Controlling Photocatalytic Reactions and Hot Electron Transfer by

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Controlling Photocatalytic Reactions and Hot Electron Transfer by Rationally Designing Pore Sizes and Encapsulated Plasmonic Nanoparticle Numbers Nguyen Nhat Nam, Thanh Lam Bui, Ngoc Thanh Ho, Sang Jun Son, and Sang-Woo Joo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05737 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

Controlling Photocatalytic Reactions and Hot Electron Transfer by Rationally Designing Pore Sizes and Encapsulated Plasmonic Nanoparticle Numbers

Nguyen Nhat Nam,§,† Thanh Lam Bui,#,† Ngoc Thanh Ho, # Sang Jun Son, §,* Sang-Woo Joo#,* §Department

#Department

of Chemistry, Gachon University, Seongnam 13120, Republic of Korea of Information Communication, Materials, Chemistry Convergence Technology,

Soongsil University, Seoul 06978, Republic of Korea.

* Address correspondence to [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Controlled self-assembly of different numbers of gold nanoparticles (AuNPs) in the highly ordered cylindrical anodic aluminum oxide (AAO) nanopores was achieved by a facile ultrasonication method. The surface plasmon resonance bands became redshifted as increasing in the number of nanoparticles (N) on the top view from monomer of M1 (N=1) to multimers of M2 (N=3±1), M3 (N=5±1), and M4 (N=7±1). The numerical calculation of the M2 model showed the best match of the 2-nm nanogap among 28 nm diameter AuNPs in 53 nm diameter nanopores. When the number of AuNPs inside the nanopores extended, the more hotspots were generated, which induced the plasmon-driven photocatalysis from AuNPs clusters at the incident visible light of 633 nm. The enhanced photocatalytic reaction of NBT was observed after sequentially raising the number of AuNPs, which began at M3 and were maximized for M4. The M3 configuration could be a magic number of AuNPs clusters for the nanogap-induced photocatalysts under the 633 nm irradiation (~0.2 mW) for 12 min. Our methods should be helpful in adjusting photocatalysis by varying the numbers of nanoparticles inside the tunable nanopores.


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1. INTRODUCTION. The coupling of light and nanostructure has attracted considerable interest in plasmonassisted applications.1-5 To date, the surface-enhanced Raman scattering (SERS) technique is one of the most sensitive ways to study the plasmon driven photocatalytic reactions of analytes on the surface of nanomaterials.6-10 The visible-light-driven photocatalysis via surface plasmon resonance induced the generation of charge carriers efficiency, including energetic electrons and holes.11,12 Also, this phenomenon could be exhibited through the near-field plasmonic coupling of noble nanoparticle at the gap junctions between plasmonic nanostructures.13 Strong electromagnetic field enhancements at the gap junctions or the tips of metallic nanostructures called hotspots could be significantly effective for SERS signals.14,15 Although the SERS-active substrates have been demonstrated in various applications, it is still challenging in preparing the substrate with homogeneity and reproducibility.16-18 The facile fabrication of uniform SERS-active substrates produced by regular hotspots is prerequisite for a SERS study of plasmonic catalysis.19 The dimerization of 4-nitrobenzenethiol (NBT) into 4,4’-dimercaptoazobenzene (DMAB) by hot electrons on the surface plasmon nanomaterials has been recently studied via in situ SERS technique.20,21 The resonance of the laser wavelength and surface plasmon mode of nanostructure could facilitate the charge-transfer photochemistry through the local electromagnetic field.22 Moreover, the material, wavelength, and timescale considerably impact the plasmonic photocatalysis. There have been notable studies on the dimerization of NBT into DMAB from the plasmon-driven photocatalysis on Au,23 whereas hot electron generation plays an important role in the coupling reaction on Au.24 The experimental and theoretical modeling has demonstrated that the hotspots are a prominent factor for the generation of hot plasmonic electrons, which contribute


