Control of Dopant Adsorption from Aqueous Solution ... - Springer Link

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Journal of Sol-Gel Science and Technology

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Journal of Sol-Gel Science and Technology 13, 579–584 (1998) c 1998 Kluwer Academic Publishers. Manufactured in The Netherlands. °

Control of Dopant Adsorption from Aqueous Solution into Nanoporous Sol-Gel Films O. McCARTHY AND E.M. YEATMAN Department of Electrical and Electronic Engineering, Imperial College, London SW7 2BT, UK

Abstract. The doping of porous sol-gel films by adsorption of cations onto silica from aqueous solutions is demonstrated. The films are fabricated using the sol-gel method, giving nanometer scale porosity, high surface area, and homogeneity on the scale of optical wavelengths. After deposition by spin coating and initial heat treatment, the films are soaked in a salt solution of the desired dopant, and then given a further consolidating heat treatment. Doping with lead, aluminum and calcium has been demonstrated, and consistent and reproducible processes have been established. Results are presented showing control of doping level through pre-doping temperature and surface treatment, doping time and temperature, and doping solution pH. Potential applications in integrated optics are proposed. Keywords:

1.

thin films, doping, silica, integrated optics

Introduction

The adsorption of ions from aqueous solution has long been known, and investigated in a wide variety of fields. Applications include the removal of unwanted materials from solution, as in the nuclear and water treatment industries; concentration of certain species from solution, in chemical analysis; and the deliberate introduction of dopants to the surface of a material, e.g., for the production of optical glasses. Our interest is in the doping of films for silica-on-silicon integrated optics [1], although we believe the results described herein are of wider applicability. A potential advantage of solution doping of sol-gel glasses is that compositions may be attained that cannot be produced by melt techniques, because for instance they lead to crystallization. To benefit from this it is necessary that a product be obtained of sufficient homogeneity for the application without final processing at near melt temperatures. Solution doping of bulk sol-gel glasses was studied by Rabinovitch et al. [2], who used combined alkoxide-particulate methods to avoid stress cracking, leading to pore sizes of 10s of nm. We have

found that solution doping can be achieved for films prepared by using acid catalyzed alkoxides, which have pore sizes below 10 nm [3], and that this doping can be patterned with high resolution [4]. Here we show that this doping process is reproducible and controllable. Several models for adsorption at solid-solution interfaces have been proposed, all of which are based on the concept of a reaction between the ionic species in solution and the ionized surface sites. According to Yiacoumi [5], the surface complex formation model is the one most commonly used in the case of adsorption on hydrous oxide surfaces. The basic reaction involves the substitution of one or more of the silanol groups on the surface with the adsorbing species. It is thought that the main reacting species is not the fully ionised cation, but one of the hydrolysis products [5]. The adsorption process can be modelled by Eqs. (1) and (2) below. The first step is the hydrolysis of the cation (M), here taken to be divalent, and the second is the adsorption of this species onto the surface site represented by the silanol group (SOH). + yH+ M2+ + yH2 O ⇔ M(OH)(2−y)+ y

(1)

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McCarthy and Yeatman

SOH(x−1)+ + M(OH)(2−y)+ x y ⇔ SOH(x−z) − M(OH)(1+x−y−z)+ + zH+ y

(2)

The amount of dopant incorporated is determined both by the number of adsorption sites available, according to the surface area and surface morphology, and the number of these sites at which adsorption occurs, according to the process chemistry. The equilibrium conditions are determined by many parameters, including the solution pH, the number of surface sites, the cation valency, the oxide surface charge, the metal salt concentration, the presence of electrolytes, and other thermodynamic factors such as pressure and temperature.

2.

