Nanoparticles and Thin Films of Silver from Complexes of Derivatives

Mohammad Afzaal,§ Madeleine Helliwell,§ Paul O'Brien,*,§ Elmira R. Shakirova,#. Maria G. Babashkina,† and Axel Klein†. †Institut f .. ur Anor...
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Chem. Mater. 2009, 21, 4233–4240 4233 DOI:10.1021/cm901200h

Nanoparticles and Thin Films of Silver from Complexes of Derivatives of N-(Diisopropylthiophosphoryl)thioureas Damir A. Safin,*,† Phumlane S. Mdluli,‡ Neerish Revaprasadu,‡ Kibriya Ahmad,§ Mohammad Afzaal,§ Madeleine Helliwell,§ Paul O’Brien,*,§ Elmira R. Shakirova,# Maria G. Babashkina,† and Axel Klein† :: :: :: :: Institut fur Anorganische Chemie, Universitat zu Koln, Greinstrasse 6, D-50939 Koln, Germany, Department of Chemistry, University of Zululand, Private bag X1001, KwaDlangezwa 3886, South Africa, § The School of Chemistry and Manchester Materials Science Centre, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom, and #A.M. Butlerov Chemistry Institute, Kazan State University, 420008, Kremlevskaya Street 18, Kazan, Russian Federation †

‡

Received April 30, 2009. Revised Manuscript Received July 24, 2009

The derivatives of N-(diisopropylthiophosphoryl)thiourea RC(S)NHP(S)(OiPr)2 (R = C5H11N, C5H6N2 or C10H7NH2) followed by their complexation with silver are reported. All complexes are decomposed in hot hexadecylamine (HDA) to give HDA-capped silver nanoparticles. The absorption spectra of the HDA-capped silver nanoparticles exhibit surface plasmon resonance (SPR) absorption in the 400-420 nm region. Transmission electron microscopy (TEM) images of all particles are close to spherical in shape; with sizes ranging from 17 to 20 nm. The X-ray diffraction (XRD) patterns of the silver nanoparticles obtained from all three complexes could be indexed to face centered cubic silver. Scanning electron microscopy (SEM) image confirmed the spherical shape of the particles. The silver complex of 1-naphthylamine was also used to deposit thin films of silver by the aerosol-assisted chemical vapor deposition (AACVD). Introduction The use of single molecular precursors is becoming a common route for preparing nanostructured materials.1-8 Various approaches have been taken to the preparation of thin films and finely divided particles of metals such as silver or gold; but there have been relatively few reports on the use of specific precursor chemistry for metal nanoparticles. The most commonly reported routes to such species are from solutions of readily available metal salts which are reduced at room temperature, in the presence of stabilizing agents to give surface derivatized metal particles. The dimethyl formamide (DMF) reduction method is, for example, well-established for silver nanoparticles of diverse *Corresponding author.

(1) Pickett, N. L.; O’Brien, P. Chem. Rec. 2001, 1, 467. (2) Malik, M. A.; O’Brien, P.; Revaprasadu, N. Phosphorus, Sulfur Silicon 2005, 180, 689. (3) Malik, M. A.; Revaprasadu, N.; O’Brien, P. Chem. Mater. 2001, 13, 913. (4) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843. (5) Burda, C.; Chen, X.; Narayanan, R.; El-Syed, M. A. Chem. Rev. 2005, 105, 1025. (6) Li, Y.; Li, X.; Yang, C.; Li, Y. J. Mater. Chem. 2003, 13, 2641. (7) Revaprasadu, N.; Mlondo, S. N. Pure Appl. Chem. 2006, 78, 1691. (8) Bruce, J. C.; Revaprasadu, N.; Koch, K. R. New J. Chem. 2007, 31, 1647. (9) Pastoriza-Santos, I.; Liz-Marzan, L. M. Pure Appl. Chem. 2000, 72, 83. (10) Deivaraj, T. C.; Lala, N. L.; Lee, J. Y. J. Colloid Interface Sci. 2005, 289, 402. (11) Suber, L.; Sondi, I.; Matijevic, E.; Goia, D. V. J. Colloid Interface Sci. 2005, 288, 489. (12) Mdluli, P. S.; Revaprasadu, N. J. Alloys Compd. 2009, 469, 519. r 2009 American Chemical Society

