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Synthesis of Ordered Hexagonal Mesostructured Nickel Oxide Sumit Banerjee,† Ashwin Santhanam,† Aruna Dhathathreyan,*,‡ and P. Madhusudhan Rao*,† Department of Chemical Engineering, Sathyabama Institute of Science and Technology, Chennai 600 119, India, and Chemical Laboratory, Central Leather Research Institute, Chennai 600 020, India Received March 12, 2003. In Final Form: April 16, 2003 An ordered hexagonal mesoporous nickel oxide was prepared for the first time using sodium dodecyl sulfate as the surfactant and urea as the hydrolyzing agent. The resulting oxide showed mesoporous structure with pore size ranging from 4 to 7 nm. Calcination at different temperatures was carried out, and the mesoporosity did not collapse even after calcination at 773 K although the surface area decreased drastically (from 279 to 72 m2/g) and a broader pore size distribution was observed.
Introduction The discovery of M41S materials by the supramolecular templating mechanism ushered in a new era in synthesis chemistry.1 These mesoporous materials exhibit high surface area, narrow pore size distribution, large pore volume, and high thermal and hydrothermal stability. The semicrystalline nature of the walls owing to longrange ordering, unlike the rigid crystalline walls of zeolites, allows the tuning of the pore sizes. Several mechanisms such as liquid crystal templating,2-4 cooperative templating,2,4 neutral templating,2,3 and recently a ligandassisted templating5 have been proposed to account for the occurrence of mesophasic structures. The extension of this synthesis protocol led to the preparation of nonsiliceous mesoporous oxides with high surface areas.4 The early synthesis procedures predominantly yielded mesophases with lamellar structures attributed to a high packing factor.2 However, innovative synthesis conditions coupled with appropriate choice of precursors and surfactants resulted in the preparation of hexagonally ordered mesoporous oxides such as zirconia, alumina, tantala, niobia, hafnia, and tin oxide. Two common features associated with the mesophasic structures of non-siliceous materials are their low thermal stability and difficulty in removing the surfactant without destroying the structural integrity.6-8 Unlike silica, the inorganic precursors of other oxides do not network extensively. Methods to increase the thermal stability of non-siliceous materials include the use of silica as a cosupport,9 hydrothermal treatment,9 partial crystalliza* Corresponding authors. E-mail:
[email protected] (Aruna Dhathathreyan);
[email protected] (Madhusudhan Rao). † Sathyabama Institute of Science and Technology. ‡ Central Leather Research Institute. (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K.D.; Chu, C. T. W.; Olson, D. H.; Shepard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Sayari, A.; Liu, P. Microporous Mater. 1997, 12, 149. (3) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (4) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1997, 8, 1147. (5) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Intl. Ed. Engl. 1995, 34, 2014. (6) Reddy, J. S.; Sayari, A. Catal. Lett. 1996, 38, 219. (7) Ayyappan, S.; Rao, C. N. R. Chem. Commun. 1997, 575. (8) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 1685.
tion,9 and functionalization of the walls with sulfate or phosphate groups.10,11 Recently, Wong et al.9 prepared mesoporous tungstated zirconia, stable up to ca. 973 K, by nanoparticle/surfactant templating using colloidal zirconia and a triblock copolymer as the surfactant. Additionally, it is important to control the hydrolysis rate of the inorganic precursor to achieve high-quality materials. Antonelli and Ying12 have demonstrated the importance of controlling the hydrolysis rate of the inorganic precursor to achieve a uniform hexagonal phase. They prepared mesoporous titania in the presence of acetylacetone which inhibited the hydrolysis rate. Similarly Sachtler et al.11 used acetylacetone as a hydrolysis inhibitor in the preparation of mesoporous zirconia. Cationic and anionic templates failed to give SnO2 and TiO2 mesophases.8 The mesoporic structure formed with dioctyl sulfo succinate collapsed on calcining at 700 K for 12 h. Moreover, the surfactant could not be removed by solvent extraction. Unlike mesoporous silica, the smallangle X-ray scattering (SAXS) investigations on mesoporous non-siliceous materials did not yield X-ray detectable ordering although the mesoporous nature of the materials was confirmed by Brunauer-Emmett-Teller (BET) measurements.13-16 It has been reported that functionalization of the oxide surface by sulfate preserves the textural properties of oxides.11,13 Hexagonal mesoporous zirconia was prepared using sodium lauryl sulfate and ammonia as the templating and hydrolyzing agents, respectively. Ayyappan and Rao7 reported the synthesis of mesoporous aluminum borates using sodium dodecyl sulfate while the use of (9) Wong, M. S.; Jeng, E. S.; King, J. Y. Nano Lett. 2001, 1, 637. (10) Ciesla, U.; Schacht, S.; Stucky, G. D.; Unger, K. K.; Schu¨th, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 541. (11) Huang, Y. Y.; McCarthy, T. J.; Sachtler, W. M. H. Appl. Catal. 