Sonochemical Synthesis of Mesoporous NiTiO3 ... - ACS Publications

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Sonochemical Synthesis of Mesoporous NiTiO3 Ilmenite Nanorods for the Catalytic Degradation of Tergitol in Water Sambandam Anandan,*,†,‡ Teresa Lana-Villarreal,§ and Jerry J. Wu*,‡ ‡

Department of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Trichy 620 015, India § Institut Universitari d’Electroquímica, Departament de Química Física, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain †

ABSTRACT: Ilmenite type titanates have very wide applications in various fields. In this paper, we report the synthesis of mesoporous NiTiO3 ilmenite nanorods via a sonochemical approach. The method is very facile and effective allowing the preparation of NiTiO3 nanoparticles with a high yield and uniform size distribution. The reaction intermediate and particularly the final product have been well characterized by various analytical tools (IR, XRD, Raman, TEM, BET, XPS). The final NiTiO3 nanoparticles can be described as a hierarchical structure composed of small nanoparticles interconnected generating nanoporous rods. In comparison with anatase TiO2 nanoparticles, the catalytic removal rate for the pollutant Tergitol (NP-9) by a process combined with ozonation is two and half-fold higher in the presence of mesoporous NiTiO3 ilmenite nanorods.

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INTRODUCTION Environmental pollution is a serious day-to-day problem faced by the developing and the developed nations in the world. Air, water, and solid waste pollution due to anthropogenic sources contribute to the overall imbalance of the ecosystem. Therefore, strict environmental legislations on the use of recalcitrant pollutants and their safe disposal are driving the research community to develop clean and green processes to degrade the pollutants before they are admitted into the atmosphere and water bodies.1−3 To address water contamination, researchers and scientists have been developing innovative treatments called advanced oxidation processes (AOP). To achieve Intimate contact between materials, flat facets have been facilitated that generate some new catalytic properties.4−6 These new catalytic properties could be related to the diffusion of one material into others, which is responsible in altering the crystallographic and physical properties of the obtained heterostructures by changing their morphology and particle size. That is, lattice distortion has an important impact on the dipole and electronic band structure of a crystal, and it can even influence the behavior of photogenerated charge carriers, including excitation and charge transfer during the catalytic process. Therefore, developing such new materials with well-defined facets has been greatly anticipated in recent years.7,8 Among the various materials, perovskite-like compounds (ABO3) are stable crystalline structures with wide applications in many fields, such as solid oxide fuel cells, gas sensors, and metal air barriers, and they have also been used as high performance catalysts for the complete oxidation of hydrocarbons or CO and NO reduction, etc.9−18 In general, an ideal cubic perovskite (ABO3) has the metal cations, A and B, coordinated with 12 and 6 O anions, respectively, and the radii of A cation is generally larger than that of B cation. Further eight BO6 octahedra with shared corners form a cubic threedimensional framework, the center of which is occupied by an © 2015 American Chemical Society

A site cation. The radii of cations varies due to their chemical nature and oxidation state. Therefore, compared to this ideal cubic perovskite structure, the real perovskites ABO3 can exhibit lattice distortions to a varying degree, thereby resulting in the transformation of crystal phases in the following sequences: orthogonal, rhombohedral, tetragonal, monoclinic, and triclinic phase. With the crystal structure, the dipole and the electronic band structure change result in different catalytic properties.19−21 In this regard, a series of ABO3 with crystal structures have been prepared by using different elements and synthetic routes, such as sol−gel, the flux method, coprecipitation, solid-state reaction, electrospinning, and the Pechini process.22−28 In this work, we have prepared ilmenite NiTiO3 nanoparticles using an ultrasound irradiation approach. The use of ultrasound has several advantages, such as the possibility of obtaining well-crystallized materials without using high temperatures and/or different nanoparticle morphologies. The NiTiO3 nanoparticles prepared by an ultrasound-assisted method have been analyzed by Fourier transform infrared red spectroscopy (FT-IR), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, and adsorption isotherm measurements. The catalytic activity of these nanoparticles has been tested in a heterogeneous catalytic ozonation process intended to remove a wastewater pollutant, such as Tergitol (NP-9), since it is the most commonly used nonionic surfactant in pharmaceutical and personal care products that may cause estrogenic effects in some living organisms.29−31

