Morphology Control and Structural Characterization of Au Crystals

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Morphology Control and Structural Characterization of Au Crystals: From Twinned Tabular Crystals and Single-Crystalline Nanoplates to Multitwinned Decahedra Li Zhang,† Cheng Zhi Huang,*,†,‡ Yuan Fang Li,† and Qing Li§

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 3211–3217

Education Ministry Key Laboratory on Luminescence and Real-Time Analysis, College of Chemistry and Chemical Engineering, College of Pharmaceutical Sciences, and College of Materials Science and Engineering, Southwest UniVersity, Chongqing 400715, China ReceiVed NoVember 17, 2008; ReVised Manuscript ReceiVed April 21, 2009

ABSTRACT: In this contribution, a general one-step route to synthesize Au crystals with the shape of twinned tabular crystals, single-crystalline nanoplates, and multitwinned decahedra is proposed. By employing HAuCl4 in an aqueous medium as the oxidizing reagent, tetracycline hydrochloride as the reducing agent, and cetyltrimethylammonium bromide (CTAB) as the capping agent, we found it is very easy to get the three types of Au crystals by modulating the molar ratio of CTAB with HAuCl4. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-resolution TEM (HRTEM) have been employed to characterize the three types of symmetric morphologies. Further investigations involve side-face analysis and growth mechanism of Au twinned tabular crystals, and the optical properties of the obtained crystals. Absorption and dark field light scattering images demonstrate their potential applications in cancer cell diagnostics and photothermal therapy. Moreover, a tentative explanation for the growth mechanism of Au crystals with different morphologies has been made.

1. Introduction 1

2

The size- and shape-dependent optical, electronic, and catalytic properties3 of metal crystals have intriguing applications in photonics,4 catalysis,5 biological sensing,6 chemical sensing7 and electronic devices.8 Up to now, a great deal of effort has been devoted to the control of particle size and shape in order to tune their properties, and thus a series of shapes including spheres,9 rods,10 wires,11 belts,12 plates,13 flowers,14 branched structures,15 and polyhedrons16 have been prepared through various routes or approaches, but there have been no general synthetic routes and involving mechanisms reported thus far. In the past decades, the synthetic approaches have mainly focused on the growth in vacuum and a series of advanced nanostructures such as cubes, octahedra, rhombic dodecahedra,17 decahedra,18 and tabular crystals19 have thus been obtained with low yields and poor monodispersity. Solution-phase methods were systematically proposed a few years ago with surfactants or polymers as capping agents, as well as the addition of some inorganic species to control the anisotropic growth, and thus various morphologies from single-crystalline to multitwinned crystals can be obtained.20,21 For example, Xia and co-workers have proposed a polyol process for shape control in metallic nanostructures in the presence of poly(vinyl pyrrolidone) (PVP). The excellent features of polyol facilitate the synthesis of various shapes of nanostructures,11,13 and additional inorganic species such as NaCl, HCl, and NaBr could substantially enhance the yield of some specific shapes.22 Despite continuous efforts up until today, the mechanism underlying the nucleation is still unclear, and thus rational control of crystal structure is still a major challenge. Although some success has been shown in fabricating decahedra and icosahedra,23,24 experiments demonstrate that it is difficult to * Corresponding author. E-mail: [email protected]. Fax: +86 23 68866796. Tel: +86 23 68254659. † College of Chemistry and Chemical Engineering. ‡ College of Pharmaceutical Sciences. § College of Materials Science and Engineering.

