J. MATER. CHEM., 1991,1(3), 409-413 Electron Microscopic Study of the Morphology of Lead Sulphide and Silver Sulphide Crystals obtained by the Silica Gel Crystal Growth Technique Pilar Aragon-Santamaria, Maria Jesus Santos-Delgado,* Amalia Maceira-Vidan and Luis Maria Polo-Diez Departamento de Quimica Analitica, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain An electron microscopic study has been performed of the morphological changes in PbS and Ag,S crystals during their growth by the silica gel technique in different inorganic acidic gel media using U-tubes. The formation and growth of the PbS and Ag,S crystals were followed by a binocular lens and an optical microscope. The morphological changes of the PbS crystals obtained in an HCI gel medium were the same as those observed for the Ag,S crystals in HCIO, and HNO, gels.The sequence began with the formation of single crystals showing dendritic growth, and was followed by growth along the [loo] direction. The changes were remarkably different for PbS crystals obtained in HCIO, and HNO, gel media. Whereas the first steps also involved the formation of dendritic crystals, subsequently the crystals grew along [lll]. Therefore, morphological changes are due to different nucleus formation and growth rates, which depend on the nature and concentration of the acidic medium. The most perfect cubic structure is obtained for the slowest rate. These changes were confirmed by electron microscopy, and the compounds obtained were identified by X-ray diffraction.Using these techniques it was observed that PbS and Ag,S crystals grew independently in gels which diffused thioacetamide and a mixture of Pb and Ag ions, instead of yielding PbS-containing Ag' ions as an impurity. Keywords: Electron microscopy; Crystal growth; Crystal morphology; Lead sulphide; Silver sulphide The crystal growth technique in gels has become very import- ant because it is straightforward and can be used at room temperature in similar conditions to those under which crys- tals grow naturally.' Metal sulphides, especially lead and silver sulphides, are of potential interest in (a)the development of ion-selective electrodes,2 and (b) the sensitization of other conductors, e.g. exploiting the photovoltaic properties of PbS in solar cells.3 The potential applications increase the import- ance of procedures able to produce large single crystals.In previous ~ork~,~ we established the optimum conditions for obtaining single PbS and Ag,S crystals by precipitation in a homogeneous phase using thioacetamide (TAA) as a sulphide reagent; this technique yielded high-purity crystals. Lead sulphide (galena) presents a cubic structure of the NaCl type, its unit-cell parameter according to the ASTM standard 5-0592 is a =5.92 A. Silver sulphide is found in two forms, argentite (a-phase) which belongs to the cubic system, and acantite (b-phase) of the monoclinic system. According to 4-0774 and 14-72 ASTM standards, the Ag2S cubic system has a=4.899 A and the Ag2S monoclinic system has a=4.229 A, b =6.931 A,c =7.862 A and /3 =99.61'. In this paper we describe an electron microscopic study of the morphological changes of PbS and Ag,S crystals during their growth by the silica gel twin-diffusion technique in different inorganic acidic gel media using U-tube devices.The crystals have also been characterized by X-ray diffraction. Experimental Apparatus A Zeiss D-7082 optical microscope with Zeiss DCR and SR binocular lenses was used. An MUC-83 scanning electron microscope with a 25 kV acceleration voltage was also used. A Philips PW 1729 X-ray generator, a PW 2253-00 X-ray tube of 2 kW and a 6 cm diameter Laue chamber (Unican) were employed for the single-crystal technique, and a Siemens D-500 X-ray diffractometer was used for powder diffraction studies; in all studies Cu-Karadiation was employed.Glass U-tubes measuring 14 or 27cm between their arms and of 1 cm internal diameter were used for crystal growth. Reagents The following reagents were employed: Lead@) nitrate; sil- ver(1) nitrate; TAA; sodium silicate (p =1.37 g cm-3); hydro- chloric, sulphuric, perchloric and nitric acids; sodium hydroxide; the disodium salt of ethylenediaminetetraacetic acid (Na,-EDTA). All the reagents used were of analytical reagent grade. The solutions were prepared by weighing or titration against standards. Inert silica gel was prepared by mixing an Na2Si03 solution (p = 1.06 g cm-3), initially at pH 11.20, with a suitable volume of 3 mol dm-3 acid (HCl, HzS04, HC104 or HN03) to obtain the desired pH of 0.5-2.0.Procedures Preparation of