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EXPERIMENTAL DETAILS All chemicals were of the highest purity available and were used as received without further purification. Nickel acetate Received: Revised: Accepted: Published: 2983

January 4, 2015 March 1, 2015 March 5, 2015 March 5, 2015 DOI: 10.1021/acs.iecr.5b00027 Ind. Eng. Chem. Res. 2015, 54, 2983−2990

Article

Industrial & Engineering Chemistry Research

Figure 1. FTIR and XRD spectra of NiTiO3 nanoparticles before (A, C) and after calcination (B, D).

[Ni(CH3COO)2], titanium isopropoxide (Ti(OC3H7)4), and Tergitol (NP-9) were purchased from Sigma-Aldrich. All the solutions were prepared using Millipore DDI water (18.2 MΩ). Ultrasound-Assisted Synthesis of Mesoporous NiTiO3 Ilmenite Nanorods. Ultrasound-assisted synthesis of onedimensional (1D) NiTiO3 porous nanorods was carried out by adapting the hydrothermal method described by Qu et al.32 Briefly, 2.48 g (0.1 M) of nickel acetate was dissolved in 60 mL of ethylene glycol and subsequently, 3.4 mL (0.12 M) of titanium iso-propoxide was added dropwise under constant stirring at room temperature. The resultant green color suspension was irradiated with a high-intensity ultrasonic horn (Ti-horn, 20 kHz, 100W/cm2) at ambient air for 20 min. During sonication, the color of the suspension turned into light blue. The precipitate was recovered by vacuum filtration, washed with ethanol several times, and dried under vacuum at 80 °C for 1 h. Finally, the precipitate was thermally treated at 600 °C for 2 h in air obtaining a yellow powder composed of mesoporous NiTiO3 nanorods. Characterization Techniques. The FT-IR spectra from 550 to 4000 cm−1 of the as-prepared nanorods (prior and after the thermal treatment) were measured at room temperature by a Thermo Nicolet iS5 FT-IR spectrophotometer. Raman spectra were recorded with a Bruker Raman spectrometer employing a 1064 nm argon ion laser as the excitation source. X-ray diffraction (XRD) patterns between 10° and 80° were

recorded using a Rigaku Ultima III diffractometer (Japan) using Cu Kα radiation. Field emission scanning electron microscopy images (FE-SEM) and high resolution transmission electron microscopy (HR-TEM) images were recorded using a JEOL 7401F and a JEOL JEM-2010 microscope, respectively. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Physical Electronics PHI 5600 XPS instrument employing as excitation source monochromatic Al Kα radiation (1486.6 eV). The surface area of the prepared samples was measured with the assistance of SA3100 surface area and pore size analyzer from Beckman Coulter. Evaluation of the Catalytic Activity. The ability of the mesoporous NiTiO3 nanorods combined with ozone to enhance the degradation of Tergitol (NP-9), a target wastewater pollutant, was assessed in this study. The degradation of NP-9 was carried out under atmospheric conditions (25 °C) at neutral pH in a 500 mL capacity borosilicate glass reactor. The amount of catalyst (250 mg per 250 mL) and the concentration of NP-9 (5.0 × 10−4 M) were fixed for all the experiments. For the ozone-based processes, ozone was introduced through a porous fritted diffuser that can produce fairly fine bubbles with a diameter less than 1 mm, which was determined using a camera with a close-up lens and image analysis software called Matrox Inspector 2.0. Ozone was produced from pure oxygen by corona discharge using an ozone generator (Ozonia, model no. LAB 2B), producing a 2984

DOI: 10.1021/acs.iecr.5b00027 Ind. Eng. Chem. Res. 2015, 54, 2983−2990

Article


Figure 2. SEM (A), HRTEM (B−D), SAED (E), and EDX (F) images of NiTiO3 nanoparticles after calcination. TEM (G) and SAED (H) images of uncalcined NiTiO3 nanoparticles.

