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Catalysts 2018, 8(9), 392; doi:10.3390/catal8090392

Article
A Facile Synthesis of Visible-Light Driven Rod-on-Rod like α-FeOOH/α-AgVO3 Nanocomposite as Greatly Enhanced Photocatalyst for Degradation of Rhodamine B
State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing Engineering Center for Hierarchical Catalysts, Beijing University of Chemical Technology, Beijing 100029, China
*
Author to whom correspondence should be addressed.
Received: 22 August 2018 / Accepted: 7 September 2018 / Published: 12 September 2018

Abstract

:
In this work, we have synthesized the rod-on-rod–like α-FeOOH/α-AgVO3 nanocomposite photocatalysts with the different amounts of solvothermally synthesized α-FeOOH nanorods via a simple co-precipitation method. The as-synthesized photocatalysts were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, UV−Visible diffuse reflectance spectroscopy, scanning electron microscopy (SEM), element mapping, high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS) analysis. The observed SEM images show that both α-AgVO3 and α-FeOOH exhibits the rod-shaped morphology with nano size. Furthermore, the photocatalytic activities of the obtained photocatalysts were evaluated towards the degradation of Rhodamine B (RhB) under visible-light irradiation. It is demonstrated that the 3 mg α-FeOOH added to the α-FeOOH/α-AgVO3 nanocomposite exhibited an enhanced photocatalytic performance as compared with the pure α-AgVO3 and α-FeOOH. This significant improvement can be attributed to the increased photo-excited electron-hole pair separation efficiency, large portion of visible-light absorption ability and the reduced recombination of the electron-hole pair. The recycling test revealed that the optimized nanocomposite exhibited good photostability and reusability properties. In addition, the believable photodegradation mechanism of RhB using α-FeOOH/α-AgVO3 nanocomposite is proposed. Hence, the developed α-FeOOH/α-AgVO3 nanocomposite is a promising material for the degradation of organic pollutants in an aqueous environment.
Keywords:
nanocomposite; co-precipitation method; photodegradation; rhodamine B; visible-light

