Ti@TiO2 Nanowire Electrode with Polydisperse Gold Nanoparticles

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Ti@TiO2 Nanowire Electrode with Polydisperse Gold Nanoparticles for Electrogenerated Chemiluminescence and Surface Enhanced Raman Spectroelectrochemistry Cailing Xu,†,‡ Hongwei Geng,† Robert Bennett,† Daniel A. Clayton,† and Shanlin Pan*,† †

Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336, United States Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu, China 730000

‡

S Supporting Information *

ABSTRACT: We present a multifunctional nanostructured electrode with Ti core and TiO2 shell (Ti@TiO2) nanowires (NWs) decorated with Au nanoparticles (NP) for studying spectroelectrochemistry (e.g., electrogenerated chemiluminescence, ECL) and surface enhanced Raman scattering (SERS). The so-called N3 dye (cis-[Ru (4,4′-COOH-2,2′bpy)2(NCS)2]) is used as probe molecule for studying ECL and SERS enhancement capability of this nanostructured electrode. The SERS enhancement of N3 dye is determined by surface coverage and particle size of Au NPs and the surface selfassembly configuration of N3 on this new nanostructured electrode. ECL and in-situ SERS spectroelectrochemistry studies suggest that Au NP decorated Ti@TiO2 NW electrode can serve as a new spectroelectrochemistry platform for helping understand redox reaction mechanism and quantitative analysis with the combined methods of optical spectroscopy and electrochemistry.

1. INTRODUCTION Nanostructured semiconductor−noble metal hybrid electrodes have progressively attracted great attention recently because of their potential applications in the area of ultrasensitive molecular sensing and electrochemical catalysis (e.g., catalytic systems for alternative energy conversion). For example, a nanostructured Au surface can enhance light absorption and photoelectrochemical reaction rates when coupled to an iron oxide1 and TiO22 thin film electrode. Interesting spectroscopic features and electroluminescence enhancement characteristics can be obtained by coupling Au nanostructures to a semiconductor photocatalyst.3,4 Our recent study showed that Au nanoparticles (NPs) electrodeposited onto a planar TiO2 electrode can also serve as miniaturized light-emitting sites.5 In addition to the above-mentioned applications in electrochemical systems, nanostructured Au and Ag provide intense surface plasmon resonance corresponding to the collective oscillating motions of electrons near these metallic nanostructured surfaces. For example, spherical NPs,6−9 nanorods,10,11 and nanowires (NWs)12−14 of Au have strong surface plasmon resonance in the visible and/or near-infrared range. Such plasmon absorption and optical wavelength response are strongly dependent on particle size, shape, and relative distances between these nanostructures. Experimental and theoretical studies show that the localized EM field between two adjacent Au nanoparticles can be dramatically increased when they are approximated closely to each other along the polarization direction of incident light.15 Such strong coupling © 2012 American Chemical Society

between nanostructured plasmonic structures produces localized EM field between the particles. Therefore, these strong coupled nanostructures can be used to enhance the light absorption cross-section and Raman scattering of a molecule when the molecule is placed in the enhanced EM field.16−18 Recent studies show that plasmonic nanostructures with optimal configuration are capable of enhancing Raman signals by a factor of 107 or higher.19 Here we present a multifunctional nanostructured electrode which is made of Ti@TiO2 NW with polydisperse Au NPs. This new structure is prepared on Ti substrate using combined methods of hydrothermal reaction and double-pulse potential electrodeposition. The Ti@TiO2 NW structure provides (1) conductive Ti NWs integrated onto a conductive substrate for charge collection and Au NP electrodeposition, (2) an ultrathin TiO2 outer layer on Ti NWs for self-assembly of SERS- and ECL-active dye molecules, and (3) strong plasmon coupling of polydisperse Au NPs with 3D spatial arrangement for SERS and spectroelectrochemistry studies.

