Enhanced catalytic performance of β-FeOOH by coupling with single-walled carbon nanotubes in a visible-light-Fenton-like process

2.1 Catalyst preparation

All chemicals used in this work were of analytical grade, purchased from Shanghai Chemical Reagent Co., Ltd. (China), and used without any further purification.

Four CNTs/β-FeOOH materials, with the different content of single-walled CNTs, were obtained by impregnation-precipitation method. FeCl₃·6H₂O in the amount of 3.2435 g was added to a solution of 2.8800-g urea in 100-ml deionized water under permanent magnetic stirring for 30 min. Without adjusting the pH value, the required mass of single-walled CNTs was added to the above mixed solution. After additional stirring for 24 h, the obtained suspension was transferred into a Teflon-lined stainless steel autoclave (200 ml). The sealed autoclave was maintained at 90°C for 8 h and then cooled to room temperature naturally. The sample was collected by centrifugation, washed several times with absolute ethanol and deionized water, and finally dried at 60°C for 24 h. By applying this procedure, four samples with the single-walled CNTs weight of 0.010, 0.050, 0.10, and 0.20 g in the initial suspensions (denoted as 0.94%CNTs/β-FeOOH, 4.7%CNTs/β-FeOOH, 9.4%CNTs/β-FeOOH, and 18.8%CNTs/β-FeOOH) were prepared. The sample obtained by the same procedure but without addition of single-walled CNTs was denoted as β-FeOOH. For comparison, the photocatalyst denoted as 18.8%CNTs/β-FeOOH-mixing was also prepared by a simple mechanical mixing method. That is, β-FeOOH and CNTs were dispersed into 20 ml of anhydrous ethanol solution and ultrasonicated to make them mingle well. After that, this mixture was fully dried at 60°C in an oven for 24 h.

2.2 Photocatalytic degradation procedures

The photocatalytic degradation of MO was carried out in an XPA-7 photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China). The temperature of reaction suspension was kept at 25±2°C by cooling water circulation. A 500-W xenon lamp was used as an irradiation source of visible light (VL). The initial pH values of suspensions were adjusted to 4.5 with dilute sulfuric acid solution and sodium hydroxide solution. The initial concentrations of MO and H₂O₂ were 80 mg l^-1 and 0.3 g l^-1, respectively, with a catalyst loading of 0.4 g l^-1. All catalysts were dipped in MO solution and stirred in the dark for 60 min to establish adsorption/desorption equilibrium between the solution and the catalysts before irradiation. At given irradiation time intervals, a small quantity of the suspension was taken and centrifuged to separate the catalyst particles from the suspension. The concentration of MO was determined by using a UV-vis spectrophotometer (Beijing Ruili Corp., UV-9100) at 464 nm.

2.3 Catalyst characterization

The powder X-ray diffraction patterns (XRD) were recorded at a scanning rate of 4° min^-1 in the 2θ range of 10–70° using a Bruker D8 Advance instrument with Cu-Kα radiation (λ=1.5406 Å) at room temperature. The morphologies and nanostructures of synthesized products were further observed using a Hitachi H-7650 transmission electron microscope (TEM) at the acceleration voltage of 80 kV. Fourier transform infrared (FTIR) spectrum measurements were performed on a Bruker Vector 22 FT-IR spectrophotometer at room temperature [23], with scanning from 4000 to 400 cm^-1 using KBr pellets. The light reflectance property was studied with a Cary50 UV-vis-NIR spectrophotometer (Varian Australia PYT Ltd.), in which BaSO₄ was employed as the internal reflectance standard. The photoluminescence (PL) spectra for solid samples were investigated on a Cary Eclipse Fluorescence spectrophotometer with an excitation wavelength of 360 nm.

3.1 Structural and morphological characterization

Figure 1A depicts the XRD patterns of β-FeOOH and CNTs/β-FeOOH composites with different weight addition ratios of CNTs (0.94%, 4.7%, 9.4%, and 18.8%) to indicate the crystalline phase of akaganeite in the composites and the effects of CNTs on the crystalline of akaganeite. All the peaks produced by the β-FeOOH sample matched the diffraction of tetragonal akaganeite (JCPDS card No. 34-1266) very well with cell constants of a₀=10.51 Å, b₀=10.51 Å, and c₀=3.033 Å, and no impurity peak can be detected. No typical diffraction peaks of CNTs are obviously observed in the CNTs/β-FeOOH composites. This is because the characteristic peak of CNTs (002) planes at 26.2° is probably overlapped with the (310) diffraction peak of akaganeite, which is consistent with the previous reports [16]. It is also clear to see that the diffraction peaks of akaganeite phase become weaker and broader when increasing the weight addition ratios of CNTs in the composites, which means that the akaganeite particle size reduces.

