Au-BINOL Hybrid Nanocatalysts: Insights into the Structure-Based

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Materials and Interfaces

Au-BINOL Hybrid Nanocatalysts: Insights into the StructureBased Enhancement of Catalytic and Photocatalytic Performance Shashank Reddy Patlolla, Chen-Rui Kao, Guan-Wei Chen, Yu-Cheng Huang, Yu-Chun Chuang, Brian T Sneed, Wu-Ching Chou, Tiow-Gan Ong, Chung Li Dong, and Chun-Hong Kuo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06489 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Industrial & Engineering Chemistry Research

Au-BINOL Hybrid Nanocatalysts: Insights into the Structure-Based Enhancement of Catalytic and Photocatalytic Performance Shashank Reddy Patlolla,†,§,△ Chen-Rui Kao,† Guan-Wei Chen,†,|| Yu-Cheng Huang, Yu-Chun Chuang,‡ Brian T. Sneed,# Wu-Ching Chou, Tiow-Gan Ong,*,† Chung-Li Dong,*,˄ and ChunHong 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, Taipei 11529, Taiwan △Department ||Department

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

of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu30013, Taiwan

Department

of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan

‡National

Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

#Cabot ˄Department Institute

Microelectronics, Aurora, Illinois 60504, United States of Physics, Tamkang University, New Taipei 25137, Taiwan

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

To whom correspondence should be addressed. *(C.-H. K.) [email protected]; *(T.G. O.) [email protected]; *(C.-L. D.) [email protected] 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Advances in new systems of organic-inorganic hybrid nanocomposites are less prevalent, owing to a lack of facile strategies for precise control of their structures, compositions, and hence their properties. In this work, Au-BINOL hybrid nanocomposites with eccentric and concentric nanostructures were produced. The hybrid nanocomposites containing two distinct moieties of inorganic Au nanocrystals and organic BINOL nanospheres were applied to catalytic hydrogenation of 4-nitrophenol with NaBH4 in the aqueous phase with and without illumination of visible light. Here we demonstrate that the existence of Au-BINOL interfaces offers benefits to their performance. The eccentric nanostructures made with CTAC show the superior activity arisen from large Au-BINOL interfaces formed between the BINOL nanospheres and the faces of Au nanoplates. They further exhibit a high Au LSPR-enhancement effect on photoreduction of 4nitrophenol, which is attributed to the strong LSPR absorption of exposed Au nanocrystals unaffected by the coverage of BINOL shells.

KEYWORDS: gold, BINOL, plasmonic, catalysis, XAS

2 ACS Paragon Plus Environment

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INTRODUCTION Organic-inorganic hybrid nanocatalysts have been given special attention in recent years as they consist of two or more components with distinctive and complementary properties.1 Notestein et al. have demonstrated that active site optimization of synthetic heterogeneous catalysts is needed to synthesize arrangements in which the surface of the inorganic substance acts synergistically with other organic ligands to stabilize specific catalyst structures or a given transition state. Such synergy in heterogeneous catalysts consisting of hybrid organic–inorganic interfaces creates features that are uncommon in most homogeneous or biological catalysis.2 Accordingly, these organic–inorganic hybrid materials are expected to show startling physics through new properties emerging at the interfaces that are nonexistent in either of the individual organic or inorganic building blocks.3 To thoroughly understand the interface-dependent properties for potential applications, precise control on the interfacial geometry is a crucial prerequisite.4 In spite of the clear goal, insightful studies were blocked owing to the challenge in operational conditions for simultaneous control of both growth kinetics of organic molecules and inorganic substances.5 In other words, unifying two different types of materials into a hybrid structure, where weakly interacting molecules in one moiety and strong covalent bonding at the interface with the other, creates a new level of complexity in the description of crystal-growth phenomena. Moreover, the anisotropy of the organic and inorganic moieties leads to poor control over the subtle interplay between the two components.6 In our former work, we have demonstrated a one-pot and one-step strategy for the precise-controlled synthesis of Au-BINOL hybrid nanocomposites.7 The structures contain inorganic Au nanocrystals formed from HAuCl4 reduction by a portion of BINOL and organic BINOL nanospheres coming from π-stacking of aromatic rings. By adjustment of



