Synthesis of nanosized silver particles by chemical reduction method

Synthesis of nanosized silver particles by chemical reduction method

Materials Chemistry and Physics 64 (2000) 241–246 Synthesis of nanosized silver particles by chemical reduction method 夽 Kan-Sen Chou∗ , Chiang-Yuh...

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Materials Chemistry and Physics 64 (2000) 241–246

Synthesis of nanosized silver particles by chemical reduction method



Kan-Sen Chou∗ , Chiang-Yuh Ren Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan Received 10 November 1999; received in revised form 24 December 1999; accepted 28 December 1999

Abstract Synthesis of nanosized silver particles by chemical reduction using formaldehyde in aqueous solution was studied in this work. Effects of several processing variables such as quantities of alkaline solution and formaldehyde and reaction time were investigated. Effects of protective agents such as PVP (polyvinyl-pyrrolidone) and PVA (polyvinyl alcohol) were also included. The products were mainly characterized for its particle size distribution to provide information on the optimal conditions of synthesis and sufficient stability against coagulation. Our results indicated that for [AgNO3 ]=0.01 M solution, the addition of [formaldehyde]/[AgNO3 ]=4, [NaOH]/[AgNO3 ]=1 and [Na2 CO3 ]/[Ag]=1, together with PVP/Ag weight ratio of 9.27 or PVA/Ag weight ratio of 3.37, could produce silver colloids having sizes between 7 and 20 nm. The suspension was stable for at least 24 h. Silver particles can be easily collected through centrifuge and filtration when the solution is mixed with sufficient amount of acetone. The wet precipitate can be redispersed for further applications with increase in particle size. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Chemical reduction; Formaldehyde; Silver colloids

1. Introduction Silver powders having a very fine and uniformly distributed sizes are desirable in many fields of industrial applications. One such example would be as the major constituent of conductive inks, pastes and adhesives for various electronic components [1]. A smaller particle size would in theory reduce the necessary quantity for providing some desired electrical conductivity. The subject of preparing nanosized metal colloids has attracted a great deal of attention in recent years. Both physical and chemical methods had been reported in the literature. For chemical methods, the choice of the reducing agent is of course the major factor. Examples include ␥-radiation [2], hydrazine [3], sodium boron hydride [4] among others. The reducing ability will determine the formation kinetics and hence reaction temperature. The reaction can be carried out in either aqueous solution or in organic solvent such as the polyol process [5]. All these methods involve the reduction of relevant metal salts in the presence of a suitable protecting agent, which is necessary in controlling the growth of metal colloids through agglomeration. A popular example would 夽 Part of this paper was presented at APCChE’99 in Seoul, South Korea on 18 August 1999. ∗ Corresponding author. Tel.: +886-3-571-3691; fax: +886-3-517-5408. E-mail address: [email protected] (K.-S. Chou)

be the polyvinyl-pyrrolidone (PVP). According to Zhang et al. [3], PVP can form a complex with silver ions and also be strongly absorbed on the silver particles. The objective of this article is to present experimental results using formaldehyde in aqueous alkaline solution as the reducing agent. We hope to obtain systematic results regarding the formation of nanosized silver colloids from this reaction system. Since the reducing ability of formaldehyde is a function of pH [6], we will also investigate the effects of alkaline concentration on the conversion and subsequent stability of silver colloids. Two protecting agents, i.e. PVP and PVA (Polyvinyl alcohol) will be examined and compared for their effects on the particle size distributions.

2. Experiment Silver nitrate (Showa) solution of 0.01 M (pH=5.18) was used throughout this work as the precursor. A predetermined quantity of PVP (molecular weight (MW)=10,000, Acros Organics) or PVA (4–6 mPa s at 4% and 20◦ C; Riedel-de Haen) was then added to AgNO3 . According to the relationship between viscosity and MW offered by Bassner and Klingenberg [7], the molecular weight of PVA is estimated to be between 31,000 and 50,000. Using a 2 M formaldehyde stock solution prepared from a source of 37% solution (Tedia), we tested at first the effect of quantity of reducing agent

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between the ratio of 1 and 120. It showed, however, only negligible effects on particle size distributions. We, therefore, used a relative low ratio of [formaldehyde]/[AgNO3 ]=4 for the rest of experiments. However, if only formaldehyde was used, the reduction rate would be too slow at room temperature due to low pH. We then added different quantities of alkaline solution consisting of NaOH and/or Na2 CO3 . The formation of silver colloids can be achieved in less than 1 h. To sum up, the specific effects of PVP, PVA and Na2 CO3 were reported in this paper. Products of silver colloids were characterized mainly on its particle size distribution using a laser particle sizer (LPA 3100, Photal). Variations in size distribution were followed along with time. A number of conditions were selected to understand the stability of these colloids. XRD, SEM and TEM were also applied to obtain information on the structure and morphology of these colloids. In order to analyze the conversion of this chemical reduction method, we used a ultrafiltration membrane (YM 100, Amicron) to separate silver colloids from the solution. The clear filtrate was then analyzed for residual silver content by ICP-AES (Jarrell-Ash, ICP 900).

