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C: Physical Processes in Nanomaterials and Nanostructures

Microstructure and Photoelectric Response of Gold Nanocrystalline on TiO Nanotube Arrays 2

Ying Zhao, Nils Hoivik, and Kaiying Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08608 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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The Journal of Physical Chemistry

Microstructure and Photoelectric Response of Gold Nanocrystalline on TiO2 Nanotube Arrays Ying Zhao, † Nils Hoivik, † and Kaiying Wang*† †

Department of Microsystems, Faculty of Technology, Natural Sciences and Maritime Sciences,

University College of Southeast Norway, Horten, 3184, Norway

ACS Paragon Plus Environment

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ABSTRACT: In this paper, we report photoelectrical response of gold nanocrystalline on TiO2 nanotubes with Au morphology ranging from nanoparticles, thin nanoring to thick nanoring. The gold nanopcrystallines are loaded on the TiO2 nanotubes (TNT) by facile magnetron sputtering technique. Comparing with pristine TiO2 nanotubes, the gold-decorated TNTs have shown obvious light absorption at wavelength of 380 ~ 850 nm as well as dependence of surface plasmonic resonant (SPR) on Au morphology.

Photocurrent response under white light

illumination presents around 4 times enhancement on all gold-decorated nanotubes at applied voltage of 0.2 V. Transient current measurement reveals different ramping up behavior on nanoparticle and nanoring coated TNTs at the onset of illumination. Full-field electromagnetic wave simulation based on finite element analysis (FEA) indicates that the shifting of SPR peaks is a joint effect of size/aspect ratio, morphology and amount of the nanocrystalline Au. Meanwhile, the FEA indicates that instant current response upon illumination on gold nanoparticles and nanoring structures are dependent on the electric field distribution, the behavior of charge carriers along longitudinal dimension of the TiO2 nanotube arrays. This study provides a perspective on SPR effect related to the enhanced photocurrent response on TiO2 nanotube arrays.


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INTRODUCTION: Due to outstanding properties such as strong chemical stability against photo-corrosion, low cost and earth-abundance, titanium dioxide (TiO2) material has become one of the most widely studied materials with applications aiming at photochemical water splitting1,2, hazardous organic wastes remediation3,4, dye-sensitized solar cells5,6 and biological devices7,8. Compared to nanoparticulated forms, nanotubular TiO2 arrays possess larger surface area and improved electronic properties owing to quantum size effects and contribution of surface reconstruction and surface curvature.9 In addition, it has been reported that absorption spectrum of the TNTs can be slightly red-shifted with respect to disordered TiO2 nanomaterial by adjusting periodicity of the nanotube arrays10, which a sufficient charge-separation is achievable throughout the light penetration length on nanotubes with thin enough walls.2 Yet, further modification strategies are still needed to make sufficient use of visible light and exhibit high photocurrent response. In this regard, several attempts have been put forward such as non-metal doping11,12, defect creation12-15 and narrow-band-gap semiconductor hybridization16-18. As an alternative, introducing noble metal, especially gold, to TNTs has drawn specific attention because of the fact that gold does not suffer from corrosion under photocatalytic conditions while largely extending cutoff wavelength of the catalyst due to SPR effect.19-21 SPR is a phenomenon occurring on certain structured metal materials when the frequency of incoming photons matches the natural frequency of electron oscillation within the metal.2 The frequency where SPR occurs is largely dependent on periodicity of the metallic nanostructure as well as refractive index of the metal and neighboring materials.22 According to earlier studies, the combination of TiO2 and Au exhibits resonant frequency between 500 nm and 650 nm19-21, which matches the wavelength range of visible light. One of the most commonly accepted