1. Introduction

Well cleaned water is one of the most important factors for human life. The Earth is covered by around 70% water; however, only 2.5% of water is accessible for drinking, agriculture and other industrial consumption [1,2,3]. Currently, environmental protection and remediation are a tremendous threat to the world owing to the increased industrialization, environmental pollution and global warming [4,5]. Currently, large amounts of pollutants from synthetic organic dyes are easily mixed with the aquatic environment due to the rapid development of various industrial sectors such as textiles, dyeing and printing industries. Generally, this colored waste-water contains large numbers of organic compounds which are very toxic to human health as well as microorganisms [6,7]. In order to resolve these issues, it is a very necessary action to eradicate these organic pollutants from wastewater. Several techniques such as adsorption [8], sonolysis [9], oxidation [10], (photo)-Fenton process [11], photocatalytic degradation [12,13] and ozonation [14] have been adapted for the eradication of organic effluents from waste-water. Among them, the semiconductor-based photocatalysis technique has received considerable attention owing to its low-cost, high photostability and eco-friendly nature towards the photocatalytic degradation of organic pollutants via freely available solar energy [15,16,17]. In addition, it can degrade the hazardous organic pollutants into nontoxic molecules such as CO2, H2O and mineral acids without creating secondary pollution and hence, the photocatalysis has been called green technology in recent times [18].
In general, the transition metal oxide-based semiconductor materials are recognized as sustainable and prospective photocatalysts for the removal of organic pollutants in wastewater [19,20,21]. Among the different metal oxides, the TiO2 is the most frequently used photocatalyst for environmental remediation due to its outstanding properties of low cost, non-toxicity, high stability and relatively suitable band gaps for photocatalytic reaction [22,23]. However, the practical application of TiO2 photocatalyst has been restricted so far in visible-light irradiation because of its limited response to solar light spectrum and fast recombination of electron-hole charge carriers for the large band-gap of 3.2 eV [24]. Therefore, the design and development of highly visible-light responsive semiconductor-based photocatalysts is an urgent requirement for its practical applications.
More recently, silver-based metal oxides such as Ag2WO4 [25,26], Ag3PO4 [27,28], Ag3VO4 [29], AgVO3 [30], Ag2MoO4 [31] and Ag2CrO4 [32,33] have gained extensive consideration because they have shown an excellent photocatalytic activity in visible-light irradiation. Among them, AgVO3 is widely reported as an emerging photocatalyst for degradation of the visible-light organic pollutants in waste-water due to its unique properties such as strong absorption in the visible light range, simple preparation, non-toxicity, high electrical conductivity and good crystallinity [34,35]. Besides, AgVO3 has relatively good photostability like typical vanadium oxide compounds due to its narrow bandgap of less than 2.5 eV, displaying a superior photocatalytic activity in visible-light [36,37]. Unexpectedly, the photocatalytic activity of pure AgVO3 is still limited due to its low separation efficiency and rapid recombination of photo-excited electron-hole pairs as a result of narrow band gap energy [30]. To tackle this issue, it is essential to explore sufficient ways to further improve the separation efficiency of photo-excited charge carriers as well as photocatalytic performance. Over the past few decades, researchers have modified the AgVO3 via preparation of heterojunction composite materials with other semiconductors which have suitable band potentials to reduce the recombination of charge carriers and hence, the photocatalytic activity has been improved. In addition, many AgVO3-based composite photocatalysts, such as AgCl/AgVO3 [38], Ag3PO4/AgVO3 [39], BiVO4/AgVO3 [40,41], MoS2/AgVO3 [42], graphene/AgVO3 [43], g-C3N4/Ag/AgVO3 [44,45], Ag2CrO4/AgVO3 [46] and Ba5Ta4O15/AgVO3 [47] have been reported to have the superior photocatalytic performance because of their increased electron-hole separation efficiency as compared with pure AgVO3.
On the other hand, α-FeOOH is another important semiconductor material in the field of photocatalysts because of its extensive absorption band position in the visible light region, inexpensiveness, non-toxicity, and corrosion resistance [48,49,50]. Moreover, the α-FeOOH can easily couple with other semiconductors to enhance their photocatalytic performance in visible-light irradiation, thus obtaining the heterojunction photocatalysts [51,52]. A few attempts have been described on α-FeOOH-based composite photocatalyst for the degradation of organic pollutants from the aquatic environment. For example, Feng et al. [53] synthesized α-FeOOH/Ag3PO4 nanocomposites with higher photocatalytic activity towards elimination of Rhodamine B (RhB) solution in irradiation visible light than that for the pure Ag3PO4. Yang et al. [54] reported the α-FeOOH/g-C3N4 composite with superior photocatalytic ability for the elimination of organic pollutants such as RhB, Methylene Blue (MB) and Methyl Orange (MO) under visible-light irradiation. More recently, Malathi [52] et al. also prepared α-FeOOH/BiOI nanocomposites by a facile wet-chemical method, which greatly improved photocatalytic activity for the visible-light degradation of RhB as compared with pure BiOI. In addition, the α-FeOOH is an ideal material to couple with AgVO3 to inhibit the recombination of photo-excited electron-hole pairs through their well matched band gap positions, resulting in the overall expected properties to enhance its photocatalytic activity. Therefore, in the present study, the α-FeOOH was selected as co-catalyst to fabricate α-FeOOH/α-AgVO3 nanocomposite photocatalysts via a facile co-prepetition method at room temperature. Also, to the best of our knowledge, the α-FeOOH/α-AgVO3 nanocomposite has not yet been reported as a photocatalyst for environmental remediation in literatures.
Herein, we have synthesized well-uniform α-FeOOH nanorods using solvothermal method. Apart from that, a series of the rod-on-rod–like α-FeOOH/α-AgVO3 nanocomposites were synthesized with different amounts of α-FeOOH by using a facile co-precipitation method at room temperature. The photocatalytic abilities of the as-syntheszied photocatalyts were assessed toward the discharging of RhB in waste-water under visible-light irradiation. The acquired degradation results revealed that the α-FeOOH/α-AgVO3 nanocomposites were found to have the superior photocatalytic activity when compared with pure α-FeOOH and α-AgVO3. Furthermore, the nanocomposite possesses an excellent photo-stability and reusability nature. Lastly, the possible charge-transfer mechanism for the increased degradation of RhB using α-FeOOH/α-AgVO3 nanocomposites has also been proposed. Consequently, the present study shows that the α-FeOOH/α-AgVO3 nanocomposite will be a promising candidate for waste-water treatment and other environmental remediation.

2. Results and Discussion

2.1. Schematic Representation of the Synthesis Process

Figure 1 shows the schematic representation of the synthesis process of α-FeOOH/α-AgVO3 nanocomposite. First, the α-FeOOH was synthesized using a solvothermal method and it exhibited the nanorods-shaped morphology. Afterwards, the α-FeOOH/α-AgVO3 nanocomposites were synthesized through a facile co-precipitation method at room temperature. The as-obtained material shows that the α-FeOOH nanorods are well deposited on the surface of α-AgVO3 nanorods. This structure is beneficial to the synergistic effect between the α-FeOOH and α-AgVO3. In addition, this nanocomposite exhibits the enhanced visible-light adsorption ability as compared with pure α-AgVO3 which can support to increasing the photocatalytic degradation efficiency of RhB under visible-light illumination by reducing the recombination of photo-excited electron-hole pairs. Hence, the α-FeOOH/α-AgVO3 can be recognized as a potential materials as photocatalyst for the removal of organic pollutants in wastewater.

