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TiO2 nanoparticles: low-temperature hydrothermal synthesis in ionic liquids/water and the photocatalytic degradation for o-nitrophenol ab

b

b

b

Jingtao Dai , Ruiyu He , Yuan Yuan , Wei Wang & Dong Fang

ab

a

Jiangsu Provincial Key Laboratory of Coastal wetland Bioresources and Environmental Protection, Yancheng Teachers University, Yancheng 224002, People's Republic of China; b

College of Chemistry and Chemical Engineering, Yancheng Teachers University, Yancheng 224002, People's Republic of China Published online: 11 Oct 2013.

Click for updates To cite this article: Jingtao Dai, Ruiyu He, Yuan Yuan, Wei Wang & Dong Fang (2014) TiO2 nanoparticles: low-temperature hydrothermal synthesis in ionic liquids/water and the photocatalytic degradation for o-nitrophenol, Environmental Technology, 35:2, 203-208, DOI: 10.1080/09593330.2013.822522 To link to this article: http://dx.doi.org/10.1080/09593330.2013.822522

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Environmental Technology, 2014 Vol. 35, No. 2, 203–208, http://dx.doi.org/10.1080/09593330.2013.822522

TiO2 nanoparticles: low-temperature hydrothermal synthesis in ionic liquids/water and the photocatalytic degradation for o-nitrophenol Jingtao Daia,b , Ruiyu Heb , Yuan Yuanb , Wei Wangb and Dong Fanga,b∗ a Jiangsu

Provincial Key Laboratory of Coastal wetland Bioresources and Environmental Protection, Yancheng Teachers University, Yancheng 224002, People’s Republic of China; b College of Chemistry and Chemical Engineering, Yancheng Teachers University, Yancheng 224002, People’s Republic of China

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(Received 20 November 2012; final version received 11 June 2013 ) The well-crystallized anatase TiO2 -IL nanoparticles were synthesized by one-step routes under low-temperature using room temperature ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate as an additional solvent with water. The photocatalytic properties of TiO2 nanoparticles were evaluated by photocatalytic degradation experiments of o-nitrophenol. The TiO2 nanoparticles show a higher photocatalytic activity than the TiO2 with pure water and commercial TiO2 (P25), which may be related to the high crystallinity. The TiO2 -IL nanoparticles still hold a high photocatalytic activity after the catalyst was recycled nine times. Chemical oxygen demand removal was achieved under optimum experimental conditions. Keywords: TiO2 nanoparticle; ionic liquid; photocatalysis; degradation; o-nitrophenol

Introduction The widespread disposal of industrial wastewater containing organics onto land and water bodies has led to serious contamination in many countries worldwide. Global consumption of water is doubling every 20 years, more than twice the rate of human population growth. To help ensure that water supplies will not dwindle, polluted water, including wastewater from sludge, must be reclaimed.[1] Because of the recalcitrant nature of synthetic organics and the high salinity of wastewater containing organics, conventional biological treatment processes are ineffective.[2] Adsorption and coagulation practices are also applied to treat organics in wastewater, which always result in secondary pollution. Consequently, a more promising technology, photocatalysis has been studied. In a photcatalysis process, when a photocatalyst is illuminated with light of an appropriate wavelength, it generates electron/hole pairs which can further produce free radicals like OH. These radicals are among the more oxidant species which attack many organic molecules into CO2 and water under ambient conditions.[3–6] Amongst various photocatalysts, titanium dioxide has proven to be the most widely used studied and used in environment because of its high activity, chemical stability, robustness photo-degradation, low toxicity, low pollution load and availability at low cost.[7–13] At room temperature, TiO2 exists in three phases: rutile, anatase and brookite. Remarkably, anatase titania obtains much more attention than the others because it is a highly efficient

∗ Corresponding

author. Email: [email protected]

