1. Structure and morphology study
The structure, composition, and crystallinity of the different powders powder synthesized by the hydrothermal process are examined by X-ray diffraction (XRD) (Fig. 1). The XRD result of the powder obtained after the first hydrothermal treatment (Fig. 1a) shows the existence of very fine and intense peaks and these various peaks are indexed to the crystalline phase Nb2O5 (JCPDS data file 00-27-1003) [24]. The absence of any additional peaks from other phases or impurities indicates that Nb2O5 with high purity can be synthesized via hydrothermal synthesis at 180°C for 72 h. Whereas the XRD analysis of the powder obtained after the second hydrothermal process (Fig. 1b) shows a slight increase in the width of the XRD peaks. The diffractogram highlights the presence of a series of peaks that are perfectly indexed to the crystal phase Nb2O5 while a new peak located at 23.6° was detected. This peak could be identified as the reduced graphene oxide (rGO) phase. This result confirms the presence of composite materials based on Nb2O5 and (rGO).
The average crystallite size of the as-synthesized materials was calculated using Scherrer’s formula:
L = 0.9λ/(βcosƟ) (Eq. 1)
where L is the average crystallite size, λ = 0.154056 nm, β is the half maximum peak width and Ɵ is the diffraction angle in degrees. The average crystallite size value calculated from XRD patterns of Nb2O5 and Nb2O5/rGO composite is about 30 ± 5 nm and 80 ± 10 nm respectively.
The structure information of the two materials (Nb2O5 and Nb2O5/rGO composite) was studied by Raman spectroscopy (Fig. 2). For the Nb2O5 (Fig. 2a) the analysis of the Raman spectrum exhibits well-defined bands located at 124, 233, 308, and 698 cm− 1. According to the literature, the band located at 124 cm− 1 is assigned to the υ1 stretching vibrations of octahedrons as a whole [25–27]. The Raman band at 233 and 308 cm− 1 observed for all materials is characteristic of the bending modes υ2 and υ3 vibrations of cations located inside the octahedrons and tetrahedrons [25–27]. The broad peak located at 698 cm− 1 was attributed to the Nb-O-Nb bridging bond of distorted NbO6 [25–27]. Figure 2b shows the Raman spectrum of the composite materials Nb2O5/rGO. The analysis of the spectrum reveals the presence of the first series of peaks that are attributed to the different vibrations characteristic of the Nb2O5 phase and we show also the presence of two broad peaks characteristic of the rGO bands. The two predominant peaks appear at about 1331 and 1596 cm− 1 in the spectrum, assigned as the D band originating from the disordered carbon and the G band corresponding to the sp2 hybridized carbon, respectively [28, 29]. The G band is associated with the vibrations of sp2 carbon atoms in reduced graphene while the D band is assigned to the vibrations of sp3 carbon atoms. These two bands confirm the presence of reduced graphene (rGO) in the niobium oxide nanocomposite.
Figure 3 show the corresponding SEM images of the Nb2O5 and Nb2O5/rGO composite powders elaborated by the hydrothermal process. The analysis of SEM images of the Nb2O5 nanoparticles (Fig. 3a) shows that the Nb2O5 phase is composed of a homogeneous phase with an agglomeration of the particles to form spherical urchins like morphology. However, the SEM image of the composite material (Fig. 3b) shows that the Nb2O5 /rGO composite consists of two types of morphologies: spherical particles due to the presence of Nb2O5 and platelets fused between them to form a reduced graphene block. The results observed on the composite material confirm the existence of two morphology distributions (niobium and reduced graphene). The SEM result confirms the result obtained by XRD and Raman spectroscopy. The textural properties including specific surface area BET (SBET), pore volume (Vpor), and average pore size (dpor) of Nb2O5 nanospheres and Nb2O5/rGO composite were investigated by the studies of nitrogen adsorption-desorption using the Brunauer-Emmett-Teller method (BET). Figure 4 show the N2 adsorption-desorption isotherms for Nb2O5 and Nb2O5/rGO, respectively. The isotherms show a typical IUPAC-type IV-like adsorption/desorption behavior with H1 hysteresis, indicating cylindrical pores. For Nb2O5, the obtained results are SBET = 65 m2.g− 1, Vpor = 76x10− 3 cm3 g− 1 and dpor = 91.65 Å. In contrast, for Nb2O5/rGO, SBET = 105.25 m2.g− 1, Vpor = 87x10− 3 cm3 g− 1 and dpor = 94.92 Å. The analysis of the experiences results reveals that the surface area increases with adding of the rGO.
