Interface-Controlled Synthesis of Au-BINOL Hybrid Nanostructures

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Interface-Controlled Synthesis of Au-BINOL Hybrid Nanostructures and Mechanism Study Shashank Reddy Patlolla, Chen-Rui Kao, Ai-Hsuan Yeh, Hung-Min Lin, Yu-Chun Chuang, Yuh-Sheng Wen, Brian T Sneed, Wen-Ching Chen, Tiow-Gan Ong, and Chun-Hong Kuo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02857 • Publication Date (Web): 20 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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Interface-Controlled Synthesis of Au-BINOL Hybrid Nanostructures and Mechanism Study Shashank Reddy Patlolla,†,§,△ Chen-Rui Kao,† Ai-Hsuan Yeh,† Hung-Min Lin,†,|| Yu-Chun Chuang,‡ Yuh-Sheng Wen,† Brian T. Sneed,# Wen-Ching Chen,*,† Tiow-Gan Ong,*,†,△,|| and Chun-Hong Kuo*,†, †Institute §Sustainable

of Chemistry, Academia Sinica, Taipei 11529, Taiwan

Chemical Science and Technology, Taiwan International Graduate Program, Academia Sinica and National Chiao Tung University, Taiwan

△Department

of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan

||Department ‡National

of Chemistry, National Taiwan University, Taipei 10617, Taiwan

Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

#Cabot Institute

Microelectronics, Aurora, Illinois 60504, United States

of Materials Science and Engineering, National Central University, Jhongli 32001, Taiwan

KEYWORDS: gold, BINOL, nano, hybrid, surfactant 1 ACS Paragon Plus Environment

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ABSTRACT The combined functionality of components in organic-inorganic hybrid nanomaterials render them efficient nanoreactors. However, the development in this field is limited due to a lack of synthetic avenues and systematic control of the growth kinetics of hybrid structures. In this work, we take advantage of an ionic switch for regio-control of Au-BINOL(1,1'-Bi-2-naphthol) hybrid nanostructures. Aromatic BINOL molecules assemble into nanospheres, concomitant with the growth of the Au nanocrystals. The morphological evolution of Au nanocrystals is solely controlled by the presence of halides in the synthetic system. Here we show that quaternary ammonium surfactants (CTAB or CTAC), not only bridge Au and BINOL, but also contribute to the formation of concentric or eccentric structures when their concentrations are tuned to the range of 10‒5 to 10‒3 M. This facile strategy offers the potential advantage of scalable production, with diverse functional organic-inorganic hybrid nanocomposites being produced based on the specific archetype of Au-BINOL hybrid nanocomposites.

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INTRODUCTION Organic-inorganic hybrid materials have received much attention for their unique synergistic physical and chemical properties. Materials scientists can integrate thermal stability, ordering, and rigidity of inorganic components into more pliable organic components.1 Miniaturizing these hybrid composites to nanoscale makes new properties possible.2 Most importantly, large interfacial areas with a close proximity between components amplifies synergistic effects in hybrid nanocomposites.3 Despite promising potentials, the development of hybrid nano-composites with organic adducts is still hampered by a lack of rational synthesis strategies to control and modulate their morphologies and structures. Partial masking4-6 and selective growth7-10 are viable approaches used for preparing these types of materials. However, these methods suffer drawbacks associated with multi-step synthetic procedures in restrictive and/or harsh experimental conditions and poor product homogeneity in terms of particle sizes and structural morphology. 1,1’-Binaphthalene-2,2’-diols (BINOLs) have been widely used in organic synthesis and organometallic chemistry because they are readily coordinated to transition metals through robust oxygen binding sites, serving as effective ligands for asymmetric catalysis.11 On the other hand, nanoparticles of gold based on colloidal hierarchical organic templates like BINOL remain greatly underexplored, despite a handful of reports on thiol derivatives-based templates leading to the realization of gold nanoparticles;12-15 however, fusing unique features of organic molecules with metallic particles remains to be demonstrated. Of particular interest is the aromaticity and axial chirality of BINOL that could potentially bridge synergistically together with metallic nanocrystals like Au. This would allow assembly of a well-organized architectures for creating a new physical and optical functions,16 which are promising toward advanced material and biological applications. Herein, we report a new proof-of-concept of a rational synthetic strategy to establish a new kind


