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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Nanometer Scale Confined Growth of Single-Crystalline Gold Nanowires via Photocatalytic Reduction Seonhee Lee, Changdeuck Bae, and Hyunjung Shin* Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea S Supporting Information *

ABSTRACT: Single-crystalline gold nanowires (Au NWs) are directly synthesized by the photocatalytic reduction of an aqueous HAuCl4 solution inside high-aspect-ratio TiO2 nanotubes (NTs). Crystalline TiO2 (anatase) NTs are prepared by the template-assisted atomic layer deposition technique with a subsequent annealing. Under the irradiation of ultraviolet light, photoexcited electrons are formed on the surfaces of TiO2 NTs and could reduce Au ions to create nuclei without using any surfactant, reducing agent, and/or seed. Once nucleation occurred, high-aspect-ratio Au NWs are grown inside the TiO2 NTs in a diffusion-controlled manner. As the solution pH increased, the nucleation/growth rate decreased and twin-free (or not observed), single-crystalline Au NWs are formed. At a pH above 6, the nucleation/growth rates increased and Au nanoparticles are observed both inside and outside of the TiO2 NTs. The confined nanoscale geometries of the interior of the TiO2 NTs are found to play a key role in the controlled diffusion of Au species and in determining the crystal morphology of the resulting Au NWs. KEYWORDS: photocatalytic reduction, confined growth, gold nanowire, titanium dioxide nanotube, single crystalline

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INTRODUCTION Nanostructures and nanomaterials offer unique anisotropic properties compared with their bulk counterparts. For example, nanowires (NWs), nanorods (NRs), and nanobelts of noble metals such as Au, Ag, Pt, and Pd exhibit longitudinal localized surface plasmon resonance,1−4 opening up new applications, including bioimaging,5,6 photothermal therapy,5−7 sensing,8,9 and photoactive devices.10,11 The most promising application of high-aspect-ratio (AR) gold NWs (Au NWs) is the photothermal therapy of cancer cells. Au nanoparticles (NPs) have often been used for photothermal therapy of cancer cells. However, one-dimensional nanostructures are known to exhibit more efficient cancer cell capture performance than NPs in bloodstreams.12 The resulting physical, electrical, and optical properties strongly depend on their size, shape, surrounding dielectric media,13,14 composition,15,16 and crystal structure.17 Therefore, obtaining a controlled synthesis is of paramount importance. Au has a valence electron configuration of 5d106s1 and is naturally isotropic when formed as a solid. Growing anisotropic Au in the form of NWs is thus challenging. Several attempts have been reported. Electrochemical deposition using hard templates, such as anodic alumina membranes (AAMs) or polycarbonate membranes,18,19 and the seed-mediated growth technique have been used for preparing Au NWs.20−23 In the seed-mediated growth technique, the surfactant (typically cetyltrimethylammonium bromide, CTAB) plays an important role in the anisotropic growth. Three possible mechanisms have been proposed to describe anisotropic growth: (i) CTAB-Ag+ © XXXX American Chemical Society

adsorbs on certain Au facets, usually Au(110) and (100) planes, which have a higher surface energy than the (111) plane, and hinders the addition of Au ions, resulting in the preferential growth of Au;22,24,25 (ii) CTAB molecules form cylindrical micelles in the presence of Ag ions, which provide a soft template from which Au NWs can grow;26−28 and (iii) the underpotential deposition of a monolayer or submonolayer of Ag on certain Au facets obstructs the addition of Au ions, resulting in the preferential growth of Au.29,30 The seedmediated growth method usually produces single-crystalline Au NRs in high yield but with quite low ARs (typically below 20). In this process, the AR and morphology of Au NRs are easily influenced by small changes in the concentrations of the reducing agent, surfactant, and precursor ions. On the other hand, electrochemical deposition could result in high-aspectratio Au NWs in high yield. In most cases, however, as-grown Au NWs are polycrystalline. Only a few studies on the electrodeposition of Au NWs having large crystal grains have been reported, as finding the optimum conditions is extremely difficult. The photocatalytic synthesis of one-dimensional Au nanostructures, in which the surfactants also act as soft templates, has not been well-reported.31−33 Moreover, when surfactants are not employed, the resulting Au NWs have irregular shapes and are polycrystalline.34,35 Recently, a Received: February 9, 2018 Accepted: May 29, 2018

