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Nonstoichiometry of nanocrystalline monoclinic silver sulfide† S. I. Sadovnikov,a A. I. Gusev*a and A. A. Rempelab
Received 2nd February 2015, Accepted 18th April 2015 DOI: 10.1039/c5cp00650c www.rsc.org/pccp
Powders of silver sulfide have been synthesized by chemical bath deposition from aqueous solutions of silver nitrate and sodium sulfide in the presence of sodium citrate or EDTA–H2Na2. Colloid solutions have been prepared by a chemical condensation method from the same aqueous solutions. Synthesized silver sulfide nanopowders have a monoclinic (space group P21/c) acanthite-type structure but the occupancy of the metal sublattice sites by Ag atoms is smaller than 1. Unlike coarse-crystalline silver sulfide Ag2S, silver sulfide nanopowders with particles sizes of less than B50 nm are nonstoichiometric, contain vacant sites in the metal sublattice and have a composition of BAg1.93S.
Determination of the influence of nanoparticle size on their nonstoichiometry is a fundamental scientific problem. The relative content of atoms on the surface of nanoparticles is high. Surface atoms are more weakly bonded than atoms inside the nanoparticle. This circumstance can lead to nonstoichiometry of a nanoparticle. For example, the appearance of nonstoichiometry is observed in nanocrystalline cerium dioxide because of the excessive increase in surface-to-volume ratio with the decrease in nanocrystallite size.1 Up to now the problem of the appearance of nonstoichiometry in nanoparticles of silver sulfide has not been studied at all. The present study deals with the elucidation of the effect of particle size on the possible deviation of the silver sulfide composition from stoichiometry. Quantum dots of sulfides including ZnS, PbS, CdS, and Ag2S have begun to be used as fluorescent labels for the identification of biological objects and for application in medical diagnostics and biotechnology.2–7 However PbS and CdS quantum dots are not suitable for application in biology and medicine because of the toxicity of lead and cadmium. It is important to prepare quantum dots with low toxicity and high emission. Silver sulfide
a
Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, Ekaterinburg 620990, Russia. E-mail: [email protected] b Ural Federal University named after the First President of Russia B.N. Yeltsin, Ekaterinburg 620002, Russia † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cp00650c
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is an ideal semiconducting material for preparing low-toxicity quantum dots. Usually, fluorescent Ag2S quantum dots are synthesized in toxic organic solutions. Aqueous colloid solutions of Ag2S nanoparticles are not toxic and hold particular promise for biological applications. According to the phase diagram of the system of Ag–S,8,9 silver sulfide Ag2S has three polymorphic modifications: (1) low-temperature monoclinic phase a-Ag2S (acanthite) existing at temperatures below B450 K; (2) b-Ag2S phase (argentite) with a body centered cubic (bcc) sublattice of sulfur atoms existing in the temperature range 452–859 K; and (3) high-temperature face centered cubic (fcc) phase g-Ag2S stable from B860 K up to its melting temperature. It is thought that monoclinic a-Ag2S phase is stoichiometric, whereas cubic b-Ag2d S and g-Ag2d S with d D 0.002–0.012 are nonstoichiometric phases having either a small deficiency or small excess of Ag.10–14 The powders of silver sulfide Ag2S were synthesized by chemical deposition from an aqueous solution of silver nitrate AgNO3 and sodium sulfide Na2S containing sodium citrate Na3C6H5O7RNa3Cit or the sodium salt of ethylenediaminetetraacetic acid Na2H2(CH2OO)4(CH2N)2REDTA–H2Na2RTrilon B as complexing and stabilizing agents. The as-synthesized matrix colloid solutions have been prepared from the same aqueous solutions. It is a well-known, simple and reliable method which allows the preparation of non-toxic colloidal solutions. Besides, colloidal solutions were obtained by the washing of deposited Ag2S powders with distilled water and then subsequent removal of the powder by filtration. The solubility product Ksp of silver sulfide Ag2S is very small (according to ref. 15, at 298 K, Ksp = 6.3 1050), and when the content of Na2S in the reaction mixture is sufficient, deposition of silver sulfide Ag2S occurs almost instantly, in fractions of a second. Sodium citrate serves as a complexing and stabilizing agent. Besides, in aqueous solutions with a small amount of S2 ions, sodium citrate and Trilon B can reduce Ag+ ions giving rise to nanoparticles of metallic silver.16,17 That is why for deposition of silver sulfide without a Ag impurity it is necessary to use reaction mixtures with excessive sodium sulfide Na2S.
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Synthesis was carried out at room temperature. Deposition of silver sulfide occurred by the following reaction schemes Na3 C6 H5 O7
2AgNO3 þ Na2 S ! Ag2 S # þ 2NaNO3
(1)
or EDTAH2 Na2
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2AgNO3 þ Na2 S ! Ag2 S # þ 2NaNO3 :
(2)
When the reagents are mixed together, silver sulfide is formed almost immediately. As a result, the reaction mixture becomes black at first, and then, after one hour, Ag2S particles deposit and the solution becomes transparent. For the sulfidization reaction to be complete, the deposit was kept in the matrix solution for one day. The study showed that the size of Ag2S particles in colloidal solutions and in deposited powders depends on the initial concentrations of silver nitrate, sodium sulfide and the complexing agents. The concentrations of AgNO3, Na2S and Na3Cit in the reaction mixture C for the synthesis of a coarse-crystalline Ag2S powder with particle sizes of B500 nm were 0.05, 0.5 and 0.005 mol l1, respectivley. Silver sulfide nanopowders with particle sizes of smaller than 60 nm have been synthesized from reaction mixtures I, II, III, and IV. Concentrations of AgNO3 and Na2S in all the reaction mixtures were equal to 0.05 and 0.025 mol l1, respectively. Concentration of Na3Cit in reaction mixtures I and II was equal to 0.025 and 0.0125 mol l1, respectively. Concentration of Trilon B in reaction mixtures III and IV was equal to 0.05 and 0.035 mol l1, respectively. The attempts to deposit nanocrystalline silver sulfide powder with particle size below 20 nm using Na3Cit or Trilon B as complexing agents failed because the nanoparticles with sizes r20 nm form a stable aqueous colloid solution. In this colloid solution, nanoparticles do not deposit after 12 months or longer. All deposits were examined by the X-ray powder diffraction (XRPD) method on a Shimadzu XRD-7000 diffractometer (CuKa1 radiation, primary and secondary Soller slits with a width of 1.0 and 0.15 mm, secondary graphite monochromator, Bragg–Brentano geometry). The XRPD measurements were performed with an angle interval of 2y = 8–951 with a step of D(2y) = 0.021 and scanning time of 10 s in each point. The instrumental resolution function FWHMR(2y) = (u tan2 y + v tan y + w)1/2 of the Shimadzu XRD-7000 X-ray diffractometer was found in a special diffraction experiment on cubic lanthanum hexaboride LaB6 (NIST Standard Reference Powder 660a) with a lattice constant of a = 0.415692 nm. The resolution function FWHMR(2y) measured in degrees has the parameters u = 0.005791, v = 0.004627 and w = 0.010201. The determination of the crystal lattice parameters and the final refinement of the structure of synthesized silver sulfide powders were carried out with the use of the X 0 Pert Plus program package.18 The microstructure, particle size and element chemical composition of silver sulfide powders were studied using scanning electron microscopy (SEM) on a JEOL-JSM LA 6390 microscope coupled with a JED 2300 Energy Dispersive X-ray Analyzer. High-resolution-transmission electron microscopy (HR-TEM) images were recorded on a JEOL JEM-2100 transmission electron microscope with a resolution of 0.14 nm.