Fig. 1. Solution pH as a function of time with different additions of [Na2 CO3 ]/[AgNO3 ]= (a) 0; (b) 1; (c) 2 (other conditions: [AgNO3 ]=0.005 M, [HCHO]/[AgNO3 ]=4, [NaOH]/[AgNO3 ]=1, PVP/AgNO3 =9.01).

3. Results and discussion 3.2. Effect of sodium carbonate concentration

3.1. Possible reaction mechanism The stoichiometric reaction between formaldehyde and silver ion in an alkaline solution can be written as follows: 2Ag+ + HCHO + 3OH− → 2Ag + HCOO− + 2H2 O (1) 2Ag+ + HCHO + OH− → Ag + HCOOH + 1/2H2

(2)

The hydroxyl ion may undergo a nucleophilic addition reaction to formaldehyde producing hydride and formate ions. The hydride ions then reduce silver ion to silver atom and may become hydrogen itself as a by-product. The relative ratio of formaldehyde to silver was actually kept at 4 throughout this study. This ratio should be sufficient to reduce all silver ions in the solution. Shown in Fig. 1 are the changes of solution pH versus time after different additions of sodium carbonate solutions. The pH decreased as reaction proceeded. It is consistent with the proposed chemical reactions. For curve a, where only [NaOH]/[AgNO3 ]=1 was added (i.e. Na2 CO3 was not added), the pH decreased quickly to below 5. The reducing power of formaldehyde was poor under this condition and as a result the conversion was 72.1% after 60 min and 73.7% after 200 min. For comparison, when [Na2 CO3 ]/[AgNO3 ]=1 was added in addition to NaOH, the pH of the solution stayed above 7 all the time. The conversion was 99.7% after 60 min of reaction. These results demonstrated the importance of solution alkalinity to the reducing power of formaldehyde.

Although a higher pH is favored for higher reducing power, it, however, showed an adverse effect on particle size. When we added more NaOH to the reaction system, the silver colloids became large precipitate and settled to the bottom of solution. As a compromise, we therefore substitute a weak base of sodium carbonate for NaOH. The intention here was to release hydroxyl ions only when the pH became lower than certain values. The effect of the quantity of sodium carbonate on the average colloidal size, as well as the standard deviation of size distribution is now exhibited in Fig. 2. These data were taken after 200 min of reaction. Clearly there existed an optimal condition with the [Na2 CO3 ]/[AgNO3 ] ratio between 1 and 1.5 for obtaining smaller particle sizes. When more Na2 CO3 was added, the solution pH would become higher and that was detrimental to the stability of these silver colloids. The silver colloid would grow from 10–20 to 80–100 nm and have a wider distribution at the same time as indicated by the bar shown in Fig. 2. The detrimental effect of adding too much sodium carbonate can be clearly demonstrated by following the stability of these silver colloids. Those results are exhibited in Fig. 3. Here, the particle size distribution was measured for up to 24 h. The results showed that when the ratio ([Na2 CO3 ]/[AgNO3 ]) was 2, silver colloid would eventually grew into micron in sizes even in the presence of PVP, while for the case of ratio being 1, its particle size remained unchanged for up to 24 h. This set of data suggested that

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may explain why under a high pH condition, the silver colloids would coagulate into larger particles quickly probably due to less free PVP molecules. We also noticed that in curve b of Fig. 1, the solution pH decreased to below 8 after about 40 min of reaction, and then increased back gradually. A possible explanation for this phenomenon would be the release of the combined hydroxyl ions from PVP under this pH condition. More work is necessary to look into the details of interactions between PVP, hydroxyl ion and silver colloids. 3.3. Comparison of different protective agents

Fig. 2. Effect of [Na2 CO3 ]/[AgNO3 ] ratio on silver average size and its standard deviation (other conditions: [AgNO3 ]=0.005 M, [HCHO]/[AgNO3 ]=4, [NaOH]/[AgNO3 ]=1, PVP/AgNO3 =9.27).

the pH of solution also affect the interaction of protective agent and silver colloids. When PVP was added to the starting AgNO3 solution, we noticed that the pH would decrease to some extent. An acid–base titration performed on PVP solution indicated that PVP is actually a weak acid. This result suggested that PVP molecules are capable of combining with hydroxyl ions. This interaction would then reduce the effective quantity of PVP, toward protecting silver colloids. This possible competition

Next shown in Figs. 4 and 5 are particle size distributions under different quantities of protective agents of PVP and PVA, with other reaction conditions being the same. It seemed that for PVP with the weight ratio of 9.27 and for PVA the weight ratio of 3.37 would be sufficient in terms of producing nanosized silver colloids. In all cases, a small fraction of particles would inevitably agglomerate into larger sizes. In general, if too much PVP (e.g. g PVP/g Ag>10) is used in the reaction, the conversion of silver ion into silver atoms will be much too slow for any practical uses. In addition to its protective role against agglomeration, PVP may also be considered as a barrier to the growth of these silver colloids. From the earlier figures, we can conclude that both PVP and PVA can function satisfactorily as protective agents for silver colloids. However, according to our experience, PVP is somewhat better (in terms of longer stability) than PVA for protecting silver colloids from agglomeration.

