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Synthesis of silver nanostructures by simple redox under electrodeposited copper microcubes and the orient attachment growth of 2D silver
Applied Surface Science 357 (2015) 583–592
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of silver nanostructures by simple redox under electrodeposited copper microcubes and the orient attachment growth of 2D silver Guoxing Wu, Sanjun Yang, Qiming Liu ∗ School of Physics and Technology, Key Lab of Artificial Micro- and Nanostructures of Ministry of Education, Wuhan University, Wuhan 430072, PR China
a r t i c l e
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Article history: Received 28 June 2015 Received in revised form 2 September 2015 Accepted 5 September 2015 Available online 8 September 2015 Keywords: Copper microcubes Silver nanostructures 2D growth Oriented attachment
a b s t r a c t Copper microcubes about 500 nm were electrodeposited on ITO glasses. Silver nanoparticles, netty consist of short nanorods or nanowires, nanosheets with thickness about 40 nm, were successfully obtained by immersing ITOs with different concentration of AgNO3 solution. XRD, SEM and TEM were applied to characterize the products. Silver ions were initially reduced on the surface of copper, and then gradually decomposed the copper. Cuprous oxide intermediate was found to participate in the redox reaction. Both agglomerates on the cubes and escaped reductant nanoparticles act as the positions for anisotropic growth. Based on the experimental results, the roles of three kinds of cubes are discussed in preparing the nanosheets before proposing the possible growth process. Oriented attachment influenced and controlled the final shapes, such as layered nanonets, nanoplates and nanosheets. Big nanoparticles were inclined to link as nanowires, netty and even 2D porous structure consisted of ‘nanosnakes’, small reductant nanoparticles with silver around absorbed on the edge of silver nanoplate and further accelerated the extension of nanoplate until worked out, holes on the nanoplate confirmed that the adsorbed matters could be reductant nanoparticles. Reductant nanoparticles can also be exhausted before silver nanoparticles and nanoflakes absorbed on the growth positions. © 2015 Published by Elsevier B.V.
1. Introduction Morphology-specific nanostructures have gained increasing attention because of their potential applications in the fields of catalysis, delivery vehicles, photonic materials, optics and sensors, and so on. Silver, as one of the most widely studied metal, has received great attention for their strong plasmon absorption in the NIR region and SERS substrates. Experimental studies have proven that the pointed nanostructures are critical to attain highly Ramanenhanced substrates, due to the possession of abundant ‘hot sites’ which could enhance the electromagnetic field (EM) near the laserirradiated noble metal particle surfaces [1]. Ag nanocrystals can take most of the shapes that have been observed of any other facecentered cubic (fcc) metals [2]. Many methods have been developed to prepare nanostructures such as surfactant-mediated synthesis [3–6], self-organized on highly oriented pyrolytic graphite (HOPG), template-assisted synthesis [7], seeded growth synthesis [8,9],
∗ Corresponding author. E-mail address: [email protected] (Q. Liu). http://dx.doi.org/10.1016/j.apsusc.2015.09.055 0169-4332/© 2015 Published by Elsevier B.V.