PbS and Ag,S Crystals by the Silica Gel Twin-difusion Growth Technique Glass U-tubes of 1 cm internal diameter and 14 or 27 cm between their arms were used to prepare the PbS or Ag2S crystals, respectively. The central part of the U-tube was filled with the silica gel and gelation was allowed up to complete solidification at room temperature. To obtain the PbS crystals 1% TAA and 1.0 x lo-' mol dm-3 Pb(N03), solutions were added to each tube arm; when the gel contained HCl the Pb(N03), solution was prepared in EDTA. To obtain Ag,S crystals, 1.O x 10-'mol dm- AgN03 solution was added to the cation arm and allowed to diffuse about two-thirds of the way across the central part of the U-tube before adding the TAA solution to the other arm.The above procedure was followed to prepare PbS crystals in the presence of Ag', the Pb(N03)2 solution being added at the same time as the TAA. The Pb(N03)2, AgN03 and TAA solutions were renewed from the U-tube arms every 8 and 15 days, respectively (after the appearance of the first crystal nucleus). Crystals were recovered by dissolving the gels in 1 mol dm-3 NaOH and then washing the crystals with distilled water to completely remove the gel and NaOH. Characterization of PbS and Ag2S Crystals by X-Ray Diflraction (a) Single-crystal Method. This method was used to charac- terize the lead sulphide crystals obtained in the HCl gel medium at pH 1.3 using a chamber sample of 30 mm radius.(b) Powder Method. The PbS and Ag2S crystals obtained in HC104 and HN03 gel media at pH 1.52 were characterized by the powder method using a nickel filter. The working conditions were: goniometer speed 2" min-l, sensitivity 1 x 10 and silicon as internal standard. Electron Microscopy The PbS crystals obtained in HC104 and HCl gel media at pH 0.7 and 1.3, respectively, were placed on the alumina sample holder, adhered with a sticking film and metallized with gold. They were observed under the scanning electron microscope at a 25 kV acceleration voltage. Results and Discussion Morphological Development of Lead and Silver Sulphide Crystals in Different Acidic Media First, the effect of different acidic media (HCl, HC104 and H2S04) on solidification of the silica gel was studied. It was found that the gelation time rose from 3 to 10 days when the acid concentration was increased from 1.0 x 10-to 2.0 x lo-' mol dm-3, independent of the acid anion.Optical Microscopy The formation and growth of the PbS and Ag2S crystals were examined every 24 h over the first 12 weeks using the optical microscope and the binocular lens. In all cases the crystals obtained were black-grey with a metallic sheen. PbS crystals were visible after 8 days in HCl gel media, and after 7 days in HC104 and HN03 media. The nucleation density in the HCl gel medium was similar over the whole pH range (0.7-2.0) studied, because Pb" was complexed by EDTA which was added to prevent PbC1, precipitation.The complexing effect seemed to be predominant at higher pH over the increasing hydrolysis rate of TAA. In aqueous solution a sudden increase in nucleation rate was observed when the pH was increased from 1 to 2.4 However, in HC104 and HN03 gel media the nucleation density began to increase at pH 1.0, because of the increasing hydrolysis of TAA as the acid concentration decreased. In the case of the Ag,S crystals, nucleation units appeared after 10 days in the HN03 or HC104 media at pH 1.52. It was necessary to allow the AgN03 solution to diffuse two-thirds of the way across the central part of the tube before adding the TAA to the other arm of the U-tube; this was required because the S2-ion diffuses at a higher rate than the Ag' ion and the gel turned black, preventing crystal growth and observation. In HzS04 gels, no germ growth appeared during the observation period of 12 weeks.The morphological changes of the PbS crystals obtained in HCl gel medium were different from those observed in HC104 and HN03 media, as indicated in Table 1. In all these J. MATER. CHEM., 1991, VOL. 1 gel media, single dendritic crystals appeared first, but whereas the crystals obtained in the HCl medium grew along the [loo] direction, those obtained in HC104 and HN03 media grew along [ll 1). In the last two media the growth went on to form crystals with eight vertices, while in the HCl medium the octahedral [1 1 13 direction was also developed, yielding cubo-octahedral crystals with (1 1l} and { 100) faces.The growth of the { 11 l} was followed by a decrease in the relative growth rate of the (loo}. Finally, single crystals of cubic morphology (100) with up to 0.6 mm edge