2. METHODS AND MATERIALS 2.1. Ti@TiO2 NW Preparation. Ti@TiO2 NW substrate preparation was based on a modified method from a previous report.20 As shown in the schematic of Figure 1A, 99.0% Received: November 2, 2012 Revised: December 13, 2012 Published: December 21, 2012 1849

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Figure 1. (A) Schematic of Ti NW preparation on Ti plate followed by thermal anenaling to form TiO2 layer and Au NP electrodeposition using two-stage stepwise potential method. (B) Cyclic voltammetry of Ti@TiO2 NW electrode in 0.1 M NaCl aqueous solution containing 1.0 mM AuCl3. Scan rate: 50 mV/s. (C) Two-stage stepwise potential function applied to the Ti@TiO2 NW electrode for polydisperse Au NP growth and typical current response. E1 = −1.6 V and E2 = −0.8 V (vs Ag/AgCl) are selected potentials from panel B for growing nucleation sites of Au and particle growth with duration time t1 and t2, respectively.

titanium plates with size of 5.0 mm × 1.5 cm and thickness of 0.50 mm were first cleaned with acetone in an ultrasonic bath for 20 min and then were rinsed with a large amount of water. After drying in air, all substrates were chemically etched in 18 wt % HCl solution at 85 °C for 15 min to remove the oxide layer naturally formed in air to provide a fresh Ti surface for the hydrothermal reaction. In a typical procedure, titanium plates were loaded into a 23 mL Teflon-lined stainless steel autoclave (Parr Instrument) filled with 10 mL of 2.5 wt % HCl aqueous solution and kept at 190 °C for 12 h to complete hydrothermal reaction. After being cooled to room temperature, the asprepared samples were completely washed with distilled water and dried in air. The as-prepared Ti NW substrates were then annealed at 450 °C for 10 h to form a thin TiO2 shell layer on Ti NW. 2.2. Electrodeposition of Au NPs onto Ti@TiO2 NWs. As shown in the schematic of Figure 1A, Au NPs were electrodeposited onto Ti@TiO2 NWs obtained previously. Typically, a piece of Ti@TiO2 NW electrode was immersed in 0.1 M NaCl containing 1 mM AuCl3 for cyclic voltammogram (CV) measurement using a biopotentiostat (CHI 760, TX). As shown in Figure 1B, cathodic current density increases in the negative scan direction due to Au3+ reduction, and the back scan does not show a reversible oxidation peak, indicating the deposition of Au onto Ti@TiO2 NW surface. Two consecutive potential steps (E1 and E2) were selected from the CV for electrodepostion of Au NP as shown in Figure 1B. E1 is located near −1.6 V (vs Ag/AgCl), and E2 is −0.8 V (vs Ag/AgCl). E2 is near the turn-on potential for Au reduction at a Ti@TiO2 NW electrode. E2 is more negative than the thermodynamic potential of Au because of the sluggish kinetics of Au reduction at Ti@TiO2 NW surface. As shown in Figure 1C, E1 and E2

were applied sequentially using the pulse potential function of the potentiostat for Au NP growth on a fresh Ti@TiO2 NW surface. The duration time t1 and t2 for E1 and E2, respectively, were adjusted to control the NP density and size.21 A short potential pulse with amplitude of E1 is applied for growing Au nucleation sites on the surface of Ti@TiO2 NWs. One would improve Au NP density by either increasing E1 and/or duration time of this potential (t1). Large Au NP size can be obtained by increasing the duration time of t2 at E2. 2.3. Nanostructure Characterization. A JEOL 7000 FESEM was used to characterize samples. Transmission electron microscopy (TEM) samples were prepared by removing the NWs from the Ti@TiO2 NW substrate by scraping with a razor blade and suspending them in deionized (DI) water prior to being transferred onto a holey carbon film on 200 mesh copper grid (Electron Microscopy Sciences, Hatfield, PA). The samples were then imaged using a FEI Tecnai F-20 TEM (FEI, Hillsboro, OR). 2.4. Electrogenerated Chemiluminescence (ECL). Ti@ TiO2 NW substrates were immersed in 0.3 mM N3 ethanol solution for 16 h before or after electrodeposition of Au NPs for ECL and SERS studies. ECL was obtained using a homebuilt ECL setup. Detailed description of this setup can be found in our earlier publication.5 Briefly, a bipotentiostat CHI 760C (CH Instruments, Inc., Austin, TX) was used as a potential control source with an auxiliary signal input from a 1931-C high-performance low-power optical meter (Newport Corporation, Irvine, CA), which was used for collecting and amplifying electroluminescence signal through a photomultiplier tube (PMT). 2.5. Raman Measurements. Raman spectra were collected using a Raman spectrometer (Jobin Yvon, HR800 UV). A 633 1850

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nm He−Ne laser was employed as the excitation source in all Raman measurements. In-situ Raman spectroelectrochemistry measurements were done using 5 s collection time per spectrum when the substrate potential was controlled using a potentiostat (CHI604d).