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Furthermore, the average particle size D (to be more precise, the average size of the coherent scattering regions (CSR)) of the synthesized silver sulfide powders was estimated by the XRPD method from the diffraction reflection broadening using the dependence of reduced reflection broadening b (2y) = [b(2y)cos y]/l on the scattering vector s = (2 sin y)/l.19–22 The specific surface area Ssp of the synthesized silver sulfide powders was found by the BET method on a Gemini VII 2390t Surface Area Analyzer. In the approximation that all particles have a similar size and spherical shape, the average particle size D = 6/rSsp was estimated from the value of the specific surface area Ssp (r is the density of Ag2S). The size (hydrodynamic diameter Ddls) of the Ag2S nanoparticles in the colloidal solutions was determined by noninvasive Dynamic Light Scattering (DLS) on a Zetasizer Nano ZS facility (Malvern Instruments Ltd) at 298 K. The XRPD patterns of the synthesized silver sulfide powders with different average particle sizes are presented in Fig. 1. According to the BET data, the particle size of the coarsecrystalline powder is 515 15 nm. Nanostructured silver sulfide has been studied extensively over the past two decades. However, until now there is no experimental work on the determination of the crystal structure of synthetic nanocrystalline silver sulfide. The crystal structure of this phase was determined experimentally mostly on natural coarse-grained samples of the acanthite mineral.23,24 According to a study,24 the unit cell of acanthite is primitive monoclinic and belongs to the space group P21/n (P121/n1). Later it was shown that the monoclinic unit cell of natural acanthite mineral a-Ag2S belongs to the space group P21/c (P121/c1).23 Recently,25 we have determined the crystal structure of artificial coarse-crystalline silver sulfide powder with an average particle size of B500 nm. According to ref. 25, synthesized coarse-crystalline silver sulfide powder has a monoclinic (space group no. 14 – P21/c (P121/c1)) structure of a-Ag2S acanthite type and is stoichiometric. Unit cell parameters are equal to a = 0.42264(2) nm, b = 0.69282(3) nm, c = 0.95317(3) nm and b = 125.554(2)1 and are in good agreement with the data.23 All experimental conditions and characteristics of the refinement for the synthesized artificial coarse-crystalline Ag2S phase with acanthite-type crystal structure and the list of the observed diffraction reflections for this refined crystal structure are given in supplementary materials26 of the article.25 Preliminary analysis revealed that all the synthesized nanocrystalline powders have a monoclinic (space group P21/c) a-Ag2S acanthite-type structure too. The diffraction reflections of the nanopowders are broadened and therefore the reflections located close to each other overlap. The average size D of coherent scattering regions estimated from broadening of non-overlapping diffraction reflections (102), (110), (113), (104), (031) and (014) is presented in the insets to the XRPD patterns of the nanopowders (Fig. 1). From these estimates, the average size D in the examined silver sulfide nanopowders, deposited from the initial reaction mixtures I, II, III and IV, is 43 6, 46 8, 58 8, and 59 8 nm, respectively. Note that silver sulfide nanopowders synthesized with Trilon B contain Ag impurities with an amount of 3.0–4.5 wt%.
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Fig. 1 The XRPD patterns of monoclinic (space group P21/c) Ag2S powders: (a) coarse-crystalline powder deposited from the reaction mixture C; (b)–(e) nanopowders deposited from the reaction mixtures I, II, III, and IV, respectively. The insets present the estimate of the average size of the coherent scattering regions from the broadening of non-overlapping diffraction reflections.
In spite of the large width and overlapping of many diffraction reflections on the XRPD patterns of silver sulfide nanopowders, and the presence of silver impurities in the nanopowders, we have tried to refine the crystal structure of silver sulfide nanopowders. In the first approximation we supposed that the nanopowders have
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the same monoclinic (space group P21/c) crystal structure as a coarse-crystalline silver sulfide with particle sizes of B500 nm. As an example, the observed Iobs, calculated Icalc and difference (Iobs Icalc) values corresponding to the refinement of the XRPD pattern for the silver sulfide nanopowder deposited from reaction
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Fig. 2 The experimental ( ) and calculated ( ) XRPD patterns of Ag1.93S nanopowder deposited from the reaction mixture I. The difference between the experimental and calculated XRPD patterns (Iobs Icalc) is shown in the lower part of the figure. The XRPD pattern is recorded in CuKa1 radiation.
