Electrodeposition of Vertically Aligned Silver Nanoplate Arrays on

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Electrodeposition of Vertically Aligned Silver Nanoplate Arrays on Indium Tin Oxide Substrates Qingyong Wu, Peng Diao, Jie Sun, Tao Jin, Di Xu, and Min Xiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05109 • Publication Date (Web): 07 Aug 2015 Downloaded from http://pubs.acs.org on August 17, 2015

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The Journal of Physical Chemistry

Electrodeposition of Vertically Aligned Silver Nanoplate Arrays on Indium Tin Oxide Substrates Qingyong Wu a, Peng Diao*,a, Jie Sun a, Tao Jina, b, Di Xu a, and Min Xiang a.

a

School of Materials Science and Engineering, Beihang University, Beijing 100191, P.R. China.

b

China Special Vehicle Research Institute, Aviation Key Laboratory of Science and Technology

on Structural Corrosion Prevention and Control, Jingmen 448035, Hubei, P.R.China.

KEYWORDS nanoparticles, electrodeposition, silver crystal, capping agent

ABSTRACT Vertically aligned Ag nanoplates (NPs) were fabricated on indium tin oxide (ITO) substrates by electrodeposition growth in the AgNO3 solution using citrate anions as the shape-controlling agent. The factors affecting the deposition process, such as the potentials applied to the ITO substrate and the concentration of the precursors and citrate, were systematically investigated. We found that the potentials applied both for nucleus generation and for nucleus growth play important roles in tuning the morphology of the Ag NPs. It was also found that the concentration ratio of capping agent to precursor (R) is a critical factor, only when R is relatively low (R < 1.0) could the well-aligned Ag NPs be formed. However, a high R value 1

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will lead to the isotropic growth of the Ag crystal. A concentration-gradient-induced growth mechanism of vertically aligned Ag NPs is proposed on the basis of experiment results obtained. An Ag NPs/Ag3PO4 composite electrode was fabricated by a electrodeposition method. The photoelectrochemical oxygen evolution reaction (OER) activity of the Ag NPs/Ag3PO4 electrode is about 15.6 times higher than that of a control electrode, which was fabricated on the basis of a Ag electrode whose morphology was irregular polyhedrons.

INTRODUCTION Metal nanoparticles have appealing properties in the fields such as optical,1,2 electronic,3 chemical sensing,4-6 biological7,8 and catalytic9-12 applications. These properties are largely determined by size, shape and the exposing facet(s) of a nanoparticle.13,14 Therefore, efforts have been made to investigate the influence of synthetic conditions that affect the morphology of the metal nanoparticles. As a coinage metal, silver, has attracted considerable attention due to its durability and high conductivity.15In the past decades, Ag nanoparticlesofvarious shapes have been synthesized, including nanocubes, nanoshperes, nanoprisms, nanoplates, nanorods, nanobelts and nanowires.16-27 Generally, capping agents, such as poly (vinyl pyrrolidone) (PVP),24 sodium citrate19 and N,N-Dimethylformamide (DMF)26 are involved to control the growth rate of different facets of

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Ag nanoparticles. For example, PVP adsorb most strongly to silver atoms on the {100} facet,28 so when PVP is used as the capping agent, crystal growth on {100} planes is greatly restrained. The most slowly growing facet is the most likely to be preserved in the final product, so nanocubes bounded mainly by {100} facets are formed when PVP is involved.29It has been found out that the concentration ratio of capping agent to precursor is crucial to the final shape of Ag nanoparticles.27,30 For example, Wiley et al.24,27 focused on the polyol synthesis, ethylene glycol (EG) served both as the solvent and reducing agent, silver nitrate (AgNO3) and poly PVP were provided as silver precursor and capping agent, respectively. By varying the capping agent/precursor ratio (R), they obtained Ag nanoparticles of different shapes: nanospheres, nanocubes and nanowires. In most of the works, Ag nanoparticles were synthesized in homogeneous solutions. This means the products have to be transferred onto solid substrates in many applications, such as electrocatalysis and sensing,31-33 when an external potential bias is to be applied. Therefore, the direct growth of Ag nanostructure on a conducting substrate is highly desired. However, the introducing of a substrate turns out to be a double-edged sword because on one hand, the controlling of size and shape is more complicated on the heterogeneous interface. What’s more, the purification methods commonly used in homogeneous synthesis, such as centrifugation, filtration and electrophoresis are not available. On the other hand, by using a conducting substrate, one could easily adjust the reaction conditions by changing the applied potential.34At 3



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least three factors are affected by the applied potential: (1) the growth rate of Ag crystal, (2) the thickness of the precursor concentration gradient layer and (3) the adsorption/desorption equilibrium of the capping agent. In previous works, we have reported the shape-controlled electrodeposition of rare metals such as Au,35 Pd36 and Rh.12Silver nanoplates (NPs) have also been reported to be obtained on Au coated Si substrate,21 but in that work the Ag NP arrays could not be formed without Au coating. Ag NPs were also obtained on n-type or p-type GaAs wafers.37 Direct growth of Ag NPs on ITO substrates by electrodeposition were also reported.38-41 However, the influence of deposition parameters, such as the growth potential, the concentration of the precursor, the growth time and especially the concentration ratio of capping agent to precursor (R) is not clear. We herein report the electrodeposition of Ag NPs on ITO by a two-step electrodeposition method, which involves a short nucleation step at a high negative overpotential and a long particle growth step at a mild deposition potential. The two-step deposition method have been proved to be successful in the electrodeposition of many noble metal NPs, such as Rh12 and Pd.36 In this work, the resulting Ag NPs stand vertically to the substrate and distribute uniformly on the ITO substrate, forming a vertically aligned NP array. The influential factors on the size and morphology of the Ag NPs, such as the nucleation potential, growth potential and capping agent/precursor ratio (R) were clarified. We also propose a mechanism for the vertically oriented growth of Ag NPs based on experimental observations. 4


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Ag3PO4 was found to exhibit high activity as a semiconductor material photoelectrochemical (PEC) for the oxygen evolution reaction (OER).42 Usually, Ag3PO4 was firstly formed in aqueous solution and then turned to a conducting substrate.43,44 In this work, as a major advantage of the electrodeposited Ag nanostructures over those fabricated in a homogeneous synthesis, Ag3PO4 was formed in situ on the Ag NPs by an electrochemical method. The surface Ag of the Ag NPs was further electrochemically oxidized to Ag3PO4 in a Na3PO4 solution, forming a Ag NPs/Ag3PO4 composite electrode. Owing to the conductive connection between the substrate and the semiconductor, the composite photoanode exhibit good photoactivity. EXPERIMENTAL Chemicals.

