Well-wrapped reduced graphene oxide nanosheets on Nb₃O₇(OH) nanostructures as good electron collectors and transporters for efficient photocatalytic degradation of rhodamine B and phenol

Abstract

Graphene-based nanocomposites have attracted considerable attention in photocatalytic research owing to their remarkable potential for the photodegradation of environmental pollutants. However, despite the progress made in this field, the development of visible-light-active photocatalysts with high activity and durability remains a challenge. In this work, bunches of Nb₃O₇(OH) nanorods wrapped in reduced graphene oxide (RGO) nanosheets were prepared by using a hydrothermal method. The photocatalytic activity of the as-synthesized Nb₃O₇(OH)-RGO nanocomposites was significantly enhanced compared to that of the pure Nb₃O₇(OH) nanostructures owing to their improved visible-light absorption and separation of photogenerated electron–hole pairs. Photoluminescence studies strongly supported the proposed charge separation and charge transport mechanism. Moreover, the photocatalytic efficiency was strongly dependent on the concentration of RGO in the nanocomposites. The highest photodegradation rate was obtained using the nanocomposite prepared with a graphene loading of 3 mg mL⁻¹, and when the RGO loading exceeded 3 mg mL⁻¹, the photodegradation efficiency decreased. This occurred because excess RGO nanosheets aggregated and hindered the absorption of incident light. We believe that this work provides invaluable information for the design of new efficient visible-light-active reduced graphene oxide-based photocatalysts to be used in water remediation through the oxidative degradation of organic dyes and toxic phenols.

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Introduction

A clean and plentiful drinking water supply is essential for the foundation of prosperous communities. However, the rapid industrialization of rural areas and the lack of efficient and economic water-treatment technologies are causing a reduction in the availability of clean drinking water. These industries lead to pollution of water resources by the discharge of wastewater containing toxic chemicals or disease-carrying organisms.¹ Established purification techniques, such as coagulation, filtration, screening, centrifugation, sedimentation, adsorption, and distillation, are widely available;² however, these techniques are unsuitable for the removal of low-level persistent organic pollutants. To overcome this difficulty, advanced photo-oxidation processes using semiconductor nanostructures under light irradiation have been recently developed.³ Using these advanced techniques, low-level persistent organic pollutants can be easily degraded into harmless mineralized end products, such as CO₂, H₂O, and NH₃.⁴ TiO₂ semiconductor nanostructures are among the most widely used catalysts for photocatalytic oxidation. However, a major drawback of TiO₂ is its large band gap (3.2 eV), which means that it only utilizes UV light, thus seriously hindering its practical application.⁵ Consequently, the design of efficient visible-light-active semiconductor nanostructures for the degradation of organic pollutants is a lucrative prospect and an important challenge for photocatalytic research.

Over the past few years, several novel visible-light-active nanostructures, such as Ag₃PO₄, AgI, BiOCl, AgCl, and Nb₃O₇(OH), have been synthesized, and show superior photocatalytic activity under visible-light irradiation.^6–10 Among these photocatalysts, Nb₃O₇(OH) nanostructures are a promising material for catalytic applications because of their non-toxicity, high stability under light irradiation, and high corrosion resistance in both acidic and alkaline media.^11,12 However, the visible-light photocatalytic activity of bare Nb₃O₇(OH) is generally inadequate owing to its relatively low surface area and the high recombination rate of its photogenerated electron–hole pairs. It has been generally recognized that for an efficient photocatalyst, long term stability against light irradiation, high life time chare carriers, appropriate energy level offsets and charge trapping centers play an important role for efficient photocatalytic selective degradation reactions. Thus, ameliorating the usage efficiency of photogenerated electrons and prolonging the life time of carriers are expected to improve the catalytic activity.

