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Articles Preparation of Chromium Hydroxide Sub-micro- and Nanoparticles by Microwave Dielectric Heating J. Go´mez Morales,*,† J. Garcı´a Carmona,† R. Rodrı´guez Clemente,‡ and D. Muraviev§ Instituto de Ciencia de Materiales de Barcelona and Departamento de Quı´mica UAB, Campus UAB, 08193 Bellaterra, Spain, and Delegacio´ n del CSIC en Andalucı´a, C/Alfonso XII, 16, 41002 Sevilla, Spain Received February 24, 2003. In Final Form: June 12, 2003 This paper reports the preparation of spherical amorphous chromium hydroxide particles by the forced hydrolysis method combined with microwave dielectric heating. The sols were obtained by aging both chrome alum (KCr(SO4)2‚12H2O) solutions and solutions of Cr(NO3)3 - K2SO4 with different initial [Cr3+]/ [SO42-] ratios, at the boiling temperature. In the first case the experiments yielded sols composed of submicroparticles. The aging times were shorter (two order of magnitude less) than those reported in the literature by using conventional heating. In the second case the mean size of particles decreased drastically from the submicrometric range for [Cr3+]/[SO42-] ratios of 0.5,0.8 and 1.0 to the nanometric one for ratios 1.6 and 2.0. This finding is the first successful demonstration of applicability of the forced hydrolysis method to obtain nanoparticles of this material. A mechanism of nanoparticles formation is proposed.
The hydrolysis of Cr3+ and the precipitation of colloidal chromium hydroxide particles (also called chromium hydrous oxide) is an important process occurring in soils and natural waters. The considerable interest in monodispersed chromium hydroxide particles is determined by their numerous technological applications, i.e., in catalysis, as a pigment, or as a model system in colloid science.1-4 The interest to nanosized powders is increasing by technological reasons, for example, because they favor the sintering process of chromium oxide ceramics. Among methods used for preparation of colloidal chromium hydroxide particles from acidic Cr3+ solutions,5-7 the more extensively studied is the forced hydrolysis by Matijevic´ et al.5,6 This method consists of aging of aqueous solutions formed either by chrome alum (KCr(SO4)2‚ 12H2O) or by Cr(NO3)3 in the presence of K2SO4, at moderate temperatures (60-90 °C) for long periods (from 18 h to several days). Under certain conditions it permits one to obtain amorphous monodisperse spherical particles of a polymeric nature. Its main disadvantage is the long aging periods needed to obtain sols of narrow size distribution. * To whom correspondence should be addressed. Telephone:+3493-5801853. E-mail:
[email protected]. † Instituto de Ciencia de Materiales de Barcelona, Campus UAB. ‡ Delegacio ´ n del CSIC en Andalucı´a. § Departamento de Quı´mica UAB, Campus UAB. (1) Corke, J. M.; Evans, J., Rummey, J. M. Mater. Chem. Phys. 1991, 29, 201. (2) Manceau, A.; Charlet, L. J. Colloid Interface Sci. 1992, 148, 425. (3) Schraml-Marth, M.; Wokaun, A.; Curry-Hyde, H. E.; Baiker, A. J. Catal. 1992, 133, 415. (4) Sprycha, R.; Matijevic´, E. Colloids Surf. 1990, 47, 195. (5) Bell, A.; Matijevic´, E. J. Phys. Chem. 1974, 78, 2621. (6) Sprycha, R.; Jablonski, J.; Matijevic´, E. Colloids Surf. 1992, 67, 101. (7) Avena, M. A.; Giacomelli, C.; de Pauli, C. P. J. Colloid Interface Sci. 1996, 180, 428.