photochemical synthesis [10–12], hydrothermal methods [13], and polyol synthesis [14]. Cu2 O is a weak inorganic reducing agent and has been used as a template agent for preparing different rare metals in various shapes, such as Au, Pt, Pd, and especially Ag which nanostructures synthesis are epidemic recently [1,15–18]. Wang et al. prepared nanosheets-assembled silver hollow microcubes with strong highly sensitive surface enhanced Raman scattering (SERS) using cubic Cu2 O template-assisted method. Yang et al., presented Cu2 O/Ag composite nanoframes with hollow truncated cube Cu2 O nanoframes synthesized hydrothermally as template [17]. In order to discuss SERS properties of silver nanostructures, Yang prepared Ag nanoparticles (NPs) on different lattice planes and interfaces of octahedron and truncated octahedron [16]. Spherical NPs is another template for preparing various Ag nanomaterials, Chen et al., reported a rapid synthesis of silver nanowires and network structures using cuprous oxide nanospheres as reducing agent and growth substrate or template [13]. They found that the SERS intensity of the network silver reached maximum value in the shape of spherical, short nanorods and dendrites, for which the explanation was ascribed to the electromagnetic (EM) enhancement on the “rough surface”. Similar to the Cu2 O, copper is another reducing
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agent but not widely used because of their fast oxidation when and after manufacture. Silver films with rough face, made up of dendrite coral-like or linked NPs, were generally used to cover on copper bulk and alloy by facile galvanic replacement reaction to discuss the water-repellent property [19,20] and enhanced antibacterial activity [21]. It is not precise to study the detailed reaction on bulk copper due to its rough surface, thus the preparation of smooth nano- or micro-copper seems to be necessary and of priority. A number of methods have been reported for fabricating the copper nanostructures, such as polyol reduction [22], chemical [23], UVlight [24], ␥ irradiation [25], sonoelectrochemical, and so on [26]. They either need higher energy or explosive, contaminative organic additives. Chen [13] and Liu [27] used Cu2 O and commercial Cu nanospheres as reductants respectively to prepare silver nanostructures are two typical synthetic examples to discuss the redox characteristic. Chen thought that the nanonets were formed due to the (Ostwald ripening) OR after silver ions were reduced by small Cu2 O NPs decomposed from nanospheres, and Liu proposed that the nanobatteries initiated the galvanic displacement, and then silver were reduced on the top of protruding silver “seeds” on their surface, but it was still unclear for its zonal growth. Although the growth mechanisms of the nanobelts obtained by Cu spherical NPs were proposed in Liu’s report [27], and he also elaborated the galvanic displacement reaction between silver ions and copper nanospheres, but he did not mention the Cu2 O in the reaction, and growth of the huge nanosheets was also not mentioned. Two-dimension (2D) template-directed strategy, surfactantmethods, CO or halide ion confined growth, seeded growth, hydrothermal and solvothermal methods, photochemical synthesis, are the most popular methods for preparing 2D nanosheets. Oriented attachment (OA) plays significant roles in tailoring materials with controllable sizes and morphologies. Disordered building blocks spontaneously organized with each other by relatively weak interactions (e.g. van der Waals, hydrogen bonds, p–p, and so on), have long been used for designing a series of ordered structure, and many inorganic and organic materials have been successfully obtained by this self-assembly method, such as CdTe, Co9 S8 . Different from the self-assembly method, OA requires small inorganic nanoparticles as building blocks to connect with each other in a confined 2D space or NPs nearby to share a common crystallographic face to minimize their high surface energy and meet a thermodynamic balance for the formation of thick 2D crystals or nanonets [28,29]. Photoinduced conversion of silver nanospheres to nanoplates was firstly reported early this century [30]. Later, a series of silver nanostructures such as nanorices [31], nanowires [32], dendritic silver [33], nanosheets and so on [34], were synthesized utilizing OA method, including huge [35] and ultrathin nanosheets [36] and films [37] of other inorganic materials. In view of these concerns, the development of an environmenttemperature, simple solution-based method for large-scale preparation of silver hierarchical structure remains attractive. The