Ionic Oxides : Distinction Between Mechanisms and Surface Roughening Effects in the Dissolution of Magnesium Oxide BY ROBERT L. SEGALL, ROGER ST. C . SMART* AND PETER S. TURNER School of Science, Griffith University, Nathan, Queensland 41 1 1, Australia Received 27th April, 1978 The mechanism of the rate determining step (r.d.s.) for dissolution of well characterised, very perfect MgO “smoke” crystals, has been re-evaluated by studies of the dependence of log (rate) on pH. Surface potential barrier modification is the most likely r.d.s. when solution diffusion is not limiting. Electron microscopic studies of changes in surface structure of partly dissolved crystals have been related to rate changes during dissolution. There is a doubling of surface area per unit mass in the first 10 % of dissolution, due to initial attack at defect sites, after which the increase in surface area per unit mass (to more than ten times the initial area) is due to both decreasing particle size and surface roughening.The complex set of factors that affect oxide dissolution, particularly those con- cerned with the metal oxide and the electrolyte solution, have been reviewed by Diggle and recently, by Valverde and Wagner.2 A theoretical treatment of the kinetics of dissolution has been proposed by Engell.3 Whilst this treatment has been accepted for ionic oxides, the extension of its application to semiconducting oxides has been ~riticised.~ In fact, much of the previous experimental work has been carried out with oxides that may be regarded as semiconducting, e.g., Fe0,39 Fe304,3* CUO,~? COO,^ Cr203,5 Ni0.6p In these oxides the electronic properties of the metal oxide, in particular, charge carrier concentration and mobilities, and the extent of charge depletion layers at the surfaces, may be of prime importance in the dissolution rate.The simplest case in which to study dissolution kinetics is that of a predominantly ionic oxide where the charge characteristics are not rate determining. The major factors of importance are then, for the solid, atomic surface detail, surface morphology, bond strength and, for the solution, diffusion, pH and electrolyte concentrations. In this paper, we present new results for MgO which lead to a critical re-examina- tion of the reliability of experimental tests of the theoretical predictions. We have used well characterised, very perfect MgO, the surface structure of which is examined before and during dissolution.The effect of diffusion of protons in solution on the dissolution rates is confirmed. The importance of the solid surface, in addition to the factors identified in the previous theoretical treatment, is established and quantita- tive estimates of rate changes due to surface roughening are given. EXPERIMENTAL MATERIALS Magnesium oxide was made from pure magnesium turnings. These were ignited in a slow stream of air and the MgO “ smoke ” collected on clean glass slides E 2 cm from the point of ignition. The oxide was immediately transferred to a small closed glass phial and kept in a dessicator. A.R. grade nitric acid and deionised, distilled water were used in dissolution experiments.29072908 DISSOLUTION OF IONIC OXIDES DISSOLUTION STUDIES The dissolution rates were measured by introducing precisely known masses of MgO to a stirred solution of known pH between 2 and 7. The mass was adjusted (e.g., from 0.5-20 mg in 50 cm3 of solution) to give accurately measurable rates over the pH range. The pH was recorded during dissolution with a Townson digital pH meter and the output recorded on a chat recorder. The solution and powder were contained in a 200 cm3 Pyrex beaker con- taining a Teflon-covered magnetic stirrer. Vermilyea s has shown that there is no difference in dissolution rates with air or inert gas atmosphere above the solution. The initial pH was successively set at 2.0, 2.5, 3.0, 3.5 and 4.0, and rose as the oxide dissolved.The dis- solution rate for both the initial well-chaacterised MgQ, and the partially dissolved oxide, could then be calculated from the rate of change of pH. A single run produced rates for the initial and higher pH values. Experiments were conducted at 25°C. We encountered the same problems with sluggish response in the pH range 6 to 8, as those discussed by Vermilyea.8 This was not a major concern as our discussion relates to the pH range below 5 in which theoretical predictions may be tested. Above pH 5, solution diffusion obscures the mechanism of dissolution of the MgO. Dissolution rates were calculated by a method similar to that used by Vermilyea.8 TECHNIQUES