Fig. 3. Stability of silver colloid size distribution for [Na2 CO3 ]/[AgNO3 ]= (a) 1; and (b) 2 (other conditions: [AgNO3 ]=0.005 M, [HCHO]/[AgNO3 ]=4, PVP/AgNO3 =9.01).

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Fig. 6. XRD pattern of silver colloids.

Fig. 4. Effect of PVP/Ag weight ratio on silver colloid size distribution (other conditions: [AgNO3 ]=0.005 M, [HCHO]/[AgNO3 ]=4, [NaOH]/[AgNO3 ]=1, [Na2 CO3 ]/[AgNO3 ]=1, time 200 min).

3.4. Other characteristics of silver colloids Finally, we like to report briefly other characteristics of these silver colloids. XRD analysis shown in Fig. 6

indicated that the silver colloid is well crystallized showing sharp peaks of (111), (200), (220) and (222). On the other hand, TEM pictures (Fig. 7) indicated that these colloids are spherical in shape and also confirmed that the colloid is nanometer in size (about 5–10 nm). Due to its small size, the colloid is quite difficult to separate from the mother solution by an ordinary centrifuge technique. However, we found out that these colloids can be easily separated from the solution through centrifuge and filtration after the solution was mixed with at least the same volume of acetone. Presumably, due to poor solubility of PVP in acetone, the PVP protected silver colloids would agglomerate under this condition and thus make the separation easier. If the precipitate was kept wet after separation, it can then be re-dispersed by de-ionized water. Shown in Fig. 8 are

Fig. 5. Effect of PVA/Ag weight ratio on silver colloid size distribution (other conditions: [AgNO3 ]=0.005 M, [HCHO]/[AgNO3 ]=4, [NaOH]/[AgNO3 ]=1, [Na2 CO3 ]/[AgNO3 ]=1, time 200 min).

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Fig. 7. TEM picture of silver colloids (bar=50 nm).

the original particle size distribution and after re-dispersion. The size indeed increased somewhat, yet it still remained within the nanometer range. Finally, the sinterability of these silver colloids was briefly tested. The colloids were collected on an alumina Anodic disc (0.02 ␮m, Whatman) and then heated at 400◦ C for 1 h. From the SEM picture shown in Fig. 9, we can clearly notice that the colloids have been coalesced into a large grain if Fig. 9. SEM micrographs of (a) original silver colloids collected on an Anodic disc; (b) after sintering at 400◦ C for 1 h.

compared with the original feature exhibited in Fig. 9a. From XRD patterns (not shown here), we also found the FWHM (full width at half maximum) of the (111) peak decreased from 0.182◦ (for the room temperature sample) to 0.172◦ (after being heated to 200◦ C for 1 h), 0.119◦ (300◦ C, 1 h) and finally to 0.087◦ (400◦ C, 1 h). This decrease of FWHM was consistent with the SEM observation, suggesting the growth of grains even after a low temperature sintering. This result clearly illustrates one of the desired characteristics of nanosized particles.

4. Conclusion

Fig. 8. Particle size distribution of (a) original colloid; and (b) after re-dispersion.

Well-dispersed crystalline silver particle with 7–20 nm size and spherical shape has been prepared by reducing silver nitrate with formaldehyde in the presence of PVP or PVA as protective agent. It was found that the addition of sodium carbonate played a very important role in determining silver conversion, its particle size and stability. Increase of solution pH would increase the reducing power of formaldehyde and the rate of conversion. However, too much alkaline solution would reduce the effectiveness of these protective agents. An optimal range of conditions therefore existed in this reaction system for the synthesis of nanosized silver particles.

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Acknowledgements The authors wish to thank National Science Council of ROC for financial support of this work (grant No. NSC 88-2214-E007-011). References [1] J.C. Lin, C.Y. Wang, Mater. Chem. Phys. 45 (1996) 136. [2] Y. Zhu, Y. Qian, M. Zhang, Z. Chen, Mater. Lett. 17 (1993) 314.

[3] Z. Zhang, B. Zhao, L. Hu, J. Solid State Chem. 121 (1996) 105. [4] K. Torigoe, Y. Nakajima, K. Esumi, J. Phys. Chem. 97 (1993) 8304. [5] P.Y. Silvert, R. Herrera-Urbina, N. Duvauchelle, V. Vijayakrishnan, J. Mater. Chem. 6 (1996) 573. [6] G.O. Mallory, J.B. Hajdu (Eds.), Electroless Plating: Fundamentals and Applications, Am. Electroplaters and Surf. Finishers Soc., Orlando, FL, USA, 1990. [7] S.L. Bassner, E.H. Klingenberg, Am. Cer. Soc. Bull. 77 (6) (1998) 71.