Electroless Deposition of Silver Nanostructures by Redox Reaction of

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Electroless Deposition of Silver Nanostructures by Redox Reaction of Copper Oxide Sanjun Yang, Qiming Liu, and Min Tan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12179 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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The Journal of Physical Chemistry

Electroless Deposition of Silver Nanostructures by Redox Reaction of Copper Oxide Sanjun Yang, Qiming Liu*, Min Tan Key Laboratory of Ariticial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, China

Abstract: We synthesized Cu2O microcrystal by solution phase reduction and drop-casted it on various substrates as reductant. Different structures of Ag，including nanowires, fold lines and dendrites so on, were obtained by immersing the Cu2O substrates into different concentrations of AgNO3. The products were characterized by XRD, SEM, TEM, etc. and their morphology is proposed to be determined by the interplay between tiny Cu2O nanoparticles and the distribution of Ag+ concentration affected by Mullins-Sekerka instabilities. Using optical microscope, we directly observed the real-time growth process of Ag nanowire and found its growth speed to be fluctuating around 5µm/min. We also investigated the effects exerted on the formation of Ag nanostructures by other parameters, including crystal morphology of Cu2O, heating, light irradiation and stirring the precursor. From the experiment results, we deduced that the {111} facet of Cu2O played a larger part than {100} facet in promoting the growth of Ag nanostructures and increase in the reaction or diffusion rate can enhance the Ag nanostructure growth.

Introduction Ag is one of the most profoundly researched metal due to its non-toxic, abundant nature and, most importantly, excellent physical and chemical properties. Consequently, a great many applications have been developed based on various Ag nanostructures. For example, due to its highest conductivity and also better oxidation resistance compared with Cu, Ag is leading in the progress of novel transparent conductive membrane constructed by nanowires, the products of which are already used in commerce1-3. Moreover, Ag is more sensitive to Localized Surface Plasmon Resonance (LSPR)4, enabling it to be one of the most popular candidates in the corresponding applications, such as Surface-Enhanced Raman Scattering(SERS)5,6, chemical7 and biological sensing8,9. In the pursuit of the extensive applications of Ag, various synthesis methods have been developed, including polyol synthesis3,10,11, template method12, electrochemical deposition13-15, galvanic displacement16-19, and so on. When utilizing Ag nanostructures in the diverse applications, the ability to control their morphology is pivotal, as the properties of Ag structures vary according to their morphology. For instance, the Ag nanowire transparent conductive membrane prefers high aspect ratio nanowire20, which exhibits better electro-optic property than that of the lower one. On the other hand, the effect of SERS will be intensified if the obtained products of Ag possess rugged structures that can act as “hot spots” to stimulate the LSPR19,21. Naturally, to fully control the morphology of Ag products, a clear appreciation of their growth processes and behind mechanisms is essential. Various mechanisms have been proposed to explain the growth of different Ag nanostructures. Recently, when using electrochemical method to prepare Ag nanostructures, Yang et al.15 found that the interplay between diffusion efficiency and reaction rate determined the final morphology of Ag, ranging from particles, nanorods to dendrites. Electroless deposition, which usually involves reactions occurring at a specific interface, can combine surface patterning, solution-phase flexibility, and controllability of top-down design22, and thus is extensively used in the synthesis of Ag nanostructures. S1

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Wen et al.16 used Zn microparticles to reduce Ag+ into Ag dendrites and proposed that Ag nanostructures grew under the control of diffusion and oriented attachment of Ag nanocrystal precursors. Cu has also been used as reductant. Liu et al.18 proposed that Cu and protruding Ag seeds, serving as anode and cathode respectively, formed short-circuited nanobattery, from which various Ag nanostructures evolved with their morphology determined by Nernst equation. In contrast, Avizienis et al. used Mullins-Sekerka (MS) instabilities and argued that the Nernst equation is only applicable to equilibrium reaction, which was not reached under the circumstance of ELD. We have synthesized Ag nanostructures by galvanic displacement reaction using electrochemical deposited Cu microcubes on ITO as reductant21. Cu2O was found to take part in this reaction. Although Cu2O-Ag composite has been widely reported to exhibit excellent performance in SERS and photocatalysis23,24, synthesis of Ag nanostructures by Cu2O is rare19 compared with metal. In view of this, we conducted the experiments of electroless deposition of Ag by Cu2O to further reveal the growth mechanism. First, we synthesized Cu2O microcrystals with various morphology including octahedral, cube-octahedral, and cube by solution-phase reduction. The Cu2O microcrystals were drop-casted on glass to create reductive substrates, which were immersed into different concentrations of AgNO3 to initiate the ELD. With the increase of concentration, the synthesized Ag morphology transformed from nanowire to fold line and dendrite. We believe the transformation arouse from the interplay between tiny Cu2O nanoparticles and the distribution of Ag+ concentration affected by Mullins-Sekerka instabilities. From the observation of the different Ag products obtained by changing Cu2O crystal habit, we found the {111} and {100} facets of Cu2O behaved differently in ELD. The influence on the Ag morphology by light irradiation, heating and stirring the AgNO3 solution were also investigated respectively and corresponding explanations were proposed. With the help of optical microscope, real-time growth of Ag nanowires was observed and the growth speed was calculated based on the observation..