To overcome the practical issues in Nb₃O₇(OH), the design of Nb₃O₇(OH)-based nanocomposites containing other low-band-gap semiconductor nanostructures or metal-free nanostructures, such as g-C₃N₄, MoS₂, or reduced graphene oxide (RGO), is a novel stagey to reduce the recombination of charge carriers and improve stability and photocatalytic activity.^13–15 Among these, the synthesis of RGO-based nanocomposites has gained increasing attention, and the nanocomposites formed are widely used for the degradation of various organic compounds in water, taking advantage of the excellent adsorption efficiency, high electrical conductivity, and large surface area of RGO sheets.¹⁶ When RGO nanosheets are used as a support for semiconductor nanostructures, the RGO nanosheets act as a mat between active semiconductor nanostructures that can help to shuttle the photogenerated charge carriers to retrain the electron–hole recombination.¹⁶ Additionally, the large two-dimensional layered RGO structures prevent the aggregation of the semiconductor nanostructures through interfacial interactions, thus increasing the number of catalytically active sites.¹⁶ However, to the best of our knowledge, there are no reports on the visible-light photocatalytic properties of Nb₃O₇(OH)-RGO nanocomposites.

Motivated by the lack of research on RGO-based Nb₃O₇(OH) nanostructures and their photocatalytic dye degradation for water remediation, here we present the first report of the design and preparation of a nanocomposite comprising Nb₃O₇(OH) nanorods wrapped in RGO nanosheets as a highly efficient visible-light-induced photocatalyst. The Nb₃O₇(OH)-RGO nanocomposite, prepared using hydrothermal methodology, exhibits enhanced photostability and visible-light photocatalytic activity toward degradation of the colored organic pollutant rhodamine B (RhB) and the colorless organic pollutant phenol compared to those of bare Nb₃O₇(OH) nanostructures. Moreover, the catalytic efficiency is strongly dependent on the RGO content of the nanocomposites, with the highest photodegradation rate being obtained using a nanocomposite prepared with a graphene loading of 3 mg mL⁻¹. Furthermore, a possible photocatalytic mechanism for the Nb₃O₇(OH)-RGO nanocomposites is proposed with the aid of photoluminescence (PL) studies and reactive-species-trapping experiments.

Experimental and characterizations

Pure Nb₃O₇(OH) and Nb₃O₇(OH)-RGO (1, 2, 3, or 4 mg mL⁻¹) nanocomposites were synthesized using hydrothermal methodology. The reactants were niobium(V) chloride (NbCl₅, 99.9%), and graphene oxide (GO). The solvent used was 6 M hydrochloric acid (HCl). The GO was prepared from natural graphite flakes according to a modification of the Hummers method, similarly to in our earlier reports.^17,18 In a typical Nb₃O₇(OH)-RGO nanocomposite preparation, 0.324 g of NbCl₅ and the required amount of GO (e.g., 1 mg mL⁻¹) were mixed with 20 mL of HCl, and the mixture was sonicated for 30 min. The solution was then transferred into a Teflon-lined 100 mL autoclave, which was then sealed and maintained at 210 °C for 20 h before being allowed to cool to room temperature naturally. The resulting precipitate was filtered and washed with deionized water and ethanol several times, and then dried in an oven at 80 °C for 24 h. Nb₃O₇(OH)-RGO nanocomposites with different GO contents were prepared following the same method using 1, 2, 3, or 4 mg mL⁻¹ of GO as the reactant, and were designated Nb-RGO-1, Nb-RGO-2, Nb-RGO-3, and Nb-RGO-4, respectively. The bare Nb₃O₇(OH) nanostructures were prepared by the same procedure, but without the addition of GO.

The microstructural properties of the materials were investigated using a JEOL JEM-2100F transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Phase determination of the as-prepared powders was performed using a Bruker D8 Avance X-ray diffractometer with CuKα as the X-ray source. X-ray photoelectron spectroscopy (XPS) measurements were recorded using a monochromatic AlKα X-ray source (hν = 1486.6 eV) at an energy of 15 kV/150 W. Ultraviolet-visible diffuse reflectance absorbance spectra (UV-Vis DRS) of the solid powder samples were recorded in the range from 200 to 800 nm using a UV-1800 double-beam spectrophotometer (Shimadzu) with BaSO₄ as the reflectance standard. The Fourier transform infrared (FTIR) spectra were recorded in transmission mode from 400 to 4000 cm⁻¹ at a resolution of 4 cm⁻¹ using a Nicolet 380 FTIR spectrometer. In addition, PL measurements were performed at room temperature using a Hitachi F-7000 fluorescence spectrophotometer.