establish a general synthetic route and mechanism for the twinned crystal preparation. Take twinned tabular crystals as an example, research development has made slow progress since the first crystal model was proposed to describe its morphology in 1957.25 To the best of our knowledge, the only studies on twinned tabular crystals consist of side-face analysis,26 and the efforts on a general preparation route are far from comprehensive and mature. Hence, we suppose that finding an approach to synthesize twinned crystals with controllable structures is a significant challenge. In this contribution, we demonstrate a novel approach to fabricate Au crystals using tetracycline hydrochloride, which acts as an alternative reducing agent, and cetyltrimethylammonium bromide (CTAB), which acts as the capping agent and structure-directing agent. Crystal structures from twinned tabular crystals and nanoplates to multitwinned decahedra could be rationally prepared by modulating the molar ratio of CTAB/ HAuCl4. It is worth noting that the Au crystals with the twinned tabular morphology were not observed in previous reports, although similar morphology was observed for AgBr and later for Ag by Bo¨gels et al.19,26 In addition, we found that Au nanoplates with an edge size of several micrometers demonstrate strong NIR absorption as wall as amazing dark field light scattering features, exhibiting morphology by surface-scattering. By monitoring the strongly scattered light from functionalized Au crystals incubated with malignant cells and further exposure to laser irradiation,27,28 a single nanoplate is expected to be a useful tool for malignant cell diagnostic and photothermal therapy.

2. Experimental Section 2.1. Synthesis of Au Crystals. In a typical synthesis for twinned tabular Au crystals, 0.8 mL of 0.1 M CTAB solution and 6.4 mL of 1 × 10-3 M tetracycline hydrochloride solution were added to 30.8 mL of boiling water, then 2 mL of 0.024 M HAuCl4 solution was rapidly injected into the boiling mixture. The reaction system was kept boiling under strong stirring for 10 min. To collect the tabular crystals, the resulting solution was centrifuged at 10 000 rpm for 5 min, the

10.1021/cg801265y CCC: $40.75  2009 American Chemical Society Published on Web 06/02/2009

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Figure 1. (a) Representative SEM image of Au tabular crystals. (b-d) SEM images of side-face structure of tabular crystals of group 1, group 2, and group 3, respectively. The side faces have been indexed, and the white arrows labeled TB indicate the twin boundary where the twin planes are expected to be located. supernatant was removed, and the precipitate was redispersed in deionized water. For the other two morphologies of Au crystals, a similar synthetic procedure was employed except for the CTAB concentration: 6 mM (CTAB/HAuCl4 molar ratio ) 5:1) for nanoplates, 12 mM (CTAB/HAuCl4 molar ratio ) 10:1) for decahedra, and the reaction time for nanoplates and decahedra was 10 and 15 min respectively. 2.2. Characterization. Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800 microscope (Tokyo, Japan) operated at 20 kV. The transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were taken using a FEI TECNAI-20 microscope (America) operated at an acceleration voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL JEM-2100F field emission electron microscope (Japan). X-ray diffraction (XRD) patterns were measured on an XD-3 X-ray diffractometer (Beijng, China) using Cu KR radiation (36 kV, 20 mA). Dark field light scattering images were acquired using an Olympus BX51 Microscope (Tokyo, Japan) with a highly numerical dark field condenser (U-DCW, numerical aperture ) 1.2-1.4) for particle illumination and a 100× variable aperture oil immersion objective (UPLANFLN, numerical aperture ) 0.6-1.3) for subsequent collection of a particle’s scattered light. The dark field pictures were taken on an Olympus E-510 digital camera. The samples were prepared by placing a few drops of the colloidal solutions either on copper grids coated with lacey carbon film for TEM, HRTEM, and SAED or on small pieces of aluminum foil for SEM. The UV-vis-nearinfrared (NIR) absorption spectra were obtained on a Shimadzu UV3600 spectrophotometer. Infrared spectra were obtained on a PerkinElmer Spectrum GX Fourier transform infrared (FT-IR) spectrometer in KBr wafer. The obtained twinned tabular crystals are too thick (about 400 nm) to get their detailed structural information from SAED and HRTEM, so we revealed the side-face structure by observing the angle between the faces and judging the shape of the faces. Hexagonal (or trapeziform) shaped side faces were indexed as {111} faces and rectangular shaped ones as {100} faces.19