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mechanisms that SPR effect enhances photocurrent response suggests that the enhanced electric field near the metal/TNT interface facilitates the generation of surface electron/hole pairs within TiO2 material.20 However, in contrast to numerous studies reporting enhanced photocatalytic activities of Au-TNT system, research into the physics behind these enhancements are less frequently reported. In this study, we loaded gold nanocrystalline of different morphologies (compact nanoparticles, thin nanoring and thick nanoring structures) on electrochemical anodized TNT samples by magnetron sputtering technique. Light absorption spectra of all samples were measured using UV-VIS spectrophotometer to see how the response is altered after gold decoration. Pristine TNT and gold-TNT samples were then utilized as working electrode for photoelectrochemical measurements at a bias voltage of 0.2 V in order to examine the influence of gold nanocrystalline on photoelectrochemical reactions on TNTs. Furthermore, FEA studies have been carried out to observe how the behavior of SPR is affected by electric field strength and distribution on TNTs coating with different gold morphologies. EXPERIMENTAL DETAILS: Preparation of samples. Self-ordered TiO2 nanotube arrays were prepared by electrochemical anodization using titanium (Ti) and platinum (Pt) as electrodes. Ti foils of 2 cm in length, 1 cm in width and 0.5 mm in thickness were connected to anodic side of the power source (DC-direct current, 50 V) after degreased and cleaned by sonication (Finn-Sonic M12) in acetone, isopropanol and deionized (DI) water for 5 mins each. Pt electrode of the same size was connected 3.5 cm apart from Ti foil and immersed together with Ti into electrolyte consisting of 0.5 wt.% NH4F, 3 vol.% deionized H2O and 97 vol.% ethyleneglycol. In order to keep the anodization process within a homogenous electrolyte, a magnetic bead was used to stir the bath


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during reaction. Anodization lasted for 2 h before the as-prepared TNTs were rinsed with ethanol in ultrasonic bath (BRANSOIC 3510E-MTH). Ti/TNTs were then transferred to high temperature furnace (LENTON WHT 6) at 450 ºC for annealing to obtain the anatase TiO2 material. Metallic gold was subsequently introduced to three of the four identical TNT samples via magnetron sputtering (AJA International, Inc.) with deposition time equal to 1 min, 2 mins and 3 mins, respectively. Photoelectrochemical measurement and characterization. Surface morphology of the samples was characterized by field emission scanning electron microscopy (FE-SEM, Hitachi SU 8230) at an acceleration voltage of 15 kV. Transmission electron microscopy (TEM, JEM2100F) was used for understanding of size distribution and crystal structure after gold coating. UV-VIS spectrophotometer (UV-2600) from SHIMADZU (BaSO4 powder as reference) was used to measure light absorption spectrum in the range of 220 ~ 850 nm. All photoelectrochemical and electrochemical measurements were implemented using Zahner elektrik IM6 electrochemical workstation and carried out in 0.5 M Na2SO4 solution in a standard three-electrode-configured quartz cell (PINE Research Instrumentation) where TNT samples were used as working electrode, Ag/AgCl (saturated KCl filling solution) as reference electrode and a platinum wire as counter electrode. The testing samples were covered with opaque insulating tape to obtain an exposed area of 1 cm2 before illumination by a 100 mW/cm2 Schott KL1500 LCD light source. Emission spectrum of the light source is provided in Fig. S1. Post data analysis was done in Matlab. SIMULATION SETUPS: Numerical models of TiO2 nanotubes loaded with gold nanoparticles and nanoring structures are built in COMSOL Multiphysics modeling software. TNTs is modeled as cellular network


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with inner and outer diameters of 120 nm and 140 nm in accordance with experimental TNT samples. On top of TiO2 surfaces are different gold nanostructures, i.e. compact nanoparticles and nanoring. Periodic conditions and perfectly matching layers (PML) are applied to boundaries. Finite element analysis (FEA) study is performed through electromagnetic wave module under frequency domain for qualitative study of distinct SPR effect induced by nanocrystalline of different morphologies. Dispersive refractive indices of TiO2 and Au required by simulation are adopted from reference.23,24 RESULTS AND DISCUSSION: Morphology Characterization. Surface morphology of pristine and Au-loaded TNTs has been studied by using FE-SEM. Figure 1a-d shows the top views of pristine TNT (a) and goldTNT (b, c and d) samples under magnification of 200 K. The samples are labeled as S0nm, S3nm, S8nm and S15nm according to the estimated thickness of gold coating of each sample. It is observed from Figure 1a that the pristine TNTs are cellular arranged with average inner diameter of around 120 nm and wall thickness of less than 10 nm. The length of the nanotubes is measured to be about 15 µm. At sputtering time of 1 min, granular Au crystalline grows on the mouth area of the TNTs with compact nanoparticle morphology, where wall thickness is about 15 nm (Figure 1b). When sputtering time increases to 2 mins, gold particles merge on top surface and forms thin nanoring structures (Figure 1c). At sputtering time of 3 mins, smooth and thicker nanorings can be seen on top of the TNTs (Figure 1d). Insets in Figure 1b-d show TEM images of Au distribution along single nanotubes from corresponding samples. It can be seen from the images that the quantity of Au nanoparticles decreases gradually from top to bottom. Larger accumulation can be observed near the top (opening) of nanotubes on S8nm and S15nm which correlates to the top-view SEM images.


