Enhancement of photocatalytic decomposition of perfluorooctanoic acid on CeO₂/In₂O₃

Abstract

CeO₂-doped indium oxide photocatalysts (xCeO₂/In₂O₃) with various CeO₂ doping amounts were synthesized and systematically characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis spectra, photoluminescence spectroscopy (PL) and photocurrent measurements. The as-obtained xCeO₂/In₂O₃ photocatalysts were used in photocatalytic degradation of PFOA for the first time and much higher photocatalytic activity of xCeO₂/In₂O₃ than CeO₂ and In₂O₃ was obtained under UV light irradiation. The excellent photocatalytic activity of 0.86% CeO₂/In₂O₃ could be mainly derived from effective inhibition of recombination of photo-induced electron-holes caused by the charge transfer between CeO₂ and In₂O₃. Moreover, the 0.86% CeO₂/In₂O₃ catalyst showed excellent photocatalytic stability in cycling runs for the photocatalytic degradation of PFOA, providing a great potential of CeO₂/In₂O₃ catalysts for PFOA treatment.

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1. Introduction

Perfluorooctanoic acid (PFOA, C₇F₁₅COOH), as a class of perfluorocarboxylic acids (PFCAs), is widely used as a surface treatment agent in manufacturing, aerospace, automotives, electronics, semiconductors and textiles.¹ The chemical structure of PFOA (Fig. S1†) shows that the PFOA is chemically inert due to its strong C–F bonds (530 kJ mol⁻¹).² As a result, PFOA is extraordinarily thermally and chemically stable. In recent years, many studies have showed that PFOA is environmentally persistent and bioaccumulative,^3,4 and is frequently detected in waters, animals and even humans beings.^5–7 Furthermore, toxicological studies demonstrate that exposure to PFOA can lead to developmental and reproductive toxicities, liver damage and possibly cancer.^8,9 Therefore, developing effective methods to eliminate PFOA from the environment is urgent.

It has been found that natural decomposition processes and many conventional techniques, including biological degradation, oxidation and reduction, are not effective to destruct PFOA under mild conditions.^10,11 In the last few years, several treatment methods have been developed to decompose PFOA, such as electrochemical treatment,^12,13 sono-chemical methods,^14–16 hydrothermal treatment^17,18 and photocatalysis.^19–22 Among these methods, UV-based photocatalysis have been demonstrated to be an effective method for PFOA decomposition under mild conditions.²³ In the previous reports, TiO₂ has been widely investigated to degrade most organic pollutants into less harmful products by hydroxyl radicals (OH˙) formed under ultraviolet (UV) irradiation. However, TiO₂ has low photocatalytic activity to decompose PFOA.^24–28 Recently, it is reported that β-Ga₂O₃ ^29–31 and In₂O₃ ^32–35 showed effective activities for PFOA decomposition under UV irradiation and the degradation production was found to be CO₂ and F⁻.

Indium oxide (In₂O₃), with a direct band gap of 3.6 eV and an indirect band gap of 2.8 eV,³⁶ has been widely applied in the development of semiconductor gas sensors, optoelectronic devices, solar cells and photocatalysis.^37,38 Moreover, In₂O₃ has proved to be an efficient catalyst to decompose PFOA under UV irradiation. PFOA could be decomposed on In₂O₃ via direct oxidation because of high adsorption capacity of In₂O₃ and tight coordination with PFOA.³² Moreover, the high redox potential of hole in In₂O₃ is suitable for oxidation of PFOA. However, the fast recombination of photo-generated electrons and holes limited the photocatalytic performance of In₂O₃. Therefore, considerable attempts have been used to modify In₂O₃ nanostructure and improve the photocatalytic activity for PFOA decomposition. For instance, In₂O₃ nanostructures with different morphologies, such as porous microspheres, nanocubes and nanoplates, have been developed to PFOA decomposition and high photocatalytic activities were found in In₂O₃ material with higher oxygen vacancy defects.³³ Moreover, In₂O₃–graphene composites were synthesized and showed an enhanced photocatalytic activity for PFOA decomposition because of the efficient separation of photogenerated hole–electron pairs.³⁹ To take better advantage of In₂O₃, it is crucial to explore effective modification methods to further optimize its photocatalytic activity toward PFOA degradation.