The photocatalytic activities of the Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites were evaluated via the degradation of RhB under simulated solar irradiation. A solar simulator equipped with an AM 1.5G filter and a 150 W Xe lamp (Abet Technologies) fitted with a <425 nm cut-off filter was used as the visible-light source. The photocatalyst (50 mg) was suspended in a 100 mL aqueous solution of RhB or phenol with an initial concentration (C_o) of 10 mg L⁻¹. Before turning on the light source, the reaction system was magnetically stirred in the dark for 30 min to allow the absorption–desorption equilibrium between the photocatalyst and dye to be established. Magnetic stirring was continued after irradiation in order to keep the photocatalyst particles suspended throughout the measurements. At given illumination-time intervals, ca. 2 mL aliquots of the mixture were removed and centrifuged at 5000 rpm for 15 min to separate the photocatalyst from the reaction solution. A UV-visible spectrum of the supernatant was then recorded to monitor the adsorption and degradation behavior. The characteristic absorption peak for RhB at 554 nm was used to assess the extent of degradation.

Results and discussion

The surface morphological properties of the as-synthesized Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites produced by hydrothermal treatment at 210 °C for 20 h using NbCl₅ and GO as the source materials, were examined by field emission scanning electron microscopy (FESEM) and are presented in Fig. 1(a)–(c) and (f)–(h). The low-magnification FESEM image of the bare Nb₃O₇(OH) nanostructures (Fig. 1(a)) shows that they are roughly spherical in shape with an average diameter of about 1–5 μm. The magnified FESEM images (Fig. 1(b) and (c)) clearly show that the microspheres are formed by a mass of radially arranged nanorods. It is clear that, the nanorods are highly crystalline and formed with some ill defined nanoparticles. The average size of the nanorods is measured to be 50–200 nm in length, 5–10 nm in thickness, and 3–7 nm in width. Fig. 1(f) and (g) shows FESEM images of Nb-RGO-3 nanostructures produced by hydrothermal treatment at 210 °C for 20 h with GO contents of 3 mg mL⁻¹ as the reactant. From these images, it can be seen that the Nb₃O₇(OH) nanorods are densely and tightly covered with two-dimensional RGO sheets. The wrapping of the Nb₃O₇(OH) with the RGO may be explained by the interaction of the –OH and –COOH groups of the RGO and their electrostatic interaction with the surfaces of the Nb₃O₇(OH) nanostructures.¹⁹ During hydrothermal synthesis, the removal of these oxygen functional groups results in unpaired π electrons, which more rapidly bind to the surface atoms of the Nb₃O₇(OH) nanostructures. This strong chemical bonding interaction promotes efficient charge transfer and enhanced photocatalytic activity.²⁰ Furthermore, there is no aggregation of the nanorods in the nanocomposites; all the nanorods are separated by thin RGO nanosheets. This prevention of aggregation by the RGO is due to interfacial interaction and increases the number and availability of active reaction sites, thus increasing the photocatalytic degradation ability of the material. Moreover, there are no radially arranged nanorods in the nanocomposites, with all the nanorods being randomly distributed on the surface of the nanostructures. This indicates that the wrapping of the nanorods by RGO nanosheets prevents their assembly into microspheres. The content of graphene concentrations in nanocomposites was analyzed by energy dispersive spectrometer. The estimated weight ratios of graphene in Nb₃O₇(OH)-RGO nanocomposites are 4.12 (Nb-RGO-1), 7.85 (Nb-RGO-2), 8.62 (Nb-RGO-3) and 12.01 (Nb-RGO-4) respectively.