Table 1 Refined crystal structure of monoclinic (space group no. 14 – P21/c (P121/c1)) Ag1.93S nanopowders deposited from the reaction mixtures I and II: a-Ag2S acanthite-type structure, Z = 4, a = 0.4234(3) nm, b = 0.6949(3) nm, c = 0.9549(5) nm, and b = 125.43(6)1
Atomic coordinates
Atom
Position and multiplicity
x
y
z
Occupancy
Biso 104 (pm2)
Ag1 Ag2 S
4(e) 4(e) 4(e)
0.0715(4) 0.7264(3) 0.4920(2)
0.0151(0) 0.3240(9) 0.2339(8)
0.3093(9) 0.4375(0) 0.1321(1)
0.97(1) 0.96(4) 1.00(0)
10.05(5) 7.44(6) 1.96(0)
mixture I are shown in Fig. 2. The coordinates of Ag and S atoms and the unit cell parameters for the Ag2S nanopowder (Table 1) are close to those for coarse-crystalline Ag2S. However, the occupancy of crystallographic positions 4(e) by Ag1 and Ag2 atoms is somewhat smaller than 1 and is equal to 0.97 and 0.96, respectively (Table 1). This means that silver sulfide nanoparticles with a size of less than B50 nm are nonstoichiometric, have a composition of BAg1.93S and contain vacant sites in the metal sublattice. Results on the nonstoichiometry and crystal structure of nanocrystalline monoclinic silver sulfide are interesting for comparing to the structural data for b-Ag2S argentite. According to a neutron diffraction study,27 the unit cell of b-Ag2S argentite has a cubic (space group Im3% m) structure in which 4 Ag atoms are statistically distributed in 18 positions 6(b) and 12(d). The occupation of these positions depends on the temperature. The probabilities of occupation of the 6(b) and 12(d) positions by Ag atoms are equal to 0.135 and 0.266 at 459 K and to 0 and 0.333 at 533 K. The temperature 459 K is only 9 K higher than the temperature of the polymorphic phase transition of monoclinic acanthite a-Ag2S to bcc argentite b-Ag2S. The silver atoms statistically occupying the 6(b) and 12(d) positions of the bcc structure of argentite are concentrated in the positions of the monoclinic structure of acanthite. This allows us to suggest that the probabilities of th eoccupation of 4(e) positions by Ag atoms in the structure of acanthite should also differ from 1.
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In this case, the composition of silver sulfide with the acanthite-type structure can deviate from stoichiometry. The axes of the unit cells of a-Ag2S acanthite can be represented as a combination of axes abcc, bbcc and cbcc of the unit cell of bcc argentite. Phase transformation of monoclinic acanthite a-Ag2S to bcc argentite b-Ag2S is accompanied by distortion of bcc sublattice of S atoms to the monoclinic sublattice. Thus, the a-Ag2S acanthite structure can be considered as a result of the distortion of the b-Ag2S argentite structure. Fig. 3a displays the arrangement of S atoms in the bcc sublattice of argentite and the contours of monoclinic unit cells of a-Ag2S acanthite.24,25 It is seen that axes a aP21/c = aP21/n = (abcc + bbcc cbcc)/2, b bP21/c = bP21/n = (abcc bbcc), cP21/n = (abcc + bbcc + 3cbcc)/2 and cP21/c = 2cbcc. The monoclinicdistorted nonmetallic sublattice constructed with allowance for found coordinates of S atoms is demonstrated in Fig. 3b. The arrangement of Ag and S atoms in the unit cell of nanocrystalline monoclinic (space group P21/c) silver sulfide Ag1.93S with an acanthite type structure is displayed in Fig. 4. The least distances between S and Ag1 atoms in the crystal lattice of nonstoichiometric silver sulfide Ag1.93S are equal to 0.25193 and 0.25267 nm, and those between S and Ag2 atoms are 0.25546 and 0.25962 nm. These interatomic distances are a little more than the distances for coarse-crystalline stoichiometric silver sulfide. It means that nanocrystalline nonstoichiometric
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Fig. 3 The distortion of sulfur atom sublattice at the transition from argentite b-Ag2S to acanthite a-Ag2S: (a) the arrangement of S atoms in bcc sublattice of argentite; (b) displacement of S atoms from bcc sublattice positions and their arrangement in the monoclinic sublattice of acanthite. The undistorted bcc sublattice is shown by thin short dashed lines, the contours of the unit cells with space groups P21/c and P21/n are shown by thick solid lines and thick long dashed lines, respectively. (J) and ( ) are S atoms located outside and inside the monoclinic (space group P21/c) unit cell of acanthite a-Ag2S, respectively. Ag atoms are not shown.
Fig. 4 The arrangement of Ag and S atoms in the unit cell of synthesized nanocrystalline monoclinic (space group P21/c) silver sulfide Ag1.93S with an acanthite structure: shown are only the atoms entering the unit cell and the nearest bonds between them, Ag1–S and Ag2–S, with a length of 0.25193 and 0.25546 nm, respectively.
sulfide Ag1.93S has a less dense structure in comparison with stoichiometric silver sulfide Ag2S. According to the Energy Dispersive X-ray analysis, the content of silver Ag and sulfur S in the synthesized dried silver sulfide nanopowder deposited from reaction mixture II is 86.2 0.4 and 13.1 0.1 wt%, which corresponds to sulfide BAg1.95–1.98S. Besides silver and sulfur, the EDX spectrum contains a Ka line of carbon from the carbon planchet, onto which the examined powder was applied. In earlier work28 it was found that silver sulfide nanoparticles with an acanthite structure and size of B10 and B6 nm deposited
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from sewage sludge had a nonstoichiometric composition of Ag1.7S and even Ag1.1S. The authors28 did not examine the crystal structure of the nanoparticles and therefore supposed that silver deficiency in the nanoparticles is caused not by silver nonstoichiometry, but by possible chemosorption of sulfur on the surface of the nanoparticles. But this supposition is in conflict with the data of the same authors about the presence of nanoparticles of the Ag2.1S composition. It is more probable that the authors28 observed nanoparticles of nonstoichiometric silver sulfide Ag2d S both with a deficiency and excess of silver. Our data on nonstoichiometry of silver sulfide nanoparticles with an acanthite structure agree with the results28 and with the data27 on nonstoichiometric distribution of Ag atoms in argentite b-Ag2S near the acanthite–argentite transition temperature. The size of silver sulfide nanoparticles was measured in colloid solutions too. The DLS measurements showed that the particle size in unfiltered solution I decanted from the deposit and then stored for 60 days varies from 8 to 30 nm, and the average size of nanoparticles is equal to B13 nm (Fig. 5a). Similar size distribution of Ag2S particles in the unfiltered colloidal solution II 60 days after synthesis is displayed in Fig. 5b: the particle size varies from 9 to 35 nm, and the average size of nanoparticles is equal to B16 nm. From side on, solution II looks translucent. However a sight check shows that it has a light-brown color and is completely transparent. The optical absorption band maximum of the solution corresponds to 385 nm. Opalescence of this solution is indicative of noticeable fluctuations of its density, at which light scattering occurs. The fluctuations are due to the presence
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Notes and references
Fig. 5 The size distribution of silver sulfide nanoparticles in unfiltered colloid solutions 60 days after synthesis: (a) solution I; (b) solution II. The inset presents a TEM image of a separate silver sulfide nanoparticle from the stable colloidal solution II.
of nanoparticles with sizes under 20 nm in the solution. This agrees with the DLS measurements (Fig. 5b). According to HR-TEM data, stable colloidal solutions I and II contain silver sulfide nanoparticles with sizes from B15–18 nm down to B5–8 nm. As an example, a TEM image of a separate silver sulfide nanoparticle from a stable colloidal solution II is shown in Fig. 5 (inset). The stability of colloidal solutions is confirmed by the turbidity measurements. For example, the turbidity of the decanted unfiltered colloid solution II, 60 and 240 days after a filtration, was equal to 195 2 and 196 2 NTU, respectively.