Cerium oxide (CeO₂), a well-known rare earth oxide, has been widely used in catalysis, electrochemistry, photochemistry and materials science due to its proper electronic structure and high stability.^40–42 It has been investigated that CeO₂ can be used as an efficient cocatalyst because of its wide band gap (E_g = 2.92 eV), which can absorb light near UV region.⁴³ For example, Liu et al.⁴⁴ reported the efficient photocatalytic activity of CeO₂/TiO₂ composite on degradation of methylene blue. Wang et al.⁴⁵ found that ZrO₂/CeO₂ nanocomposite possessed excellent photocatalytic activity toward RhB decomposition. And Song et al.⁴⁶ also found that CeO₂/Ag₃PO₄ composite exhibited enhanced photocatalytic activity to decompose methylene blue and RhB. Thus, it was proposed that CeO₂ could also be used as cocatalyst for In₂O₃ to form CeO₂-doped In₂O₃ composite. Although there is no report using CeO₂-doped In₂O₃ composite as photocatalyst so far, the CeO₂–In₂O₃ composite has been used as an effective gas sensor. As a sensor, a charge transfer from In₂O₃ to CeO₂ has been found in the CeO₂–In₂O₃ composite.⁴⁷ Therefore it is reasonable to speculate that CeO₂–In₂O₃ composite can benefit the electron transfer and promote the separation of the electron–hole pairs, leading to an enhanced photocatalytic activity on PFOA degradation.

In this study, CeO₂-doped In₂O₃ composite (xCeO₂/In₂O₃) with different loading amounts of CeO₂ were synthesized and used for PFOA decomposition under UV light irradiation for the first time. The effect of CeO₂ on photocatalytic activity of xCeO₂/In₂O₃ composite was investigated and the possible photocatalytic mechanism was discussed. The results revealed that the CeO₂/In₂O₃ composite possessed a much higher photocatalytic activity to decompose PFOA as compared to CeO₂ and In₂O₃.

2. Experimental

2.1 Preparation of catalyst

Indium oxide (In₂O₃, 99.99%) was obtained from Aladdin Chemical Reagent Co. The CeO₂/In₂O₃ catalysts with different CeO₂ loading amounts were prepared by the impregnation method. In detail, 1 g of In₂O₃ was impregnated in Ce(NO₃)₃ solution, and the mixture was evaporated to dryness under stirring at 80 °C. The obtained products were dried overnight and calcined at 600 °C for 4 h in the oven. The final products were donated as xCeO₂/In₂O₃, where x was the weight percent of CeO₂ as detected by ICP-AES.

2.2 Characterization of catalysts

Powder X-ray diffraction (XRD) patterns were obtained from a Bruker D8 advanced diffraction-meter with Cu Kα radiation and the scanning angle ranging from 10° to 80°. Transmission electron microscopy (TEM) images were acquired using JEOL JEM2100 microscopy at an acceleration voltage of 200 kV. N₂ adsorption–desorption isotherm was measured on a BEL Belsorp-MAX adsorption analyzer. The samples were outgassed at 280 °C for 6 h prior to the nitrogen adsorption measurement. The specific surface areas of the catalysts were calculated by the BJH model. X-ray photoelectron spectra (XPS) were obtained on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV). UV-vis diffuse reflectance spectroscopy was taken on a Hitachi U-3010 UV-vis spectrometer. Photoluminescence spectra were carried out on a jobinYvon SPEX Fluorolog-3-P spectroscope. Photocurrent was obtained on a CHI 660B electrochemical workstation in a standard three-electrode system. The zeta potential was analyzed using a Brookhaven zeta PALS.