The use of microwave heating (MW) instead of conventional heating (CH) in precipitation from homogeneous aqueous solutions permitted reduction of the synthesis time, and, at the same time, a narrow particle size distribution (PSD) was produced in a few studied systems.8,9 This paper reports the results obtained by studying the synthesis of chromium hydroxide sols in both submicrometric and nanometric size ranges by the forced hydrolysis method using MW and a possible mechanism responsible for the nanoparticles formation. Experimental Section The equipment included a microwave furnace Prolabo Maxidigest MX 350 (frequency, 2.45 GHz; maximum power, 300 W) provided with a 250 cm3 quartz reactor and a programmer controlling both the operating power and the radiation time. Two types of experiments were performed: (1) Chrome alum (CrK(SO4)2‚12 H2O) aqueous solutions were used to investigate the effect of the power of the furnace (30-90 W) and radiation times (20-60 min) on the PSD and the shape of the particles. The initial concentration of chrome alum ([Cr3+]o ) 5 × 10-3 mol‚dm-3) and the initial pH (pHo ) 3.5) were chosen from preliminary experiments. (2) Cr(NO3)3‚9H2O plus K2SO4 (Merck grade reagents) solutions were used to investigate the effect of the initial ([Cr3+]o/ [SO42-]o ratio (from 0.5 to 2.0 at [Cr3+]o ) 5 × 10-3 mol‚dm-3) and the evolution with time of residual [Cr3+], pH, and mean size of the PSD on heating solutions of compositions [Cr(NO3)3]o ) 4 × 10-4 mol‚dm-3 and [K2SO4]o ) 2 × 10-4 mol‚dm-3, and initial pHs (pHo ) 3, 4, and 5). The green precipitate samples were filtered (0.1 µm, Millipore), washed with distilled water, and dried in the oven at 50 °C. The (8) Rodrı´guez-Clemente, R.; Go´mez-Morales, J. J. Cryst. Growth 1996, 169, 339. (9) Go´mez-Morales, J.; Lo´pez-Macipe, A., Garcı´a-Carmona, J.; Saadoun, L.; Torrent, J.; Ayllo´n, J. A.; Domingo, C.; Rodrı´guez-Clemente, R. In Recent Research Development in Crystal Growth Research; Pandalai, S. G., Ed.; Transworld Research Network: Trivandrum, India, 1999; Vol. 1, p 353.
10.1021/la0343167 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/24/2003
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Table 1. Precipitation of Amorphous Spherical Cr(OH)3 Hydrosols from Homogeneous Solutions of Chrome Alum (Runs 1-10) or Cr(NO3)3 plus K2SO4 (Runs 11-15) with Initial pH 3.5, Using MW run no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
[CrK(SO4)2], mol‚dm-3 5.0 × 10-3 5.0 × 10-3 5.0 × 10-3 5.0 × 10-3 5.0 × 10-3 5.0 × 10-3 5.0 × 10-3 5.0 × 10-3 5.0 × 10-3 5.0 × 10-3
[Cr3+]o/ [SO42-]oa
power, W
t, min
[Cr3+]f,b mol‚dm-3
[SO42-]f,b mol‚dm-3
pHf
mean size (LLS),c nm
diam range (SEM),d nm
0.5e 0.8e 1.0e 1.6e 2.0e
30e 45e 60e 75e 90e 75 75 75 75 75 75 75 75 75 75
40 40 40 40 40 40 e 45 e 50 e 55 e 60 e 40 40 40 40 40
4.7 × 10-3 4.6 × 10-3 4.5 × 10-3 4.5 × 10-3 4.6 × 10-3 4.6 × 10-3 4.6 × 10-3 4.6 × 10-3 4.5 × 10-3 4.4 × 10-3 4.4 × 10-3 4.1 × 10-3 3.7 × 10-3
9.5 × 10-3 9.8 × 10-3 9.4 × 10-3 9.9 × 10-3 9.7 × 10-3 9.8 × 10-3 9.9 × 10-3 9.9 × 10-3 10.0 × 10-3 6.24 × 10-3 4.96 × 10-3 3.06 × 10-3 2.48 × 10-3
2.52 2.09 2.24 2.34 2.27 2.75 2.01 2.19 2.17 2.27 1.99 1.82 2.01 2.05 2.08
242 327 453 248 292 611 789 1173 778 366 390 74 84
200-300 300-400 300-400 100-200 200-300 400-550 400-500 400-500 400-450 150-200 200-250 30-50 10-20
a [Cr3+] and [SO 2-] : final concentrations of Cr3+ and SO 2- respectively. b Solutions with [Cr(NO ) ] ) 5.0 × 10-3 mol‚dm-3 and f 4 f 4 3 3 o [K2SO4]o variable. c Mean size of particle size distribution analyzed by laser light scattering. d Aproximated diameter range of the particles analyzed by scanning electron microscopy. e Variable parameter.
final pH and the residual Cr3+ and S concentrations were measured with a pH electrode (ORION ROSS) and by ICP technique (Thermo Jarrel Ash Mark IV spectrometer), respectively. The particles were inspected in a Hitachi S 570 SEM microscope, and their PSDs were evaluated with a laser light scattering (LLS) Malvern Zeta-Sizer device, after the particles were treated in an ultrasonic bath for several minutes. The powder was analyzed by X-ray diffraction (XRD) with a Siemens D-500 diffractometer in the 5° < 2θ < 60° range using the Cu KR radiation (1.5418 Å). For FT-IR analysis the powder was mixed with KBr and the spectrum was recorded over the wavelength range 400-4000 cm-1 with a Nicolet 710 FTIR spectrometer.