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to the plasmon-driven photocatalysis efficiency.25 The self-assembly of spherical AuNPs into confined templates can provide a versatile construction for controllable structure. Recently, the design of various Au nanostructures is proved for the formation of dynamic SERS hotspots via, an example, DNA hybridization, ligand exchange, π-π interaction, or molecular cage.26,27 The critical drawbacks for the assembly of particles are the dynamic and Brownian motion, which lead to the deformation of the assembled nanostructure and cannot expose the docking site of hotspots.28,29 The uncontrollable of hotspots by over-aggregation of particle also cause the limit of SERS application. The nanoporous AAO (anodic aluminium oxide) has been reported to take the advantages over soft-template technique, in term of a highly uniform template and precisely controllable of dimension.30 An AAO template-assisted assembly of plasmonic nanoparticles has provided a method of forming nano-patterned structures in a cost-effective way.31 In previous reports, we employed the AAO template as the intermediate material for the AuNPs encapsulation.32,33 The silica nanotubes fabricated via AAO were used for the internalization of AuNPs through the capillary force during the wet-dry process on a rocking platform under vacuum. Herein, different structures of AuNPs@AAO (M1, M2, M3, and M4) were successfully fabricated as SERS-active substrates. A facile technique for the self-assembly of AuNPs inside the nanocylinder of AAO (AuNPs@AAO) by ultrasonication was investigated. As a confined number of AuNPs inside the AAO nanopore increased, the more hotspots were generated on the plasmonic substrate. The dimerization of NBT into DMAB by obtaining SERS spectra was also studied as a typical reaction to verify the efficiency of hotspots to photocatalysis.


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2. EXPEMENTAL SECTION 2.1. Materials. Aluminum foils (Alfa Aesar, 99.99%), gold (III) chloride hydrate (Aldrich, 99.999%), oxalic acid (OCI company, 99.5%), per-chloric acid (DC Chemical, 70%), phosphoric acid (Daejung, 85%), oleylamine (ACROS, 80−90%), toluene (DC Chemical, 99.9%), ethanol (DC Chemical, 99.9 %) and 4-nitrobenzenthiol (Aldrich, 80%) were used without further purification. 2.2. Preparation of Two Different Sized AuNPs. Briefly, HAuCl4 (70 mg) was dissolved in toluene and oleylamine (1:1.2, v/v). Then, the solution was quickly injected to a preheated (80 ℃) mixture of toluene and oleylamine (10:1, v/v). The mixture was then refluxed at 120℃ for 2 h, and AuNPs were collected and purified by centrifugation and stored in toluene solvent. The whole reaction cycle was repeated 3 times and 5 times to obtain the AuNPs diameter of ~28 nm and ~43 nm, respectively. 2.3. Preparation of the Tunable Pore Diameters in AAO Templates. Typically, asannealed aluminium foils (3 cm x 10 cm, 0.25 mm thick) were chemically electropolished in a mixture solution of perchloric acid and ethanol (1:5, v/v) at 5 ℃ and 15 V. (i) The first anodization was performed in a 0.3 M oxalic acid solution at 10℃ and 40 V for 7 h and was then etched away by a solution of phosphoric acid (6 wt. %) and chromic acid (1.8 wt. %) at 60 oC. (ii) The length of AAO nanopores was controlled by the second anodization under the same condition of first anodization. The diameter of the nanopores was adjusted by pore-widening step in phosphoric acid (5 wt. %) at 30 ℃. To obtain ~50 nm and ~60 nm in length nanopore, the second anodization time was 50 s and 70 s, respectively. To obtain ~46, ~53, ~62, and ~84 nm in diameter nanopore, the pore-widening time was 13, 16, 20 and 30 min, respectively.