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The samples are further investigated by high-resolution TEM. Figure 1e and f show HRTEM images of S3nm and S8nm with insets demonstrating their Fast Fourier Transform (FFT) from the selected area (I or II). Lattice spacing of 0.24 (and 0.236) nm and 0.2 nm can be observed from Au crystalline, which corresponds to the (111) and (200) lattice spacing, respectively. Well resolved lattice fringe of TiO2 (400) (d=0.24 nm) is also observed in Figure 1e. Figure 1f is partial of a well preserved single nanoring structure (as shown in inset), thus no TiO2 lattice is present. In addition, Au nanocrystallines on both samples exhibit FFTs where small spots make up rings. Therefore, the Au layer formed in our experiment is confirmed to be polynanocrystalline.25

Figure 1 Top view of TNTs samples with a: 0 nm, b: 3 nm, c: 8 nm and d: 15 nm Au coating layer. Insets in b-d are TEM images of side view from corresponding samples. e and f are HRTEM images of samples in b and c. Insets in e are FFTs from selected areas and insets in f are FFT from selected area (top) and full view of a single nanoring structure (bottom). Light Absorption properties. Figure 2 presents the UV-visible light absorption spectrum of the TNT samples. A reference spectrum of TiO2 powder is plotted in the figure for comparison.


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It can be seen from the figure, all four TNT samples present relative enhancement of light absorption from 380 nm to 850 nm, indicating the existence of periodicity TNT arrays.10 Comparing gold-loaded TNTs with the pristine one, the former exhibits higher absorption in entire visible light region (380 ~ 850 nm) and absorption peaks can be found at certain wavelengths. The presence of absorption peaks confirms the occurrence of SPR effect on Auloaded TNTs under visible light illumination. Among these TNTs, S3nm shows a broadband resonance peak (500 ~ 650 nm). As gold structure changes from nanoparticles to nanoring, the absorption peak narrows down and is blue-shifted to around 500 nm. According to Awang et al.’s report 26, SPR induced by gold nanograting can be shifted from ~550 nm down to ~540 nm when the gap distance of

is increased from 5 nm to 30 nm. Following this trend, it is

understandable that our nanoring structure with average inner diameter of ~100 nm exhibits absorption peaks of around 500 nm. Besides the variation of morphology that changes the absorption behavior of each sample, the grain boundary is also found to greatly enhance electron-phonon interactions which can also contribute to the peak shifting in Figure 2 27.

Figure 2 UV-visible light absorption spectrum of TiO2 powder and samples S0nm, S3nm, S8nm and S15nm.


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Photoelectrochemical properties. I-V characteristic of pristine and Au-loaded TNTs were recorded in dark and under 100 mW/cm2 illumination at applied voltage range of -0.5~0.5 V. Under dark condition, all of them behave analogically to diode where current increases exponentially with increased forward bias but is nearly zero under reversed bias (Figure 3a). The contact between titanium substrate and TiO2 nanotubes is Ohmic in nature and Schottky contact exists only at the interface between TNTs and Au/electrolyte.28 Unlike the dark current in Figure 3a which is caused by external bias, the current under illumination is on a large extent attributed to the photo-generated electron/hole pairs within TiO2 material. It can be seen from Figure 3b that the charge carriers induced in TiO2 by irradiation drift to titanium back contact (electron) and electrolyte (hole) under positive applied voltage, forming positive current. While Audecorated TNTs exhibit constant (or slightly increased) current, sample S0nm shows more rapid current increase when applied voltage reaches around 0.3 V. This is attributed to that break down voltage at TiO2/electrolyte interface is lower than that at TiO2/Au interface. The inset in Figure 3b is a closed look at the current response in the voltage range of 0 ~ 0.2 V, where an over four times increase in current is observed at V = 0.2 V on the TNTs with gold. The enhancement is induced by increasing usage of incident light and efficient separation and transfer of charge carriers. Electrochemical impedance spectroscopy (EIS) described below will further discuss this issue.