Silver

nitrate

(AgNO3),

potassium

nitrate

(KNO3),

sodium

citrate

(C6H5O7Na3·2H2O, herein after, referred to as Na3Cit), trisodium phosphate anhydrous (Na3PO4) and potassium phosphate monobasic (KH2PO4) were all purchased from Beijing Chemical Reagents Industry. ITO substrates were purchased from Nanbo Co., Ltd. (Shenzhen, China). All reagents are analytical grade and are used without further purification. Double-distilled water was used throughout the experiments. The square resistance of the indium tin oxide (ITO) coated glass slides was < 10 Ω sq-1. Electrodeposition of Ag NPs. The electrodeposition was performed on a CHI 660A electrochemical workstation (CH Instruments Co.). We used a conventional three-electrode cell at room temperature. An ITO substrate, a Pt foil and a saturated calomel electrode (SCE) were 5



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employed as working, counter and reference electrode, respectively. All potentials mentioned are quoted to SCE. Before use, the ITO substrates were cleaned by sonication successively in 0.5 M KOH and acetone for 10 min and 15 min, respectively. In a typical electrodeposition, 10ml aqueous solution containing 5 mM AgNO3, 200 mM KNO3 and 1mM Na3Cit was used as electrolyte. We adopted a two-step multi-potential method. At first, the nucleation step, silver seeds were obtained under -0.400 V in 20 ms, this process was repeated for four times to get a higher seed density. Secondly, the growth step, Ag NPs grew under 0.250 V for another 1800 s. The as synthesized Ag NPs coated ITO was then cleaned with double-distilled water for several times, dried in a steam of high-purity N2. The preparation of the Ag NPs/Ag3PO4 composite photoanode. The as-fabricated Ag NPs arrays were used to prepare Ag NPs/Ag3PO4 composite electrodes. The Ag NPs array electrodes were biased at 0.600 V for 150 s in 0.2 M Na3PO4 solution. The applied potential of 0.600 V is positive enough to oxidize surface Ag to Ag3PO4 in phosphate solution. As a control experiment, a Ag/Ag3PO4 electrode was also prepared by simply removing the Na3Cit from the deposition electrolyte of Ag. Characterization. The morphology and size of Ag NPs were observed by field-emission scanning electron microscopy (SEM) (Hitachi S-4800). The height of the Ag NPs was characterized by tapping-mode atomic force microscopy (AFM, Multimode NanoScopeIIIa, Veeco Instruments, USA). High resolution transmission electron microscopy (HRTEM) and 6


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selected area electron diffraction (SAED) were conducted on a JEM-2010F, JEOL Ltd., Japan instrument. The crystal structure of Ag NPs were also investigated on a X-ray diffraction (XRD) (Rigaku, Rint 2000 advance theta-2theta powder diffractometer) with Cu Kα radiation. The photoelectrochemical measurements. The photoelectrochemical oxygen evolution reaction activities of the composite photoanodes were tested in a traditional three-electrode cell, with the composite photoanode as the working electrode, and a Pt foil and a SCE were used as the counter and reference electrodes, respectively. A 300 W xenon lamp was used as the light source and the incident light intensity on the electrode surface was 100 mW⋅cm-2. A chopper chopped the light between the electrode and the light source at an on/off interval of 5 s/5 s. A constant bias potential of 1.0 V was applied to the electrode in a 0.2 M KH2PO4 solution (pH = 4.3), and the current-time response of the electrodes under the chopped light were recorded.

RESULTS Electrodeposition of Ag NP arrays. In this work, AgNO3 was used as the precursor, and the main reaction involved in the electrodeposition is the reduction of Ag+ anions to neutral Ag: Ag+ + e-→ Ag The standard redox potential of this reaction is 0.558 V vs. SCE. The electrolyte used in this work contains 5 mM AgNO3, therefore, according to the Nernst Equation, the equilibrium potential of the Ag+/Ag redox couple was calculated to be 0.420 V. The cyclic voltammogram 7



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(CV) of an ITO substrate in the deposition which contains 5 mM AgNO3 and 200 mM KNO3 is presented in Figure 1a. An oxidation and a reduction peak are clearly distinguished. We believe this oxidation/reduction peak couple could be assigned to the oxidation/reduction reaction of the Ag+/Ag redox couple on the ITO/solution interface. The peak potential of the reduction wave (Ep (reduction))

locates at ca. 0.258 V, and the peak potential of the oxidation wave (Ep (oxidation)) located

at 0.604 V. The equilibrium potential (Eq) could be calculated by Eq = (Ep (oxidation) + Ep (reduction))/2 = 0.431 V. This value is in good agreement with that predicted from the Nernst Equation. Na3Cit was added as capping agents to direct the growth of Ag NP arrays in this work. Citrate anions could coordinate strongly to Ag+ anions, forming a variety of coordinate species,15,45 which significantly reduce the concentration of free, isolated Ag+ anions. After Na3Cit was introduced into the electrolyte, the open-circuit potential of the ITO substrate shifted negatively from 0.400 V to 0.315 V, which means the reduction of Ag+ to Ag is retarded. As shown in Figure 1b, Ep of the reduction and oxidation waves also shifted negatively by tens of millivolts, to 0.208 V and 0.506 V, respectively, and Eq was calculated to be 0.357 V. The negative shifts of the peak and equilibrium potentials indicated that the reduction of Ag+ to Ag is greatly retarded by the adding of citrate anions, as a more negative potential is now required to reduce Ag+ ions from the precursor. According to the CV curve in Figure 1b, to conduct the electrodeposition, the applied potential should be lower than 0.357 V. The lower the applied potential, the larger the deposition driving 8


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force and the faster the growing of Ag crystal, and vice versa. In this work, we chose -0.400 V, with a large overpotential, as the seeding potential to form Ag seeds rapidly and 0.250 V, with a small overpotential, to induce the growth of Ag NPs mildly. The effects of different seeding and growth potentials would be discussed in the following sections.

Figure 1. Cyclic voltammogram (CV) curves of an ITO substrate in (a) 5 mM AgNO3 and 200 mM KNO3 and (b) 5 mM AgNO3, 200 mM KNO3 and 1mM sodium citrate. The potential sweep rate is 50 mV s-1. The arrows indicate the circling direction of the CVs.

Morphology and Crystal Structure. Figure 2a shows the SEM image of the Ag NPs electrodeposited on ITO substrate. The Ag NPs are flake-like and they are standing vertically to the substrate. The low magnification image demonstrates that Ag NPs disperse uniformly on the 9



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whole substrate with a high density. The high magnification (inset of Figure 2a) indicates that the Ag NPs are of a few microns in edge length and about 25 nm in thickness. The AFM images and an SEM image obtained with the sample mounted on a slant (Figure S1 in supporting information) also confirm the vertically standing nature of the Ag NPs, and the average height of the Ag NPs is calculated to be ca. 600 nm. Optical photographs of both the front and back side of a Ag NPs coated ITO are displayed in Figure S2. The front side is gray, and is totally opaque, while the back side is like a reflective mirror. It’s worth to note that the Ag NPs adhere so strongly to the ITO substrate that they would not peel off under ultrasonic vibration even in ethanol.21

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Figure2. (a) scanning electron microscope (SEM) image of vertically aligned Ag NPs array fabricated on ITO. Inset: local magnification. (b) X-ray diffraction (XRD) pattern of the Ag NPs. The lower line spectrum is the standard diffraction of Ag powders (JCPDS No. 65-2871). (c) high-resolution transmission electron microscope (HRTEM) image of a single Ag NP lying flat on the TEM grid, inset of (c) shows the TEM image of the corresponding Ag NP. (d) selected area electron diffraction (SAED) spots correspond to the very Ag NP in (c), the image contrast of one of the spots is adjusted to see more clearly.