Fig. 1 FESEM images of (a–c) Nb₃O₇(OH) and (f–h) Nb₃O₇(OH)-RGO nanocomposites. (d and e) TEM and HRTEM images of the Nb₃O₇(OH) nanostructures (i–l) TEM and HRTEM images of Nb-RGO-1 and Nb-RGO-3 nanocomposites, respectively.

The microstructures and growth characteristics of the Nb₃O₇(OH) nanostructures and Nb₃O₇(OH)-RGO nanocomposites were further investigated by TEM and HRTEM. Fig. 1(d) and (e) show the TEM and HRTEM images of the pure Nb₃O₇(OH). From Fig. 1(d), it can be clearly seen that the morphology of the nanorods is regular and uniform, but that their lengths and diameters vary. The nanorods are measured to be 30–200 nm in length, 3–5 nm in thickness, and 2 nm in width. The HRTEM image in Fig. 1(e) shows that all the Nb₃O₇(OH) nanostructures are highly crystalline. The spacing between adjacent fringes is measured to be 0.271 nm, which is close to the (111) inter-planar distance of orthorhombic Nb₃O₇(OH) nanostructures. The TEM and HRTEM images of Nb₃O₇(OH)-RGO nanocomposites (Fig. 1(i)–(l)) clearly show that the nanorods are well-wrapped in RGO nanosheets. These results confirm that the nanocomposites are composed of both RGO and Nb₃O₇(OH) nanorods. The HRTEM image in Fig. 1(i) shows that the spacing between adjacent fringes is 0.375 nm, which is close to the (110) inter-planar distance of orthorhombic Nb₃O₇(OH) nanostructures.

Fig. 2(a) shows the XRD patterns of Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites with different RGO loadings synthesized hydrothermally at 210 °C for 24 h. It is evident that all the diffraction peaks in the pattern of the as-synthesized Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites can be fully indexed as standard orthorhombic Nb₃O₇(OH) (JCPDS file 31-0928), with the characteristic reflection phases (200), (400), (001), (110), (600), (310), (510), (111), (601), (511), (1000), (002), and (020). Moreover, due to the small amount of RGO and the relatively low diffraction intensity of the RGO in the nanocomposites, no distinctive diffraction peaks for the carbon species are observed. Furthermore, no new peaks related to secondary phases of niobium are observed, indicating that the synthesized composites have high purity. The growth direction can be ascertained using the intensity of the diffraction signals due to different planes. The patterns clearly show that in pure Nb₃O₇(OH) nanostructures, the diffraction peak at 2θ = 32.41 is much stronger and narrower than the other peaks, whereas in the nanocomposites, the diffraction peak at 2θ = 23.94 is much stronger, indicating that the preferred growth directions for the bare Nb₃O₇(OH) and the Nb₃O₇(OH)-RGO nanocomposites are along the (111) and (110) crystal planes of the orthorhombic structure, respectively. Average nanocrystallite sizes were estimated using the Debye–Scherrer formula, D = 0.89λ/β cos θ, where λ, β, and θ are the wavelength of the CuKα radiation, the diffraction angle, and the full width at half maximum (FWHM) of the diffraction peak for the major diffraction plane, respectively. The estimated average crystallite sizes are in the range of 50–95 nm, and are slightly smaller for the nanocomposites, suggesting that the presence of a certain amount of RGO improves the distribution of the nanoparticles, due its large surface area and thin, two-dimensional layered structure.²⁰

Fig. 2 (a) X-Ray diffraction patterns of the Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites; (b) XPS survey spectra of GO and the Nb-RGO-3 nanocomposites; (c) and (d) narrow-scan C 1s spectra of GO and the Nb-RGO-3 nanocomposites; (e) and (f) narrow-scan spectra of Nb 3d and O 1s. 3d for the AgI-2 nanocomposites.

