Electrochemical Properties of Zinc Oxide-Aqueous Electrolyte Solution Interface

Indian Journal of Chemistry Vol. 16A, August 1978, pp, 675·678

K. C. RAY & S. K. ROY Department of Chemistry, Visva-Bharati, Santiniketan, West Bengal and S. KHAN Department of Chemistry, Jadavpur University, Calcutta 32

Received 29 September 1977; accepted 1 February 1978

The interfacial properties of zinc oxide-aqueous electrolyte system have been investigated and characterized using several independent methods. Surface charge -pH curves have been obtained at 350 ± 0.10 by potentiometric acid-base titration method at several ionic strengths maintained by sodium nitrate. By measurtng the equilibrium distribution of potential-determining ions, the following interfacial properties of the oxide-solution system, (i) the point of zero charge, (ii) differential capacity of the electrical double layer and (Hi) the effect of surface charge density and ionic strength on the interfacial free energy, have been studied. Some of these measurements have been confirmed and amplified by microelectrophoretic mobility measurements. The experimental point of zero charge (pzc) has also been studied by consider- ing the solubilities of different ionic hydroxo complex species of zinc in water. It has been found that the experimental pzc of zinc oxide coincides fairly well with the isoelectric point of the solution which remains in equilibrium with the solid zinc oxide. It has also been found that the pH of the solution having minimum solubility of the oxide corresponds fairly well to the pzc of the oxide.

N an earlier communication", we reported the in the slow region (30-90 min after each addition of results of investigation on the tin dioxide- acid or base depending on the pH of the solution). Iaqueous solution interface. Potentiometric a<:id- By allowing sufficient time for the ZnO surface base titrations were used to obtain fast adsorption to come to solubility equilibrium, the point of isotherms leading to the evaluation of the important zero charge (pzc) of the ZnO-aqueous electrolyte characteristics of electrical double layer. However, solution interface has been predicted from a con- the solubility of zinc oxide is fairly higher than sideration of the solubility of different zinc-bearing those of other sparingly soluble oxides such as species at different equilibrium pH of the solution. haematite, tin dioxide and rutile, therefore, deter- Microelectrophoretic mobility measurements of sus- mination of the surface properties by adopting fast pended zinc oxide particles at various ionic strengths adsorption technique may not be correct in the maintained by NaCl have also been made. These present case. The dissolution of the oxide material measurements are useful for the comparison of the is expected to result in pH drift which will continue result obtained by slow acid-base titration method for a considerable length of time. The equilibrium used in the present study. pH will be attained only if requisite time is allo:ved for the completion of a number of slow reactions Materials and Methods such as hydrolysis, complex formation, dissociation of the dissolved complexes of the surface, and The zinc oxide sample used in the present study precipitation or colloid formation. These slow re- was prepared by adding sodium hydroxide solution actions may reasonably be believed to follow. ~he to zinc chloride solution with constant stirring at dissolution of the oxide, however small the solubility room temperature. The precipitate thus obtained of the oxide in question be. Hence, if the fast was boiled under reflux for one week. Surface adsorption method is used for the study of surface impuri~ies should be Ieached out during this refluxing behaviour of ZnO, the system might not be in operation. The precipitate was subjected to repeated equilibrium pH corresponding to adsorption density centnfugation, washing with conductivitv water of potential-determining ions (H+ and OH-) in the and decantation until specific conductivity of the adsorption isotherms (adsorption density versus supernatant liquid attained a value less than that of equilibrium PH). With a view to studying the to-3M ReI at the same temperature. Three different surface behaviour of ZnO under equilibrium condi- sa!Dples of zinc oxide prepared in this way were tion after the completion of a number of slow mIx:d ~~gether, stored in a polythene bottle (to reactions as mentioned above, we have adopted aVOId silicate contamination) and the mixture was acid-base potentiometric titration of ZnO suspension allowed to age for a period of 15 days.