© 2013 Taylor & Francis

photocatalyst.[14–16] The pure anatase phase is thermodynamically less stable than rutile at room temperature, and it can undergo thermal conversion into the rutile phase in the temperature range of 700–800◦ C.[16] Traditional methods such as sol–gel, hydrothermal processes, chemical/physical vapour deposition and electrodeposition have been extensively reported, but most of them involve high-temperature annealing to improve the crystalline, which can cause the decrease in specific surface area. A lot of studies have shown that particle size, morphology, crystallinity, phase purity and surface area have been shown to exert a significant influence on the photoctalytic activity of TiO2 .[17–19] Hence, more attention needs to be focused on the low-temperature preparation of nanocrystalline anatase titania. Recently, attempts have been made to fuse two broad areas of research, the nanoparticles and ionic liquids (ILs) are an exceptional type of solvent consisting virtually only of ions. ILs have practically no vapour pressure and possess tunable solvent properties.[20,21] Now, the investigation indicated that ILs for the preparation of hybrid nanomaterials have novel properties.[22–24] Many common ILs consist of nitrogen-containing organic cations and inorganic anions. Their chemical and physical properties can be tuned to a range of potential applications by varying the nature of cations and anions. Furthermore, the desired structures are easily synthesized and controlled. In particular, imidazolium-based are favourable because of their air, water and electrochemical stability and wide liquid range.

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Very recently they are increasingly finding applications in nanoscience.[11] The potential application of ILs in the synthesis of metallic architectures [17] and metal compounds [18] is particularly interesting because their electrostatic and coordination effects should influence, and may even facilitate this synthesis. ILs seems to be ideal for the stabilization of nanoparticles. The intrinsic ionic charge, high polarity, high dielectric constant and supramolecular network of ILs provide an electrostatic protection according to the Derjaugin–Landau–Verwey–Overbeek theory in the form of a protective shell of metal nanoparticles. More studies have shown that imidazolium ILs may oxidatively add to coordinatively unsaturated low-valent metal centres,[25–27] implying that the ILs might stabilize metal nanoparticles not only electrostatically but also by coordination involving the cations. In this study, we used an imidazolium-based ILs as a template in sol–gel low hydro-treatment methods to synthesize highly active titania photocatalyst. This approach involves the use of room temperature IL 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4 ]) as an additional solvent with water. The low vapour pressure of the IL could prevent reduction of surface area. Furthermore, photo-degradation efficiencies in o-nitrophenol (ONP) wastewater catalysed by TiO2 nanoparticles were evaluated. In addition, the degradation was also assessed in terms of chemical oxygen demand (COD). COD is defined as the number of oxygen equivalents consumed in the oxidation of organic compounds using strong oxidizing agents such as dichromate or permanganate.[28] Experimental Materials Tetrabutyl titanate, 1-methylimidazole, 1-bromobutane and sodium fluoborate were purchased from the Shanghai Chemical Reagent Corporation. The commercial TiO2 (P25) was purchased from Degussa Corporation. All reagents were used as received without further purification. The water used was purified through a Millipore system. Preparation of IL The IL of [BMIM][BF4 ] was synthesized using a method reported previously.[29] The compound was analysed by 1 H nuclear magnetic (1 H NMR), 13 C nuclear magnetic 13 ( C NMR), and Fourier transform infrared (FT-IR) spectroscopy, and the spectral data was found to tally with the structure. Synthesis of TiO2 -IL nanoparticles About 1.6 mL of tetrabutyl titanate was added dropwise to a mixture with 7 mL [BMIM][BF4 ] and 7 mL water under rigorous stirring. White precipitates of hydrous oxides were produced instantly, and the mixture was stirred for

2 h at room temperature. The mixture was then transferred to a Teflon-lined stainless steel autoclave and heated at 100◦ C for 24 h. After cooling, the product (TiO2 -IL) was filtered, washed with deionized water several times and dried in air. For comparison, another TiO2 sample (TiO2 -W) was prepared in pure water under the same conditions. A feature of ILs is their ability to be reused many times. Because of this advantage, ILs have made significant contributions to green chemistry and have been used widely as a reaction medium in organic chemistry.[30,31] Consequently, after the reaction was completed, the reaction mixtures were cooled to room temperature, the TiO2 nanoparticles were isolated by filtration and the IL in filtrate could be recovered easily by evaporation at 80◦ C in vacuum for several hours. The recovered IL could be used for the same reactions directly in TiO2 -IL prepared. The thermogravimetric analysis (TGA) analyses show that [BMIM][BF4 ] is thermally stable up to 320◦ C, which is consistent with the literature.[29,30]

Photocatalytic activities In order to evaluate the photocatalytic activity of TiO2 -IL samples, photocatalytic degradation of ONP was undertaken. A typical experiment constituted 100 mL of 45 mg/L ONP solution and 0.4 g of catalyst taken in a glass reactor. The mixture was stirred 30 min in the dark to establish the adsorption equilibrium between the ONP molecules and the catalyst surface prior to the photocatalytic test. The mixture was irradiated by a 250 W high-pressure Hg lamp and solar radiation to result in the degradation of ONP. During the reaction, the mixture was magnetically stirred in a water bath at 25◦ C. After definite time intervals, 2.2 mL of the sample solution was withdrawn and centrifuged. The residual concentration of ONP in each sample was measured by a UV–vis spectrophotometer at the characteristic absorption band of ONP. The maximal absorbency of ONP solution abides with the Lambert–Beer law and decolorization ratio (C0 − C)/C0 , can be simply calculated from equation (A0 − A)/A0 .

