Article pubs.acs.org/IECR
Synthesis, Photocatalytic and Electrocatalytic Activities of Wormlike GdFeO3 Nanoparticles by a Glycol-Assisted Sol−Gel Process Li Li,†,‡ Xiong Wang,*,† Yan Lan,† Wen Gu,† and Silan Zhang† †
School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Department of Chemistry and Chemical Engineering, Huainan Normal University, Huainan 232001, China
‡
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S Supporting Information *
ABSTRACT: Wormlike GdFeO3 (GFO) nanoparticles were synthesized by a glycol-assisted sol−gel rapid calcination process. The as-synthesized GdFeO3 was characterized by X-ray diffraction, transmission electron microscopy, differential scanning calorimeter and thermogravimetric analysis, Fourier transformed infrared spectroscopy, and UV−vis absorption spectroscopy. The visible-light-responsive photocatalytic activity of GdFeO3 nanoparticles was evaluated by the photodegradation of Rhodamine B under visible light. The synergism of semiconductor-photocatalyzed oxidation and heterogeneous photo-Fentonlike reaction can markedly promote the photodegradation with a booming catalytic activity (k = 1.2814 h−1). Compared with the bulk, the catalytic activity of the GFO−H2O2 system was improved about 80 times. Meanwhile, the glass carbon electrode decorated with the resulting nanoparticles was used to examine the electrocatalytic behavior for p-nitrophenol reduction in a basic solution. The results show the obtained GdFeO3 nanoparticles with excellent photocatalytic and electrocatalytic activities.
1. INTRODUCTION Rare-earth based perovskite orthoferrites with composition LnFeO3 (Ln = rare-earth element) have been intensely studied since the 1960s because of their interesting opto-magnetic properties.1−3 They are a class of materials having potential in various applications such as gas separators, sensors, fuel cells, catalysis, magneto-optic materials, etc.4−7 As one of the interesting perovskites, GdFeO3 (GFO) is important because it is the prototype material for the GdFeO3 (Pbnm space group) family structures, which commonly occurrs among peroskites. At the same time, GdFeO3, belonging to the class of orthoferrite (G-type magnet), possesses a strong uniaxial anisotropy. It has attracted much attention because of their high coercivity and Faraday rotation and thus is an interesting material for magnetooptical data storage devices.8−10 Nanoscaled materials have unique properties conferred by particles of very small dimensions ( 400 nm) using a 500 W Xe lamp. Figure 5 shows the temporal evolution of the absorption spectrum of the RhB aqueous solution using GdFeO3 nanoparticles as photocatalyst. During the photodegradation, the color changed from initial red to essentially 9132
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Figure 5. Absorption changes of RhB solution in visible-light-induced photocatalytic process for (a) GFO nanoparticles (inset: degradation efficiency of RhB with irradiation time), (b) the bulk GFO (inset: magnification of absorption curve of RhB), and (c) nanoparticulate GFO−H2O2 (inset: degradation efficiency of RhB), respectively. (d) The corresponding photocatalytic kinetics.
Therefore, the photocatalytic activities can be evaluated by the apparent first-order rate constant k. As compared with that of the bulk GdFeO3 (k = 0.0160 h−1), the photocatalytic activity of GFO nanoparticles was improved by about 21 times (k = 0.3427 h−1), which might be related with the higher surface area (5.1 m2 g−1, see the Supporting Information, Figure S2). It has been confirmed that larger surface area endows higher photocatalytic activity for the increased reactive sites and the promoted electron−hole separation efficiency.29,30 When a small amount of H2O2 was introduced into the photoreaction suspension, the interfacial Fe atoms (denoted as FeIII) of GdFeO3 nanoparticles with the extremely high surface potential can react with H2O2 to form •OH radicals through Fenton-like reaction (eqs 1−3). Simultaneously, when the photocatalyst is illuminated by light with energy equals to or greater than the band gap, electrons are excited from the valence band to the conductor band, thus generating electron− hole pairs (eq 4). The electron is captured by H2O2 molecules as an efficient electron scavenger, producing highly reactive and nonselective hydroxyl radicals which can induce mineralization of organic pollutants (eqs 5−8). Therefore, the synergism of semiconductor−photocatalyzed oxidation and heterogeneous Fenton-like reaction can markedly promote the degradation of RhB with a booming catalytic activity (k = 1.2814 h−1).
colorless, the characteristic absorption at wavelength of 553 nm decreased remarkably with the irradiation time and nearly 76% RhB in the solution was decomposed after 4 h. In comparison, about 5.7% of RhB were photodecomposed after 4 h for the bulk GFO according to the results of Figure 5a,b. During the photodegradation, RhB underwent a stepwise deethylation and a synchronously hydroxylation/oxidation process25,26 (see the Supporting Information, Figure S1). As can be observed from the present RhB/GFO system, the main absorption at a wavelength of 553 nm decreased remarkably with the irradiation time, and the depletion of the absorbance happened without an evident hypsochromic shift related to the deethylation process, which indicates that the hydroxylation/ oxidation of RhB is a predominant procedure in this condition. For GFO nanoparticles with the addition of H2O2 in Figure 5c, RhB in the solution was totally decomposed within 2.5 h, and the absorption band experienced a slight hypochromatic shift, corresponding to the de-ethylation process of RhB.27,28 Figure 5d exhibits ln C0/C against time profiles under visible light irradiation, where C was the concentration of RhB at the irradiation time t and C0 was the concentration after the adsorption equilibrium on GdFeO3 at t = 0. The ln C0/C linearly increased, indicating that the photodegradation of RhB over GdFeO3 could be described as first-order reaction. 9133
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Figure 6. Recycling test of the nanoparticulate GFO catalyst. Reaction conditions: 0.01 g of nanoparticles and 1 mL of 3% H2O2, 10 mL solution of 1.0 × 10−5 M RhB. Each run of photocatalytic reactions lasted for 2.5 h.