The XRD pattern of the Ag NPs coated ITO is shown in Figure 2b. The four diffraction peaks at 2θ = 38.18 °, 44.48 °, 64.60 ° and 77.54 ° could be indexed to the (111), (200), (220) and (311) reflections of the face-centered cubic (FCC) structure (JCPDS No. 65-2871) of metallic silver, which demonstrates that the Ag NPs are FCC crystals. All the other distinguishable peaks 11



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in the XRD pattern could be attributed to the ITO substrate. The intensity ratio of the diffraction peak of (200) plane to that of the strongest (111) plane is 0.28, lower than the value obtained from the JCPDS card (0.46). This implies that the Ag NPs are abundant in the {111} plane. Figure2c and d show the HRTEM and SAED images of a single Ag NP scraped from ITO, as seen from inset of Figure 2c. Due to the surface flatness, the scraped Ag NPs tend to lie on the TEM grid. The electron beam was directed perpendicular to the planar surface of the NP. According to Figure 2c, the lattice spacing is calculated to be 0.249 nm, which could be ascribed to the normally forbidden 1/3{422} lattice. Such forbidden reflection was also observed in previous works, usually in plate-like silver crystal.21,27 It has been revealed that the forbidden reflection could be attributed to the {111} stacking faults parallel to the flat surfaces. The lattice spacing of 0.144 nm, which corresponds to the {220} plane of the FCC Ag crystal, was also observed. The SAED (Figure 2d) pattern indicates that each NP is a single crystal. The inner spots (to see more clearly, the image contrast of one of the spots was adjusted) are indexed to the 1/3{422} Bragg reflections, with a d-spacing of 2.5 Å, which is in agreement with the lattice spacing shown in Figure 2c. The second set of spots, with a d-spacing of 1.4 Å, is indexed to the {220} Bragg reflections. The six-fold symmetry displayed by the SAED spots indicates that the planar surface of the NP is bounded with {111} planes. Influence of the deposition conditions

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(A) The concentration ratio of capping agent to precursor (R). Citrate anions have long been used as the capping agent when synthesizing metal nanostructures, especially for Ag.45,46Citrate anions can preferentially bind to the {111} plane of Ag, therefore slowing down its growth rate, which leads to the anisotropic growth of the Ag NPs. To illustrate the significance of the introduction of citrate anions, Ag NPs were prepared in absence of Na3Cit. Figure 3a shows that without citrate anions, isotropic irregular polyhedrons were obtained. This result indicates that citrate anion plays a key role in controlling the morphology of the Ag NPs. To investigate the effect of the concentration of citrate anions, we prepared Ag NPs with a variety of Na3Cit concentrations, with the concentration ratio of capping agent to precursor (R) ranging from 20 to 0.02. Figure 3b-f and Figure S3 show the SEM images of samples prepared with different Na3Cit concentrations while keeping the AgNO3 concentration at 5mM and the other conditions unchanged. It was unexpected that at a high capping agent/precursor ratio (Figure 3b, R= 20), we still could not obtain the vertically aligned Ag NPs array. A rough coating of Ag was formed, few small standing Ag NPs sporadically dispersed on the film. This result indicates the predominantly isotropic growth of Ag NPs at such high R value. When the ratio was set at R = 2.0 (Figure 3c), thick and small Ag NPs were formed, most of which were vertically aligned, indicating the starting of the anisotropic and oriental preferred growth of Ag NPs. Well-aligned Ag NPs arrays could only be obtained when R is lower than 1.0 (Figure 3d to 3f, R= 1.0, 0.4 and 0.02, respectively, see also Figure S3d and S3e). In this R value range, the 13



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anisotropic growth of Ag NPs becomes more obvious, high aspect ratio nanoplates were formed as the dominant product. However, if the R value was too low (Figure 3f), there was nonuniformity in size distribution. Moreover, to separately investigate the effect of Na3Citeither in the nucleation step or in the growth step, Ag NPs were prepared under the following two conditions: (1) the nucleation step was conducted in absence of Na3Cit and Na3Cit was added to the solution in the growth step; and (2) Na3Cit was added to the solution in the nucleation step but it was removed from the solution in the growth step. All the other conditions remained unchanged. Figure S4a shows the SEM image of the resulting product of condition (1), small NP clusters with low distribution density were obtained, each cluster was composed of many low-aspect-ratio NPs. As shown in Figure S4b, sporadically dispersed polyhedrons are formed under condition (2), which is very similar to those in Figure3a when Na3Cit is absent in both the nucleation step and the growth step. These results indicate that Na3Cit is indispensible in both the two electrodeposition steps. As the growth step has a much longer duration than the nucleation step, Na3Cit plays a more important role in the growth step than in the nucleation step.

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Figure3. (a) SEM image of a sample prepared in absent of sodium citrate. (b) to (f) show the SEM images of samples prepared with different surfactant/precursor ratio, (b) R=20.0, (c) R=2.0, (d) R=1.0, (e) R=0.4 and (f) R=0.02, with all the other conditions remain unchanged.

(B) The nucleation step and nucleation potential. As mentioned above, we adopt a two-step electrodeposition method to grow Ag NPs, which included a short nucleation step carried out at a high negative overpotential and a relatively long growth step performed at a mild deposition potential. The high overpotential provided larger driving force for the generation of large amount of Ag nuclei on ITO substrates. The nucleation step should be extremely short in time due to its high overpotential. If the duration of nucleation is too long, the generation of new nanoseeds will be accompanied by the fast growth of existing nuclei, leading to a nonuniformity in the size of Ag NPs. To obtain the required density of seeds, this fast nucleation step can be repeated for several times. Ag seeds were generated, and they would serve as active seeds in the growth step. The SEM image of these Ag seeds is shown in Figure S5. These nuclei dispersed uniformly on the surface of ITO. They are randomly piled up Ag spheres, with the diameter of ca. 15 nm, indicating that the nucleation step with a high overpotential led to the isotropic growing of Ag. This nucleation step is indispensible to the formation of Ag NPs on ITO. Figure 4a shows the typical SEM image of the Ag NPs prepared by direct electrodeposition growth without the nucleation step, a flower-like Ag nanostructure was obtained, which was composed of randomly oriented Ag NPs. 16