Specimens of partially dissolved oxide were prepared for electron microscopy by dipping an uncoated gold specimen support grid into the solution, removing excess fluid on filter paper and immediately inserting the grid into the microscope.The initial, unattacked oxide was examined by both coating a grid during burning of the magnesium and ultrasonically dispersing some of the stored powder in A.R. petroleum ether and depositing onto a grid. Specimens were examined on a JEM lOOC microscope at 100 keV. Surface areas were measured by adsorption of nitrogen (a = 16.2 A2) at - 196°C using the continuous flow method of the Perkin Elmer Model 212D sorptometer. More than 5 adsorption/desorption cycles were measured. The surface area of initial oxide was found to be between 9.8 and 10.9 m2 8-l. RESULTS AND DISCUSSION CHARACTERISATION OF MAGNESIUM OXIDE MgO prepared by burning magnesium in air produces small cubic crystals with a very high degree of perfection (fig.1). The edge length of the crystals ranges from about 50 nm to over 200 nm. No dislocations are evident in any of the crystals ; they are clearly very well annealed compared with MgO crystals prepared by de- composition of the hydroxide, carbonate etc. This is borne out by the relatively low surface area (about 10 m2 g-l) compared with that formed from the hydroxide by decomposition at 600°C in air (67 m2 g-l). The atomic surface detail of this material has been extensively examined by Moodie and Warble,9 who heated small crystals to desorb gas and examined the surface by high resolution electron microscopy using phase contrast.Their micro- graphs show growth steps of one or two unit cell height, as isolated cubic projections with a volume of a few unit cells. The steps occur roughly within 50 A of each other. On the smoother surfaces, Moodie and Warble have estimated, in some cases, a surface roughness of only 0.05 using the definition of Burton et aZ.lo In contrast, their work shows that MgO formed by decomposition is considerably less perfect, with high index mean faces composed of closely spaced {loo} steps, dislocations and sintered intergrowth showing both perfect and imperfect alignment. The obvious sites for attack in the smoke crystals are the sharp edges, corners and the kink sites (i.e., ions with fewer nearest neighbour ions than the ions in a perfect { 100) surface) associated with the projections.FIG. 1.-Transmission electron micrograph of MgO smoke crystals.The edge length of the cubic crystals in the powder is in the range 50-200 nm. [To face page 2908FIG. 3. F~G. 4. FIG. 3.-Transmission electron micrograph of MgO smoke crystals after initial attack to < 5 total dissolution from initial pH 2.5. FIG. 4.-Transmission electron micrograph of MgO smoke crystals after 95 % dissolution from initial pH 2.5. ofPLATES A AND B.-Electron micrographs of collapsed micelles isolated by the spreading-drop technique from micelle solutions of the polystyrene-polyisoprene block copolymer in DMA. Specimens shown on both plates were stained in solution with Os04 and the specimen shown on Plate A was To firrepage 3355.1 in addition lightly shadowed with C/Pt.The scale marks are ( A ) 200 and ( B ) 400 nm.R. L. SEGALL, R. ST. C . SMART AND P. S . TURNER 2909 RATE CONTROL I N MECHANISM OF DISSOLUTION The theoretical treatment of oxide dissolution has been developed by EngelL3 Vermilyea and Digg1e.l Their predictions, for the experimental value of the slope of log (rate) against pH dependence, for different mechanisms in MgO dissolution are summarised in table 1. In a previous experimental study of the dissolution of MgO and Mg(OH)2, Vermilyea considers that, at low proton concentration between pH 5 and 7, proton diffusion in solution is the r.d.s. Our work has confirmed the importance of solution diffusion in rate measurements even down to pH 2. Stirring rates directly affected the measured rate in all experiments.Rates discussed below were always measured with stirring rates above the level at which no further increase in dissolution rate could be measured simply by increasing the stirring rate. At pH > 5, this could not be achieved; proton diffusion is clearly limiting. TABLE 1.