NW, and the lattice of the Au NP is clearly resolved. The estimated oxide layer is around 2 nm varying from wire to wire. To illustrate the oxide outer layer of Ti NWs, Raman spectra of as-synthesized Ti NW, thermally annealed Ti NW, and NWs decorated with Au NPs are illustrated in Figure 3. It clearly

3. RESULTS AND DISCUSSION 3.1. Ti@TiO2 NW and Au NP Characterization. Ti plates coated with Ti@TiO2 NWs show gray color after thermal treatment in air because of the strong light scattering, absorption, and light interference of the thin film top layer of Ti@TiO2 NWs. Figure 2A shows a typical SEM image of Ti

Figure 3. Raman spectra of as-synthesized Ti NWs, thermally annealed Ti NWs to form Ti@TiO2 core−shell structure, and the Ti@TiO2 NW coated with Au NPs.

shows that the Ti@TiO2 NWs have Raman lines at around 150, 242, 448, and 613 cm−1 which we believe correspond to the Eg, B1g, A1g or B1g, and Eg modes22 of the anatase phase of TiO2, respectively. Only a weak Raman signature of TiO2 is obtained for as-synthesized Ti NWs because their surfaces are naturally oxidized in air at room temperature. Au NP coated Ti@TiO2 NWs show well-resolved TiO2 Raman scattering features along with strong photoluminescence (PL) background of Au NPs. Figure 4 illustrates a series of SEM images of Au NPs obtained with various particle development times (t2) at E2 while the Au NP nucleation time t1 is fixed (cf. Figure 1C). It clearly shows that longer t2 produces large particles. We also investigated the effect of t1 on Au NP size and distribution while t2 is fixed. High-density Au NPs were obtained with longer nucleation time t1 (Figure S1, see Supporting Information). It should also be noted that Au NPs sitting on top of the NWs near the electrode surface are larger in their sizes than the ones at the bottom of the NWs. Such polydisperse Au NP formation can be explained by the unique redox concentration profile of Au3+ around the NW structure. Higher redox concentration near the NW top surface is expected than the bottom and/or the interstitials formed by NWs due to the fast mass transfer process at the nanostructured surface, yielding large Au NPs on top of the NWs. Such polydisperse Au NP distribution is an important structural feature for surface enhanced spectroscopy (e.g., Raman and photoluminescence) as it can provide a broad range of plasmon resonance wavelength region. Variation of Ti NW sizes and spatial heterogeneity in size distribution from sample to sample shown in the SEM images are believed to be essentially determined by surface roughness of the Ti substrate used for NW preparation. The effect of Ti purity and surface roughness on ordering of Ti NW growth needs further investigation. To quantitatively show the effect of electrodeposition conditions on the particle size and polydispersity, ImageJ23,24 was used to analyze Au NP size distribution in SEM images. Particle size distribution analysis was obtained by converting SEM images to binary images by setting a threshold in ImageJ

Figure 2. (A) SEM image of Ti@TiO2 NWs prepared by hydrothermal reaction on Ti substrate. (B) SEM image of Au NPs on Ti@TiO2 NW electrodes prepared at t1 = 16.5 s and t2 = 600 s. (C) Cross-section SEM of Au NP-coated Ti@TiO2 NWs. (D) Typical high-resolution TEM showing the lattice structure of Au NPs and Ti NW.