Experimental Materials and preparation All chemical reagents were obtained from the Sinopharm Chemical Reagent Co., Ltd. and used without further purification. Ultrapure water was used in experiments as solvent. Cube and octahedral Cu2O microcrystals were prepared by a modified method reported by Wang 25

et al. Cube-octahedral Cu2O was prepared following our previous work26. Firstly 25 ml CuSO4(0.49 g) solution and 25ml NaOH(0.22 g) solution was mixed to form Cu(OH)2 gel, followed by the addition of 1g PVP under vigorous stirring. After that, in 15 min, 25 ml glucose(0.45 g) solution was evenly added into the gel in a 700C waterbath under stirring. The obtained products were centrifuged and washed with water and ethanol for three times. Then the clean products were dried in a vacuum oven and dissolved in ethanol with a density of 0.5 mg/ml. To synthesize Ag nanostructures, Cu2O suspension (20 µL) with specific morphology was drop-casted on a microscope slide and dried under ambient condition. Then the slide was put in a 50 ml centrifuge tube, which was filled with 20 ml AgNO3 (0.1 mM, 1 mM, 10 mM, 50 mM) and then kept in dark for 30min. To examine the effect of heating, 20 ml AgNO3 in a 50 ml beaker was put in a watebath at 700C for 15 min. Then the Cu2O slide was put in the beaker for 30 min. The influence of light irradiation was explored by putting the Cu2O slide in a culture dish, which was filled with 20 ml AgNO3 solution, and then illuminating the culture dish with a 30 W white led light for 30 min. We investigated the effects of stirring the precursor by putting a magnetic stirring bar in the bottom of the S2


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centrifuge tube and the rotating rate was set to be 100 rpm. All the above microscope slides, after reaction, were washed with water and ethanol before characterization. The observation of real-time growth of nanowires was conducted by putting a Cu2O slide in a smaller culture dish, which was set at the microscope stage. The amount of AgNO3 solution in the culture dish was reduced to 10 ml due to the limited space between objective lens and microscope stage.

Characterization Bruker D8 advance X-ray diffractometer by Cu Kα radiation (λ = 1.5406 A) was used to obtain XRD results. SEM images were obtained using a FEI Sirion FEG operating at 20 kV and FE-SEM images was acquired from ZEISS-∑IGMA working at 10 kV. The samples were sputted with gold for 60 sec before observation. JEM-2010FEF(UHR) was utilized to obtain TEM and HRTEM images. Optical video and images were recorded using Olympus BX51 microscope.

Results and discussion The effect of the AgNO3concentration Optical microscope proves powerful in gaining insight into the growth of nanowires18,27, and therefore, we utilized it widely in this work. We used cube-octahedral Cu2O microcrystal in the following experiments unless otherwise noted. The characterizations of Cu2O cube, cube-octahedral and octahedral were shown in Fig. S1. Fig. 1a-c shows the SEM images of Ag nanostructures obtained when the concentrations of AgNO3 are 1 mM, 10 mM, and 50 mM. It is clearly that the morphology of Ag transformed from nanowires（1 mM）to fold lines (10 mM) and dendrites (50 mM). We also show the corresponding optical images in Fig. S2 a-c and these images, in bright field mode, generally exhibit bright white color, as have been reported28. Fig 1d shows the XRD of nanowires and dendrites, marked as A and B, respectively. All the peaks can be well indexed to pure Cu2O (JCPDS no. 05-0667) and Ag (JCPDS no. 04-0783).From the XRD pattern, it can be identified that, with the morphology of Ag transforming from nanowire to dendrite, the peaks of Ag mount at the expense of Cu2O peak, implicating that the Cu2O is consumed in higher degree by ELD, as can be seen in Fig. 1c. One interesting characteristic of the XRD pattern is that, except the (111) and (222) peaks of Ag, no other characteristic peaks, such as (200), can be identified. This is in great contrast with the case of ELD of Ag in solution19,22. Avizienis et al.22 discovered that restricted volume tended to increase the (111):(200) ratio. Hence, we believe that the reaction occurring near the surface tends to deposit products on the surface, which is more thermodynamically favored by reducing the surface energy. In this experiment, the Ag monomers stacked up and their (111) planes were parallel to the substrate, thereby leading to a dominant (111) ratio in the XRD pattern.