A

DOI: 10.1021/acsami.8b02473 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) FE-SEM image of dispersed TiO2 NTs after fully dissolving the AAMs. The TiO2 NTs show smooth surfaces with a uniform diameter along the length. (b) HRTEM image of an anatase TiO2 NT upon annealing at 400 °C in ambient atmosphere. The lattice fringes of 0.25 and 0.48 nm indicate the {101} and {200} interplanar spacings of the anatase polymorph, respectively. (c) Corresponding SAED patterns in panel (b) show a zone axis of [100], and the (011) and (004) reflections of the anatase phase are indexed. to the AAM for use as templates. Titanium tetraisopropoxide (at 60 °C), purchased from UP Chemical (South Korea), and H2O (at room temperature) were used as the Ti and O source, respectively. Argon (5 N) was used as the carrier (50 sccm) and the purge (150 sccm) gas. A full ALD cycle consisted of 30 s pulse times for the reactants, followed by 120 s of purging, and the substrate temperature was 160 °C. The TiO2 wall thickness was determined by the number of deposition cycles, which was fixed at 300 cycles (∼10 nm). After the deposition, TiO2 was annealed at 400 °C for 1 h in air for crystallization in the anatase form. Then, the TiO2 NTs were removed from the AAO membranes by using a 1 M NaOH solution and dispersed on Si substrates. Growth of Au NWs. Gold chloride trihydrate (HAuCl4·3H2O, 99.9%) was purchased from Sigma-Aldrich. First, 5 mM HAuCl4 was dissolved in deionized water, and the pH was adjusted by adding a 1 M NaOH aqueous solution. The acquired HAuCl4 aqueous solution was stirred and stabilized for 1−2 days. The temperature of HAuCl4 solution was maintained at 5 °C for the synthesis of Au NWs. A lowpressure mercury lamp array with a power of 6 W and a wavelength centered at approximately 254 nm was used as the ultraviolet (UV) light source. To make the surface of the TiO2 NTs hydrophilic, all samples were treated with UV light (UV/Ozone Cleaner, 60 W, Bioforce) prior to reaction. Then, the samples were immersed in Au solution and irradiated 20 cm away from the samples. The UV intensity was 0.4 mW/cm2 at the sample position. Characterization. SEM was carried out using a scanning electron microscope (JSM-7000F, JEOL, Japan). TEM investigations were performed using a transmission electron microscope (at 400 kV, JEM4010, JEOL, Japan).

synthesis method combining a seed-mediated method with a hard template has also been reported.36 Recently, we reported a novel method for synthesizing single-crystalline Au NWs with high ARs, and an unusual polymorphism under nanoscale confinement provided by TiO2 nanotubes (NTs) was studied.37 Here, we have investigated the selective nucleation and growth of the Au NWs under nanoscale confinement by considering the interaction between the surfaces of TiO2 NTs and Au ions. The growth behavior resembles the natural biomineralization process in that diffusion-controlled processes determine the resulting structures. A well-known example is sea urchins, whose spines consist of single-crystalline calcite.38 The growth mechanism is not fully understood, and it is believed that physical confinement is one of the key factors in providing nucleation sites and tailoring the diffusion of ions.39−41 The templateassisted atomic layer deposition (ALD) technique was used to fabricate TiO2 NTs as photocatalytic nucleation sites, enabling the precise control of the length, diameter, and wall thickness. Therefore, the dimensions of the Au NWs fabricated inside TiO2 NTs are controllable. The nanoscale confinement provided by the hollow interiors of the TiO2 NTs allows for the successful photoreduction and tailored crystallinity without any surfactants or reducing agents. The resulting structures and morphologies were characterized using high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FE-SEM). The present study offers a model photocatalytic crystallization in a confined system.