Conclusion Synthesized silver sulfide nanopowders have a monoclinic (space group P21/c) a-Ag2S acanthite-type structure. The occupancy of the metal sublattice sites by silver atoms in silver sulfide nanopowders is somewhat smaller than 1 and is equal to B0.96–0.97. Thus, for the first time it is established experimentally that the monoclinic silver sulfide nanopowders are nonstoichiometric, contain vacant sites in the metallic sublattice and have a composition of BAg1.93S. By varying the ratio between the concentrations of silver nitrate, sodium sulfide and complexing agent in the initial reaction mixtures it is possible to deposit Ag2S nanoparticles with preassigned average sizes ranging from B500 down to 30–40 nm. Stable aqueous colloidal solutions of Ag2S nanoparticles have been synthesized for the first time. The size of Ag2S nanoparticles in stable colloidaal solutions varies from B18–20 nm down to B5–8 nm.
Acknowledgements This research work is financially supported by the Russian Science Foundation (Grant 14-23-00025) through the Institute of Solid State Chemistry of the Ural Division of the RAS. We thank Ms Yu. V. Kuznetsova for assistance in the measurement of the hydrodynamic diameter of the Ag2S nanoparticles in the colloid solutions using the Dynamic Light Scattering method. The authors are grateful to Dr E. Yu. Gerasimov for help in the high-resolution transmission electron microscopy study.
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´, Z. Dohcˇevic´-Mitrovic ´, A. Kremenovic ´, N. Lazarevic´, 1 S. Asˇkrabic ´, J. Raman Spectrosc., 2012, 43, V. Kahlenberg and Z. V. Popovic 76–81. 2 A. Chatterjee, A. Priyam, S. C. Bhattacharya and A. Saha, J. Lumin., 2007, 126, 764–770. 3 W. J. Parak, D. Gerion, T. Pellegrino, D. Zanchet, C. Micheel, S. C. Williams, R. Boudreau, M. A. Le Gros, C. A. Larabell and A. P. Alivisatos, Nanotechnology, 2003, 14, R15–R27. 4 S. V. Rempel, A. A. Podkorytova and A. A. Rempel, Fiz. Tverd. Tela, 2014, 56, 549–552 (in Russian) (Engl. transl.: Phys. Solid State, 2014, 56, 568–571). 5 P. Jiang, C.-N. Zhu, Z.-L. Zhang, Z.-Q. Tian and D.-W. Pang, Biomaterials, 2012, 33, 5130–5135. 6 C. Li, Y. Zhang, M. Wang, Y. Zhang, G. Chen, L. Li, D. Wu and Q. Wang, Biomaterials, 2014, 35, 393–400. 7 S. Yamamoto, N. Manabe, K. Fujioka, A. Hoshino and K. Yamamoto, IEEE Trans. Nanobioscience, 2007, 6, 94–98. 8 Gmelin’s Handbuch der anorganischen Chemie, Verlag Chemie GmbH, Weinheim, 5nd edn, 1973. 9 R. S. Sharma and Y. A. Chang, Bull. Alloy Phase Diagrams, 1986, 7, 263–269. 10 H. Reye and H. Schmalzried, Z. Phys. Chem., 1981, 128, 93–100. 11 H. Rau, J. Phys. Chem. Solids, 1974, 35, 1553–1559. 12 G. Bonnecaze, A. Lichanot and S. Gromb, J. Phys. Chem. Solids, 1978, 39, 299–310. 13 A. Ditman and I. N. Kulikova, Zh. Fiz. Khim., 1979, 53, 260–261 (in Russian). 14 K. Mitteilung, Z. Phys. Chem., 1980, 119, 251–255. 15 P. Patnaik, Dean’s Analytical Chemistry Handbook, McGrawHill, New York, 2nd edn, 2004, Table 4.2. 16 P. C. Lee and D. Meisel, J. Phys. Chem., 1982, 86, 3391–3395. 17 R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz and J. G. Zheng, Science, 2001, 294, 1901–1903. 18 X 0 Pert Plus Version 1.0. Program for Crystallography and Rietveld analysis Philips Analytical B. V. r 1999 Koninklijke Philips Electronics N. V. 19 W. H. Hall and G. K. Williamson, Proc. Phys. Soc., London, Sect. B, 1951, 64, 937–946. 20 A. I. Gusev and A. A. Rempel, Nanocrystalline Materials, Cambridge Intern. Science Publ., Cambridge, 2004, p. 351. 21 A. I. Gusev and A. S. Kurlov, Metallofiz. Noveishie Tekhnol., 2008, 30, 679–694 (in Russian). 22 S. I. Sadovnikov and A. I. Gusev, J. Alloys Compd., 2014, 586, 105–112. 23 R. Sadanaga and S. Sueno, Mineral. J., 1967, 5, 124–148. 24 A. J. Frueh, Z. Kristallogr., 1958, 110, 136–144. 25 S. I. Sadovnikov, A. I. Gusev and A. A. Rempel, Superlattices Microstruct., 2015, 83, 35–47. 26 S. I. Sadovnikov, A. I. Gusev and A. A. Rempel, Superlattices Microstruct., 2015, DOI: 10.1016/j.spmi.2015.03.024. 27 R. J. Cava, F. Reidinger and B. J. Wuensch, J. Solid State Chem., 1980, 31, 69–80. 28 B. Kim, C.-S. Park, M. Murayama and M. F. Hochella, Environ. Sci. Technol., 2010, 44, 7509–7514.
