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Synthesis of Submicron Silver Powder by the Hydrometallurgical Reduction of Silver Nitrate with Hydrazine Hydrate and a Thermodynamic Analysis of the System DINABANDHU GHOSH and SAMUDRA DASGUPTA A silver powder of submicron size was produced from the aqueous solutions of its compounds. The silver compounds tried out were silver nitrate and silver oxide, and the reducing agents employed were dimethyl formamide (DMF), hydrazine hydrate, and sodium azide. The solvent mediums were distilled water for the reductions with DMF and sodium azide, and a 2:1 (by volume) mixture of distilled water and ethanol for the reductions with hydrazine hydrate. Of the three reductants, hydrazine hydrate (N2H4ÆH2O) alone was successful in reducing both the silver compounds to a submicron (<500 nm) metallic silver powder, as revealed by X-ray diffraction (XRD) studies and scanning electron microscopy (SEM) analyses. Additionally, the thermo- dynamic equilibrium of the system AgNO3-N2H4ÆH2O in the water–ethanol mixture (2:1) was studied at 298 K; the equilibrium constant data so generated was found to compare very well with those derived from the established data of enthalpies and free energies of formation, and half-cell potentials. The following activity coefficient (Raoultian)–composition relationship for hydrazine hydrate in its dilute solution in water (plus ethanol) at 298 K is proposed: 2 lnðcN2H4:H2OÞ¼1862ð371Þ2055ð424Þð1 À XN2H4ÁH2OÞ DOI: 10.1007/s11663-007-9123-5 Ó The Minerals, Metals & Materials Society and ASM International 2008 I. INTRODUCTION used are: sodium borohydride in the presence of dodecanethiol (for synthesizing gold particles 1 to POWDERS of noble metals such as gold, silver, and 3 nm in size with a surface coating of thiol),[2] tetrakis palladium have of late gained tremendous industrial (hydroxymethyl) phosphonium chloride (for preparing acceptance, owing to some exciting and pathbreaking a hydrosol of gold clusters),[3] and hydrazine dissolved applications. Silver powders of submicron scale find in the mixture of di(2-ethyl-hexyl) sulfosuccinate, extensive use as catalysts and in conductive adhesives, isooctane, and water (for synthesizing silver nanod- display devices, and the fabrication of thick film isks).[4] A number of more recent studies[5–16] exclu- materials. To meet the requirements of these applica- sively on silver synthesis have been concerned with a tions, it is necessary to suitably change the chemical nanosized silver powder,[5,9,10–12,16] nanostructures,[6] and physical properties of the products during their [8,13,15] [1] and composites. Many of these works followed preparation. One such special requirement may be to the liquid-phase reduction route,[6,7,11–14] using reduc- have monosized submicron or nanoparticles of the ing agents like polyglycol[6] and ethylenediamine tetra- final product. This type of ultrafine metal powder can acetic acid.[7] In the present work, to produce submi- be prepared by a variety of means, such as sol-gel cron silver powder from silver compounds such as synthesis, spray pyrolysis, plasma synthesis, inert gas silver nitrate and silver oxide, the different reducing condensation, electrodeposition, etc., in addition to the agents tried out are: sodium azide (NaN3) in aqueous wet chemical synthesis route, which is followed in the medium, dimethyl formamide (DMF) in aqueous present work with aqueous (with or without ethanol) medium, and hydrazine hydrate (N2H4ÆH2O) in a solvents. The presence of ethanol in the medium is water–ethanol mixture (2:1, by volume). Sodium azide expected to prevent the formation of bigger particles in is supposed to be a powerful reducing agent with the the final product in a low-temperature recovery pro- low oxidation number (-1/3) for nitrogen (N) in it. (It cess. Some of the important reducing agents previously may help to recall that N can possess up to a +5 oxidation number.) The possible mechanism of the DINABANDHU GHOSH, Professor, SAMUDRA DASGUPTA, reduction by DMF is that DMF breaks the water formerly Undergraduate Student, are with the Department of Metal- molecule into oxygen and hydrogen; the latter, in turn, lurgical Engineering, Jadavpur University, Kolkata 700032, India. reduces the metallic salt. The third reducing agent, Contact e-mail: [email protected] SAMUDRA DASGUPTA, Scientist, is with the Aeronautical Development Establishment, Defence hydrazine hydrate (N2H4ÆH2O), is also expected to be a Research and Development Organisation, Bangalore 560075, India. good reducing agent, with the low oxidation number Manuscript submitted November 13, 2006. (-2) of N. Article published online February 5, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 39B, FEBRUARY 2008—35 In addition to the synthesis of submicron silver 3000P and PHILIPS* PW 1710/1840) and scanning powder, the thermodynamic equilibrium of the system AgNO3-N2H4 Æ H2O in the water-ethanol mixture (2:1) is studied in the present work and the resulting *PHILIPS is a trademark of Philips Electronic Instruments Corp., thermodynamic data (the standard free energy change Mahwah, NJ. of the reduction reaction) is compared with the estab- lished data.