surfactant concentration, the relative position of Au nanocrystal and BINOL nanosphere were shifted to form eccentric and concentric hybrid nanostructures. Although the formation mechanism of the Au-BINOL hybrid nanostructures were investigated in our former work, the interplay between BINOL and Au for catalysis remains unclear. Au nanocrystals (Au NCs) are excellent photocatalysts by virtue of their chemical durability and high capability of visible energy conversion via localized surface plasmon resonance (LSPR).8-9 When integrated with organic compounds into a hybrid nanostructure, the adsorbed organic moiety may offer additional benefits aside from the role in the overall structural integrity of the Au NCs.10 For example, Liu et al. reported a high-performance catalytic membrane designed for continuous-flow catalytic reactions composed of ultra-small gold nanoclusters (Au NCs) and high-aspect-ratio carbon nanotubes (CNTs). In this membrane, Au NCs served as superior catalysts for hydrogenation of 4-nitorphenol, while the ligands function as surfactants to dissolve CNTs, and as an efficient protector for the Au NCs to avoid agglomeration.11 Reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride is a benchmark reaction widely investigated on metal-polymer hybrid nanocomposites and bimetallic nanomaterials, by which the interplay between two components or elements in a nanostructure can be estimated.12-15 In this present work, we specifically investigated the possible interplay in the Au-BINOL hybrid nanocomposites by getting insights into their catalytic behaviors in hydrogenation of 4-nitrophenol along with NaBH4 in dark and under illumination of visible light. As a result, we realized the importance in creation of Au-BINOL interfacial regions toward catalysis through examining the reaction activation energies and activities with different types of Au-BINOL hybrid nanocomposites as catalysts. The eccentric nanostructures made with CTAC addition have better performance due to larger exposure of Au-BINOL interfaces. Au moieties in


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the hybrid nanostructures, whether inside or outside BINOL shells, serve as photo-absorbers via LSPR to transfer photon energy into the catalytic reaction. By examining the LSPR enhancement phenomenon, eccentric nanostructures were again confirmed as the superior photocatalysts for 4nitrophenol reduction, which we attribute to a strong, uninterrupted Au LSPR-effect. Finally, the charge transfer of these hybrid nanocomposites with and without illumination of visible light were studied by X-ray absorption spectroscopy to further confirm the advantageous existence of AuBINOL interfaces toward catalysis.

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%), 4-nitrophenol (C6H5O3, 99%, Alfa Aesar), sodium borohydride (NaBH4, > 98%, Sigma Aldrich). Ultrapure deionized water DI water (18.2 MΩ cm-1) was used for all solution preparations. Synthesis of Au-BINOL Hybrid Nanocomposites. The synthetic procedures of eccentric and concentric hybrid nanostructures were carried out by referring to those in our earlier work.7 Briefly, 0.01 M HAuCl4 stock solution was prepared by dissolving hydrogen tetrachloroaurate(III) trihydrate in DI H2O. Meanwhile, 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. Next, a solution was made by 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 prepared in a 22 mL glass vial which was then placed in an oven at 55 °C for 6 h. The stock solution of surfactant was prepared by



adding CTAB or CTAC powder into 5 mL of DI H2O and then was treated with sonication for complete dissolution of the powder. To obtain eccentric and concentric nanostructures, the added weights of CTAB and CTAC were changed to get the concentrations of 10−3 and 10−5 M, respectively. The concentrations of 10−3 M surfactants were used to synthesize eccentric AuBINOL hybrid nanostructures, and those of 10−5 M surfactants for concentric ones. After reacting for 6 h, the suspended greenish-brown colloid in the resulting solution was collected by centrifuging at 6000 rpm for 12 min (Eppendorf Centrifuge 5804) and redispersed in 5 mL DI H2O. This rinsing process was repeated two more times to ensure that any excess surfactant was removed. Finally, the resulting clear supernatant was discarded and the colloids were stored in 5 mL of DI H2O. Catalytic Reduction of 4-Nitrophenol. The reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in an aqueous solution was performed to explore the interfacial and the AuLSPR effects of the two types of hybrid nanostructures via recording the degradation kinetics of the 4-NP featured absorption peak at 400 nm in dark and under illumination of visible light. Typically, a solution containing 1.0 mL of DI water, 1 mg of catalysts, 1.5 mL of 0.01 M NaBH4, and 1.0 mL of 0.15 mM 4-nitrophenol was prepared in a quartz cuvette with an inner edge length of 1 cm. The cuvette was placed in a water bath controlled at constant temperatures of 25, 30, 35, and 40 °C throughout the reactions. The reactions were carried out in dark and under the illumination of visible light using a solar simulator providing a true incident light power of 1000 w/m2 to the sample at a distance of 25 cm. HITACHI U-3310 UV-visible spectrometer was set to monitor the absorbance in the range of 280 to 550 nm where the λmax of 4-NP is at 400 nm and 4AP at 305 nm. For comparison, pure Au nanoparticles with an average size of 13.6 nm and pure BINOL nanospheres with an average size of 156.0 nm were used as references. The calibration


