A.C.), as can be seen in Figure 6. It can be observed that Nb₂O₅ A.H. sample is composed of a spherical agglomeration with size around 50 nm, formed by smaller particles with size around 10 nm. It can be observed two kinds of morphology, spherical and nanorod particles. The calcination of ammonium oxalate complex of niobium (Nb₂O₅ A.C.) resulted in the increase of the particles size (ca. 100 nm), due to the calcination process at high temperatures. It agrees with SEM images and specific surface area results. In Figure 6e is presented the high-resolution transmission electron microscopy (HRTEM) image of the Nb₂O₅ A.H. samples. It exhibits spherical and elongated particles with length size around 10 nm. This sample also presents interlayer distance of 0.37 nm, which is characteristic of the (001) plane of the pseudo-hexagonal crystalline phase of Nb₂O₅, according to XRD results.

Figure 6
TEM images of the samples (a) and (b) Nb₂O₅ A.H., (c) and (d) Nb₂O₅ C.H., (e) HRTEM image of the Nb₂O₅ A.H.

XPS survey spectra confirmed the presence of Nb, O and C in both Nb₂O₅ surface samples (Figure S2a, ^SI section). Carbon peak located at 284.8 eV was used to calibrate the samples. In Figure S2b, the Nb 3d high-resolution XPS spectra show peaks at 207.6 and 210.3 eV, corresponding to Nb 3d_3/2 and Nb 3d_5/2 levels, respectively.³³,³⁵,⁵⁸ It confirms that both samples exhibit niobium in high valence (Nb⁵⁺), as suggested by XRD and Raman spectroscopy results. The XPS results confirmed that it was not observed any segregation of metallic Nb at the surface of the sample.

Photocatalytic performance of Nb₂O₅ samples for organic pollutants degradation

The photocatalytic performance of the synthetized samples was investigated using two organic pollutants, Rhodamine B (dye) and amiloride (drug). Figure 7 shows the RhB photodegradation curves catalyzed by the Nb₂O₅ samples under UV radiation. The direct photolysis caused insignificant degradation of the dye. All the Nb₂O₅ were photoactive to promote the dye degradation. The photocatalytic performance of the synthetized samples was compared to the commercial Nb₂O₅. It can be observed that the hydrothermal treatment on commercial Nb₂O₅ precursor did not increase the photocatalytic activity of the Nb₂O₅ C.H., probably because the hydrothermal treatment did not cause any significant change in the sample’s physical chemical properties. In addition, the precursor calcination of niobium ammonia complex presented a lower photocatalytic performance than the commercial Nb₂O₅. On the other hand, Nb₂O₅ A.H. showed an increased photoactivity in comparison to the reference sample. More than 60% of RhB was degraded after 180 min under UV irradiation. It can be concluded that Nb₂O₅ A.H. sample exhibited an increased photocatalytic activity due to the higher specific surface area and higher amount hydroxyl groups adsorbed on the surface of the material that may act as intermediaries on the hydroxyl radicals formation.⁵⁹ It is important to highlight that Nb₂O₅ samples exhibited different crystalline phases, and the observed order of photoactivity may be also related to the intrinsic activity of each crystalline phase.

Figure 7
(a) Photodegradation curves of Rhodamine B (5 mg L^-1) catalyzed by the Nb₂O₅ samples under UV radiation; (b) second-order kinetics for the RhB degradation catalyzed by the Nb₂O₅ samples.

For a better comprehension of the kinetic process and the photoactivity order, the photodegradation kinetics of the reaction was also studied. Through this study is possible to verify if the reaction follows the same mechanism during the evaluation time or if there was a change in the mechanism, in addition to verify if the different catalysts lead the reaction by the same mechanism.⁴⁸ Then, considering that the light source is constant and the number of active sites of the photocatalyst is dependent only on the superficial area and since no poisoning occurs during this process, it is expected that the reaction follows a first (equation 2) or second order kinetics (equation 3):

(2) $$\mathrm{v}=-\frac{\mathrm{d}[\mathrm{R}\mathrm{h}\mathrm{B}]}{\mathrm{d}\mathrm{t}}=\mathrm{k}[\mathrm{R}\mathrm{h}\mathrm{B}]$$

(3) $$\mathrm{v}=-\frac{\mathrm{d}[\mathrm{R}\mathrm{h}\mathrm{B}]}{\mathrm{d}\mathrm{t}}=\mathrm{k}[\mathrm{R}\mathrm{h}\mathrm{B}{]}^2$$

where, v is the reaction rate; [RhB] is the dye concentration; t is the time and k is the rate constant. Integrating the velocity laws the following linear equations are obtained:

(4) $$\ln \frac{[\mathrm{R}\mathrm{h}\mathrm{B}]}{[\mathrm{R}\mathrm{h}\mathrm{B}{]}_0}=-\mathrm{k}\mathrm{t}$$