3. Results and Discussion 3.1. Morphology Control and Structure Analysis. A typical SEM image of the tabular crystals prepared with the molar ratio of 5:3 CTAB/HAuCl4 (2 mM CTAB) shows three different morphologies as shown in Figure 1. Similar to the side-face analysis of tabular AgBr crystals reported by Bo¨gels et al.,26 these twinned tabular crystals could be divided into three groups. Group 1 has the outside shape in top view as a perfect hexagon, and most of the as-prepared crystals belong to this group (see Figure 1b). These crystals are bound by two large {111} top and bottom faces and have the morphology of truncated cuboctahedra for which the {100} and {111} side faces touch the twin boundary at the central part. Namely, the crystal side consists of a {111} face and a {100} face in a ridged structure. The continual growth of cuboctadedral nuclei with facet truncation makes it possible for the formation of such crystals (see further discussion below). A nucleation twin plane in a cuboctahedron can be considered as a rotation over 60° of the top with regard to the bottom of the crystal,29 and thus two twin planes will give the same morphology as a (truncated) cuboctahedron (shown in Figure 1b, where the twin boundary is indicated by an arrow labeled with TB). Different from group 1, which has an even number of twin planes, groups 2 and 3 have an odd number of twin planes. An example of group 2 with a single-twin structure is shown in Figure 1c, and it can be clearly seen that two adjacent {111} faces or {100} faces locate in a ridged structure and the outside shape is triangle or truncated triangle in top view, which results from the rotation over 60° of one nucleation twin plane.19 According to the substep mechanism,26 the sides with two adjacent {100} faces have a higher growth rate than the sides

Morphology Control and Structure of Au Crystals

Figure 2. XRD patterns of tabular crystals, nanoplates, and decahedra.

with adjacent {111} sides, and thus triangular or intermediate shaped crystals can be observed. SEM observation shows that there are also some crystals with triple twin planes (ranged as group 3), and the difference between group 3 and group 2 exists in the structure of the side faces. Group 3 has the side structure consisting of two {100} faces with a {111} face between, as clearly indicated in Figure 1d, and they could be considered as the result of adding an accidental twin plane to group 1.26 Twinning is one of the most common lattice defects in crystals, and probably the twinned structure determines the shape of crystals. It should be noticed that the distances between the different twin planes is very small for the majority of the tabular crystals, and thus the separation could not be determined. Therefore, we assume that parallel twinning occurs during the nucleation stage. The XRD pattern of the tabular crystals is consistent with the standard JCPDS card (No. 04-0784) except for the peak intensities (Figure 2). The relative intensities for {111}, {200}, and {220} are 1:0.5:0.3 for a conventional powder sample of face-centered cubic (fcc) Au (JCPDS card, No. 04-0784), while the tabular crystals have the intensity ratios of 1:0.3:0.15. The difference of the tabular crystals from that of the conventional fcc Au powder could be explained by the structure of the crystals, which is bounded by two large {111} top and bottom faces with the side faces consisting of {111} planes and small {100} planes. That is to say, the crystals are composed of primary {111} planes and a small amount of {100} planes, which results in the dominant orientation of {111} planes parallel to the supporting substrate and enhances the {111}