morphology.9-12 Alternative methods for the formation of silver nanorods using solid-liquid phase arc-discharge or ultraviolet irradiation-photoreduction have been reported by Zhou and co-workers.13 A microwave polyol reduction method, in which the molecular weight of PVP is varied, in the presence of nucleation agents such as H2PtCl6 6H2O, also produces nanowires.14 Murphy and co-workers have also synthesized silver nanorods and nanowires by using a rodlike micelle template of cetyltrimethylammonium bromide (CTAB).15 Green and co-workers reported the synthesis of trialkyl phosphine oxide/amine stabilized silver nanocrystals.16 Nath et al. have synthesized hexadecylamine (HDA)-capped silver organosols which were stable for over a year.17 Chen et al.,18 developed an important method using tri-n-octylphosphine (TOP) as the reducing agent, solvent, and surfactant. Thin films of silver have been deposited by various methods including sputtering,19 thermal evaporation, (13) Zhou, Y.; Yu, S. H.; Cui, X. P.; Wang, C. Y.; Chen, Z. Y. Chem. Mater. 1999, 11, 545. (14) Tsuji, M.; Nishizawa, Y.; Matsumoto, K.; Kubokawa, M.; Miyamae, N.; Tsuji, T. Mater. Lett. 2006, 60, 834. (15) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (16) Green, M.; Allsop, N.; Wakefield, G.; Dobson, P. J.; Hutchison, J. L. J. Mater. Chem. 2002, 12, 2671. (17) Nath, S.; Praharaj, S.; Panigrahi, S.; Kundu, S.; Ghosh, S. K.; Basu, S.; Pal, T. Colloids Surf., A 2006, 274, 145. (18) Chen, Z.; Gao, L.::Mater. Res. Bull. 2007, 42, 1657. (19) Hauder, M.; Gstottner, J.; Hansch, W.; Schmitt-Landsiedel, D. Appl. Phys. Lett. 2001, 78, 838. (20) Carter, A. C.; Chang, W.; Qadri, S. B.; Horwitz, J. S.; Leuchtner, R.; Chrisey, D. B. J. Mater. Res. 1998, 13, 1418.

Published on Web 08/25/2009

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Scheme 1. Synthesis of Aryl N-Thiophosphorylated Thioureas and Their Silver Complexes

Figure 1. Thermal ellipsoid representation of the complex [Ag(PPh3)2L(3)]. Ellipsoids are drawn at the 50% probability level.

electron beam evaporation,20 and chemical vapor deposition (CVD).21 Among these various techniques, CVD has the advantage of potentially superior step-coverage, and is a single-step process that can easily be scaled up to deposit high purity films over large areas. The conventional CVD route has been largely limited by the requirement that the precursors have to be very volatile. These limitations have been overcome by the development of the aerosol-assisted CVD method. This technique involves the vaporization of the precursor dissolved in an organic solvent. After transportation into a reactor where the glass substrate is housed, the precursor is then decomposed to give the required material. The reported silver precursors for CVD are silver(I) salts (AgF,22 AgI,23 CH3COOAg24), organometallic and coordination compounds with fluorinated and nonfluorinated β-diketonates, carboxylates along with neutral donor ligands such as olefins, thioethers, alkyl isocyanides, and tertiary phosphines (PMe3, PEt3 and PPh3) adducts. However, these precursors either have poor volatility or (21) Grodzicki, A.; Lakomska, I.; Piszczek, P.; Szymanska, I.; Szlyk, E. Coord. Chem. Rev. 2005, 249, 2232. (22) Voorhoeve, R. J. H.; Merewether, J. W. J. Electrochem. Soc. 1972, 119, 364. (23) Shapiro, M. J.; Lackey, W. J.; Hanigofsky, J. A.; Hill, D. N.; Carter, W. B.; Barefield, E. K. J. Alloys Compd. 1992, 187, 331. (24) (a) Lu, Y.-F.; Takai, M.; Nagatomo, S.; Kato, K.; Namba, S. Appl. Phys. A: Mater. Sci. Process. 1992, 54, 51. (b) Lu, S. Y.; Lin, Y.-Z. Thin Solid Films 2000, 376, 67. (25) Yuan, Z.; Dryden, N. H.; Li, X.; Vittal, J. J.; Puddephatt, R. J. J. Mater. Chem. 1995, 5, 303. (26) Piszczek, P.; Szlyk, E.; Chaberski, M.; Taeschner, C.; Leonhardt, A.; Bala, W.; Bartkiewicz, K. Chem. Vap. Depos. 2005, 11, 53. (27) (a) Dryden, N. H.; Vittal, J. J.; Puddephatt, R. J. Chem. Mater. 1993, 5, 765. (b) Yuan, Z.; Dryden, N. H.; Vittal, J. J.; Puddephatt, R. J. Chem. Mater. 1995, 7, 1696. (c) Chi, K.-M.; Chen, K.-H.; Peng, S.-M.; Lee, G.-H. Organometallics 1996, 15, 2575. (d) Szlyk, E.; Piszczek, P.; Lakomska, I.; Grodzicki, A.; Szatkowski, J.; Blaszczyk, T. Chem. Vap. Depos. 2000, 6, 105. (e) Szlyk, E.; Piszczek, P.; Grodzicki, A.; Chaberski, M.; Golinski, A.; Szatkowski, J.; Blaszczyk, T. Chem. Vap. Depos. 2001, 7, 111. (f) Chi, K.-M.; Lu, Y.-H. Chem. Vap. :: Depos. 2001, 7, 117. (g) Haase, T.; Kohse-Hoinghaus, K.; Atakan, B.; Schmidt, H.; Lang, H. Chem. Vap. Depos. 2003, 9, 144. (h) Schmidt, :: H.; Shen, Y.; Leschke, M.; Haase, T.; Kohse-Hoinghaus, K.; Lang, H. J. Organomet. Chem. 2003, 669, 25. (i) Zanotto, L.; Benetollo, F.; Natali, M.; Rossetto, G.; Zanella, P.; Kaciulis, S.; Mezzi, A. Chem. Vap. Depos. 2004, 10, 207.