The reduction of GO during the hydrothermal process and the oxidation states of the synthesized nanostructures was further characterized by XPS. Typical survey scan XPS spectra of the GO and the Nb-RGO-3 nanocomposite are shown in Fig. 2(b). There are two dominant peaks in the GO spectra, which represent the graphitic C 1s and O 1s orbitals. However, for Nb-RGO-3, in addition to the C 1s and O 1s peaks, a new peak related to Nb is observed. On further analysis, the C 1s region contains peaks at 284.56, 286.48, and 288.83 eV, assignable to the non-oxygenated ring C, the C in C–O bonds, and the carboxylate carbon (O–C=O) respectively (Fig. 2(c)).²¹ The presence of similar species is also indicated in the C 1s region of Nb-RGO-3 (Fig. 2(d)). However, the peak intensities of the various oxygenated carbon species drastically decrease compared to those in GO. Moreover, the peak corresponding to the C=O group vanishes, and new peak at 289.36 eV, ascribed to the O–C=O group, is observed.²² This suggests that the GO has been reduced to RGO during the formation of the nanocomposites under hydrothermal conditions. The high-resolution Nb 3d XPS spectrum shows peaks at 210.43 and 207.63 eV, which correspond to Nb 3d_3/2 and Nb 3d_5/2, respectively (Fig. 2(e)).²³ Fig. 2(f) shows the O 1s deconvolution narrow scan spectra. Two intense peaks are observed at binding energies of 531.23 and 532.88 eV, which are assigned to oxygen vacancies and dissociated oxygen species, respectively.²⁴ These results confirm the existence of Nb₃O₇(OH) and RGO in the composites.

In order to determine the presence of RGO and investigate the reduction phenomena during the hydrothermal process, the GO and nanocomposite was further characterized by FT-IR spectroscopy. The results of the FT-IR analysis of the as-prepared GO and Nb-RGO-3 samples are shown in Fig. 3(a). The characteristic peaks of GO appear at 3409 (O–H stretching band), 1742 (C=O vibrations of the –COOH groups), 1625 (C–C skeletal vibrations of the aromatic domains), 1348 (bending absorption of the O=C–O carboxyl group), 1248 (O–H bending vibrations), and 1065 cm⁻¹ (C–O stretching vibrations).²⁵ All of these vibrations reveal the existence of a high number of oxygen-containing functional groups at the edges and basal plane of the nanosheets. For the FTIR spectra of Nb-RGO-3 nanocomposites, the GO-related stretching bands of the C–O and carboxyl groups are completely absent, and the peaks for C–O and C–C are still observed after reduction, but are much smaller than those in GO.²⁶ These results indicate that GO is effectively reduced by Nb₃O₇(OH) to generate RGO nanosheets during the hydrothermal reaction.

Fig. 3 (a) FT-IR spectra of pure GO and Nb-RGO-3 nanocomposites. (b) Optical absorption spectra of GO, Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites prepared with different RGO concentrations. (c) Emission spectra of Nb₃O₇(OH) and the Nb₃O₇(OH)-RGO nanocomposites.

To investigate the optical properties and estimate the band gap the as-synthesized Nb₃O₇(OH), Nb₃O₇(OH)-RGO nanocomposites and pure RGO nanostructures were analyzed by UV-Vis diffuse reflectance spectra (DRS) and the results are shown in Fig. 3(b). The spectrum of the bare Nb₃O₇(OH) nanostructures shows the absorption edge at 436.5 nm (2.84 eV).¹⁰ This behaviour indicates that the nanostructures can absorb visible light radiation of wavelengths less than 440 nm, while the pure RGO absorbs UV and visible light at wavelengths less than 800 nm. Nb₃O₇(OH)-RGO nanocomposites exhibit much stronger response in the visible region compared to the Nb₃O₇(OH) nanostructures, with stronger absorbance from 450–800 nm, which indicates that the synthesized nanocomposites may utilize more visible light during the photocatalytic reaction. The enhanced visible-light absorbance range in the Nb₃O₇(OH)-RGO nanocomposites may be due to the cooperative effect between the RGO and the Nb₃O₇(OH) nanostructures.