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Figure 3 I-V characteristic of four samples under dark (a) and illumination (b) conditions. Insets (a): equivalent circuit of electrochemical system under dark condition and (b): magnification of voltage range from 0 to 0.2 V.

Figure 4 shows transient photocurrent response of the samples at applied voltage of 0.2 V. The illumination interval of transient response is ON (20 s)/ OFF (40s), and all the samples were kept under illumination for 10 s before recording started. A similar study on surface plasmon enhanced photocatalysis of Au/TiO2 nanopillar (TiO2 NP) arrays has been carried out in 2016 by Shuang et al..29 Comparing our bare TNT structure with their as well pristine TiO2 NP arrays, we find that the former exhibit photocurrent density of one order of magnitude larger at half the bias


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voltage and illumination intensity of the latter. Such advantage of TNT can be largely originated from the increased surface area as compared to nanopillars. On top of that, the three TNT samples with gold coating in our experiment present photocurrent of 4 to 5 times larger than that of bare TNTs upon irradiation while Shuang et al. reported only 1.25 and 2 times enhancement after varied amount of metallic deposition.29 SEM images of their Au-TiO2 NPs structure show uniform but relatively loose distribution of gold nanoparticles. This phenomenon thus indicates that the photocurrent response on TiO2/Au-nanoparticle system can be enhanced by increasing the quantity of gold particles. In addition, S8nm and S15nm show nearly straight lines from t = 0 s to t = 20 s during the first cycle (t = 0~60 s), but S3nm appears to decay in all three light-on intervals. The decay behavior of S3nm under illumination agrees with Liu’s report which was explained by the releasing of charge trapped at the TiO2 surface upon irradiation.20 However, nanoring Au-loaded TNTs reveal different ramping up behavior at the onset of illumination from nanoparticle ones. It is believed that the difference is induced from the distinct metallic structure on the surface of TNTs. The detailed explanation can be found in numerical simulation section.

Figure 4 Time transient measurement of all samples with light on for 20 s followed by light off for 40 s.


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EIS under potentiostatic regulation has been used to evaluate impedance of the system over frequency range of 10-2 - 106 Hz (potential perturbation = 5 mV). Figure 5 is the Nyquist (a) and bode (b) plot of the EIS data acquired under white light illumination. All samples exhibit two dispersion arcs on negative imaginary impedance plane (capacitive feature) and one arc on positive imaginary impedance plane (inductive feature) in Figure 5a. The inductive feature at the frequency region (> 10 kHz) is due to the measuring instruments and wire connections. The small arc occurring on each curve at the frequency region (0.1~10 kHz) is caused by the reactions happening at the counter electrode, and the large arc (0.01~100 Hz) is originated from the diffusion of electron and charge transfer process on the working electrode. By fitting with equivalent circuit model (ECM) shown in Figure 5c, we find that the value of charge transfer resistance on the working electrode (denoted as Rct,A) reaches minimum on S3nm (1.21 kΩ) and S8nm (1.03 kΩ), which is only ¼ of that on S0nm (4.89 kΩ). This is consistent with the current response characteristic above. Wang et al. indicate that the frequency where valley occurs in bode plot is the characteristic frequency (fc) and its value is corresponded to the electron lifetime within the working electrode 30. It can be seen from Figure 5b that the fc for four samples are fc ≈ 0.3 Hz, fc_np ≈ fc_nr8nm ≈ 0.8 Hz and fc_nr15nm ≈ 0.5 Hz, respectively. In our electrochemical system where electron/hole pairs are main current carriers, higher fc values indicate that electrons are efficiently transferred away from working electrode to join chemical reaction and contribute to photocurrent response. Thus, the transferring speed of electrons within samples S3nm and S8nm is nearly three times as fast as the speed in S0nm and nearly twice as fast as in S15nm. We assume that a proper amount of gold, despite the different shape and form, on TNTs contributes to optimize the TNT/electrolyte interface properties and accelerates the transfer of photo-generated electrons.