low thermal stability or cause contamination (usually carbon or phosphorus) in the as-deposited silver films thereby limiting their suitability for conventional CVD process.25-28 In an attempt to overcome these difficulties, the synthesis and evaluation of new classes of silver precursors are under investigations. Recently polycrystalline cubic-phase silver films have been deposited by the AACVD using tris(phosphino)borato silver(I) complexes as precursors.29 O’Brien and co-workers have reported the use of a silver complex of the dithioimidodiphosphinate ligand, [Ag3{(SPiPr2)6N3}]2 as a suitable precursor to deposit thin films of silver sulfide and silver using the AACVD method.30 The deposition temperature determined whether a mixture of silver sulfide and silver or silver only films were obtained. Molloy and co-workers have reported silver films by AACVD using silver carboxylates, fluorocarboxylates, β-diketonates, β-diketoiminates, and aryl oxides as phosphine adducts to give useful precursors which were, however, considered unsuitable for conventional CVD.31 In previous work many complexes of N-(thio)phosphorylated (thio)amides and (thio)ureas RC(E)NHP(E0 )R0 2 (E, E0 =O, S; R = alkyl, aryl; R0 =alkyl, aryl) (HL) (28) (a) Bailey, A.; Corbitt, T. S.; Hampden-Smith, M. J.; Duesler, E. N.; Kodas, T. T. Polyhedron 1993, 12, 1785. (b) Xu, C.; Corbitt, T. S.; Hampden-Smith, M. J.; Kodas, T. T.; Duesler, E. N. J. Chem. Soc., Dalton Trans. 1994, 2841. (c) Yuan, Z.; Dryden, N. H.; Vittal, J. J.; Puddephatt, R. J. Can. J. Chem. 1994, 72, 1605. (d) Doppelt, P.; Baum, T. H.; Ricard, L. Inorg. Chem. 1996, 35, 1286. (e) Chi, K.-M.; Chen, K.-H.; Lin, H.-C.; Lin, K.-J. Polyhedron 1997, 16, 2147. (f) Gao, :: L.; Harter, P.; Linsmeier, Ch.; Wiltner, A.; Emling, R.; Schmitt-Landsiedel, D. Microelectron. Eng. 2005, 82, 296. (29) McCain, M. N.; Schneider, S.; Salata, M. R.; Marks, T. Inorg. Chem. 2008, 47, 2534. (30) (a) Panneerselvam, A.; Malik, M. A.; O’Brien, P.; Raftery, J. J. Mater. Chem. 2008, 18, 3264. (b) Panneerselvam, A.; Malik, M. A.; O'Brien, P.; Helliwell, M. Chem. Vap. Depos. 2009, 15, 57. (31) (a) Edwards, D. A.; Harker, R. M.; Mahon, M. F.; Molloy, K. C. J. Mater. Chem. 1999, 9, 1771. (b) Edwards, D. A.; Harker, R. M.; Mahon, M. F.; Molloy, K. C. Inorg. Chem. Acta 2002, 328, 134. (c) Edwards, D. A.; Mahon, M. F.; Molloy, K. C.; Ogrodnick, V. J. Mater. Chem. 2003, 13, 563. (d) Edwards, D. A.; Mahon, M. F.; Molloy, K. C.; Ogrodnick, V. Inorg. Chim. Acta 2003, 349, 37. (32) Sokolov, F. D.; Brusko, V. V.; Zabirov, N. G.; Cherkasov, R. A. Curr. Org. Chem. 2006, 10, 27. (33) Sokolov, F. D.; Brusko, V. V.; Safin, D. A.; Cherkasov, R. A.; Zabirov, N. G. Coordination Diversity of N-Phosphorylated Amides and Ureas Towards VIIIB Group Cations. In Transition Metal Chemistry: New Research; Varga, B., Kis, L., Eds.; Nova Science: Hauppauge, NY, 2008, p 101.