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The flower-like Ag NPs sporadically disperse on the substrate, with most of the ITO surface exposed. This phenomenon demonstrates that the nucleation step plays a key role in the final morphology of the Ag NPs. The nucleation potential is also an important influential factor for the formation of the well-aligned Ag NPs arrays. Generally, as the nucleation process is essentially a reduction reaction, the lower the nucleation potential, the larger the driving force for the generation of Ag seeds, and then the higher the seed distribution density on the substrate. The higher nuclei density leads to higher Ag NPs density in the resulting Ag NPs array. Figure4b-d show the SEM images of the samples prepared under different nucleation potential with the other conditions remained the same. As shown in Figure 4b, when the nucleation step is conducted at 0.000 V, there are still “islands” of flower-like Ag NPs, however the density of the NPs flowers increases a lot compared to that without a nucleation step. When the nucleation potential is set at -0.400 V (Figure 4c), well-aligned Ag NPs array is obtained after the growth step. The whole surface of the substrate is coated with the Ag NPs array, indicating the high density of Ag NPs. Further negatively shifting the nucleation potential makes the Ag NPs to grow thicker (Figure 4d). We are not sure why this happens, it is probably because that the extremely negative nucleation potential may lead to some changes in the structure of Ag seeds. Further investigation is needed. The extremely negative nucleation potential also makes the final product to be easier to peel off from the substrate, which is expected because the extremely high nuclei density results in high 17



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stress among the Ag NPs. The SEM images of the resulting Ag NPs arrays with the nucleation potential of -0.200 V and -0.600 V are presented in Figure S6. It is revealed that well-aligned Ag NPs array could only be obtained in a nucleation potential range of -0.200 V~ -0.600 V.

Figure4. (a) SEM image of a sample prepared without the nucleation step. Inset: local magnification. (b) to (d) show a series of SEM images of samples prepared with different nucleation potentials, (b) 0.000 V, (c) -0.400 V and (d) -0.800 V, with all the other conditions remain unchanged. 18


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(C) The growth potential. The second step is the growth step. This step is conducted under a higher potential, which is near to the equilibrium potential, than in the nucleation step. This means the growth step is performed at low overpotential with small deposition driving force, and the growth of the Ag NPs is a mild process. It has been found in earlier works that the substrate potential is a key parameter in the electrodeposition procedure,12,35,47 and our previous works has demonstrated that there is usually a narrow potential range only within which the well-aligned anisotropic nanostructure of metal could be obtained.12,36In order to investigate the effect of the electrodeposition potential to the growth of Ag NPs arrays, we applied different potentials to the Ag nuclei modified ITO substrate (with the nucleation potential fixed at - 0.400 V). Figure 5 shows the typical SEM images of the Ag NPs arrays prepared under different growth potential. The growth step was performed at 0.350 V, 0.300 V and 0.200 V, respectively for 1800s.When the growth potential was 0.350 V (Figure 5a), clusters of Ag NPs with low growth density were formed. The plate-like morphology could be distinguished, but most of the plates are randomly oriented, unlike the Ag NPs arrays obtained with lower growth potential (e.g. 0.250 V), which are vertically aligned to the substrate. As the growth potential was decreased to 0.300 V, a Ag NPs array with high distribution density was obtained (Figure 5b). The Ag NPs are perpendicular to the substrate, and the sizes of them are basically similar to each other. This indicates the beginning of anisotropic and vertically oriented growth. If the growth potential is shifted to 0.200 V (Figure 5c, the well-aligned Ag NPs array obtained at a growth potential of 0.250 V has 19



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already been displayed in Figure 2a, we do not display it here again), there will be a nonuniformity in the size of Ag NPs. In addition, the distribution density of the Ag NPs becomes lower, compared with that of the Ag NPs deposited under 0.300 V or 0.250 V. All these evidence indicate that the growth potential is a key factor controlling the size and morphology of the Ag NPs arrays. It is also revealed that there exists a potential range only in which the well-aligned Ag NPs array could be obtained. Unlike our previous work of the electrodeposition of Rh NPs,12 where the potential range is as narrow as tens of millivolts, the potential range of Ag NPs is about from 0.300 V to 0.200 V. We believe that the growth potential may affect both the reduction rate of the precursor and the adsorption/desorption behavior of the capping agent, and thus would influence the formation of Ag NPs. The detailed mechanism will be discussed later.

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Figure5. SEM images of samples prepared under different growth potentials, (a) 0.350 V, (b) 0.300 V and (c) 0.200 V, with all the other conditions remain unchanged.

(D) The growth time. We stopped the electrodeposition at different growth time, and the time evolution of the morphology of the Ag NPs was illustrated in Figure 6. At the primary stage, when the growth time was 300 s (Figure 6a), the Ag NPs orientated randomly. Ag NPs both perpendicular to and lying on the substrate could be observed. And the sizes on each direction were nearly the same. The distribution density of the Ag NPs was relatively low; a certain amount of the ITO surface was still uncoated. With the growth time prolonged to 900 s (Figure 6b), both the size and the distribution density increased. Meanwhile, the Ag NPs started to show a preferential growth direction which was perpendicular to the substrate (see also Figure 2a).When the growth time was as long as 3600s (Figure 6c), extremely large plates were found, indicating that a too long growth time would lead to the nonuniformity of the Ag NPs size.

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Therefore, the best growth time for the formation of vertically aligned Ag NPs array is ca. 900 s to 1800 s.

Figure 6. SEM images of samples prepared with different growth time, (a) 300 s, (b) 900 s and (c) 3600 s, with all the other conditions remain unchanged.

The vertically aligned Ag NPs arrays as a framework for a Ag NPs/Ag3PO4 composite electrode for PEC water splitting. Ag3PO4 was found to have shown high activity toward OER. 22


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In this work, the as synthesized well-aligned Ag NPs arrays were used as the framework to fabricate a Ag NPs/Ag3PO4 composite electrode. Ag3PO4 were deposited onto the Ag NPs by electrochemically oxidizing the surface Ag of the Ag NPs in a 0.2 M Na3PO4 solution under a bias potential of 0.600 V for 150 s. This potential is positive enough to oxidize Ag to Ag3PO4.The PEC water oxidation activity of the Ag NPs/Ag3PO4 composite photoanode was investigated by a three-electrode cell in 0.2 M KH2PO4 (pH = 4.36) solution. A control experiment was also conducted simply by removing Na3Cit from the Ag electrode deposition solution, in which the composite photoanode was denoted to as Ag/Ag3PO4. The current-time response of both two electrodes under chopped illumination of 100 mW cm-2, at an applied potential of 1.0 V, are shown in Figure 7. Both electrodes showed difference in current density between under illumination and in the dark, indicating the electrodes are photoactive. The photo-current density (the difference between the photo-current and the dark-current) of the Ag NPs/Ag3PO4 electrode slowly increased to maximum value of ca. 1.25 mA cm-2 and then slowly decreased to ca. 1.00 mA cm-2 at the end of the 200 s test. However, the photo-current density of the Ag/Ag3PO4 electrode was only ca. 0.08 mA cm-2. The photo-current density of Ag NPs/Ag3PO4 is 15.6 times of that of Ag/Ag3PO4.