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RESULTS AND DISCUSSION

We first prepared the TiO2 NTs by the template-directed ALD method. Figure 1a shows a FE-SEM micrograph of the resulting TiO2 NTs dispersed on a Si substrate after removing the AAMs. The HRTEM image in Figure 1b shows a TiO2 NT with an average diameter of approximately 50 nm and a hollow interior. The NTs completely crystallized into the anatase polymorph upon annealing. The lattice fringes of 0.35 and 0.48 nm are clearly observed and are consistent with the {101} and {200} interplanar spacings of the anatase polymorph, respectively. The corresponding selected-area electron diffraction (SAED) pattern, taken along the [100] zone axis, is shown in Figure 1c. The detailed structure and properties of our ALDgrown TiO2 NTs were described in our previous works.43−47 The well-crystallized anatase TiO2 NTs with high AR were used as hard templates for preparing Au NWs. TiO2 NTs can effectively provide photoelectrons to reduce Au cations in close

EXPERIMENTAL SECTION

Preparation of Anodic Aluminum Membranes. The AAMs used in this work were prepared in a two-step electrochemical anodization process.42 A high-purity (99.999%) aluminum foil (Goodfellow, UK) was electropolished in a solution of perchloric acid and ethanol (HClO4/C2H5OH = 1:5). The foil was anodized in a 0.3 M oxalic acid solution at 10 °C under a constant voltage of 40 V. After the first anodization process, the alumina layer was removed in a mixed solution of chromic acid (1.8 wt %) and phosphoric acid (6 wt %). Then, the aluminum foil was anodized again for 5 h under identical conditions to those used in the first process. The remaining aluminum foil was removed in a saturated HgCl 2 solution. Subsequently, the barrier layer was removed by using a 5 wt % phosphoric acid solution. Atomic Layer Deposition. TiO2 NTs were applied by a showerhead-type ALD reactor (OZONE-100A, ForAll, South Korea) B

DOI: 10.1021/acsami.8b02473 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

and thus resulting morphologies of Au NWs. The hydrolysis of AuCl4− ions has been studied by other researchers.56−60 As the pH increases, Au species are hydrolyzed, replacing the Cl− ions with OH−. On the basis of the previous studies, different pH values of 2.3, 4.0, 4.5, 6.0, and 7.5 were selected to examine the influence of the pH on the formation of Au NWs. Figure 3

proximity to the semiconductor surface. The reduction potential of AuCl4−/Au (1.002 V vs NHE)48 is more positive than the conduction band of TiO2 (−0.1 to −0.6 V vs NHE in a pH range of 1−6),49−52 which implies that the excited photoelectrons in the conduction band of TiO2 can easily reduce cations. The reduction of Au ions was performed below room temperature (at ca. 5 °C) to prevent the thermal deposition and precipitation of Au NPs on the TiO2 NT surfaces. The comparative study of using anatase and amorphous TiO2 NTs for growing Au NWs resulted in clear differences, as shown in Figure 2. Figure 2a displays the FE-SEM secondary

Figure 2. (a,c) FE-SEM SE image and (b,d) corresponding BE image of anatase and amorphous TiO2 NTs, respectively. The brighter contrast of the corresponding BE image clearly shows the resulting Au NWs in the anatase NTs. When the Au NWs reach the distal ends of the TiO2 NTs, the growth of Au is no longer anisotropic. No Au NWs were formed when using amorphous TiO2 NTs (UV irradiation = 2 h).

electron (SE) and corresponding backscattered electron (BE) images of Au wires inside of anatase TiO2 NTs in Figure 2b over the same area. The BE image in Figure 2b has a brighter contrast, clearly shows that the Au NWs were grown in the TiO2 NTs with different lengths from 1 to 8 μm, and indicates that nanoparticulate golds are rarely observed. Interestingly, Au ions were not reduced at all in the amorphous TiO2 NTs (Figure 2c,d). It has been shown that amorphous TiO2 has a lower photocatalytic performance than anatase TiO2.53,54 This observation further suggests that the photoexcited electrons from TiO2 NTs mainly contribute to the reduction of Au ions and the internal growth of Au NWs. Although no scavengers were used in this study, Cl− in the HAuCl4 solution is considered to act as a hole scavenger.55 The average length of Au NWs was measured after 5, 10, and 15 min UV irradiation (Figures S1 and S2). Because nucleation occurs randomly, the standard deviation is considerably large; however, the increase in average length is clearly observed. Depending on the solution pH, different species of aqueous HAuCl4 exist. The species affect nucleation/growth processes