COD measurements COD is one of the most commonly used measurable parameters in assessing water quality and an important index for the control of operation of wastewater treatment plants.[31] COD content indicates the amount of organic containments. COD measurements were carried out by the dichromate titration method on the ONP solutions before and after 1 h of light irradiation. The detail is presented as follows. The sample was refluxed with a known volume of standard K2 Cr2 O7 , AgSO4 and H2 SO4 for 2 h and titrated with standard ferrous ammonium sulphate (FAS) using a ferroin indicator. A blank titration was carried out with distilled water instead of

Environmental Technology sample. COD was calculated using the following equation: COD =

[(V0 − V1 ) × C × 8 × 1000] , V

(1)

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where V0 is the blank titre value, V1 is the sample titre value, C is normality of FAS and V is volume of the sample.

Characterization of TiO2 nanoparticles Transmission electron microscope (TEM) analysis was conducted on a Philips Tecnai-12 (Philips) field emission transmission electron microscope at 120 kV. The high-resolution TEM images were obtained by a Philips model JEM-2100F electron microscope at 200 kV. X-ray diffraction (XRD) measurement was carried out on PANALYTICAL/X PERT PROMPD diffractometer at 40 kV with Cu Kα radiation (λ = 0.1542 nm). Ultraviolet and visible (UV–vis) absorption spectra were analysed by a TU-1810 SPC spectrophotometer. TGA were recorded with SDT-Q600. The irradiation source of photocatalytic reactions was a 250 W high-pressure mercury lamp.

Results and discussion Characterization of product Figure 1 shows the XRD patterns of the TiO2 nanoparticles prepared under different conditions. The anatase phase was produced in TiO2 -IL nanoparticles prepared from the mixture solvent of IL and water (Figure 1(a)). However, it is clear that titania prepared from pure water contains brookite phase at 2θ = 31◦ (Figure 1(b)) besides the anatase phase. The XRD patterns of TiO2 -IL nanoparticles show broad peaks, indicating a small crystallite size. This result indicates that when the IL was added to the system, the brookite phase almost disappears. This can be attributed to [BMIM][BF4 ] as an additional solvent with water.

a

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Figure 1.

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XRD diffraction patterns of TiO2 -IL (a), TiO2 -W (b).

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Figure 2 illustrates the TEM images of TiO2 nanoparticles prepared from different solvents. Nanostructured TiO2 of IL-assisted preparation (Figure 1(a), (c)) was a ca. 10 nm. It is clearly seen that most of the TiO2 nanoparticles are highly crystalline in shape, as shown by clear lattice fringes in the HRTEM images (Figure 2(d)). TiO2 -IL nanoparticles with lattice fringes of d = 0.351 nm that are well matched with the crystallographic plane of anatase TiO2 (101) are observed in Figure 2(d). The diffraction pattern of selected area electron diffraction (inset) for the sample also clearly indicates a highly crystalline structure. However, the nanoparticles prepared from pure water TiO2 -W (Figure 2(b)) are distorted, aggregate and lower crystalline. Photocatalytic activity Figure 3 shows the variation of ONP concentration with time in the presence of TiO2 -IL nanoparticles under the irradiation of the high-pressure Hg lamp. The linear relationships of ln(AS0 /A) versus time as indicated in Figure 3 revealed that the photocatalytic degradation reaction followed the pseudo-first-order kinetics ln(A) = kt + C, where A is the absorbency, the slope k is the apparent reaction rate constants. The apparent photocatalytic degradation rate constant was calculated to be 0.06132 min−1 . For comparison, Degussa P25, a commercially available TiO2 was also tested as a reference catalyst for photocatalytic ONP. After UV irradiation from a 250 W high-pressure Hg lamp for 120 min, about 99.0% of the ONP is degraded using the TiO2 -IL nanoparticles as a photocatalyst. In contrast, under the same irradiation conditions, only about 85.2% are degraded using the commercial photocatalyst (P25). It indicates that the TiO2 -IL nanoparticles are a highly efficient photocatalyst, even superior to the commercial P25. The excellent photocatalytic performance of TiO2 -IL may be ascribed to the highly crystalline, small size and the enhanced absorbability associated with the existence of [BMIM][BF4 ] on the TiO2 nanoparticles surface and then transfers it to TiO2 . This can result in a the high efficiency lowered e− /h+ recombination. The recycling of performance of TiO2 -IL catalyst for photodecolorization of ONP The photocatalytic experiments were repeated nine times with the same catalyst at 100 mL ONP in the presence of TiO2 -IL catalyst at 298 K and 120 min in high-pressure Hg lamp radiation. After each experiment, the catalyst was filtered, washed and dried. The results showed that the photocatalytic activity of TiO2 -IL had a slight decrease after four cycles and the results are shown in Figure 4. Although the decolorization efficiency for the nine cycling reuse was 36.1%, the degradation rate is 81.5% in 180 min. The decrease in decolorization efficiency may be on the one hand to the loss of the catalyst during washing and filtering, on the other hand to aggregation of the nanoparticles. These