2.3 Photocatalysis experiments

The photocatalytic decomposition of PFOA was conducted in a quartz tube reactor at 25 °C. In a typical experiment, 0.08 g catalyst was added into 200 mL of 100 mg L⁻¹ PFOA solution. The mixture was kept stirring in dark for 30 min to reach adsorption–desorption equilibrium before a 500 W Hg lamp was turned on. At given time intervals of irradiation, 2 mL of suspension was withdrawn and then filtered. The concentrations of PFOA in the filtrate were determined by a high performance liquid chromatography (e2695, Waters) equipped with a 432 conductivity detector. The reaction products were separated on an X-Bridge C18 column (4.6 × 250 mm). The mobile phase was the mixture of methanol/0.02 M NaH₂PO₄ (75/25, v/v) at a flow rate of 1.0 mL min⁻¹ and the injected sample volume was 100 μL. The concentration of fluoride ion (F⁻) was determined by an ICS-2000 Dionex ion chromatography. The total organic carbon (TOC) was measured using an Elementar vario TOC analyzer.

3. Results and discussion

3.1 Catalyst characterization

Fig. 1 depicts the XRD patterns of In₂O₃ and xCeO₂/In₂O₃ catalysts. For In₂O₃, the diffraction peaks appeared at 21.42°, 30.60°, 35.59°, 45.50°, 50.96° and 60.76° were correspond to the (211), (222), (400), (134), (440) and (622) planes of In₂O₃, respectively (JCPDS no. 65-3170). All of these peaks could be seen in the XRD patterns of xCeO₂/In₂O₃, indicating that loading of CeO₂ did not destroy the crystal structure of In₂O₃. Interestingly, the sharper peaks could be observed in xCeO₂/In₂O₃ as compared to In₂O₃, revealing that fine crystallite of In₂O₃ was obtained in xCeO₂/In₂O₃. During the preparation process of xCeO₂/In₂O₃, the catalysts were calcined at 600 °C, which may influence the crystallinity of In₂O₃. Thus, XRD pattern of In₂O₃ after calcined at 600 °C was also measured and it was found that the calcined In₂O₃ showed better crystallinity than In₂O₃ (Fig. S2†), indicating that calcination at high temperature was beneficial to crystal formation. In addition, no diffraction peaks of CeO₂ species were observed, because the CeO₂ particles were dispersed well on the surface of the xCeO₂/In₂O₃ catalysts.

Fig. 1 XRD patterns of In₂O₃ and CeO₂/In₂O₃ samples with different CeO₂ contents.

BET surface areas of In₂O₃ and xCeO₂/In₂O₃ were calculated from the results of N₂ adsorption–desorption isotherms. The BET surface area of In₂O₃ was 39.7 m² g⁻¹, while the surfaces areas of xCeO₂/In₂O₃ (25.7–32.1 m² g⁻¹) were slightly smaller than that of In₂O₃. It revealed that the introduction of CeO₂ may slightly reduce the BET specific surface of In₂O₃, for some pores of In₂O₃ were blocked by CeO₂ particles.

TEM characterization was used to investigate the microstructure of xCeO₂/In₂O₃ catalysts. Uniform distribution of CeO₂/In₂O₃ nanoparticles with porous structure could be seen from TEM image (Fig. 2a) of 2.33% CeO₂/In₂O₃. The average particle size of CeO₂/In₂O₃ is about 30–40 nm. The high-resolution (HRTEM) image of 2.33% CeO₂/In₂O₃ is shown in Fig. 2b. The basal distances of the lattice fringes are calculated to be about 0.15 nm and 0.29 nm, matching well with the lattice spacing of (622) and (211) planes of In₂O₃, respectively. And the lattice fringes of 0.27 nm is corresponded to the interplanar spacing of (200) planes of CeO₂. It demonstrated that the conjunctions were well-formed without changing the crystal of CeO₂ and In₂O₃.

Fig. 2 (a) TEM image of 2.33% CeO₂/In₂O₃ and (b) HRTEM image of 2.33% CeO₂/In₂O₃.