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Figure 7. The current-time response curves of two different composite photoanodes. (red) a composite photoanode fabricated based on a well-aligned Ag NPs array, and (black) a Ag/Ag3PO4 composite photoanode fabricated based on a Ag electrode, which was electrodeposited on an ITO substrate in the absence of capping agents (Na3Cit). An external bias potential of 1.0 V were applied to the electrodes.

DISCUSSION General discussion of the mechanism of the anisotropic growth of metal nanostructure. The growth of metal nanostructures depends greatly on the thermodynamic conditions of different facets. The newly formed (usually reduced from the precursor) metal atoms tend to 24


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enter into the lattice of each facet of the existing nanoparticles. Meanwhile, the difference of the growth rate on different facets would lead to different shapes in the final nanostructure product. The facets that grow slower will be preserved in the resulting product, and nanostructure will be bounded by these slowly growing facets. This means the kinetic control of the growth rate of the different facets is of great significance in tuning the final shape of a metal nanostructure.48,49 Capping agents are often introduced to direct the growth of the metal nanostructure because they could specifically absorb on some of the facets of the metal lattice. By this means the growth on the special facets is retarded, while the growth rate of the other facets are unaffected or be slowed down more slightly than the special ones. This difference in growth rates between different facets results in the anisotropic growth of metal nanostructure. The kinetic control of the growth rate of the different facets is of great significance in tuning the final shape of a metal nanostructure. Obviously, it is easier to control the shape of NPs by slowing down the reduction rate. For this purpose, a mild overpotential was applied to slow down the reduction rate in the growth step. It’s worth to note that citrate anions were also used to slow down the deposition rate because citrate ions can coordinate with Ag+ and reduce the concentration of free Ag+ ions. Moreover, citrate ions have different adsorption strength on different facets of Ag, for example, citrate anions bind most strongly to the {111} facet of Ag,45,50 The facets on which citrate ions have strong adsorption will grow much slower than the facets on which citrate ions have week adsorption, resulting in the anisotropic growth of Ag 25



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particles. This anisotropic growth can be achieved only in mild overpotentials because a high negative overpotential will desorb all citrate ions from Ag surface due to the negatively-charged nature of citrate ions. Mechanism of how Na3Cit and the concentration ratio of capping agent to precursor (R) influence the growth of the Ag NPs arrays.Na3Cit plays two main roles in the formation of the Ag NPs arrays. Firstly, as mentioned above, the coordination effect of citrate anions with Ag+ ions would reduce the concentration of free, isolated Ag+, thus reducing the reduction rate, which would facilitate the kinetic control of the anisotropic growth. Secondly, citrate anions act as the capping agent during the electrochemical growth of the Ag NPs. Citrate anions adsorb most strongly to the {111} facet of Ag,45,50 which further slows down the growth rate of the {111} facets. Therefore, the {111} facet was preserved in the final product, and the NPs are bounded by two parallel {111} planes, as is demonstrated by the HRTEM, SAED and XRD results. So, the introduction of Na3Cit is crucial to the formation of the plate-like Ag nanostructure. In addition, we carefully studied the effect of the concentration ratio of the capping agent to the precursor, R, and found out that a small R value (R< 1.0) is unexpectedly needed for the formation of the well-aligned Ag NPs arrays. Previous works28,51,52 concerning shape-controlled synthesis of nanoparticles often employed a R value in the range of R> 1.0 to make sure that there were enough capping agent molecules (or anions) to bind to certain facets of nanocrystals. For example, the fabrication of gold nanorods reported by Murphy’s group51 and El-sayed’s 26


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group52required very high concentration of capping agent (cetyltrimethlyammonium bromide). And in Xia’s work,28the molar ratio of polyvinylpyrrolidone (a capping agent) to AgNO3 was kept at 1.5 to form the Ag nanocubes. Excess amount of capping agent was required because more surface was generated as particles grew. However, in this work, the best (highest aspect-ratio, highest uniformity in size and most uniform dispersion) Ag NPs arrays were obtained when R< 1.0, as shown in Figure 3. It seems that just very small amount of citrate anions are enough to direct the anisotropic growth of Ag NPs. We believe that there are two possible reasons that may lead to an unexpected small R value. (1) When R value is high, the citrate anions are adsorbed on all facets of Ag nanocrystals. In this case, the difference of the adsorption affinityon different facet is not as prominent as that with small R value. The growth rate of all facets will slow down, resulting in the similar growth rates among different facets. Therefore, the growth of the Ag nanostructure proceeds in an isotropic manner. (2) In previous works, shape-controlled gold and silver nanoparticles were obtained in homogeneous solutions and high concentrations of capping agents were needed to passivate the growing surfaces. Unlike the these works, the fabrication of Ag NPs in this work was conducted by an electrodeposition method. The mole quantity of Ag that actually deposited on ITO is very small compared with the mole quantity of citrate anions in the bulk solution. To be specific, the volume of the electrolyte was 10 mL (with the citrate concentration of 1 mM) and the electrode had a surface area of 0.25 cm2. The integrated quantity of electric charge during the 0.5 h electrodeposition was ca. 0.13 C. 27



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Assuming a Faraday Efficiency of 100%, the mole ratio of Ag crystal to citrate anion is ca. 1 : 7.42. Moreover, only a small part of Ag atoms are located on the crystal surface. Therefore, there are quite enough citrate anions to block the {111} surface of Ag NPs. The samples prepared with different R values are characterized by XRD and the results are shown in Figure S7a.The dependence of the normalized peak intensity of {200} diffraction to the R value is shown in FigureS7b (the intensity of {111} diffraction is normalized to be 100.0). The {200} peak intensity decreased gradually as R value decreased. This result is in agreement with the former discussion that as the R value increase, the growing process is more likely to proceed in an isotropic manner. We fixed R at 0.2 and varied the absolute concentration of both AgNO3 and Na3Cit in order to study whether R is the only factor that affects the morphology of Ag NPs under certain conditions (which means the same nucleation potential, the same growth potential and the same growth time). Figure 8 shows the SEM images of the samples prepared under the different conditions. The well-aligned Ag NPs arrays were only obtained when AgNO3 concentration is equal to or higher than 5 mM. When AgNO3 concentration was lowered to 2.5 mM, a layer of irregular Ag bricks was formed on the substrate. And when the AgNO3 concentration was further lowered to 1 mM, isotropic Ag nanospheres were the main product on the substrate. This phenomenon is quite similar to that when independently changing the AgNO3 concentration (see Figure S8). This is because a low AgNO3 concentration leads to low growth rate on every facets, 28


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the difference of the growth rates between different facets is not large enough to induce the anisotropic growth of Ag. These observations also suggest that the R value DO NOT determine the morphology of Ag NPs INDEPENDENTLY. A proper AgNO3 concentration is required to form Ag NPs.

Figure8. SEM images of different samples while keeping the surfactant/precursor ratio R=0.2: with the solution containning (a) 0.2 mMNa3Cit and 1mMAgNO3, (b) 0.5mMNa3Cit and 29



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2.5mMAgNO3, (c) 2 mM Na3Cit and 10 mM AgNO3. All the other conditions remain unchanged.

