Gold Micrometer Crystals Modified with Carboranethiol Derivatives

The preparation and characterization of micrometer gold and silver single-crystals of well-defined shapes are reported here. The shapes of the crystal...
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14446

J. Phys. Chem. C 2008, 112, 14446–14455

Gold Micrometer Crystals Modified with Carboranethiol Derivatives Toma´sˇ Basˇe,*,† Zdeneˇk Bastl,*,‡ Miroslav Sˇlouf,| Mariana Klementova´,† Jan Sˇubrt,† Aliaksei Vetushka,§ Martin Ledinsky´,§ Antonı´n Fejfar,§ Jan Macha´cˇek,† Michael J. Carr,¶ and Michael G. S. Londesborough† Institute of Inorganic Chemistry of the Academy of Sciences of the Czech Republic, V.V.i., 250 68 Husinec-Rˇezˇ 1001, Czech Republic, J. HeyroVsky´ Institute of Physical Chemistry of the Academy of Sciences of the Czech Republic, V.V.i. DolejsˇkoVa 3, 182 23 Prague 8, Czech Republic, The Institute of Macromolecular Chemistry of the Academy of Sciences of the Czech Republic, V.V.i. HeyroVske´ho na´m. 2, 162 06 Prague 6, Czech Republic, The Institute of Physics of the Academy of Sciences of the Czech Republic, V.V.i. CukroVarnicka´ 10, 162 53 Prague 6, Czech Republic, and The Department of Chemistry and Biochemistry, Baylor UniVersity, Waco, Texas 76798-7348 ReceiVed: March 16, 2008; ReVised Manuscript ReceiVed: June 11, 2008

The preparation and characterization of micrometer gold and silver single-crystals of well-defined shapes are reported here. The shapes of the crystals can be described as plates, polyhedra, and wires. The orientation of the crystal faces was studied using electron and X-ray powder diffraction techniques, and a (111) orientation of the large faces of gold plates was experimentally shown. The surface morphology of the crystal faces was studied by atomic force microscopy. Modifications of gold microplates with the thiolated carborane clusters 1,2-(HS)2-1,2-C2B10H10 (1), 9,12-(HS)2-1,2-C2B10H10 (2), and 1,12-(HS)2-1,12-C2B10H10 (3) are described. The carboranethiol molecules 1 and 2 show dipole moments of 4.1 and 5.9 D. In comparison, the thiolate derivative of compound 1 has a dipole moment of 4.7 D in the opposite direction to 1, and the thiolate form of compound 2 has a dipole moment of 16.7 D in the same direction. On the basis of X-ray photoelectron spectroscopy (XPS) analyses and values of work functions, we revealed that the molecules of 1 and 2 attached to the gold surface have similar electron distribution and dipole moments as within the free thiol derivatives. Following the modification of microplate gold crystals with 3, a monolayer of gold nanoparticles was attached on top of the carborane moieties. The composition of the surface species was studied using XPS. Dynamic contact angles of water on the modified gold surfaces are also discussed. Introduction Metal-organic interfaces approximating molecular dimensions have over the last few decades attracted significant attention.1 More particularly, the systems composed of two-dimensional self-assemblies2–12 are under intensive investigation because of their potential application as quantum-electronic components in molecular devices. The attachment of thiol molecules to a metal surface leads to interesting properties at the metal-organic interface either because of the inherent dipole moment of the molecules13 or through the interaction between the bare gold surface and the anchoring groups of the attaching molecules.14 Various organic molecules attached via thiol groups to gold surfaces have been described, and it is generally recognized that these molecules bind to gold surface as thiolate units.2–12 Among the thiolated hydrocarbon-based compounds, a special class is represented by aromatic thiols.12,15–17 Examples of these molecules, studied on modified metal surfaces, are positional isomers of benzene-dithiol.12,15 The structure of a benzene-1,4dithiol molecule on a gold flat surface was theoretically studied * To whom correspondence should be addressed. E-mail: (T.B.) tbase@ iic.cas.cz; (Z.B.) [email protected]. † Institute of Inorganic Chemistry of the Academy of Sciences of the Czech Republic. ‡ Institute of Physical Chemistry of the Academy of Sciences of the Czech Republic. | Institute of Macromolecular Chemistry of the Academy of Sciences of the Czech Republic. § Institute of Physics of the Academy of Sciences of the Czech Republic. ¶ Baylor University.

to reveal the electron transport through the single molecule connecting two gold electrodes.12 It was shown that both the orientation of the molecule and the anchoring groups influence the conductance of the molecular junction.12 Recently, we published a study on carboranethiol modified surfaces of gold colloids and films.18 This study provided a basis for comparison with the structure and properties of thiolated hydrocarbon-based compounds. The carboranethiol derivatives represent inorganic cluster species with a geometry derived from an icosahedron.19,20 The maternal 12-vertex icosahedral C2B10H12 cluster exists as three positional isomers, each made unique by the location of the carbon atoms in the skeleton: 1,2 (ortho-), 1,7 (meta-) and 1,12 (para-).21 The location of the two carbon atoms gives the ortho- and meta- isomers high inherent dipole moments of 4.522,23 and 2.85 D,22 respectively. These species are stable toward heating and oxidation and do not decompose under X-ray irradiation.18 Flat gold surfaces are densely packed with the carboranethiol derivatives; approximaly 8 surface gold atoms are occupied by one molecule.18 This study is a logical continuation of our previous work. The first part of this report deals with the preparation of micrometer gold single-crystals and their basic characterization. Of the three differently shaped crystals, that is, plates, polyhedra, and strips, particular attention is paid to the gold plates, as it is these that make suitable substrates for studies of self-assembled monolayers (SAMs) of the carboranethiol derivatives. These crystal plates have large and flat (111) faces characterized by severaltechniquesincludingelectronandatomicforcemicroscopies.