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Figure 5 (a) Nyquist plot with fitting. Inset: details in high-frequency region; (b) bode plot: impedance (solid lines) and phase (dashed lines); (c) Equivalent circuit model for fitting nyquist plot: Rb denotes the bulk resistance which includes resistance of electrolyte and connection wires, RTNT-Au/EL and CPETNT-Au/EL in parallel model the diode feature at the interface of TNT and gold (or electrolyte), Rct,A (Rct,C) and CPEdl,A (CPEdl,C) stand for the charge transfer resistance


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and double layer capacitance on the anode (cathode) respectively, warburg element (We-TNT) is used to describe the diffusion process of electrons in nanotubes. Stability of the samples is tested by recording the photocurrent density for 104 s under illumination. Data recording started 20 s after illumination began. Figure 6 shows that S3nm and S8nm demonstrate stable photocurrent of approximately half the initial/maximum value after ~1000 s, which indicates a 50% decrease from initial to stable state. Compared to these samples with relatively less quantity of Au coating, S15nm exhibits less decay from initial ~38 µA cm-2 to stable ~24 µA cm-2, corresponding to a 37% decrease. We have repeated the same measurement on these samples after one week and got graphic curves with similar trends and values as in Figure 6 (See Fig. S2). Such result indicates that the reduction of current density within the first 1000s in Figure 6 is not permanent, meaning no irreversible structure or morphology change on the samples. The less current decay in S15nm can be attributed to more grain boundaries of the polycrystalline Au in the thick ring structure which provides more surface reacting sites and results in stable state of higher photocurrent. However, further research to identify the grain boundaries in polycrystalline Au of different morphology is needed regarding to this phenomenon. In addition, we believe that the high absorption of S15nm in the entire visible light range has, to an extent, compensated its low electron-transferring speed and slightly large charge transfer resistance (as are indicated in Figure 5). As a result, S15nm exhibits comparable stablephotocurrent response as S3nm even although there is no obvious SPR peak observed for S15nm in Figure 2. This has also indirectly reflected that, among the two factors which limit the photoelectrochemical property of TiO2 nanotubes (low usage of visible light and slow charge transfer process), the former is the major limitation.


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Figure 6 Photocurrent stability of all samples for 104 s.

Numerical simulation verification. In order to better understand the difference of SPR effect generated by nanoparticles and nanoring structures, two 3D models of identical TiO2 nanotubes decorated with compact nanoparticles (Mnp) and nanoring (Mnr) have been assessed (See Figure 7a). For simplifying the shape of the Au nanoparticles, which should be whole or partial sphere, cylindrical structure (nanorod) is used to model the particles. Liu et al. pointed out that the enhancement of local electric field (E) at the interface of TiO2 and gold is proportional to the photon absorption.20 As a result, the surface-maximum of the electric field intensity (Es-max) on TNT top surface has been used as the equivalent of the SPR effect. The electric field distribution within nanotube unit cell is shown in Figure 7b (Mnp) and c (Mnr) with both plasmonic metallic structures being ~5 nm in height. The radius of the cylinder in the Mnp model is 3 nm. The height of the cylindrical/nanorod structure is defined as 5 nm after balancing the difference of its vertical dimension, surface area and volume from a sphere nanoparticle of the same radius. It has been observed that the Mnp exhibits Es-max = 2.8E10 V/m at λ = 800 nm, while the Es-max for Mnr has a value of only 1/10 of the former and occurs at λ = 600 nm. The SPR wavelength blue-shifts about ~200 nm from Mnp to Mnr. The quantitative mismatch between simulation and experiment