2.2. Structural, Optical and Morphological Studies

The phase formation and purity of the as-synthesized photocatalysts were determined by X-ray diffraction (XRD) analysis. The XRD patterns of the pure α-FeOOH, pure α-AgVO3 and different α-FeOOH/α-AgVO3 nanocomposites are shown in Figure 2a,b. It is shown that the XRD peaks of pure α-FeOOH are well consistent with the orthorhombic structure of α-FeOOH (JCPDS card No. 81-0462) [52,53]. The XRD pattern of pure α-AgVO3 is well indexed to the standard monoclinic structure of α-AgVO3 (JCPDS card No. 89-4396) [41,55]. Additionally, the characteristic XRD peaks of the α-FeOOH were not found in the case of α-FeOOH/α-AgVO3 nanocomposites due to the presence of a low amount of α-FeOOH content in the nanocomposites. However, there are no new peaks detected in the nanocomposite, indicating the high purity of the as-synthesized samples.
The FT-IR spectra of pure α-FeOOH, pure α-AgVO3 and different α-FeOOH/α-AgVO3 nanocomposites are presented in Figure 3a. The pure AgVO3 exhibits the two absorption bands at 647 cm−1 and 928 cm−1 due to the symmetric stretching vibration of VO3 and the bands appeared at 773 893 cm−1 and 893 cm−1, which are related to the anti-symmetric stretching vibrations of VO3 [37]. Also, the band located at 556 cm−1 is due to the symmetry stretching mode of V-O-V. The two broad absorption bands detected at 1633 cm−1 and 3429 cm−1 are attributed to the physical absorption of water molecules on the surface of the AgVO3 [56]. Additionally, the pure α-FeOOH shows the two vibrational bands at 893 cm−1 and 799 cm−1, corresponding to the bending vibrations of the Fe-O-H bonds of α-FeOOH [52]. The absorption bands observed at 634 cm−1 and 497 cm−1 are related to the stretching vibration of the Fe-O bonds of α-FeOOH. A broad absorption band at 3138 cm−1 is due to the stretching vibration of the O-H group of α-FeOOH. However, in the case of α-FeOOH/AgVO3 nanocomposites, the α-AgVO3 peaks intensities are found to be decreased by the addition of α-FeOOH, which is due to the formation composites with α-FeOOH and also the merging of vibrational peaks of α-FeOOH and α-AgVO3.
The optical absorption nature of the fabricated photocatalysts was determined through UV-vis diffusion reflectance spectroscopy analysis and the obtained UV-vis diffuse reflectance spectra of pure α-FeOOH, pure α-AgVO3 and FeAVO-3 nanocomposite photocatalysts are given in Figure 3b. As seen from Figure 3b, the absorption edges (λg) of pure α-FeOOH, pure α-AgVO3, FeAVO-1, FeAVO-2, FeAVO-3, FeAVO-4 and FeAVO-5 nanocomposites are estimated to be around 645 nm, 557 nm, 526 nm, 564 nm, 622 nm, 602 nm and 584 nm, respectively. It can be seen that the absorption edges of FeAVO-3 nanocomposite are much higher than that of pure α-AgVO3. That is owing to the slow recombination of photo-excited electrons and holes, which can assist the strong absorption in the visible light region to degrade more organic pollutants [17]. Furthermore, the band-gap energy (Eg) of the as-synthesized photocatalysts can be calculated via the equation, Eg = 1240/λg (eV), where, λg is the absorption edge, which is obtained from the intercept between the tangent of the absorption curve and abscissa [57]. The Eg values of pure α-FeOOH, pure AgVO3, FeAVO-1, FeAVO-2, FeAVO-3, FeAVO-4 and FeAVO-5 nanocomposites are calculated to be 1.92 eV, 2.22 eV, 2.35 eV, 2.19 eV, 1.99 eV, 2.05 eV and 2.12 eV, respectively. It is shown that FeAVO-3 nanocomposite has the much lower Eg value when compared with α-AgVO3, indicating the higher absorption efficient light in the visible light region which may be favorable for its photocatalytic performance [28,31].