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Fig. 1. SEM images of Ag (a) nanowire, (b) fold line, (c) dendrite. (d) XRD spectra of A nanowire, B dendrite. The TEM images of the Ag nanowires and fold lines are shown in Fig. 2a and Fig. 2b respectively. The morphology of nanowire and fold lines is not completely mutually exclusive, as can be observed from Fig. S2a and Fig. S2b. Therefore we believe they may share an analogous formation mechanism. Chen et al.

19

reported that higher concentration of Ag+ resulted in an increase in the width of the

nanowires when synthesize Ag nanostructures by adding Cu2O in AgNO3 solution. They found that small particles attached on the nanowire in a time scale study, indicating the occurrence of an Ostwald ripening (OR) process. Similarly, in our experiment, we also discovered the evidence of OR and demonstrated it in Fig. S3a. Moreover, in Fig. 2b, the fold line is composed of two parts: the right part is wider with straight edge, the left part thinner with rugged edge. The difference in their width and morphology implies that they may form at different moment or the right part consumed the majority of the precursor. In any case, a process of growth from thinner wires to fold lines occurs. Besides, it has been reported that, slowing down the reaction rate favors the kinetic growth and results in the formation of triangular plates29. Interestingly, in Fig. 2b, a triangular structure protrudes from the fold lines（red circle）, indicating that a slower, kinetic controlled growth occurred after the formation of rugged fold lines. Example of other fold lines containing similar structures is shown in Fig. S3b. From the above discussion, we can conclude that the fold lines are evolved from thinner wires, which explains the co-existence of them on some occasions. In contrast, these two structures are essentially different with the dendrite nanostructure both in morphology and in behind formation mechanism, analysis of which will be carried on subsequently.

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Fig. 2. (a) TEM image of Ag nanowire. (b) TEM image of Ag fold line.

The formation mechanism of Ag nanostructures Utilizing optical microscope, we have directly observed the growth of nanowires (see the video in the Supporting Information; the video is accelerated by 5 times). In a previous work reported by Chen et al. 19, Cu2O was decomposed in AgNO3 solution into small particles, which, acting as both reducing agent and growth substrate, promoted the growth of Ag nanowires. However, in this experiment, it can be seen from the video that nanowires sprout from the Cu2O microcrystals and extend to dozens of micrometers. This growth mode appears to be akin to the above “short-circuited nanobattery”18 growth mode, which proposed that, when using Cu to displace Ag, the potential was transferred from Cu to the growth front of Ag through the already formed Ag nanostructures and the front can consequently kept extending18,22. Nevertheless, a closer examination reveals that a vast difference exists between these two growth modes. Fig. 3a shows the section of the nanowires approaching the Cu2O microcrystal. Interestingly, although the nanowires maintain their profiles, they become segmented. Also, in Fig. 3b, we magnified a part of the dendrite in Fig. 1c (marked in red rectangle) and found it discontinuous. This feature renders the growth of Ag nanostructures in our experiment impossible to be consistent with the “short-circuited nanobattery” theory, as potential cannot be transferred to the growth front through the discontinuous nanostructure. Since the reaction solution contains only Cu2O and Ag+, it is obvious that small Cu2O nanoparticles were decomposed from the Cu2O microcrystals and, serving as both reducing agent and growth substrate, reduce Ag+ into various Ag nanostructures. This reduction process is of localized nature. S5



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Fig. 3. (a) The nanowires near the Cu2O crystal. (b) A magnified image of the dendrite. (c) Partly segmented nanowire. (d) A magnified section of the segmented nanowire. Diffusion-limited aggregation(DLA) model proposed by Witten and Sander22,30 is inappropriate to be applied in solution containing aggregates as it ideally approximates the solution approaching zero density31. On the contrary, by hypothesizing the existence of an appropriate concentration gradient, Mullins -Sekerka(MS) instability predicted the formation of dendrite32 and also the transition of morphology from nanowire to dendrite15,22. This theory proposed that the growth front of nanostructure, by protruding further into the solution, receives more reactants and thus keeps growing. The essence of this theory is that, when the depletion rate of the reactants outpaces the restoring forces in the vicinity of the growth front, perturbation of precursor concentration (Ag+ in this case) occur, which will arouse the instability of the precursor diffusion. The disturbed diffusion process makes the Ag+ transferred not only to the growth front, but also to the sides of the nanostructure, thereby leading to branch growth. Through self-propagating of this growth mode, the dendrite forms. In our experiment, the interaction between growth front, Ag+, and small Cu2O nanoparticles determined the final morphology. As shown in Fig. 4Aa, when the Ag+ concentration is low, perturbations aroused by the consumption of Ag+ near the growth front are diminished by local restoring forces. Therefore, the Ag+ steadily transfer to the growth front, and, along with Cu2O nanoparticles, deposit on the growth front. We believe the depositions are mixtures of Ag and Cu2O nanoparticles surrounded by Ag+. This is evidenced in Fig. S4, in which we present a growing nanowire adsorbed by nanoparticles. A careful examination shows that, while many nanoparticles seem to be amorphous, some, especially the ones with black dots, are crystallized with lattice connected with the nanowire. This indicates the occurrence of epitaxial growth or OA. EDS of the nanoparticle (Fig. S4c) illustrates that it incorporates Cu element with atomic ratio to Ag approximately one to three. Intuitively, more Cu2O nanoparticles are available near the Cu2O microcrystal, which will facilitates S6