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2.4. Insertion of AuNPs into AAO Nanopores. AuNPs@AAO substrates were prepared using ultrasonication processes. Typically, an AAO template (3 cm x 1 cm) was immersed into AuNPs solution (1mg mL-1) in an Eppendorf tube. The tube was then sonicated in a sonication bath (2510E-DTH, Branson, 60Hz) for 60 s at 25 oC. The AAO template was treated again with pure toluene by ultrasonication for other 60 s to remove the AuNPs that bound to its surface. The inserted procedures were repeated 2-3 times to ensure the AAO nanopores fully internalized with AuNPs. 2.5. SERS Measurement of NBT. The as-prepared SERS substrate was functionalized with NBT by incubating in a solution of NBT (10-4 M) for an hour. The substrate was then washed and rinsed with ethanol to remove any surface residuals. The treated substrate was dried under room condition before shining. The SERS measurements were performed by focusing on 633 nm excitation laser, 0.2 mW for 12 min of the total expose time. The Raman spectra were recorded by a Renishaw RM 1000 microspectrometer and the laser spot diameter was ~2 µm diameter. The 35 mW laser power (25 LHP 928, Melles Griot) became reduced to 0.2 mW at the sampling position. The linearly polarized beam was aligned parallel on the sample plane. Since the random orientations of AuNPs inserted in AAO pores, we have not attempted any polarization dependent experiments. 2.6 Characterization. AuNPs@AAO substrates were characterized using a field emission scanning electron microscope (FE-SEM, JSM 7500F, JEOL). AuNPs were measured by using a transmission electron microscope (TEM, Tecnai G2 F30, FEI). The UV-Vis absorption spectra were obtained using a Shimadzu 2550 (or UV-3600 plus) spectrometer equipped with an ISR 603 integrating sphere attachment. For the crosssectional SEM of the substrates, the focus ion beam was attempted to obtain the images


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using a Zeiss crossbeam 540 equipment to show the leakage of AuNPs. The AAO substrate was deformed by hand pressure to fold the specimen. The folded AAO plate was placed onto the SEM stub, making sure that the target surface was on the outside the brittle part. The cross-sectional images were taken by focusing on the breakable brittle part, which exposed the cross-sectional area. 2.7. Computational Calculations. Finite-difference time-domain (FDTD) numerical simulation was performed using a Lumerical software to yield the scattering and absorption spectra of the AuNPs aggregates including the electromagnetic enhancements between the nanogaps. To estimate the 3D configurations of AuNPs in a confined geometry of nanocylinders, multiphysics modelling was introduced to estimate the electromagnetic field enhancements using a COMSOL vol. 5.3 software. NBT and DMAB calculations were performed via GAUSSIAN 09 at the B3LYP level with either 6-31G(d) basis set or the LanL2DZ basis set for the Au atoms.


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3. RESULTS AND DISCUSSION

(a)

d

M1: D/d~1.1 (b) D

d

M2: D/d~1.9 D

h

h

M1

(c)

d

M3: D/d~2.2 (d) D

d

h

M4: D/d~3.0 D

h

N=5±1

Scheme 1. Schematics illustrating the fabrication of monomeric a) M1 and multimeric b) M2, c) M3, and d) M4 AuNP configurations inside AAO nanopores by sonication-induced self-assembly. The length and diameter of nanopore were controlled by the second anodization and pore-widening step, respectively. The two sizes of AuNPs were prepared by the solvothermal method. The number from the top view of M1, M2, M3, and M4 were measured to be 1±1, 3±1, 5±1, and 7±1, respectively, with the homogeneity of 100, 91, 91, and 86%. Scheme 1 is to illustrate the SERS-active substrate fabrication with the different number of AuNPs (N) inside AAO nanopore (AuNPs@AAO) via ultrasonication technique and the detail process was described in the experimental section. The number of AuNPs located in the AAO was strictly dependent on the size of AuNPs (d) and the orifice diameter of the AAO (D). To further demonstrate this point, the monomeric AuNP@AAO (M1 D/d~1.0) and multimeric


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AuNPs@AAO (M2 D/d~1.9, M3 D/d~2.2, and M4 D/d~3.0) substrates were produced using two sizes of AuNPs. The AuNPs with different diameters obtained through solvothermal method were inserted inside AAO nanopores by ultrasonication-induced self-assembly.32,33 The monomeric substrate (M1) is the combination of AuNPs (~43 nm) and the AAO nanopores with ~46 nm of pore diameter, ~50 nm of pore length (Figure 1a,c and Table S1-S2). The multimeric substrates (M2, M3, and M4) were obtained by using smaller AuNPs (~28 nm) and larger nanopores diameter.