Figure 3. FE-SEM BE images of Au NWs grown after 30 min, 1, and 2 h of UV irradiation at solution pHs of 2.3 (a−c), 3.5 (d−f), 4.0 (g−i), 4.5 (j−l), 6.0 (m−o), and 7.5 (p−r). As the pH decreases (between 2.3 and 4.5), the nucleation rate increases and branched or platelike Au NPs are formed at the ends of the TiO2 NTs. Because the growth rate is quite fast, once the Au ions are nucleated, high-aspect-ratio Au NWs are formed after 2 h of irradiation, even though the initial nucleation rate is slow. At a pH above 6 (m−r), rather short Au NRs are formed inside of the TiO2 NTs and aggregated Au NPs are formed even at the outside of the TiO2 NTs.

illustrates the FE-SEM BE images of Au NWs grown in TiO2 NTs after 0.5−2 h under UV irradiation at solution pHs of 2.3 (Figure 3a−c), 3.5 (Figure 3d−f), 4.0 (Figure 3g−i), 4.5 (Figure 3j−l), 6.0 (Figure 3m−o), and 7.5 (Figure 3p−r), respectively. When the pH was between 2.3 and 4.5, the production yield and AR of the Au NWs seemed to decrease as the pH increased in the initial 30 min. After UV illumination for 2 h, the Au NWs grew to a satisfactory length (AR > 100). It is C

DOI: 10.1021/acsami.8b02473 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) Mosaic of TEM images of single-crystalline Au NW grown at pH 5.0. The Au NW was 15 μm in length, 50 nm in diameter, and an AR of more than 300. (b) Electron diffraction of Au NW. The zone axis of this electron diffraction pattern is [011]. (c,d) HRTEM image of Au NW.

by the dashed circles in panel e), which indicated the structural continuity between the Au plates and NWs. Figure 5f is a HRTEM lattice image of the Au NW region, showing Au(111) and (200) planes with lattice spacings of 0.24 and 0.20 nm, respectively. Although not all of the Au NWs were perfect single crystals, it is clear that the formation of twin defects was reduced at pH 4.5 compared with that at pH 2.3 and 3.5. Above pH 6, rather short Au NWs are formed inside the photocatalytic tubes, whereas Au NPs with irregular sizes and shapes are formed at the inner and outer surfaces of the TiO2 NTs (Figure 5g,h). These NWs were not compact within the hollow interiors compared to the wires grown at pH 4.5 (Figure 5e). The resulting Au NPs were also observed on the outer surfaces of the TiO2 NTs (Figure 3m−r), again exhibiting that Au ions were reduced mainly on the NT surface, not by precipitation from solution. Recently, we reported the Au polymorph, nonclose-packed hexagonal (ncp-2H) structure (i.e., a = b = 2.88 Å, c = 7.15 Å).37 Our previous study demonstrated that twins are not formed in ncp-2H Au NW. Interestingly enough when the twinning is suppressed at pH 4.5, the twin-free ncp-2H Au NWs are observed (Figure 6). A low-magnification TEM image of Au NW grown at pH 4.5 as shown in Figure 6a, and the corresponding SAED pattern taken at the tip of the Au NW with the zone axis of [1−213] as the inset. The HRTEM images and the corresponding fast Fourier transform (FFT) patterns taken at the edge of the Au plate and NW are shown in Figure 6b,c and 6d,e, respectively, implying the continuity of the Au plate and NW. It is possible for Au ions to be reduced in water without any reducing agent. Kurihara et al. observed the transient spectra of Au solutions after irradiation with nanosecond laser excitation.62 They proposed the following excitation processes