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diffraction intensity. However, it is worth noting that the ratio between the intensities of {111} and {200} diffraction peaks is only 1.7-fold higher than the conventional one, which could be attributed to the {200} diffraction signal originating from the {100} planes located at the side faces. Furthermore, the diffraction peaks for {220} and {311} could also be observed obviously, probably due to the polyhedra concomitant with the regular crystals. Further increase of CTAB/HAuCl4 ratio to 5:1 (6 mM CTAB), however, results in the formation of single-crystalline nanoplates (Figure 3a), which is evident from SAED analysis (Figure 3b). The SAED pattern from a single triangular nanoplate is obtained by aligning the electron beam perpendicular to the face of selected nanoplate. The hexagonal symmetrical diffraction spots imply that each Au nanoplate is a single crystal, whose surfaces are bounded by {111} planes. Two sets of spots can be identified based on the lattice spacing (d) of the fcc Au crystal. The spots with the strongest intensity could be indexed to {220} planes (circle), and the inner spots with relatively weak intensity correspond to the normally forbidden 1/3{422} planes (square), which could be observed when the {111} stacking faults parallel to the {111} surface extend across the entire nanoplate.30 All these results are consistent with previous reports on the nanostructure of nanoplates bound by atomically flat {111} planes.31 In XRD data (Figure 2), the overwhelmingly intensive diffraction at 2θ ) 38.03° assigned to the {111} lattice planes of fcc Au crystal indicates that the basal planes of the crystals should be {111} planes, and diffractions from other planes such as {200}, {220}, and {311} are insignificant, indicating that the {111} planes are preferentially oriented parallel to the surface of supporting substrate, which is in agreement with previous studies32,33 and former SAED results. A higher CTAB/HAuCl4 ratio of 10:1 (12 mM CTAB) suppresses particle growth and yields small particles, in which 70%-80% of the Au nanoparticles exhibit pentagonal symmetrical decahedral shape with edge size of about 100 nm, with a small amount of truncated tetrahedra remaining (Figure 4a). It has been known that a decahedron can be considered as the twinned assembly of five tetrahedra, forming a perfect pentagonal bipyramid of equal equilateral-triangle faces. The 5-fold twining with a vertex and five sharp edges for each decahedron can be observed distinctly from a high-magnification SEM image in Figure 4b. The HRTEM image taken at one edge shows distinct lattice planes with a d-spacing of 2.37 Å assigned to the {111} planes of fcc gold (Figure 4c). Another HRTEM image (Figure 4d) taken at the twin boundary exhibits inhomogeneous broadening (marked by arrows), which should be attributed to the deviation of {111} twinning angle between two adjacent twin variants in equilibrium (70.53°) from 360°/5 (72°). Previous reports have demonstrated that the presence of dislocations such as splitting in the pentagonal axis34 and broadening in the twin boundary35 could adjust the lattice mismatch and avoid the gap of 7.35°. The SAED pattern from a single decahedron (Figure 4e,f) demonstrates a 5-fold axis symmetry, in which 10 pairs of diffraction spots could be characterized. They are five pairs (triangle) with a d-spacing of 2.37 Å, and five others (square) with a d-spacing of 2.06 Å, corresponding to {111} and {200} planes, respectively. Furthermore, the pattern exhibits an interpenetrated set of five individual diffraction patterns, in which one set corresponds to a {110} zone axis (labeled with circles) and a rotation by 72° yields the diffractions for subsequent ones. The pattern is in good agreement with the

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Figure 3. (a) SEM image of Au nanoplates. (b) TEM image of a triangular Au nanoplate and its corresponding SAED pattern. The strongest spots (circle) could be indexed to {220} reflections, and the inner spots (square) correspond to the 1/3{422} reflections.

Figure 4. (a, b) SEM images of decahedra with low magnification and higher magnification, respectively, (c) HRTEM image from the edge, displaying the lattice fringes, (d) HRTEM from the twin boundary, clearly exhibiting inhomogeneous broadening (labeled by arrows), and (e, f) TEM image of a Au decahedron and its corresponding SAED pattern. The {200} reflections are marked with squares, while {111} reflections are marked with triangles. The circles indicate a diffraction pattern corresponding to a {110} zone axis.

simulated image of a copper decahedron in the 5-fold orientation.36 The XRD pattern of the decahedra is very similar to that

for nanoplates (Figure 2). The diffraction peaks for {111}, {200}, {220}, and {311} planes are all present, but the ratio

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Figure 5. (a) UV-vis-NIR spectra of twinned tabular crystals, nanoplates, and decahedra (curves 1-3, respectively); (b-d) dark field light scattering images of twinned tabular crystals, nanoplates, and decahedra, respectively. The scale bar is 5 µm.