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RESULTS AND DISCUSSION In this work, ilmenite NiTiO3 nanoparticles were synthesized by a hybrid hydothermal method employing ultrasonic irradiation. Ultrasound can favor the generation of wellcrystallized materials without using high temperatures and/or different nanoparticle morphologies and/or size distribution. In the procedure employed in this work, first nickel acetate reacts with ethylene glycol to form nickel−ethylene glycol (Ni−EG), which is a green intermediate complex. The addition of titanium iso-propoxide induces the generation of new linear polymeric chains type Ni−Ti−EG as revealed by the color change (from green to light blue). This light blue polymer coagulates to form a uniform rodlike precursor by van der Waals interactions.33,34 This reaction route has been accepted by a number of research groups.26−28 It has been proposed that polyols can serve as ligands to form chain-like coordination

maximum ozone concentration of ca. 6% (by volume) in the oxygen-enriched gas stream. The gas flow rate was regulated at 200 mL min−1 by a gas flow controller (Brooks 5850E), and the inflow ozone concentration was adjusted to 40 mg/L. The gaseous ozone concentrations were determined spectrophotometrically measuring the absorbance of ozone in a 2 mm flowquartz cuvette at 258 nm. An extinction coefficient of 3000 M−1 cm−1 was used to convert the absorbance into concentration units. Reverse-phase HPLC analyses were carried out on a spectral system (Chromquest 100 model) with UV detection at 210 nm using an ultra-aquo C18 column (250 mm × 4.6 mm) with an isocratic mobile phase of acetonitrile/water (55:45 v/v, flow rate = 1 mL/min). 2985


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Figure 3. Raman spectrum (A) and BET isotherm of NiTiO3 nanoparticles (B) (inset: BET pore size distribution diagram) and XPS spectrum corresponding to Ni2p (C) and Ti2p (D) of NiTiO3 nanoparticles.

The crystal structure of mesoporous NiTiO3 nanorods was determined by powder X-ray diffraction (XRD). As in the case of the FTIR spectra, the observed XRD patterns (Figure 1C,D) of NiTiO3 particles prior and after calcination (600 °C) are entirely different. Before being calcinated the pattern shows peaks that can be ascertained to both pure NiO (JCPDS No. 69-2901) and anatase TiO2 (JCPDS No. 71-2774) crystal structures. In fact, the strongest diffraction peak is clearly seen at 2θ = 25° belonging to the (101) plane which is in good agreement with the anatase TiO2 standard value (JCPDS No. 71-2774). However, the calcinated sample shows characteristic XRD peaks at 24.1°, 33.1°, 35.6°, 40.8°, 49.5°, 53.9°, 62.5°, and 64° which can be attributed to the (012), (104), (110), (113), (024), (116), (018), (214), and (300) reflections for ilmenite NiTiO3 (rhombohedral structure; JCPDS No. 33-0960). Also, the sharp high intense peaks reveal the high crystallinity of the sample. In addition, there is a very small amount of rutile and anatase TiO2 present as impurities which are marked in the spectrum. The morphology of the as-synthesized NiTiO3 nanoparticles before and after calcination was studied by FE-SEM and HRTEM (Figure 2). FE-SEM and HR-TEM images (Figure 2A−

complexes because polyols lower the hydrolysis rate of transition metal alkoxides. The formation of such a Ni−Ti−EG polymer was confirmed in our case by Fourier transform infrared (FT-IR) analysis recorded in the region 550−4000 cm−1 using the KBr pellet technique (Figure 1A). The observed characteristic absorption band in the range 2800−2950 cm−1 can be attributed to symmetric and asymmetric stretching vibrations of glycolate CH2 groups. The hydroxy groups (C−OH) of the glycolate were detected by the stretching vibration band at 1060 cm−1, confirming the complex formation. In addition, the band at around 1650 cm−1 can be ascribed to acetate (Ac−) anions wrapped around the hierarchical structure wall. In addition, the characteristic broad absorption bands observed in the range from 3400 to 3550 cm−1 can be attributed to absorbed water molecules. Upon calcination at 600 °C, the spectrum drastically varies. The absence of peaks demonstrates the complete removal of organic residues (Figure 1B). Exclusively, strong absorption bands were noticed at 640 and 576 cm−1 which can be attributed to the stretching vibration of Ti−O and Ni−O.35 Thus, the observed FT-IR spectra supports the formation of ilmenite NiTiO3 nanorods. 2986


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Figure 4. 3-D plots showing the evolution of the HPLC chromatogram as a function of NP-9 ozonation (5 × 10−4 M) in the presence of NiTiO3 (A) and anatase TiO2 (B) nanoparticles. The degradation plot (C) for NP-9 ozonation with NiTiO3 nanorods, anatase TiO2, and without any catalyst.