suggested that although the nucleation rate varied with the pH, the Au NWs grow fast once nuclei are formed. Moreover, nucleation occurs in a random fashion because not all of the TiO2 NTs were filled with Au NWs even after UV irradiation for 2 h. Note that nucleation was suppressed as the pH increased from 2.3 to 4.5. After the reaction for 30 min, much shorter Au NWs are found at the pH 4.0 (Figure 3g). At a pH of 4.5, no Au NWs were initially found inside the tubes, as shown in Figure 3j. Interestingly, Au NWs grew only inside of the TiO2 NTs up to a pH of 4.5. On the other hand, at a pH above 6.0 (Figure 3m−r), the Au NPs grew both inside and outside of the TiO2 NTs and Au NWs grown inside the TiO2 NTs have much lower ARs. Different species of Au complexes are formed depending on the pH of the solution, and the resulting species are responsible for the nucleation and growth processes. A typical Au NW with high AR (in this case is more than 300) is shown in Figure 4a. The Au NW is of the length of more than 15 μm and the diameter of 50 nm. The SAED pattern with the zone axis of [011] indicates single-crystalline Au, as shown in Figure 4b, and the corresponding highresolution TEM lattice images show a Au(111) interplanar spacing of 0.24 nm, as shown in Figure 4d. The pH affected not only the nucleation and growth rates but also the crystal morphologies. The HRTEM images in Figure 5 unambiguously show the crystal structures and morphologies of the resulting Au NWs. At pH 3.5, multipletwinned Au NWs were found (Figure 5a−d). Figure 5a shows the low-magnification TEM image of Au NWs grown at pH 3.5. The corresponding SAED patterns taken from the black circle in panel (a) show typical diffraction patterns of multipletwinned Au NWs with a face-centered cubic structure (Figure 5b). Figure 5c,d are the HRTEM images, showing a number of {111}/⟨112⟩ twin planes. Multiple-twinned Au NWs are rarely reported, except for penta-twinned Au NWs produced by the seed-mediated growth method.18,61 At around pH 4.5, twin-free (or not observed) Au NWs were formed and occasionally hexagonal or truncated triangular platelets were obtained at the end of the TiO2 NTs (Figure 5e,f). Figure 5e indicates that the wires and plates are single crystalline, that is, linked without any grain boundaries, and have the same growth direction of [112]. Three SAED patterns were taken from different areas (marked

hν

D

(HAu 3 +Cl4) → (HAu 3 +Cl4)*

(1)

(HAu 3 +Cl4)* → (HAu 2 +Cl3···Cl)

(2)

(HAu 2 +Cl3··· Cl) → HAu 2 +Cl3 + Cl

(3)

2HAu 2 +Cl3 → HAu 3 +Cl4 + HAu+Cl 2

(4)

DOI: 10.1021/acsami.8b02473 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. (a) Low-magnification TEM image of a Au NW grown at pH 3.5 and (b) corresponding SAED pattern (from the area indicated by the black circle in panel a), which is similar to the typical diffraction pattern of a penta-twinned crystal. (c,d) HRTEM lattice images showing several {111}/⟨112⟩ twin planes. (e) Low-magnification TEM image of a Au NW grown at pH 4.5, and the SAED patterns (below) taken from different areas marked by the dotted circles. The resulting diffraction exhibited identical patterns, indicating the structural continuity between the Au plate and NW. (f) HRTEM lattice image taken from the Au NW region indicated in (e), showing Au(111) and (200) planes with lattice spacings of 0.24 and 0.20 nm, respectively. (g) Low-magnification TEM image of Au NWs grown at pH 7.5. Au NWs grow inside the TiO2 NTs, whereas Au NPs also grow outside of the TiO2 NTs. Au NWs are quite short and not compact within the interior of the TiO2 NTs. (h) Magnified TEM image taken from the rectangle area indicated in panel (g). Au NPs formed both inside and outside of the TiO2 NTs and have irregular shapes and sizes.