between the intensities of {111} and {200} is much larger than the standard powder diffraction value, implying the preferential orientation of {111} planes on the surface, providing strong evidence of the decahedral nanostructure covered with {111} faces. 3.2. Optical Properties of the As-Prepared Au Crystals. The obtained Au crystals with different morphologies exhibit amazing optical properties, which were testified by measuring UV-vis-NIR extinction spectra and dark field light scattering images. The typical extinction spectrum for the twinned tabular crystals shows a broad band centered at around 650 nm (curve 1 in Figure 5a); as the CTAB/HAuCl4 ratio increases from 5:3 to 5:1, the broad band decreases and the plasmon resonance in the NIR region shows a continual increase (curve 2 in Figure 5a), which might be attributed to the formation of nanoplates with large edge size and morphological diversity.31,37 The strong NIR absorption of the nanoplates makes them potentially applicable in thermotherapy.38 The extinction spectrum of the decahedra exhibits two characteristic peaks (curve 3 in Figure 5a), and the peak located at 550 nm could be assigned to the polar-dipolar plasmon mode, which corresponds to the oscillation pattern along the 5-fold axis, while the peak at 660 nm should be associated with the azimuthal dipolar plasmon mode, which corresponds to the oscillation pattern along the equatorial plane of the decahedral nanoparticle. It is worth mentioning that quadrupolar or higher multipolar resonance cannot be observed in ensemble measurements possibly due to inhomogeneity and impurities in the sample.39 The bump at about 820 nm for all the samples should be attributed to the detector change during the measurements. To explore the applications of Au crystals in malignant cell diagnostic, we further investigated the dark field light scattering features (Figure 5b-d). It can be seen clearly that the strong

light scattering mainly originates from the edge of nanoplates, exhibiting amazing crystal morphology (Figure 5c). For tabular crystals (Figure 5b) and decahedra (Figure 5d), the light color rather than the morphology could be distinguished due to their small size. It is worth noting that both tabular crystals and nanoplates scatter in yellow, while decahedra with edge size about 100 nm scatter in orange. The strongly enhanced absorption and scattering due to the strong electric fields at the surface demonstrate the potential use of these particles as novel optical reagents for molecular imaging and photothermal cancer therapy.28 3.3. Growth Mechanism for Three Types of Crystals. As stated above, controlling the concentration of CTAB is very important for the formation of Au crystals ranging from twinned tabular crystals and single-crystalline nanoplates to multitwinned decahedra (detailed experimental results, see Supporting Information); therefore, the role of CTAB should be considered. The FTIR spectra of the Au nanoplates identify the bound CTAB on the gold surface (see Supporting Information), which is consistent with previous investigations that CTA+ could selectively adsorb on {111} planes, inhibiting the random growth of the particle but favoring that of a single crystal with {111} facets extending.40 However, CTA+ alone is not sufficient for shape control of crystals, especially when the concentration of CTAB is relatively low and then Br- is possibly involved to clarify the crystal formation. Filankembo et al. have demonstrated that a small amount of halide anions such as Cl- and Br- could selectively adsorb on {100} or {111} planes to direct crystal anisotropic growth. For example, low Br- concentration favors its preferential adsorption on {100} planes, leading to the decrease of {111} faces and the formation of {100} faces.41 In this case, it is reasonable to speculate that the binding between Au and CTAB, combining both CTA+ and Br- adsorption, leads

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Scheme 1. Schematic Mechanism Proposed to Account for the Formation of Au Crystals with Different Morphologies from Three Types of Nuclei at Different CTAB Concentrationsa

a

Red and blue represent the {111} faces and {100} faces, respectively.