Mechanism of how the growth potential influences the growth of the Ag NPs arrays. The growth potential has the three following effects on the growth of Ag NPs arrays. (1) The growth potential DIRECTLY determines the reduction rate of Ag+. The lower the potential is, the faster the reaction will be. In this work, we chose a relatively high potential, which is slightly lower than the equilibrium potential, to slow down the reduction rate of the precursor, which is in favor of the kinetic control over the Ag NPs growth. (2) The growth potential may INDIRECTLY influence the concentration gradient when diffusion from bulk solution to the electrode/solution surface is relatively slow. The thickness of the concentration gradient layer is correlated to the reaction rate on the surface. The faster the reaction is, the thicker the concentration gradient layer will be. This concentration gradient layer is very important to the growth of the vertically aligned Ag NPs arrays. Because, when this layer is established, the concentration of Ag+ on the surface of the substrate would be lower than that in the bulk solution. This means the vertically standing Ag NPs, which protrude into the bulk solution, have more access to Ag+ ions than those that are lying on the substrate. As a result, the standing Ag NPs grew faster than the lying ones. Eventually, the standing Ag NPs grew taller and taller, which in turn causes the gradient layer to become thicker. Ag NPs that are lying on the substrate and those who fail to grow tall enough will grow slower and slower until the surface of the substrate is depleted of Ag+, when the lying 30


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Ag NPs and the shorter Ag NPs totally stop growing. Finally, the vertically standing Ag NPs predominate the resulting product. To summarize, this concentration gradient layer is indispensable to the formation of a vertically aligned Ag NPs array because it favors the vertical growth of Ag NPs, but retards the growing on the direction parallel to the substrate. As the thickness of this concentration gradient layer is in close relationship with the reaction rate, it is also indirectly dependent on the growth potential. As shown in Figure 5a, when the growth potential is as high as 0.350 V, the reduction rate is very low, and the concentration gradient layer is thin, there is no remarkable difference between the concentration on the surface of substrate and in the bulk solution. The standing and the lying Ag NPs will grow in similar rates, so the Ag clusters, rather than a vertically-aligned Ag NPs array, were formed. In fact, the evolution of the concentration gradient layer is also time-relevant. As shown in Figure 6, as the growth time increases, the preferential growth on the vertical orientation is more and more obvious. This means the gradient layer is growing thicker as the deposition proceeded.(3) The third effect of the growth potential is that the adsorption/desorption behavior of the capping agent is greatly influenced by the applied potential. Citrate anion fully deprotonate upon adsorption on noble metal, these negatively charged species will totally desorb at a negatively shifted potential. Desorption of citrate anions leads to the failure to adjust the growth rate of different facets, which eventually result in the isotropic growth of Ag nanoparticles. As is shown in Figure 5c, when the growth potential was 0.200 V, the Ag NPs started to grow thicker, which 31



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indicated the emergence of the tendency of isotropic growing. It has been reported21 that large Ag particles with equal axial shape were obtained when a very high deposition current density is applied. We believe if we further decrease the growth potential, similar results will be obtained. Mechanism for the difference in PEC catalytic activity between Ag NPs/Ag3PO4 and Ag/Ag3PO4 photoanodes. The SEM images of the composite photoanodes are shown in Figure 9. The Ag NPs were coated with a thick layer of Ag3PO4, but the NP structure remained unchanged (Figure 9a). As to the Ag/Ag3PO4 (Figure 9b), a smaller amount of Ag3PO4 was deposited than on Ag NPs. Most of the Ag polyhydons were not affected by the electrodeposition. The vertically aligned Ag NPs arrays provide much more surface area than the isotropic Ag polyhedrons (Figure 3a), which benefits both the light absorption and the OER, leading to the highly enhanced photocurrent density. This result indicates that the vertically aligned Ag NPs could be rationally designed as a framework to form a Ag based composite nanostructure for a variety of usage.

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Figure 9. SEM images of the composite photoanodes fabricated based on Ag NPs (a) and on irregular Ag polyhydrons (b). Inset of (b) is a local magnification of the region in the black rectangular.

General growth mechanism of vertically aligned Ag NPs arrays. According to the observation and discussion above, we conclude that two factors are essential for the growth of vertically aligned Ag NPs arrays. (1) R value determines how the Ag crystal is grown, anisotropic or isotropic, in other words, NPs or isotropic nanocrystals. (2) The presence of a concentration gradient layer causes the Ag NPs to grow vertically to substrate. Based on these two conclusions, we propose here a general growth mechanism of the vertically aligned Ag NPs, as is illustrated in Figure10. The instantaneous nucleation step conducted under highly negative potential (-0.400 V) produces uniformly distributed Ag nanoseeds on the ITO substrate (Figure 10a). A proper R value was chosen (R = 0.2) to guarantee the anisotropic growth of the Ag crystal. Anisotropic growth of Ag crystal was conducted as the {111} facets are blocked by citrate anions, and the growth of these facets is greatly restrained. At primary stage, there’s no significant concentration decrease of precursor near the surface of ITO. So Ag NPs protrude toward each direction randomly and the sizes on each direction are approximately the same (Figure 10b). As the deposition proceeds, a Ag+ concentration gradient layer starts to form. Ag+ concentration on the surface of ITO substrate is lower than that of bulk solution. As a result, the vertically standing 33



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NPs grow faster than the lying NPs (Figure 10c). These vertically standing NPs extend higher and higher into the bulk solution, which in return makes the gradient layer to grow thicker (Figure 10d). The Ag+ ions near to the surface will eventually be exhausted, growth of the lying NPs is completely stopped (Figure 10e). The growth of the vertically standing NPs is favored because Ag+ is reduced on the active facets of these NPs as soon as Ag+ ions are diffused to them. Finally, vertically standing NPs become the main product and a vertically aligned Ag NPs array is formed.

Figure10. Growth mechanism of vertically orientated Ag NPs. The black arrows (solid line) indicate the growth orientation of the Ag NPs, the length of the arrows represents the growth rate on each orientation.