Au2+ disproportionates to Au+ and Au3+. Then, Au+ is subsequently reduced by absorbing a photon. Nucleation occurs when the concentration of Au0 exceeds a certain level called the critical concentration. The concentration of the HAuCl4 solution used in this study is sufficiently low that nucleation does not occur without TiO2. As shown in Figure 2, Au NWs were not formed in amorphous TiO2 NTs. These results imply that the nucleation from the photoirradiation of

hν

HAu+Cl 2 → Au 0 + HCl + Cl

(5)

nAu 0 → (Au 0)n

(6)

These mechanisms suggest that photoirradiation of the HAuCl4 solution resulted in the dissociation (eq 4) and disproportionation (eq 5) of the Au complex. Au3+ is excited by irradiation and then reduced to Au2+. Because of its instability, E

DOI: 10.1021/acsami.8b02473 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 6. (a) Low-magnification TEM image of Au NW grown at pH 4.5, and the corresponding SAED pattern taken at the tip of the Au NW (inset). (b) HRTEM lattice image taken from the edge of the Au plate and (c) corresponding FFT pattern. (d) HRTEM lattice image of Au NW and (e) corresponding FFT pattern. The black circles in (a) indicate the regions of HRTEM images. The zone axes and reflections are indexed in SAED and FFT patterns, respectively.

Figure 7. (a) Schematic illustration for the confined growth of Au NWs in TiO2 NTs. (b) Schematic depiction of the concentration of Au ions in a TiO2 NT as a function of distance from the surface of a 1 nm thick Au segment (C0 is the concentration of Au complexes). When a 1 nm thick Au segment forms inside a TiO2 NT with an inner diameter of 40 nm in a 5 mM solution, the concentration of Au complexes is lower than C0 throughout the TiO2 NT length.

bulk solution because the nanoscale confinement limits the diffusion of Au ions from the outside into the NTs. If the concentration of Au0 drops below the critical concentration, no additional nucleation occurs in the depletion region. Au ions diffuse into the TiO2 NTs and absorb on the surface of the Au nuclei; photogenerated electrons are continuously provided by the TiO2 NTs and injected into the Au nuclei simultaneously. In addition, Au NWs grow along with the axial direction of the NTs, resulting in high-quality crystals (see Figure 7). When Au NP nucleates in a TiO2 NT with a 40 nm pore diameter in 5 mM solution and Au ions are uniformly distributed throughout the inner volume, the corresponding length, which contains Au ions equivalent to the number of atoms in a 1 nm thick Au segment, is approximately 24 μm.37 The length of the diffusion layer would be longer than the length of NTs, and the concentration gradient becomes more significant in smaller

the Au complexes is negligible. During UV irradiation of TiO2 NTs, the dissociation and disproportionation of Au3+ are more prevalent than those in the case of without TiO2 NTs. More Au0/Au+ (a form that can be easily reduced to Au0) are present inside the TiO2 NTs than outside. The TiO2 NTs facilitate the reduction of Au complexes by providing excited photoelectrons and hinder Au0/Au+ to diffuse out to the bulk solution as a nanoscale confinement. Therefore, the concentration of Au0 can be exceeded locally in the TiO2 NTs, and nucleation can occur. On the outer surface of TiO2 NTs, Au complexes can also be reduced by excited photoelectrons from the TiO2 NTs. However, it may diffuse out to the bulk solution by Brownian motion, and the concentration of Au0 cannot be reached to the critical concentration for nucleation. Once nucleation occurs, the concentration of Au ions in the TiO2 NTs quickly drops near the nuclei compared with that in F

DOI: 10.1021/acsami.8b02473 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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directly affected by the pH of the solution. Au ions nucleated inside the NTs only at low pH (pH 2.3−4.5) and grew into high-aspect-ratio Au NWs. The occurrence of twins was more frequent at lower pH. Multiple-twinned Au NWs were observed at pH 2.3 and 3.5, whereas at a pH between 4.0 and 4.5, the number of twins was significantly reduced and single-crystalline Au NWs were observed. Over pH 6, Au ions nucleated both inside and outside of the NTs, and consequently, rather short Au NWs were formed, whereas Au NPs were nucleated outside of the TiO2 NTs. It is apparent that nucleation is facilitated in the TiO2 NTs, resulting in the selective growth of Au NWs with high ARs. We suggest that the point of supersaturation of Au0/Au+ species which have a higher reduction potential than other species may be photocatalytically reached inside of the TiO2 NTs compared with that outside, and the limited diffusion of Au ions allows control over the resulting structures. The present study provides further understanding of the nucleation and growth mechanisms during crystallization in a confined area.