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are resulted from the simplified model and the adapted refractive index of bulk TiO2 material instead of periodic nanotube arrays. By varying the thickness of the Au nanoring structure in Mnr model, Es-max is found to occur at λ = 650 nm for thickness of 3 nm and remain at λ = 600 nm with increasing thickness (up to 10 nm). The result explains the overlapping peaks of S8nm and S15nm in the absorption spectrum. From the normalized color bars in Figure 7b,c, we can further observe that the electric field distributes more uniformly and spreads deeper into the nanotube walls on Mnr than that on Mnp. In fact, if the maximum surface average of the electric field (Esavg)

is used to identify SPR rather than Emax, it is at λ = 750 nm where SPR occurs for Mnp rather

than 800 nm as is mentioned earlier (where Emax occurs).

Figure 7 (a) Numerical model of TNT unit cell with gold nanoparticles (Mnp) and nanoring (Mnr) on top; Electric field distribution on TNT unit cell with (b) compact nanoparticles and (c)


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nanoring of 5 nm in height as plasmonic generation layer. Both models are at their own SPR frequencies and color bars of two models are normalizaed.

We have already shown above that the photocurrent response on TiO2/Au-nanoparticle system can be enhanced by increasing the quantity of gold particles. Further simulation has been carried out to evaluate the relationship among SPR, distribution uniformity and particle size. Table 1 lists the SPR wavelength and corresponding Es-avg with regard to morphology and size/thickness of the Au nanostructures. It can be seen from the table that (1) for the same size of nanoparticles (group c vs. d and e), larger amount (i.e. coverage) leads to greater Es-avg value and red-shift of the resonant wavelength; (2) for the same amount of gold particles (group c vs. f), larger particle size gives greater Es-avg value and red shifts the SPR wavelength; (3) for the same contact area between Au and TiO2 (groups d and e vs. f), smaller particles with higher amount exhibits greater Es-avg. In addition, high uniformity can also help to enhance the electric field intensity in the vicinity of Au/TiO2 interface. Considering the dimension limitation of a real full or partial spherical, the height of the cylindrical structure used in the model cannot be larger than its diameter (indicated by NA - Not Applicable in the table). It is obvious that SPR changes following the variation of ratio between height and diameter of the modelled Au nanorod. Such phenomenon suggests that when loading Au nanoparticles to TiO2 (or other semiconductors), different contact angles will lead to SPR changes in both strength and position. However, it is hard for all nanoparticles to keep identical contact angles, which explains the wide band absorption spectrum in Figure 2 and corresponding reference.31 Table 1. Plasmonic resonant wavelength with regard to morphology and size/thickness of the Au nanostructure a Au thickness (nm)

2

3

4

5

6

7

8


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700 (7.5e8)

650 (1.1e9)

600 (4.4e8)

600 (6.3e8)

600 (5.2e8)

600 (4.1e8)

600 (3.5e8)

Compact c

750 (3.8e9)

800 (1.4e9)

750 (1.8e9)

750 (1.8e9)

700 (2.7e9)

NA

NA

Loose + uniform d

700 (9.4e8)

750 (8.5e8)

700 (7.7e8)

700 (7.3e8)

650 (8.3e8)

NA

NA

Loose + nonuniform e

700 (6.3e8)

750 (6.2e8)

750 (6.8e8)

700 (6.3e8)

650 (7.7e8)

NA

NA

Uniform + small f

700 (7.8e8)

750 (9.7e8)

750 (8.4e8)

NA

NA

NA

NA

Nanoring structure

Nanoparticlesb

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a

number without parentheses is the wavelength (unit: nm) where SPR occurs; number with parentheses is the electric field intensity at corresponding wavelength (unit: V/m).

b

different size, amount and distribution of gold nanoparticles are considered: size of particles in c, d, and e are identical and larger than the size of f; amount of particles in c and f are equal, which is twice as much as that of d and e; the contact area of gold and TiO2 in d, e and f are the same, which is half the value in c.