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line of 4-NP was carried out (Figure S1a) to obtain the extinction coefficient (262.4 L·mol‒1·cm‒1) with the Beer’s law for calculating the specific conversion of 4-NP. Characterization. To prepare samples for SEM or TEM, the collected products were concentrated to 0.2 mL 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 and allowed to dry 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 and HAADFSTEM images were taken by a JEOL JEM-2100F microscope operating at 200 kV. Energy dispersive X-ray spectroscopy (EDS) spectrum images were acquired in the modes of mapping and line scanning. UV–Vis absorption spectra were measured on a HITACHI U-3310 spectrophotometer. The X-ray diffraction experiments were carried out using Taiwan Photon Source (TPS) at beamline 09A in the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The diffraction data were collected using 15 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 FTIR spectroscopic analysis for nanocomposites were recorded by using an FT-IR spectrometer (Perkin Elmer Spectrum 100). The surface chemical states of Au-BINOL nanocomposites were measured with a PHI QuanteraII instrument (ULVAC-PHI, Inc.) using a single optical scanning device (scanning monochromated Al anode) as the X-ray source and 180º hemispherical analyzer along with 128-channel detector for energy analysis. The thermostability of samples were recorded by Pyris 1 TGA thermogravimetric analyzer (PERKIN ELMER) under N2



atmosphere condition. The catalytic conversion reaction of 4-NP to 4-AP under irradiation was performed by solar simulator (model: XEF 300, 007, SAN EI Electric). The X-ray absorption spectra at carbon (C) K- and gold (Au) L3-edge were carried out at BL20A and BL17C, respectively, at the National Synchrotron Radiation Research Center, Hsinchu, Taiwan.

RESULTS AND DISCUSSION Eccentric and Concentric Hybrid Nanostructures. The formation of Au-BINOL hybrid nanostructures was accomplished via heating mixed solution of HAuCl4 precursors and BINOL molecules reported in our previous work.7 We showed that BINOL not only served as the reducing agent for HAuCl4, but also acted as an organic functionality in these hybrid nanostructures. In the synthetic process, the ionic surfactants CTAB and CTAC, both bearing long alkyl-chain CTA+ and halide anions, served as nanostructure stabilizers by virtue of their capability of directing the Au nanocrystals in an orderly fashion via capping of metallic surfaces with CTA+/halide bilayers. The regioselective formation of eccentric and concentric Au-BINOL nanostructures was governed by the switch of [CTA+X−] (X =Br, Cl). Adjusting the [CTA+X−] could alter the states of dynamic adsorption on Au surfaces by BINOL and CTA+ molecules in the early stage of Au nucleation. When the [CTA+X−] was close to the values of critical micelle concentrations (CMC) in the solution, eccentric nanostructures dominated the growth process because of an incomplete or heterogeneous capping of CTA+ monolayers on the surfaces of Au nuclei, by which the adsorption of BINOL exhibited site selectivity. When the [CTA+X−] was much lower than CMC, the surfaces of Au nuclei were more likely surrounded by incomplete monolayers of CTA+ that caused further self-assembling of BINOL molecules along the hydrophobic chains, eventually constructing the concentric Au-BINOL nanostructures with small Au particles.


