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J. Phys. Chem. C 2010, 114, 10396–10402
Photoinduced Size-Controlled Generation of Silver Nanoparticles Coated with Carboxylate-Derivatized Thioxanthones Jean-Pierre Malval,*,† Ming Jin,‡ Lavinia Balan,*,† Raphae¨l Schneider,§ Davy-Louis Versace,| He´le`ne Chaumeil,⊥ Albert Defoin,⊥ and Olivier Soppera† Institut de Sciences des Mate´riaux de Mulhouse, LRC CNRS 7228, UniVersite´ de Haute Alsace, 15 rue Jean Starcky, 68057 Mulhouse, France, Institute of Functional Polymer Materials, School of Materials Science and Technology, Tongji UniVersity, 1239 Siping Road, 200092 Shanghai, China, De´partement de Chimie Physique des Re´actions, UMR CNRS 7630, UniVersite´ de Nancy, 1 rue GranVille, 54001 Nancy, France, De´partement de Photochimie Ge´ne´rale, FRE CNRS 3252, UniVersite´ de Haute Alsace, ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse, France, and Laboratoire de Chimie Organique, Bioorganique et Macromole´culaire, FRE CNRS 3253, UniVersite´ de Haute Alsace, ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse, France ReceiVed: March 10, 2010; ReVised Manuscript ReceiVed: May 4, 2010
We report a new strategy for a rapid photoinduced synthesis of monodisperse ligand-coated silver nanoparticles. Such nanoparticles are produced through a very fast reduction of Ag+ by R-aminoalkyl radicals which are first generated from hydrogen abstraction toward an aliphatic amine by the excited triplet state of the 2-substituted thioxanthone series (TX-O-CH2-COO- and TX-S-CH2-COO-). The quantum yield of this prior reaction is tuned by the substituent effect on thioxanthones and leads to a kinetic control of the conversion of Ag+ to Ag(0). Combined with the capping role of a carboxylate function linked to the chromophores, a size regulation of the growing nanoparticles is both promoted and optimized because of a concomitant kinetics adjustment between the photoreduction process and the subsequent functionalization of the nanoparticles. We demonstrate that the optimal adjustment is then obtained with TX-S-CH2-COO-. 1. Introduction
SCHEME 1: Molecular Structures of the Thioxanthones
The development of synthetic routes devoted to the size and shape control of silver nanoparticles continues to be the subject of an intense research focus. Such morphological requirements constitute the key issues for optical, electronic, magnetic, and catalytic properties which are clearly size-dependent.1–7 Chemical methods as opposed as photochemical ones have been extensively described. In these multicomponent reactions, a chemical reduction of a silver cation is achieved in the presence of various reducing agents such as sodium borohydride,8–12 hydrazine,13,14 dimethylformamide,6,15 alcohol,3,16,17 or sodium citrate,18 for instance. Furthermore, the addition of a large amount of stabilizers is necessary to obtain a narrow particle size distribution. The most commonly used stabilizers are thiol derivatives,10,19–21 ionic or neutral surfactants,14,17,22 alkylamines,13,23 or carboxylic compounds.8,13,24 Alternately, the photochemical method should constitute an original route for a fast lighttriggered generation of nanoparticles. The use of plasmon excitation to direct nanostructure growth has been clearly demonstrated.2,25–27 For instance, the photoreduction of silver cations by citrate can be catalyzed by silver seeds and leads to the generation of disk-shaped silver nanoparticles.26 In this plasmon-mediated synthesis, the irradiation wavelength constitutes a key parameter which determines the final shape of the * To whom correspondence should be addressed. E-mail:
[email protected] or
[email protected]. † Institut de Sciences des Mate´riaux de Mulhouse, LRC CNRS 7228, Universite´ de Haute Alsace. ‡ Tongji University. § Universite´ de Nancy. | De´partement de Photochimie Ge´ne´rale, FRE CNRS 3252, Universite´ de Haute Alsace. ⊥ Laboratoire de Chimie Organique, Bioorganique et Macromole´culaire, FRE CNRS 3253, Universite´ de Haute Alsace.