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Table 1. Selected Bond Lengths (A˚) and Bond Angles (deg) for [Ag(PPh3)2L(3)] Bond Lengths Ag(1)-S(1) Ag(1)-S(2) Ag(1)-P(2) Ag(1)-P(3) S(1)-P(1) S(2)-C(1)

2.640(6) 2.545(7) 2.519(4) 2.473(5) 1.970(2) 1.745(3)

P(1)-O(1) P(1)-O(2) P(1)-N(1) N(1)-C(1) N(2)-C(1)

1.585(8) 1.579(2) 1.622(2) 1.313(3) 1.360(3)

S(1)-P(1)-O(2) S(1)-P(1)-N(1) O(1)-P(1)-O(2) O(1)-P(1)-N(1) O(2)-P(1)-N(1) P(1)-N(1)-C(1) S(2)-C(1)-N(1) S(2)-C(1)-N(2) N(1)-C(1)-N(2)

114.64(8) 117.97(9) 97.72(1) 103.98(1) 109.84(1) 124.51(2) 126.6(2) 114.50(2) 118.9(2)

Bond Angles S(1)-Ag(1)-S(2) S(1)-Ag(1)-P(2) S(1)-Ag(1)-P(3) S(2)-Ag(1)-P(2) S(2)-Ag(1)-P(3) P(2)-Ag(1)-P(3) Ag(1)-S(1)-P(1) Ag(1)-S(2)-C(1) S(1)-P(1)-O(1)

92.69(2) 114.27(2) 103.68(2) 113.82(2) 121.84(3) 109.08(2) 101.84(3) 98.07(9) 110.24(8)

were described.32,33 Only a few of these deal with complexes of the coinage metals.34-40 Among the reported coinage metal complexes, those containing silver(I) cations are particularly poorly represented,34 which may be due to the propensity of silver(I) to be easily reduced in polynuclear compounds. Mononuclear silver(I) complexes with the same type of ligands are generally easily synthesized and isolated.34 Additional donor ligands in these complexes (e.g., OTf, PPh3) prevent the reduction of the silver. Silver(I) complexes of ligands of the type HL might be good precursors for silver, because they are volatile due to their neutral character and they contain both the metal and chalcogenide in one molecule and therefore, in a fixed ratio (single-source precursors). Also, the readily accessible systematic chemistry of the ligands should allow to investigate the effects of functionality on deposition systematically.32 In this paper we report the synthesis of HDA capped nanoparticles and thin films of silver using [Ag(PPh3)2L(1-3)] complexes as single-molecule precursors. Experimental Section Synthesis of HL(1) (piperidine adduct). A solution of piperidine (1.0 g, 12 mmol) in chloroform (25 mL) was added dropwise to a solution of O,O-diisopropylthiophosphonic acid isothiocyanate (3.1 g, 13 mmol) in the same solvent (15 mL). The mixture was stirred at room temperature for 5 h. The solvent was then removed in vacuo. The residue was recrystallized from a dichloromethane-n-hexane mixture 1:5 (v/v). The product was obtained as colorless crystals. Yield: 3.2 g, 80%. Mp 84 °C. Anal. (34) Crespo, O.; Brusko, V. V.; Gimeno, M. C.; Tornil, M. L.; Laguna, A.; Zabirov, N. G. Eur. J. Inorg. Chem. 2004, 423. (35) Verat, A. Y.; Sokolov, F. D.; Zabirov, N. G.; Babashkina, M. G.; Krivolapov, D. B.; Brusko, V. V.; Litvinov, I. A. Inorg. Chim. Acta 2006, 359, 475. (36) Sokolov, F. D.; Babashkina, M. G.; Safin, D. A.; Rakhmatullin, A. I.; Fayon, F.; Zabirov, N. G.; Bolte, M.; Brusko, V. V.; Galezowska, J.; Kozlowski, H. Dalton Trans. 2007, 4693. (37) Sokolov, F. D.; Babashkina, M. G.; Fayon, F.; Rakhmatullin, A. I.; Safin, D. A.; Pape, T.; Hahn, F. E. J. Organomet. Chem. 2009, 694, 167. (38) Babashkina, M. G.; Safin, D. A.; Szyrwiel, Le.; Kubiak, M.; Sokolov, F. D.; Strikov, Y. V.; Kozlowski, H. Z. Anorg. Allg. Chem. 2009, 635, 554. (39) Safin, D. A.; Babashkina, M. G.; Bolte, M.; Sokolov, F. D.; Brusko, V. V. Inorg. Chim. Acta 2009, in press. (40) Meijboom, R.; Bowen, R. J.; Berners-Price, S. J. Coord. Chem. Rev. 2009, 253, 325.

Figure 2. Absorption spectra of HDA silver nanoparticles synthesized from [Ag(PPh3)2L(1)] (A), [Ag(PPh3)2L(2)] (B), and [Ag(PPh3)2L(3)] (C).