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Figure 1 displays the collection of TEM and SEM images of Au-BINOL hybrid nanocomposites. In Figure 1a-d, the results show the eccentric hybrid nanostructures synthesized with the stock solution of 10−3 M CTAC (Figure 1a-b) and 10−3 M CTAB (Figure 1c-d). These “Janus” type eccentric nanostructures possess well-defined heterogeneous domains of inorganic Au nanocrystals and organic BINOL assemblies. The hybrid nanostructures made with CTAC contain a high proportion of flat Au nanoplates (50‒200 nm), but those with CTAB mostly have faceted twinned nanoparticles, which is attributed to the halide effect to influence the nanocrystal growth.16-17 On observation, most formed a “droplet-on-plate” morphology (Figure S1). This infers that the strong interplay between CTA+ and BINOL in the competition of Au surface adsorption is crucial in the growth process. Turning the concentrations of surfactants down to 10−5 M, the concentric nanostructures of core-shell Au-BINOL formed instead of the eccentric ones (Figure 1e-h). The sizes of Au nanocrystals inside the BINOL nanospheres are efficiently confined to between ~2-5 nm. This result infers that the lack of CTA+ molecules (not enough to form bilayers) causes monolayer adsorption on the surfaces of Au nanocrystals with their organic tails exposed to attract and facilitate assembly of aromatic BINOL molecules.7 Structural Features of Hybrid Nanostructures. Figure 2 shows XRD patterns of eccentric, concentric Au-BINOL hybrid nanocomposites, and reference Au nanoparticles (Figure S2a) that were acquired by15 keV synchrotron X-ray. All patterns have a set of peaks at 38.32°, 44.54°, 64.84°, 77.87°, and 82.03°, which correspond with the (111), (200), (220), (311), and (222) crystal planes of the face-centered cubic Au crystal structure (JCPDS 00-004-0784). The result indicates the hybrid nanostructures are composed by crystalline Au moieties and amorphous BINOL counterparts. To characterize the BINOL moieties, the colloids of Au-BINOL nanostructures were dried and made into pellets with dried KBr powder for FT-IR spectroscopy.



Figure 3 gives the FT-IR spectra of CTAB, CTAC, commercial BINOL, and Au-BINOL hybrid nanostructures. In the spectra of CTAB and CTAC, the characteristic IR absorption peaks include aliphatic C-H (2918 and 2849 cm−1), N-H (3018 cm−1), and C-N (1488 cm−1).18 These peaks don’t show up in the spectra of Au-BINOL hybrid nanostructures, owing to the cleaning steps by centrifugation. Instead, most of the characteristic peaks of BINOL, e.g. O-H (3487 and 3403 cm−1), C=C (1618 cm−1), and aromatic C-H (827 and 751 cm−1), can be observed. It suggests the organic component of the hybrid structures comprises BINOL molecules, assembling through π-π stacking and intermolecular hydrogen bonding. Figure 4 displays HAADF-STEM images and EDS analytical results of a selected eccentric (Figure 4a-d) and concentric (Figure 4e-h) Au-BINOL nanostructures. According to the EDS maps of local composition (Figure 4b-c, 4f-g), the signal distributions of Au and C can be well-distinguished as two individual domains. Moreover, their corresponding EDS line-scan profiles (Figure 4d, 4h) show sharp boundaries at the edges of the Au signal plateau. This reveals the interface between Au and BINOL domains, and further verifies the common inter-diffusion phenomenon at heterogeneous interfaces19-21 (i.e. metal-metal or metal-semiconductor) is inhibited. Apart from the interfaces, the presence of intact Au domains at nanoscale is also an advantage to generate the localized surface plasmon resonance (LSPR) after excitation by visible light, which is able to enhance the surface reactivity of the nanocatalysts and therefore the catalytic performance.22 Figure S3 collects the HRXPS spectra of Au-BINOL samples to show their chemical states. In the spectra, their Au 4f peaks (Figure S3a) all exhibit shifts to lower binding energy while the C 1s peaks (Figure S3b) move toward higher energy in comparison to those of pristine Au nanoparticles (4f5/2: 87.6 eV and 4f7/2: 84.0 eV) and pristine BINOL nanospheres. The results reflect that significant charge redistributions take place between Au and BINOL which exhibit