Room temperature PL spectra of pure Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites were recorded in order to investigate the possible interactions between the Nb₃O₇(OH) nanorods and the RGO nanosheets, and are presented in Fig. 3(c). It is clear that the bare Nb₃O₇(OH) nanostructure exhibits two emission bands: one in the UV region at ca. 400 nm and the other in the blue region at ca. 464 nm. The observed emission peaks may be attributed to electron transitions from the valence band to the conduction band, trap sites, and oxygen vacancies.¹² The emission spectra of Nb₃O₇(OH)-RGO nanocomposites also exhibit similar emission bands to those of Nb₃O₇(OH). However, the emissions are greatly suppressed compared to those of the bare nanostructures, which is probably due to charge transfer from the Nb₃O₇(OH) nanostructures to the RGO nanosheets. In general, the PL emission occurs from the recombination of excited electrons and holes under light irradiation, with a higher PL intensity indicating a higher recombination rate of photoexcited electrons and holes, and a commensurate decrease in the photocatalytic activity. Conversely, a lower PL intensity indicates a lower recombination rate of photoexcited electrons and holes, suggesting that more photoexcited holes and electrons can participate in the oxidation and reduction reactions, thus enhancing photocatalytic performance. In our case, the decrease in PL intensity is due mainly to the presence of RGO nanosheets in the nanocomposites. The RGO sheets act as photoluminescence-quenching centers, and effectively inhibit electron–hole pair recombination owing to their high specific surface area and high electrical conductivity, resulting in non-radiative recombination.²⁷

The photocatalytic activities of the as-synthesized bare Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites were investigated by the degradation of RhB (10 mg L⁻¹) under visible-light (λ > 425 nm) irradiation. Fig. 4(a) shows the UV-Vis absorption spectra of RhB solutions irradiated under visible light for 5 h in the presence of bare Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites. It is evident that the absorption peak at 554 nm drastically decreased in the presence of all the nanocomposites. The spectra confirm that the synthesized nanostructures are effective for dye degradation under visible-light irradiation. In the absence of the photocatalyst (control experiment) showed that the concentration of RhB was marginally reduced when exposure to the experimental radiation for 5 h. This indicates that the degradation of RhB was negligible in the absence of the photocatalyst. To further systematically investigate the photocatalytic activities of the synthesized samples, their degradation efficiencies towards RhB, given by

D = (A(RhB)₀ − A(RhB))/A(RhB)₀ × 100

where A(RhB)₀ and A(RhB) are the absorbances at 554 nm of RhB solutions kept in the dark and treated under light irradiation at t min, respectively, were determined, and are presented in Fig. 4(b). As the spectra show, pure Nb₃O₇(OH), Nb-RGO-1, Nb-RGO-2, Nb-RGO-3, and Nb-RGO-4 exhibit degradation efficiencies of 67.73%, 74.97%, 83.26%, 89.33% and 85.53%, respectively. For comparison, the RhB degradation over graphene also carried out under identical conditions. For graphene, there is 58% of adsorption and photocatalytic degradation rate was noticed and it is due to its excellent electric conductivity and large specific surface area. The maximum degradation is observed for the Nb-RGO-3 nanocomposite after 5 h of irradiation. These results indicate that the introduction of RGO efficiently enhances photocatalytic activity, which may be ascribed to the increased number of active adsorption sites and photocatalytic reaction centers offered by RGO. The RhB molecules are adsorbed onto the surface of the RGO via offset face-to-face π–π interactions between the RhB molecules and the aromatic regions of the RGO nanosheets, leading to improved adsorption of RhB compared to that of bare Nb₃O₇(OH). This phenomenon is crucial for the improved photodegradation ability observed with RGO-based nanocomposites.²⁸ Furthermore, the RGO nanosheets facilitate the transfer of photoinduced electrons away from the conduction band of Nb₃O₇(OH), inhibiting their recombination with the holes located in the valence band of Nb₃O₇(OH) and the resulting regeneration of ground-state species.^29,30 However, when the concentration of RGO increases above 3 mg mL⁻¹, the photocatalytic degradation ability of the nanocomposite decreases. This may be due to the formation of thicker RGO layers on the surface of the Nb₃O₇(OH) nanorods, resulting in reduced visible-light absorption, and a commensurate decrease in photocatalytic degradation ability.²⁹ However compared to the commercial TiO₂ photocatalyst, all the synthesized bare and RGO based nanocomposites have higher photocatalytic activity.