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Nonstoichiometry of nanocrystalline monoclinic silver sulfide† S. I. Sadovnikov,a A. I. Gusev*a and A. A. Rempelab
Received 2nd February 2015, Accepted 18th April 2015 DOI: 10.1039/c5cp00650c www.rsc.org/pccp
Powders of silver sulfide have been synthesized by chemical bath deposition from aqueous solutions of silver nitrate and sodium sulfide in the presence of sodium citrate or EDTA–H2Na2. Colloid solutions have been prepared by a chemical condensation method from the same aqueous solutions. Synthesized silver sulfide nanopowders have a monoclinic (space group P21/c) acanthite-type structure but the occupancy of the metal sublattice sites by Ag atoms is smaller than 1. Unlike coarse-crystalline silver sulfide Ag2S, silver sulfide nanopowders with particles sizes of less than B50 nm are nonstoichiometric, contain vacant sites in the metal sublattice and have a composition of BAg1.93S.
Determination of the influence of nanoparticle size on their nonstoichiometry is a fundamental scientific problem. The relative content of atoms on the surface of nanoparticles is high. Surface atoms are more weakly bonded than atoms inside the nanoparticle. This circumstance can lead to nonstoichiometry of a nanoparticle. For example, the appearance of nonstoichiometry is observed in nanocrystalline cerium dioxide because of the excessive increase in surface-to-volume ratio with the decrease in nanocrystallite size.1 Up to now the problem of the appearance of nonstoichiometry in nanoparticles of silver sulfide has not been studied at all. The present study deals with the elucidation of the effect of particle size on the possible deviation of the silver sulfide composition from stoichiometry. Quantum dots of sulfides including ZnS, PbS, CdS, and Ag2S have begun to be used as fluorescent labels for the identification of biological objects and for application in medical diagnostics and biotechnology.2–7 However PbS and CdS quantum dots are not suitable for application in biology and medicine because of the toxicity of lead and cadmium. It is important to prepare quantum dots with low toxicity and high emission. Silver sulfide
a
Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, Ekaterinburg 620990, Russia. E-mail: [email protected] b Ural Federal University named after the First President of Russia B.N. Yeltsin, Ekaterinburg 620002, Russia † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cp00650c
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is an ideal semiconducting material for preparing low-toxicity quantum dots. Usually, fluorescent Ag2S quantum dots are synthesized in toxic organic solutions. Aqueous colloid solutions of Ag2S nanoparticles are not toxic and hold particular promise for biological applications. According to the phase diagram of the system of Ag–S,8,9 silver sulfide Ag2S has three polymorphic modifications: (1) low-temperature monoclinic phase a-Ag2S (acanthite) existing at temperatures below B450 K; (2) b-Ag2S phase (argentite) with a body centered cubic (bcc) sublattice of sulfur atoms existing in the temperature range 452–859 K; and (3) high-temperature face centered cubic (fcc) phase g-Ag2S stable from B860 K up to its melting temperature. It is thought that monoclinic a-Ag2S phase is stoichiometric, whereas cubic b-Ag2d S and g-Ag2d S with d D 0.002–0.012 are nonstoichiometric phases having either a small deficiency or small excess of Ag.10–14 The powders of silver sulfide Ag2S were synthesized by chemical deposition from an aqueous solution of silver nitrate AgNO3 and sodium sulfide Na2S containing sodium citrate Na3C6H5O7RNa3Cit or the sodium salt of ethylenediaminetetraacetic acid Na2H2(CH2OO)4(CH2N)2REDTA–H2Na2RTrilon B as complexing and stabilizing agents. The as-synthesized matrix colloid solutions have been prepared from the same aqueous solutions. It is a well-known, simple and reliable method which allows the preparation of non-toxic colloidal solutions. Besides, colloidal solutions were obtained by the washing of deposited Ag2S powders with distilled water and then subsequent removal of the powder by filtration. The solubility product Ksp of silver sulfide Ag2S is very small (according to ref. 15, at 298 K, Ksp = 6.3 1050), and when the content of Na2S in the reaction mixture is sufficient, deposition of silver sulfide Ag2S occurs almost instantly, in fractions of a second. Sodium citrate serves as a complexing and stabilizing agent. Besides, in aqueous solutions with a small amount of S2 ions, sodium citrate and Trilon B can reduce Ag+ ions giving rise to nanoparticles of metallic silver.16,17 That is why for deposition of silver sulfide without a Ag impurity it is necessary to use reaction mixtures with excessive sodium sulfide Na2S.
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Synthesis was carried out at room temperature. Deposition of silver sulfide occurred by the following reaction schemes Na3 C6 H5 O7
2AgNO3 þ Na2 S ! Ag2 S # þ 2NaNO3
(1)
or EDTAH2 Na2
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2AgNO3 þ Na2 S ! Ag2 S # þ 2NaNO3 :
(2)
When the reagents are mixed together, silver sulfide is formed almost immediately. As a result, the reaction mixture becomes black at first, and then, after one hour, Ag2S particles deposit and the solution becomes transparent. For the sulfidization reaction to be complete, the deposit was kept in the matrix solution for one day. The study showed that the size of Ag2S particles in colloidal solutions and in deposited powders depends on the initial concentrations of silver nitrate, sodium sulfide and the complexing agents. The concentrations of AgNO3, Na2S and Na3Cit in the reaction mixture C for the synthesis of a coarse-crystalline Ag2S powder with particle sizes of B500 nm were 0.05, 0.5 and 0.005 mol l1, respectivley. Silver sulfide nanopowders with particle sizes of smaller than 60 nm have been synthesized from reaction mixtures I, II, III, and IV. Concentrations of AgNO3 and Na2S in all the reaction mixtures were equal to 0.05 and 0.025 mol l1, respectively. Concentration of Na3Cit in reaction mixtures I and II was equal to 0.025 and 0.0125 mol l1, respectively. Concentration of Trilon B in reaction mixtures III and IV was equal to 0.05 and 0.035 mol l1, respectively. The attempts to deposit nanocrystalline silver sulfide powder with particle size below 20 nm using Na3Cit or Trilon B as complexing agents failed because the nanoparticles with sizes r20 nm form a stable aqueous colloid solution. In this colloid solution, nanoparticles do not deposit after 12 months or longer. All deposits were examined by the X-ray powder diffraction (XRPD) method on a Shimadzu XRD-7000 diffractometer (CuKa1 radiation, primary and secondary Soller slits with a width of 1.0 and 0.15 mm, secondary graphite monochromator, Bragg–Brentano geometry). The XRPD measurements were performed with an angle interval of 2y = 8–951 with a step of D(2y) = 0.021 and scanning time of 10 s in each point. The instrumental resolution function FWHMR(2y) = (u tan2 y + v tan y + w)1/2 of the Shimadzu XRD-7000 X-ray diffractometer was found in a special diffraction experiment on cubic lanthanum hexaboride LaB6 (NIST Standard Reference Powder 660a) with a lattice constant of a = 0.415692 nm. The resolution function FWHMR(2y) measured in degrees has the parameters u = 0.005791, v = 0.004627 and w = 0.010201. The determination of the crystal lattice parameters and the final refinement of the structure of synthesized silver sulfide powders were carried out with the use of the X 0 Pert Plus program package.18 The microstructure, particle size and element chemical composition of silver sulfide powders were studied using scanning electron microscopy (SEM) on a JEOL-JSM LA 6390 microscope coupled with a JED 2300 Energy Dispersive X-ray Analyzer. High-resolution-transmission electron microscopy (HR-TEM) images were recorded on a JEOL JEM-2100 transmission electron microscope with a resolution of 0.14 nm.