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of hybrid nanostructures based on mixing the organic component, BINOL, with Au(III) complexes in a simple one-pot aqueous solution. At the same time, we also deploy an ionic switching mechanism to induce two morphologically different nano gold hybrid structures. Previously, we demonstrated successfully this ionic switching approach in nano-synthesis for the first time, controlling evolution of Au-Pd alloy to core-shell icosahedral nanocrystals via modulation of an anionic ratio between Cl- and Br-. In this work, we also found that the selective morphological evolution of Au nanocrystals is dictated regionally by the choice of alkyl ammonium surfactants and halides. The mechanism and kinetics of the formation of hybrid Au-BINOL are investigated in order to understand a subtle balance among Au, BINOL, and cetyltrimethylammonium bromide (CTAB) or chloride (CTAC) and their complex interaction. These results also reveal that the concentration of the mixtures greatly governs the regioselectivity in the Au-BINOL formation, leading to concentric and eccentric morphology of the nanostructures. By virtue of these findings, this work provides a fundamental understanding of factors for the nanostructure fabrication, which offers useful guidance in the facile and scalable production of related diverse functional organicinorganic hybrid nanocomposites. EXPERIMENTAL SECTION Chemicals. Hydrogen tetrachloroaurate-(III) trihydrate, (HAuCl4·3H2O, Alfa Aesar, 99.99 %), (±)-1,1´-Bi(2-naphthol), (BINOL, Alfa Aesar, 99 %), cetyltrimethylammonium bromide (CTAB, TCI, > 95%), cetyltrimethylammonium chloride (CTAC, TCI, > 95%), ethanol (C2H5OH, Echo chemicals, > 95%), Silica gel (Si 60, 70-230 mesh, 0.063-0.200 mm Merck), dichloromethane (Duksan pure chemicals, 99.5%), n-hexane (Uni region biotech, 95%), deuterochloroform (CDCl3, Sigma-Aldrich, 99.8%,). Ultrapure deionized water DI water (18.2 MΩ cm‒1) was used for all solution preparations. 4 ACS Paragon Plus Environment

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Synthesis of Au-BINOL Hybrid Nanocomposites. Before the synthesis, 0.01 M HAuCl4 stock solution was prepared by dissolving the crystals of hydrogen tetrachloroaurate-(III) trihydrate in DI H2O. A 0.01 M BINOL stock solution was prepared by dissolving racemic BINOL powder in a mixed solvent of ethanol and DI water with a volume ratio of 1:1. In the typical synthesis, the 22 mL glass vial containing a solution made by well mixing 5 mL of surfactant aqueous solution, 1 mL of 0.01 M HAuCl4, and 1 mL of 0.01 M BINOL stock solutions was placed in an oven at 55 °C for 6 hours. The surfactant aqueous solution was prepared by adding CTAB or CTAC powder into 5 mL of DI H2O followed by sonication to entirely dissolve the powder. For control experiments, the added weights of CTAB and CTAC were changed to get the concentrations of 10–2, 10–3, 10–4, and 10–5 M. After reaction for 6 hours, the solutions turned turbid into a greenish brown color owing to the suspended colloids. They were collected by centrifuging at 6,000 rpm for 12 min (Eppendorf Centrifuge 5804) and redispersed in 5 mL DI H2O. The washing process was repeated two more times to ensure that any excess surfactant was removed. Lastly, the resulting clear supernatant was discarded and the colloids were stored in 5 mL of DI H2O. Characterization. To prepare samples for SEM or TEM, the collected products were concentrated to 200 μL in DI H2O and stored in a 1.5 mL centrifuge tube. Next, 5 μL of concentrated sample solutions was dropped onto silicon wafers in the size of 0.3 × 0.3 cm2 or carbon-coated copper grids with slowly drying at room temperature. Centrifuging steps were done using Eppendorf Centrifuge 5804 and Thermo Scientific Heraeus Pico 17. SEM images were recorded by a ZEISS ULTRA PLUS equipped with an OXFORD EDX detector, operated at the accelerating voltage of 10 KeV. Low-and high-magnification TEM bright-field images were taken by a JEOL JEM-2100F microscope operating at 200 kV. UV–Vis absorption spectra were


