APPLIED PHYSICS LETTERS 88, 083122 共2006兲
Significance of solubility product in the solution growth of Pb1−xMxS „M = Fe, Co, Cd, and Mn… nanoparticle films Rakesh K. Joshia兲 Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India
共Received 3 November 2005; accepted 17 January 2006; published online 24 February 2006兲 A model is proposed to explain the observed decrease of average grain size with an increase of Fe concentration in the Pb1−xFexS 共0.25艋 x 艋 0.75兲 and Co concentration in the Pb1−xCoxS 共0.15艋 x 艋 0.35兲 nanoparticle films. Higher solubility product for MS 共M = Fe and Co兲 as compared to PbS is observed to be responsible for the decrease of grain size with an increase of M concentration in the films. The model was found true for Pb1−xCdxS 共0.45艋 x 艋 0.85兲 and Pb1−xMnxS 共0.03艋 x 艋 0.37兲 films. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2179111兴 We have studied the growth, structural, electrical, and optical properties of solution grown ternary Pb1−xFexS 共0.25艋 x 艋 0.75兲 and Pb1−xCoxS 共0.15艋 x 艋 0.35兲 nanoparticle films and reported a decrease of average grain size with an increase of Fe concentration in Pb1−xFexS films1,2 and Co concentration in Pb1−xCoxS films.3 The grain size variations for the ternary films are shown in Fig. 1. The deposition method and conditions for the ternary films are described elsewhere.1–3 The Pb1−xFexS 共0.25艋 x 艋 0.75兲 films were deposited at a pH of 9.75 and temperature of 30 ° C. The growth conditions like pH and temperature were same for all values of x. Similarly the fixed growth parameters, pH of 9.5 and temperature of 30 ° C, were chosen for Pb1−xCoxS films. The average grain size for the films was estimated with the help of transmission electron microscopy and x-ray diffraction studies. In this letter, we explain the observed decrease of average grain size with the concentration of M 共Fe and Co兲 in the ternary films on the basis of solubility product values for the two sulfides. Fixed temperature and pH conditions for the growth of films help to understand independently the effect of concentration on the grain size. It is proposed that the difference of solubility products 共Ksp兲 for PbS 共Ksp = 3 ⫻ 10−28, at 25 ° C兲 and MS 共Ksp = 6 ⫻ 10−19 for FeS and Ksp = 4 ⫻ 10−21 for CoS, at 25 ° C兲 plays a key role to control the grain size in our ternary Pb1−xM xS nanoparticle films. It is known that the solution growth of thin film is an outcome of controlled precipitation though ion by ion recombination on the substrates.4–6 The precipitation of metal sulfides is a well known phenomenon, which occur through primary nucleation of particles and starts when the ionic product of reacting ions overcomes the solubility product in the solution. This results in the formation of a lattice in the solution or on the substrate. Since the solubility product 共Ksp兲 of MS is more than the Ksp of PbS, it is believed that the value of pH 共low value兲 which is enough for the primary nucleation and further growth of PbS is not sufficient for starting the primary nucleation of MS 共higher Ksp兲. Now, if the interaction which can be a force of attraction between Pb++ and S−− ions is p then because of the higher solubility product of MS the interaction between M++ and S−− can be represented as p
− ⌬p 共Fig. 2兲. Because of this the value of the pH 共low兲 of the solution is not enough to increase the interaction/rate of reaction, between M ++ and S−−. So, the net interaction for simultaneous reaction of Pb++ and M++ ions with S−− ions in the beaker at the vicinity of the substrate will be 共p + p − ⌬p兲 / q 共there will be a force generated by the slow reaction of M with S which will decrease the net rate of formation of the ternary alloy on the substrate兲. The value of q should depend on the growth parameters and the rate of reaction between the anion and cation. Since the growth parameters like pH and temperature are fixed therefore, in our case the value of q can be slightly less or greater than 2, because two positive ion species are reacting with one negative ion species. Therefore, if Pb++ and M++ ions are the same in number and both have the same reactivity with S−−, then the value of q should be 2. In other cases its value will be more 共more M++ ions兲 or less 共more Pb++ ions兲 than 2 on varying the concentration of Fe from 0.75 to 0.25 in the Pb1−xFexS films and 0.35 to 0.15 in the Pb1−xCoxS films. This suggests that the decreased rate of reaction in presence of M++ 共Fe++ and Co++兲 in solution is responsible for the decrease of grain size with an increase of M concentration in the ternary films. The above proposed model was found to be true for the Pb1−xCdxS and Pb1−xMnxS solution grown films. In the case of Pb1−xCdxS 共0.45艋 x 艋 0.85兲 the average grain size nearly
Present address: Faculty of Engineering Sciences, University of DuisburgEssen, Campus Duisburg Bismarstr. 81, Duisburg 47057, Germany; electronic mail: [email protected]
FIG. 1. Variation of average grain size with the concentration of Fe and Co in Pb1−xFexS and Pb1−xCoxS nanoparticle films, respectively.