-THEORETICAL PREDICTIONS OF THE SLOPE OF LOG (RATE) AGAINST pH DEPENDENCE rate potential barrier determining reaction of anions with protons4 modification by [H+].1 step OH-+ H+ -+ H20 02-+H+ + OH- 0 2 - +2H+ + H2O anion removal cation removal slope of log (rate) - 0.67 - 0.5 - 1.0 -0.5 - 0.5 against pH Vermilyea has proposed that, at low pH, MgO first reacts to form Mg(OH)2 and that dissolution of the hydroxide is then rate limiting. Below pH 5, for Mg(OH)2 he has found a slope of approximately -0.47 for a log (rate) against pH dependence.However, his curves were reproducible to w 50 % and results below pH 5 were only considered correct as to order of magnitude. Part of the reason for this un- certainty arises from surface area estimates. Vermilyea's MgO powders were obtained by sieving (10-30 pm fraction) ground, fused optical grade MgO crystals, sedimentation in alcohol, dried at 400°C and stored in a dessicator. The surface area was then calculated from the particle size assuming smooth surfaces, and presumably perfect cubes. The surface area was corrected for the amount dissolved whereas, as discussed below, the area per unit mass actually increases markedly during dissolution. TABLE 2.-DIsso~uTIoN RATES FOR MgO IN NITRIC ACID AT 25°C PH ratelmol cm-2 s-1 2.0 9.5 x 10-l0 2.5 6.3 x 10-lo 3.0 3 .0 ~ 10-lo 3.5 1.8 x 10-lo 4.0 1.05 x 10-lo We have used highly perfect MgO smoke crystals of known surface area and surface morphology. Table 2 lists initial rates at different pH from a number of runs between pH 2 and 4. These values are at least one order of magnitude lower than those found by Vermilyea. However, replotting these results as log (rate) against pH as in fig. 2, it can be seen that the error in the measurements, even with well characterised, relatively perfect oxide, is sufficient to give a slope between -0.4 and - 0.6 with a mean value of - 0.5. It is clear that dissolution is controlled by a surface mechanism, not solution2910 DISSOLUTION OF IONIC OXIDES diffusion, in this pH range.Vermilyea has suggested that it is surface reaction, i.e., a slow reaction of surface hydroxide with a second proton and, hence, expects a slope of -3 for the log (rate) against pH plot. He explains the lower value in his work (i.e., -0.47) as being due to a low value of the transfer coefficient a+ for the cation (i.e.y - 0.35 instead of 0.5). In support of Vermilyea, infrared studies 11* l2 have shown that the MgO surface is covered with both free, noninteracting hydroxyl and hydrogen bonded hydroxyl groups immediately after exposure to air but this does not constitute a brucite Mg(OH)2 structure. It is not possible to avoid surface hydroxylation without preparation and transfer in vacuum. It seems reasonable to assume that surface hydroxylation to form OH groups takes place virtually instantaneously in solution.If surface reaction (Le., reaction of OH- to form H20) is rate limiting, a slope of - 3 would, indeed, be expected on this assumption. -9.c -9.2 - 9.4 n c1 .s % -9.6 - -9.8 -10.0 I I I I - 2-0 2.5 3.0 3.5 4-0 P H is -0.49. FIG. 2.-Log (rate) against pH for MgO smoke crystals in nitric acid at 25°C. The slope of the graph Infrared studies ll. l 2 also show very rapid formation of water molecules on the surface. Electron microscopy, from our work and that of Moodie and Warbley9 shows rapid surface etching by water vapour. It seems more likely, since both protonation reactions are very fast, that the rate is limited in solution by modification of the surface potential barrier by the protons. This would suggest a slope of -0.5, in reasonable agreement with Vermilyea’s and our results.This result does not distinguish between rate control by cation or anion removal. The ion responsible for rate control would, however, have a transfer coefficient very close to 0.5. Again, in view of the very fast hydroxylation, it seems more likely that cation removal is rate limiting.R. L. SEGALL, R. ST. C. SMART AND P. S. TURNER 291 1 In reassessing distinction between different r.d.s. on the basis of log (rate) against pH data, it does not appear likely that, even for well characterised material, a definite prediction of reaction mechanism can be made. Part of the reason for this uncertainty lies in differences in surface properties between different preparations, and in initial surface attack.SURFACE CHANGES DURING DISSOLUTION Fig. 3 shows the initial stages of attack on the small cubes of MgO at < 5 % of total dissolution. The edges and corners appear to be shaved off leaving an average (110) facet. Steps are observed across these facets (e.g., at A) suggesting that material has been removed through dissolution at steps which move along the facets in the (100) directions. The { 100) faces of the original cubes show considerable roughening suggestive of the development of pits or channels (e.g., at B). Further attack leads to far less regularly shaped crystals (fig. 4 at 95 % dissolution) which show the development of distinct steps and ledges as dissolution proceeds. The surface of these particles is obviously very rough with a high concentration of kink sites at steps, ledges, corners etc.The surface detail from all micrographs obtained between 10 and 95 % dissolution is consistent with the model, demonstrated by Moodie and Warble for growth of MgO sinters, in which the various (hkl) crystal faces are made up of (100) steps down to the unit cell level, e.g., an average (110) face is actually a series of closely packed ( 100) steps. % dissolution FIG. 5.