XPS experiments were carried out to examine the surface component and chemical state of In₂O₃ and 0.86% CeO₂/In₂O₃ catalysts. As depicted in Fig. 3a, the XPS spectra of In3d measured from pure In₂O₃ were spilt into double peaks centered at binding energies of 443.7 and 451.3 eV, respectively. Compared with In₂O₃, 0.86% CeO₂/In₂O₃ showed slight shift to high binding energy, possibly owing to the electron transport from In₂O₃ to CeO₂. Fig. 3b shows the XPS spectra of O1s in In₂O₃ and 0.86% CeO₂/In₂O₃. The two peaks in 443.9 and 531.6 eV were corresponded to the lattice oxygen (O′) and chemisorbed oxygen (O′′) in crystalline In₂O₃, respectively. The ratio of lattice oxygen (O′/(O′ + O′′)) in 0.86% CeO₂/In₂O₃ was larger than that of In₂O₃, indicating the CeO₂ doping could improve the degree of crystallinity of In₂O₃. The result was consistent with the results of XRD.

Fig. 3 XPS spectra of (a) In3d and (b) O1s for In₂O₃ and 0.86% CeO₂/In₂O₃.

The photoelectrochemical properties of CeO₂/In₂O₃ were clarified through UV-visible absorption spectroscopy, photoluminescence spectroscopy (PL) and transient photocurrent response measurements. As shown in UV-vis absorption spectra (Fig. 4a), 0.86% CeO₂/In₂O₃ exhibited higher absorption intensity in the UV light region as compared to pure In₂O₃. The absorption of 0.86% CeO₂/In₂O₃ showed a slight red-shift than that of In₂O₃. The PL spectra (excited in 250 nm) of In₂O₃ and 0.86% CeO₂/In₂O₃ are shown in Fig. 4b. In comparison of In₂O₃, the 0.86% CeO₂/In₂O₃ catalyst exhibited a lower emission intensify, indicating the recombination of the photogenerated charge carrier was inhibited. Moreover, the results of transient photocurrent responses (Fig. 4c) showed that the photocurrent of 0.86% CeO₂/In₂O₃ (5.35 μA) was much higher than that of In₂O₃ (2.32 μA). The phenomena suggested that doping CeO₂ could improve the separation efficiency of photogenerated electrons and holes, probably due to the interfacial charge transformation from In₂O₃ to CeO₂.

Fig. 4 (a) UV-vis absorption spectra; (b) photoluminescence spectra and (c) transient photocurrent responses of In₂O₃ and 0.86% CeO₂/In₂O₃.

3.2 Photocatalytic decomposition of PFOA

Fig. 5a shows the photocatalytic decomposition process of aqueous PFOA (100 mg L⁻¹) in the presence of 0.08 g of xCeO₂/In₂O₃ catalyst under UV light irradiation. The decomposition efficiencies in 60 min were 43.2%, 53% and 88% for P25, CeO₂ and In₂O₃, respectively, while the xCeO₂/In₂O₃ catalysts possessed higher degradation efficiency. The control experiment without light irradiation is exhibited in Fig. S3.† It is clear that little perfluorooctanoic acid has been adsorbed by xCeO₂/In₂O₃, indicating adsorption is not the main procedure in the photocatalytic decomposition process.

Fig. 5 (a) Photocatalytic degradation of PFOA over P25, CeO₂, In₂O₃ and xCeO₂/In₂O₃ catalysts and (b) the pseudo-first-order rate constant (k_obs) for the photocatalytic degradation of PFOA by P25, CeO₂, In₂O₃ and xCeO₂/In₂O₃ catalysts.

The kinetic of the photocatalytic decomposition of PFOA could fit well to the pseudo-first-order model, which could be described as follows:

ln(C₀/C_t) = kt (1)