Fig. 4 (a) Changes in the UV-Vis absorption spectra of an aqueous RhB solution following visible-light irradiation in the presence RGO, TiO₂, Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites. (b) Degradation efficiency towards RhB of RGO, TiO₂, Nb₃O₇(OH) and the Nb₃O₇(OH)-RGO nanocomposites. (c) Rate constant (k) towards RhB of Nb₃O₇(OH) and the Nb₃O₇(OH)-RGO nanocomposites. (d) Inhibition of the photocatalytic degradation of RhB with different active-species scavengers (EDTA: disodium ethylenediaminetetraacetate, BQ: benzoquinone, and TA: tert-butyl alcohol). (e) The reusability of the Nb-RGO-3 nanocomposite in the photocatalytic degradation of RhB. (f) Absorption spectra of a phenol solution following visible-light irradiation in the presence of Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites. (g) Degradation efficiency towards phenol of Nb₃O₇(OH) and the Nb₃O₇(OH)-RGO nanocomposites. (h) Rate constant efficiency towards phenol of Nb₃O₇(OH) and the Nb₃O₇(OH)-RGO nanocomposites.

Further, to improve the understanding of the reaction kinetics of RhB organic degradation over all the photocatalysts, the results were fitted to a pseudo-first order correlation (ln(C_o/C_t) = kt, where k is the apparent reaction-rate constant, and C_o and C_t are the RhB concentrations initially and at time t, respectively). The calculated apparent k values for bare and RGO nanocomposites are shown in Fig. 4(c). The highest photocatalytic mineralization performance was achieved with the Nb-RGO-3 nanocomposite: its k value is about 5.14 times greater than that of the bare Nb₃O₇(OH) nanostructures. Therefore, we can conclude that the formation of the RGO nanocomposite enhances the visible light photocatalytic activity.

To further study the photocatalytic mechanism and identify the main oxidative species (h⁺, ˙O₂⁻, and ˙OH) in the photocatalytic process, we studied the effect of active-species scavengers on the decolorization of RhB, and the results are presented in Fig. 4(c). Ethylenediaminetetraacetic acid (EDTA), benzoquinone (BQ), and t-butyl alcohol (TBA) were used as scavengers for h⁺, ˙O₂⁻ radicals, and ˙OH radicals, respectively. It is clear from Fig. 4(d) that the addition of EDTA slightly affected the photocatalytic degradation of RhB over the Nb-RGO-3 nanocomposite, suggesting that the h⁺ reactive species is only partially involved in the photocatalytic process. Conversely, the degradation efficiency decreased considerably after the addition of BQ and TBA, which indicates that ˙O₂⁻ and ˙OH radicals are the main active species in the photocatalytic degradation of RhB under visible-light irradiation.³¹

Considering the effect of scavengers on the photocatalytic reaction, the observed PL quenching, and the band gap structure of Nb₃O₇(OH), the enhanced photodegradation ability of the nanocomposites is rationalized as follows: under visible-light irradiation, the synthesized nanostructures utilize light energy to generate electrons and holes. The photogenerated electrons and holes in Nb₃O₇(OH) can move freely into the RGO nanosheets owing to their close interfacial contact. Hence, coupling with the RGO provides different transfer possibilities for the photogenerated holes and electrons. These promote the effective separation of the photoexcited electron–hole pairs, decreasing the probability of electron–hole recombination at the interface of the nanocomposites, and promoting high photocatalytic performance. The photogenerated electrons participate in reduction reactions with electron acceptors, such as adsorbed oxygen molecules, to generate hydroxyl and superoxide radicals upon protonation. The photoinduced holes either can oxidize organic compounds directly or be trapped by electron donors, such as OH⁻, to produce OH radicals. The superoxide radical anions and hydroxyl radicals are responsible for the decomposition of the RhB dye into non-toxic products, such as CO₂, NO₃, NO_x, and mineral acids.³² A schematic illustration of the charge transfer and enhanced photocatalytic activity of the Nb₃O₇(OH)-RGO nanocomposites is shown in Fig. 5.