-Area ratio (expressed as part dissolved area per unit mass/initial area per unit mass) against percent of total dissolution. Full line from experimental results, broken line calculated from area ratio change with decreasing particle size during dissolution. The mechanism for dissolution is dependent on the surface structure in that protons diffusing to the surface attack preferentially at kink sites.It is possible, from the pH changes, to follow the change in surface area per unit mass during dissolution. For example, from an initial pH of 2 with initial oxide of known area, the rate at pH 2.5, 3.0, 3.5 and 4.0 may be calculated in mol g-l s-l,2912 DISSOLUTION OF IONIC OXIDES since the mass remaining at each pH can be calculated. These rates are then com- pared with initial rates, for the same initial oxide, at the respective pH value, i.e., in mol cm-2 s-l. This allows the ratio of the area of the partially dissolved oxide to the initial area to be computed as a function of percent of total dissolution. Results are shown in fig. 5. There is a doubling of the area per unit mass within the first 10 % of dissolution.This would appear to be associated with the expected, and observed (fig. 3), initial attack at edges, corners and kink sites associated with the surface projections. The area gradually increases until, at more than 70 % dissolution, the very irregular crystals give an increase in area to more than five times the initial area. At this stage the irregular crystals (fig. 4) provide a great variety of kink sites for preferential attack. In considering fig. 5, it is clear that, in addition to the effects of changing kink site density, there is an increase in surface area per unit mass associated with decreasing partick ,yize. For cubic particles, of initial mass M,, the area ratio (expressed as part-dissolved area per unit mass/initial area per unit mass) is given by (Mx/Mo)-* where M, is the mass remaining at x % dissolution. Assuming that the increase in surface area due to this decreasing particle size effect is additional to the doubling of area due to initial attack (within the first 10 % of dissolution) we have plotted the particle size area increase from 10 % onwards as the broken line in fig.5. Our results suggest, that after 10 % dissolution, the two factors causing an increase in surface area per unit mass, namely (i) particle size decrease and (ii) surface roughening (fig. 4) associated with production of an increasing density of kink sites, may be roughly equal in magnitude. Each effect separately appears to give a factor of roughly 1.5 after about 60 %. It is clear, from these results, that correction during dissolution for loss of surface area due to mass loss alone is not adequate. In determining dissolution rates for ionic oxides, the importance of atomic surface detail and particle size is at least as great as that of theoretical considerations of the mechanism of the solid/solution interface. Support from the Australian Research Grants Committee and the Australian Institute for Nuclear Science and Engineering is gratefully acknowledged. J. W. Diggle, Dissolution of Oxide Phases in Oxides and Oxide Films, ed. J. W. Diggle (Marcel Dekker, N.Y., 1972), vol. 2. N. Valverde and C . Wagner, Ber. Bunsenges. phys. Chem., 1976, 80, 330. H. J. Engell, 2. phys. Chem., 1956, 7, 158. D. A. Vermilyea, J. Electrochem. SOC., 1966, 113, 1067. N. Valverde, Bey. Bunsenges. phys. Chem., 1976, 80, 333. C . F. Jones, R. L. Segall, R. St. C . Smart and P. S. Turner, J.C.S. Faraday I, 1977, 73, 1710. C . F. Jones, R. L. Segall, R. St. C . Smart and P. S. Turner, J.C.S. Faraday I, 1978, 74, 1615. A. F. Moodie and C . E. Warble, J. Cryst. Growth, 1971, 10, 26. W. K. Burton, N. Cabrera and F. C. Frank, Phil. Trans. A, 1951, 243,299. R. S t . C. Smart, T. L. Slager, L. H. Little and R. G. Greenler, J. Phys. Chem., 1973,77, 1019. * D. A. Vermilyea, J. Electvochem. SOC., 1969, 116, 1179. l 2 J. V. Evans and T. L. Whateley, Trans. Faraday SOC., 1967, 63, 2739. (PAPER 8/786)