Calcd for C12H24N2O2PS2 (323.1): C, 44.6; H, 7.5; N, 8.7. Found: C, 44.5; H, 7.7; N, 8.6. Synthesis of HL(2) (3-aminopyridine adduct). This was prepared by a method similar to that described for the ligand (1) but with 3-aminopyridine (3-AP) (1.02 g, 12 mmol). The product was obtained as colorless crystals. Yield: 2.6 g, 87%. Mp 112 °C. Anal. Calcd for C12H18N302PS2 (331.3): C, 43.5; H, 5.5; N, 12.7. Found: C, 43.5; H, 5.4; N, 12.7. Synthesis of HL(3) (1-naphthylamine adduct). This was prepared by a method similar to that described for ligand (1) but with 1-naphthylamine (1-NA) (1.05 g, 12 mmol). The product was obtained as colorless crystals. Yield: 2.9 g, 86%. Mp 120 °C. Anal. Calcd for C21H21N2O2PS2 (380.5): C, 53.7; H, 5.6; N, 7.4. Found: C, 53.7; H, 5.6; N, 7.3. General Methodology for the Synthesis of the Silver Complexes. All silver complexes as shown in Scheme 1 were synthesized following a general method reported by Sokolov et al.36 The complex [Ag(PPh3)2NO3], prepared by reacting silver nitrate with triphenylphosphine in acetonitrile, followed by recrystallization from chloroform,40 which was reacted with the ligand and KOH (described in the following). Synthesis of [Ag(PPh3)2L(1)]. A suspension of HL(1) (0.98 g, 3 mmol) in aqueous ethanol (25 mL) was mixed with an ethanolic solution of KOH (0.19 g, 3.3 mmol). A solution of [Ag(PPh3)2NO3] (2.93 g, 3.0 mmol) in dichloromethane (25 mL) was added dropwise under vigorous stirring to the solution. The mixture was stirred at room temperature for one more hour and the precipitate was filtered off. The filtrate was concentrated until crystallization started. The residue was recrystallized from a dichloromethane-n-hexane mixture 1:5 (v/v). Complex [Ag(PPh3)2L(1)] was obtained as colorless crystals. Yield: 1.92 g, 72%.

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Figure 3. (A, C, and E) TEM images of HDA-capped silver nanoparticles synthesized from complexes [Ag(PPh3)2L(1-3)], respectively, and (B, D, and F) their corresponding particle size histograms.

Mp 148 °C. Anal. Calcd for C48H54AgN2O2P3S2 (955.88): C, 60.3; H, 5.7; N, 2.9. Found: C, 60.3; H, 5.7; N, 2.9. Synthesis of [Ag(PPh3)2L(2)]. Complex [Ag(PPh3)2L(2)] was prepared similar to the method that is described for complex [Ag(PPh3)2L(1)] but with HL(2) (0.77 g, 3.0 mmol). Complex [Ag(PPh3)2L(2)] was obtained as colorless crystals. Yield: 2.07 g, 80%. Mp 120 °C. Anal. Calcd for C48H48AgN3O2P3S2 (963.84): C, 59.8; H, 5.0; N, 4.4. Found: C, 59.8; H, 5.0; N, 4.3. Synthesis of [Ag(PPh3)2L(3)]. Complex [Ag(PPh3)2L(3)] was prepared similar to the method that is described for complex [Ag(PPh3)2L(1)] but with HL(3) (1.04 g, 3.0 mmol). Complex [Ag(PPh3)2L(3)] was obtained as colorless crystals. Yield: 2.19 g, 75%. Mp 121 °C. Anal. Calcd for C53H51AgN2O2P3S2 (1012.91): C, 62.9; H, 5.1; N, 2.8. Found: C, 63.8; H, 5.1; N, 2.7.

Synthesis of HDA-Capped Silver Nanoparticles. The silver complexes [Ag(PPh3)2L(1-3)] (0.3 mmol) were each dissolved in TOP (6 mL) and injected into hot HDA (6 g, 2.5 mmol) at 150 °C. An initial decrease in temperature from 150 to 130 °C was observed. The solution was then allowed to stabilize and the reaction was continued for 45 min at 150 °C. After completion, the reaction mixture was allowed to cool to 70 °C; methanol was added to precipitate the nanoparticles. The solid was separated by centrifugation and washed three times with methanol. The resulting solid precipitates of HDA-capped silver nanoparticles were dispersed in toluene for further analysis. Deposition of Films. In a typical deposition run, 0.10 g (0.1 mmol) of the precursor was dissolved in 10 mL of toluene in a round-bottom flask. Argon at flow rate of 150 sccm was used as a carrier gas. The argon flow rate was controlled by a Platon

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Figure 4. Powder X-ray diffraction patterns of HDA-capped silver nanoparticles synthesized from (A) [Ag(PPh3)2L(1)], (B) [Ag(PPh3)2L(2)], and (C) [Ag(PPh3)2L(3)].