The isotherm (Figure 2c) can be identified as a type III, which is in correspondence to the characteristic of a mesoporous material. The BET surface area and the average pore diameter of the NiTiO3 nanorods are about 18.83 m2/g and 29.87 nm, respectively (Figure 3B). To further support the formation of mesoporous NiTiO3 nanorods, XPS analysis was performed. The Ti 2P and Ni 2p high resolution spectra are shown in Figure 3. The XPS spectra of NiTiO3 nanorods show the characteristic peaks for Ni, Ti, and O. Nickel peaks were found (Figure 3C) at 855 eV (Ni 2P3/2) and 872.4 eV (Ni 2P1/2) and in addition a satellite peak is also noticed around 861 eV. Such binding energies indicate the occurrence of nickel exclusively as Ni2+.38 Similarly, titanium peaks (Figure 3D) were found at 456 (Ti 2P3/2) and 462 eV (Ti 2P1/2) which indicate that Ti exists as Ti4+. A single peak was noticed for oxygen (O 1s) at 530 eV that belongs to the O2− contribution (not shown). The catalytic activity of mesoporous NiTiO3 nanorods was tested for the degradation of wastewater pollutant containing Tergitol (NP-9) by a process combined with ozonation. The HPLC chromatograms recorded during the ozone degradation of Tergitol with mesoporous NiTiO3 nanorods and anatase TiO2 were compared and are shown in Figure 4 A,B as a 3-D plot. It is clear from the figure that mesoporous NiTiO3 nanorods underwent more adsorption compared to anatase TiO2 due to its larger pore size, which indirectly indicates

D) for NiTiO3 after calcination show mesoporous nanorod like structures consisting of interconnected nanocrystallites. From HR-TEM (Figure 2D), the regular d-spaces between lattice fringes can be measured, being 2.5 Å. This distance corresponds well to the (110) planes of rhombohedral NiTiO3 structure. The observed selected area electron diffraction (SAED) pattern is entirely different compared to pure NiO and anatase TiO2, confirming the formation of mesoporous NiTiO3 nanorods (Figure 2E,H). No other impurity was seen in the EDX (Figure 2F). It should be mentioned that the thermal treatment is required to generate the NiTiO3 nanoparticles. In fact before calcination, the nanoparticles cannot be distinguished, as expected for amorphous polymer chains (Figure 2G). To have further information about the crystal structure and chemical nature of the mesoporous NiTiO3 nanorods, Raman spectroscopy analysis was performed. NiTiO3 nanorods (Figure 3A) show 10 Raman active modes (184, 226, 238, 284, 338, 389, 458, 608, 705, and 760 cm−1) which can be assigned to the NiTiO3 rhombohedral structure (C23i symmetry and R3̅ space group).36,37 These results are in good agreement with the XRD pattern (Figure 1). In addition, the stretching vibrations found in the range 500−830 cm−1 can be related to TiO6 octahedra. Thus, the Raman spectrum confirms that the as-prepared NiTiO3 by ultrasound irradiation has an ilmenite structure. On the other hand, the porosity of the NiTiO3 nanorods was investigated using N2 adsorption and desorption isotherms. 2987


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Figure 5. LC mass spectra analysis of NP-9 (5 × 10−4 M) upon ozonation in the presence of NiTiO3 at various time intervals ((A) 0, (B) 10, and (C) 30 min). Inset shows structure of NP-9.