All the discussion above has been focused on the interface between Au and TiO2. However, the effect of SPR can spread deeper into the TiO2 material. The enhanced electric field distribution in the vicinity of Au/TiO2 interface is thus investigated in this section and the difference between nanoparticle- and nanoring- generated SPR effect will be presented. To this end, the following four models are built: Mnp_3nm – TNT with gold nanoparticles of 3 nm in particle height, Mnp_5nm – TNT with gold nanoparticles of 5 nm in particle height, Mnr_5nm - TNT with gold nanoring of 5 nm in thickness and Mnr_8nm - TNT with gold nanoring of 8 nm in thickness. A series of cut planes (a typical cross-section plot in COMSOL where planes are created through a 3D in a 2D geometry to visualize a quantity as a family of plots) are created under the top surface of TNT unit cell. Es-avg on these planes is then calculated so that the effect of SPR effect in the vicinity of TNT top surface can be analyzed. It is worth noting that the highest Es-avg occurs at different wavelength for the three aforementioned models. Figure 8a


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shows a series of surface plots of electric field distribution on the cut planes which are 2 nm, 4 nm and 6 nm under TNT top surface on Mnp_5nm and Mnr_5nm. In addition to the more uniformly distributed and deeper spread electric field on Mnr, the location of the highest and lowest electric field intensity on Mnp and Mnr is also different. For nanotube coated with nanoparticles, the nanotube walls that are perpendicular to incident electric wave experience the least effect from SPR. On the contrary, the same nanotube walls on Mnrs demonstrate the strongest enhancement of electric field under the same polarization for incident electric field. Surface average of electric field intensity at SPR is subsequently calculated for every single cut plane and is plotted as a function of distance between Au/TiO2 interface and corresponding cut planes in Figure 8b. Interestingly, the curves in Figure 8 show similar trend as the transient current upon illumination in Figure 4. The highest Es-avg locates on the Au/TiO2 interface for nanoparticles, whereas below and near the Au/TiO2 interface for nanostructure. This is also true even for the same thickness of different morphologies (Mnp_5nm and Mnr_5nm). Comparing the coordinates used in Figure 4 and Figure 8, we can easily relate the current density to the amount of charge carriers generated under the enhanced electric field. While charge carriers drift to the back contact or electrolyte, there are several competing activities taking place at the same time, such as the scattering and reabsorption of incident photons, trapping of electrons, recombination and re-stimulation and etc.. Therefore, the generation sites of the charge carriers can affect the ramping up behavior by defining the travelling path of the charges. As a result, we correlate the two curves in Figure 4 and 8(b) and suggest that the instant current response upon illumination might be related to how the electric field is distributed and how the charge carriers travel and recombines along the longitudinal dimension of the TiO2 layer.


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Figure 8 (a) Intensity of electric field distribution on the cut planes which are 2 nm, 4 nm and 6 nm under TNT top surface; (b) Dependence of Es-avg versus position of cut planes where Es-avg is calculated. Each Eavg is calculated at the resonant frequency of each model.

CONCLUSIONS:


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This study reported SPR effect and understanding of gold nanocrystalline with different amount and morphology on TNT electrodes. The SPR effect has been observed on TNTs with gold coating from UV-vis absorption spectrum. Photocurrent response is enhanced by over 4 magnitudes after compact gold nanoparticle decoration, which, as compared to previous report, indicates that the photocurrent response on TiO2/Au-nanoparticle system can be enhanced by increasing the quantity of gold particles. Such phenomenon can be explained by the increased contact area of Au and TiO2 which boosts more near-surface e-/h+ pairs to be generated. As compared to Au nanoparticles, the lateral dimension and periodicity of the nanoring is defined by the underneath TiO2 nanotube arrays, and the thickness can be controlled by varying sputtering time. Therefore, the nanoring structure can be more favorable in terms of large-scale application due to the precise control of physical dimensions. Furthermore, stability measurement indicates that all samples experience reversible, quantity/morphology-dependent photocurrent decay until reaching a stable state after approximately 1000 s. In addition to experimental results, numerical simulation has been conducted and suggests that the shift in resonant frequency observed on different samples is related to the size, amount and morphology of gold nanocrystalline layer. FEA study has also suggest that the distinct ramping up behavior at the onset of illumination on different metallic structures is related to how the electric field is distributed and how the charge carriers travel and recombines along the longitudinal dimension of the TiO2 layer. In summary, a detailed study on the effect of metallic nanostructures (size, distribution and morphology) on SPR has been carried out both theoretically and experimentally. A new prospective is brought up on the ramping up behavior of different plasmonic structures. This research has also provided an easier way to manufacture nanoring as a plasmonic structure and given further guidance when it comes to real applications: metallic nanoparticles offer slightly better photocurrent density