10.1021/jp802281s CCC: $40.75  2008 American Chemical Society Published on Web 08/15/2008

Modified Gold Micrometer Crystals We have investigated and compared the dipole moment values of the free carboranethiol derivatives and their respective thiolate forms. The experimental results show that the carboranethiol derivatives, once attached to the gold flat surface, have a similar orientation of the dipole moments as is in the free thiols. These results are supported by X-ray photoelectron spectroscopy, which is a technique used throughout the study for chemical composition analysis. The single-crystal X-ray diffraction structures of 2 and 3 are presented, and space-filling models of the single molecules placed on a (111) gold surface are shown. All three dithiol derivatives behave as dibasic acids and, therefore we discuss the extent to which one thiol group can intramolecularly influence the character of the second. The attachment of gold colloidal particles on the faces of gold microplates after treatment with 3 leads to the preparation of a molecular junction between two gold surfaces. Experimental Section AuCl3 (99.9%) and AgNO3 (99.9%) were purchased from Safina Ltd. (Czech Republic). Na2S · 9H2O (99.99+ %) was obtained from Aldrich (U.S.A.). The carboranethiol derivatives, 1,2-(HS)2-1,2-C2B10H10 (1), 9,12-(HS)2-1,2-C2B10H10 (2), and 1,12-(HS)2-1,12-C2B10H10 (3), were prepared according to the literature.24–26 Poly(vinyl pyrrolidone), K 90, powder, MR ∼ 360000 (PVP), was obtained from Fluka (Germany). All solvents were purchased as analytical grade from Lachema Ltd. (Czech Republic). An aqueous solution of citrate-stabilized gold colloids with an average diameter of 20 nm was received from British BioCell Int. Ltd. (U.K.) (product code: EM.GC20). The approximate number of gold particles per 1 mL was 7 × 1011. Gold colloidal particles linked together with 3 were prepared using the same procedure as described in the literature.27 Gold coatings used for the measuring of contact angles were obtained from Arrandee (Dr. Dirk Schro¨er, Germany) and were flameannealed before use according to the procedure reported previously.18 The measurements of the work function of the carboranethiol self-assembled monolayers were carried out on flame-annealed gold films from Platypus Technologies (Madison, WI). Sodium salts of 1, 2, and 3 were prepared as follows: To a vigorously stirred methanol (20 mL) solution of a carboranethiol derivative (100 mg of either 1, 2, or 3) was added 3.3 mL of an aqueous solution of NaOH (0.29 M) dropwise over a period of an hour. Once finished, the mixture was stirred for an additional 30 min and the solvent was then evaporated under reduced pressure. A quantitative amount (0.12 g) of the white solid product was obtained. In this report, these sodium salts are for simplicity referred to as 1-, 2-, and 3-. Single-crystal X-ray diffraction structures for 2 and 3 are provided. Attempts to grow a single-crystal of 1 suitable for X-ray analysis were not successful. Crystal Data and Structure Refinement for 9,12-(HS)21,2-C2B10H10 (2) and 1,12-(HS)2-1,12-C2B10H10 (3). Crystals suitable for single-crystal X-ray crystallography were obtained Via slow diffusion of hexane into a concentrated solution of dichloromethane. Data were collected at 110(2) K on a BrukerNonius X8 APEX CCD area-detector diffractometer, using Mo KR X-radiation (λ ) 0.71073 Å). Several sets of narrow data “frames” were collected at different values of θ for various initial values of φ and ω and using 0.5° increments of φ or ω. The data frames were integrated using SAINT;28a the substantial redundancy in data allowed an empirical absorption correction (SADABS)28a to be applied, based on multiple measurements of equivalent reflections. Structures were solved by direct