ozone molecules, which effectively degrade the pollutants compared to ozone alone processes.39 Under the HPLC conditions used, NP-9 was eluted at 7 min whereas shorter chained NP-9s (chain length of NP-9 less than 9) were eluted at 3.5 and 5 min.40,41 Upon ozonation, the peak area corresponding to NP-9 (7 min) decreases with the reaction time, whereas the shorter chained NP-9 peaks increase. The same trend was observed upon performing LC mass spectroscopy analysis (Figure 5A−C). That is the amount of molecules with m/z > 450 (probably NP-3 to NP-9) decreases as the time prolongs from 0 to 30 min, whereas the amount of molecules with m/z 287 (single chain nonylphenol; NP1) increases, indicating that high molecular mass compounds undergo the degradation to form NP-1 upon heterogeneous ozonation. Thus, this promising preliminary result shows that a

mesoporous NiTiO3 nanorods is a better catalyst. The change in the concentration of NP-9 as a function of reaction time was calculated by integrating the area under the entire peak of NP-9 eluted at 7 min. Figure 4 C shows the diminution of the concentration (Ct/C0) as a function of reaction time for ozonation of NP-9 in the presence of NiTiO3 nanorods, anatase TiO2, and without any catalyst. First order rate constants were calculated from the slopes of −ln(Ct/C0) versus time plot (figure not shown) for NP-9. The observed rate constants for NiTiO3 nanorods, anatase TiO2, and those without any catalyst are 12.17 × 10−4 s−1, 4.83 × 10−4 s−1, and 3.83 × 10−4 s−1, respectively, which indicates a two and half fold higher degradation rate for mesoporous NiTiO3 nanorods compared to that of anatase TiO2. this occurs because the heterogeneous catalyst generates a superoxide ion species by adsorption with 2988


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(9) Peña, M. A.; Fierro, J. L. G. Chemical structures and performance of perovskite oxides. Chem. Rev. 2001, 101, 1981−2017. (10) Shi, J. W.; Guo, L. J. ABO3-based photocatalysts for watersplitting. Proc. Natl. Acad. Sci. U.S.A. 2012, 22, 592−615. (11) Kim, H. G.; Borse, P. H.; Jang, J. S.; Ahn, C. W.; Jeong, E. D.; Sung, J. Engineered nanorod perovskite film photocatalysts to harvest visible light. Adv. Mater. 2011, 23, 2088−2092. (12) Wendt, H.; Imarisio, G. Nine years of research and development on advanced water electrolysis. A review of the research programme of the Commission of the European Communities. J. Appl. Electrochem. 1988, 18, 1−14. (13) Yamamoto, O.; Takeda, Y.; Kanno, R.; Noda, M. Perovskitetype oxides as oxygen electrodes for high temperature oxide fuel cells. Solid State Ionics 1987, 22, 241−246. (14) Shimizu, Y.; Uemura, K.; Miura, N.; Yamzoe, N. Chem. Lett. 1988, 67, 1979. (15) Skoglundh, M.; Lowedalh, L.; Janson, K.; Dahl, L.; Nygren, M. Appl. Catal. 1970, 53, 56. (16) Obayashi, H.; Sakurai, Y.; Gejo, T. Perovskite-type oxides as ethanol sensors. J. Solid State Chem. 1976, 17, 299−303. (17) Dharmaraj, N.; Park, H. C.; Kim, C. K.; Kim, H. Y.; Lee, D. R. Nickel titanate nanofibers by electrospinning. Mater. Chem. Phys. 2004, 87, 5−9. (18) Murugan, A. V.; Samuel, V.; Navale, S. C.; Ravi, V. Phase evolution of NiTiO3 prepared by coprecipitation method. Mater. Lett. 2006, 60, 1791−1792. (19) Lin, W. H.; Cheng, C.; Hu, C. C.; Teng, H. S. NaTaO3 photocatalysts of different crystalline structures for water splitting into H2 and O2. Appl. Phys. Lett. 2006, 89, 211904. (20) Hu, C. C.; Lee, Y. L.; Teng, H. S. Efficient water splitting over Na1‑xKxTaO3 photocatalysts with cubic perovskite structure. J. Mater. Chem. 2011, 21, 3824−3830. (21) Li, P.; Ouyang, S. X.; Xi, G. C.; Kako, T.; Ye, J. H. The effects of crystal structure and electronic structure on photocatalytic H2 evolution and CO2 reduction over two phases of perovskite-structured NaNbO3. J. Phys. Chem. C 2012, 116, 7621−7628. (22) Ni, Y.; Wang, X.; Hong, J. Nickel titanate microtubes constructed by nearly spherical nanoparticles: Preparation, characterization and properties. Mater. Res. Bull. 2009, 44, 1797−1801. (23) Lopes, K. P.; Cavalcante, L. S.; Simões, A. Z.; Gonçalves, R. F.; Escote, M. T.; Varela, J. A.; Longo, E.; Leite, E. R. NiTiO3 nanoparticles encapsulated with SiO2 prepared by sol−gel method. J. Sol−Gel Sci. Technol. 2008, 45, 151−155. (24) Mohammadi, M. R.; Fray, D. J. Mesoporous and nanocrystalline sol−gel derived NiTiO3 at the low temperature: Controlling the structure, size and surface area by Ni:Ti molar ratio. Solid State Sci. 2010, 12, 1629−1640. (25) Wang, J.; Li, Y.; Byon, Y.; Mei, S.; Zhang, G. Synthesis and characterization of NiTiO3 yellow nanopigment with high solar radiation reflection efficiency. Powder Technol. 2013, 235, 303−306. (26) Tahir, A. A.; Mazhar, M.; Hamid, M.; Wijayantha, K.G. U.; Molloy, K. C. Photooxidation of water by NiTiO3 deposited from single source precursor [Ni2Ti2(OEt)2(l-OEt)6(acac)4] by AACVD. Dalton Trans. 2009, 3674−3680. (27) Salvador, P.; Gutiérrez, C.; Goodenough, J. B. Photoelectrochemical properties of n-type NiTiO3. J. Appl. Phys. 1982, 53, 7003−7013. (28) Lopes, K. P.; Cavalcante, L. S.; Sim̃ oes, A. Z.; Varela, J. A.; Longob, E.; Leite, E. R. NiTiO3 powders obtained by polymeric precursor method: Synthesis and characterization. J. Alloy. Compd. 2009, 468, 327−332. (29) Amaral Mendes, J. J. The endocrine disrupters: A major medical challenge. Food Chem. Toxicol. 2002, 40, 781−788. (30) Knudsen, F. R.; Pottinger, T. G. Interaction of endocrine disrupting chemicals, singly and in combination, with estrogen-, androgen-, and corticosteroid-binding sites in rainbow trout (Oncorhynchus mykiss). Aquat. Toxicol. 1998, 44, 159−170. (31) Van der Putte, I.; Groshart, C.; Okkerman, P. Towards the Establishment of a Priority List of Substances for Further Evaluation of