1996, 148, 135. (12) Antonelli, D. M.; Ying, J. Y. Chem. Mater. 1996, 8, 874. (13) Larsen, G.; Lotero, E.; Nabity, M.; Petkovic, L. M.; Shobe, D. S. J. Catal. 1996, 164, 246. (14) Parvulescu, V. I.; Bonnemann, H.; Enduschat, U.; Rufinska, A.; Lehmann, Ch. W.; Tesche, B.; Poncelet, G. Appl. Catal., A 2001, 214, 273. (15) Pacheo, G.; Zhao, E.; Garcia, A.; Sklyaruv, A.; Fripiat, J. J. Chem. Commun. 1997, 497. (16) Kundo, J. N.; Takahara, Y.; Domen, K. Chem. Mater. 2001, 13, 1200.
10.1021/la034420o CCC: $25.00 © 2003 American Chemical Society Published on Web 05/14/2003
Ordered Hexagonal Mesostructured Nickel Oxide
cationic surfactants or long-chain amines did not result in the mesophase. Nickel oxide is a material extensively used in catalysis, battery cathodes, gas sensors, electrochromic films, and magnetic materials. In this paper, we have synthesized mesostructured nickel oxide, which has been shown to be an excellent electrode material for energy storage applications. In the present study, we have attempted to apply the current understanding on non-siliceous materials to synthesize mesostructured nickel oxide. We have used an anionic sulfate based surfactant to understand its role in improving thermal stability and employed urea hydrolysis to control the hydrolysis rate.
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Figure 1. Adsorption isotherms of the as-synthesized (leached), calcined, and unleached samples of mesoporous nickel oxide.
Experimental Section Synthesis. The synthesis protocol adopted in the present study is as follows: Nickel chloride, (Indian Scientific, AR Grade), sodium dodecyl sulfate (SDS) (Sigma Chemicals USA, >99% pure), urea (Indian Scientific, AR Grade), and double-distilled water were taken in the mole ratios of 1:2:30:60. These were stirred at 313 K for 1 h to yield a transparent solution. The contents were then heated to 353 K and maintained at that temperature for 1 h. The resulting mixture was cooled to room temperature to prevent further hydrolysis of urea. After centrifugation, the solid was washed with water a few times and then dried in air. To remove the excess surfactant, the solid was mixed with a 0.05 M ethanolic solution of sodium acetate and stirred at 313 K for an hour. The resulting solid was washed with ethanol extensively and then dried in air. The samples were then calcined at 673 K for 4 h and at 773 K for 2 h. Characterization. Nitrogen adsorption and desorption experiments were carried out at 77 K on a Quantachrome instrument, Nova1000 series analyzer. Surface area was calculated using the BET equation. Pore size distributions were calculated by the BJH (Barrett, Joyner, and Halender) method using the desorption branch of the isotherm. Transmission electron micrographs were taken at a operating voltage of 200 kV using a Philips STEM instrument, and the sample was prepared by dipping a copper grid, coated with a holey carbon film, into a colloidal suspension of particles dispersed in methanol which was then air-dried and stored in a vacuum chamber. Infrared spectra were recorded on a Perkin-Elmer spectrum RXI series Fourier transform infrared (FTIR) spectrophotometer using KBr pellets containing 1% weight sample in KBr. Thermogravimetric analysis (TGA) was performed on a DuPont 951 thermogravimetric analyzer. The samples were heated at a rate of 10 K/min. In this study, we decided on a sulfate-based surfactant because the presence of sulfate or phosphate groups preserves the porous structure upon calcination.11,15 Another factor was the facile removal of SDS. Larsen et al.13 reported that SDS could be removed by repeated washing with ethanol and water. Earlier reports had mentioned the difficulty in removing the surfactant.7 A high surfactant/Ni ratio was employed since it is reported that sodium dodecyl sulfate undergoes a spherical to rod transition at high concentrations leading to high-quality materials.13 Precipitation of the inorganic precursor had been widely performed with liquid ammonia. Yada et al.17 employed urea as the hydrolyzing agent in the preparation of mesoporous alumina. In the present study, the hydrolysis rate of the nickel oxide precursor was controlled differently by adopting the urea hydrolysis method. At ca. 353 K, ammonia is generated in situ due to urea decomposition.