(5) $$\frac{[1]}{[\mathrm{R}\mathrm{h}\mathrm{B}]}=\frac{[1]}{[\mathrm{R}\mathrm{h}\mathrm{B}{]}_0}-\mathrm{k}$$

The two linearized equations were applied to the data of RhB dye photodegradation kinetics. It was verified that the second-order kinetics presented a better fit to the experimental data than the first-order one. Therefore, the second-order rate constants are presented in Table 3, where the fitting is shown by the values of coefficient of determination (R²), which corroborates the RhB photodegradation mechanism follows the second-order kinetics. The photoactivity order previously verified is confirmed by numerical values comparation of the rate constant obtained, where the sample Nb₂O₅ A.H. exhibited a k value 3 times higher than the commercial one.

Table 3
Rate constant of the RhB photodegradation reaction (k_RhB) and the coefficient of determination (R²) for the Nb₂O₅ samples

Sample	kRhB × 10-3 / min-1	R2
Nb2O5 A.H.	8.9	0.99
Nb2O5 A.C.	0.7	0.91
Nb2O5 C.H.	1.4	0.91
Nb2O5 C	2.7	0.98

To verify the versatility of the Nb₂O₅ samples for photocatalytic applications, they were applied to the amiloride photodegradation under ultraviolet radiation. Amiloride is a colorless organic pollutant; it is a drug extensively used for the treatment of hypertension, belonging to a class of pharmaceutical compounds that are often found in wastewater and can adversely affect water quality.³⁴Figure 8 shows the amiloride photodegradation curves catalyzed by the Nb₂O₅ samples under UV radiation. All the samples were also active in the amiloride degradation. The Nb₂O₅ A.H. sample also showed a higher photocatalytic performance in comparison to the other samples, with 70% of drug degradation. The other samples were able to degrade only 50% after 120 min. The increased specific surface area and surface hydroxylation of Nb₂O₅ develop a key role in the photocatalytic activity of the samples for organic pollutant degradation. The importance of these parameters is closely related to the adsorption ability of the materials, since a higher specific surface area provides more active sites for molecule adsorption and can also affect the degradation mechanism via indirect or direct mechanism.⁶⁰,⁶¹

Figure 8
Photodegradation kinetics of amiloride (10 mg L^-1) catalyzed by the Nb₂O₅ samples under UV radiation.

The reaction mechanisms responsible for the pollutant’s degradation were investigated by the photocatalytic essays in the presence of main active species scavengers, hydroxyl radical (^•OH), superoxide radical (O₂^-•) and photogenerated hole at valence band of the semiconductor. There are three main mechanisms that may be involved in the photocatalytic degradation process of pollutants: direct mechanism, indirect mechanism and photosensitization process.⁵⁹ Then, the degradation mechanism was investigated using the following main active species scavengers: 1-butanol (^•OH scavenger), EDTA (h⁺ scavenger) and benzoquinone (O2^-• scavenger).⁶²,⁶³ The sample Nb₂O₅ A.H. was chosen to be applied in the study of the reaction mechanisms, because this sample presented the highest photoactivity, as observed by photodegradation experiments in Figures 7 and 8. Figure 9 shows that all scavengers employed promoted a decreasing in the photoactivity of the Nb₂O₅ A.H. sample. This is indicative that two radicalar species (^•OH and O₂^-•) and the hole photogenerated in the VB take part in the organic pollutant photodegradation. However, it can be observed that benzoquinone was the most efficient scavenger to decrease the photoactivity of Nb₂O₅ A.H. sample, which demonstrates that O₂^-• radical is the main radicalar species responsible for the organic pollutant photodegradation. Additionally, it can be observed that 1-butanol and EDTA caused similar reduction in the photocatalyst’s activity, which means that the hole photogenerated in the VB did not present an important role, because it is just an intermediate in the formation of ^•OH radicals. Therefore, this result agrees that the indirect mechanism is the main responsible for the organic pollutant degradation, in agreement with a previous work of our group.²⁰

Figure 9
Photodegradation kinetics of RhB (5 mg L^-1) catalyzed by Nb₂O₅ A.H. samples in the presence of 1-butanol (100 μmol L^-1), EDTA and benzoquinone (50 μmol L^-1).