J. Phys. Chem. C, Vol. 112, No. 37, 2008 14447 methods using SHELXS-9728b and refined by full-matrix leastsquares on F2 using SHELXL-97.28c All non-hydrogen atoms were assigned anisotropic displacement parameters. The locations of the cage carbon atoms were verified by examination of the appropriate internuclear distances and the magnitudes of their isotropic thermal displacement parameters. All hydrogen atoms were set riding on their parent atoms in calculated positions and were assigned calculated isotropic thermal parameters [Uiso(H) ) 1.2 × Uiso(parent)]. 9,12-(HS)2-1,2-C2B10H10 (2): C2H12B10S2, M ) 208.34, monoclinic (colorless prism, 0.26 × 0.25 × 0.16 mm), space group P21/c, a ) 12.154(3), b ) 7.6888(19), c ) 13.983(4) Å, β ) 123.342(18)°, V ) 1091.6(5) Å3, Z ) 4, µ ) 0.425 mm-1, Rint ) 0.0553, R1 ) 0.055 for 2885 reflections with I > 2σ (I) and wR2 ) 0.1261 for all 3559 reflections collected. CCDC reference number 672252. 1,12-(HS)2-1,12-C2B10H10 (3): C2H12B10S2, M ) 208.34, monoclinic (colorless plate, 0.25 × 0.16 × 0.05 mm), space group P21/c, a ) 7.102(3), b ) 7.166(3), c ) 11.490(4) Å, β ) 113.86(2)°, V ) 534.8(4) Å3, Z ) 2, µ ) 0.434 mm-1, Rint ) 0.0437, R1 ) 0.044 for 1016 reflections with I > 2σ (I) and wR2 ) 0.1531 for all 1225 reflections collected. CCDC reference number 672253. Gold and Silver Single Crystals. A modified procedure which is generally referred to as “poly ol” was used.29–31 The Preparation of Gold Single-crystal Microplates. A 3.34 g sample of PVP and 3.17 g of AuCl3 were separately dissolved, both in 75 mL of 1,3-propanediol. The dissolution of PVP required gentle heating. These two solutions were then mixed together, resulting in the precipitation of a yellow solid. With vigorous stirring, this mixture was placed in an oil bath heated to 165 °C. The yellow solid dissolved in ∼10 min to give a bright orange solution that was stirred with continuous heating for an additional 2 h. After ∼30 min, a suspension of micrometer gold crystals could be observed with a characteristic gold shimmering. Once finished, the oil bath was removed and the mixture stored at room temperature for further use. The gold crystals can be decanted with methanol thus replacing 1,3propanediol by a less viscous solvent. The Preparation of Gold Single-Crystal Micropolyhedra. The use of the procedure described above with approximately one-half the amount of AuCl3 (1.5 g), the same amount of PVP (3.34 g), and glycerol as a solvent led to the formation of polyhedral gold crystals (Figure 1, picture B). The Preparation of Gold Single-Crystal Microwires. The use of 1,5-pentanediol as a solvent, instead of 1,3-propanediol, and the same experimental conditions as described in the procedure for the preparation of microplates led to a mixture that contained in addition to the microplates a significant number of microwires. The suspension can be further enriched for wires by faster sedimentation of the plates. The experimental details for the preparation of silver microwires and polyhedra are covered by an international patent application.32 Self-Assembly Processes on Gold Crystal Plates. One drop of a methanol suspension of the gold crystal microplates was placed on a glass or mica slide of dimensions approximately 1 cm × 1 cm, and the solvent was allowed to evaporate at room temperature (sample A). The crystals strongly adhered to the substrate surface and were not removable by rinsing in various solvents (e.g., water, methanol, dichloromethane, hexane, acetone). This procedure ensured the secure deposition of the crystals on the glass or mica slides and simplified their further treatment. The deposited crystals were further treated as follows:

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Figure 1. SE micrographs of (A) gold microplate crystals, (B) gold micropolyhedra, and (C) silver microwires. (D) SE micrograph showing very thin plates. (E) The geometrical shapes of the polyhedral crystals mostly observed. (F) SE micrograph showing gold strips.

A substrate slide with the deposited gold crystals was annealed with a hydrogen flame to remove any remaining molecules of PVP and subsequently immersed in a CH2Cl2 solution (5 mL) of 1, 2, or 3 (123 mg, 0.6 mmol) for an hour. The samples were finally rinsed with an excess of the pure solvent and dried in a stream of Ar. These samples are further referred to as 1 SAM, 2 SAM, and 3 SAM. Gold Plates Modified with Gold Colloids. A drop of a gold colloidal solution (British BioCell Int., EM.GC20) was placed on a sample of A and left for 18 h. In order to avoid premature drying and to maintain a consistent volume, further drops were added during this time. After this period of 18 h, the slide was rinsed in an excess of distilled water and dried in a stream of Ar (sample B). To prepare sample C, a sample of A was first modified with 3 (analogically to 3 SAM but without annealing) and subsequently with a gold colloidal solution using the same method as described above for the preparation of B. Modification of Gold Flat Films. A macroscopic flameannealed gold film purchased either from Arrandee (Germany) or Platypus Technologies (Madison, WI) was immersed in an aqueous solution (5 mL) of PVP (0.51 g), or sodium sulfide (0.51 g), or 1, 2 or 3 (0.12 g) for an hour. The sample was then rinsed with an excess of pure solvent and dried in a stream of Ar. Methods Atomic Force Microscopy (AFM). Measurements were performed in the contact mode using a scanning probe microscope Digital Instruments Nano-Scope III (Santa Barbara, CA, USA) equipped with a silicon nitride cantilever NP-S Veeco (Camarillo, CA, USA) with spring constant 0.12 N m-1. The images were taken under ambient conditions on air. The scan rate was set to 0.5 Hz (images A and C), 0.3 Hz (image B), and 4 Hz (image D). The operating set point was 2 V. High Resolution Transmission Electron Microscopy (HRTEM). Transmission electron microscopy (TEM) was carried out on a JEOL JEM 3010 microscope operated at 300 kV (LaB6 cathode, point resolution 1.7 Å). Images were recorded on a CCD camera with resolution 1024 × 1024 pixels using the Digital Micrograph software package. A drop of a dilute