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the nucleation to an extent that the compounds of Ag and Cu2O self-nucleate and form new growth front. This new growth front, if sufficiently large, can block the diffusion of Ag+ to the old growth front and thereby forming segmented nanowires, as shown in Fig. 3a. Interestingly, if the self-deposition is not large enough, the Ag+ partly bypass it and reach the old growth front, which can keep extending, as shown in Fig. 3a(marked with green rectangle), Fig. 3c, and Fig. 3d (a magnified section of the Fig. 3c). Schematic illustrations of the segmented and partly segmented growth modes of nanowires are shown in Fig. 4Ac and Fig. 4Ab respectively. In contrast, a higher concentration of Ag+ accelerates the reduction and deposition rate to outpace the local restoring force, which will arouse M-S instability as discussed above. Consequently, branch growth begins, and, facilitated by the high Ag+ concentration, results in dendrite morphology. We show the schematic illustrations for the growth process of dendrite and fold line in Fig. 4C and Fig. 4B respectively. It is also noteworthy that, multiple morphologies may co-exist, due to the localized nature of the redox reaction. For example, we have observed the existence of nanodisks surrounding Cu2O microcrystals when synthesizing dendrites, such as shown in Fig. 1c. Since in the case of synthesizing

dendrites, the concentration of Ag+ was high. Also, the growth of Ag nanostructure near the microcrystal should last a longer time than the ones in the periphery. Plus, the continuous supplement of Cu2O nanoparticles in the vicinity of the Cu2O microcrystal provides enough growth substrates and reductant, enabling individual Cu2O nanoparticles carried on reducing Ag+ and finally formed the nanoplates in large scale probably through coalescence.

Fig. 4. (A) Schematic illustration of the growth mechanisms of (a) continuous nanowire, (b) partly segmented nanowire, (c) segmented nanowire. Schematic illustration of the growth processes of (B) fold line, (C) dendrite.

Real-time growth of Nanowires Using optical microscope, experiments were conducted to reveal the growth of nanowires in real S7



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time. The AgNO3 concentration was set to be 1mM. Fig. 5a-c show the growth of a nanowire at 0, 10, 20 min respectively and the starting point is marked in Fig. 5a (green arrow). Based on the observation, the growth rate of the nanowire can be calculated (note that we have recorded more snapshots in the growth process). Fig. 5d shows the length of the nanowire and the growth speed at one-minute intervals. The growth speed fluctuates between 2 and 6 µm/min. As the interactions between Cu2O nanoparticles, Ag+, and growth front determine the growth of nanowire, the fluctuation in speed may be aroused from this complicated situation. A linear fit implies that the average growth speed is 4.47 µm/min (or 74 nm/s), which is slower than the one of nanobelt synthesized by ELD when using Cu as reductant (143-345 nm/s)18. We attribute this decrease in speed to the weaker reductive power of Cu2O. The growth speed of nanobelt gradually decreased with the growth of it, which was ascribed to the depletion of Cu particle and larger resistance from Cu to the growth front caused by the extending nanobelt18. Interestingly, this is not the case in our experiment. In spite of the fluctuations, the growth speed basically maintains 5µm/min. Moreover, when a secondary growth of the nanowire occurs, the growth speed drops greatly (marked with blue circle in Fig. 5d; probably due to the depletion of precursor around the starting point), but raises in one minute to the average speed. All these observations indicate that the growth of nanowire is the result of interactions between Cu2O nanoparticles, Ag+, growth front and is, most importantly, of localized nature.