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response while nanoring structure provides much steadier ramping up behavior upon illumination. Supporting Information. Spectrum of the Schott KL1500 LCD light source. Repeated photocurrent stability test one week after the first test.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *Telephone number: +47 3100 9317 ACKNOWLEDGMENTS This work is financially supported by KD program at University College of Southeast Norway, Norwegian Research Council-FRINATEK programme (231416/F20), EEA-Poland (237761) and partial funding for this work was obtained from the Norwegian PhD Network on Nanotechnology for Microsystems, which is sponsored by the Norwegian Research Council, Division for Science, under Contract no. 221860/F40.


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Understanding

the

Enhancement

Mechanisms

of

Surface

Plasmon‐Mediated

Photoelectrochemical Electrodes: A Case Study on Au Nanoparticle Decorated TiO2 Nanotubes. Adv. Mater. Interfaces 2015, 2, 1500169.


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Table of Contents (TOC) Graphic


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Figure 1 Top view of TNTs samples with a: 0 nm, b: 3 nm, c: 8 nm and d: 15 nm Au coating layer. Insets in b-d are TEM images of side view from corresponding samples. e and f are HRTEM images of samples in b and c. Insets in e are FFTs from selected areas and insets in f are FFT from selected area (top) and full view of a single nanoring structure (bottom). 81x37mm (300 x 300 DPI)


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Figure 2 UV-visible light absorption spectrum of TiO2 powder and samples S0nm, S3nm, S8nm and S15nm. 64x51mm (300 x 300 DPI)



Figure 3 I-V characteristic of four samples under dark (a) and illumination (b) conditions. Insets (a): equivalent circuit of electrochemical system under dark condition and (b): magnification of voltage range from 0 to 0.2 V. 126x193mm (300 x 300 DPI)


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Figure 4 Time transient measurement of all samples with light on for 20 s followed by light off for 40 s. 58x41mm (300 x 300 DPI)



Figure 5 (a) Nyquist plot with fitting. Inset: details in high-frequency region; (b) bode plot: impedance (solid lines) and phase (dashed lines); (c) Equivalent circuit model for fitting nyquist plot: Rb denotes the bulk resistance which includes resistance of electrolyte and connection wires, RTNT-Au/EL and CPETNT-Au/EL in parallel model the diode feature at the interface of TNT and gold (or electrolyte), Rct,A (Rct,C) and CPEdl,A (CPEdl,C) stand for the charge transfer resistance and double layer capacitance on the anode (cathode) respectively, warburg element (We-TNT) is used to describe the diffusion process of electrons in nanotubes. 164x329mm (300 x 300 DPI)


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Figure 6 Photocurrent stability of all samples for 104 s. 58x45mm (300 x 300 DPI)



Figure 7 (a) Numerical model of TNT unit cell with gold nanoparticles (Mnp) and nanoring (Mnr) on top; Electric field distribution on TNT unit cell with (b) compact nanoparticles and (c) nanoring of 5 nm in height as plasmonic generation layer. Both models are at their own SPR frequencies and color bars of two models are normalizaed. 103x131mm (300 x 300 DPI)


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Figure 8 (a) Intensity of electric field distribution on the cut planes which are 2 nm, 4 nm and 6 nm under TNT top surface; (b) Dependence of Es-avg versus position of cut planes where Es-avg is calculated. Each Eavg is calculated at the resonant frequency of each model. 169x348mm (300 x 300 DPI)



Table of Contents (TOC) Image 44x24mm (300 x 300 DPI)


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Nanotube Arrays - ACS Publications - American Chemical Society

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