suspension was placed on a holey-carbon-coated copper grid and allowed to dry by evaporation at ambient temperature. Scanning Electron Microscopy (SEM). The micrographs were collected with a microscope Quanta 200 FEG (FEI, Brno, Czech Republic), which is equipped with a field emission gun. The samples were observed using secondary electrons in lowvacuum mode (pressure inside the chamber was 50 Pa) at 30 kV and with the smallest spot size to minimize sample damage and maximize resolution. X-ray Photoelectron Spectroscopy. The photoelectron spectra were measured using an ESCA 310 (Scienta, Sweden) electron spectrometer equipped with a rotating anode, quartz crystal monochromator, and a hemispherical electron analyzer operated in a fixed transmission mode. All measurements were done with the experimental setup reported previously.18 Work Functions (WF). WF were measured using a Kelvin Probe (KP Technology Ltd., U.K.) equipped with KP 4.7 software. Contact Angle Meter. The dynamic contact angles of water on macroscopic gold coatings (Arrandee, Germany) were measured using an OCA20 instrument (DataPhysics Instruments GmbH, Filderstadt, Germany) equipped with 6-fold zoom lens (0.5-4.5-fold magnification) with an integrated continuous fine focus (+(-) 6 mm), high light transmitting capacity CCDcamera with a resolution of maximum 768 × 576 pixels, and a high performance image processing system with 132 Mbytes/s data transfer rate. The system is equipped with SCA 20 software. Computational Methods. The dipole moments of the carboranethiol derivatives, 1 and 2, and their respective thiolate forms, 1- and 2-, were calculated using the Gaussian 98 package.33 The geometries of the species were optimized first in the Hartree-Fock approximation34 and then at the second order of Møller-Plesset perturbation theory.35 Dipole moments are reported for both the HF and MP2 optimized structures at the respective levels of theory. In all the calculations, a 6-31G* basis36 of Gaussian type orbitals with polarization functions at all the non-hydrogen atoms was used. The dipole moments were also calculated using a Dalton package.37

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Figure 2. TEM observation. (A) Gold plate (up) and strip (down) and the corresponding electron diffraction patterns showing a (111) orientation of the top face. (B) Gold wire with the respective selected-area diffraction patterns.

Results and Discussion Experiments with the carboranethiol species described in this report were conducted either on macroscopic gold films or gold single-crystals. The gold single-crystals were especially prepared for this purpose, and therefore their basic characterization is provided. The crystal shapes reported in this study correspond to those observed in the literature29–31 and can be described as triangular or hexagonal plates, polyhedral crystals, and wires (Figure 1, panels A, B, and C). However, the relatively high concentrations of Au3+ ions that we used in our preparations lead to the suspensions of gold micrometer crystals while the literature to date deals mainly with colloidal solutions. We also observed the formation of gold crystals without any PVP in the reaction mixture. The absence of PVP resulted in the formation of crystal aggregates, and thus the role of PVP molecules is to separate individual crystals and avoid the aggregation. According to the literature,29–31 a colloidal form of these crystals, their shapes and dimensions, can be effectively influenced by both the presence and amount of PVP. The micrometer dimensions of the crystals also simplify the investigation of their shapes. We used both scanning and transmission electron microscopes to follow some selected aspects of the crystalline objects. We observed that the gold crystals preferably grow either as plates or polyhedra. The plates have top faces 10 to 80 µm large and their thickness is about 300 nm. The preferred geometry of the polyhedral crystals can be derived from a cuboctahedron (Figure 1E, top); however some other geometrical motifs have also been observed, such as a pentagonal pyramid shown at the bottom in Figure 1E. This shape, and its potential mechanism of growth, is more thoroughly discussed in the literature.38 The X-ray diffraction (XRD) pattern, typical of gold plates, exhibits preferred orientation with enhanced intensity of a 111 diffraction line. This corresponds to large {111} faces of the plates. In comparison, the XRD pattern of gold polyhedral crystals resembles a polycrystalline gold sample. Both XRD patterns are shown as insets in panels A and B of Figure 1 and also indicate the face centered cubic (fcc) structure of the studied crystals. Gold crystals can also grow in the form resembling wires. The majority of the prepared wires formed in straight-lines, although a small fraction of them were bent in various ways as shown in Figure 1F. Their width was usually in the range 0.5-1 µm, thickness was approximately 300 nm, and length was from 10 to 200 µm. With respect to

these dimensions, these gold wirelike crystals can be considered rather as thin strips cut along the edge of the crystal plates. A small fraction of the wires were observed that displayed a pentagonal cross-section. The electron diffraction analysis shows that the top faces of both the crystal plates and the thin strips are (111) oriented (Figure 2). Regarding the pentagonal crosssection wires, the electron diffraction analysis shows superposition of κ and R crystallographic orientations. This observation is consistent with five {111} boundaries arranged radially to the [101] directions of elongation and is in agreement with the results described elsewhere.39 The preparation of silver microcrystals resulted entirely in the formation of wires and polyhedra similar to those of gold whereas plates were not observed. Figure 3B shows the surface of a gold crystal plate modified with PVP molecules, which form spherical objects approximately 50 nm in diameter. A comparison with the literature shows that each of these spherical objects should be a single PVP molecule.40 To remove this polymer surfactant from the surface, gold crystals deposited on a glass or mica slide were annealed with a hydrogen flame. XPS showed the disappearance of carbon, and AFM showed a flat surface with only a few monatomic steps (Figure 3A). These bare gold crystals were subsequently modified with the carboranethiol derivatives 1, 2, or 3. Attempts were made at obtaining single-crystals of species 1, 2, and 3 so as to obtain geometric information via X-ray diffraction studies. Although it proved difficult to obtain suitable crystals for 1, the solid-state structures of 2 and 3 were established by single-crystal X-ray diffraction experiments and are shown in Figure 4. Further details of these studies can be found in the Supporting Information. Carboranethiol Derivatives. The carboranethiol derivatives 1 and 2 are schematically shown in Figure 5 along with their respective thiolate forms. The maternal carborane cluster, 1,2C2B10H12, is one of three positional isomers of the icosahedral cluster C2B10H12 proposed by Hofmann and Lipscomb in 1962.21 Figure 5 shows also a schematic of 1,2-C2B10H12 and its numbering system. As in the molecules of 1 and 2, there are four different boron atom environments in this molecule. The electron density located at boron vertexes generally increases with increasing distance from the carbon atoms. Therefore, boron atoms at the 9, 12 and 8, 10 positions are the most negatively charged while the most positive boron vertexes are

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Figure 3. AFM image of (A) the gold crystal surface after annealing with a hydrogen flame, (B) the gold crystal surface in a dry state with PVP molecules, (C) 20 nm gold colloids on mica surface, and (D) the crystal microplate surface modified with Au colloids. The white dashed lines indicate positions of the cross section analyses.