substrate coated with Ti@TiO2 NWs. Ti NWs with rectangular prism shape are obtained by a hydrothermal reaction in HCl, and the rectangular shape does not change after annealing at 450 °C. The NW length is estimated to be around 500 nm, and the edge length of the NW varies from a few nanometers to a 100 nm. The formation of the interesting NW structure is hypothesized to be dependent on the selective chemical etching of Ti under high temperature and pressure in HCl. A detailed reaction mechanism is yet to be fully understood. After thermal annealing in air, a thin layer of anatase TiO2 is formed on the Ti NW surface. Polydisperse Au NPs were obtained on the NW surface using an electrodeposition method with two consecutive potential steps. As shown in Figure 2B, polydisperse Au NPs are electrodeposited directly to each of the Ti@TiO2 NWs and arranged in a vertical direction from top to the bottom of the wires. Figure 2C is the cross-section image of Ti@TiO2 NWs decorated with Au NPs. Figure 2D shows a highresolution TEM image of one Au NP attached to a Ti@TiO2 1851

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be collected from the bare Ti@TiO2 NW surface in the absence of Au NPs under the same laser excitation conditions. To quantitatively analyze the Raman enhancement without signal being obscured by the PL background, we subtracted the PL background from all our Raman spectra for data analysis as shown in Figure 5A. Raman shifts at 800, 1316, and 1481 cm−1 corresponding to the fluorescence background, C−O, and bipyridine ring stretching are selected to study how their intensities are dependent on the nucleation time t1 and development time t2 for Au NP growth. As shown in Figure 5B, SERS intensity increases dramatically with the particle development time t2 and the enhancement peaks at t2 = 1200 s with a nucleation time t1 of 0.5 s. All Raman intensities are calculated by averaging SERS spectra of 15 different sites of a sample with their PL background subtracted. The standard deviation of the average is shown as an error bar to reflect the variation of SERS enhancement from site to site on the Au NP-decorated Ti@TiO2 NW substrate. Such spatial variation in SERS intensity is very common for a 5 mm by 1.5 cm sample because of the spatial heterogeneities in Au NP sizes and morphology/size of the underlying Ti@TiO2 NWs as shown by Figure 4. Accurate and precise quantitative analysis may suffer from the spatial heterogeneity in SERS enhancement and can be further improved if the SERS spatial heterogeneities are minimized by obtaining ordering and uniform NW structures. The falloff in both PL and Raman enhancement capability beyond t2 = 1200 s is attributed to the formation of large Au NPs because the Raman excitation laser wavelength is off the plasmon resonance wavelength of large Au NPs and Au NP aggregates. Appreciable SERS enhancement can still be seen for t2 longer than 2100 s because of the rough surface of Au NPs that support EM field enhancement in contrast to a bare Ti@TiO2 NW electrode. Figure 5C illustrates the effect of nucleation time t1, which determines the density of Au seeds toward Au NP formation, on SERS intensity. It is shown that the SERS enhancement peaks at the electrodeposition nucleation time of t1 = 10.5 s with particle growth time of t2 = 600 s. This is because strong surface plasmon coupling starts contributing to local EM enhancement when particles are close to each other at long nucleation period. 3.3. Spectroelectrochemistry of N3 Dye at Au NP Decorated Ti@TiO2 NW Electrode. In addition to the SERS enhancement capability, the Au NP decorated Ti@TiO2 NWs would allow spectroelectrochemistry studies to reveal both electrochemical and spectroscopic information of N3 dye. 3.3.1. Electrogenerated Chemiluminescence (ECL) of N3 and Its SERS Spectroelectrochemistry at Au NP Decorated Ti@TiO2 NW Electrode. Oxidation reaction of N3 and its oxidative-reduction ECL generation in presence of a coreactant (e.g., tri-n-propylamine, TPrA) were measured at the multifunctional Ti@TiO2 NW electrode. As shown in Figure 6A, the presence of Au NPs on Ti@TiO2 NW electrode can enhance the faradaic current of TPrA oxidation and ECL in the PBS solution containing 0.1 M TPrA and 10 μM N3, indicating the formation of conductive nanoelectrodes on the Ti@TiO2 NW structure with good electrical contact to the Ti substrate. No interesting redox features were obtained at bare Ti@TiO2 NW electrode because of the sluggish reaction kinetics of TPrA and N3 at a metal oxide surface. Ti@TiO NWs decorated with Au NPs provide higher current density than the bare Au disc electrode because of the high surface area of Au nanostructured Ti@TiO2 electrode. Similar to the planar Au disc electrode as shown in Figure 6B, two ECL waves with different light

Figure 4. SEM images of Au NP coated Ti@TiO2 NW electrodes with the same Au nucleation time t1 of 0.5 s and different particle size development time t2 of 300 (A), 600 (B), 900 (C), 1200 (D), 1500 (E), and 2000 s (F).