3-fold higher heterogeneous catalytic ozonation degradation rate was observed by using mesoporous NiTiO3 nanorods as compared to ozonation only, which encourages us to study more about the mesoporous NiTiO3 nanorods in our ongoing research.

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CONCLUSIONS This research has led to several conclusions that can be summarized as follows: (i) mesoporous ilmenite NiTiO3 nanorods were synthesized at room temperature by a facile sonochemical approach combined with a thermal treatment in air. The chemical nature and crystal structure were confirmed by various characterization methods. (ii) The experimental results indicate that the heterogeneous catalytic ozonation of NP-9 degradation by mesoporous NiTiO3 nanorods is two and half fold faster when compared to anatase TiO2 nanoparticles. (iii) Finally, this reliable method makes it possible to produce highly pure mesoporous NiTiO3 nanorods at low cost, thus offering a great opportunity for the scale-up manufacturing.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +91-431-2303639. Fax: +91431-2300133. *E-mail: [email protected]. Tel.: +886-4-24517250, x5206. Fax: +886-4-24517686. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.A. thanks Feng Chia University in Taiwan for the Visiting Professor appointment. S.A. and T.L.V. thank DST, New Delhi, for the sanction of an India−Spain collaborative research grant (DST/INT/Spain/P-37/11 dt.16th Dec 2011) and Generalitat Valenciana for the financial support through ACOMP/2014/ 137. In addition, acknowledgement is given to the partial financial support of National Science Council (NSC), Tawian, Grant No. NSC-101-2221-E-035-031-MY3.

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Sonochemical Synthesis of Mesoporous NiTiO3 ... - ACS Publications

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