Fig. 5. Nanowire growth at (a) 0 min, (b) 10 min, (c) 20 min. (d) nanowire length and growth speed versus time. As mentioned above, we believe the initially formed nanowire is composed of Ag and Cu2O nanoparticles. When the growth of nanowires in Fig. 5 ended, we washed the glass slide with copious water and ethanol. The slide dried under ambient condition in approximate 30 min and then was observed by optical microscope. Fig. 6a and Fig. 6b show the transmission and bright field images of the nanowire in Fig. 5. Fig. 6c and Fig. 6d are the in-situ images obtained after 6 hours. Interestingly, the nanostructures changed significantly. In transmission field(Fig. 6a), the nanowire assumes a half-transparent dark green form that strongly resembles the small dispersive dots (probably formed by aggregation of nanoparticles shown in Fig. S4) around the nanowires (marked with red arrow in Fig. 6a). This morphology indicates that they are not completely crystallized, which is also evidenced by the various color reflected by the nanowire in the bright field (Fig. 6b). However, in Fig. 6c, the nanowire S8


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turns to be opaque, the dispersive dots disappeared, and the nanowire exhibits bright white color in the bright field(Fig. 6d), all indicating that a process of crystallization occur to fully transform the polycrystalline compounds into Ag. Moreover, in Fig. 6c, Ag nanostructures (marked with green arrow) evolved from places where polycrystalline dots previously aggregated. Since we have proved that, in our experiment, small Cu2O nanoparticles are indispensable to complete the reduction of Ag, we can safely conclude that the initiative nanowire in Fig. 6a-b is composed of Cu2O@Ag+ and Ag compounds and a slower post-crystallization occurs after the formation of the primary nanowire. This deduction can be further proved by the EDS results of the growing nanowire in Fig. S4d-e. They show that the nanowire has Cu atoms with a proportion of 1/4 of its total metal atoms. Using HRTEM, a careful survey of the nanowire indicated that it was poorly crystallized. So these Cu atoms, serving as reductant, triggered the following post-crystallization. The panoramic view of the nanowire and another example of the post-crystallization are shown in Fig. S5.

Fig. 6. (a) Transmission field and (b) bright field image of the Ag nanowire 30 min after reaction. In-situ (c) transmission field and (d) bright field image of the Ag nanowire 6 hours after reaction. The influence of other parameters Cube-octahedral Cu2O crystal is composed of eight {111} facets and six {100} facets. Since these facets have different atom arrangement and commonly perform differently in various applications33, we changed the Cu2O microcrystal from cube-octahedral to octahedral and cube, which are composed of eight {111} facets and six {100} facets respectively (see their characterization in Fig. S1). Fig. 7a shows that the Ag nanostructures grow from Cu2O cube are very limited in size when the AgNO3 concentration is 1mM. In fact, the corresponding optical image in the top-right corner shows that some cubes cannot generate any Ag nanostructures. Moreover, Ag dendrites cannot be prepared by raising S9



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the concentration to 50mM, as shown in Fig. 7b (insert in top-right corner shows the corresponding optical image). In contrast, synthesized with 1mM AgNO3, the Ag wires in Fig. 7c sprouting from the Cu2O octahedral are wider than those from cube-octahedral. Also, we obtain Ag dendrites with 50mM AgNO3, as shown in Fig. 7d, and a closer view of a part of the dendrite (marked with red rectangle in Fig. 7d) shows that it is also segmented, as shown in Fig. S6. Obviously, compared with the Ag nanostructures electrolessly deposited by Cu2O cube-octahedral, the changes in morphology of the nanostructures synthesized by cube are opposite to those synthesized by octahedral. This can be intuitively attributed to the different surface structures between {100} and {111} facets. Because it has been proved that Cu2O nanoparticles, released from the microcrystal, participate in the redox reaction of Ag as both reductant and growth substrate, naturally, from Fig. 7, we infer that the {111} facet facilitates the growth of Ag nanostructures by releasing more Cu2O nanoparticles. It is noteworthy that the size of octahedral Cu2O has a large distribution, but it, unlike the case of synthesizing Ag nanostructures by Cu18,22, plays a minor role in determining the morphology of Ag nanostructure. By centrifugation, smaller octahedral Cu2O (approximately 2 μm) was separated to synthesize Ag nanowire and dendrite, which, as shown in Fig. S7, exhibits no noticeable difference with the ones synthesized by large octahedral or cube-octahedral Cu2O microcrystals. The different effect of size between GR by metal and GR by metal oxide reflects the fundamental difference of sources of M-S instabilities. It also further confirms the difference in Ag nanostructures synthesized by cube and octahedral stems from the different atomic structures of {100} and {111} facets instead of different sizes. The {100} and {111} facets of Cu2O have been widely researched. While {100} facet is non-polar with Cu and O atoms stacked layer by layer, the {111} facet contains, in each two Cu atoms, one under-coordinated Cu atom, which possess a dangling bond33-35. The higher surface energy of {111} facet makes it on one hand preferred to be deposited by metal cations36, and on the other hand incurs more etch in alkaline solution37,38. Therefore, we believe the small Cu2O nanoparticles are much more easily to be broken off from the {111} facets of Cu2O microcrystal and then diffuse outward. In contrast, the redox reaction in the case of cube occurs mainly in the vicinity of {100} facets. A careful examination shows that, in 1mM AgNO3 solution, while the cube-octahedral and octahedral Cu2O in Fig. 3a and Fig. 7c are apparently solid, the Cu2O cube in Fig. 7a is hollow (red arrow) and covered by small particles with size approximate 30 nm. These observations are well consistent with “Kirkendall Effect”39,40. This theory predicts that Ag+ absorbing on the surface of the cube tend to diffuse inward and the internal

Cu+ diffuse outward due to their different gradient distribution. The higher diffusion

speed of Cu+ leads to the hollow structure.