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measured on a HITACHI U-3310 spectrophotometer. The time-dependent UV-vis spectra were acquired by measuring aliquots of 200 µL supernatant in a 3 mL U.V quartz cuvette taken from the solution under reaction every 10 minutes for a serial time duration from 0 min to 60 min. The absorbance of the BINOL (0.01 M) is recorded within the range of wavelength from 350 nm to 260 nm. The X-ray

diffraction experiments were performed at BL01C2 in National Synchrotron Radiation Research Center (NSRRC). The diffraction data were collected using 18 keV X-rays (0.82656 Å in wavelength) and Mar345 image plate detector with Debye-Scherrer geometry. The patterns were converted by GSAS -II program and the angle calibration was performed according to LaB6 (SRM 660c) standard. The composition of elements in supernatants was analyzed using a Varian 720 ES, an ICP optical emission spectrometer (ICP-OES). High-resolution electron ionization (HREI) mass spectra were acquired with a JEOL JMS-700 (EI) double focusing mass spectrometer using 70 eV electrons to ionize compounds. 1H and 13C-NMR analyses of the BINOL components in the Au-BINOL hybrid nanostructures were carried out by BRUKER AVANCE 400 NMR spectrometer using the residual proton of the deuterated solvent for reference (CDCl3, 1H-NMR: δ7.26 ppm, 13C-NMR: δ77.0 ppm). The peaks of the eccentric hybrid nanostructures maded with CTAB 10-3 M are listed as following. 1H-NMR (400 MHz, CDCl3): δ7.98 (d, J= 8.8 Hz, 2H), 7.90 (d, J= 8.0 Hz, 1H), 7.40–7.36 (m, 1H), 7.33–7.29 (m, 1H), 7.15 (d, J= 8.4 Hz, 1H), 5.04 (s, 2H, OH),