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Furthermore, the average particle size D (to be more precise, the average size of the coherent scattering regions (CSR)) of the synthesized silver sulfide powders was estimated by the XRPD method from the diffraction reflection broadening using the dependence of reduced reflection broadening b (2y) = [b(2y)cos y]/l on the scattering vector s = (2 sin y)/l.19–22 The specific surface area Ssp of the synthesized silver sulfide powders was found by the BET method on a Gemini VII 2390t Surface Area Analyzer. In the approximation that all particles have a similar size and spherical shape, the average particle size D = 6/rSsp was estimated from the value of the specific surface area Ssp (r is the density of Ag2S). The size (hydrodynamic diameter Ddls) of the Ag2S nanoparticles in the colloidal solutions was determined by noninvasive Dynamic Light Scattering (DLS) on a Zetasizer Nano ZS facility (Malvern Instruments Ltd) at 298 K. The XRPD patterns of the synthesized silver sulfide powders with different average particle sizes are presented in Fig. 1. According to the BET data, the particle size of the coarsecrystalline powder is 515 15 nm. Nanostructured silver sulfide has been studied extensively over the past two decades. However, until now there is no experimental work on the determination of the crystal structure of synthetic nanocrystalline silver sulfide. The crystal structure of this phase was determined experimentally mostly on natural coarse-grained samples of the acanthite mineral.23,24 According to a study,24 the unit cell of acanthite is primitive monoclinic and belongs to the space group P21/n (P121/n1). Later it was shown that the monoclinic unit cell of natural acanthite mineral a-Ag2S belongs to the space group P21/c (P121/c1).23 Recently,25 we have determined the crystal structure of artificial coarse-crystalline silver sulfide powder with an average particle size of B500 nm. According to ref. 25, synthesized coarse-crystalline silver sulfide powder has a monoclinic (space group no. 14 – P21/c (P121/c1)) structure of a-Ag2S acanthite type and is stoichiometric. Unit cell parameters are equal to a = 0.42264(2) nm, b = 0.69282(3) nm, c = 0.95317(3) nm and b = 125.554(2)1 and are in good agreement with the data.23 All experimental conditions and characteristics of the refinement for the synthesized artificial coarse-crystalline Ag2S phase with acanthite-type crystal structure and the list of the observed diffraction reflections for this refined crystal structure are given in supplementary materials26 of the article.25 Preliminary analysis revealed that all the synthesized nanocrystalline powders have a monoclinic (space group P21/c) a-Ag2S acanthite-type structure too. The diffraction reflections of the nanopowders are broadened and therefore the reflections located close to each other overlap. The average size D of coherent scattering regions estimated from broadening of non-overlapping diffraction reflections (102), (110), (113), (104), (031) and (014) is presented in the insets to the XRPD patterns of the nanopowders (Fig. 1). From these estimates, the average size D in the examined silver sulfide nanopowders, deposited from the initial reaction mixtures I, II, III and IV, is 43 6, 46 8, 58 8, and 59 8 nm, respectively. Note that silver sulfide nanopowders synthesized with Trilon B contain Ag impurities with an amount of 3.0–4.5 wt%.
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Fig. 1 The XRPD patterns of monoclinic (space group P21/c) Ag2S powders: (a) coarse-crystalline powder deposited from the reaction mixture C; (b)–(e) nanopowders deposited from the reaction mixtures I, II, III, and IV, respectively. The insets present the estimate of the average size of the coherent scattering regions from the broadening of non-overlapping diffraction reflections.
In spite of the large width and overlapping of many diffraction reflections on the XRPD patterns of silver sulfide nanopowders, and the presence of silver impurities in the nanopowders, we have tried to refine the crystal structure of silver sulfide nanopowders. In the first approximation we supposed that the nanopowders have
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the same monoclinic (space group P21/c) crystal structure as a coarse-crystalline silver sulfide with particle sizes of B500 nm. As an example, the observed Iobs, calculated Icalc and difference (Iobs Icalc) values corresponding to the refinement of the XRPD pattern for the silver sulfide nanopowder deposited from reaction
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Fig. 2 The experimental ( ) and calculated ( ) XRPD patterns of Ag1.93S nanopowder deposited from the reaction mixture I. The difference between the experimental and calculated XRPD patterns (Iobs Icalc) is shown in the lower part of the figure. The XRPD pattern is recorded in CuKa1 radiation.