Then, it can be proposed that photogenerated hole (h⁺) at valence band of Nb₂O₅ could react with adsorbed water or hydroxyl group to produce hydroxyl radical (^•OH). In the presence of dissolved oxygen, the photogenerated electron at conduction band (e^-) could be transferred to oxygen molecule, leading to the generation of superoxide radical (O₂^•-) in the subsequent reaction.⁵⁶ The conduction band edge (E_CB) of Nb₂O₅ presents a more negative reduction potential than the oxygen molecule reduction potential, therefore, the electron transfer is spontaneous.⁵⁸,⁶⁴

The stability experiment (Figure 10) was performed under five cycles of photocatalytic experiment for RhB degradation using the Nb₂O₅ A.H. sample that presented the higher photoactivity (Figures 7 and 8). It can be observed that Nb₂O₅ A.H. sample shows a slight deviation in the efficiency through the five cycles, with a RhB dye degradation percentage ranging from 60 to 70%. However, it ultimately maintained constant efficiency across the five cycles, proving its reusability for several heterogeneous photocatalytic cycles.

Figure 10
Stability of the Nb₂O₅ A.H. sample under five photocatalytic experiments cycles for RhB dye degradation.

Photocatalytic performance of Nb₂O₅ for H₂ production

The photodegradation mechanism results demonstrated that Nb₂O₅ A.H. sample exhibited the ability to produce O₂^-• radicals. It is an indicative that this sample is also able to promote the H₂ evolution reaction since the hydrogen formation exhibits a lower electrochemical reduction potential than the O₂^-• radicals formation. Therefore, Nb₂O₅ A.H. sample was employed as a photocatalyst to promote the H₂ evolution reaction, using 20% v/v methanol/water mixtures under UV-Vis light (λ > 350 nm). For comparison, Nb₂O₅ A.H. samples with and without Pt nanoparticles were also evaluated under the same conditions. The pristine Nb₂O₅ A.H. sample was not able to produce significative amounts of H₂; however, the Pt nanoparticles (1 wt.%) modified sample was photoactive to produce H₂, as can be seen in Figure 11. The low photoactivity of the pristine Nb₂O₅ A.H. sample can be related to the fast recombination of the photogenerated electrons on conduction band of the semiconductor and due to the weak interaction between H⁺ and photocatalyst surface. Then, the modification of the surface of the sample by Pt nanoparticles deposition can overcome both drawbacks. Pt is considered a cocatalyst and it is an excellent electron acceptor, which results in an increased charge carrier lifetime of the photocatalyst. Additionally, Pt can also act as proton reduction sites, due to its good interaction with H⁺.⁶⁵,⁶⁶ Nb₂O₅ A.H. modified with Pt was able to produce around 4 mmol g^-1 of H₂ after 8 h of reaction. Our sample presented a similar result to Nb related compounds, for example, modified hexaniobate (K_4-xH_xNb₆O₁₇) with different cocatalysts, such as Co, Fe and Au.³⁷,⁶⁷

Figure 11
Photocatalytic hydrogen evolution from H₂O/methanol mixtures (20% v/v) catalyzed by Nb₂O₅ A.H. sample with and without Pt nanoparticles.

Conclusions

In summary, the synthesis method and the Nb precursor employed played an important role in the structural, morphologic, and surface properties of the Nb₂O₅ samples obtained. All samples were photoactive for organic pollutant photodegradation (Rhodamine B dye and amiloride drug). Nb₂O₅ sample obtained by hydrothermal method using ammonium niobium oxalate as Nb precursor exhibited the highest photocatalytic performance for RhB dye and amiloride drug degradation because this sample has the smaller particle size, higher specific surface area and concentration of hydroxyl groups on the surface. The photodegradation mechanism evaluation showed that indirect mechanism mediated by radicalar species (O₂^-• and OH^•) was the main responsible for the organic pollutant degradation. The modification of Nb₂O₅ with Pt resulted in a material with good performance for H₂ production. Therefore, Nb₂O₅ is a semiconductor with excellent photocatalytic properties to be applied in water effluent treatment (organic pollutants photodegradation) and for renewable energy production (H₂ production).

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br as PDF file.

Acknowledgments

The authors acknowledge FAPEMIG (Minas Gerais Research Foundation, grant: APQ-00282-21, BPD 00811 22, BPD-00165-22 and RED-00175-22) and CNPq (Brazilian National Council for Scientific and Technological Development, grants: 407497/2018 8, 405819/2022-6 and 150371/2023-3) for the financial support. The authors thank to GMIT group and MSc Lucas Leão (UFU) for N₂ physisorption measurements. O. F. Lopes also acknowledges Alexander von Humboldt Foundation by Return Fellowship Grant.

• This manuscript is part of a series of publications in the Journal of the Brazilian Chemical Society by young researchers who work in Brazil or have a solid scientific connection with our country. The JBCS welcomes these young investigators who brighten the future of chemical sciences.

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Edited by

Editor handled this article: Izaura C. N. Diógenes (Associate)

Publication Dates

• Publication in this collection
  26 Feb 2024
• Date of issue
  2024

History

• Received
  30 Apr 2023
• Accepted
  20 Oct 2023