nitrophenol reduction was performed only with a current peak value of 19.04 μA at a potential of −1.01 V, as shown in Figure 7a. The peak current was rather low, which suggested that the GCE exhibited poor electrocatalytic activity for p-nitrophenol reduction in basic solution. However, the nanoparticulate GFOmodified GCE exhibits obvious catalytic activity for pnitrophenol reduction. Compared with the bare GCE, the peak current value had a distinct increase from 19.04 to 39.00 μA while the peak potential had almost no change. The experiments demonstrate that the as-prepared GdFeO3 sample could be used to catalyze p-nitrophenol reduction, for its higher electrocatalytic activity and the stability for p-nitrophenol reduction. To investigate the effect of scanning rate on the electrocatalytic activity, different rates were tested for the GFOmodified GCE. Figure 7b shows the cyclic voltammograms of the modified GCE for p-nitrophenol reduction by increasing scanning rate (v). The peak current (Ipc) of p-nitmphenol reduction (Figure 7c) increases linearly with the square root of the scanning rate (v1/2) in the range of 0.02−0.12 V s−1, indicating that the p-nitrophenol reduction on the GFOmodified GCE is attributed to diffusion-controlled reaction.31
Compared with the bulk, the catalytic activity of the nanoparticulate GFO−H2O2 system was improved about 80 times. Within 2 h, RhB in the solution was thoroughly decomposed. The main reactions involved in the whole photocatalytic reaction can be expressed as below ≡Fe III + H 2O2 → ≡Fe II + •OOH + H+
(1)
≡Fe III + •OOH → ≡Fe II + O2 + H+
(2)
≡Fe II + H 2O2 → ≡Fe III + •OH + OH−
(3)
GdFeO3 + hν → GdFeO3
(e− + h+)
(4)
e− + H 2O2 → OH− + •OH
(5)
dye + hν → dye*
(6)
dye* + h+ → dye+
(7)
dye (or dye+) + •OH → degradation products
(8)
To examine the photostability of the as-prepared GdFeO3 nanoparticles, the nanoparticles after photocatalytic reaction were collected for the subsequent photocatalytic reaction cycles. As well as having high activity, the GdFeO3 photocatalyst can be effectively recycled and used repetitively at least four times without an apparent decrease in photocatalytic activity (Figure 6), which demonstrates their excellent stability. Additionally, the Fe concentration of the final aqueous solution after the degradation experiment was determined by ICP-AES. The concentration of leaching iron was as lower as 0.17 mg L−1. It may be ascribed to the heterogeneous Fentonlike reaction involved in the process, i.e., surface-catalyzed reaction. Compared with the homogeneous Fenton reaction, this process is environmentally benign, and the generation of iron sludge can be avoided. Cyclic voltammograms (CV) of the bare glassy carbon electrode (GCE) and the GCE modified with GdFeO 3 nanoparticles in presence of p-nitrophenol in a basic solution are shown in Figure 7. When a bare GCE was used, p-
4. CONCLUSIONS In conclusion, the pure-phase wormlike GdFeO3 nanoparticles with average particle size of 80 nm are successfully prepared by the glycol-assisted sol−gel process. The visible-light-induced photicatalytic activity was also investigated. It exhibits excellent photocatalytic properties in the visible light region due to their perovskite structures and large surface area, which predicts their promising application in water treatment. The as-prepared GdFeO3 product decorated on a GCE was used to reduce pnitrophenol. The results indicate that p-nitrophenol could be reduced with a higher peak current value at an invariable peak potential on the GCE modified with the GdFeO3 samples. As a whole, the GdFeO3 nanoparticles show excellent visible-lightresponsive photocatalytic behavior and higher electrocatalytic activity for p-nitrophenol reduction in a basic solution. 9134
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Figure 7. (a) Cyclic voltammorgrams of bare GCE (1) and the GCE modified with GdFeO3 nanoparticles (2) at scanning rate of 0.02 V s−1. (b) CV curves of the GFO-modified GCE at diffent scanning rates. (c) The corresponding Ipc ≈ v1/2 plot.
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ASSOCIATED CONTENT
S Supporting Information *
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Deethylation and hydroxylation/oxidation of RhB and nitrogen adsorption−desorption isotherm. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel/Fax: +86-25-84313349. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
ACKNOWLEDGMENTS
We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (21001064), the Natural Science Foundation of Jiangsu Province (BK2010487), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20103219120045), the NUST Research Funding (2011ZDJH27), and the Natural Science Foundation by the Advanced Education Agency of Anhui Province of China (KJ2012Z379). 9135
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