The SEM measurement was performed to explore the surface morphology of the as-synthesized photocatalysts. The SEM images of the pure α-FeOOH, pure α-AgVO3 and the optimized FeAVO-3 nanocomposite photocatalysts at different magnifications are presented in Figure 4a–f. As seen from Figure 4a,b, the pure α-FeOOH exhibits a well uniform nanorod-shaped morphology. Meanwhile, the pure α-AgVO3 also exhibited the nanorod-shaped morphology with some irregular shaped nanoparticles as shown in Figure 4c,d. From Figure 4e,f, the SEM images of FeAVO-3 nanocomposite show that the α-FeOOH nanorods are attached on the surface of the nanorod-shaped α-AgVO3. This result confirms the formation of α-FeOOH/α-AgVO3 nanocomposite material. Also, the existence of good interfacial contact between the α-FeOOH and α-AgVO3 may facilitate reducing the hole-electron recombination and improving the photocatalytic activity of the nanocomposite. For further investigation of the surface morphology of the α-FeOOH/α-AgVO3 nanocomposite photocatalyst, the HR-TEM analysis was carried out for the optimized FeAVO-3 nanocomposite photocatalyst. The HR-TEM images of FeAVO-3 nanocomposite at different magnifications with its corresponding selected area (electron) diffraction (SAED) pattern are shown in Figure 5a–d). Obviously, the HR-TEM images demonstrate that the α-FeOOH nanorods are well attached on the surface of α-AgVO3 nanorods. This observed HR-TEM result is well consistent with the SEM analysis. In addition, the SAED pattern of FeAVO-3 nanocomposite contains proper crystalline structure of both α-FeOOH and α-AgVO3, which clearly indicates the successful synthesis of α-FeOOH/α-AgVO3 nanocomposite photocatalysts.
The elemental compositions of the as-synthesized nanocomposite were assessed from element mapping analysis. The typical element mapping images of the optimized FeAVO-3 nanocomposite are given in Figure 6. It is shown that the elements of Ag, V, Fe and O only appear for the as-synthesized nanocomposite, indicating the successful synthesis of α-FeOOH/α-AgVO3 nanocomposite without any impurities. The XPS was also employed to extensively verify the chemical composition and the chemical states of the as-synthesized α-FeOOH/α-AgVO3 nanocomposite. The XPS spectrum of the optimized FeAVO-3 nanocomposite photocatalyst: (a) Full survey spectrum, (b) Ag 3d, (c) V 2p, (d) O 1s and (d) Fe 3d are displayed in Figure 7. The high resolution XPS spectra in Figure 7a indicate the FeAVO-3 nanocomposite is composed of the elements of Ag, V, O, and Fe, which also prove the high purity of the synthesized samples. In Figure 7b, two XPS peaks of Ag 3d concentrate at 367.9 eV and 374.1 eV, corresponding to the Ag 3d5/2 and Ag 3d3/2 binding energies of Ag+, respectively [46]. Also, the peaks were not observed at the binding energies of 369.2 eV and 375.8 eV, indicating no formation of Ag0 metal in the nanocomposite. The two XPS peaks appeared at 516.9 eV and 524.4 eV in Figure 7c which are assigned to the V 2P5/2 and V 2P3/2 binding energies of V5+, respectively [30]. Whereas, in the case of O 1s, two XPS peaks detected in Figure 7e at 530.02 eV and 531.42 eV are attributed to the V-O bond in the AgVO3 and the Fe-O bond in the α-FeOOH, respectively [43]. As shown in Figure 4f, the two XPS peaks are located at 711.5 eV and 719.5 eV due to the binding energies of Fe 2p3/2 and Fe 2p1/2, respectively, which indicates that the Fe3+ is present in the nanocomposite. The results confirm the successful formation of α-FeOOH/α-AgVO3 composites.