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Experimental Procedure

Film fabrication conditions were designed to maximize porosity while maintaining good optical quality. The main factors contributing to these properties, i.e., molar ratio R of water to TEOS (tetraethyl orthosilicate), type and concentration of catalyst, and heat treatment after spinning, were previously studied [6, 7]; from these results, optimum conditions were determined and used as the starting point for this work. The silica sol was made by mixing TEOS and ethanol in equal volumes, and adding water, as 0.1 M HCl, to R = 2. The solution was then refluxed at 70◦ C for 2 hours while stirring continuously. The resulting sol was stored in a sealed container at room temperature for a maximum of two weeks before use. Thin films were fabricated by spin coating; sol was dispensed through a 0.1 micron PTFE filter onto a silicon wafer, which was spun at 3000 rpm for 30 s. This gel film was then immediately baked in air in an electric tube furnace at 500◦ C for 30 min. Doping solutions were made by dissolving acetate salts in de-ionized water, to a molarity of 0.05 M. All chemicals were 99% pure and obtained from SigmaAldrich Ltd. Solutions were made no more than 2 hours before use, with pH adjusted with ammonia water and acetic acid as required. Films were doped by soaking in the salt solution, rinsed thoroughly in deionized water, and dried with a nitrogen gun. Where required, doping was carried out in a temperature controlled water bath. Unless otherwise specified, films were doped at room temperature. Pre-soak treatments were carried out immediately prior to doping; films were soaked in solutions of either ammonia

water or acetic acid, and then dried without rinsing. Finally, films were heated in an electric tube furnace until they reached densification; this was defined as the point at which there was no residual porosity, measured according to the method described in [3]. The densification temperature ranged between 800◦ C and 1000◦ C depending on the dopant concentration. 3.

Results and Discussion

The three dopants investigated were Pb, Ca and Al, all common glass constituents extensively described in the literature. The thickness and index of films were measured using a Rudolph AutoEL ellipsometer with precision of ±0.002 in index and a few tens of Angstroms in thickness. Energy dispersion spectroscopy (EDS) was used to qualitatively determine the atomic constituents of the films. After initial experiments to establish approximate doping conditions, samples with the highest doping levels, as indicated by refractive index, were analysed by EDS. The presence of the desired dopants was confirmed, and no undesired species were detected. A summary of the results is shown in Table 1. As Pb doping gave the largest index change, it was used for detailed characterization of the process. We believe that all the lead is in the form of PbO; the absorption spectrum of our material after annealing at 800◦ C, shown in Fig. 1, is similar to that of Magruder et al. [8], who found that for a similar Pb doping level, ion implanted silica films were almost identical to bulk lead silicate glass after annealing at 800◦ C. Based on this assumption the doping level can then be estimated from the refractive index, for the results that follow, by correlation with data from commercial lead silicate glasses [9], as shown in Fig. 2. In all results below, the measured index is given after lead doping and densification. To test the effect of surface hydroxylation, doping was carried out on films pre-soaked in solutions Table 1. Refractive indices of films doped with Ca, Al and Pb (index of pure silica is 1.459). EDS analysis confirmed the presence of each dopant. Dopant

Doping solution

Doping time

Index

EDS

Ca

0.05 M CaAc, pH 8.75

30 min

1.472

X

Al

0.05 M AlAc, pH 3.75

24 h

1.480

X

Pb

0.05 M PbAc, pH 6.1

30 min

1.536

X

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Nanoporous Sol-Gel Films

Figure 1.

Absorption spectrum of Pb doped silica film, annealed at 800◦ C.

Figure 2.

Refractive index vs. mole% PbO for lead silicate glasses, after [9]. The line shows a 3rd order polynomial fit to the data.

of different pH. As can be seen in Fig. 3, a marked increase in index was seen after pre-soaking in solutions above pH 6.0. This may be a consequence of increased hydroxylation, or of increased pore surface area by etching. Etching is indicated by the corresponding decrease in thickness, but significant etching of silica is not expected below pH 9 [10], so the effect of soaking in pH 7 seems to indicate increased hydroxylation. In the following results, only the doping conditions were varied. Figure 4 shows the effect of pH on the

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doping solution. This is an important factor, due to both its effect on the surface charge and the degree of hydrolyzation of the dopant. According to Schindler et al. [11], the relationship between pH and adsorption is expected to be s-shaped, with a rapid transition over a narrow pH range. Figure 4 shows a linear relation in this transition region; below pH 5.3 doping was negligible, while above 6.3 precipitation of the solution occurred. To test repeatability, results were obtained for two equivalent sols; no difference is seen within the error range of the measurements.