675 INDIAN J. CHEM., VOL. 16A, AUGUST 1978

O'IN solutions of NaOH, HCI and HNOa were ions have been plotted as a function of pH in used as titrants in the potentiometric titration of accordance with the above reactions (1-5). With a the ZnO suspension. NaN03 and NaCl were used view to correlating the pzc of the oxide with the as supporting electrolytes to maintain ionic strengths solubility of the oxide, total solubility of ZnO (10-321,[ to 5 X 10-lM). Titrations were performed represented as logarithm of the sum of concen- at 350 ± 0.10 in an atmosphere of purified N2 by trations of different Zn-bearing species was then adding to the dispersion (about 2 g/300 ml) the plotted as a function of pH. appropriate aliquots of acid or base at intervals of 30-90 min. The reagents used were of AR grade. Results and Discussion Other details of the titration procedure including the Although adsorption measurements of potential- titration cell (glass and calomel electrode) were the determining ions (H+ and OH-) have been made same as those reported in previous publicationsv". on ZnO surface in the presence of NaNOa and Two sets of adsorption isotherms (TH+-rOH- NaCl as supporting electrolytes, the data for NaNOa versus PH) were constructed from the calculated have only been presented in Fig. 1. Adsorption data of adsorption density and the corresponding isotherms corresponding to NaCl are identical in equilibrium pH of the solution measured with the all respects with those obtained for NaNOa. It is help of an expanded scale pH meter (model No. pH evident from Fig. 1 that the isotherms of different 823, Electronics Corporation of India). ionic strengths intersect at pH 9·2 which may be The specific surface area of the oxide, as deter- identified with the pzc of the interface. The mined by BET (N2) method, was found to be electrophoretic mobility of the suspended ZnO 20·2 m2 g-l. The pH at which all curves of a particles is found to be zero at pH 9·2 (Fig. 2) particular family meet is set as the point of zero in the solutions having ionic strengths 0·025, 0·05 charge (pzc) of the oxide. Microelectrophoretic and O'IM (NaCI). This pH of zero mobility agrees mobility measurements of the suspended ZnO well with the pzc determined by slow adsorption particles in NaCI solution were measured in a measurement technique. microelectrophoretic cell manufactured by Arthur Using fast adsorption technique, Blok and de- H. Thomas & Co. Details of the experimental Bruyn" have reported different pzc values, for the procedure were the same as given in an earlier ZnO system, which are located in the alkaline pH publication''. The pH-mobility curves corresponding range. The highest one, as determined by the to different ionic strengths were constructed. The intersection of the low ionic strength curves, lies pH of zero mobility was determined and compa~ed with that obtained by adsorption of potential- IO~------~ determining ions on the oxide surface. Predictions of the surface behaviour of ZnO system 4 from the standpoint of its solubility behaviour 8 were compared with the results obtained by slow acid-base potentiometric tit~ation me.thod. Solid 6 zinc oxide, when suspended m water, 1S known to exist in equilibrium with Zn2+, Zn(OH)2' (aq.), Zn(OH)+, HZnOz, and ZnO~-. The solubility .of these species including the hydroxo complex species (both positive and negative) at different pH may be calculated from the equilibrium constants of the reactions represented by Eqs. (1)-(5). ZnO (s)+2H+;;:: Zn2++H20; pK = -11·53 . .. (1) ZnO (s)+H+;;:: Zn(OH)+; pK = -1·9 ... (2) ZnO (s)+OH-;;:: HZnOi; pK = 2·9 ... (3) ZnO (s)+20H-;;:: ZnO~-+H20; pK = 1·1 ... (4) (ref. 4) ZnO (s) +H20(1) ;;:: Zn(OH)2(aq.); PK = 5·4 z ... (5) 9 -4 The following thermodynamic data regarding free energy of formation (6.Gj) were used to calculate the c:~ -6 pK values of reactions (1-5): 6.G'l (Zn2+)= -35·184 kcal/mole'', 6.Gj (ZnO) = -76·05 kcal/rnole", toG'} -8 (H20) = -56·69 kcaljmole", 6.G'} (OH-) = -37,595 kcal/rnole", 6.Gj [Zn(OH)+] = -78·8 kcal/mole", 6.GJ 2 _~L- __~ ~ -L ~ (HZn02) = -109·56 kcal/mole'', 6.GJ (Zn0 -) = 7·5 8 9 10 11 -93·03 kcal/mole", 6.GJ [Zn(OH)2J = -132·6 kcal/ pH mole", Fig. t - Adsorption density of potential determining ions From a knowledge of the pK values of the as function of pH and ionic strength with sodium nitrate reactions representing the solubility .of ZnO at as supporting electrolyte at 35G [Curve 1, [NaNO.) = 1·0 3 2 varying pH, concentrations of the different Zn- Xto- M; curve 2, [NaN03) = 1 x 10- M; curve 3, [NaNOs) bearing aqueous species including hydroxo complex = 1 X 10-1M; curve 4, [NaNOs] = 5 X to-1M]