Figure 1:

XRD patterns of the as-prepared β-FeOOH and CNT/β-FeOOH composites (A); characteristic parts of the FTIR spectra of CNTs and CNTs/β-FeOOH composites (B); TEM images of 0.94%CNTs/β-FeOOH (C); 4.7%CNTs/β-FeOOH (D); 9.4%CNTs/β-FeOOH (E); 18.8%CNTs/ β-FeOOH (F); UV-vis DRS of β-FeOOH and CNTs/β-FeOOH composites (G).

The as-prepared photocatalysts were analyzed by FTIR for further identification. As shown in Figure 1B, the absorption peaks at wavelength 842, 696, 649, and 492 cm^-1 were assigned to the vibration modes of the FeO₆ coordination octahedron [24]. The presence of the absorption peaks at 1639 cm^-1 was assigned to O-H bending and aromatic C=C stretching (skeletal ring vibration) [15]. A band of wavenumber at 1402 cm^-1 is most probably due to sorbed water and overlapping bands in region characteristic for carboxyl C-O groups [25]. Features at 1454 cm^-1 attributed to CNTs vibrational modes (C-C stretching) are also apparent [25]. The FTIR spectrum of CNTs/β-FeOOH composites shows absorption peaks at 1656 and 1612 cm^-1 corresponding to the stretching vibration of C=O from the carboxylic groups [26]. Moreover, the strong absorption bands observed in the range of 400–1000 cm^-1 corresponded to Fe-O-Fe bonding, remarkably weakened with the increase of the weight addition ratios of CNTs. It suggests the presence of Fe-O-C bonds in the composites, indicating the chemical interaction between surface hydroxyl groups of akaganeite and functional groups of CNTs [13, 27].

Figure 1C–F display TEM images of the CNTs/β-FeOOH composites. As seen in these figures, the particle size of β-FeOOH decreases with the increase of the weight addition ratios of CNTs. This means that added CNTs have significant impact on the product particle size. During the synthesis of composites, CNTs can serve as heterogeneous nuclei for the growth of akaganeite, while the existence of CNTs can also inhibit the growth of akaganeite nanoparticles. It is in accordance with the XRD result. In particular, it is worth noting that the sample of 18.8%CNTs/β-FeOOH consists of long CNTs decorated with akaganeite nanoparticles on the surface (Figure 1F). The direct contact interface between akaganeite and CNTs is beneficial for the charge carriers transfer and separation, which might improve the photocatalytic performance of the 18.8%CNTs/β-FeOOH composite. Similar structure has been reported in a previous study [28]. However, the added amount of CNTs is much larger than that of the reference [28]. There are two possible reasons. One is that the concentration of iron ions in the reaction suspension is quite high. The other is that the diameter of single-walled CNT is too small compared with that of akaganeite. Therefore, the nanoparticles of akaganeite may grow and assemble on multiple parallel single-walled CNTs.

Figure 1G shows the ultraviolet-visible light diffuse reflectance spectra (UV-vis DRS) of β-FeOOH and CNTs/ β-FeOOH nanocomposites. Apparently, the visible light absorption increases with the increase of CNTs amount. The results demonstrate the significant influence of CNTs on the optical properties of CNTs/β-FeOOH composites. Similar to the case of TiO₂-CNT or TiO₂ chemically converted graphene composites, the phenomena in this study can be ascribed to the formation of Fe-O-C chemical bonding in the prepared composites [10, 13], which is also confirmed by the FTIR results.

3.2 Photocatalytic activity of CNTs/β-FeOOH nanocomposites

Figure 2 shows the photocatalytic activity of bare β-FeOOH and CNTs/β-FeOOH nanocomposites with different weight ratios of CNTs toward oxidation MO under visible light irradiation. It can be observed that the weight addition ratio of CNTs exhibits a significant influence on the photocatalytic activity of CNTs/β-FeOOH composites. Even with a small-amount addition of CNTs (0.94%), the photocatalytic activity of sample is noticeably increased. The degradation efficiency of MO follows the following order: 18.8%CNTs/β-FeOOH>9.4%CNTs/β-FeOOH>4.7%CNTs/ β-FeOOH>0.94%CNTs/β-FeOOH>bare β-FeOOH. The degradation efficiency of MO facilitated by 18.8%CNTs/ β-FeOOH was 88.9% after 120 min. Compared with previous studies (Table 1), the Fenton-like composite catalyst of 18.8%CNTs/β-FeOOH prepared in this work shows quite good VL catalytic performance.