Fig. 5 Illustration of the proposed reaction mechanism for the photocatalytic degradation of RhB in aqueous solution over Nb₃O₇(OH)-RGO nanocomposites under visible-light irradiation.

In addition to their photocatalytic activity, the stability of nanostructures is an important criterion for photocatalysts. Therefore, we carried out stability tests on the as-synthesized Nb-RGO-3 nanocomposite using four successive recycling experiments. As shown in Fig. 4(e), after four consecutive runs the degradation efficiency of the sample is still 77.99%. This remarkable performance is attributed to the higher number of reaction sites and the chemical interactions between the RGO nanosheets and the Nb₃O₇(OH) nanostructures.

To rule out the photosensitization effect under visible-light irradiation, we also evaluated the photocatalytic performance of Nb₃O₇(OH) and Nb₃O₇(OH)-RGO nanocomposites toward the degradation of phenol, which is colorless, because the photosensitization effect of phenol is negligible under visible light.^33,34 Fig. 4(f) shows the absorption spectra of aqueous solutions of phenol (5 mg L⁻¹) after visible-light irradiation for 5 h in the presence of 50 mg of one of the synthesized catalysts. The characteristic absorption peak of phenol at 270 nm was chosen to monitor the progress of the photocatalytic degradation process. From Fig. 4(f), it can be clearly seen that the absorption peaks corresponding to phenol decrease with irradiation time, which indicates rapid phenol decomposition. Fig. 4(g) shows the phenol degradation rate. Pure Nb₃O₇(OH), Nb-RGO-1, Nb-RGO-2, Nb-RGO-3, and Nb-RGO-4 exhibit degradation efficiencies of 45.6%, 62.8%, 73.4%, 83.1%, and 76.7% respectively. The rate constants were 0.0166, 0.02263, 0.0293, 0.03555 and 0.03293 min⁻¹ for Nb₃O₇(OH), Nb-RGO-1, Nb-RGO-2, Nb-RGO-3, and Nb-RGO-4, respectively (Fig. 4(h)). It is clear that the Nb-RGO-3 nanocomposite promotes a significantly higher level of phenol photodegradation after 300 min of irradiation compared with the bare Nb₃O₇(OH) nanostructures. The enhanced photocatalytic activity mechanism for phenol is similar to that for the organic dyes.

Conclusions

In summary, we have successfully synthesized Nb₃O₇(OH)-RGO nanocomposites via a hydrothermal method. The as-synthesized nanocomposites were used as photocatalysts for the oxidative degradation of RhB and phenol. The synthesized nanostructures exhibited excellent photocatalytic degradation ability and recyclability compared to bare Nb₃O₇(OH) nanostructures under visible-light irradiation. The enhanced catalytic activity was mainly due to the increased charge separation and transportation between the RGO nanosheets and the Nb₃O₇(OH) nanostructures. The proposed charge-transfer mechanism was supported by PL results. We believe that the work presented here illustrates the promising scope of visible-light-active, RGO-based photocatalysts in organic dye degradation and toxic phenol oxidation for the purification of polluted water resources.

Acknowledgements

This work was supported by National Research Foundation of Korea (NRF) grants, funded by the Korean government (MEST and MSIP) (2013S1A2A2035406, 2013R1A1A2009575 and 2014R1A4A1001690). This work was also supported by the Max Planck POSTECH/Korea Research Initiative Program [Grant no. 2011-0031558] through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning.

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