particles and offers a flexible tool to generate anisotropic nanostructures such as triangles,27 prisms,2 cubes,28 or bipyramids.25 Another method which does not required the preliminary addition of any metal nanoparticle seeds corresponds to the direct reduction of a silver cation by an excited chromophore or by an intermediate reactant which is generated in situ during excitation.29–34 The homogeneous growth of nanoparticles is then regulated by the presence of stabilizers similar to those used for chemical methods. However, the presence of large concentrations of stabilizers with respect to that of the chromophore can affect the efficiency of the photoreduction through a dynamic quenching process, for instance. The interfering role of this side reaction becomes all the more significant as the photoreduction of silver precursors proceeds through a bimolecular multistep mechanism. In this precise case, the stabilizer concentration cannot be modulated as conveniently as for chemical methods. Herein, we propose to overcome this problem by a direct integration of the stabilizing functionality into a 2-substituted thioxanthone series used as photoactive chromophores (Scheme 1). We also demonstrate that the substitution effect on the thioxanthone ring has a direct effect on the
10.1021/jp102189u 2010 American Chemical Society Published on Web 05/26/2010
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TABLE 1: Electrochemical and Spectroscopic Data Relative to the S0, S1, and T1 States of the Thioxanthones in Acetonitrile S0
TX TO TS a
S1
T1
λabs nm
εabs M-1 · cm--1
Eox V vs SCE
Ered V vs SCE
ES eV
Φfluo
ET eV
ΦISC
τT µs
kqb 108 M-1 · s-1
378 395 385
2040 5700 4035
1.75a 1.60a 1.50a
- 1.66 - 1.65 - 1.63
3.17 3.02 2.97
0.006 0.11 0.09
2.76 2.54 2.47
0.65 0.70 0.80
6.70 2.70 0.13
30 6 2
Irreversible oxidation wave. b Quenching rate constant of T1 state by MDEA.
photoreduction efficiency, which strongly influences the final silver nanoparticle size distribution. 2. Experimental Section 2.1. Materials. N-Methyl diethanol amine (MDEA), silver nitrate (AgNO3), and thioxanthone (TX) were supplied by Aldrich. All of the solvents employed were Fluka spectroscopic grade. The synthesis of 2-(carbomethoxy)thioxanthone (TO) and 2-thioxanthone-thioacetic acid (TS) are described in reference.35 1 H NMR (400 MHz) and 13C NMR (100.6 MHz) spectra were measured on a Bruker Avance serie 400 at 295 K. Chemical shifts are reported in ppm relative to SiMe4. IR spectra (cm-1) were recorded on a BRUKER Avatar spectrometer. 2-(Carbomethoxy)thioxanthone (TO). 1H NMR (D6, DMSO): δ ) 4.84 (2H), 7.47 (1H, H3, dd, J ) 8.8 Hz, J ) 3 Hz,), 7.59 (1H, H7, ddd, J ) 8.2 Hz, J ) 7 Hz, J ) 1.3 Hz), 7.77 (1H, H6, ddd, J ) 8,2 Hz, J ) 7 Hz, J ) 1.5 Hz), 7.82 (1H, H4, d, J ) 8.8 Hz), 7.855 (1H, H1, d, J ) 3 Hz), 3.856 (1H, H5, dd, J ) 8.1 Hz, J ) 1.2 Hz), 8.47 (1H, dd, H8, J ) 1.5 Hz, J ) 8.1 Hz), 12-14 (1H, CO-OH, singlet large) ppm. 13C NMR (D6, DMSO): δ ) 64.8 (CH2), 111.3 (C1), 122.6 (C3), 126.55 and 126.6 (C5, C7), 127.7 (C8a), 128.1 (C4), 128.6 (C4a), 129.1 (C8), 129.3 (C1a), 132.8 (C6), 136.7 (C5a), 156.7 (C2), 169.9 (CO-OH), 178.4 (CO) ppm. IR (KBr) νmax: 3430, 3057, 2975, 1736, 1633, 1590, 1473, 1414, 1323, 1262, 1220, 1102, 745 cm-1. 2-Thioxanthone-thioacetic Acid (TS). 1H NMR (D6, DMSO): δ ) 4.93 (2H), 7.6 (1H, H7, ddd, J ) 8.1 Hz, J ) 7 Hz, J ) 1.2 Hz), 7.76 (1H, H3, dd, J ) 8.6 Hz, J ) 2.3 Hz,)), 7.85 (1H, H6, ddd, J ) 8.1 Hz, J ) 7 Hz, J ) 1.5 Hz), 7.83 (1H, H4, d, J ) 8.6 Hz), 7.86 (1H, H5, dd, J ) 8.2 Hz, J ) 1.2 Hz), 8.335 (1H, H1, d, J ) 2.3 Hz), 8.465 (1H, dd, H8, J ) 1.5 Hz, J ) 8.1 Hz), 12-14 (1H, CO-OH, singlet large) ppm. 13C NMR (D6, DMSO): δ ) 34.98 (CH2), 126.86 and 126.88 (C1 and C7), 127.3 (C4), 128.2 and 128.6 (C1a and C8a), 129.1 (C8), 132.3 (C3), 133.1 (C6), 133.9 (C4a), 135.2 (C2), 136.5 (C5a), 170.3 (CO-OH), 178.1 (CO) ppm. IR (KBr) νmax: 3470, 3056, 2944, 1701, 1623, 1578, 1432, 1393, 1318, 1285, 1204, 1176, 1114, 745 cm-1. 