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Figure 3. Effect of pre-soaking on index and thickness of doped films. The control sample had no presoak treatment before doping. Lines are for ease of viewing only.

Figure 4.

Refractive index vs. doping solution pH, for two equivalent sols. Doping time was ∼15 min. Lines show best linear fit to data.

In Fig. 5 the logarithmic dependence of doping level on time is shown. In Fig. 6 an approximately linear dependence on doping solution temperature is indicated. By isolating and controlling major factors, repeatability can be achieved. This is indicated by the correlation of the experiments of Figs. 4–6. For example, Fig. 6 for room temperature (22◦ C), gives an index n = 1.52, as does Fig. 5 for a time of 10 min. Figure 5 for 15 min gives n = 1.53, as does Fig. 4 for pH = 6.0.

4.

Conclusions

Surface adsorption from aqueous solution can be used to dope nanoporous acid catalyzed sol-gel films. The doping level can be widely and systematically controlled by doping time, temperature and solution pH, and by treatment of the film prior to doping. A variety of dopants can be exploited. This method has potential applications in optical waveguide fabrication,

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Figure 5.

Refractive index vs. doping time. Line indicates best logarithmic fit to data. Doping conditions: pH 6.0, temperature 22◦ C.

Figure 6.

Refractive index vs. doping solution temperature. Line indicates best linear fit to data. Doping conditions: pH 6.0, time 10 min.

particularly as patterned doping can be obtained. Future work will include investigation of additional dopant species; one possible application is in the monolithic integration of passive and amplifying waveguides, by selective area doping with rare earth ions. Acknowledgments We are grateful to the European Commission for financial support, under the CAPITAL Project (ACTS

AC047) and through a Human Capital and Mobility Fellowship for O. McCarthy, and to Drs. Emma Dawnay and Munir Ahmad for technical advice. References 1. E.M. Yeatman, in Sol-gel and Polymer Photonic Devices, edited by S.I. Najafi and M. Andrews (SPIE Crit. Rev. Proc. CR-68, 1997), pp. 119–142. 2. E.M. Rabinovitch, A.J. Bruce, N.A. Kopylov, and P.L. Trevor, J. Non-Cryst. Solids 160, 126 (1993).

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3. E.M. Yeatman, M. Green, E.J.C. Dawnay, M.A. Fardad, and F. Horowitz, J. Sol-Gel Science and Technology 2, 711 (1994). 4. O. McCarthy and E.M. Yeatman, Selected area doping of porous sol-gel films for integrated optics, Optics Letters 22, 1864 (1997). 5. S. Yiacoumi and C. Tien, Kinetics of Metal Ion Adsorption from Aqueous Solutions (Kluwer Academic Publishers, Boston, 1995), chap. 2. 6. M.A. Fardad, E.M. Yeatman, E.J.C. Dawnay, M. Green, and F. Horowitz, J. Non-Cryst. Solids 183, 260–267 (1995).

7. M.A. Fardad, Catalysts and the structure of silica sol-gel films, J. Materials Science, in press. 8. R.H. Magruder, D.O. Henderson, S.H. Morgan, and R.A. Zuhr, J. Non-Cryst. Solids 152, 258 (1993). 9. N.P. Bansal and R.H. Doremus, Handbook of Glass Properties (Academic Press, New York, 1986), p. 547. 10. R. Iler, The Chemistry of Silica (Wiley, New York, 1979), chap. 1. 11. P.W. Schindler, B. Furst, R. Dick, and P.U. Wolf, J. of Colloid and Interface Science 55, 469 (1976).