were obtained; owing to their compact structure they were easy to handle when the experiment finished after 8 weeks. By contrast, in HC104 and HN03 gel media hopper-shaped single crystals of cubic morphology {loo} were obtained, whose empty sites continued to be filled by the addition of new growth units. Although the crystals obtained in the HC104 medium were very large (edge length up to 1.5 mm), their structural growth was incomplete and they were fragile and disintegrated easily. In the HN03 medium the crystal size was smaller (0.6-0.8 mm edge) because the gel aged more quickly than in the other media studied, especially at high acid concentrations.The Ag,S crystals showed the same morphological changes as observed for the PbS single crystals in the HCl gel medium; they had cubic morphology {loo}, up to 0.1 mm edge length, and were difficult to handle. Electron Microscopy The above observations of the sequence of morphological changes during PbS and Ag2S crystal growth was confirmed by electron microscopy. The growth over time of a PbS single crystal was followed. To study the sequence in HCl gels, their pHs were adjusted to 1.30. Fig. 1 shows the initial dendritic morphology with the six-tip crystals growing preferentially along [1001, i.e.normal to { 100) F faces (in contradiction to the Periodic Bond Chain Theory of Hartman6). The next morphological stage (Fig. 2) shows several secondary branches, which grow along directions normal to the axis and which, because the available interdendritic spaces are very small and fill rapidly, apparently have a continuous structure. Consequently, the cross-section normal to [100) was cross-shaped. After reaching a 'critical' size, the preferential directions of growth changed drastically from [loo] to [lll]. Fig. 3 shows the base of each dendrite branch, with the cross-shaped axis appearing in the centre of the picture. The morphology obtained by preferential growth along [111) can be distinguished clearly on the outside.With continued growth, the (111) faces of the small crystals that form the dendrite disappear. Further growth yields a cube- like crystal with centred flat faces (plateaux), Fig. 4. The plateau on each face of the cube shows macro-step growth layers, which are slightly depressed at the centre. All the edges are very sharp and the crystal does not show cubo-octahedral faces. In the next morphological step (Fig. 5), {loo} and (1 1 l} faces can be distinguished as well as (100) edges which show serrated profiles due the presence of (111) surfaces. Sub- sequently a cubo-octahedral shape is observed in Fig. 6, having smooth {loo}faces and rough { 11 l} surfaces. Growth continues by the piling of blocks on the (1 1l} surfaces, until finally cubes with slightly concave {loo} faces, are obtained (Fig.7). Fig. 8-10 show a sequence of morphological changes observed in PbS single crystals obtained in the HC104 medium at pH 0.70. Fig. 8 shows a cross-section of a dendrite whose branches consist of small crystals which increase in size towards the end of the primary branches. These small crystals show {loo), { 1 lo} and { 11l} faces, and a further increase in size gives rise to only (100) faces, i.e. the final equilibrium shape begins to predominate. Fig. 9 shows hopper-shaped J. MATER. CHEM., 1991, VOL. 1 411 Table 1 Morphological changes of PbS crystals observed by optical microscopy initial phase medium face direction 'Incomplete hopper shape.Fig. 1 Dendritic PbS crystal growing preferentially along [loo] Fig. 2 Intermediate morphology of a growing PbS single crystal Fig. 3 Internal base of the dendrite branch in Fig. 2 faces, indicating that two-dimensional nucleation along the edges is the main growth mechanism. Fig. 10 is a typical hopper-shaped crystal clearly showing its growth units. The differences observed between the above two growth sequences in HC1 and HC104 media confirm the influence of intermediate phase final phase face direction face Fig. 4 Plateau. Morphology during PbS single-crystal growth Fig. 5 Intermediate morphology of PbS single crystal Fig. 6 Intermediate morphology of growing PbS single crystal the growth rate on crystal morphology. In HCl gels the PbS growth rate is slower than in HC104 gels because Pb" is complexed with EDTA.In HC104 gels this growth rate yields imperfect crystals which do not complete their structure. Because of the slow diffusion of Ag' ions in HN03 or HC104 J. MATER. CHEM., 1991, VOL. 1 Fig. 7 Final morphology of PbS single crystal Fig. 8 Cross-sectionof PbS dendrite gels, the growth pattern of Ag2S is similar to that