Figure. 1. The characterization of as-prepared (a) ~43 nm AuNPs, (b) ~28 nm AuNPs, (c) AAO for the fabrication of monomeric M1 (D/d~1.0) and multimeric M2 (D/d~1.9), M3 (D/d~2.2), and M4 (D/d~3.0). The size distribution of small AuNPs (~28 nm) and the nanopores diameters for M2 (~53 nm), M3 (~62 nm), and M4 (~84 nm) were shown in Figure 1b, c and Table S1, S2. Figure 2 and Figure S1-S5 exhibit the top-view and side-view SEM images of as-prepared SERS-


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active substrates. The number of AuNPs in single nanopore could be estimated by the ratio of the nanocylinder pore and the AuNPs volume (Figure S6), which the packing efficiency was clarified in previous reports.34-36

Fig. 2. The SEM images of top-view (upper) and side-view (below) of M1, M2, M3, and M4. The amount of encapsulated AuNPs released when the AAO nanopore was opened for the side-view measurement. All scale bars are 100 nm. A spherical particle filling model and representative magnified the view of the cross-sectional SEM image to estimate the number of AuNPs in each pore (bottom). In this regard, the top layer of AuNPs with hotspots array was focused on the study of SERS behaviour and photocatalytic reaction of NBT.37 The statistically results show the skewed distribution of the multimer, which indicated the number of AuNPs observed on


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the surface was 3±1 NPs/pore (91 %), 5±1 NPs/pore (92 %), and 7±1 NPs/pore (86 %) for M2, M3 and M4, respectively. The large area of AAO templates could entirely accommodate AuNPs, which was induced by ultrasonication-wave. The side-view SEM images revealed that the AuNPs were fulfilled inside the cylindrical pores of AAO. The single AuNP (M1) and AuNP clusters (M2, M3, and M4) were precisely separated from each other by approximately measuring the edge-to-edge distances of AAO nanopores, accounting by ~59 nm, ~52 nm, ~43 nm, and ~20 nm for M1, M2, M3, and M4 respectively (Figure 3a, and Table S3). The self-assembly of oleylamine-AuNPs was demonstrated to form the monolayers of a particle with ~2 nm of the inter-particle gap based on the chain length of oleylamine (C18H37N) as reported with the values ranging from 1.5–1.8 nm in most cases.38,39 The integrated spatial interaction between oleylamine chains reduced the centre-tocentre distances between the AuNPs. Despite the difficulties of measuring the nanogap distances, we could estimate the values according to the numerical calculation. Figure 3b shows the top-view FDTD simulation for the electromagnetic field distribution of a single AuNP and AuNPs clusters based on the modeling of prediction of the self-assembly of AuNPs inside tapered and curved bottom cylinder shapes. Considering the length of oleylamine was around 1.5-1.8 nm,38,39 AuNPs can hardly touch each other to form the gap less than 1 nm (nonclassical quantum regime). The occurrence of the touching particles are expected to rare in the presence of the organic layers. FDTD simulation also predicted the cluster with the gaps of ~2 nm.


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Fig. 3. (a) The number of AuNPs, hotspot density inside single nanopore of M1, M2, M3, and M4 (b) The photograph shows their visual color, the COMSOL simulation of electromagnetic field distribution at 633 nm showing increasing nanogaps and hotspots from M2 to M4. k and E represent the directions of the wave vector and electric field, respectively. (c) The calculated spectra of three dimensional M2 structures with a series of gap junction between AuNPs from 1 nm to 2.5 nm (left), the experimental (middle), and the calculation of UV-vis absorption spectra of M1, M2, M3, and M4 (right). The different gap junctions from 1.0 nm to 2.5 nm in the M2 structure were simulated via FDTD calculations. The calculated spectra depending on the nanogap values were best