Figure 5. (A) SEM image and (B) EDAX spectrum of HDA-capped silver nanoparticles from [Ag(PPh3)2L(1)].

flow gauge. Six glass substrates (approximately 1  3 cm) were placed inside the reactor tube and they were annealed at the desired temperature for 10 min before carrying out the deposition. The precursor solution in a round-bottom flask was kept in a water bath above the piezoelectric modulator of a PIFCO ultrasonic humidifier (Model No. 1077). The aerosol droplets of the precursor thus generated were transferred into the hot wall zone of the reactor by carrier gas. The reactor was placed in a Carbolite furnace. Decomposition of the precursors took place resulting in deposition of thin films on the substrate. Single-Crystal X-ray Diffraction Study. Crystal data for [Ag(PPh3)2L(3)]: C53H52AgN2O2P3S2, Mr = 1013.89 g mol-1, triclinic, space group P1, a = 12.9095(8) A˚, b=13.4462(9) A˚, c= 16.0728(10) A˚, R = 74.343(1)°, β = 69.868(1)°, γ=66.795(1)°, V=2378.3(3) A˚3, Z=2, Dc =1.416 g cm-3, μ(Mo-KR)=0.656 mm-1, F(000)=1048, T=100 K. Total numbers of measured and observed independent reflections are 15145 and 10587 (Rint =0.0208). R1 =0.0478; wR2 =0.0936. Single-crystal X-ray diffraction data for the compounds were collected using graphite monochromated Mo KR radiation (λ= 0.71073 A˚) on a Bruker APEX diffractometer. The structure was solved by direct methods and refined by full-matrix least-squares41 (41) Sheldrick, G. M. SHELXS-97 and SHELXL-97; University of :: :: Gottingen: Gottingen, Germany, 1997.

on F2 All non-H atoms were refined anisotropically. H atoms were included in calculated positions, assigned isotropic thermal parameters, and allowed to ride on their parent carbon atoms. All calculations were carried out using the SHELXTL package.42

Results and Discussion N-thiophosphorylated thioureas HL(1-3) were prepared by the addition of corresponding amine to O,O0 diisopropylthiophosphoric acid isothiocyanate (iPrO)2P(S)NCS (Scheme 1). Reaction of the potassium salts of HL(1-3) with [Ag(PPh3)2NO3] in aqueous EtOH/ CH2Cl2 leads to the mononuclear [Ag(PPh3)2L(1-3)] (Scheme 1). The complexes obtained are colorless crystalline powders that are soluble in acetone, benzene, dichloromethane, DMSO, and DMF and insoluble in n-hexane. 1 H and 31P{1H} NMR and IR data indicated that the deprotonated thioureas L(1-3) are 1,5-S,S0 -ligands in all the present cases studied. Single crystals of the complex [Ag(PPh3)2L(3)] were obtained from a dichloromethane-n-hexane solution. The solid-state structure has been determined by X-ray (42) SHELXTL version 6.12; Bruker AXS Inc.: Madison, WI, 2001.