13C-NMR

(100 MHz, CDCl3): δ152.75, 133.49, 131.32, 129.45, 128.40, 127.47, 124.28,

124.03, 117.81, 111.04 ppm. The peaks of the eccentric hybrid nanostructures formed with CTAC 10-3 M are describe as following. 1H-NMR (400 MHz, CDCl3):δ8.00 (d, J= 8.8 Hz, 2H), 7.91(d, J= 8.0 Hz, 1H), 7.42–7.38 (m, 1H), 7.35–7.31 (m, 1H), 7.18 (d, J= 8.4 Hz, 1H), 5.08 (s, 2H, OH).13C-NMR (100 MHz, CDCl3) δ 152.76, 133.45, 131.39, 129.47, 128.41, 127.48, 124.24, 124.04, 117.78, 110.93 ppm. The FTIR spectroscopic analysis for nanocomposites were recorded by using an FT-IR spectrometer (Perkin Elmer Spectrum 100). 6 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION Formation of Eccentric and Concentric Au-BINOL Nanocomposites. The formation of Au-BINOL nanocomposites was achieved via thermal reduction of HAuCl4 precursors to Au nanocrystals, where BINOL not only served as the reducing agent, but also acted as an organic functionality in this hybrid nanostructure. In the synthesis process, ionic surfactants, either CTAB or CTAC, bearing long alkyl-chain CTA+ and halide anions were used as nanocrystal stabilizers, owing to their ability to cap (trap) metallic surfaces, generating nanocrystals in an orderly fashion via the repulsive force created along bilayer-CTA+.17-19 In this juncture, we observed that halide ions play a critical role in shaping the types of Au-BINOL nanostructures. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images in Figure 1 display the two morphologies of Au-BINOL nanostructures, eccentric and concentric. The average sizes of these eccentric and concentric nanostructures are 197.1 nm with a 13.4% deviation (Figure S1a) and 405.0 nm with a 20.1% deviation (Figure S1c). Figure 1a and b contain SEM and TEM images of the eccentric Au-BINOL nanocomposite obtained with the 10‒3 M CTAB stock solution, for which the brighter (Figure 1a) and darker (Figure 1b) regions are identified as isotropic Au nanoparticles with irregular morphology (50‒200 nm). The phenomenon is further confirmed by the synchrotron X-ray diffraction patterns (Figure 1e). Conversely, the nanosphere in slight contrast appears as a colloid aggregation with poor crystallinity, which we believe is composed of organic BINOL derivatives. Keeping in view of a poor encapsulation in this hybrid nanocomposite system, the “eccentric” definition is preferred over a common term “Janus”, since the Au nanocrystals are only partially enveloped within the cavity of nanospheres. Upon lowering the concentration of CTAB to 10‒5 M, hybrid nanocomposites containing two different variants were still obtained, but these nanocomposite materials now have a markedly concentric structure. Figure


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1 c and d show that concentric structures are composed of a batch of smaller Au nanoparticles (2‒5 nm) closely distributed inside the carbon-based nanospheres. It is important to note that Au nanoparticles appeared in random sizes and shapes when CTAB is excluded in the synthesis (Figure S2). Moreover, the vast majority of metallic particles here appear to physically adsorb on the surfaces of the nanospheres. Thus, it can be inferred that the existence of CTAB exhibits beneficial effects based on two aspects: (1) size and shape controlled growth of inorganic Au nanocrystals, and (2) regio-growth of Au-BINOL inorganic-organic hybrid nanostructures for concentric and eccentric morphology. Generally, the concentration of CTAB dictates the size of the Au nanocrystals’ formation.20 Nonetheless, varying the concentration of Br‒ ions could also affect the reduction rate of Au3+ complexes, which are derived from AuBr4‒ complexes, leading to different epitaxial growth modes for Au nanocrystals.21 To further clarify the role of halides in the nanocrystal formation, an experiment using CTAC as Cl‒ source with a concentration of 10‒3 M stock solution was applied to prepare the Au-BINOL nanocomposite. Figure 2a-d shows distinct images and the STEM-EDS maps of the nanocomposite formed. The structures are eccentric comprising of anisotropic triangular Au nano-plates synchronously fused with the carbon-based nanospheres. Their average size is 579.6 nm with a 21.1% deviation (Figure S1b). In viewing of a single structure on its side at a tilted angle (yellow arrow, Figure 2a), the eccentric structure does not appear to originate from a simple physical adsorption of the organic nanosphere on the anisotropic Au nanoplates, but more as outgrowths of each other. Interestingly, when the concentration CTAC was adjusted to the lower value of 10-5 M, a similar result as in the 10‒5 M CTAB case (Figure 2e and f) was observed as the concentric hybrid structures with an average size of 594.6 nm and a 13.4% deviation (Figure S1d). This outcome highlighted that the influence of halide ions on the nanocrystal morphology at low