use of Cu or Cu2 O as reductants is considerable. Electrodeposition offers a promising way to prepare the inorganic metallic oxide and metals because it is low-cost, simple, and environmental friendly [38]. Well-defined Cu or Cu2 O nanostructures such as nanocubes, dendritic, flower-like, NPs have been prepared through the electrochemical deposition method [39–41]. In fact, Cu2 O/Cu core–shell nanocubes, cubooctahedrons, nanospheres can also be obtained by electrodeposition [42]. Considering the simplicity and efficiency of this approach, in this paper, we prepared silver nanostructures (especially, 2D nanoplates and nanosheets) using Cu cubes without any added surfactant molecule capping agent and toxic reagent. The process of their growth was also discussed. The experimental results revealed that the growth of silver NPs, nanosheets and nanonets emitted from the surface of the cubes was mainly
influenced by the concentration distribution of Ag ions near the substrate. A probable growth mechanism of the nanosheets and nanonets was presented. Three kinds of the cubes played roles in preparing Ag nanosheets are also proposed. OA influenced and controlled the final shapes, the 2D OA was observed during the nanosheets’ growth [34]. 2. Experimental 2.1. Materials and preparation All the chemical reagents were obtained from the Sinopharm Chemical Reagent Co., Ltd. and used without further purification. ITO (ITO-p001) glasses were purchased from Zhuhai Kaivo Optoelectronic Technology Co. Ltd. The working area was about 8 mm × 10 mm. Sheet resistance was less than 10 /䊐. A typical three-electrode cell containing an Ag|AgCl (3 M KCl) reference electrode (RE) and a platinum counter sheet electrode (CE, 2 cm × 2 cm) was used for electrochemical measurements and Cu cubes electrodeposition. The CE electrode was cleaned ultrasonically for 2 min before the deposition experiments. The working electrode (WE) and ITO were cleaned ultrasonically by acetone (>99.5%), ethanol and deionized water for 5 min, and then dried under 50 ◦ C for several minutes in the oven sequentially. In a typical synthesis, 20 ml electrolyte was prepared with 10 mM CuSO4 , 1 mM KCl, 1.5 ml npropanol (>99%) and deionized water without any other additives. The constant voltage was −0.3 V vs. Ag|AgCl was sustained for 450 s. Cu was treated with different concentrations of AgNO3 (>99.8%, 4 ml) for 40 min if it was without other states. 2.2. Characterization The results of XRD were collected on a Bruker D8 advance X-ray ˚ The acceleratdiffractometer by Cu K␣ radiation ( = 1.5406 A). ing voltage and current were 40 kV and 40 mA, respectively. The results were obtained at a step of 4◦ /min and the range (2) was from 30◦ to 80◦ . SEM images were obtained using a FEI Sirion FEG operating at 20 kV; TEM images were acquired from a JEM-2100 microscope (200 kV). TEM images were acquired from a JEM-2100 microscope (200 kV). The Raman scattering spectra of all the samples were acquired using a micro-Raman system (Lab RAM HR800). Ar laser (488.0 nm) was used as the excitation source with a power kept at 5.6 mW. The prepared composite substrates were immersed in Rhodamine B (RB) solutions for 10 h. The concentration of the RB was 10−5 M. The substrates were douched several times with deionized water and dried before Raman measurement. 3. Results and discussion 3.1. Preparation of Cu microcubes Fig. 1a shows a typical SEM image of the as-prepared copper microcubes by electrodeposition, the insert at the top-right corner is the insight view of the cubes. From the XRD pattern of the Cu cubes (Fig. 1b), the diffraction peaks at 2 = 43.3◦ , 50.4◦ are corresponding to the {1 1 1}, {2 0 0} crystal planes of face-centered cubic (fcc) Cu, respectively. It is different from the inside-out seed growth in hydrothermal synthesis where (2 0 0) diffraction is the strongest peak [43], the microcubes tended to show (1 1 1) diffraction as the strongest peak while other peaks were weak or even covered by the peaks of ITO, due to their preferential orientation with {1 1 1} planes parallel to the substrate and low coverage rate [44]. Fig. 1c shows the typical TEM image of the silver nanoplate sprouting from the agglomeration. Some raised parts were found on the edge of the nanoplate, and one edge is folded inward. The selected area electron
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Fig. 1. (a) SEM and (b) XRD patterns of copper microcubes prepared by electrodeposition containing 0.010 M CuSO4 and 1 mM 1-propanol, (c) typical TEM image of silver nanoplate made by using the copper as reductant (d) selected-area electron diffraction pattern image of the single nanoplate.