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Fig. 7. Ag nanostructures synthesized by Cu2O cube with (a) 1 mM AgNO3 (top-right insert shows the corresponding bright field image), (b) 50 mM AgNO3 (top-right insert shows the corresponding bright field image). Ag nanostructures synthesized by Cu2O octahedral with (c) 1 mM AgNO3, (d) 50 mM AgNO3. Although the surface of the Cu2O microcrystals became rugged in the synthesis process, the facet-selectivity retained in this experiment timescale. Similar phenomenon has been reported before. For example, in photocatalysis, though the facet transformation occurred in prolonged light irradiation41, initially the {111} and {110} facet showed better photo-catalytic performance than {100} facet33,34. Also, the {100} facet was more stable under the etch of alkaline solution37,38. In this study, we illustrated the facet-selectivity of Cu2O microcrystal leaded the reaction on {111} and {100} facet to occur following different path. A prolonged reaction may transform the cube into hollow structure40, but a systematic study is outside the scope of this paper. We also explored the influence of light irradiation, heating and stirring the AgNO3 solution on the morphology of Ag nanostructures. They were anticipated to promote the growth of Ag nanostructures by affecting the reaction rate or diffusion rate. Light have been used to promote the growth of metal nanostructures42-44 and the proposed mechanism is that light can photo-generate holes and electrons, which will accelerate the redox reaction. The reactions have been proposed as follows44. Cu2O+hv→Cu2O(e-+h+) e-+Ag+→Ag Although detailed reaction process, in our opinion, is much more complicated, especially when taking the different facet properties and crystal structure of Cu2O into consideration, it can be deduced that light irradiation can affect the Ag morphology by promoting the reaction rate. Heating can increase the reaction rate and diffusion rate of precursor45, as can be determined from Butler-Volmer formulation46 and Stokes–Einstein equation. S11



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where kS is the standard rate constant of the redox couple, and α is the transfer coefficient. F represents the Faraday constant and n is the stoichiometric number of electrons. kB, T, r, and µ are Boltzmann constant, absolute temperature, the radius of diffusive materials, and the dynamic viscosity of the solvent respectively. It can be easily deduced that both kf (reaction rate constant) and D (diffusivity coefficient) increase when T mounts, indicating that heating promotes both the reaction rate and diffusion rate. Intuitively, stirring the precursor will facilitate the diffusion of Ag+, thereby increasing the diffusion rate. It is noteworthy that the higher concentration of Ag+ can also promote the reaction rate (Nernst equation)18 and diffusion rate by increasing the concentration gradient (Fick' s first law). Consequently the influence of high concentration of Ag+ on the Ag morphology is mingled with the influences of the three interferences (light irradiation, heating, stirring). Therefore, to highlight the effects of the three interferences, we went a step further to reduce the influence of high concentration of Ag+ by reducing the AgNO3 concentration to 0.1mM using Cu2O octahedral as reductant. Firstly, we characterized the obtained Ag nanostructures without the three interferences. Fig. 8 shows the products obtained after 30 min of reaction. Fig. 8a is a bright field image and Fig. 8b is the corresponding transmission field image. They both clearly exhibit randomly deposited aggregations forming a circle with the Cu2O microcrystal in its center. The shallow red color in bright field image (Fig. 8a) and the blurred green in transmission field image (Fig. 8b) of the aggregations indicate that they incorporate polycrystalline Cu2O. Fig. 8c is the SEM image of the product and Fig. 8d is a closer view of a part of the product (red circle). In Fig. 8d, the aggregation assumes no noticeable crystal morphology (the approximately 10 nm nanoparticles are probably granular sputtered gold films). We infer that small Cu2O nanoparticles, due to the limited Ag+, were only partly oxidized, and along with the deposited Ag on their surface, they aggregated probably following OA mechanism to reduce their surface energy. These observations are well consistent with Fig. S4 and our growth mechanism discussed above.