2. Photocatalytic studies
The photocatalytic performance of the two catalysts-based niobium oxide (Nb2O5 and Nb2O5@rGO composite) was evaluated for photodegradation of an aqueous methylene blue (MB) as a probe pollutant under sunlight irradiation (Fig. 5). The photodegradation of the MB in presence of the nano-Nb2O5 as photocatalyst under different intervals of time (0–200 min) as shown in Fig. 5a. The analysis of the results shows that only 40% of MB was degraded after 200 min of illumination. For the Nb2O5@rGO composite, we can see that the MB dye are almost totally adsorbed and decomposed after 200 min. The Improvement of photocatalytic properties observed for Nb2O5@rGO composite is due to the bigger surface-to-volume ratio leading to a greater number of available active sites and, consequently, to an enhancement of the photocatalysis process. Figure 6 shows the variation of the C/C0 of an aqueous solution of the MB vs. irradiation time in presence of the different catalysts-based Nb-oxide. The direct irradiation of MB in presence of the Nb2O5 commercial as photocatalyst resulted in about 38% degradation after 200 min irradiation under sunlight irradiation. However, in the case of homemade Nb2O5 catalyst we observe a degradation of the order of 60% after 200 min under sunlight. This enhancement in the photocatalytic properties between Nb2O5 commercial and homemade Nb2O5 can be explained by the nanometric aspect of the synthesized Nb2O5 which favors an important surface reactivity compared to the commercial Nb2O5. In the presence of the Nb2O5@rGO composite as catalyst we observe a total degradation of the MB after 200 min irradiation under sunlight (Fig. 6).
The reaction kinetics can be observed by plotting linear curves for the concentration ratio, ln(C/Co), against the irradiation time (Fig. 7). When the data are plotted as logarithms of normalized dye concentration versus time, linear plots are obtained (Fig. 7). The photocatalytic experimental data indicates that the decomposition follows the first-order kinetics model, based on the Langmuir-Hinshelwood equation. This model is defined by:
Ln(C/C0) = kt (Eq. 2)
with C is the concentration at the instant t, C0 is the initial concentration and k is the kinetic constant. The k values were 3.23x10− 4, 3.64x10− 3, and 4.79x10− 2 min− 1, for Nb2O5 commercial, homemade-Nb2O5, and Nb2O5@rGO composite respectively. According to the kinetic analysis, we show that the introduction of rGO in Nb2O5 phase enhanced the photocatalytic properties of the Nb2O5 catalyst. In fact, the low degradation of the MB in the presence of commercial Nb2O5 is explained by the micrometric size of the particles which prevents a total dissolution of the catalyst. Such values are nicely compared with values reported in the literature. Recently, Rathnasamy et al. [30] prepared sphere-like niobium pentoxide (Nb2O5) nanostructures through a one-pot solvothermal method and employed this nanoparticle as a photocatalyst for removal of methylene blue (MB) and rose bengal (RB) dyes under ultraviolet (UV) light. This catalyst-based Nb2O5 nanostructure degrade up to 87% and 62% of MB and RB dye after irradiation for 90 and 180 min, respectively. In the same context, Liu et al [31] reported that the using of the hexagonal-like Nb2O5 nanoplates as a photocatalyst leads to 92% for methylene blue degradation.
5. Mechanism
Niobium pentoxide (Nb2O5) is an n-type semiconductor characterized by band gap energy value 3.0-3.5 eV. Nb2O5 is used as a photocatalyst thanks to its redox activities. Several studies show that the performance of Nb2O5 as a catalyst for the degradation of pollutants depends on several parameters such as the particle size, the morphology of the materials, and band gap energy. Based on previous studies and the results obtained results through this study, a possible degradation mechanism of MB can be proposed (Fig. 8).
Under visible light illumination, electrons (e-) absorb light energy in the Valence Band (VB). If the incoming energy is greater than the bandgap energy, electrons will go from the VB to the Conduction Band (CB). In this process, empty holes (h+) will be left in the VB. From the CB, electrons can move to the surface of the Nb2O5 photocatalyst to react with dissolved oxygen molecules, producing reactive oxygen species, which include superoxide radical O2•−. At the same time, holes moving to the surface from the VB react with water molecules to give hydroxyl radicals (OH•). Finally, holes themselves and both generated radicals oxidize MB dye molecules to produce intermediate species, which can be further oxidized to form carbon dioxide and water. Notably, holes and electrons can recombine to produce heat as an energy. The overall photocatalysis reaction scheme can be written as follows:
Nb2O5 + hν → h + + e- (Eq. 3)
H2O + h+ → OH• + H+ (Eq. 4)
O2 + e- → O2•− (Eq. 5)
h + + e- → Energy (heat) (Eq. 6)
h + + MB → Intermediates → H2O + CO2 (Eq. 7)
OH•+ MB → Intermediates → H2O + CO2 (Eq. 8)
O2•−+ Pollutant → Intermediates → H2O + CO2 (Eq. 9)