2.2. General Techniques. The absorption measurements were carried out with a Perkin-Elmer Lambda 2 spectrometer. Steady-state fluorescence and phosphorescence spectra were collected from a FluoroMax-4 spectrofluorometer. Emission spectra are spectrally corrected, and fluorescence quantum yields include the correction due to solvent refractive index and were determined relative to quinine bisulfate in 0.05 M sulfuric acid (Φ ) 0.52).36 Fluorescence lifetimes were obtained using a nano lightemitting diode (LED) emitting at 266 nm as an excitation source with a nano led controller module, Fluorohub from IBH, operating at 1 MHz. The detection was based on a R928P type photomultiplier from Hamamatsu with a high sensitivity photoncounting mode. The decays were fitted with the iterative reconvolution method on the basis of the Marquardt-Levenberg algorithm.37 Such a reconvolution technique38 allows an overall
time resolution down to 0.2 ns. The quality of the exponential fits was checked using the reduced χ2 (e 1.2). The determination of the intersystem crossing quantum yields of chromophores (ΦISC) were performed by the triplet-triplet energy transfer method using camphorquinone (CQ) as energy acceptor (ET ) 2.23 eV, ΦISC ) 1).39 The ΦISC of thioxanthones were measured by a relative method40 comparing the integrated phosphorescence spectra of CQ excited at 355 nm in the presence of thioxanthones (A) with that in presence of benzophenone (A°) which is used as reference system. The intersystem crossing quantum yield of thioxanthones (ΦISC) is then derived from the following relation:
(1 - 10-OD)ΦISC (1 - 10-OD°)Φ°ISC
)
A A°
where OD and OD° denote the absorbances of mixing systems thioxanthone/CQ and benzophenone/CQ, respectively. All solutions were prepared in acetonitrile and were purged with argon for 15 min prior to photophysical studies. The concentration of the energy donor compounds was sufficiently high (c > 5.10-4 M) to assume a quantitative energy transfer. Finally, ΦISC° corresponds to the intersystem crossing quantum yield of benzophenone and has a value close to unity. The cyclic voltammetry experiments41 (using a computercontrolled Princeton 263A potentiostat with a three-electrode single-compartment cell; a saturated calomel electrode (SCE) in methanol used as a reference was placed in a separate compartment) were performed at 300 K, in Ar-degassed acetonitrile with a constant concentration (0.1 M) of n-Bu4BF4. Ferrocene was used as an internal reference. The bimolecular quenching rate constant of the triplet state of chromophores by MDEA was measured by laser flash photolysis. Transient absorption experiments were carried out by laser flash photolysis at the nanosecond time scale with an Edinburgh Analytical Instruments LP900 equipped with a 450 W pulsed Xe arc lamp, a Czerny-Turner monochromator, and a fast photomultiplier. The samples were irradiated with the third harmonic (λ ) 355 nm, ∼10 ns, 5 mJ per pulse) of a Nd/YAG Powerlite 9010 from Continuum. The sample concentration was adjusted to get an optical density of ∼0.3 at the excitation wavelength and purged with argon for 15 min prior to photophysical studies. The photogeneration of silver nanoparticles were achieved under irradiation at 377 nm with a laser diode from Coherent (cube type). Typically, a 1 × 1 cm quartz cell containing a solution of acetonitrile with the thioxanthone derivative (2 × 10-4 M), AgNO3 (5 × 10-3 M), and MDEA (5 × 10-2 M) is first purged with argon for 15 min then irradiated at 377 nm (P ) 9 mW). The growth of the surface plasmon band is monitored by UV-vis absorption. The transmission electron microscopy (TEM) measurement was carried out at 200 kV using a Philips CM20 instrument with Lab6 cathode.