where k is the rate constant (min⁻¹), C₀ and C_t are the concentrations of PFOA aqueous solution at irradiation times of 0 and t min, respectively. As described in Fig. 5b, the rate constants for PFOA decomposition over P25, CeO₂, In₂O₃, 0.61% CeO₂/In₂O₃, 0.86% CeO₂/In₂O₃, 1.34% CeO₂/In₂O₃ and 2.33% CeO₂/In₂O₃ were found to be 0.54 h⁻¹, 0.72 h⁻¹, 2.10 h⁻¹, 2.22 h⁻¹, 3.78 h⁻¹, 2.82 h⁻¹ and 2.4 h⁻¹, respectively. It can be concluded that the activities of xCeO₂/In₂O₃ catalysts depended on the composition of CeO₂ since a gradual increase in CeO₂ content led to an enhancement of decomposition efficiencies with 0.86% CeO₂/In₂O₃ exhibiting the highest photocatalytic activity. Further increase in CeO₂ content led to a decrease of decomposition efficiency, probably due to the agglomeration of CeO₂ caused a weaker UV light harvesting. The defluorination curve of perfluorooctanoic acid (PFOA) with 0.86% CeO₂/In₂O₃ is displayed in Fig. S4a.† The defluorination ratio of PFOA is calculated by the formulation:where C_F⁻ is the molar concentration of fluoride ion, and C₀ is initial molar concentration of PFOA. It can be observed that the defluorination ratio of PFOA is 53.3% in 60 min. Fig. S4b† depicts the TOC removal curve of perfluorooctanoic acid (PFOA) with 0.86% CeO₂/In₂O₃, in which 54.8% of the PFOA was effectively mineralized after irradiation for 60 min. The result confirmed that PFOA was decomposed to inorganic production and F⁻, and the mechanism was discussed below.

In this study, xCeO₂/In₂O₃ catalysts displayed much higher photocatalytic activities as compared to CeO₂ and In₂O₃, due to a synergistic effect between CeO₂ and In₂O₃ in xCeO₂/In₂O₃ catalysts. As known, the conduction band (CB) bottom and the valence band (VB) top of In₂O₃ lie at −0.63 and 2.17 eV versus NHE, respectively.⁴⁸ Meanwhile, the CB bottom and the VB top of CeO₂ lie at −0.39 and 2.50 eV versus NHE, respectively.⁴⁹ Thus, charge transfer could occur between CeO₂ and In₂O₃ for the interfacial electric field under the UV light irradiation. The photoexcited electrons on the CB of In₂O₃ could easily transfer to the CB of CeO₂. At the same time, photoinduced holes in VB of CeO₂ transferred to In₂O₃. The charge transfer process allowed CeO₂ to scavenge electrons to promote the separation of charge carriers. Moreover, the lifetime of charge carriers related to the radiative process was also prolonged for xCeO₂/In₂O₃, and thus improved the photocatalytic activity of decomposition of PFOA. It has been described by Panchangam et al.⁵⁰ that PFOA was decomposed in a stepwise way.

Firstly, an electron of C₇F₁₅COO_ads⁻ was attracted by h_vb⁺, producing C₇F₁₅COO˙ radical:

h_vb⁺ + C₇F₁₅COO_ads⁻ → C₇F₁₅COO˙ (3)

Then this unstable C₇F₁₅COO˙ radical was transformed to C₇F₁₅ radical after Kolbe decarboxylation reaction:

C₇F₁₅COO˙ → C₇F₁₅ + CO₂ (4)

The C₇F₁₅ radical reacted with ˙OH radicals to form thermodynamically unstable C₇F₁₅OH. And C₇F₁₅OH underwent hydrolysis and HF elimination, which formed C₆F₁₃COO⁻ with the loss of CF₂:

C₇F₁₅OH + H⁺ → C₆F₁₃COF + 2HF (5)

C₆F₁₃COF + ˙OH → C₆F₁₃COO⁻ + HF (6)

Similarly, C₆F₁₃COO⁻ was decomposed into C₅F₁₁COO⁻ by repeating this process. PFOA was finally mineralized to CO₂ and fluoride ions in the stepwise manner. The proposed PFOA decomposition mechanism by CeO₂/In₂O₃ is exhibited in Fig. 6. The main oxidative species in the photocatalytic reaction were further investigated by adding radical scavengers, using KI, tert-butanol (t-BuOH) and benzoquinone (BQ) as hole, hydroxyl radical and superoxide radical scavengers, respectively. As shown in Fig. 7, the photocatalytic reaction of PFOA was greatly inhibited by adding KI and BQ, while slight inhibited by adding t-BuOH, revealing that photogenerated holes and superoxide radicals were the main active species in the photocatalytic process.