Figure 2:

Dynamic curves of MO degradation over bare β-FeOOH and CNTs/β-FeOOH composites with different weight additions of CNTs under VL irradiation; (A) 18.8%CNTs/β-FeOOH+tert-butyl alcohol; (B) β-FeOOH; (C) 9.4%CNTs/β-FeOOH-mixing; (D) 18.8%CNTs/β-FeOOH without H₂O₂; (E) 0.94%CNTs/β-FeOOH; (F) 4.7%CNTs/β-FeOOH; (G) 9.4%CNTs/β-FeOOH; (H) 18.8%CNTs/β-FeOOH. Test conditions: pH=4.5; [H₂O₂]=0.3 g l^-1; catalyst loading=0.4 g l^-1.

3.3 Possible mechanism involved in the oxidation of MO

Previous studies have demonstrated that ·OH radicals produced in H₂O₂-iron oxide systems showed a strong capacity to oxidize organics [32–34]. In order to identify whether ·OH was the key oxidant in the oxidation of MO catalyzed by CNTs/β-FeOOH composites, the photocatalytic degradation of MO was repeated with the addition of excess 2-propanol as an ·OH scavenger. Addition of tert-butyl alcohol (2% v/v) in the 18.8%CNTs/β-FeOOH/VL system inhibited the degradation of MO (Figure 2A), which confirms the critical role of hydroxyl radical for MO degradation. However, OH radicals were possibly derived from the photocatalytic process. Therefore, additional experiment in absence of H₂O₂ was carried out and the results are shown in Figure 2D. We can clearly see that MO was degraded by photocatalytic reaction, which exhibited the contribution of about 43% for the degradation of MO in the presence of H₂O₂. This means that both photocatalytic reaction and photo-Fenton-like reaction are responsible for the degradation of MO, which is shown in Figure 3. Based on previous studies [35–38], the degradation process of MO catalyzed by CNTs/β-FeOOH composites can be explained by the following mechanisms (“≡” signified the surface structure of catalysts):

Figure 3:

The photocatalytic schematic diagram of CNTs/βFeOOH samples.

3.4 Analysis of improved catalytic performance of β-FeOOH

On the basis of the above characterizations and discussions, the significant enhancement of photoactivity after the hybridization of β-FeOOH with CNTs can be attributed to the following three factors. Firstly, the photogenerated electrons from β-FeOOH under irradiation can be accepted by CNTs [14, 19], thereby decreasing the recombination probability of the photoexcited electron-hole pairs. To better understand the importance of interfacial interaction between β-FeOOH and CNTs on the degradation efficiently of MO, the sample of 18.8%CNTs/β-FeOOH-mixing was prepared. As seen in Figure 2C, 18.8%CNTs/β-FeOOH-mixing shows much worse photoactivity than 18.8%CNTs/β-FeOOH toward the oxidation of MO under identical reaction conditions. This result suggests that the interfacial interaction at the direct contact between β-FeOOH and CNTs can efficiently contribute to the oxidation of MO, whereas this effect cannot be obtained over 18.8%CNTs/β-FeOOH-mixing with poor interfacial interaction. Actually, CNTs have 1D structure of carbon-based nanocylinders, where the electrons can move freely without any scattering from atoms or defects [27]. Furthermore, 18.8%CNTs/ β-FeOOH composite shows decreased PL intensity than that of bare β-FeOOH (displayed in Figure 4A), suggesting the effectively reduced charge recombination. Secondly, the presence of CNTs in the composite materials is able to enhance the visible light absorption intensity as reflected by the above UV-vis DRS, which is beneficial to both photocatalytic and photo-Fenton-like reaction. Thirdly, CNTs act as a dispersing template or support to decrease the particle size of β-FeOOH in the synthesis process as confirmed by the above TEM images. It is well known that the small-sized catalyst particles have larger specific area, which ensures efficient light absorption and provides more photoreactive sites for degradation reactions [14].

Figure 4:

Photoluminescence spectra of CNTs, 18.8%CNTs/β-FeOOH and β-FeOOH (A); repeated use of 18.8%CNTs/β-FeOOH in the photocatalytic oxidation of MO under visible light irradiation (B).

3.5 Stability and reusability of 18.8%CNTs/ β-FeOOH composite

The stability and reusability of 18.8%%CNTs/β-FeOOH catalyst were evaluated by four successive recycling tests for the oxidation of MO under visible light irradiation, as displayed in Figure 4B. After each cycle, the 18.8%%CNTs/β-FeOOH catalyst was directly centrifuged and entered into the next cycle without any other post treatment. After four cycles of repeated use, 83.8% percentage of MO can still be degraded, which was basically close to the level of the first trial. This indicates that the reuse of the composite catalyst is cost-effective because the catalyst does not have to be replaced over a relatively long period.