Figure 4. Structures of 2 and 3 as determined by single crystal X-ray diffraction studies. Thermal ellipsoids are drawn with 40% probability, and the crystallographic labeling schemes are shown. Symmetry code (a) for 3: -x, -y, -z - 1.

Figure 5. Carboranethiol molecules 1 and 2 and their respective thiolate forms, 1- and 2-. The arrows along the C2 axis of the molecules show the orientation of the dipoles. Hydrogen atoms at vertexes of the carborane icosahedra have been omitted for purposes of clarity. The larger black dots at vertexes indicate the positions of carbon atoms. Picture on the right is a schematic representation of 1,2-C2B10H12 isomer and numbering of positions at its vertexes.

at positions 3 and 6, next to carbon atoms (Supporting Information). The dipole moment value for 1,2-C2B10H12 was calculated to be 4.53 D22 and experimentally determined to be 4.50 D.23 The positive part of this molecular dipole is located near the carbon atoms, and the negative part of the molecule is at the opposite 9 and 12 boron vertexes. The charge distribution

provides an explanation for the trends observed in the experimentally determined acidities discussed at a later stage in this study and also helps to understand the dipole moment values of the studied molecules. The dipole moment values of the carboranethiol derivatives and their respective thiolate forms are displayed in Table 1.

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TABLE 1: Dipole Moment Values (in Debye) for Carboranethiol Derivatives and the Respective Thiolate Formsa 1 12 2-

HF

MP2

Dalton

4.07 5.67 5.92 16.75

4.10 4.75 5.88 16.45

4.12 4.65 5.88 16.05

a HF, Hartree-Fock approximation (G03, HF/6-31G*); MP2, Møller-Plesset theory (G03, MP2/6-31G*); Dalton, Dalton package (MP2/6-31G*).

TABLE 2: The pKa Values of the Three Studied Dithiols and a Comparison with their Respective Monothiol Analogues pKa41

pK1, pK2 1 2 3

4.47, 8.87 5.5, 10.45 6.4, 10.3

1-(HS)-1,2-C2B10H11 9-(HS)-1,2-C2B10H11 1-(HS)-1,12-C2B10H11

3.30 10.08 5.85

The electronegative sulfur atoms generally attract electrons to themselves. Thus, the dipole moment value of 1 in which thiol groups are attached to the “positive pole” of the maternal molecule decreases by 0.4 to 4.1 D in comparison with the maternal 1,2-C2B10H12. Concordantly, an increase of the dipole moment value for about 1.4 D to ∼5.9 D was observed for 2 in which thiol groups are attached to the cluster at the negative 9 and 12 boron vertexes. With regards to both carborane thiolate derivatives 1- and 2-, their dipole moments are orientated with the negative pole at the thiolate sulfur atoms. There is a synergic effect between the negative charge associated with thiolate sulfur atoms and the negative part of the carborane skeleton in 2-. This leads to the dipole moment value of approximately 16.7 D. It is worthy to note that the orientation of this dipole moment remains the same as in 2, and its value shifts for about 10.8 D. The thiolate sulfur atoms in 1- are attached at positions 1 and 2 and, therefore, the negative charge associated with them is attached to the positive pole of the maternal molecular dipole 1,2C2B10H12. This arrangement turns over the orientation of the dipole in comparison with 1 and gives the thiolate molecule the value of 4.7 D. This theoretical investigation affords a question: To what extent might these changes also occur on gold surfaces? According to the binding energies of boron, carbon, and sulfur atoms in the cluster, we concluded that the situation on a gold surface is similar to that with free thiol derivatives. This is further discussed in the XPS Results and supported by a comparison of the charge distributions within the carboranethiol derivatives and their respective thiolate forms and also by work functions measured of the modified gold films. All three dithiol derivatives 1, 2, and 3 behave as dibasic acids. The values of pK1 and pK2 provide the information about the strength of the thiol groups and exhibit how effectively the thiol groups are linked through the quasi-aromatic carborane cage. The data are summarized in Table 2, and a comparison with the respective monothiol derivatives is provided. The higher pK1 and pK2 values of derivative 2 in comparison with 1 indicate that 2 is the weaker acid. In 2, thiol groups are attached at the negative 9 and 12 vertexes at which the negative charge of thiolate sulfur atoms is less effectively stabilized by its delocalization. It follows that a thiol group in 1 becomes a thiolate more preferably because the negative charge from the thiolate sulfur atoms is stabilized by delocalization within the positive pole of the maternal dipole. The pK2 for all three

Figure 6. The side view of space filling models of 2 and 3 on a gold (111) surface.