Fig. 6 Proposed photocatalytic mechanism of PFOA degradation over 0.86% CeO₂/In₂O₃ under UV light irradiation.

Fig. 7 Effects of scavengers on PFOA degradation over 0.86% CeO₂/In₂O₃.

3.3 Effect of pH

Fig. 8a exhibits the impact of initial pH on the catalytic decomposition of PFOA over 0.86% CeO₂/In₂O₃. With the initial pH increasing from 2.84 to 9.51, the photocatalytic degradation efficiency of PFOA gradually decreased from 100% to 54.9%. Results of zeta potentials of 0.86% CeO₂/In₂O₃ (Fig. 8b) indicated that zeta potentials continuously decreased with the increasing of solution pH and the points of zero charge (pH_pzc) of 0.86% CeO₂/In₂O₃ was 3.8. PFOA in the aqueous solution are in an anionic form in the range of tested pH because the pK_a of PFOA is around −0.1.⁵¹ When the solution pH values were lower than 3.8, the surfaces of the 0.86% CeO₂/In₂O₃ were positively charged owing to the protonation. Herein, PFOA could strongly interact with catalyst by electrostatic interaction, resulting in a high PFOA removal efficiency. At pH 3.8–9.51, the negative charges on the surfaces of 0.86% CeO₂/In₂O₃ were observed due to the deprotonation, leading to an electrostatic repulsion between PFOA and 0.86% CeO₂/In₂O₃. Thus, the degradation efficiency of PFOA on 0.86% CeO₂/In₂O₃ decreased sharply with the increase of solution pH.

Fig. 8 (a) Influence of the initial pH on PFOA degradation by 0.86% CeO₂/In₂O₃ sample and (b) zeta potentials of 0.86% CeO₂/In₂O₃.

3.4 Stability of photocatalyst

In addition to photocatalytic activity, the stability of photocatalysts is another key point for their practical applications. Therefore, the stability of 0.86% CeO₂/In₂O₃ was investigated by recycling experiments to decompose PFOA. As illustrated in Fig. 9, the catalytic efficiency of 0.86% CeO₂/In₂O₃ decreased slightly after recycling tests for 4 times. XRD patterns of the 0.86% CeO₂/In₂O₃ samples before and after photocatalysis were measured and the results are shown in Fig. S5.† It revealed that the crystal structure of 0.86% CeO₂/In₂O₃ had no change after photocatalytic reaction, reflecting that the 0.86% CeO₂/In₂O₃ photocatalyst had a remarkable stability, which could greatly favor the practical application to eliminate the PFOA from wastewater.

Fig. 9 Cycling runs for the photocatalytic degradation of PFOA over 0.86% CeO₂/In₂O₃.

4. Conclusions

In summary, CeO₂-doped In₂O₃ catalysts with different amounts of CeO₂ (xCeO₂/In₂O₃) were fabricated via the conventional impregnation method. The catalysts possessed higher photocatalytic activities than pure CeO₂ and In₂O₃ for the PFOA decomposition under UV light irradiation. The remarkably increased photocatalytic activity of xCeO₂/In₂O₃ could be ascribed to the complementary heterostructure between CeO₂ and In₂O₃, which promoted the separation of charge carriers and prolonged the lifetime of charge carriers. Furthermore, the 0.86% CeO₂/In₂O₃ exhibited excellent stability in cycling tests, demonstrating its promising application in the degradation of PFOA from aqueous phase.

Acknowledgements

The financial supports from the Natural Science Foundation of China (No. 51178223, 51208257, 51478223 and 21307103) and the Natural Science Foundation of Jiangsu Province (BK2012405), China Postdoctoral Science Foundation (No. 2013M541677), Jiangsu Planned Projects for Postdoctoral Research Funds (1202007B) and the Fundamental Research Funds for the Central University (No. 30915011308) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09856h

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