observed for PbS in HCl gels. Effect of Ag' Ions on PbS Crystal Morphology Crystals of PbS in the presence of Ag' ions were obtained under optimum experimental conditions for both sulphides [pH 1.52 in HN03 or HC104 using a 1: 1 (v:v) Pb(N03)2:AgN03 solution, obtained from 1.0 x lo-' mol dmA3 solutions]. Three growing zones were observed with the binocular lens in the U-tube. In the first zone, close to the TAA solution, the crystals presented the same morphological sequence as the PbS crystals obtained in HN03 and HC104 gels; growth along [111] resulted in dendritic crystals with eight tips, whose edges grew up to 0.5 mm.A black precipitate appeared in the central zone (see Fig. 9 Intermediate phase of PbS single-crystal growth Fig. 10 Single hopper-shaped crystal changes as Ag2S crystals obtained in HNOJ and HC104 media. From these results it may be supposed that pure PbS and Ag2S crystals were formed in the first and the third zones, respectively. Characterization by X-Ray Diffraction Lead Sulphide Some crystallographic parameters of the lead sulphide crystals obtained in HCl gels were determined by the rotatory crystal method.The value of the unit-cell parameter was 5.936 A, which according to ASTM Standard 5-0592 is due to PbS (galena). The lead sulphide crystals obtained in HC104 and HN03 media are difficult to handle because they break easily; these crystals were characterized using the powder method. The diffraction data allow these crystals to be identified by ASTM below in the X-ray study). In the third zone some very small Standard 5-0592 as PbS (galena), which belongs to the cubic crystals appeared, which showed the same morphological system. Table 2 Unit-&ll parameters obtained by X-ray diffraction simp!e crystal cubic system, a/A acid medium PbS Ag2S HCl 5.936 - HNOJ 5.937 4.892 5.942" 5.937b HC104 5.931 4.991 5.94" 5.934b precipitate monoclinic system Ag,S -~=4.17A, b=6.92 A c=7.95 A, B= 99.8" a=4.22 A, b=6.92 A c=7.83 A, j3=99.9" a=4.21 A b =6.94 A ~=7.86A, B=99.69" a=4.19 A, b=6.93 A ~=7.83A, B=99.76" " PbS in the presence of Ag' ions in zone I; PbS in the presence of Ag' ions in zone 11.J. MATER. CHEM., 1991, VOL. 1 Silver Sulphide The powder diffraction data from silver sulphide single crystals obtained in the HNOJ and HC104 gels at pH 1.52 indicate that these crystals are ol-Ag2S which belong to the cubic system, ASTM Standard 4-0774. From diffractograms of the silver sulphide precipitate, the d-spacing and relative intensities obtained indicate that the compound obtained is /3-Ag2S (acantite) which belongs to the monoclinic system, according to ASTM Standard 14-72.Lead Sulphide in the Presence of Ag' Ions The diffraction data for the crystals which appeared in the first zone mentioned above, showed that the compound formed in both acidic media was PbS (galena) (cubic system, ASTM Standard 5-0592). Diffraction data for the precipitates which appeared in the second zone indicate a mixture of PbS (cubic system) and Ag2S (monoclinic system), according to ASTM Standard 5-0592 and 14-72, respectively. The value of the relative intensities indicates that PbS predominates. Table 2 shows the unit-cell parameters for the compounds obtained in different acidic media. These results confirm that Ag' ions are not incorporated into the PbS crystals and do not appear as an impurity in these crystals, but that crystals of both sulphides, PbS and Ag2S, grow independently.The conductivity of PbS does not change with the presence or absence of Ag' ions, which supports this concl~sion.~ The authors thank Dr. J. M. Garcia-Ruiz and the Department of Crystallography and Mineralogy, Geology Faculty and Dr. J. M. Gonzalez-Calbkt of the Inorganic Chemistry Depart- ment, Chemistry Faculty, Complutense University, for their invaluable collaboration in the present work. References J. M. Garcia-Ruiz and J. L. Amoros, Bol. R. SOC.Espafiola Hist. Nut. (Geol.), 1979, 77, 101. R. K. Paramguru, S. K. Bose and S. C. Sircar, Trans. Inst. Min. Metall., Sect. C, 1979, 88, 197. H. Hirata and K. Date, Anal. Chem., 1971, 43, 279. P. Aragon-Santamaria, M. J. Santos-Delgado, A. Maceira-Vidan, R. Gallego-Andreu and J. M. Garcia-Ruiz, Ann. Quim., 1984, 80B, 134. M. J. Santos-Delgado, P. Aragon-Santamaria, J. A. Quiroga- Estevez, A. Maceira-Vidan and R. Gallego-Andreu, XX Reunibn Bienal R.S.E.Q. Castellon, 1984, pp. 8-178. P. Hartman, Structure and Morphology in Crystal Growth, An Introduction, North Holland, Amsterdam, 1973. P. Aragon-Santamaria, Doctoral Thesis, Complutense Univer- sity, Madrid, 1987. Paper 0/05508E; Receioed 6th December, 1990