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Fig. 8. (a) Bright field and (b) transmission field image of Ag nanostructures synthesized by octahedral with 0.1 mM AgNO3. (c) SEM image of Ag nanostructures synthesized with 0.1 mM. (d) A magnified part of the Ag nanostructures in (c). Then, with 0.1 mM AgNO3 the influences of light irradiation, heating and stirring on the formation of Ag were investigated and the results were shown in Fig. 9a1–a3, respectively. Fig. 9a1 shows that increase in the reaction promote the anisotropy growth. While the Ag nanowires near the microcrystal was thin due to the low concentration of Ag+ there, Ag nanoplates and fold lines formed in the periphery where more Ag+ were available. This observation confirms our deduction that light irradiation mainly promotes the reaction rate. Fig. 9a2 exhibits that by heating, fold lines can be synthesized. As we have illustrated above, formation of fold lines involves a process of lateral growth, which requires an acceleration of diffusion of Ag+ in 0.1 mM AgNO3. So clearly heating increases both reaction rate and diffusion rate. Fig. 9a3 shows that large pieces of Ag disks deposit around the Cu2O by stirring and the top-right insert is the corresponding bright field image, which clearly exhibits that the Ag disks surround the Cu2O, forming a circle that almost duplicate the shape and dimension of the aggregations shown in Fig. 8c. This implies that Ag+ diffused at a much faster speed to the aggregations of partly oxidized Cu2O nanoparticles under stirring. However, low intensity stirring exerts little influence on the Cu2O aggregations determined from the similar dimensions between Ag disks and aggregations. In fact, the stirring rate is pivotal as we have conducted experiments with high stirring rate and found no Ag disks were synthesized, the cause of which we infer to be dissipation of Cu2O aggregations under high speed stirring. In conclusion, the stirring mainly promoted the diffusion rate and therefore Ag disks nearly isotropically deposited surrounding the Cu2O microcrystal. Next, we explored the influences of light irradiation, heating and stirring on the morphology of Ag Nanostructures synthesized with 1 mM and 50 mM AgNO3. Fig. 9b1-b3 show the products obtained S13



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with 1 mM AgNO3. Fig. 9b1 exhibits that the nanoplates in the periphery of the Cu2O microcrystal become larger than the ones in Fig. a1 when more Ag+ ions are available. Fig. 9b2 shows that wide nanobelts are formed along with thin nanowires, which may form at a later stage. There are nanoparticles absorbing on the edge of the nanobelts, which, similar to the case of fold lines as shown in Fig. s3, illustrates an OR growth in the lateral direction. Fig. 9b2 illustrates that by heating, the synthesized Ag nanostructure with 1 mM AgNO3 maintain its anisotropy nature, namely, one dimensional nanostructure, but the lateral growth is enhanced due to the increase in diffusion rate. The effects of stirring is shown in Fig. 9b3 and unlike nanobelts in Fig. 9b2, the nanowires grow wider with sawtooth on both edges. These triangular protuberances indicate a kinetic control growth as we have stated above29, which can be well explained since stirring almost exclusively increase the diffusion rate. Fig. 9c1-c3 show the Ag nanostructures synthesized with 50 mM AgNO3 by light irradiation, heating, and stirring respectively. Although the Ag nanostructures all maintain dendrite appearance, the components in them are very different: light irradiation (Fig. 9c1) – large nanoparticles, heating (Fig. 9c2) – willow leaf like nanobelts, stirring (Fig. 9c3) – nanoplates. So, compared with the dendrites formed without the three interferences as shown in Fig. 7d, the nanoparticles that composed the dendrites in Fig. 7d became larger by reducing limited amount of Ag+ surrounding the nanoparticles, since light irradiation mainly promote reaction rate. By heating, they formed one dimensional willow leaf like nanobelts due to the increase of diffusion rate and reaction rate. As stirring mainly promote the diffusion rate, the nanoparticles grew isotropically and nanopaltes appeared as shown in Fig. 9c3. To summarize, under the interferences of light irradiation, heating and stirring, the Ag nanostructures, though subjected to modifications, still retained their basic features as nanowires and dendrites respectively with 1 mM and 50 mM AgNO3, which were dominated by M-S instabilities as shown in Fig. 4. The differences in changes of morphology clearly are related to the different working mechanism of the three interferences, namely, light irradiation increasing reaction rate, stirring promoting diffusion rate, and heating enhancing both of them.

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Fig. 9. Ag nanostructures synthesized with 0.1 mM AgNO3 by (a1) light irradiation, (a2) heating, (a3) stirring the precursor solution. Ag nanostructures synthesized with 1 mM AgNO3 by (b1) light irradiation, (b2) heating, (b3) stirring the precursor solution. Ag nanostructures synthesized with 50 mM AgNO3 by (c1) light irradiation, (c2) heating, (c3) stirring the precursor solution. All the top-right inserts are the corresponding bright field optical microscope images and the scale bars in them are 10 µm. The bottom-right insert shows a magnified image of a part of c1 (red square) and the scale bar in it is 1 µm.