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concentration is not significant until reaching a certain critical concentration. The observation is also further validated with additional experiments by testing the concentration of CTAB and CTAC solution ranging from 10‒4 and 10‒2 M. Figure S3a and S3b collect the TEM images of AuBINOL hybrid nanostructures prepared with 10‒4 M CTAB and CTAC. Both of the conditions exhibit concentric structures within which the isotropic Au nanospheres are formed. This reflects a marginal effect of halide ions in low concentrations. However, distinct images are observed upon increase of concentrations of CTAB and CTAC to 10‒2 M. Figure S3c shows the result gained with 10‒2 M CTAB, for which large Au crystals (> 500 nm) were obtained with a wide distribution of shape. Figure S3d shows the large Au crystals formed with 10‒2 M CTAC. In contrast to those of CTAB, the Au crystals here own well-defined facets and preferred anisotropic morphology such as nanoplates. This confirms a significant shape-control effect by an appropriate concentration of halide ions. Identification of Organic Moieties in Au-BINOL Nanocomposites. Following from this information, we were interested to discover whether the nanospheres of organic component can be obtained through the synthesis without gold complexes. A control experiment without HAuCl4 and CTAB showed that the organic part of nanospheres was still observed in the synthesis (Figure S4). Therefore, we strongly believe the nanospheres are BINOL-based molecular assemblies, owing to their intra-/intermolecular interactions via π-π stacking and hydrogen bonding.22 Specific to the question to the reduction of HAuCl4, we postulate that the two hydroxyl groups of BINOL coordinate to the highly electrophilic Au(III) centers, forming seven-membered ring organometallic Au complexes, previously reported.23 Similar to the other phenolate complexes, the two consecutive O-anionic centers of binaphthol ligand are electron-rich, which render them effective reductants in the synthesis.24 Although EtOH and CTA+ long-chain surfactants in reaction


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mixture also could be regarded as a possible weak reducing agents,25-26 the Au3+ ions were not prone to undergo any reduction in such a mild temperature of 55 °C in the absence of BINOL (Figure S5). To further clarify the secondary role of BINOL as the reductant, the supernatant of a resulting Au-BINOL solution (10‒3 M CTAB) was separated for analysis. The supernatant solution was isolated from the colloids by centrifuging, and filtered through running column chromatography (silica gel, Si 60, 70-230 mesh, 0.063-0.200 mm). Subsequently, this filtered solution was dried and analyzed by HREI mass spectrometry. Figure S6a shows the mass spectrum of the byproducts derived from supernatant solution containing four sets of peaks. A group of peaks corresponding to the m/z value of 298.06 is detected, which is unlikely to be the compound BINOL (see Figure S6b). The successful characterization of this byproduct by single crystal Xray diffraction found this component to be naphthofuranoquinone (NFQ, dinaphtho[2,1-b:2',3'd]furan-8,13-(8H, 13H)-dione), as shown in Figure S7 and Table S1-S4. The formation of NFQ is the direct result of the chemical oxidation of BINOL, which has been reported previously by Farrugia et al. in 1996.27 In addition, the identity of this byproduct is further confirmed with the mass spectrum of the standard compound of NFQ, with signature peak values of 270.067 and 298.062 (Figure S6c). Thus, it can be concluded that BINOL plays a critical role in reduction of gold precursor. Given the fact that BINOL undergoes oxidation to NFQ, liberating electrons for the reduction of HAuCl4, it is necessary to examine if the organic moieties in the hybrid nanostructures contain any NFQ. For this reason, the dried solid eccentric Au-BINOL nanostructures were redispersed in EtOH solution to allow dissolution of organic nanospheres. Subsequently, the solutions were analyzed by HREI mass spectrometry. Figure S8 shows the mass spectra of eccentric Au-BINOL nanostructures under CTAB (Figure S8a) and CTAC (Figure S8b) conditions.