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matched for the experimental UV-vis absorption spectra, when the gap distances between AuNPs for M2 were ~ 2 nm (Figure S7). The UV-Vis absorption spectra of the multimeric substrates were also observed and compared with the monomeric AuNP@AAO. The UVvis absorption spectra of experimental results and FDTD calculations indicated the significant redshifts from M1 (~530 nm), M2 (~545 nm), M3 (~570 nm) to M4 (~630 nm) due to the increase in AuNP numbers with close packed hotspot array on the SERS surfaces. The blue-green color was observed for the M4 template, whereas the monomeric AuNP exhibited a red-violet color contributing to the strong absorbance at 530 nm. The pore density of AAO (Pp) was demonstrated to be ~150 pores/µm2 in our previous report.31 Thus the total number (NA) of AuNPs and the total hotspot number (NH) on the surfaces of the SERS substrates (top layer) could be calculated: NA ≈ nA × PP × S

(1)

NH ≈ nH × PP × S

(2)

where the nA and nH are the numbers of AuNPs and hotspots in the individual nanopore, respectively. S is the surface area of the SERS-active substrate. To clarify the regularity of AuNPs@AAO substrates, the in situ SERS monitoring of the pesticide acetamiprid was observed on the monomeric M1, multimeric M4 and compared with the citrate-reduced AuNPs (Au-citrate). Figure 4 shows the SERS spectra of 500 nM of acetamiprid adsorbed on M4, M1, and Au-citrate, respectively. The main characteristic peaks of acetamiprid were obtained at 632, 1105, and 2190 cm-1 (Figure 4a).


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Fig. 4. (a) The SERS spectra of 500 nM of acetamiprid absorbed on M4, M1, and Au-citrate. (b) The stick diagram is showing the reproducibility by the CN peak at 2190 cm-1 for M4, M1, and aggregated Au-citrate. Total 15 different spots were selected randomly and tested for the Raman intensities. (c) TEM images of aggregated Au-citrate coated with acetamiprid.

The M4 substrate exhibited the highest Raman intensity due to the highest hotspots density on the surface. Also, the resonance of both incident laser wavelength and surface plasmon mode increases the capability of M4 in comparison with another structure. Acetamiprid solutions with various concentrations from 1000 nM to 20 nM were used to estimate the sensitivity of M4 substrate. The limit of detection for M4 was demonstrated to be ~20 nM (Figure S8). The reproducibility and stability of the SERS signal on AuNPs@AAO substrates were considered by


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measuring the intensity from randomly chosen 15 different points. The relative standard deviation (RSD) of the peak intensities at 2190 cm-1 was measured and compared as illustrated in Figure 4b. The RSD values for the M4 and M1 were found to be 6.64% and 5.46%, respectively, while that of the Au-citrate was as large as 22.48%. Hence, the improvement of reproducibility and stability of SERS intensities was observed on AuNPs@AAO, while the non-uniformity of the gap distance or irregular aggregation of the Au-citrate resulted in the variation and fluctuating SERS signal (Figure 4c). Also, the uncontrollable hotspots were demonstrated to limit the reliability of the SERS signal.41 This ultrasonication induced self-assembly of AuNPs inside AAO templates, which could avoid the over-aggregation and large agglomeration by fixing the separation distance between the AuNP (M1) or AuNPs clusters (M2, M3, and M4). The distribution of hotspots on the AuNPs@AAO array could increase the exposure of the docking sites, where the analytes could be trapped with a specific molecular orientation. To verify the efficiency of plasmon-driven photocatalysis40 of M1, M2, M3, and M4 substrates, NBT was chosen as a probe to observe the dimerization of NBT into DMAB using in situ SERS monitoring. The SERS measurement of the photocatalytic reaction was performed at the same condition including the excitation of 633 nm laser, 0.2 mW of laser power, within 12 min of reaction timescale. The Raman spectrum of NBT powder matches well with the simulated spectra with the typical peaks at 1084 (1074), 1325 (1320), and 1575 (1571) cm-1, which can be associated with the C-S stretching (ѵC-S), NO2 stretching (ѵNO2), and parallel C-C stretching (ѵC-C) of the molecule, respectively, (Figure S9). The simulated spectra were presented in (Figure S10) along with their optimized geometry (Figure S11).