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flows of valence electrons from BINOL to Au. It is worth noting that the sample of CTACeccentric Au-BINOL nanostructures gives the least shift in Au 4f peaks. However, there are weak shoulders aside the two main 4f peaks. Figure S3c shows its corresponding fitting spectrum where two pairs of 4f peaks representing two components were fitted. The minor component (purple) has the 4f peaks at 86.7 and 83.0 eV, the same to those of all other Au-BINOL nanocomposites in Figure S3a. The major component (green) has the 4f peaks at 87.6 and 83.9 eV which well correspond with those of pure Au nanoparticles. According to the result, we estimate that the mixed XPS signals should come from the blended nanostructures of Au nanoparticle (NP)- and Au nanoplate (PLT)-BINOL in the sample. The Au nanoplates are big in size (300-500 nm) whose chemical state would be close to that of pure Au because of the relatively high exposed Au surface area without contacting BINOL. On the other hand, the pair of weak 4f peaks is possibly arisen from the minor amount of Au NP-BINOL in the same sample. Figure S4 shows the TGA curves of Au-BINOL nanocomposites and pristine BINOL nanospheres. For all samples, there are slow drops of weight by 200 °C that can be attributed to the evaporation of adsorbed moisture. After 200 °C, the drastic decreases in weight are the result of BINOL degradation. The consistency in the degradation temperature for all samples reveals that the Au-BINOL nanostructures hold the original thermostability of BINOL, which causes the limitation in heating temperature for thermal catalysis. Catalytic Properties of Au-BINOL Nanocatalysts. To study structure and Au LSPR effects of Au-BINOL hybrid nanocomposites upon their catalytic performances, the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by sodium borohydride was chosen as a model reaction. Apart from the catalysts of eccentric and concentric Au-BINOL nanocomposites, pure Au nanoparticles prepared by reducing HAuCl4 with sodium citrate at 100 °C in water phase and 11 ACS Paragon Plus Environment


pristine BINOL nanospheres synthesized without HAuCl4 and any surfactant were used as references. As shown in Figure S2, the Au nanoparticles were mondipersed spheres in an average size of 13.6 nm with a percent size deviation of 11.0% (Figure S2a-b), and the BINOL nanospheres had an average size of 156.0 nm with a percent size deviation of 15.4% (Figure S2c-d). Reducing 4-NP to produce 4-AP is a vital reaction in that 4-AP is the final intermediate in the industrial synthesis of paracetamol (also called acetaminophen or APAP), a common medicine used to treat pain and fever. The reaction has been demonstrated to follow Langmuir-Hinshelwood mechanism statistics that both reactant molecules adsorb onto the surfaces of catalysts before undergoing the reaction.23 4-NP is a strong visible-light absorber owning an absorption peak at 400 nm, yet the reaction product 4-AP has absorption at 305 nm. In the presence of excess NaBH4, the reaction kinetics correspond with pseudo first order which is merely dependent upon the concentration of 4-NP. Hence, the slope in the plot of ln(A0/At) vs time, where A is the absorbance at 400 nm, represents the reaction rate k. Figure S5 is the calibration line of 4-NP to check the relationship between absorbance and [4-NP]. The linear correlation indicates that it correpsonds with Beer’s law which can be used to estimate the time-dependent [4-NP] for further calculation of the reaction activation energy. Figure S8a-d collects the time-dependent UV-Vis spetcra of 4-NP reduction in the solution of NaBH4 in dark without catalyst (blank test). By recording the decreasing intensity of 4-NP peak at different temperatures of 25, 30, 35, and 40 °C, the values of k were obtained as 1.49 × 10‒4 min‒1, 8.26 × 10‒4 min‒1, 3.05 × 10‒3 min‒1, and 4.85 × 10‒3 min‒1 in order, from the plots of ln(A0/At) vs time (Figure S8a). Such low rate values with a little increment for temperature– dependent reveal a poor efficiency in reduction of 4-NP without catalysts. The activation energy of the blank reaction is estimated to be 180.88 kJ/mol as shown in Figure 7a, based on the plot of