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Appl. Phys. Lett. 88, 083122 共2006兲
Rakesh Kumar Joshi
FIG. 2. Representation of the phenomena of decreasing the interaction during simultaneous reaction of Pb++ and M++ ions with S−− ions.
remains the same while in Pb1−xMnxS 共0.03艋 x 艋 0.37兲 the average grain size decreases with an increase of x. This is to mention that the solubility product 共Ksp兲 for CdS is equal to 8 ⫻ 10−28 and for MnS is equal to 3 ⫻ 10−14. On comparing these solubility product values to the value for PbS 共Ksp = 3 ⫻ 10−28兲 the experimentally observed grain size variations for the ternary films are observed to be in accordance with the model. The thin films of Pb1−xCdxS were grown at pH = 10.0 and temperature= 75 ° C using lead acetate, cadmium acetate, and thiourea aqueous solutions while Pb1−xMnxS films were grown at pH = 9.5 and a temperature of 35 ° C using lead acetate, manganese chloride 共MnCl2兲, and thiourea aqueous solutions. The average grain size was estimated using transmission electron microscopy for the Pb1−xMnxS films and x-ray diffraction peak broadening method for the Pb1−xCdxS films.
The grain size is observed to decrease from 25 nm to 16 nm on changing the x from 0.03 to 0.37 in Pb1−xMnxS films grown from the solution bath at pH of 9.5 and temperature of 35 ° C. The Pb1−xCdxS films show an average grain size which is ⬃160 nm for all values x in the range 0.45艋 x 艋 0.85. It is observed that the good quality films of Pb1−xCdxS could be grown only at higher temperature 共⬃75 ° C兲. In order to see the validity of the model for other material systems we mention the work reported by Sutrave et al.,7 for chemically grown Cd1−xZnxSe films. In their study the crystal size was reported to decrease with an increase of Zn concentration in the films. This decrease of crystal size was expected according to our model due to the higher solubility product of ZnSe 共Ksp = 3.6⫻ 10−26兲 in comparison to CdSe 共Ksp = 10−33兲. In summary, a model has been proposed to understand the significance of the difference in solubility product values of two binary compounds for the growth of ternary films in extended composition range by the solution growth technique. According to the model a higher difference in solubility product of two binaries will lead to a stronger dependence of grain size of the ternary films on the composition of the binary constituents. This role of solubility product for the growth of ternary films from solution growth has never been addressed in the past. R. K. Joshi and H. K. Sehgal, Nanotechnology 14, 592 共2003兲. R. K. Joshi, A. Kanjilal, and H. K. Sehgal, Nanotechnology 14, 809 共2003兲. 3 N. Mathur, R. K. Joshi, G. V. Subbaraju, and H. K. Sehgal, Physica E 共Amsterdam兲 23, 56 共2004兲. 4 N. C. Sharma, R. C. Kainthla, D. K. Pandya, and K. L. Chopra, Thin Solid Films 60, 55 共1979兲. 5 S. Gorer, A. Albu-Yaron, and G. Hodes, J. Phys. Chem. 99, 1644 共1995兲. 6 C. Voss, Y.-J. Chang, S. Subramanian, S. O. Ryu, T.-J. Lee, and C.-H. Chang, J. Electrochem. Soc. 151, C655 共2004兲. 7 D. S. Sutrave, G. S. Shahane, V. B. Patil, and L. P. Deshmukh, Mater. Chem. Phys. 65, 298 共2000兲. 1 2
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