2.3. Photocatalytic Performance

The photocatalytic performances of the as-synthesized photocatalysts were assessed by the photodegradation of pollutant RhB under visible-light illumination. In order to find out the photostability nature of RhB in the visible-light region, the blank experiments were also performed without any photocatalysts for 90 min. The obtained degradation results of pure α-FeOOH, pure α-AgVO3 and different α-FeOOH/α-AgVO3 nanocomposite photocatalysts are presented in Figure 8a,b. From Figure 8a,b, it can be seen that the concentration of RhB is not changed after 90 min of blank experiments under illumination of visible-light, indicating that the RhB has almost no self-degradability. The pure α-FeOOH and pure α-AgVO3 shows only 23.9% and 38.8% of degradation efficiency, respectively. Whereas, the α-FeOOH/α-AgVO3 nanocomposites displayed the greatly enhanced photocatalytic performance when compared with pure α-FeOOH and pure α-AgVO3. The improvement in the nanocomposite photocatalysts is mainly attributed to the lower band-gap energy and the extended visible-light absorption ability [32,33]. However, among the different nanocomposites, the FeAVO-3 nanocomposite achieves the highest degradation efficiency of 88.0% within 90 min, superior to the other nanocomposites. The decreased photocatalytic performance of the nanocomposites beyond the addition of 3 mg of α-FeOOH is probably ascribed to the poor catalytic activity of α-FeOOH, clearly indicating that the amount of α-FeOOH also creates the major impact on the photocatalytic performance of α-FeOOH/α-AgVO3 nanocomposite.
In addition, the obtained photodegradation results were fitted with the following first order kinetics Equation (1) to determine the degradation rate of the reaction.
Ln(C0/C) = kt
where, C0 and C denote the RhB concentration at the initial stage and at a certain time, respectively, k denotes the pseudo-first-order rate constant (min−1) and t denotes irradiation time (min). The linear relationship between Ln(C0/C) and t towards the degradation of RhB and the acquired rate constant values corresponding to different samples are illustrated in Figure 8c,d. It is clearly revealed that the RhB degradation follows the pseudo-first-order kinetics. As seen from Figure 8d, the FeAVO-3 nanocomposite exhibits the highest rate constant value when compared with other samples.
The optimized FeAVO-3 nanocomposite was used to investigate the influence of catalyst dosage on photodegradation of RhB. The photodegradation experiment was carried out by changing the catalyst amount from 0.25 g L−1 to 1.50 g L−1 under visible-light eradiation and the result is shown in Figure 9a. This shows that the RhB degradation is significantly increased with increasing the catalyst dosage from 0.25 g L−1 to 1.00 g L−1. This improvement of degradation may be owing to the availability of the more active surface catalyst sites to adsorb a large number of dye molecules on the surface of the photocatalyst and to extend visible-light absorption, resulting in the increase of photocatalytic activity [52]. Even though, the decreased RhB degradation is observed beyond the addition of 1.00 g L−1, that may be reason for the turbidity of solution and scattering of irradiated light [58]. Lastly, this result shows that the higher catalyst dosage is not appropriate for degradation of RhB.
The active species which are more responsible for the degradation process of RhB by FeAVO-3 nanocomposite is also another important factor for its practical application. Hence, the different types of radical scavengers specifically benzoquinone (BQ), ammonium oxalate (AO) and isopropanol (IPA) were introduced into the reaction solution as scavengers of superoxide radical (•O2), hole (h+) and hydroxide radical (•OH), respectively, for carrying out radical trapping measurements as given in Figure 9b. As shown in Figure 9b, no considerable change is observed in the degradation of RhB with the addition of IPA, indicating non-participation of •OH in the degradation process. In addition, the obvious decrease in the degradation of RhB is observed when introducing AO and BQ into the reaction solution. This result reveals that the h+ and •O2 are important active species and play an essential role in the degradation process of RhB over α-FeOOH/α-AgVO3 nanocomposite under visible-light excitation.
The excellent photostability and reusable nature of the as-synthesized photocatalysts are the most crucial factors for its large-scale applications. Therefore, the four successive recycling experiments were performed for the optimized FeAVO-3 nanocomposite under visible-light illumination as depicted in Figure 9c. At the end of every cycle, around 70% of the photocatalyst sample was recovered from the degraded solution via centrifugation followed by being dried at 80 °C for 3 h. After that, the recovered samples were weighed again and followed by replenishing the lost portion and introduced into fresh RhB solution for the next batch. Figure 9c reveals that the photocatalytic activity of the FeAVO-3 nanocomposite remains constant after four cycles. Moreover, the changes in the crystalline structure of the FeAVO-3 nanocomposite were analyzed by XRD before and after recycling experiments and the acquired XRD patterns are given in Figure 9d. It is indicated that there is no considerable change in the crystalline structure of the sample before and after recycling. These observed results infer that the α-FeOOH/α-AgVO3 nanocomposite exhibits excellent photostability and reusability properties. Consequently, the as-synthesized α-FeOOH/α-AgVO3 nanocomposite is a potential material and can be used in long-term practical application for the removal of organic pollutants from waste-water. Additionally, the comparison of photocatalytic performance of the as-synthesized α-FeOOH/α-AgVO3 nanocomposite towards the degradation of RhB with that of some previously reported silver-based photocatalysts and commercially available P25 TiO2 nanoparticles is shown in Table 1. It is revealed that the α-FeOOH/α-AgVO3 nanocomposite exhibits a superior photocatalytic activity to those previously reported in the literatures and P25 TiO2 [30,31,41,53,59,60].