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lnk vs 1000/T by referring to the Arrhenius equation,24 k = Ae-Ea/RT (or ln k = (‒Ea/R)(1/T) + lnA), where k is rate coefficient, A is a constant, Ea is activation energy (kJ/mol), R is universal gas constant (8.314 × 10‒3 kJ/mol·K), and T denotes temperature (in Kelvin). Figure 5b summarizes all values of activation energy for 4-NP reduction with different catalysts. By using BINOL nanospheres (BINOL NSs) and Au nanoparticles (Au NPs) as catalysts (Figure S8e-l, S8b-c), we found that the values of activation energy were reduced to 109.95 and 60.36 kJ/mol, demonstrating a more effective catalysis by inorganic Au NPs than those of organic BINOL NSs. Similarly, all prepared Au-BINOL hybrid nanocomposites were also examined as catalysts for 4-NP reduction (Figure S7, S8d-g). The activation energy were 55.92 kJ/mol for CTAC-concentric, 19.23 kJ/mol for CTAC-eccentric, 47.22 kJ/mol for CTAB-concentric, and 34.51 kJ/mol for CTAB-eccentric nanostructures. Thus, the hybrid nanostructures of Au nanocrystals and BINOL nanospheres have enhancement in their catalytic performance through possible synergistic cooperation in either surface reactivity or electronic activity. In this regard, three are three interesting phenomena worth being noted. First, eccentric nanostructures showed lower activation energy than those of concentric ones. Second, the surfaces of BINOL nanospheres in the concentric nanostructures exhibit a much higher catalytic ability than those of pure BINOL nanospheres. Although Au nanocrystals were buried inside BINOL nanospheres, the existence of conductive domains of Au crystals could further assist the electron transfer during catalysis.25 Third, distinct morphologies of Au nanocrystals (CTAC vs CTAB) in a similar eccentric nanostructures exhibit a rather difference in activation energy of 4-NP reduction. As shown in Figure 1 as well as in our former discussion,7 the synthesis of the eccentric Au-BINOL hybrid nanocomposite with CTAC promotes the formation of triangular Au nanoplates (Au TPs). Although the Au TPs cause a much lower surfaceto-volume ratio than that of Au nanocrystals in the case of CTAB-eccentric nanostructures, the



sharp corners and edges of the Au TPs are highly active sites for catalysis. Moreover, the shape of plate makes the hybrid nanostructure form the “droplet-on-plate” morphology which possesses a large interfacial interaction between Au and BINOL, facilitating effective electron transport in catalysis. The durability of Au-BINOL hybrid nanocomposites was evaluated under these catalytic conditions. These hybrid nano-materials were isolated and examined via centrifugation process after the 4-NP reduction reaction. Figure S9 displays the SEM images of four different Au-BINOL hybrid nanocomposites after catalytic reaction, exhibiting well-maintained in their structural integrity without any significant morphological evolution taking place in either Au or BINOL moieties. The stability of hybrid nanocomposites after catalysis was further characterized and validated separately by XRD (Figure S10a) and FT-IR (Figure S10b), exhibiting no much chemical alteration from the original form that is freshly prepared from synthesis. LSPR-enhanced Photocatalytic Activity of Au-BINOL Nanocatalysts. Localized surface plasmon resonance (LSPR) is the phenomenon of coherent oscillations of conduction electrons in a metallic nanocrystal excited by electromagnetic radiation at a metal-dielectric interface. The spectral position of the resonance and the magnitude of the near field of LSPR depend on the size, shape, composition, and local dielectric environment of the nanostructure. Linic et al. has reported that the unique capacity of plasmonic nanostructures, particularly Au, Ag, and Cu with the high LSPR coefficient extinction in the visible range have capability to concentrate electromagnetic fields from a scatter radiation flux, and convert the energy of photons into heat, rendering them superior photocatalysts.22 Wang et al. have also reviewed the benefits of LSPR enhancement in driving energy conversion, CO2 reduction, and N2 fixation by integration of plasmonic metals and semiconductors into hybrid nanostructures.26-27 Nevertheless, there is a lack


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of information regarding the benefits of metal’s LSPR incorporated into organic-inorganic hybrid nanocomposite catalysts. In this respect, we’re curious that how LSPR-enhanced photocatalytic properties governed by different Au-BINOL hybrid nanocomposites has its role in the reaction of 4-NP reduction. Concerning the relationship between the structural composites and LSPR, Figure S11 shows the UV-Vis spectra of various Au-BINOL hybrid nanocomposites as well as a pure Au nanoparticles with absorption bands at the range of 500-650 nm attributed by the typical Auinduced-LSPR effect. Interestingly, the concentric nanostructures exhibited a very weak LSPR compared to eccentric ones, which we attributed to two main reasons: (i) the small crystal of Au with 2-5 nm size and (ii) complete coverage of BINOL shells over Au surfaces in concentric nanostructures compromised the LSPR phenomenon. It has been reported that small nanoparticle size leads to weaker intensity of the LSPR magnitude due to limited electronic density,28-29 while the LSPR phenomenon would also be interrupted by the thickness of BINOL coverage that hampered the exposure of incidence light. In addition, the larger thickness of CTAB-concentric (53.8 nm) compared to the CTAC-concentric (29.5 nm) has consistently diminished the Au LSPR’s strength as the CTAB-concentric effectively blocks off some of visible incidence light (the average thickness of BINOL shells was approximately estimated by the difference of the outer diameters of concentric nanostructures and the inner distribution lengths of Au cluster groups insides, as shown in Figure S12). The photo-enhanced catalytic activities of various Au-BINOL nanostructures, blank BINOL nanosphere as well as pure Au NPs were further evaluated by acquiring data of timedependent UV-Vis spectra of 40-nitrophenol in reduction process at 35 °C in presence and absence of lights (solar simulator, ~1000 w/m2) as shown in Figure S13. All these data in Figure S13 were