676 RAY et al.: PROPERTIES OF ZnO-AQUEOUS SOLUTION INTERFACE 2.0~------~ 180 j

160 OJ: (J 7~ 140 o > ..•~ T~ '" 0.5 ~... 120 , ~ 5 ~ >- o ..>-. 100 f- ;:; :f i 4: -0.5 U 80 i oJ

Fig. 2 - Electrophoretic mobilities of zinc oxide particles Fig. 3 - Total differential capacity as a function of surface as f unction of pH and ionic strength [Curve 1. [NaCt] charge at various ionic strengths of sodium nitrate [Curve 1. 3 = O'OlM; curve 2. [NaCI) = O'OSM; curve 3. [NaCt) [NaNO,] = t x to' M; curve 2, [NaNO.] = t x to-'M; 1 = O·025M] curve 3. [NaNO.) = I X to- M; curve 4, [NaNO.) = 5 X to-1M]

around pH 9·5. By adopting slow adsorption technique in the present investigation, pzc of ZnO- to be much larger for oxide systems. Among the solution interface has been found to occur at an oxides, the amorphous modifications have lower alkaline pH (9,2) which is at least in qualitative capacity values than the more crystalline and more agreement with the values of pzc in the alkaline polar solids. Apparently, the water molecules in the region reported by Blok and deBruyn. By replacing vicinity of a solid with strong field strength are the potential axis of a polarized interface by pH, drastically modified and more counter ions are thus the Lippmann-Helmholtz equation of differential accommodated at the surface". The penetration of capacity (C."p) for a reversible electrode may be ions into the compacted part of the double layer derived from the surface charge density versus pH may enhance its capacity. Breeuwsma and Lyk- curves (Fig. 1) in the following form (Eq. 6): lerna" have attributed this phenomenon to the surface porosity of oxides. They, however, did not rule out the possibility of ion penetration into Cexp (6) some depth of the surface layer. =G;;)P'T~s=:T(O~~)P.T'~$ ... By applying the general form of the Gibbs where fJS is the chemical potential of the supporting ~dsorption equation and by assuming that the electrolyte. The computed differential capacity for Interfacial potential difference is determined by the ZnO in NaNOa supporting electrolyte is shown in bulk concentration of H+ and OH- ions by a Fig. 3. The steepness of the plots of CellP versus Nernst type equation, it is possible to derive an pH as well as the magnitude of Cexpfcm2 is a clear expression for the interfacial free energy change indication of the significant sorb ability of the ionic with respect to the pzc in the following form" species when the surface is far away from its pzc. (Eq. 7): The differential capacity values calculated from the adsorption density of potential-determining ions - 6.. Y = JE (1, dE •.. (7) clearly indicate that the double layer characteristics E. ·of hydrous oxide-electrolyte solution interfaces differ The interfacial free energy changes (-6..Y) with from those of polarized mercury-, or reversible respect to the pzc, as defined by Eq. (7), were HgI- and Ag2S-electrolyte interface. The minimum obtained by graphical integration of the adsorption -differential capacity of the electrical double layer isotherms (Fig. 1) and plotted as the function of at ZnO-solution interface has been found to be PH of the solution (Fig. 4). This does not give 37 flffcm2 at 10-3M NaNOa (Fig. 3) as compared any additional information as compared to the to 6 flf/cm2 in mercury, AgI and Ag2S systems. previous data since in contrast to the polarized In general, the capacity values have been found electrodes the absolute value of the interfacial free