Conclusion Using Cu2O microcrystals as reductants, we have synthesized various Ag nanostructures, including nanowire, fold line, dendrite, and disk. Small nanoparticles released from the Cu2O microcrystal are revealed to play a large part in the formation of the above nanostructures. The interaction between Ag+, small Cu2O nanoparticles and growth front determines the morphology of the Ag nanostructure. Their growth process and transformation between different morphology can be well predicted by M-S instability theory. Real-time observation of the nanowire growth shows that its growth speed fluctuates around 5 µm/min. {111} facet plays a critical role in the formation of Ag nanostructures as it releases more Cu2O nanoparticles than {100} facet. Light irradiation, heating and stirring the precursor solution can promote the growth of Ag nanostructures and we attribute their origins to be the increase of reaction rate, the increase of diffusion rate and the increase of both, respectively. Diverse Ag nanostructures and Ag@Cu2O composites have already been widely used in various fields such as transparent conductive electrode, sensing, SERS, and photocatalysis. We believe our research will further extend their applications. S15



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Supporting Information Real-time growth video of nanowires; Characterization of the Cu2O microcrystals (Figure S1); Optical microscope images of Ag nanostructures (Figure S2); Additional TEM images of the fold lines (Figure S3); HRTEM and EDS data of a growing nanowire (Figure S4); Other examples of nanowire recrystallization (Figure S5); Magnified images of dendrites synthesized using octahedral Cu2O (Figure S6).

Acknowledgements This research work was financially supported by the National Natural Science Foundation of China (51572202 and 51272183) and the Nanotechnology Program of Suzhou (ZXG201438) and Shenzhen basic research (JCYJ20150417142356651).

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TOC Graphic

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Fig. 1. SEM images of Ag (a) nanowire, (b) fold line, (c) dendrite. (d) XRD spectra of A nanowire, B dendrite. Fig. 1, Fig. 1a, Fig. 1b, Fig. 54x37mm (300 x 300 DPI)



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Fig. 2. (a) TEM image of Ag nanowire. (b) TEM image of Ag fold line. Fig. 2, Fig. 2a, Fig. 2b 119x201mm (300 x 300 DPI)


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Fig. 3. (a) The nanowires near the Cu2O crystal. (b) A magnified image of the dendrite. (c) Partly segmented nanowire. (d) A magnified section of the segmented nanowire. Fig. 3, Fig. 3a, Fig. 3b, Fig. 54x36mm (300 x 300 DPI)



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Fig. 4. (A) Schematic illustration of the growth mechanisms of (a) continuous nanowire, (b) partly segmented nanowire, (c) segmented nanowire. Schematic illustration of the growth processes of (B) fold line, (C) dendrite. Fig. 4, Fig. 4A, Fig. 4Aa, Fig 113x80mm (300 x 300 DPI)


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Fig. 5. Nanowire growth at (a) 0 min, (b) 10 min, (c) 20 min. (d) nanowire length and growth speed versus time. Fig. 5, Fig. 5a, Fig. 5b, Fig. 80x80mm (300 x 300 DPI)



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Fig. 6. (a) Transmission field and (b) bright field image of the Ag nanowire 30 min after reaction. In-situ (c) transmission field and (d) bright field image of the Ag nanowire 6 hours after reaction. Fig. 6, Fig. 6a, Fig. 6b, Fig. 62x48mm (300 x 300 DPI)


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Fig. 7. Ag nanostructures synthesized by Cu2O cube with (a) 1 mM AgNO3 (top-right insert shows the corresponding bright field image), (b) 50 mM AgNO3 (top-right insert shows the corresponding bright field image). Ag nanostructures synthesized by Cu2O ¬octahedral with (c) 1 mM AgNO3, (d) 50 mM AgNO3. Fig. 7, Fig. 7a, Fig. 7b, Fig. 54x36mm (300 x 300 DPI)



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Fig. 8. (a) Bright field and (b) transmission field image of Ag nanostructures synthesized by octahedral with 0.1 mM AgNO3. (c) SEM image of Ag nanostructures synthesized with 0.1 mM. (d) A magnified part of the Ag nanostructures in (c). Fig. 8, Fig. 8a, Fig. 8b, Fig. 58x42mm (300 x 300 DPI)


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Fig. 9. Ag nanostructures synthesized with 0.1 mM AgNO3 by (a1) illumination, (a2) heating, (a3) stirring the precursor solution. Ag nanostructures synthesized with 1 mM AgNO3 by (b1) illumination, (b2) heating, (b3) stirring the precursor solution. Ag nanostructures synthesized with 50 mM AgNO3 by (c1) illumination, (c2) heating, (c3) stirring the precursor solution. All the top-right inserts are the corresponding bright field optical microscope images and the scale bars in them are 10 µm. The bottom-right insert shows a magnified image of a part of c1 (red square) and the scale bar in it is 1 µm. Fig. 9, Fig. 9a1-a3, Fig. 9a1, 109x74mm (300 x 300 DPI)


Electroless Deposition of Silver Nanostructures by Redox Reaction of

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