2.4. Photocatalytic Degradation Mechanism

A predictable charge-transfer mechanism for the increased degradation of RhB using α-FeOOH/α-AgVO3 nanocomposite under irradiation of visible-light is presented in Figure 10. The different energy levels are the most important requirement for formation of the composites of two different photocatalysts, which helped to produce higher electron-hole charge carriers through reducing the recombination. The valence band (VB) and conduction band (CB) potentials of the as-obtained pure α-FeOOH and α-AgVO3 were determined through the following Equations (2) and (3) [24,61].
EVB = X − Ee + 0.5Eg
ECB = EVB − Eg
Here, EVB is the VB potential, ECB is the CB potential, X is the electronegativity and Eg is the band-gap energy of the semiconductor, respectively. Ee stands for the energy of free electrons on the hydrogen scale (−4.5 eV). Furthermore, the absolute electronegativity values of α-FeOOH and AgVO3 are 6.38 eV and 5.86 eV, respectively [45,52]. According to the above equations, the VB and CB potential values of the pure α-AgVO3 are calculated to be 2.47 eV and 0.24 eV, respectively. In parallel, the VB and CB potential values of pure α-FeOOH are calculated to be 2.84 eV and 0.92 eV, respectively, and these calculated values are also shown in Figure 10. Under illumination, the ground states of α-AgVO3 and α-FeOOH photocatalysts are turned into excited states, creating photo-excited electron-hole pairs. Afterwards, the photo-excited electron in the CB of α-AgVO3 can freely transfer into that in the CB of α-FeOOH because the CB potential of α-AgVO3 is lower than that of the α-FeOOH. Simultaneously, the photo-excited hole in the VB of α-FeOOH can readily transfer into that in the VB of α-AgVO3 because the VB potential of α-FeOOH is higher than that of the α-AgVO3. In this manner, the electron and holes are efficiently separated as a result of inhibition of the electron-hole recombination, which is beneficial for the improvement of photocatalytic performance of the α-FeOOH/α-AgVO3 nanocomposite.

3. Materials and Methods

3.1. Materials

Silver nitrate (AgNO3) was purchased from Shantou Xilong Chemical (Guangdong, China). Ammonium metavanadate (NH4VO3) was obtained from Tianjin Guangfu Fine chemical (Tianjin China). Iron nitrate nonahydrate (Fe(NO3)3•9H2O) and ammonium oxalate (AO) were purchased from Tianjin Fuchen Chemical Reagent Co., Ltd. (Tianjin, China). Isopropyl alcohol (IPA), Benzoquinone (BQ) and absolute ethanol were bought from Beijing Chemical Industry Co., Ltd (Beijing, China). Rhodamine B (RhB) was received from Tianjin Hengxing Chemical Factory (Tianjin, China). All the above chemicals and reagents were of analytical grade and used without any further purification.

3.2. Synthesis of α-FeOOH Nanorods

The pure α-FeOOH nanorods were synthesized via a simple solvothermal method. Briefly, 1 g of Fe(NO3)3•9H2O was dissolved in 30 mL of isopropanol and 30 mL of deionized (DI) water by magnetically stirring for 30 m at room temperature. The reaction solution pH was adjusted to 12.0 using 0.5 M NaOH solution, and the solution was stirred again for 30 min. Then, the obtained mixture was moved into a 100 mL Teflon-lined stainless steel autoclave followed by the solvothermal reaction carried out at 180 °C for 12 h. Lastly, the acquired precipitate was filtered and washed with DI water and absolute ethanol several times to wash out all the impurities if any and then dried at 80 °C for 3 h.

3.3. Synthesis of α-FeOOH/α-AgVO3 Nanocomposites

The rod-on-rod–like α-FeOOH/α-AgVO3 nanocomposite photocatalysts were synthesized by a facile co-precipitation method. First, the different amounts of (1 mg, 2 mg, 3 mg, 4 mg and 5 mg) of α-FeOOH nanorods and 2 mmol of AgNO3 were dissolved in a beaker using 30 mL of DI water by ultra-sonication. At the same time, 2 mmol of NH4VO3 was dissolved in another beaker using 30 mL of DI water by ultra-sonication. Then, the NH4VO3 solution was slowly dripped into the α-FeOOH-AgNO3 containing solution under stirring. After the addition of NH4VO3 the solution, the formed precipitate was continuously stirred for 3 h in indoor light-shielding conditions. Finally, the attained α-FeOOH/α-AgVO3 nanocomposites were filtered and washed with DI water and absolute ethanol regularly, several times to remove impurities if any and then dried at 80 °C for 3 h. For comparison, the pure α-AgVO3 was also synthesized by the same method without adding α-FeOOH. Furthermore, the as-synthesized α-FeOOH/α-AgVO3 nanocomposites with 1 mg, 2 mg, 3 mg, 4 mg and 5 mg α-FeOOH were denoted as FeAVO-1, FeAVO-2, FeAVO-3, FeAVO-4 and FeAVO-5, respectively in the present work.