[17,18] This allows the proposal for the electron microscopy (SEM) (JEOL** JSM 5200) activity coefficient–composition relationship for hydra- zine hydrate in its dilute solution in water (plus ethanol) at 298 K. **JEOL is a trademark of Japan Electron Optics Ltd., Tokyo. II. EXPERIMENTAL PROCEDURE analyses for phase identification and particle size deter- mination. A. Ordinary Runs with DMF, NaN3, and N2H4ÆH2O for the Synthesis of Ag B. Special Runs with N2H4ÆH2O for Thermodynamic The steps involved in the ordinary runs were as Analysis follows. (1) Preparation of solvent: 30 mL of distilled water Of the three reducing agents, hydrazine hydrate alone (for the reducing agents DMF and NaN3), or a mixture was found to successfully precipitate metallic silver and, of 20 mL of distilled water and 10 mL of ethanol (for hence, was chosen for subsequent thermodynamic anal- the reducing agent N2H4ÆH2O) was transferred to a ysis. These special experimental runs were conducted, to 100 mL beaker, with the help of 10-, 20-, or 30-mL study the thermodynamic equilibrium of the system pipettes; both beakers and pipettes were made of Pyrex AgNO3-N2H4ÆH2O in a 2:1 (volumetric ratio) water- glass (supplied by Science India, Kolkata, India). A very ethanol mixture and to determine the activity coefficient– small quantity of gelatin powder, a hydrophilic colloid, composition relationship for hydrazine hydrate in its was added to prevent the possible coagulation of the dilute solution in water (plus ethanol) at 298 K. For this precipitated particles during the subsequent reduction purpose, to a fixed quantity of silver nitrate solution stage. Now, the solvent was constantly subjected to (1.000 g, i.e., 5.88 millimol, of silver nitrate dissolved in a stirring by an electromagnetic stirrer-cum-heater (Remi solvent of 20 mL of distilled water and 10 mL of Equipments, Mumbai, India, model 2MLH) at 298 K. ethanol), taken in a 100 mL beaker, drops of hydrazine (2) Preparation of solution: Typically, 1 g of the hydrate were added from the burette, beginning with one chosen silver compound (silver nitrate or silver oxide), drop (1 drop ” 0.05 mL ” 1.03 millimol of hydrazine [19] 99.8 pct pure (Nice/Loba Chemie, Mumbai, India), was hydrate with a density of 1.03 g/mL; the clarification added to the solvent. The mass measurements were about the measurement and reproducibility of the drop made in a single-pan balance (Dhona Instruments Pvt. volume is given subsequently) and progressively increas- Ltd., Kolkata, India, model: 160 D) with the precision ing up to 10 drops, in a series of ten experiments, of 0.1 mg. The magnetic stirring was continued, to ensuring equilibrium in each case. The attainment of ensure the homogenization of the solution. equilibrium was instantaneous, since the kinetics of silver (3) Reduction: Drops of the chosen reducing agent precipitation was exceedingly fast. The wet precipitate of (DMF, sodium azide, or hydrazine hydrate), 99 pct pure silver was then, as in the ordinary runs, heated on the (S.D. Fine–Chem Ltd., Mumbai, India), were added from hotplate at a low temperature (about 323 K), to slowly a graduated Pyrex glass burette, with the smallest division drive away the liquid phase out of the beaker. The mass of 0.1 mL on it, to the homogenous solution at 298 K of the resulting dry, freeflowing silver powder was then (controlled within ±2 K by the temperature controller of measured by the balance, from which the percent the stirrer-cum-heater mentioned previously) and 1-atm reduction for each run was obtained. Considering the pressure, to examine whether any precipitates (of silver) importance of these special runs, the reproducibility of would form. If a precipitate started forming, steps (4) and the results (percent reduction) was verified with another (5) were taken on completion of the precipitation. ten-experiment set of repeat runs and found to be (4) Recovery: The resulting mixture (the remaining excellent; the deviation in each run was within ±1 pct. solution and the precipitate) was heated on a hotplate (Science India, Kolkata, India), equipped with a tem- 1. Measurement and reproducibility of the drop volume perature controller that controlled within ±2 K, at 323 Since the reducing agent hydrazine hydrate was added to 333 K, to evaporate away the aqueous phase (with or in drops into the reaction mixture, it was necessary to without ethanol), leaving behind dry, freeflowing (silver) establish the accuracy and reproducibility of the drop powder in the beaker. The conventional method of volume. This was done as follows. filtration for separating the precipitate from the solution First, the density of hydrazine hydrate was ascer- would not work here, owing to the ultrafineness of the tained by transferring a pipetted quantity (20 mL) of the precipitated particles as well as the possible loss of liquid to a clean, preweighed (1) beaker or (2) measuring material adhering to the filter paper.

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