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The peaks associated with NFQ do not appear in both samples, indicating the absence of NFQ in the nanospheres. Figure S9 and S10 are the corresponding 1H-NMR and 13C-NMR spectra of the two kinds of eccentric Au-BINOL nanostructures. Their NMR spectra reveal significant characterization carbon peaks and hydrogen distribution from the organic moieties that well correspond with the NMR result of pure BINOL in Masters’s work.28 It again proves only BINOL molecules are involved in the organic moieties in the hybrid nanostructures. To further verify, the dried eccentric Au-

BINOL nanostructures were made into pellets with dried KBr powder for FTIR spectroscopy. Figure 3 collects the FTIR spectra of the powders of CTAB, CTAC, commercial BINOL, NFQ, and the two types of eccentric nanostructures. In the spectra of CTAB and CTAC, the characteristic IR absorption peaks include aliphatic C-H (2918 and 2850 cm−1), N-H (3018 cm−1), and C-N (1488 cm−1) .29 However, they are not observed in the spectra of eccentric nanostructures. In contrast, most of the characteristic peaks of BINOL, e.g. O-H (3487 and 3403 cm−1), correspond with those in the spectra of eccentric nanostructures, while those of NFQ do not. This suggests the organic component of the hybrid structures is comprised of BINOL molecules, assembling through - stacking and intermolecular hydrogen bonding. Factors to Regio-Selective Formation of Au-BINOL Nanocomposites. We establish that the formation of Au-BINOL nanocomposites results from two distinct growths of Au nanocrystals and BINOL nanospheres. In-between the two moieties, the ionic surfactant plays a role to bring them together, forming a hybrid inorganic-organic nanostructure. In addition, it is a “switch” to tune the regioselectivity of Au and BINOL moieties. However, how the surfactant works in the growth kinetics of Au-BINOL nanostructures is still unclear, thus it is very important to know the mechanism for this synthesis process. To shed some light over this complex question in a more systematic fashion, separate investigations were carried out to understand on the growth kinetics of Au nanocrystals as well as 11 ACS Paragon Plus Environment

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BINOL nanospheres. First of all, we aimed to understand the reduction kinetics of HAuCl4 precursors with CTAB and CTAC in different concentrations. Our previous report30-31 showed that the formation of nanocrystals via the reduction of metallic salts followed a pseudo first-order reaction. Its rate constant kM can be obtained from the plot of ln(Ct/C0) vs time t, where Ct indicates the [Mn+] in the supernatant after a reaction time t and C0 is the initial [Mn+]. Again, we utilized a same method to probe the growth kinetics of Au nanocrystals. Figure S11a displays the timedependent ln(Ct/C0) plots of the reduced [Au3+] in the supernatants of reaction solutions under two different surfactant conditions, either with CTAB or CTAC (0, 10‒3, and 10‒5 M). All concentrations of Au3+ ions were measured by ICP-OES measurement. The reduction rate constant kAu elevates with the increasing [CTA+X‒], where X denotes Br or Cl. However, the value of kAu remained unchanged if the same concentration of CTAB or CTAC are used. This outcome revealed a slight dependence between the particle shape evolution and the [Au3+] reduction rate. In other words, the shape evolution is a result from the halide effect, as discussed in Figure 2. We also noticed that the particle size increases with the increasing the concentration of [CTA+X‒]. Normally, higher concentration of [CTA+] should lead to a smaller size of metallic nanocrystals owing to the quick confinement effect by CTA+ double layers capped on the surfaces of nanocrystals.20 This contradiction implies a possible competition occurred between BINOL and CTA+ for Au particle surface capping. If this were true, the formation rate of BINOL nanospheres would also largely depend on the concentration of CTA+. To verify this conjecture of the concentration effect of CTA+, the control experiments using UV-visible spectroscopy was performed to measure the damping rates of suspended [BINOL] in CTAB and CTAC with different concentration. Figure S11b is the initial UV-vis absorption spectra of BINOL with 10‒3 M CTAB (red), 10‒3 M CTAC (green), and no surfactant (black). All