3.4. Characterization Techniques

The as-synthesized α-FeOOH/α-AgVO3 nanocomposite photocatalysts were characterized by using various techniques. The X-ray powder diffraction (XRD) patterns of the samples were collected by a Rigaku D/Max2500VB2+/PCX diffractometer with Cu Kα radiation in 2θ range from 10° to 80° at a scan rate of 10° min−1. The Fourier transform infrared (FT-IR) spectra of the samples were completed via Perkin-Elmer FT-IR spectrometer in the range of 400 cm−1–4000 cm−1. The UV-Vis diffuse reflectance spectra of the samples were conducted on a Shimadzu UV-2600 (Tokyo, Japan) UV-vis spectrophotometer. The scanning electron microscopy (SEM) measurements of samples were performed on a Hitachi S-4700 scanning electron microscope at an accelerating voltage of 20 kV. The high-resolution transmission electron microscopy (HR-TEM) measurements of the samples were carried out by a H-800 JEOL JEM-2100F high-resolution transmission electron microscope operated at 300.0 kV. The X-ray photoelectron spectra (XPS) of the samples were performed on a Thermo Fisher ESCAESCALAB 250 X-ray photoelectron spectrometer with monochromated Al Kα radiation source.

3.5. Photocatalytic Degradation Experiments

The photocatalytic activities of the as-synthesized photocatalysts were evaluated towards the degradation of RhB under visible-light irradiation. The 500 W xenon lamp was used as a visible-light source which was connected with a UV light cut-off filter in order to cut wavelength below 420 nm. In each experiment, 50 mg of the photocatalyst was well dispersed in 50 mL RhB dye solution (1 × 10−5 M). Prior to irradiation, the suspension was magnetically stirred in the dark for 30 min to achieve adsorption-desorption equilibrium. During irradiation, 5 mL suspension was taken out after a proper time interval and centrifuged to eliminate the photocatalyst particles. Afterwards, the concentration of the RhB was measured via a MAPADA UV-visible spectrophotometer (Shanghai, China) based on its characteristic absorbance (λmax) at 554 nm. The degradation efficiency of the photocatalysts was calculated using Equation (4).
Efficiency ( % ) = C 0 C C 0 × 100 % = A 0 A A 0 × 100 %
Here, C0 and A0 denotes the concentration and absorption of dye at the time t = 0, respectively, and C and A denotes the concentration and absorption of the dye at certain time intervals.

4. Conclusions

In conclusion, a series of α-FeOOH/α-AgVO3 nanocomposite photocatalysts were synthesized by a facile co-precipitation method with the different contents of solvothermaly obtained α-FeOOH nanorods for the first time. The structural, morphological, optical natures and elemental compositions of the as-synthesized photocatalysts were investigated by XRD, FT-IR, DRS-UV, FE-SEM, elemental mapping, HR-TEM and XPS analysis. The photocatalytic activities of the α-FeOOH/α-AgVO3 nanocomposites were evaluated by the degradation of RhB dye under visible-light excitation. It can be demonstrated that the α-FeOOH/α-AgVO3 nanocomposite obtained from 3 mg of α-FeOOH displays a greater photocatalytic activity when compared with that of pure α-AgVO3 and α-FeOOH. This observed efficient photocatalytic activity of the nanocomposite is mainly ascribed to the increased charge separation efficiency and high visible-light absorption ability. Meanwhile, this optimized α-FeOOH/α-AgVO3 nanocomposite exhibits the outstanding photostability and reusability properties. An acceptable charge-transfer mechanism for the enhanced degradation of RhB over α-FeOOH/α-AgVO3 nanocomposite has also been proposed. Thereby, the above acquired results revealed that the α-FeOOH/α-AgVO3 nanocomposite can be a prospective degradation material for application in wastewater treatment.

Author Contributions

J.P. proposed the concept, and supervised the research work at BUCT. M.S., R.A.S. and J.P. designed the experiments; M.S., R.A.S., S.O. and A.K. performed the experiments. All authors analyzed the data and wrote the paper.