Results and Discussion The adsorption isotherms and BJH pore size distributions of mesoporous nickel oxide are shown in Figures 1 and 2, respectively. A well-defined pore filling step at 0.4P/ P0 in the oven-dried sample is indicative of a well-ordered (17) Yada, M.; Machinda, M.; Kijma, T. Chem. Commun. 1996, 769.
Figure 2. Pore size distribution plots of the as-synthesized (leached), calcined, and unleached samples of mesoporous nickel oxide.
hexagonal pore system with a narrow pore size distribution. The BJH pore size distribution is narrow, and the pore diameter is centered at 4.0 nm. Such well-defined pore filling steps have been observed previously in other oxides.7,14,15 Calcination at 673 K resulted in an expected decrease in volume absorbed and the pore-filling step shifted to higher pressure revealing the presence of pores of wide-ranging sizes. The mean BJH pore diameter was found to be 7.0 nm. However, the mesoporous nature of the oxide was preserved. Increasing the calcination temperature to 773 K did not significantly alter the shape of the isotherm nor did the pore size distribution vary markedly. The failure to observe an expected collapse of the mesoporous structure is a significant feature in this study. Parvulescu et al.14 reported a similar resistance to collapse in mesoporous zirconia after calcination at 773 K. The transmission electron micrograph of the oven-dried sample displayed in Figure 3a shows a hexagonal morphology that is ordered and periodic with the pores in the nanometer scale. The 110-lattice fringe characteristic of the hexagonal phase is clearly seen. The corresponding selective area diffraction is shown in Figure 3b. Since the sample is oven-dried after repeated washing with ethanol (drying at T ) 200 °C), any packing order seen could be only due to the hexagonal channels of the nickel oxide formed and such structures should be fairly stable even on heating. To our knowledge, this is the first instance of ordered hexagonal mesoporous nickel oxide by this synthetic route. Earlier investigations on the preparation of mesoporous nickel oxide resulted in lamellar phases.18,19 (18) Nelson, P. A.; Elliot, J. M.; Attard, G. S.; Owen, J. R. Chem. Mater. 2002, 14, 524.
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Figure 4. FTIR spectra of the as-synthesized sample. Table 1. Textural Properties of Mesoporous Nickel Oxide
samples air-dried (110 °C) 400 °C calcined (4 h) 500 °C calcined (2 h) air-dried unleached (110 °C)
Figure 3. Transmission electron micrograph of the assynthesized sample.
Liu et al.19 prepared nickel oxide using nickel sulfate in the presence of a cationic surfactant with ammonia as the precipitating agent. The contents were autoclaved for 20 h at 398 K. The lamellar phase obtained showed no X-ray detectable ordering after calcination, and the surface area was 31 m2/g after calcination at 773 K. However, direct synthesis using sodium silicate resulted in an ordered hexagonal mesophase. Nelson et al.18 synthesized mesoporous nickel/nickel oxide using the liquid crystal templating approach involving high concentrations of nonionic surfactants (>30 wt %) and nickel acetate as the precursor. The transmission electron micrographs showed lamellar structure. Sayari and Liu have postulated that assembled precursors of type S-I+ would yield lamellar phase nickel oxide.2 The textural properties of mesoporous nickel oxide are tabulated in Table 1. An expected decrease in surface area with increasing calcination temperature is seen, but the pore sizes are typical of mesoporous materials. The surface area of the samples is close to the maximum theoretical surface area calculated from the expression S ) 6000/FD (19) Liu, X.; Chu, C. M.; Aksay, I. A.; Shih, W. H. Ind. Eng. Chem. Res. 2000, 39, 684.