derivatives is increased significantly to higher value in comparison to pK1. It indicates that a change of the character of one thiol group influences significantly the character of the second. The molecules of 1 and 2 are both attached to a gold flat surface through both thiol groups, as reported recently.18 Such an arrangement provides a nearly perpendicular orientation of the dipoles to the surface, along the 2-fold symmetry axis (C2) of the molecules. On the other hand, the 5-fold symmetry axis (C5) of a molecule of 3 can be tilted from the surface normal, as we show in Figure 6. Still, the steric demands of the icosahedral skeleton and the hydrogen atoms at its vertexes counteract this tilt. The symmetrical arrangement of the carbon atoms in 3 at the opposite 1 and 12 positions results in a zero dipole moment of the free dithiol molecule. Also, the acidity of the SH groups is lower in comparison with 1 (see Table 2). The same effect was observed with the acidities of the respective monothiol derivatives (Table 2).41 The modification of gold surfaces with 3 provides free thiol functional groups at the surface for additional bottom-up assemblies of gold colloidal particles as discussed further. Gold Colloidal Assemblies. Gold particles with an average diameter of ∼ 20 nm were used to form an overlayer on a gold crystal surface premodified with 3. The carborane moiety 1,12C2B10H12 is a nearly regular icosahedron, and a molecule of the respective dithiol derivative should ideally bind to the gold crystal surface with only one of the thiol groups.18 XPS analysis of the gold crystal microplates modified with 3 showed two different types of sulfur atoms with mutual ratio 1:1 (Figure 7A, Table 4, B). The additional modification of the gold crystal surface with gold colloids shifts the thiolate/thiol ratio in the favor of the thiolate (Figure 7B, Table 4, C). It can only be rationally explained by the formation of new thiolate bonds between the free thiol groups oriented from the surface upward and the approaching gold particles. A scanning electron micrograph of the gold particles attached to the gold crystal surface is shown in Figure 8. The distance between gold crystal surface and a colloidal particle is 1.2 nm, which is the theoretical length of the carboranethiolate linker. To visualize this molecular distance, we prepared oligomers of the gold colloidal particles according to the literature.27 An example of a colloidal dimer with the respective molecular distance between two particles is shown in Figure 8B, upper inset. XPS Results. XPS results are summarized in Tables 3 and 4. To avoid confusion, we discuss the data in two parts: first, the thiolate derivatives 1-, 2-, and 3-, and the gold crystal microplates modified with 1, 2, and 3, and second, the gold crystals as prepared and subsequently modified with 3 and a gold colloidal solution. A good agreement between the measured chemical composition of the disodium salts of 1-, 2-, and 3- and their nominal stoichiometry was observed. The XP spectra of S 2p electrons in 2- and 3- exhibit a minor component with the binding energy

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Figure 8. SE micrographs of the gold colloids attached to gold crystal plate modified with (A) PVP and (B) PVP and 3. HRTEM image of two gold colloidal particles linked with 3 is in the upper inset. The schematic of 3 is in the bottom inset.

Figure 7. XP spectra of S (2p) electrons for (A) gold crystal surface modified with 3 and (B) modified additionally with a monolayer of gold colloids.

TABLE 3: Elemental Concentrations as Determined from XPS Analysis of Carboranethiolate Derivatives and Au Crystals Modified with PVP and by Carboranethiols (Atomic Concentrations)a sample

B

CCBb CHC

S

1231 SAM 2 SAM 3 SAM [Au(C2B10H10S2)2]N+Bu4 Au film with sulfide ions A B C

10 10 10 10 10 10 21.8

2.0 1.2 2.3 1.0 2.0 1.1 2.7 4.5 3.1 5.1 2.7 3.0 9.0a 19.0

2.0 2.0 2.0 2.0 1.6 2.1 4.0

1.0 3.4b 1.0 1.4b

Au

O

Na

N

2.5 2.8 0.73 2.1 12.4 11.5 12.4 1.0

0.6 0.1

1

0.2

8.7 5.8 0.26 3.3 0.27

4.2 2.2 7.2 4.6 2.5 2.0

1.0 1.2 0.4

a CCB, carbon atoms from carboranes; CHC, carbon atoms from hydrocarbons. A: Au single-crystal micro-plates stabilized with the molecules of PVP. B: Au single-crystal micro-plates treated with 3. C: Au single-crystal micro-plates treated with 3 and additionally with gold colloids. SAM: self-assembled monolayers on the Au crystal micro-plates. b The values do not correspond to nominal stoichiometry of the carborane cluster because of the C-OH and C-N moieties in which the C 1s electrons have the same binding energy as the CCB.

values of 163.4 and 163.7 eV. This component represents ∼18% of the total sample and can be assigned to disulfide molecules, which are a product of partial oxidation. XPS provided information about the electron densities associated with boron, carbon, and sulfur atoms and therefore further contributed to our understanding of the molecules 1, 2 and 3 attached to a gold flat surface. Our theoretical investigation of the dipole moments described so far in this report has provided basic information about the orientation and values of the dipoles in free thiols, 1 and 2, and their respective thiolate forms, 1- and 2-. The results of XPS analysis of 1 and 2 selfassembled on gold microplates (Tables 3 and 4) or on macroscopic gold coatings18 resemble the free molecules, 1 and 2, rather than the disodium thiolate salts of 1- and 2-. The S 2p electrons in 2- have the binding energy of 161.2 eV, which is 0.6 eV less than the S 2p electrons in a thiolate moiety of 2