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Figure 1. Evolution of the absorption spectra during irradiation of mixing solutions of acetonitrile containing thioxanthone derivatives (2 × 10-4 M), AgNO3 (5 × 10-3 M), and MDEA (5 × 10-2 M). All solutions are argon degassed prior to irradiation (λexc: 377 nm, P: 9 mW). Inserts: absorbance ratio between 465 and 600 nm during irradiation time. Corresponding TEM images of silver nanoparticles with their respective size distributions. (Statistical analysis performed from a population of 160 and 230 nanoparticles for TO and TS, respectively.)
X-ray photoelectron spectroscopy (XPS) measurements were performed at a residual pressure of 10-9 mbar using a KRATOS Axis Ultra energy analyzer operating with an Al KR monochromatic source. 3. Results and Discussion The electrochemical and spectroscopic data relative to each chromophore are collected in Table 1. By substitution of an electron-donor group in position 2, the charge density on the thioxanthone ring obviously increases. As a consequence, the oxidation potential of the chromophores is progressively lowered from 1.75 to 1.50 V versus the SCE when going from TX to TS. Interestingly, this substitution effect hardly affects the reduction potential of the thioxanthones, which suggests that the associated radical anion presents a strongly localized charge on the carbonyl function. The last absorption band of the thioxanthone derivatives is located in 320-450 nm range and corresponds to the superimposition of two electronic transitions which are energetically very close:42 a forbidden S0 f S2 transition with a strong nπ* character, which is mainly localized on the carbonyl function, and a strongly allowed S0 f S1 transition with a ππ* character which involves an electronic delocalization along the aromatic ring. The substitution of the donor group leads to a significant increase of the absorption
band due to a larger contribution of the ππ* transition. Moreover, the fluorescence and phosphorescence bands are both shifted to the low-energy region, leading to a significant decrease of the singlet (S1) and triplet (T1) states energies as compared with TX. The presence of the donor substituents also induces a strong fluorescence enhancement consecutive to a better energy separation of the relaxed S2 and S1 states which limits their vibronic interactions.43 The reduction of this so-called “proximity effect” hardly influences the intersystem crossing process (ISC) since both TO and TS exhibit high ΦISC which are in same range as TX. However, a strong reduction of the hydrogen abstractability of the T1 state is observed when going from TX to TS. For instance, the quenching rate constant of the T1 state by MDEA is at least divided by a factor of 10. As a 3nπ* state is considered to be highly reactive toward H-transfer,44 this effect should be ascribed to a decrease of the nπ* character of the T1 state when going from TX to TS. Moreover, it is noteworthy that the triplet state of TS singularly exhibits a very short lifetime of about 130 ns with respect to the microsecond range lifetimes measured for TX or TO in acetonitrile. This short triplet lifetime which also contrasts with that of the 2-mercaptothioxanthone (i.e., 21 µs45) was previously46 ascribed to an efficient quenching of the triplet state consecutive to a fast intramolecular charge transfer process between the substituent group and the thiox-
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Figure 2. Evolution of the absorption spectra of each sample after 2 days of storage under air atmosphere. (Samples were initially irradiated during 180 s.)
SCHEME 2: General Reaction Scheme
anthone moiety. At a low concentration range (