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(a)

(b)

(c)

(d)

Fig. 5. (a-d) Comparison SERS spectra of the NBT reduction to DMAB is driven by M1, M2, M3, and M4, respectively, using 633 nm laser excitation (0.2 mW of laser power and 12 min of reaction time scale). Oleylamine layers were removed for the efficient adsorption of NBT.


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In addition, on all of the SERS-active substrates, the characteristic peaks at 1080, 1336 and 1573 cm-1 were compatible with the normal Raman spectrum.42,43 The photocatalysis without hotspots was introduced when incident laser irradiated NBT powder and the remarkable peaks for the dimerization of NBT were not exposed after 12 min of illumination (Figure S12). It is acceptable to conclude that the efficiency of NBT dimerization was notably depended on hotspots of the SERS substrate as well as the number of AuNPs. In the current report, this influence was strictly considered as a primary reason for the yield of photocatalysis besides many factors. The reduction of NBT was successively examined on four different substrates (Figure S13). There was no photoconversion behaviour observed on the monomeric M1 and multimeric M2 within the reaction of 12 min (Figure 5a, b). However, the reduction of NBT was clearly observed on the M3 and M4 substrate (Figure 5c, d). The intensity of NO2 vibration (NO2) at 1336 cm-1 constantly decreased after the continuous exposure of 633 nm laser, which was recognized the hotspots-induced the dimerization of NBT. The Raman spectrum of density functional theory (DFT) calculations for Au5–DMAB–Au5 in Figure S10, S11 approved with the experimental SERS spectra obtained from the NBT adsorption on the M3 and M4 substrates during laser illumination. A peak position appeared at 1144 cm-1 was defined as a C-N stretching vibration (C

N).

The sensitive peaks of azo group

indication were detected at 1390 and 1435 cm-1 which correlated to N=N stretching vibration (N=N) of DMAB. The DFT calculation and experimental results for SERS spectra of NBT and DMAB were also consistent with many previous reports.44,45 Notably, the dimerization of NBT into DMAB was conducted without using any reducing agent.


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Figure 6a is a simplified schematic and an energy diagram of hot electron generation and transfer on the multimeric AuNPs@AAO to the adsorbates. The generation of hot electron occurred within the hotspots array enhanced the plasmon-driven photochemical reaction.24,43 In our SERS-active substrates, the increase in a number of AuNPs and hotspots induced the sequential red-shift of the surface plasmon mode and gradually led to the overlap between the absorption of M4 at ~630 nm with the incident light frequency of 633 nm.46,47 The dimerization of NBT into DMAB is well known to be a part of pseudosecond-order kinetics regarding the chemical equation, and the intensity (I) of SERS spectra is proportional to the concentration of the analysts.48,49 The IDMAB, INBT were estimated through the intensities of the peaks at 1435 cm-1 and 1336 cm1. Considering that the relative intensity changes in NO2 at 1336 cm-1 of M3, M2, and M1 became reduced to approximately 47%, 2.9%, and 2.4%, respectively, with that of M4 (Figure S13). Figure 6b displays the corresponding plot of I1435/I1336 versus 12 minutes of the NBT dimerization on M3 and M4 under excitation of 633 nm laser, respectively. The results showed that the photocatalytic reaction rate constant relative with the ratio intensity of I1435/1336, which was calculated to be 0.0161 ± 0.00084 min-1 and 0.0072 ± 0.00032 min-1 for M4 and M3, respectively. Under the irradiation of 633 nm laser (beam diameter, d~2 µm), the total AuNPs excited on the SERS surface (the top layer) were estimated to be ~470, ~1400, ~2300, and ~3300 for M1, M2, M3 and M4 respectively regarding to the equation (1). Also, the number of hotspots on the top array was around ~0, ~1400, ~2300, and ~5600 according to the equation (2). The DMAB peaks at 1144, 1390, and 1435 cm-1 were not clearly observed for M2, despite the presence of the expected existence of hotspots, whereas the dimerization on M3 and M4 seemed to occur easily as shown in Figure 6b and Figure S14. This may be due to


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rather nonuniform substrates, which made direct correlations between the number of hotspots and plasmon-induced chemistry problematic.