on gold single-crystal microplates (Table 4). This indicates a lower electron density associated with the sulfur atoms in the thiolate moieties on the gold microplates than in the thiolate sodium salts. It can be explained as follows: after the attachment of the carboranethiol derivatives to the gold surface, the negative charge from the sulfur atoms can be delocalized within the bulk metal as well as in the “pseudo” aromatic carborane skeleton. There is an experimental aspect related to this explanation; the measured binding energy of B1s electrons in the carborane clusters attached via sulfur atoms to the gold surface (2 SAM, Table 4) is 189.3 eV, a value smaller than in 2- by 0.7 eV. According to this comparison, the boron atoms in 2 assembled on a gold surface bear higher electron density than in 2-. This feature is in agreement with the calculated charges of the boron vertexes in 2 and 2- (see the Supporting Information). In 2, there is a higher negative charge associated with boron atoms, especially those localized at the 9 and 12 positions, than in 2-. The same trend is observed with the molecules of 1 and 1-. To further investigate the character of the studied species, we measured the binding energy of S 2p electrons on a macroscopic gold film modified from an aqueous solution of sodium sulfide. These “surface-sulfide” atoms have the same binding energy of S 2p electrons as sulfur atoms in 2 SAM, 161.8 eV (Table 4). In comparison, the binding energy of S 2p electrons in 1 SAM is shifted toward a higher value, 162.4 eV, than in 2 SAM by 0.6 eV. This increase can be attributed to the opposite orientation of the dipole in 1 than is in 2. The effect of the dipole orientation has been discussed earlier in this report in association with the acidities of the SH groups. In accordance with the discussion provided so far, the positive pole, located in 1 SAM close to a gold surface and sulfur atoms, can more effectively delocalize the negative charge associated with the sulfur atoms within the pseudo aromatic carborane cage. Consequently, there is a smaller negative charge on sulfur atoms in 1 SAM than in 2 SAM, and therefore the binding energy of S 2p electrons observed in 1 SAM is higher than in 2 SAM also. It is worthy to note that the character of S 2p electrons in 1 SAM is very similar to that in bulk gold (I) sulfide18 in which the S-Au bonds are generally considered as covalent and the material itself is a good conductor.42 The XPS analysis of a square-planar complex of Au3+ with two molecules of 1 as ligands ([Au(C2B10H10S2)2]- N+Bu4)43 is shown for comparison. In this compound, a molecule of 1 is attached to a single gold atom in an oxidation state of 3+. The value of the binding energy of S 2p electrons is higher than in 1 SAM. This indicates a smaller negative charge on sulfur atoms in the complex, and in comparison to bulk gold can be explained by a higher positive charge localized on a single gold atom. The gold and silver crystals are, as prepared, stabilized by molecules of PVP. The measured XP spectra of the C 1s

Modified Gold Micrometer Crystals

J. Phys. Chem. C, Vol. 112, No. 37, 2008 14453

TABLE 4: Measured Core Level Binding Energies and fwhm (in eV)a sample 123-

Na 1s

Au 4f7/2

1071.9 (1.8) 1072.3 (2.6) 1072.2 (2.0)

B 1s

S 2p3/2

C 1s

189.3 (2.1) 190.0 (2.3) 189.5 (1.6)

162.4 (1.8) 161.2 (2.4) 163.0 (2.4) 161.6 (2.1) 163.7 (2.1) 162.4 (2.2) 164.7 (2.2) 161.8 (1.7) 163.3 (1.7) 161.6 (1.8) 163.6 (1.8) 162.9 (1.5)

286.5 (1.8) 286.8 (2.3) 286.1 (1.5)

1 SAM

84.0 (1.3)

188.7 (1.9)

2 SAM

84.0 (1.2)

189.3 (1.8)

3 SAM

84.0 (1.2)

189.2 (1.5)

[Au(C2B10H10S2)2]- N+Bu4

86.8 (1.7)

189.4 (2.1)

Au film with sulfide ions

84.0 (1.2)

A

84.0 (1.2)

B

84.0 (1.2)

189.3 (1.7)

161.6 (1.9) 164.2 (1.9)

C

84.0 (1.2)

189.8 (1.9)

161.4 (2.0) 163.6 (2.0)

161.8 (2.0)

284.3 (1.7) 286.0 (1.7) 284.5 (1.8) 286.3 (1.8) 284.4 (1.6) 285.9 (1.6) 284.8 (2.1) 286.5 (2.1) 284.4 (2.1) 286.5 (2.1) 285.4 (1.9) 286.3 (1.9) 288.3 (1.9) 285.2 (1.9) 286.2 (1.9) 288.2 (1.9) 285.2 (1.9) 286.3 (1.9) 288.3 (1.9)

a A: Au single-crystal micro-plates stabilized with the molecules of PVP. B: Au single-crystal micro-plates treated with 3. C: Au single-crystal micro-plates treated with 3 and additionally with gold colloids.

TABLE 5: Dynamic Contact Angles of Water (in Degrees)

electrons resemble those of free PVP. They exhibit at least three components due to different types of carbon atoms in the structure of PVP. The first component can be attributed to carbon atoms in C-C and C-H bonds, the second can be assigned to carbon atoms in C-N and C-OH bonds, and the binding energy of the third component is consistent with carbon atoms in N-CdO bonds. The measured values of the C 1s core level binding energies of the individual components are displayed in Table 4. The spectra of C 1s electrons of the gold crystal microplates treated with the carboranethiol derivative 3 still exhibit components characteristic of the first sample. This finding indicates that substitution of the PVP molecules by 3 does not proceed completely under the used experimental conditions. The spectrum of the C 1s electrons, measured for the crystals treated with 3 and additionally modified with the gold colloids, is also dominated by contributions from PVP. Consequently, we could not exclude that some of the gold colloidal particles are bonded to PVP remaining on the crystal surface. To evaluate this hypothesis, we carried out an experiment in which a gold crystal was modified with gold colloids without the previous treatment with 3. The SE micrographs, of which typical representatives are shown in Figure 8, proved that there is a cumulative effect between PVP and molecules of 3 in anchoring the gold colloids to the crystal surface. Using the procedure described in the Experimental Section earlier on, the surface of gold plates modified only with PVP had the average of 570 colloidal particles per 1 µm2, and gold plates modified with PVP and additionally with 3 had the average of 1160 particles per 1 µm2. Work Functions. The Kelvin measurement technique was used to obtain the work function values of both the bare gold coatings and those modified with 1 or 2. The orientation and value of the dipoles attached to the gold surface influence the value of the work function. A value of 5.31 eV for a bare (111) gold crystal face was reported in the literature.44 Our experimental value, measured on a macroscopic flame-annealed gold coating, was 5.35 eV, which is in good agreement with the published value. Generally, a SAM of molecules oriented with

sample bare Au film Au film with Au film with Au film with Au film with Au film with