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of them have three intense peaks at 277, 288, and 332 nm, which are denoted as light absorption of the conjugated aromatic structures of suspended BINOL molecules. Figure S11c is timedependent UV-vis absorption spectra of BINOL without surfactant, serving as a representative example. In this case, the peak intensity dropped dramatically against time, while the appearance of solution slowly turned white and turbid, due to the precipitation of BINOL nanospheres. The rate for the formation of BINOL nanospheres is a key factor to the construction of a concentric or eccentric Au-BINOL nanostructures which is largely dependent on the concentration of the ionic surfactant. Since the identity of nanospheres have been confirmed as pure assemblies of BINOL (vide supra), their assumed formation rate (ignoring the consumption of BINOL for reducing Au3+ ions) is related to the decreasing rate of [BINOL] which is described as r = k[B] = ‒d[B]/dt, where r, k, and [B] are designated as the decreasing rate of BINOL concentration, the rate constant, and the concentration of suspended BINOL, respectively. Thus, it can be further integrated into ln([B]t/[B]0) = ‒kt in order to obtain the rate constant value k. The insets in Figure 4 are the plots of ln(At/A0) vs time obtained in different concentrations of CTAB and CTAC, where At denotes the BINOL absorbance at 332 nm after a reaction time t, and A0 is the initial BINOL absorbance at 332 nm. The ln(At/A0) is used to represent ln([B]t/[B]0) according to Beer’s law. The slope of the linear fits represents the reduction rate constant k of the peak intensity of BINOL absorbance in a specific [CTAB] or [CTAC]. Figure 4a and 4b are the plots of k vs [CTAB] (4a) and [CTAC] (4b) that exhibit the relationship between the BINOL reduction and the surfactant concentration. In both cases, there are significant drops in the k values in the initial stages of a low concentration of [CTAB] and [CTAC]. As the surfactants increase to certain critical points, in which the k value has plateaued out, the concentration of the ionic surfactant behaved like a switch to control the growth rate of BINOL moieties, instead of Au nanocrystal growth. The concentrations of CTAB


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and CTAC stock solutions at are 1.2 × 10‒3 and 1.1 × 10‒3 M, respectively (or both about 8 × 10‒4 M in the final 7 mL solution) trigger significant decrease in BINOL absorption (Figure 4). We designate both values as the “take-off concentrations” which are very close to the critical micelle concentrations (C.M.C.) of CTAB (9 × 10‒4 M) and CTAC (1.1 × 10‒3 M) in water.32-33 Thus, it can be concluded that the existing micelles of CTA+X‒ did interrupt the assembly of BINOL to form a non-regular BINOL nanospheres, when the [CTA+X‒] reaches 10‒2 M. However, when [CTA+X‒] reached a low concentration below 10-3 M, the k value increases, differentiating the momentary interaction of BINOL and Au during growing process. For this reason, the regio-selective formation of eccentric and concentric Au-BINOL is more likely a consequence resulting from the competition of dynamic adsorption on Au surfaces by BINOL and CTA+ in the early stage of Au nucleation. Scheme 1 represents our postulated growth mechanisms of the Au-BINOL nanostructures. When [CTA+X‒] reached a concentration higher than the takeoff concentrations (Figure 4), then there is abundant CTA+ available to help disperse BINOL molecules, and fully cap Au nanoclusters in the form of bilayers (Scheme 1a). They not only interrupt the self-assembling of BINOL but prevent new nucleation of Au micro-crystal from BINOL capping. As a result, a quick growth condition of Au is favored, leading to large Au nanocrystals with slow- and post-coated BINOL thin layers. However, if [CTA+X‒] is lower than the take-off concentrations, the surfaces of Au nuclei are more likely surrounded by incomplete monolayers of CTA+ (Scheme 1c). This causes further self-assembling of BINOL molecules along the hydrophobic chains, eventually constructing the concentric Au-BINOL nanostructures with small Au particles (2-5 nm). Interestingly, as the [CTA+X‒] is close to the take-off concentrations, eccentric nanostructures dominate the growing process because of an incomplete or heterogeneous capping of CTA+ monolayers on the surfaces of Au nuclei (Scheme 1b), by which the adsorption 14 ACS Paragon Plus Environment