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single-crystal crystallography and reveals the expected four-coordination (tetrahedral geometry) around the silver center formed by two sulfur atoms and two PPh3 ligands (Figure 1). The values of bond angles around Ag(I) range from 92.69(2) to 121.84(3)° (Table 1). The ring defined by Ag(1), S(1), P(1), N(1), C(1), and S(2) adopts a boat conformation with the Ag(1) [0.5147(6) A˚] and S(2) [0.6765(9) A˚] atoms, showing the greatest deviation from the least-squares plane of the six-membered metallocycle. The fragment NC(S)NP is almost planar, the sulfur atom of the thiophosphoryl group is significantly deviated from the average plane of this fragment. This deviation of the phosphoryl sulfur is common for the complexes of N-thioacylamidophosphates.32 Two different Ag-P bond lengths (Table 1) were observed in the complex [Ag(PPh3)2L(3)] (2.473(5) and 2.519(4) A˚, respectively). The shorter value of the two is very close to those found in [Ag{PhC(S)NHP(S)(OiPr)2}(PPh3)2] [2.471(1) and 2.490(7) A˚] and [Ag{PhNHC(S)NHP(S)(OiPr)2}(PPh3)2] [2.451 and 2.480(8) A˚].34 Very similar values are found in [Ag(PPh3)2NO3] [2.440(1) and 2.443(1) A˚],43 [Ag(PPh3)2NO3] 3 C6H6 [2.416(1) and 2.435(1) A˚],44 and [Ag{P(mMeC6H4)3}2NO3] [2.412(8) A˚].45 The longer value matches with distances found in [Ag(PPh3)3NO3] [2.525(1)-2.630(2) A˚],43 [Ag(PPh3)4]ClO4 [2.650(2)-2.668(5) A˚],46 [Ag(PPh3)3I] [2.533(5)-2.681(4) A˚],47 [Ag(PPh3)3X] [X = BF4, 2.506(3)-2.577(3); Cl, 2.552(1)-2.556(1); I, 2.572(4)-2.616(3) A˚],48 and [Ag{(SPPh2)2CH2}{(PPh2)2C2B10H10}]ClO4 [2.526(2) and 2.532(2) A˚].49 Also, two different Ag-S bond lengths are present in complex [Ag(PPh3)2L(3)]. The distance to the sulfur atom of thiophosphoryl group is longer [2.640(6) A˚] than the other Ag-S distance [2.545(7) A˚] (Table 1). These values can be compared with those found in other tetrahedral silver complexes such as [Ag{PhC(S)NHP(S)(OiPr)2}(PPh3)2] [2.589(3) and 2.695(4) A˚], [Ag{PhNHC(S)NHP(S)(OiPr)2}(PPh3)2] [2.537(9) and 2.664(8) A˚], [Ag{(SPPh2)2CH2}{(PPh2)2C2B10H10}]ClO4 [2.540(2) and 2.588(2) A˚], or in [Ag{S2C2(CN)2}(PPh3)4] [2.568(7) and 2.653(7) A˚].50 HDA-Capped Silver Nanoparticles. The use of single molecule precursors to synthesize nanostructured materials involves the thermolysis of the precursor in a high boiling point solvent. In nanoparticle synthesis, the choice of an effective coordinating solvent is very important, as the surface passivation of the nanoparticles by organic molecules controls their growth and size. HDA, a primary amine, is a weak base possessing an electron (43) Barron, P. F.; Dyason, J. C.; Healy, P. C.; Engelhardt, L. M.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1986, 1965. (44) Harker, C. S. W.; Tiekink, E. R. T. Acta Crystallogr., Sect. C 1989, 45, 1815. (45) Liu, C. W.; Pan, H.; Fackler jun, J. P.; Wu, G.; Wasylishen, R. E.; Shang, M. J. Chem. Soc., Dalton Trans. 1995, 3691. (46) Engelhard, L. M.; Pakawatchai, C.; White, A. H.; Healy, P. C. J. Chem. Soc., Dalton Trans. 1985, 125. (47) Hibbs, D. E.; Hursthouse, M. B.; Abdul Malik, K. M.; Beckett, M. A.; Jones, P. W. Acta Crystallogr., Sect. C 1996, 52, 884. (48) Camalli, M.; Caruso, F. Inorg. Chim. Acta 1987, 127, 209. (49) Bembenek, E.; Crespo, O.; Gimeno, M. C.; Jones, P. G.; Laguna, A. Chem. Ber. 1994, 127, 835. (50) Heinrich, D. D.; Fackler, J. P.; Lahuerta, P. Inorg. Chim. Acta 1986, 116, 15.

Safin et al. Scheme 2. Decomposition Mechanism of Silver Complexes

donating NH2 group but with small steric hindrance, allowing a good capping density.51 The reduction of the silver complexes at 150 °C in HDA resulted in a fast reaction of the silver complexes and the growth of silver nanoparticles. The absorption spectra of all HDAcapped silver nanoparticle samples (Figure 2) exhibited a strong absorption band in the 400-420 nm region. The appearance of this absorption in samples containing metallic nanoparticles is due to the oscillation of the conduction electrons induced by an interacting electromagnetic field and this resonance is called the surface plasmon resonance (SPR). The resonance can be influenced by the size, shape, and dielectric constant of the medium in which the nanoparticles are dissolved.20 TEM images (Figure 3) show that the HDA capped silver nanoparticles obtained from all complexes are close to spherical in shape. The average particle sizes as determined from the TEM images are 16 ( 2 nm (complex [Ag(PPh3)2L(1)]), 14 ( 2 nm (complex [Ag(PPh3)2L(2)]), and 20 ( 2 nm (complex [Ag(PPh3)2L(3)]), respectively. The particles obtained from complex [Ag(PPh3)2L(1)] are evenly distributed over the TEM grid. There is evidence that the particles are more polyhedral than spherical in shape. The particles obtained from complex [Ag(PPh3)2L(2)] show some degree of aggregation, whereas particles obtained from complex [Ag(PPh3)2L(3)] showing a linear arrangement of particles. We conclude that a compact layer of HDA is formed between the silver nanoparticles to prevent them from aggregation, confirming HDA to be an effective surfactant for producing well-dispersed metal nanoparticles. (51) Talapin, D. V.; Rogach, A. L.; Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 5782.

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Figure 6. UV-visible spectra of HDA-capped nanoparticles synthesized from (A) [Ag(PPh3)2L(1)] and (B) [Ag(PPh3)2L(2)], obtained between 5 and 55 min.