677 INDIAN J. CHEM., VOL. 16A, AUGUST 1978

1 0

0.36

0.72

Ni! \.08 u -8 ~ e a:: U ... 1.44 -12 \- (!'J

2.16 -20

8 9 10 t1 pH -2-4 Fig. :4 -.Interfacial free energy lowering at the zinc oxide + 6 8 10 12 14 16 solution _~nte.rface as a functio~ of pH [Curve 1, [NaNO.] pH =5xl0 M, curve 2, [NaN03J=lxl0-2M; curve 3, 2 [NaN03] = 1 Xto- M; curve 4; [NaN03] = 1 Xto-3M] Fig. 5 - .~on~entration o~ variou~ zinc-bearing species in, equifibr iurn wi th solid ZnO III aqueous solution energy at the pzc is not known. However, the general features of these curves definitely resemble ment .. The concentration of ZnO~- ion at the i.e.p. electrocapillary curves obeying Lippmann equation (9,2) IS far .below those of Zn(OH)+ arid HZr.O- for polarized mercury electrode with a maximum an~ hence Its contribution is negligible. It i: evident from the total solubility of the oxide at the pzc. In adopting slow acid-base potentiometric titration (curv~. ABC, Fig. 5) that the point of minimum method in preference to fast one, we advanced solubility of t.he oxide also coincides fairly well with arguments that ZnO being more soluble than other t?e p~c obtained by slow acid-base potentiometric oxides, it must be allowed more time to attain titra tion method. Thus the solubility behe viour of equilibrium through a number of slow reactions Zr~'0 system ha s been correla ted sa tisfa ctorily with the surface behaviour of ZnO system. resulting from the dissolution of the oxide. In order to correlate the solubility of the oxide with its Acknowledgement surface property, the variation of the concentration One of us (S.K.R.) thanks CSIR, New Delhi, (log concentration) of various Zn-bearing species in equilibrium with solid ZnO in aq. solution has been for the grant of a junior research fellowship. plotted as a function of PH (Fig. 5). The total References solubility of the oxide as a function of has PH 1. RAY, K. C. & KHAN, S., Indian J. Chem., (in press). also been represented by curve ABC (Fig. 5). It is 2. RAY, K. C. & KHAN, S., Indian]. Chem., 12 (1974) 74 evident from Fig. 5 that the solubility curves of 3. RAY, K. C. & KHAN, S., Indian]. Chem., 13 (1975),'577: Zn(OH)+ and H~n02 intersect at PH 9·2. This 4. FULTON,J. W. & SWINHART,D. F., J. Am. chem Sac 76 (1954),863. . " point of intersection of the solubility curves (where 5. LATIMER,W. M., Oxidation potential (Prentice-Hall N J ) the concentrations of Zn(OH)+ arid HZn02 are 1952, 169. ' •. equal) is called the isoelectric point (i.e.p.) of the 6. BLOK, L. & DEBRUYN,P. L., J. Call. Interface Sci 32- solution. The isoelectric PH coincides well with the (1970), 518, 533. ., 7. STUMN,W., HUANG,C. P. & JENKINS,S. R., Croat. chem pzc of ZnO obtained by two independent methods Acta, 42 (1970), 223. - of measurements, i.e. slow acid-base potentiometric 8. BREEUWSMA,A. & LYKLEMA,J., Discuss. Faraday Sac titration and microelectrophoretic mobility measure- 52 (1971), 324. "

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