Funding

This research was funded by the National Natural Science Foundation of China grant number (21676022), and the Fundamental Research Funds for the Central Universities grant number (BHYC1701A & JD1701).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. A schematic representation of the synthesis route of α-FeOOH/α-AgVO3 nanocomposite photocatalyst.
Figure 1. A schematic representation of the synthesis route of α-FeOOH/α-AgVO3 nanocomposite photocatalyst.
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Figure 2. (a) X-ray diffraction (XRD) patterns of pure α-FeOOH, pure AgVO3 and different α-FeOOH/α-AgVO3 nanocomposite photocatalysts and (b) XRD patterns of pure α-FeOOH and pure α-AgVO3 with their corresponding (hkl) values.
Figure 2. (a) X-ray diffraction (XRD) patterns of pure α-FeOOH, pure AgVO3 and different α-FeOOH/α-AgVO3 nanocomposite photocatalysts and (b) XRD patterns of pure α-FeOOH and pure α-AgVO3 with their corresponding (hkl) values.
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Figure 3. (a) Fourier-transform infrared (FT-IR) spectra of pure α-FeOOH, pure α-AgVO3 and different α-FeOOH/α-AgVO3 nanocomposite photocatalysts and (b) UV-Visible diffuse reflectance spectra of pure α-FeOOH and pure α-AgVO3 and optimized FeAVO-3 nanocomposite photocatalysts.
Figure 3. (a) Fourier-transform infrared (FT-IR) spectra of pure α-FeOOH, pure α-AgVO3 and different α-FeOOH/α-AgVO3 nanocomposite photocatalysts and (b) UV-Visible diffuse reflectance spectra of pure α-FeOOH and pure α-AgVO3 and optimized FeAVO-3 nanocomposite photocatalysts.
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Figure 4. Scanning electron microscopy (SEM) images of (a,b) pure α-FeOOH, (c,d) pure α-AgVO3 and (e,f) optimized FeAVO-3 nanocomposite photocatalysts at different magnification.
Figure 4. Scanning electron microscopy (SEM) images of (a,b) pure α-FeOOH, (c,d) pure α-AgVO3 and (e,f) optimized FeAVO-3 nanocomposite photocatalysts at different magnification.
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Figure 5. (ad) High-resolution transmission electron microscopy (HR-TEM) images of the optimized FeAVO-3 nanocomposite photocatalyst at different magnifications with the corresponding selected area (electron) diffraction (SAED) pattern.
Figure 5. (ad) High-resolution transmission electron microscopy (HR-TEM) images of the optimized FeAVO-3 nanocomposite photocatalyst at different magnifications with the corresponding selected area (electron) diffraction (SAED) pattern.
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Figure 6. Element mapping images of the optimized FeAVO-3 nanocomposite photocatalyst.
Figure 6. Element mapping images of the optimized FeAVO-3 nanocomposite photocatalyst.
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Figure 7. X-ray photoelectron spectra (XPS) of the optimized FeAVO-3 nanocomposite photocatalyst: (a) Full survey spectrum, (b) Ag 3d, (c) V 2p, (d) O 1s and (e) Fe 3d.
Figure 7. X-ray photoelectron spectra (XPS) of the optimized FeAVO-3 nanocomposite photocatalyst: (a) Full survey spectrum, (b) Ag 3d, (c) V 2p, (d) O 1s and (e) Fe 3d.
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Figure 8. (a) Photodegradation plots, (b) the degradation efficiency, (c) first-order kinetics plots and (d) the rate constant values of the pure α-FeOOH, pure α-AgVO3 and different α-FeOOH/α-AgVO3 nanocomposite photocatalysts for the degradation of Rhodamine B (RhB) under visible-light illumination.
Figure 8. (a) Photodegradation plots, (b) the degradation efficiency, (c) first-order kinetics plots and (d) the rate constant values of the pure α-FeOOH, pure α-AgVO3 and different α-FeOOH/α-AgVO3 nanocomposite photocatalysts for the degradation of Rhodamine B (RhB) under visible-light illumination.
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Figure 9. (a) Effect of catalyst dosage from 0.50 g L−1–1.50 g L−1, (b) effect of scavengers on the degradation of RhB, (c) the recycling runs for the degradation of RhB using the optimized FeAVO-3 nanocomposite in visible-light irradiation and (d) XRD patterns of the optimized FeAVO-3 nanocomposite before and after recycling test.
Figure 9. (a) Effect of catalyst dosage from 0.50 g L−1–1.50 g L−1, (b) effect of scavengers on the degradation of RhB, (c) the recycling runs for the degradation of RhB using the optimized FeAVO-3 nanocomposite in visible-light irradiation and (d) XRD patterns of the optimized FeAVO-3 nanocomposite before and after recycling test.
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Figure 10. A proposed charge-transfer mechanism for the increased degradation of RhB using α-FeOOH/α-AgVO3 nanocomposite photocatalysts under visible-light illumination.
Figure 10. A proposed charge-transfer mechanism for the increased degradation of RhB using α-FeOOH/α-AgVO3 nanocomposite photocatalysts under visible-light illumination.
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Table 1. Comparison of photocatalytic activity of the α-FeOOH/α-AgVO3 nanocomposite with that of other silver-based composite materials in the literatures.
Table 1. Comparison of photocatalytic activity of the α-FeOOH/α-AgVO3 nanocomposite with that of other silver-based composite materials in the literatures.
MaterialPollutantLight SourceIrradiation TimeDegradation Efficiency (%)Reference
InVO4/AgVO3RhBVisible200 min99.8[30]
g-C3N4/Ag2MoO4RhBVisible120 min85.0[31]
BiVO4/AgVO3RhBVisible240 min65.7[41]
α-FeOOH/Ag3PO4RhBVisible120 min76.0[53]
Ag3VO4/ZnORhBVisible450 min100[59]
Ag2WO4/WO3RhBVisible120 min94.0[60]
P25 TiO2RhBVisible90 min28.2This work
α-FeOOH/α-AgVO3RhBVisible90 min88.0This work

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