to the formation of tabular crystals limited by {111} planes and truncated by {100} planes. While Filankembo and co-workers demonstrated that increase of Br- concentration above a certain value is of no effect on the crystal anisotropic growth,41 and the increase of CTA+ concentration induces higher crystal surface coverage and enhances the adsorption on {111} planes.42 That is to say the key factor determining the growth of crystal is CTA+ other than Br- at higher CTAB concentrations. Therefore, it is understandable that the {100} side faces gradually decrease and finally disappear with increasing CTAB concentration, concomitantly with increase in particle size and decrease in particle thickness, as observed for the morphology converting from tabular crystals to nanoplates. Because the surface energy in a fcc metal is in the order γ111 < γ100 < γ110,39 it is reasonable to speculate that Au nuclei formed in the initial stage are bound by {111} or {100} planes, fluctuating between kinetic structures and thermodynamic structures.43 Similar to copper crystals, we propose that the stable nuclei for Au crystals are cuboctahedral, tetrahedral, and decahedral.36 If the CTAB concentration is very low, twinned cuboctahedral nuclei can be generated from the reaction mixture. Previous studies have demonstrated that decrease of molar ratio of retarding agent to metallic precursor (CTAB/HAuCl4) would drastically increase the reduction yield and accelerate the reduction rate, resulting in the generation of abundant Au atoms in a short time.44,45 The probability of high supersaturation of Au atoms on the {111} surface facilitates the formation of twin planes during nucleation through a layer by layer growth mechanism,19,46 and the twinning structure remains stable in this environment due to the low concentration of Br-, which prefers to adsorb on crystal planes rather than etch twin boundary defects. Once the twinned nuclei form, CTA+ as well as Br- are adsorbed on special crystal planes to promote the anisotropic growth, leading to the formation of twinned tabular crystals with the morphology of truncated cuboctahedra, as discussed above. If the CTAB concentration is increased and modulated to an appropriate value, Br- could effectively etch the twin boundary defects rather than adsorb on crystal planes, and single-crystalline nuclei such as tetrahedra are more stable in this environment.22,43 Then CTA+ selectively adsorbs on {111} planes to favor single crystal growth with {111} facets extending, and thus triangular or truncated triangular nanoplates

can be obtained. If the CTAB concentration is very high, multitwinned nuclei are stable, and the twin boundary defects could be effectively protected from etching by CTA+ capping,34 then simultaneously and regularly grow to large decahedra without facet truncation, so the decahedra are the predominant products with a small amount of truncated tetrahedra, which probably grow from the small amount of single-crystalline nuclei. This proposed mechanism is depicted schematically in Scheme 1. The effect of CTAB on Au crystal morphology control could be only applicable at relative low pH values (see Supporting Information). A possible explanation is that OH- could react with NH4+, neutralizing the positive charge of CTAB, and then weakening the interaction between C-N+ and the gold surface. In neutral or alkaline conditions, CTAB could not effectively adsorb on crystal faces, and thus anisotropic growth was prohibited.

4. Conclusions In this contribution, we propose a simple solution-phase method for rational Au crystal synthesis using tetracycline hydrochloride as reducing agent in the presence of CTAB. With the increase of CTAB concentration, the shape of Au crystals experiences a change from twinned tabular crystals to singlecrystalline nanoplates to multitwinned nanodecahedra. The CTAB concentration added in the reaction mixture plays a key role for structure fine-tuning and is responsible for the morphology variation. The possibility of achieving morphology control by simply adjusting the concentration of CTAB at relative low pH values overcomes the previous limitations such as multiple steps and complexity and shows promises for extension to other conditions. We have also proposed a rough mechanism for the nuclei formation and crystal growth that need to be improved by further studies. Novel optical properties of the Au crystals including UV-vis-NIR absorption and dark field light scattering images give them potential applications in cancer cell diagnostics and photothermal therapy. Acknowledgment. The authors are grateful for the assistance with TEM and SAED analysis of Chongqing University and HRTEM analysis of Wuhan University of Technology. We

Morphology Control and Structure of Au Crystals

thank the National Natural Science Foundation of China (NSFC, Grant No. 20425517), the Ministry of Science and Technology of the People’s Republic of China (2006CB 933100), and the Chongqing Science and Technology Commission for the Chongqing Key Laboratory on Luminescence and Real-Time Analysis (2006CA8006) and for the Natural Science Foundation (CSTC, 2008BB0248) for support. Supporting Information Available: Detailed experimental results of morphology evolution depending on the concentration of CTAB, as well as the pH values in the medium, and FTIR results of CTABcapped Au nanoplates. This material is available free of charge via the Internet at http://pubs.acs.org.

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