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of BINOL exhibits site selectivity. Such uneven surface coverage results in heterogeneous distribution of BINOL, and therefore eccentric Au-BINOL nanostructures. Once CTA+X‒ is totally excluded in the synthesis, BINOL has difficulty in capping the surfaces of Au nanocrystals owing to the negative charges of chloride anions from Au precursors. Instead, BINOL molecules quickly assemble to form nanospheres (Scheme 1d). Given the clues of the close connection between BINOL and CTA+X‒, we wanted to further understand the benefit of the BINOL molecular structure to regio-selective formation of hybrid structures. In the control experiments, 1-Naphthol and 2-Naphthol were engaged to replace BINOL in the synthetic condition of 10‒5 M CTAC. Both of these molecules produced Janus structures comprising organic nanospheres with their surfaces adsorbed by Au nanocrystals (Figure S12). Compared with that of BINOL (concentric structures), this reveals a significant benefit by the BINOL structure to form an organic-inorganic hybrid (as well as CTA+X‒). The assumed reason is that BINOL owns a more flexible structure than those of 1- and 2-Naphthol; thus, it drives stable and extendable assembling of BINOL molecules along hydrophobic chains of CTA+, finally giving Au-BINOL hybrid structures. Although 1- and 2-Naphthol cannot lead to hybrid structures, the existence of nanospheres in their final products implies many organic components in hybrid composites could be prepared with aromatic units.

CONCLUSIONS We report a facile, one-step strategy for the regio-controlled synthesis of Au-BINOL hybrid nanomaterials, where BINOL serves in both the self-assembly into nanospheres and reduction of the gold precursor. The synthesis is sensitive to the concentration of CTAB or CTAC. The [CTA+X‒] was a switch able to modify not only the reduction rate of Au precursors but also the formation rate of BINOL nanospheres. It served as a bridge and allowed the regioselective 15 ACS Paragon Plus Environment

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formation of concentric and eccentric Au-BINOL hybrid nanostructures by balancing the reaction kinetics and self-assembly. By virtue of the findings, the synthesis of Au-BINOL nanocomposites is an archetype for the future development of diverse functional organic-inorganic hybrid nanomaterials.


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Figure 1. SEM and TEM images of (a,b) eccentric and (c,d) concentric Au-BINOL nanostructures synthesized with 10-3 M CTAB (eccentric) and 10-5 M CTAB (concentric), respectively. The scale bars in the inset panels indicate 50 nm. (e) Synchrotron XRD patterns of eccentric, concentric AuBINOL nanocomposites, and pristine Au nanoparticles (13 nm) as the reference.


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Figure 2. (a) SEM image of eccentric Au-BINOL nanostructures made with 10−3 M CTAC. (b) HAADF-STEM image of a single structure and its corresponding STEM-EDS maps of (c) gold and (d) carbon. (e) SEM and (f) TEM images of concentric Au-BINOL nanostructures made with 10−5 M CTAC. 18 ACS Paragon Plus Environment

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Figure 3. FTIR spectra of eccentric Au-BINOL nanostructures, CTAC, CTAB, commercial BINOL, and NFQ.


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Figure 4. The plots of the reduction rate constant k of BINOL absorption at 332 nm vs (a) CTAB, and (b) CTAC concentrations. The inset panels show the ln(At/A0) vs time to obtain the values of k in different CTAB and CTAC concentrations.


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Scheme 1. Schematic illustration for the competitive capping of CTAB/CTAC and BINOL on Au clusters in the conditions of [CTAB] or [CTAC] (a) >> 10‒3, (b) ≈ 10‒3, (c)

Interface-Controlled Synthesis of Au-BINOL Hybrid Nanostructures

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