(Figure S2, see Supporting Information). Our analysis shows that Ti@TiO2 NWs are coated with polydisperse Au NPs from a few nanometers to 100 nm with large Au NPs mainly located on top of the NW electrode. Crossing of diffusion layers of individual Au NPs plays a major role in forming smaller particles between Ti NWs when two nucleation sites are very close to each other. 3.2. SERS of N3 Dye on Au NP Decorated Ti@TiO2 NW Electrode. To demonstrate the SERS enhancement capability of the polydisperse Au NPs, the so-called N3 dye (cis-[Ru (4,4′COOH-2,2′-bpy)2(NCS)2]) is self-assembled onto the surface of Au NP decorated Ti@TiO2 NW electrode as shown in the schematic inset of Figure 5. N3 is one of the popular dyes used for attachment onto TiO2 electrodes to form dye-sensitized organic solar cells.25,26 Raman spectroscopy has been used to study the structural change of dye on TiO2 during the interfacial charge transfer process as this molecule can covalently attach to the TiO2 surface through carboxylic acid groups.27 Figure 5 shows the Raman signature of N3 at the Au NP decorated Ti@TO2 NW electrode. Two Raman peaks at 1260 and 1316 cm−1 are attributed to C−C inter-ring and C−O stretching modes, respectively, of the carboxylic acid groups of N3. Raman peaks at 1481, 1540, and 1600 cm−1 are attributed to three stretching modes of the bipyridine ring. Background signal of the Raman spectrum is from the intrinsic PL of Au NPs. All Raman peaks of N3 are greatly enhanced in the presence of Au NPs because of the strong local field enhancement capability of the nanostructured electrode under surface plasmon resonance conditions. No SERS spectrum can 1852

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Figure 5. (A) SERS spectra of N3 before (1) and after (2) Au NP photoluminescence background subtraction. Raman shift of 800, 1316, and 1481 cm−1 corresponding to fluorescence background of Au NPs, C−O, and bipyridine ring stretching Raman modes, respectively, are selected for studying their intensity dependence on Au NP deposition conditions. Inset is the schematic for SERS of N3 dye attached onto Ti@TiO2 NW electrode in the presence of Au NPs. (B) and (C) illustrate Raman intensities of the three selected Raman shifts and their dependences on Au NP growth nucleation time (t1) and particle size development time (t2) as defined in Figure 1C. Ti@TiO2 NWs were first electrodeposited with Au NP and then treated with N3 dye.

TPrA undergoes a deprotonation reaction to form TPrA radical, which then reacts with neutral N3 dye to form N31+. The oxidized N3 dye (N33+) and reduced form (N31+) subsequently react to yield an excited state N32+*, which undergoes radiative decay to produce chemiluminescence. A detailed ECL generation mechanism needs further investigation. To test the light emission properties of N3 molecules spontaneously adsorbed onto the Au NP decorated Ti@TiO2 electrode, current and ECL responses of N3 modified electrodes are shown in Figures 6C and 6D, respectively. The difference in their current density and ECL intensity can be explained by the heterogeneities in particle sizes and spatial distribution as shown by our SEM imaging results (cf. Figure 4). It clearly shows intense ECL generation at the Ti@TiO2 NW electrode in the presence of Au NPs in contrast to the bare Ti@TiO2 NW electrode. This indicates that Au NPs have reliable electrical contact with the Ti@TiO2 NW electrode, serving as charge collector for redox reaction and for ECL generation from N3 molecules at Au NPs. The shape of ECL waves at Ti@TiO2 NW electrodes, which was first coated with Au NPs and then N3, is different from the electrodes prepared with opposite attachment sequences (N3 first then Au electrodeposition). For electrodes with Au NP modification