CONCLUSION 34


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We successfully synthesized uniformly disperse vertically standing Ag NPs arrays on ITO substrate by electrochemical deposition. A two-step multi-potential method was used, including a nucleation step and a growth step. Sodium citrate was used as the capping agent to control the morphology of Ag crystal. The obtained Ag NPs were terminated by Ag {111} facets due to the strong binding of citrate anion on Ag {111} facets. We found that the concentration ratio of capping agent to precursor R was a key factor that determined the formation of plate-like Ag crystal. Ag NPs could only be obtained when R was lower than 1.0. We demonstrated that the nucleation step performed at high negative potential was necessary to the formation of Ag NPs arrays. The nucleation and growth potential affect the morphology of Ag NPs. Well-aligned Ag NPs were only obtained with the nucleation potential within -0.200 V to -0.600 V range and the growth potential within the 0.300 V to 0.200 V range. The surface concentration gradient of the precursor (Ag+) was responsible for the perpendicular growth orientation of the nanoplates. A Ag NPs/Ag3PO4 composite photoanode was fabricated based on the Ag NPs, which exhibited a photocurrent density of 1.25 mA cm-2, 15.6 times higher than that of the composite photoanode fabricated based on the isotropic Ag polyhydrons, indicating the promising application of this Ag NPs electrode in electrocatalysis. A brief growth mechanism illustrating the origin of the formation of Ag NPs arrays was proposed based on the experimental results. This work shows insight into the preparation of anisotropic noble metal nanoparticles on conducting substrates by modulating the adsorption behavior of capping agents. 35



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ASSOCIATED CONTENT

Supporting Information. AFM images of Ag NPs, optical photographs of the as synthesized Ag NPs electrode, SEM images of Ag NPs prepared under other R values and other growth potentials, SEM images of the Ag nanoseeds and XRD patterns of different Ag NPs samples under different R values. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT We gratefully acknowledge the financial support of this work by National Natural Science Foundation of China (NSFC 21173016), Beijing Natural Science Foundation (2142020 and 2151001) ABBREVIATIONS NPs, nanoplates; ITO, indium tin oxide; XRD, X-ray diffraction; HRTEM, high-resolution transmission electron microscopic; FESEM, field-emission scanning electron microscopic; SERS, surface-enhanced Raman Scattering; Na3Cit, sodium citrate; SCE, saturated calomel electrode; AFM, atomic force microscope; CV, cyclic voltammograms. REFERENCES

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(1)

Nallathamby, P. D.; Lee, K. J.; Xu, X. H. Design of stable and uniform single

nanoparticle photonics for in vivo dynamics imaging of nanoenvironments of zebrafish embryonic fluids. ACS Nano2008, 2, 1371-1380. (2)

Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Noble metals on the nanoscale:

optical and photothermal properties and some applications in imaging, sensing, biology, and medicine. Acc. Chem. Res.2008, 41, 1578-1586. (3)

Li, Y.; Wu, Y.; Ong, B. S. Facile synthesis of silver nanoparticles useful for fabrication

of high-conductivity elements for printed electronics. J. Am. Chem. Soc.2005, 127, 3266-3267. (4)

Liao, W. Y.; Chou, T. C. Fabrication of a planar-form screen-printed solid electrolyte

modified Ag/AgCl reference electrode for application in a potentiometric biosensor. Anal. Chem.2006, 78, 4219-4223. (5)

Ray, P. C. Size and Shape Dependent Second Order Nonlinear Optical Properties of

Nanomaterials and Their Application in Biological and Chemical Sensing. Chem. Rev.2010, 110, 5332-5365. (6)

Wang, J.-Y.; Diao, P.; Zhang, D.-F.; Xiang, M.; Zhang, Q. Electrochemical sensing of

CO by gold particles electrodeposited on indium tin oxide substrate. Electrochem. Commun.2009, 11, 1069-1072.

37



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

(7)

Page 38 of 45

Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry,

Quantum-Size-Related

Properties,

and

Applications

toward

Biology,

Catalysis,

and

Nanotechnology. Chem. Rev.2003, 104, 293-346. (8)

Rosi, N. L.; Mirkin, C. A. Nanostructures in Biodiagnostics. Chem. Rev.2005, 105,

1547-1562. (9)

Mori, K.; Kumami, A.; Tomonari, M.; Yamashita, H. A pH-Induced Size Controlled

Deposition of Colloidal Ag Nanoparticles on Alumina Support for Catalytic Application. J. Phys. Chem. C2009, 113, 16850-16854. (10) Chen, J.; Tang, X.; Liu, J.; Zhan, E.; Li, J.; Huang, X.; Shen, W. Synthesis and Characterization of Ag−Hollandite Nanofibers and Its Catalytic Application in Ethanol Oxidation. Chem. Mater.2007, 19, 4292-4299. (11) Diao, P.; Zhang, D.; Wang, J.; Zhang, Q. Electrocatalytic activity of supported gold nanoparticles toward CO oxidation: The perimeter effect of gold–support interface. Electrochem. Commun.2010, 12, 1622-1625. (12) Li, Y.; Diao, P.; Jin, T.; Sun, J.; Xu, D. Shape-controlled electrodeposition of standing Rh nanoplates on indium tin oxide substrates and their electrocatalytic activity toward formic acid oxidation. Electrochim. Acta2012, 83, 146-154. (13) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Controlling anisotropic nanoparticle growth through plasmon excitation. Nature2003, 425, 487-490. 38


Page 39 of 45

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


(14) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. The Shape Transition of Gold Nanorods. Langmuir1999, 15, 701-709. (15) Chadha, R.; Maiti, N.; Kapoor, S. Reduction and aggregation of silver ions in aqueous citrate solutions. Mater Sci Eng C Mater Biol Appl2014, 38, 192-196. (16) Siekkinen, A. R.; McLellan, J. M.; Chen, J.; Xia, Y. Rapid synthesis of small silver nanocubes by mediating polyol reduction with a trace amount of sodium sulfide or sodium hydrosulfide. Chem. Phys. Lett.2006, 432, 491-496. (17) Liang, H.; Wang, W.; Huang, Y.; Zhang, S.; Wei, H.; Xu, H. Controlled Synthesis of Uniform Silver Nanospheres†. J. Phys. Chem. C2010, 114, 7427-7431. (18) Tang, B.; Xu, S.; An, J.; Zhao, B.; Xu, W. Photoinduced Shape Conversion and Reconstruction of Silver Nanoprisms. J. Phys. Chem. C2009, 113, 7025-7030. (19) Dong, X.; Ji, X.; Jing, J.; Li, M.; Li, J.; Yang, W. Synthesis of Triangular Silver Nanoprisms by Stepwise Reduction of Sodium Borohydride and Trisodium Citrate. J. Phys. Chem. C2010, 114, 2070-2074. (20) Sun, Y. Direct Growth of Dense, Pristine Metal Nanoplates with Well-Controlled Dimensions on Semiconductor Substrates. Chem. Mater.2007, 19, 5845-5847. (21) Liu, G.; Cai, W.; Liang, C. Trapeziform Ag Nanosheet Arrays Induced by Electrochemical Deposition on Au-Coated Substrate. Cryst. Growth Des.2008, 8, 2748-2752.