Table 1 Refined crystal structure of monoclinic (space group no. 14 – P21/c (P121/c1)) Ag1.93S nanopowders deposited from the reaction mixtures I and II: a-Ag2S acanthite-type structure, Z = 4, a = 0.4234(3) nm, b = 0.6949(3) nm, c = 0.9549(5) nm, and b = 125.43(6)1
Atomic coordinates
Atom
Position and multiplicity
x
y
z
Occupancy
Biso 104 (pm2)
Ag1 Ag2 S
4(e) 4(e) 4(e)
0.0715(4) 0.7264(3) 0.4920(2)
0.0151(0) 0.3240(9) 0.2339(8)
0.3093(9) 0.4375(0) 0.1321(1)
0.97(1) 0.96(4) 1.00(0)
10.05(5) 7.44(6) 1.96(0)
mixture I are shown in Fig. 2. The coordinates of Ag and S atoms and the unit cell parameters for the Ag2S nanopowder (Table 1) are close to those for coarse-crystalline Ag2S. However, the occupancy of crystallographic positions 4(e) by Ag1 and Ag2 atoms is somewhat smaller than 1 and is equal to 0.97 and 0.96, respectively (Table 1). This means that silver sulfide nanoparticles with a size of less than B50 nm are nonstoichiometric, have a composition of BAg1.93S and contain vacant sites in the metal sublattice. Results on the nonstoichiometry and crystal structure of nanocrystalline monoclinic silver sulfide are interesting for comparing to the structural data for b-Ag2S argentite. According to a neutron diffraction study,27 the unit cell of b-Ag2S argentite has a cubic (space group Im3% m) structure in which 4 Ag atoms are statistically distributed in 18 positions 6(b) and 12(d). The occupation of these positions depends on the temperature. The probabilities of occupation of the 6(b) and 12(d) positions by Ag atoms are equal to 0.135 and 0.266 at 459 K and to 0 and 0.333 at 533 K. The temperature 459 K is only 9 K higher than the temperature of the polymorphic phase transition of monoclinic acanthite a-Ag2S to bcc argentite b-Ag2S. The silver atoms statistically occupying the 6(b) and 12(d) positions of the bcc structure of argentite are concentrated in the positions of the monoclinic structure of acanthite. This allows us to suggest that the probabilities of th eoccupation of 4(e) positions by Ag atoms in the structure of acanthite should also differ from 1.
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In this case, the composition of silver sulfide with the acanthite-type structure can deviate from stoichiometry. The axes of the unit cells of a-Ag2S acanthite can be represented as a combination of axes abcc, bbcc and cbcc of the unit cell of bcc argentite. Phase transformation of monoclinic acanthite a-Ag2S to bcc argentite b-Ag2S is accompanied by distortion of bcc sublattice of S atoms to the monoclinic sublattice. Thus, the a-Ag2S acanthite structure can be considered as a result of the distortion of the b-Ag2S argentite structure. Fig. 3a displays the arrangement of S atoms in the bcc sublattice of argentite and the contours of monoclinic unit cells of a-Ag2S acanthite.24,25 It is seen that axes a aP21/c = aP21/n = (abcc + bbcc cbcc)/2, b bP21/c = bP21/n = (abcc bbcc), cP21/n = (abcc + bbcc + 3cbcc)/2 and cP21/c = 2cbcc. The monoclinicdistorted nonmetallic sublattice constructed with allowance for found coordinates of S atoms is demonstrated in Fig. 3b. The arrangement of Ag and S atoms in the unit cell of nanocrystalline monoclinic (space group P21/c) silver sulfide Ag1.93S with an acanthite type structure is displayed in Fig. 4. The least distances between S and Ag1 atoms in the crystal lattice of nonstoichiometric silver sulfide Ag1.93S are equal to 0.25193 and 0.25267 nm, and those between S and Ag2 atoms are 0.25546 and 0.25962 nm. These interatomic distances are a little more than the distances for coarse-crystalline stoichiometric silver sulfide. It means that nanocrystalline nonstoichiometric
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Fig. 3 The distortion of sulfur atom sublattice at the transition from argentite b-Ag2S to acanthite a-Ag2S: (a) the arrangement of S atoms in bcc sublattice of argentite; (b) displacement of S atoms from bcc sublattice positions and their arrangement in the monoclinic sublattice of acanthite. The undistorted bcc sublattice is shown by thin short dashed lines, the contours of the unit cells with space groups P21/c and P21/n are shown by thick solid lines and thick long dashed lines, respectively. (J) and ( ) are S atoms located outside and inside the monoclinic (space group P21/c) unit cell of acanthite a-Ag2S, respectively. Ag atoms are not shown.
Fig. 4 The arrangement of Ag and S atoms in the unit cell of synthesized nanocrystalline monoclinic (space group P21/c) silver sulfide Ag1.93S with an acanthite structure: shown are only the atoms entering the unit cell and the nearest bonds between them, Ag1–S and Ag2–S, with a length of 0.25193 and 0.25546 nm, respectively.
sulfide Ag1.93S has a less dense structure in comparison with stoichiometric silver sulfide Ag2S. According to the Energy Dispersive X-ray analysis, the content of silver Ag and sulfur S in the synthesized dried silver sulfide nanopowder deposited from reaction mixture II is 86.2 0.4 and 13.1 0.1 wt%, which corresponds to sulfide BAg1.95–1.98S. Besides silver and sulfur, the EDX spectrum contains a Ka line of carbon from the carbon planchet, onto which the examined powder was applied. In earlier work28 it was found that silver sulfide nanoparticles with an acanthite structure and size of B10 and B6 nm deposited
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from sewage sludge had a nonstoichiometric composition of Ag1.7S and even Ag1.1S. The authors28 did not examine the crystal structure of the nanoparticles and therefore supposed that silver deficiency in the nanoparticles is caused not by silver nonstoichiometry, but by possible chemosorption of sulfur on the surface of the nanoparticles. But this supposition is in conflict with the data of the same authors about the presence of nanoparticles of the Ag2.1S composition. It is more probable that the authors28 observed nanoparticles of nonstoichiometric silver sulfide Ag2d S both with a deficiency and excess of silver. Our data on nonstoichiometry of silver sulfide nanoparticles with an acanthite structure agree with the results28 and with the data27 on nonstoichiometric distribution of Ag atoms in argentite b-Ag2S near the acanthite–argentite transition temperature. The size of silver sulfide nanoparticles was measured in colloid solutions too. The DLS measurements showed that the particle size in unfiltered solution I decanted from the deposit and then stored for 60 days varies from 8 to 30 nm, and the average size of nanoparticles is equal to B13 nm (Fig. 5a). Similar size distribution of Ag2S particles in the unfiltered colloidal solution II 60 days after synthesis is displayed in Fig. 5b: the particle size varies from 9 to 35 nm, and the average size of nanoparticles is equal to B16 nm. From side on, solution II looks translucent. However a sight check shows that it has a light-brown color and is completely transparent. The optical absorption band maximum of the solution corresponds to 385 nm. Opalescence of this solution is indicative of noticeable fluctuations of its density, at which light scattering occurs. The fluctuations are due to the presence
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Notes and references
Fig. 5 The size distribution of silver sulfide nanoparticles in unfiltered colloid solutions 60 days after synthesis: (a) solution I; (b) solution II. The inset presents a TEM image of a separate silver sulfide nanoparticle from the stable colloidal solution II.
of nanoparticles with sizes under 20 nm in the solution. This agrees with the DLS measurements (Fig. 5b). According to HR-TEM data, stable colloidal solutions I and II contain silver sulfide nanoparticles with sizes from B15–18 nm down to B5–8 nm. As an example, a TEM image of a separate silver sulfide nanoparticle from a stable colloidal solution II is shown in Fig. 5 (inset). The stability of colloidal solutions is confirmed by the turbidity measurements. For example, the turbidity of the decanted unfiltered colloid solution II, 60 and 240 days after a filtration, was equal to 195 2 and 196 2 NTU, respectively.