Fig. 6. (a) The schematic and an energy diagram of hot electron generation on multimeric AuNP@AAO and their transfer to the molecule. (b) The spectral change and the corresponding plot of I1435/I1336 versus 12 min of reaction time scale observed on M1, M2, M3, and M4. Accordingly, the hot electron generation and transfer of M4 occurred faster than M3, M2, and M1. There can be two major mechanisms of (i) high rate generation of hot electrons through high-density hotspots and (ii) the fast and maximum hot-electron transfer


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efficiency through the resonance between the surface plasmon mode and the incident light.50,51 The light absorption of AuNPs was early studied and concerned in harnessing hot charge carriers for photocatalysis.52 The number of hot electrons with considerable energies was well demonstrated strongly effect by an individual hotspot.47,53 The steady electromagnetic enhancement of hotspots in single nanopore caused to the increase of the hot electron generation rate, which contributed by the classical effect of plasmonic enhancement and the quantum mechanism.52,53 The formation of the electromagnetic field of hotspots in every AuNP cluster was described via COMSOL computation (Figure S15). The formation of hotspots was observed near the AuNP-AuNP gap regions. The physical mechanism for the generation of hot electrons is that the linear momentum of an electron near the surface is not conserved and, therefore, the electromagnetic field can create excited electrons and holes with considerable energies ~ћѡ. The generation of hot electrons is a surface effect, which can be transferred directly to the reacting molecule near the surface, which was NBT in this study.54,55 4. CONCLUSIONS In summary, we introduced such a facile ultrasonication method could induce the selfassembly of AuNPs inside nanocylinder of AAO. The number of AuNPs inside the nanopore was controlled by adjusting the diameter of the AAO or AuNPs. The well-ordered and organized of monomeric AuNP (M1) and AuNPs clusters (M2, M3, and M4) were demonstrated to enhance the reproducibility and stability of SERS signals. The close packing of the AuNP cluster by AAO actively generates a high density of hotspots on the SERS substrates revealed photocatalytic reductions of NBT by the original approach of a


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new platform. Finally, our plasmon modelling could be used to develop for the multifunctional platform in the same manner.

ASSOCIATED CONTENT Supporting Information. Experimental details, theoretical calculation, and characterization. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Sang-Woo Joo (email: [email protected]) and Sang Jun Son (email: [email protected]) Author Contributions †Nguyen

Nhat Nam and Thanh Lam Bui contributed equally to this work.

ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) (No. 2017R1E1A1A01075141, NRF-2016R1D1A1B03932668). REFERENCE (1) Sprague-Klein, E. A.; Negru, B.; Madison, L. R.; Coste, S. C.; Rugg, B. K.; Felts, A. M.; McAnally, M. O.; Banik, M.; Apkarian, V. A.; Wasielewski, M. R.; et al. Photoinduced PlasmonDriven Chemistry in trans-1,2-bis(4-pyridyl)ethylene Gold Nanosphere Oligomers. J. Am. Chem. Soc. 2018, 140, 10583−10592. (2) Trujillo, M. J.; Strausser, S. L.; Becca, J. C.; DeJesus, J. F.; Jensen, L.; Jenkins, D. M.; Camden, J. P. Using SERS to Understand the Binding of N-heterocyclic Carbenes to Gold Surfaces. J. Phys. Chem. Lett. 2018, 9, 6779−6785. (3) Barrow, S. J.; Wei, X.; Baldauf, J. S.; Funston, A. M.; Mulvaney, P. The Surface Plasmon Modes of Self-Assembled Gold Nanocrystals. Nat. Commun. 2012, 3, 1275. (4) Almohammed, S.; Zhang, F.; Rodriguez, B. J.; Rice, J. H. Electric Field-Induced Chemical Surface-enhanced Raman Sspectroscopy Enhancement from Aligned Peptide Nanotube–Graphene Oxide Templates for Universal Trace Detection of Biomolecules. J. Phys. Chem. Lett. 2019, 10, 1878−1887.


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Controlling Photocatalytic Reactions and Hot Electron Transfer by

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