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Figure 2.

TEM images of TiO2 -IL (a, c), TiO2 -W (b) and HRTEM images of TiO2 -IL (d) aggregate, and lower crystalline.

0

100 lnA = –0.06132t + 1.54245

Decomposition rates (%)

R 2 = 0.98987 –2

lnA

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First-order kinetics of ONP.

results indicated that TiO2 -IL catalyst remained effective and stable under light irradiation.

The recycling of performance of IL for synthesis of TiO2 nanoparticles In order to demonstrate the efficiency and the applicability of the recovered IL, the TiO2 nanoparticles were

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Figure 4.

Reuse of TiO2 -IL for the photodecolorization of ONP.

synthesized in recovered IL medium. The recovered IL experiments were repeated five times with the same process. The efficiency of photocatalytic degradation to ONP is shown in Figure 5. It is notable that the IL recovered was stable in the air and reusable many times. The UV source is hazardous and also expensive, so solar light was used as the light source. After 200 min solar light illumination, the ONP concentration (%) in solution was

Environmental Technology

Decomposition rates (%)

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Laboratory of Coastal wetland Bioresources and Environmental Protection (JLCBE10013).

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Figure 5. Synthetic TiO2 using recovered ionic liquid for the photodecolorization of ONP.

near zero and the decomposition rate of ONP was 99%. It showed that TiO2 -IL can be also reduced by the recombination of photo-generated carriers under solar light which has only 5% of optimum energy for photocatalytic degradation of pollutants. Although the degradation time is longer under the sunlight, it is safe and cost effective. The dichromate reflux method was used to estimate COD (Standard Methods for the Examination of Water and Waste water, America Water Works Association, New York, 1989). Approximately 95% reductions in the COD of the sample were achieved after 120 and 200 min in UV/solar irradiation, respectively. The COD test indicates little organic intermediates were produced during the photocatalytic degradation of ONP.

Conclusion Nanocrystalline of anatase TiO2 -IL was obtained in the mixture of ionic liquid and water by one-pot hydro-treatment methods at low temperature. The photocatalytic activities of the TiO2 -IL nanoparticles have been studied by monitoring the degradation of ONP. Upon modifying TiO2 with the IL of [BMIM][BF4 ], the rate of photocatalytic destruction of ONP is increased in comparison with that of TiO2 from Degussa P25, a commercially available TiO2 . The TiO2 IL nanoparticles still hold a high photocatalytic activity even after the catalyst was recycled nine times. Moreover, under solar light illumination, TiO2 -IL nanoparticles exhibited higher photocatalytic activities significantly, which is helpful in environmental remediation. The COD test shows UV/solar irradiation followed the complete degradation of ONP in CO2 and H2 O under TiO2 -IL photocatalysis. Acknowledgements The work is financially supported by the Key Laboratory of Organic Synthesis of Jiangsu Province (KJS1112), Yangcheng Teachers University (11YSYJB0208) and Jiangsu Provincial Key

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water and the photocatalytic degradation for o-nitrophenol.

The well-crystallized anatase TiO2-IL nanoparticles were synthesized by one-step routes under low-temperature using room temperature ionic liquid (IL)...
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