The formation of nearly monodispersed silver nanoparticles could be attributed to the balance between stabilization and nanoparticles growth in solution. The rapid addition of the reagents to the reaction vessel raises the precursor concentration above the nucleation threshold and the solution becomes supersaturated because of the high reaction temperature. As a result, a short nucleation burst occurs and consequently the concentration of the precursors in solution drops below the critical concentration for nucleation. If the time for nanoparticle growth during the nucleation period is short compared to the subsequent growth processes, monodispersed particles are prevalent.52 In general, the particle size increases with an increase in reaction time and temperature.52 The systematic adjustment of the reaction parameters, such as reaction time, temperature, concentration, and the selection of reagents and surfactants, can be used to control the size, shape, and quality of the nanoparticles. The HDA capped silver nanoparticles retain the same morphology over a long periods of time as compared to PVP capped silver nanoparticles.12 The HDA-capped silver nanoparticles presumably have the amine functional group bound to the surface of the silver nanoparticles, with the long hydrophobic hexadecyl chains providing a hydrophobic environment around the silver nanoparticles, giving steric stabilization, isotropic growth, and stability. The X-ray diffraction patterns of the HDA-capped silver nanoparticles are shown in Figure 4. The four diffraction peaks (111, 200, 220, and 311) above 2θ = 30° are characteristic of face centered cubic silver. Figure 5A shows a representative scanning electron microscopy (SEM) image of HDA-capped silver nanoparticles synthesized from [Ag(PPh3)2L(3)], on the glass substrate activated by PVP. Isotropic HDA-capped silver nanoparticles are formed on the surface of glass substrate. Further elucidation of the HDA-capped silver nanoparticles was achieved by EDAX. The EDAX (Figure 5B) reveals that about 60% of the glass substrate constitutes (52) (a) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183–184. (b) Qu, L.; Peng, Z. A.; Peng, X. Nano Lett. 2001, 1, 333.

Figure 7. (A) X-ray diffraction pattern and (B) SEM image of silver thin films deposited from [Ag(PPh3)2L(3)].

of silver nanoparticles and the other 40% is covered by carbon. This observation can be attributed to carboncoated glass subtracted for SEM analysis. The decomposition of silver complexes probably follows the Chugaev mechanism, as shown in Scheme 2.53 There are two possible decomposition pathways, the first gives Ag2S. The second decomposition pathway is considered more likely in view of the results and gives silver metal nanoparticles. It is appropriate to study the time dependence of the decomposition. The induction time was 5 min and the absorbance was observed to gradually increase (Figure 6). This could be attributed to an increase in the amount of silver nanoparticles as indicated in the UV-visible spectra at different time intervals. These spectra did not show any trace of silver sulfide semiconductor nanoparticles. As time elapsed the absorption bands narrowed and shifted continuously to longer wavelengths, which could be attributed to the growth of silver nanoparticles (Figure 6). Silver Thin Film Growth and Characterization. The complex [Ag(PPh3)2L(3)] used for the deposition of metallic silver films on glass substrates by aerosol-assisted (53) Pradhan, N.; Katz, B.; Efrima, S. J. Phys. Chem. B 2003, 107, 13843.

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Figure 8. UV-visible spectra of Ag films grown at (a) 450 and (b) 400 °C.

chemical vapor deposition (AACVD). The precursor was not only air-stable but also highly soluble in common organic solvents making it seem ideal for AACVD. Brownish reflective films were deposited at 400-450 °C with a dynamic argon flow rate of 150 sccm. The deposited films were gray, nonadherent, and could be easily wiped off the surface, indicating that the silver particles are weakly adsorbed on the glass. X-ray diffraction (54) (a) Jona, F.; Marcu, P. M. J. Phys., Conden. Matter 2004, 16, 5199– 5204. (b) Taneja, P.; Banerjee, R.; Ayyub, P. Phys. Rev. B 2001, 64, 0334051.

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measurements (Figure 7A) revealed reflections corresponding to the cubic phase of silver (ICDD 04-0783). Cubic is by far the most common phase for metallic silver, although a metastable hexagonal phase has been reported.54 Scanning electron microscope (SEM) confirmed poor film coverage, consisting of spherical silver particles randomly distributed over the entire substrate (Figure 7B).There is also evidence for agglomeration taking place on the surface. Bare glass is also visible between the particles, showing that less than one monolayer has been deposited. Quantitative energy-dispersive X-ray analyses of the films confirmed the presence of metallic silver. The UV-visible spectra (Figure 8) of asdeposited films indicated broad SPR peaks at ca. 418 nm. No noticeable shift is observed in the SPR of films. Conclusion We have demonstrated that N-thiophosphorylated thiourea complexes of silver are suitable for the controlled synthesis of silver nanoparticles. Hexadecylamine is found to be a useful capping ligand for such nanoparticles. Moreover, HDA-capped nanoparticles retain their morphology over long periods and also their monodispersity. There is no variation of shape and size range of the HDA capped silver nanoparticles synthesized from the three precursors.