BET area (m2 g-1)
pore volume (mL g-1)
278.41 161.99 72.61 213.53
0.457 0.454 0.156 0.28
pore diameter (nm) mean avg (BJH) (4PV/SA) 3.68 7.14 6.84 3.72
6.58 10.84 8.6 5.26
where S is the surface area (cm2/g) and F and D are the density (g/cm3) and pore diameter (nm) of a spherical particle, respectively. The study shows that the pore volume of the as-synthesized sample is larger than that of any other forms. This is possibly due to the presence of nanoporous channels of the mesoporous nickel oxide formed, and these do not collapse even after calcinations at higher temperature. Previous studies involving different precursors of nickel, surfactants, and synthesis conditions have reported only the existence of the lamellar phase.18,19 Interestingly, Wirnsberger et al.20 prepared mesostructured iron oxyhydroxides using FeCl3 in the presence of sodium dodecyl sulfate and other long-chain sulfate-containing surfactants with ammonia as the precipitating agent. Initially ammonia was added to the precursor solution followed by the surfactant after 2 h of stirring. On aging, a lamellar phase was obtained, and prolonged aging times increased only the layer thickness. The FTIR spectrum of the oven-dried sample is shown in Figure 4. The prominent bands at 2850-3000 and 1370-1380 cm-1 are typical C-H symmetric stretching and bending vibrations, respectively, confirming the incomplete removal of surfactant even after extensive leaching with ethanol. However, Larsen et al.,13 employing a similar washing procedure in preparing mesoporous zirconia, could completely remove the dodecyl sulfate surfactant.13 However, in the calcined samples no peaks typical of C-H stretch vibrations corresponding to the alkyl tails of the surfactants were observed. The TGA profiles of the leached and unleached samples are shown in Figure 5. In the leached sample (Figure 5a), ca. 6 wt % of the surfactant is chemically bound to the walls since it is not removed even after extensive leaching with ethanol. The complete removal of the surfactant, a vexing problem in other studies,8 is observed at ca. 673 (20) Wirnsberger, G.; Gatterer, K.; Fritzer, H. P.; Grogger, W.; Pillep, B.; Behrens, P.; Hansen, M. F.; Bender Koch, C. Chem. Mater. 2001, 13, 1453.
Ordered Hexagonal Mesostructured Nickel Oxide
Figure 5. TGA-differential thermogravimetry (DTG) plots of (a) the as-synthesized sample and (b) the unleached sample.
K. The TGA results confirm that strongly bound surfactant is needed for the retention of mesoporosity after treatment at high temperatures.14,21 A high percentage of weight loss (ca. 55 wt %) observed in the unleached sample (Figure (21) Knowles, J. A.; Hudson, M. J. J. Mater. Chem. 1996, 6, 89.
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5b) is due to the high surfactant/Ni ratio used and skipping of the ethanol washing step. It can be seen that the bulk of the surfactant is loosely held and can be easily removed by ethanol washing. From the TGA studies, it can be inferred that the low pore volume observed in the unleached sample (Table 1) is due to pore blockage by the surfactant. The results are in agreement with studies on mesoporous zirconia that the presence of surfactant during calcination is essential to achieve improved textural properties.14,21 Crucial to the formation of the ordered mesoporous phase in non-siliceous materials is the controlled hydrolysis of the inorganic precursor, which allows a stronger interaction between the inorganic precursor and the organic template. A hydrolysis inhibitor such as acetylacetone has been employed widely.12,13,14,22 In the present study, the inorganic precursor was hydrolyzed by ammonia generated in situ by urea decomposition at 353 K. A similar control over hydrolysis rate achieved with in situ generated ammonia may be responsible for the highly ordered hexagonal phase achieved in our study. An increase in the aging period from 1 h, maintained in this study, should definitely improve the textural properties since non-siliceous materials do not form network structures easily. Aging periods of g6 h are routinely employed in the synthesis of non-siliceous materials.8,12,13 Efforts are presently on to improve the textural properties by increasing the aging period. Moreover, the effects of varying the surfactant/Ni ratio, the nature of the nickel precursor, and the surfactant on the morphology of the mesophase are under investigation. This study indicates clearly that it is possible to design ordered mesoporous structures of transition metal oxides using anionic surfactants and the use of urea for hydrolyzing the inorganic precursor seems to be an added advantage in stabilizing the template structure. Further calcinations even at high temperatures did not destroy the mesoporosity. Acknowledgment. The authors thank the Director, Central Leather Research Institute, Chennai, for extending the facilities of the institute for the experimental work. LA034420O (22) Kaneko, E. Y.; Pulcinelli, S. H.; Vteixeira da Silva; Santilli, C. V. Appl. Catal., A 2002, 235, 71.