1 2 3 PVP sulfide atoms

θA

θR

93.6 84.9 75.3 88.2 51.0 90.0

57.7 49.3 20.1 41.3 15.3 44.4

the negative pole from the surface upward increases the work function of the respective gold surface. Fittingly, we observed an increase of the WF at 1 assembled on the gold flat surface by 0.29 to 5.64 eV and a decrease at 2 by 0.09 to 5.26 eV. These results qualitatively confirm our previous discussion about the dipole moments of 1 and 2 on a gold surface. Dynamic Contact Angles. Dynamic contact angles were measured on macroscopic gold films. Recently we have reported the static contact angles of carboranethiol modified gold films.18 Herein we discuss the dynamic contact angles of water on a bare gold film and films modified with PVP, sulfide ions and the carboranethiol derivatives 1, 2, and 3. The values of advancing (θA) and receding (θR) angles are displayed in Table 5. PVP molecules contain polar groups and are thus easily soluble in water. Accordingly, a modification of gold surface with molecules of PVP engenders a hydrophilic surface with θR 15.3°. Formally, the self-assembled monolayers of 1, 2, or 3 can be described as a monolayer of sulfide atoms and a monolayer of carborane clusters additionally attached. A gold surface modified from an aqueous solution of sodium sulfide is only slightly less hydrophobic than that of a bare gold film (Table 5). Molecules of the carboranethiol derivatives are easily soluble in nonpolar solvents because of their hydrophobic character. Hence, the modifications of gold films with 1 or 3 engender surfaces with the receding angles of 49.3 and 41.3°. In comparison with the gold surfaces modified with PVP, both are shifted toward a hydrophobic character similar to that of bare gold. However, a significant difference from bare gold is observed in 2 SAM. In agreement with our previous discussion,18 the CH vertexes are acidic and can readily interact with

14454 J. Phys. Chem. C, Vol. 112, No. 37, 2008 the molecules of water. This fact is responsible for a significantly more hydrophilic surface with θR 20.1°. The space filling model of 2 on a gold (111) surface is shown in Figure 6. This molecule has C-H vertexes, indicated with gray-white color, oriented from the surface upward. The difference of ∼30° in the receding angles on a gold film modified with 1 (θR 49.3°) and 2 (θR 20.1°) demonstrates a very different character of BH and CH vertexes. Conclusions Continuing our previous study,18 herein we showed that micrometer-sized crystals can be advantageously used as novel types of supports. We prepared gold and silver micrometer crystals of three geometrical shapes, plates, polyhedra, and wires, using a modified “poly ol” procedure. In comparison to the traditional poly ol method producing colloids,29-31 our modified process gives suspensions. The micrometer dimensions of the crystals in these suspensions afford several advantages over their more usual colloidal form, in particular easier mechanical manipulation and larger flat faces. We described here the immobilization of carboranethiols, 1,2(HS)2-1,2-C2B10H10 (1), 9,12-(HS)2-1,2-C2B10H10 (2) or 1,12(HS)2-1,12-C2B10H10 (3), on large {111} oriented faces of the gold microplates produced by our modified “poly ol” method as a new type of support. The dipole moments of the carboranethiol molecules 1 and 2 were discussed here in detail and compared with those of their respective thiolate forms. Calculated charges associated with specific cluster vertices provided a basic perspective of these molecules and were complimentary to the discussion about dipole moment values. The work function values of modified gold 111 surfaces changed fittingly with the orientation of molecules 1 or 2 anchored to the surface. Derivative 3 exhibited on larger flat faces of the gold microplates the ideal mutual ratio of the thiol to thiolate sulfur atoms according to its nominal stoichiometry. The rigid molecular architecture and thiol groups attached to the 1 and 12 vertices made this derivative a suitable molecular linker for the anchoring of gold colloids to the crystal surface. This opens the possibility for the preparation of 3-D colloidal networks assembled on a gold crystal surface. The work described here suggests that carboranethiol molecules can potentially be used for tuning the properties of precious metal surfaces according to requirement. Additionally, they might be used as components of molecular electronics, introducing a dipolar character into circuits. Gold and silver crystals can be regarded as a transitional phase in the structural continuum between microsized objects and simple molecules. Gold and silver wires can be used as conductive leads, and for this purpose their specific electrical resistances were measured. Acknowledgment. We thank the Ministry of Education, Youth, and Sports of the Czech Republic (Grants LC06041 and 203/06/P398) and the Grant Agency of the Academy of Sciences of the Czech Republic (Grants KAN400480701, 1ET400400413,IAA400320601,andProjectNos.AVOZ40320502 and AVOZ40500505) as well as the Robert A. Welch Foundation (Grant AA-0006) and Baylor University for financial support. We thank to Dr. Natalya Murafa for HRTEM analyses, Dr. Petr Bezdicˇka for XRD analyses, and Dr. Jaromı´r Plesˇek for fruitful discussions. Supporting Information Available: Full details of the crystal structure analyses in CIF format. The calculated charges (Gaussian 03 NPO/MP2/6-31G*, Gaussian 03 NPO/

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