emission intensities were observed at the Au NP modified Ti@ TiO2 NW electrode. This feature is similar to the tris(2,2′bipyridine)ruthenium(II)/TPrA ECL system.28 The first ECL wave originates near the potential of TPrA oxidation that determines the total ECL intensity when N3 starts to be oxidized, and the second is near the oxidation potential of N3 dye at the electrode surface. This two-wave feature is not observed in the current response because the current signal is overwhelmed by the oxidation current of the Au electrode. The intensity of the second wave at 1.3 V (vs Ag/AgCl) for the Au NP modified Ti@TiO2 NW electrode is much stronger than that of the first wave at 0.9 V, and its turn-on voltage is much less positive than that of the bare Au planar electrode. This might be due to the different retention time of oxidized tripropylamine at the Au NP modified Ti@TiO2 NW electrode. A longer retention time of oxidized TPrA species at the nanostructured electrode would allow sufficient reaction with N33+ to produce higher ECL intensity. Stronger adsorption of N3 at the Ti@TiO2 NW electrode is another factor that would contribute to the intense ECL at the second wave rather than the first one. We hypothesize the ECL mechanism of N3 dye is similar to tris(2,2′-bipyridine)ruthenium(II) ECL process in the presence of TPrA:28 N3 dye and TPrA are oxidized at a positive electrode potential, and the produced radical cation of 1853

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Figure 6. (A) Cyclic voltammetry and corresponding ECL intensity (B) of N3 dye at Ti@TiO2 NW electrode, Au disc electrode, and Au NP decorated Ti@TiO2 NW electrode in PBS solution (pH = 7.0) containing 0.1 M tripropylamine and 10 μM N3. (C) Cyclic voltammetry and corresponding ECL responses (D) at N3 modified Ti@TiO2 NW electrode (1), Au NP coated Ti@TiO2 NW electrode treated with N3 dye (2), and N3 modified Ti@TiO2 NW electrode and then coated with Au NPs (3) in PBS solution containing 0.1 M tripropylamine. An additional control experiment of Au NPs decorated Ti@TiO2 NW electrode treated with N3 dye in PBS solution containing 0.1 M tripropylamine and 10 μM N3 is also shown (4). Scan rate: 250 mV/s. Figure 7. (A) ECL intensity at Au NP decorated Ti@TiO2 NW electrode when electrode potential is scanned for a three consecutive cycles in a narrow potential window between 0.1 and 0.8 V (vs Ag/ AgCl) at scan rate of 100 mV/s. (B) Transient ECL intensity responses at the Au NP decorated Ti@TiO2 NW electrode when electrode potential is stepped from 0.1 to 0.8 V (vs Ag/AgCl) with 1 s duration time for each step in PBS solution (pH = 7.0) containing 0.1 M tripropylamine and 10 μM N3.

first, only one wave appeared during the positive scan, and the onset potential of the ECL wave is more positive than that of the electrode with N3 modification first. This has to do with the surface adsorption of N3 as evidenced by SERS characterization (cf. Figure 5) that would decrease in the charge transfer rate until more overpotential is applied. The surface modified configuration would also decrease the overpotential of N3 due to its short oxidation distance to Au NPs so that only one ECL peak appears. SEM images of Au NPs prepared in the opposite sequence and corresponding SERS spectra are shown in Figures S4 and S5, respectively. In the PBS solution containing 0.1 M TPrA and 10 μM N3, ECL at the Au NP decorated NW electrode increases rapidly, and the intensity of the second wave becomes stronger than that of the first wave, indicating that N3 molecules are absorbed on the Au NPs for ECL generation. It should also be noted that bare Ti@TiO2 NW electrodes provide appreciable current from the redox reaction and produce weak ECL; this may be associated with the defects of the oxide layer that would allow redox reaction to take place at large overpotential for ECL generation. Relatively stable ECL generation at the Au NP decorated Ti@TiO2 NW electrode can be achieved by scanning the electrode potential less than 0.8 V (vs Ag/AgCl) as shown in Figure 7A or applying doublepulse potentials as shown in Figure 7B to avoid building a thick mass transfer diffusion layer and oxidation of Au. More stable transient ECL data can be obtained by increasing the ECL OFF time and decreasing the ECL ON time of the double-pulse potential method. 3.3.2. SERS Spectroelectrochemistry of N3 at Au NP Decorated Ti@TiO2 NW Electrode for Exploring ECL Mechanism of N3 Dye. Our ECL study above shows that the ECL intensity decrease over time for N3 modified Ti@