Results and Discussion Precipitation from Chrome Alum Solutions. From the preliminary experiments by analyzing the influences of [Cr3+]o (5 × 10-3 and 1 × 10-3 mol‚dm-3), pHo (nonadjusted or adjusted with KOH to 3.5 and 4.0) and radiation times (20-60 min) for a fixed power of the furnace (75 W), we have chosen the reference conditions ([Cr3+]o ) 5 × 10-3 mol‚dm-3; pHo ) 3.5; P ) 75 W; t ) 40 min) for further experiments. The spherical particles obtained in one of these experiments, analyzed by LLS, displayed the lowest mean size (224 nm) among samples obtained in preliminary experiments. Moreover, comparison of this value with the diameter range (100-200 nm) observed by scanning electron microscopy (SEM) of this sample reveals a low level of particle aggregation (Table 1). The interesting result obtained in preliminary experiments is that the minimum explored times yielding chromium hydroxide sols are 20 min when [Cr3+]o ) 5 × 10-3 mol‚dm-3, pHo ) 4.0, and 40 min when [Cr3+]o ) 5 × 10-3 mol‚dm-3, pHo ) 3. These times appear to be substantially lower in comparison with those when using the CH technique.5,6 Only in experiments with [Cr3+]o ) 5 × 10-3 mol‚dm-3 and pHo ) 2.98 (nonadjusted) was precipitation not visually observed within the first 60 min of heating. Using the solution of reference ([Cr3+]o ) 5 × 10-3 mol‚dm-3, pHo ) 3.5) the effects of increasing the power of the MW furnace from 60 to 90 W (runs 3-5 in Table 1) was found to shift the PSD to higher sizes. At lower powers (30 and 45 W) no precipitation was observed within the explored period of time. On the other hand, the increase of the aging time from 40 to 60 min (runs 6-10) was shown to shift the mean size of the PSD to higher sizes, while the diameter observed by SEM was practically constant after 50 min of aging, indicating that after initial particle
growth (first 50 min) the particles tended to aggregate. The consumption of small amounts of sulfate ions during the course of precipitation is a finding similar to that reported elsewhere.5,6 Under the explored conditions the final samples did not show XRD reflections, which was an indication of their amorphous nature and their FT-IR spectra (not shown) fully coincided with that for amorphous chromium hydroxide hydrate reported by Sprycha and Matijevic´.10 Comparison of the results obtained by applying MW with those obtained by Bell and Matijevic´5 when using CH reveals the following differences. When heating a 4 × 10-4 mol‚dm-3 of chrome alum solution (pH around 3.8) at 75 or 90 °C, these authors5 reported that equilibrium of the precipitation reaction is reached after 100 h of aging at both temperatures. The Cr3+ consumed is around 45% at 75 °C and 70% at 90 °C. The final pH stabilizes at pH 3.3, and the final modal size of particles after the growth period is around 370 nm at 75 °C and 320 nm at 90 °C. In our experiments (runs 6-10) the temperature reached was 100 °C, the amount of Cr3+ consumed after 40 min was 8-10% of the total Cr3+ and the diameter of the resulting particles ranged between 400 and 500 nm. An increase of aging time from 50 to 60 min resulted in aggregation without consumption of additional Cr3+ amounts. Therefore, after the growth period, the solutions remained metastable with a high Cr3+ content that can be attributed to the low power of the MW furnace used (75 W). Small increases of precipitation degrees can be achieved by increasing the power (runs 3-5). Despite a relatively low reaction yield (almost 1 order of magnitude lower than in CH case), it must be emphasized that a higher size of spherical particles is obtained in a far lower aging time (more than 2 orders of magnitude less), which must be interpreted by a higher growth rate achieved in the MW experiments. However, the main conclusion, which follows from the results obtained in these experiments is insufficient efficiency of MW. Nevertheless, a far higher efficiency can by obtained by increasing the power of the MW furnace. Precipitation from Cr(NO3)3 and K2SO4 Aqueous Solutions. The effect of varying the [Cr3+]o/[SO42-]o ratios between 0.5 and 2, when maintaining the [Cr3+]o ) 5 × 10-3 mol‚dm-3 can be also seen in Table 1 (runs 11-15). Indeed, the mean size of the PSD decreases drastically (10) Sprycha, R.; Matijevic´, E. Langmuir 1989, 5, 479. (11) Bell, A.; Matijevic´, E. J. Inorg. Nucl. Chem. 1975, 37, 907.