pores. Nanoscale confinement is thus important for the nucleation and growth of Au NWs. At lower pH values (pH 2.3−4.5), the nucleation process was promoted with decreasing pH because of the reduction potential of aqueous HAuCl4, which increases with decreasing pH.59 Therefore, the Au NWs were shown to have a faster nucleation rate at low pH. The exact nucleation and growth processes of Au NWs may be more complex. The reduction potential argument is in good agreement with the results when the pH ranged from 2.3 to 4.5. However, the nucleation rate increased when the pH reached 6. It is necessary to apply another factor affecting nucleation. That is a surface charge of TiO2 determined by isoelectric point (IEP). When the solution pH is lower than the IEP of TiO2, the surface of TiO2 is positively charged, whereas the solution pH is higher than the IEP, the surface of TiO2 is negatively charged. The IEP of TiO2 is known to be about pH 6.63 The Au species in the solution are negatively charged regardless of solution pHs.57 The higher the pH, the more Cl− in Au species is replaced by OH−. At pH 2.3, AuCl4− is dominant, whereas at pH 7.5, AuCl(OH)3− is dominant in the solution.58 Although the Au species have negative charges at all pHs, which likely that there is a repulsive force between the Au species and TiO2, Au species can adsorb on the TiO2 surface via ligand exchange between OH− and TiO− when the solution pH is above the IEP.64 Soejima et al. suggested that the photoelectron from TiO2 can efficiently reduce the Au species adsorbed by ligand exchange.64 The Au species can be absorbed both on the outer as well as inner surfaces of TiO2 by ligand exchange. Therefore, Au can be nucleated both inside and outside of the TiO2 NTs at high pH than 7.5. A summary of the relation between the solution pH and resulting crystal morphology of Au NWs is shown in Figure 8.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02473. Average lengths and FE-SEM BE images of Au NWs grown at the solution pH of 2.3 after UV irradiation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Seonhee Lee: 0000-0001-7700-7655 Changdeuck Bae: 0000-0001-5013-2288 Hyunjung Shin: 0000-0003-1284-9098 Author Contributions

S.L., C.B., and H.S. conceived the project. S.L. prepared the samples. S.L. and H.S. analyzed the structures. S.L., C.B., and H.S. wrote the manuscript. All authors reviewed the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT Future Planning (MSIP) of Korea under contracts NRF-2017R1A4A1015770 (Basic Research Laboratory Program), NRF-2016M3D1A1027664 (Future Materials Discovery Program), NRF-2018M3C1B7020716 (Nature Inspired Innovative Technology Research Program), and NRF2015R1C1A1A02037529 (Basic Science Research Program).

Figure 8. Summary of the relationship between the solution pH and resulting product morphology. The red region indicates the IEP of TiO2, which influences the surface charge of the TiO2 NTs as well as the crystal morphology of Au NWs.

Although the exact mechanisms are yet to be ascertained, the nanoscale confinement and subsequent growth in the surrounding media are crucial to the formation of singlecrystalline Au NWs with high AR. The present methodology enables us to fabricate different kinds of customized heterostructures for various potential applications and to understand confined crystallization processes in general.

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REFERENCES

(1) El-Sayed, M. A. Some Interesting Properties of Metals Confined in Time and Nanometer Space of Different Shapes. Acc. Chem. Res. 2001, 34, 257−264. (2) Anderson, L. J. E.; Payne, C. M.; Zhen, Y.-R.; Nordlander, P.; Hafner, J. H. A Tunable Plasmon Resonance in Gold Nanobelts. Nano Lett. 2011, 11, 5034−5037. (3) Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911−921.

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CONCLUSIONS In conclusion, we have photocatalytically synthesized highaspect-ratio Au NWs in the confined volume of TiO2 NTs. The crystal morphology and nucleation/growth of Au NWs were G

DOI: 10.1021/acsami.8b02473 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.8b02473 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX