Silver colloid particles preparation via reduction by organic redox system

ACTA UNIVERSITATIS PALACKIANAE OLOMUCENSIS FACULTAS RERUM NATURALIUM 1998 CHEMICA 37

Silver colloid particles preparation via reduction by organic redox system

Kvítek Libor1, Fichna Petr1, Pikal Petr2, Novotný Radko3

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Department of Inorganic and Physical Chemistry, Palacký University, Svobody 8, 771 46 Olomouc, Czech Republic 2 Precheza a.s., Dr. E. Beneše 24, 750 62 Přerov, Czech Republic 3 Department of Microscopic Methods, Faculty of Medicine, Palacký University, I. P.Pavlova 35, 771 26 Olomouc, Czech Republic Received May 29, 1998 Abstract Several colloid silver sols which differ in average particle size were prepared from complex silver compound [Ag(SO3)2]3- via reduction by means of potassium hydroquinonemonosulfonate in alkaline solution. Average particle sizes of the prepared silver particles varied from 700 to 170 nm and were significantly influenced by rate of reduction silver ions and by sulfite concentration in the used reaction system. The later factor is crucial to minimum particle size limit of prepared silver particles. Key words: Silver colloid particles preparation, hydroquinonemonosulfonate, particle size distribution. Introduction Colloid dispersions of metal particles play important role in both scientific and industrial chemistry. Typical example of specific catalytic properties of such systems is surface enhanced Raman spectroscopy (SERS),1 where, due to adsorption of studied organic molecules on colloid metal (usually silver) surface, the Raman signal is considerably amplified (approximately 107). Classical methods of colloid silver preparation are based on reduction of soluble silver salt by a convenient reduction substance, most often borohydride (NaBH4) is used.2 These methods are simple and provide silver particles from several nm to a few

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tens of nm in diameter. The particles are stabilized by BH4- ions adsorption giving a negative charge several tens of mV to the particle surface.3 The latest findings from SERS field show,4 that silver particles having specific size (approximately 110 - 120 nm) are more efficient in Raman light scattering enhancement (amplification factor is about 1015). These so called “hot particles” can not be prepared by above mentioned borohydride reduction method, as in this case it is impossible to produce such large particles by reaction conditions variation. Average size of silver particles can be increased only by primary particles coagulation.5 Some other methods are therefore investigated to allow preparation of particles with greater variability in their size. The new techniques appear in area of colloid particles preparation for SERS, for example based on mechanical force of laser on macroscopic silver precursor.6 However, possibilities of conventional methods of metal colloid particles preparation based on chemical reduction are not used up. Utilizing of organic redox systems used in photography as developers offers some possibility in the way of particle size variation. The reduction abilities of the photographic developers are simple to control by means of pH variation, because at least one redox form is always a weak electrolyte.7 The variation of redox potential of reduction system has, in accordance with electrode theory of photography developing,8 influence on reaction rate silver ion reduction which changes prepared silver particles sizes and shapes. As silver ions in photographic systems are present in bound form (halogenide or soluble complex compound) it is possible to influence the reaction velocity by variation of free Ag+ ions equilibrium concentration in solution too. Such the using of photographic developing system arrangement allow to regulate two important factors, whose modification can control the size of prepared colloid silver particles. Experimental The experimental scheme of silver particles preparation by reduction of Ag+ ions via organic redox systems was similar to model system of photographic development.9 Silver sulfite complex [Ag(SO3)2]3- was reduced by potassium hydroquinonemonosulfonate (HQS), while pH was maintained and controlled by Britton-Robinson buffer (in the range of pH = 11–13). The Na2SO3 concentration in the reaction mixture was 0.2, 0.1 and 0.04 mol dm-3, AgNO3 2.5 x 10-4 mol dm-3 and HQS 2 x 10-3 mol dm-3. All solutions used for reaction mixture were cleaned from mechanical impurities by filtering through G5 frite (pore size 1.3 mm). Progress of Ag+ ions reduction was continuously monitored by measuring turbidity of reaction system. The simple photometry instrument made up from titration supplement to spectrometer Specol (cuvette volume 30 ml) with high luminance LED diode (l = 660 nm, luminance 1500 mcd) was used. The temperature in cuvette was not stabilized, all measurements were made at laboratory temperature (approx. 20°C). Particle size distributions of prepared colloid silver samples were measured by means of Zeta Potential Analyzer Zeta Plus (Brookhaven, USA) which use dynamic light scattering method (QELS).10 The same instrument was used for estimation of

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zeta potential of the colloid particles (by electrophoretic mobility measuring). Reference measurements of silver particles were made by electron transmission microscopy method by Opton Zeiss microscope (Zeiss Jena, SRN). Results and discussion The applied method of Ag+ ions reduction allowed broad variations of reaction rate by changing pH and sulfite concentration. Reaction time for full reduction Ag+ ions present in studied system differed from minutes to hundreds of minutes. Size and partly a shape of prepared colloid silver particles was greatly influenced by reaction rate of reduction, what can be seen in electron microscopy pictures (Fig. 1).

Fig. 1: Electron microscopic photographs of the silver colloid particles (overall magnification 20400x) prepared via [Ag(SO3)2]3- complex reduction by HQS at sulfite concentration 0.04 mol dm-3 and pH a) 12.8 (overall reduction time 100 s), b) 11.7 (overall reduction time 1200 s) and c) 11.1 (overall reduction time 2400 s).

The results of silver particle size relationship on reaction rate of Ag+ ions reduction (time for reduction of all Ag+ ions in the reaction system) are summarized in Fig. 2. The average particle sizes (estimated by Zeta Plus) are computed from unimodal lognormal particle size distribution weighted by intensity of scattered light.11 If the multimodal distribution analysis was employed (the sample is supposed to consist from several distinct subpopulations) the main part of the sample had similar size as computed from monomodal distribution. At least one other part of the sample was found, which size was several magnitudes bigger than the main one, but their weight was only few percent of the main one.

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Fig. 2: Relationship between size of silver particles prepared via [Ag(SO3)2]3- complex reduction by HQS and overall reaction time for different Na2SO3 concentration in reaction system. (Concentration of sulfite: 1–0.2 mol dm-3, 2–0.1 mol dm-3, 3–0.04 mol dm-3; pH varied in the range 11–13.)

Results confirm the assumption of severe relationship between prepared silver particles sizes and the reaction rate of silver sulfite complex reduction. The sulfite concentration in the studied system can be seen itself as very important factor influencing size of prepared silver particles. If the sulfite concentration is held constant, the silver particle size limits with the increased reduction time (decreased reduction rate) to a distinct minimum value. This silver particle size threshold strongly depends on sulfite concentration and declines with its decreased value. This feature of the studied system, together with zeta potential values prepared colloid particles are summarized in Table 1. Table 1: Relation of prepared silver particle sizes to reduction time for different sulphite concentration in studied system.

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csulf

Particle size [nm] for tred[s]

zeta potential

[mol dm-3]

300

800

2400

boundary

[mV]

0.2 0.1 0.04

700 360 250

310 275 200

270 235 170

230 190 140

–85 –60 –45

It is necessary to take into account the theory of formation and growth a new phase to clarify the observed relationship between particle sizes of prepared silver particles and reduction rate of silver complex and sulfite concentration. According this theory the nuclei of a new phase form only in thermodynamically suitable conditions, characterized particularly by oversaturation. 12 The oversaturation control the frequency of new phase nuclei formation J: J = J0 exp[–Wc /(kT)],

(1)

where J0 is the pre-exponential factor and Wc is the energy of the critical nuclei formation: 15ps3(Vm)2 Wc = ————— , (2) 3 | Dm | 2 where s is the surface tension newly emerging phase, Vm is its molar volume and Dm is the difference of chemical potentials of a matter in stable macroscopic phase and in previous phase (just this difference is determined by solution oversaturation of the matter forming the new phase). Also the critical nucleus size rc (the size of nucleus allowing from thermodynamic point of view further growth) is influenced by oversaturation degree: 2sVm rc = ———— . | Dm |

(3)

Exponential growth of silver particles sizes with increase of reduction rate observed during reaction times shorter than 300 s can be qualitatively explained by high oversaturation degree caused by strong reduction ability of HQS at higher values of pH. Due to this high oversaturation the nuclei of the new phase are formed very quickly and their further rapid growth leads to equally rapid decrease of oversaturation (considering validity of the Kelvin’s equation), so further formation of the new nuclei is inhibited. The existing nuclei growth during subsequent reduction until all Ag+ ions in the reaction system are consumed. The produced silver atoms are therefore divided among fewer particles, which growth to bigger dimensions than in case of the slower reduction. In this case the lower oversaturation allowing due slow growth of the formed nuclei formation of new nuclei lasts longer in beginning moments of system development and gives greater chance to create more critical size nuclei. Observed decline of silver particles size limit with decreasing concentration of sulfite in the system can be explained by new phase formation theory too. Decreasing concentration of sulfite increases stability of the small nuclei (sulfite can oxidize silver atom clusters backward to Ag+ ions) and therefore increases frequency of new phase nuclei formation. As result, silver atoms produced by reduction process have eventu-

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ally divide into greater number of particles. Due to limited content of Ag+ ions in system leads this effect to decline of silver particles sizes. The relation of average silver particles size limit to sulfite concentration is, for observed results, linear in logarithmic coordinates and allows prediction of conditions for preparation required size silver particles (Fig.3). Mentioned conclusion is, due to limited number of experimental results, only qualitative and it requires to acquire further details to validate relationship between silver particles size and sulfite concentration.

Fig. 3: Relationship between silver particles size dp (nm) threshold, prepared via [Ag(SO3)2]3- complex reduction by HQS, and sulfite concentration cS (mol dm-3) in the studied system.

Conclusion Presented method of [Ag(SO3)2]3- complex compound reduction by means of HQS redox system shows possibility of controlled production colloid silver particles with desired size. The most important factor of this method is sulfite concentration, which limits the minimum accessible silver particles sizes. The quantitative evaluation of this relationship requires to make further experiments with increased concentration range of the mentioned most important system component.

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