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Figure 1. SEM micrographs of chromium hydroxide precipitates obtained at different [Cr3+]o/[SO42-]o ratios (r) by aging Cr(NO3)3 and K2SO4 solutions at pHo ) 3.5 during 40 min (see Table 1).
from the sub-micrometric range for [Cr3+]o/ [SO42-]o ratios of 0.5, 0.8, and 1.0 to the nanometric one, with typical mean sizes of 74 and 84 nm for ratios 1.6 and 2.0, respectively. Simultaneously the yield of the precipitation reaction increases, and, as in experiments with chrome alum, some consumption of SO42- ions is observed. This finding is the first successful demonstration of the applicability of the forced hydrolysis method to obtain nanoparticles. As seen in SEM photographs of nanoparticles (see Figure 1), they displayed a spherical form with diameters laying either within the interval of 30-50 nm for the [Cr3+]o/[SO42-]o ) 1.6 or 10-20 nm for the ratio of 2.0. The nanoparticles are rather polydisperse. They do not manifest XRD reflections, and their IR spectra coincides with those corresponding to the submicrometric material. The differences between the mean size analyzed by LLS and the diameter ranges estimated by SEM reveal a certain degree of particle aggregation. As in experiments using chrome alum, a relatively low yield (18%) in nanoparticle formation using [Cr3+]o ) 5 × 10-3 mol‚dm-3 is mainly attributed to the low pHo of solution, its decrease during heating, and also the low power of the MW furnace. In fact, a slight increase of pHo from 3 to 4 and 5 in solutions of lower concentrations ([Cr3+]o ) 4 × 10-4 mol‚dm-3 and [SO42-]o ) 2 × 10-4 mol‚dm-3 (see evolution of residual [Cr3+] in Figure 2a) permits to substantially enhance the reaction yield so that consumption of Cr3+ after 80 min of aging rises from 8.7 to 54% and 72% in experiments at pHo ) 3, 4, and 5,
Figure 2. (a) Variation of residual [Cr3+] and [SO42-], (b) pH, and (c) mean size of the particle size distributions with time in experiments carried out by aging solutions containing 4 × 10-4 mol‚dm-3 Cr(NO3)3 and 2 × 10-4 mol‚dm-3 K2SO4 at pHo 3.0, 4.0, and 5.0.
respectively. This consumption was accompanied by a small consumption of SO42- ions (see Figure 2a) and a decrease of pH (Figure 2b), testifying to the trend to reach an apparent equilibrium. Nevertheless, the resulted precipitates seem to reach a stationary PSD with mean sizes of 404 and 202 nm in experiments at pHo ) 3 and 5, while at pHo ) 4 they do not (Figure 2c). After 80 min of aging the system has not yet reached the equilibrium. The nanoparticles (mean size of 68 nm) were obtained only at low aging times (40 min) in the experiment at pHo ) 4. According to some authors12 the condition to obtain nanoparticles of a given material consist in simultaneously stimulate the nucleation while inhibit the growth process. Hence, to explain the formation of chromium hydroxide nanoparticles by forced hydrolysis, one needs to consider how this method fits these requirements. Matijevic´ et al.5,6,11 suggested that chromium hydroxide hydrosols were (12) Li, R.; Zhang, J.; Zhang, P. J. Cryst. Growth 2002, 245, 309.
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Figure 3. Dependence of [Cr(OH)SO4]o and [Cr2(OH)2SO4]2+ concentrations on the [Cr3+]tot./[SO42-]tot. ratio and pH (see text).
formed after attaining the supersaturation condition following a two-stage mechanism: (1) first polymeric basic chromium sulfate primary particles are formed according to eq 1, and (2) then they act as precursor for the formation of chromium hydroxide according to eq 2.
m[Cr2(OH)2SO4]2+ + mSO42- f [Cr(OH)SO4]m(polymer) (1) [Cr(OH)SO4]m + nCr(OH)3 + 2mH2O f [Cr(OH)3]m+n(s) + mSO42- + 2mH+ (2) To explain how the key conditions ([Cr3+]o/[SO42-]o ) 1-2, pHo ) 3-4, and T ) 100 °C) affect the nucleation rate, we calculated the evolution in the concentration of species containing Cr3+ and SO42- versus pH, in solutions with total [Cr3+]tot. ) 5 × 10-3 mol‚dm-3 and different [SO42-]tot. by using the speciation program Visual MINTEQA213 (Figure 3). The remarkable increase of the
major species [Cr2(OH)2SO4]2+ with the rise of [Cr3+]tot/ [SO42-]tot ratio at pH between 3 and 4 favors the attaining of a critical supersaturation of complexes [Cr2(OH)2SO4]2+ with respect to the primary particles, thus stimulating the nucleation process, which according to the classical nucleation theory provides an increase of the number of nuclei with a lower critical size. After nucleation, the limitation of aging time to a maximum of 40 min seems to prevent the particles trend to growth up to the submicrometric size otherwise. Acknowledgment. The authors of ICMAB belong to the Excellence Research Team 99SGR-00207 financed by the Generalitat de Catalunya (Spain). The ICP analysis of Cr3+ and S were carried out at the Servicios Cientı´ficoTe´cnicos de la Universidad de Barcelona. LA0343167 (13) Gustafsson, J. P. Visual MINTEQ 2.01, Computer Program for Calculating Aqueous Geochemical Equilibria; 2001. Adapted from MINTEQA2 (CEAM, EPA, 1999).