Conclusion Synthesized silver sulfide nanopowders have a monoclinic (space group P21/c) a-Ag2S acanthite-type structure. The occupancy of the metal sublattice sites by silver atoms in silver sulfide nanopowders is somewhat smaller than 1 and is equal to B0.96–0.97. Thus, for the first time it is established experimentally that the monoclinic silver sulfide nanopowders are nonstoichiometric, contain vacant sites in the metallic sublattice and have a composition of BAg1.93S. By varying the ratio between the concentrations of silver nitrate, sodium sulfide and complexing agent in the initial reaction mixtures it is possible to deposit Ag2S nanoparticles with preassigned average sizes ranging from B500 down to 30–40 nm. Stable aqueous colloidal solutions of Ag2S nanoparticles have been synthesized for the first time. The size of Ag2S nanoparticles in stable colloidaal solutions varies from B18–20 nm down to B5–8 nm.
Acknowledgements This research work is financially supported by the Russian Science Foundation (Grant 14-23-00025) through the Institute of Solid State Chemistry of the Ural Division of the RAS. We thank Ms Yu. V. Kuznetsova for assistance in the measurement of the hydrodynamic diameter of the Ag2S nanoparticles in the colloid solutions using the Dynamic Light Scattering method. The authors are grateful to Dr E. Yu. Gerasimov for help in the high-resolution transmission electron microscopy study.
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´, Z. Dohcˇevic´-Mitrovic ´, A. Kremenovic ´, N. Lazarevic´, 1 S. Asˇkrabic ´, J. Raman Spectrosc., 2012, 43, V. Kahlenberg and Z. V. Popovic 76–81. 2 A. Chatterjee, A. Priyam, S. C. Bhattacharya and A. Saha, J. Lumin., 2007, 126, 764–770. 3 W. J. Parak, D. Gerion, T. Pellegrino, D. Zanchet, C. Micheel, S. C. Williams, R. Boudreau, M. A. Le Gros, C. A. Larabell and A. P. Alivisatos, Nanotechnology, 2003, 14, R15–R27. 4 S. V. Rempel, A. A. Podkorytova and A. A. Rempel, Fiz. Tverd. Tela, 2014, 56, 549–552 (in Russian) (Engl. transl.: Phys. Solid State, 2014, 56, 568–571). 5 P. Jiang, C.-N. Zhu, Z.-L. Zhang, Z.-Q. Tian and D.-W. Pang, Biomaterials, 2012, 33, 5130–5135. 6 C. Li, Y. Zhang, M. Wang, Y. Zhang, G. Chen, L. Li, D. Wu and Q. Wang, Biomaterials, 2014, 35, 393–400. 7 S. Yamamoto, N. Manabe, K. Fujioka, A. Hoshino and K. Yamamoto, IEEE Trans. Nanobioscience, 2007, 6, 94–98. 8 Gmelin’s Handbuch der anorganischen Chemie, Verlag Chemie GmbH, Weinheim, 5nd edn, 1973. 9 R. S. Sharma and Y. A. Chang, Bull. Alloy Phase Diagrams, 1986, 7, 263–269. 10 H. Reye and H. Schmalzried, Z. Phys. Chem., 1981, 128, 93–100. 11 H. Rau, J. Phys. Chem. Solids, 1974, 35, 1553–1559. 12 G. Bonnecaze, A. Lichanot and S. Gromb, J. Phys. Chem. Solids, 1978, 39, 299–310. 13 A. Ditman and I. N. Kulikova, Zh. Fiz. Khim., 1979, 53, 260–261 (in Russian). 14 K. Mitteilung, Z. Phys. Chem., 1980, 119, 251–255. 15 P. Patnaik, Dean’s Analytical Chemistry Handbook, McGrawHill, New York, 2nd edn, 2004, Table 4.2. 16 P. C. Lee and D. Meisel, J. Phys. Chem., 1982, 86, 3391–3395. 17 R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz and J. G. Zheng, Science, 2001, 294, 1901–1903. 18 X 0 Pert Plus Version 1.0. Program for Crystallography and Rietveld analysis Philips Analytical B. V. r 1999 Koninklijke Philips Electronics N. V. 19 W. H. Hall and G. K. Williamson, Proc. Phys. Soc., London, Sect. B, 1951, 64, 937–946. 20 A. I. Gusev and A. A. Rempel, Nanocrystalline Materials, Cambridge Intern. Science Publ., Cambridge, 2004, p. 351. 21 A. I. Gusev and A. S. Kurlov, Metallofiz. Noveishie Tekhnol., 2008, 30, 679–694 (in Russian). 22 S. I. Sadovnikov and A. I. Gusev, J. Alloys Compd., 2014, 586, 105–112. 23 R. Sadanaga and S. Sueno, Mineral. J., 1967, 5, 124–148. 24 A. J. Frueh, Z. Kristallogr., 1958, 110, 136–144. 25 S. I. Sadovnikov, A. I. Gusev and A. A. Rempel, Superlattices Microstruct., 2015, 83, 35–47. 26 S. I. Sadovnikov, A. I. Gusev and A. A. Rempel, Superlattices Microstruct., 2015, DOI: 10.1016/j.spmi.2015.03.024. 27 R. J. Cava, F. Reidinger and B. J. Wuensch, J. Solid State Chem., 1980, 31, 69–80. 28 B. Kim, C.-S. Park, M. Murayama and M. F. Hochella, Environ. Sci. Technol., 2010, 44, 7509–7514.
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