eventually normalized and converted into plots of ln (At/A0) vs time in Figure 6a. The corresponding results are summarized in Figure 6b, unveiling a general prominent rateenhancement in all samples containing Au NPs after the exposure of visible light. The negative results in pure BINOL nanospheres with and without light illumination further confirmed that the Au-mediated LSPR was the most likely cause of the enhancement of photocatalytic activity. Notably, no significant enhancement effect was observed in the case of the CTAB-concentric AuBINOL nanocomposite. This phenomenon can be explained by weak LSPR absorption (Figure S11) resultant the shell thickness of BINOL inhibiting effective light penetration. Finally. Figure S14 shows a series of LSPR enhancement factors of k(L)/k(D) in various samples. The highest optimum LSPR enhancements are witnessed in CTAC- and CTAB-eccentric Au-BINOL nanocomposites with ratios of 1.76 and 1.79, respectively. The superior enhancement is evidence to show the advantageous role of the eccentric nanostructures in photoreduction of 4-NP, as the Au crystal plasmons are not adversely affected by BINOL shells. Validation of Catalytic and Photocatalytic Charge Transfer by XAS. Besides the beneficial consequence of Au-LSPR effect in Au-BINOL hybrid nanocomposites on the catalytic activity, we speculated that the synergistic interfacial interaction of Au and BINOL has certain degree of influences over the reaction. To elucidate the interplay between Au and BINOL, X-ray absorption spectroscopy of C K-edge and Au L3-edge were performed. Figure 7a presents the C K-edge of prepared BINOL nanospheres (NSs), mixture of Au/BINOL nanoparticles (Au NPs physically adsorb into BINOL NSs without any surfactant manipulation), and commercial BINOL power was used as a reference. In the process of X-ray absorption of C K-edge, the electron is excited from C 1s core level to the unoccupied 2p orbital associated with * and * states. As seen in the Figure 7a, the features at around 284-286 eV and 289-295 eV are, respectively, originated


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from C 1s to * transition in aromatic C=C bond, and 1s to * transition in C-C bond.30-32 The region (286-289 eV) between * and * states are attributable to 1s to * transition in carboxyl C=O(OH) and carbonyl carbon C=O.31-32 Clearly, the * states of BINOL nanospheres is decreased and broadened in comparison with that of commercial BINOL. This is ascribed to the nature of nanostructural defect/disorder in the surface and thus reducing the - stacking. As the Au is in contact with BINOL, the intensity of * states further decreases which implies strong interaction between Au and BINOL. It is known that electrons in * states are delocalized and able to move from carbon to carbon laterally. As a result, it enhances the conductivity, and therefore the chemical and catalytic activities. The decline in the intensity of * states of mixed Au/BINOL NPs in comparison with pure BINOL NSs, suggesting Au transfers some charges to * states of BINOL. Figure 7b collects the C K-edges of concentric and eccentric Au-BINOL nanocomposites made with addition of CTAB and CTAC for comparison. Both of CTAC- and CTAB-concentric Au-BINOL nanocomposites exhibit higher * states than those of the eccentric ones. It reveals that there are more electrons transferred to the * states in the Au-BINOL eccentric nanostructures. The strong interfacial interaction between Au and BINOL in the eccentric nanocomposites is possibly due to more efficient contact of Au and BINOL by virtue of larger Au crystal faces. The estimation can be supported by comparing the peak intensities of * states between the two kinds of eccentric Au-BINOL nanocomposites formed with CTAC and CTAB. The CTAC-eccentric nanostructures are composed by Au nanoplates with large 111 crystal faces contacted with big BINOL nanospheres (>200 nm as shown in Figure 1a, b), and thus exhibits a lower intensity of * states than those composed by irregular Au nanoparticles contacted with relatively small BINOL nanospheres (

Au-BINOL Hybrid Nanocatalysts: Insights into the Structure-Based

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