TiO2 NW electrodes. This could be due to the detachment of N3 dyes from the Au NPs or passivation of Au NPs with ECL product (e.g., possible TPrA electropolymerization). We use the SERS capability of the new electrode to probe how the N3 Raman intensity changes over the period of ECL reaction. Combined in-situ ECL and Raman experiment methods were used to help reveal the detailed reaction mechanism. As shown in Figure 8, linear scanning of electrode potential is used to generate ECL in the presence of TPrA in PBS buffer for Au-NP decorated Ti@TiO2 NWs, which are loaded with N3 molecules. SERS spectra were collected continuously with time resolution of 5 s per spectrum over the potential scanning period. It is clearly shown that the SERS signal of N3 decreases over the time of ECL generation. There is no Raman signature of TPrA detected at the Au NP decorated Ti@TiO2 NW electrode. No pronounced ECL and Raman signatures were further detected after the first scan segment. Therefore, we conclude that the SERS signal decrease has to do with desorption of N3 from the Au NP surface when gold was stripped off the electrode by positive electrode potential scanning. This SERS spectroelectrochemistry study provides additional evidence of SERS enhancement that requires close distances between N3 and Au NPs for local EM field enhancement. 1854

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Figure 8. Time evolution of SERS spectra of N3 modified Au NP coated Ti@TiO2 NW electrode in PBS solution containing 0.1 M tripropylamine. A typical Raman spectrum of N3 is shown along the left axis of the figure. Potential was linearly scanned from 0 to 2 V (vs Ag/AgCl) then back to zero at a scan rate of 20 mV/s. Corresponding current and ECL traces over the period of in-situ Raman collection are shown on the top of the figure. Raman data was collected for 5 s per spectrum.

Notes

4. CONCLUSIONS Polydisperse Au NPs on Ti@TiO2 NWs cover a broad plasmon wavelength region for SERS spectroelectrochemistry. Au NPs can support redox reaction and ECL generation as they have good electrical contacts with Ti@TiO2 NWs. Surface plasmon coupling between Au NPs for enhancing SERS of molecules attached directly onto Au NPs or Ti@TiO2 NW surface can be optimized as by varying the electrodeposition potential and time used in the two-step potential method. The Ti electrode nanostructured with Ti@TiO2 NWs and Au NP can serve as a new SERS platform for quantitative molecule sensing because of its SERS enhancement and ECL enhancement capabilities.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is supported in part by the National Science Foundation under award number CHE-1153120 and the University of Alabama 2010 RGC award. R.B. is under the support of the Research Stimulation Program the University of Alabama. C.L.X. is supported by the Chinese government council fellowship. D. Clayton, H. Geng and S. Pan thank the Department of Energy for partial support of this work under Award Number(s) DE-SC0005392. We acknowledge Zhichao Shan for assisting with Au NP-Ti@TiO2 NW sample preparation.

ASSOCIATED CONTENT

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S Supporting Information *

Figures S1−S5. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Gao, H. W.; Liu, C.; Jeong, H. E.; Yang, P. D. ACS Nano 2012, 6, 234−240. (2) Tian, Y.; Tatsuma, T. Chem. Commun. 2004, 1810−1811. (3) Pan, S. L.; Gupta, A. Mater. Matters 2012, 64−66. (4) Wang, C. Z.; E, Y. F.; Fan, L. Z.; Yang, S. H.; Li, Y. L. J. Mater. Chem. 2009, 19, 3841−3846.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. 1855

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3108304 | J. Phys. Chem. C 2013, 117, 1849−1856