39



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

Page 40 of 45

(22) Guo, S.; Dong, S.; Wang, E. Rectangular Silver Nanorods: Controlled Preparation, Liquid−Liquid Interface Assembly, and Application in Surface-Enhanced Raman Scattering. Cryst. Growth Des.2008, 9, 372-377. (23) Wiley, B.; Sun, Y.; Xia, Y. Polyol Synthesis of Silver Nanostructures: Control of Product Morphology with Fe(II) or Fe(III) Species. Langmuir2005, 21, 8077-8080. (24) Wiley, B. J.; Im, S. H.; Li, Z. Y.; McLellan, J.; Siekkinen, A.; Xia, Y. Maneuvering the surface plasmon resonance of silver nanostructures through shape-controlled synthesis. J. Phys. Chem. B2006, 110, 15666-15675. (25) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Photoinduced conversion of silver nanospheres to nanoprisms. Science2001, 294, 1901-1903. (26) Tsuji, M.; Maeda, Y.; Hikino, S.; Kumagae, H.; Matsunaga, M.; Tang, X.-L.; Matsuo, R.; Ogino, M.; Jiang, P. Shape Evolution of Octahedral and Triangular Platelike Silver Nanocrystals from Cubic and Right Bipyramidal Seeds in DMF. Cryst. Growth Des.2009, 9, 4700-4705. (27) Wiley, B.; Sun, Y.; Mayers, B.; Xia, Y. Shape-controlled synthesis of metal nanostructures: the case of silver. Chemistry2005, 11, 454-463. (28) Sun, Y.; Xia, Y. Shape-controlled synthesis of gold and silver nanoparticles. Science2002, 298, 2176-2179.

40


Page 41 of 45

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


(29) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics? Angew Chem Int Ed Engl2009, 48, 60-103. (30) Du, J.; Han, B.; Liu, Z.; Liu, Y.; Kang, D. J. Control Synthesis of Silver Nanosheets, Chainlike Sheets, and Microwires via a Simple Solvent−Thermal Method. Cryst. Growth Des.2007, 7, 900-904. (31) Wang, J.; Diao, P.; Zhang, Q. Dual detection strategy for electrochemical analysis of glucose and nitrite using a partitionally modified electrode. Analyst2012, 137, 145-152. (32) Roucoux, A.; Schulz, J.; Patin, H. Reduced Transition Metal Colloids: A Novel Family of Reusable Catalysts? Chem. Rev.2002, 102, 3757-3778. (33) Yuan, J. H.; Wang, K.; Xia, X. H. Highly Ordered Platinum-Nanotubule Arrays for Amperometric Glucose Sensing. Adv. Funct. Mater.2005, 15, 803-809. (34) Chen, A.; Holt-Hindle, P. Platinum-Based Nanostructured Materials: Synthesis, Properties, and Applications. Chem. Rev.2010, 110, 3767-3804. (35) Zhang, D.; Diao, P.; Zhang, Q. Potential-Induced Shape Evolution of Gold Nanoparticles Prepared on ITO Substrate. J. Phys. Chem. C2009, 113, 15796-15800. (36) Xu, D.; Yan, X.; Diao, P.; Yin, P. Electrodeposition of Vertically Aligned Palladium Nanoneedles and Their Application as Active Substrates for Surface-Enhanced Raman Scattering. J. Phys. Chem. C2014, 118, 9758-9768. 41



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(37) Sun, Y. Synthesis of Ag Nanoplates on GaAs Wafers: Evidence for Growth Mechanism. J. Phys. Chem. C2010, 114, 857-863. (38) Bian, J.; Li, Z.; Chen, Z.; Zhang, X.; Li, Q.; Jiang, S.; He, J.; Han, G. Double-potentiostatic electrodeposition of Ag nanoflowers on ITO glass for reproducible surface-enhanced (resonance) Raman scattering application. Electrochim. Acta2012, 67, 12-17. (39) Bian, J.-C.; Li, Z.; Chen, Z.-D.; He, H.-Y.; Zhang, X.-W.; Li, X.; Han, G.-R. Electrodeposition of silver nanoparticle arrays on ITO coated glass and their application as reproducible surface-enhanced Raman scattering substrate. Appl. Surf. Sci.2011, 258, 1831-1835. (40) Sandmann, G.; Dietz, H.; Plieth, W. Preparation of silver nanoparticles on ITO surfaces by a double-pulse method. J. Electroanal. Chem.2000, 491, 78-86. (41) Yang, J.; Liu, X.; Du, X. Electrodeposition of Silver Nanoparticles on ITO Films with Different Thickness and Application as LSPR Sensor. ECS Electrochemistry Letters2014, 3, B30-B32. (42) Yi, Z.; Ye, J.; Kikugawa, N.; Kako, T.; Ouyang, S.; Stuart-Williams, H.; Yang, H.; Cao, J.; Luo, W.; Li, Z., et al. An orthophosphate semiconductor with photooxidation properties under visible-light irradiation. Nat. Mater.2010, 9, 559-564. (43) Bi, Y.; Hu, H.; Ouyang, S.; Lu, G.; Cao, J.; Ye, J. Photocatalytic and photoelectric properties of cubic Ag3PO4 sub-microcrystals with sharp corners and edges. Chem Commun (Camb)2012, 48, 3748-3750. 42


Page 43 of 45

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


(44) Hou, Y.; Zuo, F.; Ma, Q.; Wang, C.; Bartels, L.; Feng, P. Ag3PO4 Oxygen Evolution Photocatalyst Employing Synergistic Action of Ag/AgBr Nanoparticles and Graphene Sheets. J. Phys. Chem. C2012, 116, 20132-20139. (45) Zeng, J.; Tao, J.; Li, W.; Grant, J.; Wang, P.; Zhu, Y.; Xia, Y. A mechanistic study on the formation of silver nanoplates in the presence of silver seeds and citric acid or citrate ions. Chem Asian J2011, 6, 376-379. (46) Wulandari, P.; Nagahiro, T.; Fukada, N.; Kimura, Y.; Niwano, M.; Tamada, K. Characterization of citrates on gold and silver nanoparticles. J. Colloid Interface Sci.2015, 438, 244-248. (47) Gu, C.; Zhang, T. Y. Electrochemical synthesis of silver polyhedrons and dendritic films with superhydrophobic surfaces. Langmuir2008, 24, 12010-12016. (48) Kochkar, H.; Aouine, M.; Ghorbel, A.; Berhault, G. Shape-Controlled Synthesis of Silver and Palladium Nanoparticles Using β-Cyclodextrin. J. Phys. Chem. C2011, 115, 11364-11373. (49) Berhault, G.; Bausach, M.; Bisson, L.; Becerra, L.; Thomazeau, C.; Uzio, D. Seed-Mediated

Synthesis

of

Pd

Nanocrystals:

Factors

Influencing

a

Kinetic-

or

Thermodynamic-Controlled Growth Regime. J. Phys. Chem. C2007, 111, 5915-5925. (50) Xia, Y.; Xia, X.; Wang, Y.; Xie, S. Shape-controlled synthesis of metal nanocrystals. MRS Bulletin2013, 38, 335-344. 43



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(51) Sau, T. K.; Murphy, C. J. Room temperature, high-yield synthesis of multiple shapes of gold nanoparticles in aqueous solution. J. Am. Chem. Soc.2004, 126, 8648-8649. (52) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. Chem. Mater.2003, 15, 1957-1962.

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For Table of Contents Use Only.

SYNOPSIS With the sodium citrate as the capping agent, the deposited Ag is plate-like. Due to the presence of a concentration gradient layer, only the standing Ag nanoplates could grow larger. These two factors lead to the formation of vertically aligned Ag nanoplates arrays.

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Electrodeposition of Vertically Aligned Silver Nanoplate Arrays on

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