diffraction (SAED) pattern of the silver nanoplate is shown in Fig. 1d, the intense spots in the [1 1 1] zone axis are allowed {2 0 0} Bragg reflection and the diffraction pattern correspond to the normally forbidden 1/3 {4 2 2} Bragg reflection [27]. 3.2. The influence of the concentration of AgNO3 The concentration of AgNO3 plays a critical role in determining the shapes of Ag nanostructures. Morphology evolution of the silver nanostructures after treated with different concentration of AgNO3 is shown in Fig. 2. Firstly, we immersed the substrate into the 0.2 mM AgNO3 solution for 40 min. A great deal of Ag NPs about 80 nm was observed on the substrate and some on the surface of the cubes. It is obvious that something reductive have been diffused away from the cubes to solution, and then reacted with Ag+ , but whether small Cu2 O or Cu NPs is hard to verify. In consideration of the particle size on and away from the cubes and the distinct gaps on the corner of the cubes, we infer that the bigger ones may be on the surface of cubes originally and then fell off for some causes. It was also interesting to find few short silver nanorods in this sample (Fig. 2a), which was not mentioned in Liu’s report [27], and was similar to Yang’s report about the Ag/Cu2 O composite heterogeneous structure. So we surmise that the executive parts on the surface layer were Cu2 O, which is confirmed from the homologous XRD diffraction pattern in Fig. 3a. In electrodepositing copper nanostructure, small NPs of Cu are inclined to nucleate and grow
away from huge bulk Cu due to the Volmer–Weber island growth [39,45], so the high grain density around the cubes reflects the state of the outward propagation of decomposed NPs from the surface of the cubes. It is credible that the existence of smaller NPs outside the cubes are due to separation of reductant NPs from escaped Cu2 O bulk after a growth process. When the concentration of AgNO3 was increased to 0.5 mM (Fig. 2b), a number of clustered NPs were formed near the cracked cubes. These NPs mostly grew out from the rod-like silver, while NPs on the ITO and the clustered silver were fewer than the former one, indicating that the silver ions were prior to be reduced on the previous reduction sites when more silver ions were added. Further increasing the concentration of the AgNO3 to 0.8 mM, as shown in Fig. 2c, the NPs were almost formed on the whole cubes, and short silver nanowires extended from the clustered NPs. In fact, a few small nanoplates were also found in this experiment condition. When the concentration was 1 mM (Fig. 2d), the cubes were broken up into pieces, and nanosheets grew on them, the diagonal diameter of the nanosheets could reach 1 m. It was interesting that two nanoplates were connected with a narrow area (arrow). So the concentration of Ag+ ions determined the decomposition level of the cubes, which can control the sizes and amount of reductive in solution. We will show it below. Fig. 3 depicts the XRD patterns of the corresponding heterogeneous nanostructures on glass substrates referred above. These composite nanostructures were identified on the basis of the clearly distinguishable diffraction peaks at 2 = 38.2◦ , corresponding to
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Fig. 2. SEM images of the shapes evolution of heterostructures prepared by different concentration of AgNO3 , the gray surface indicates formation of Ag nanostructures. (a) 0.2 mM; (b) 0.5 mM; (c) 0.8 mM and (d) 1.0 mM.
the {1 1 1} crystal plane of face-centered cubic (fcc) Ag. Weak diffraction peak of Cu2 O and gradually decreased peak of Cu (1 1 1) were also found from Fig. 3, these indicated that Cu2 O had participated in the redox, it is very likely that silver nanostructures and copper were separated by Cu2 O layer until the copper cubes drained, or Cu2 O NPs were the reductants escaping from the cubes. The nanosheets were easy to oxidize when exposed in air (Fig. 3d).
3.3. Anisotropic growth from agglomerates Ignoring the final shapes of silver, we need to know what support their growth originally is. It is well known that anisotropic growth started on the polycrystalline seeds [2]. Therefore, the formation process of seeds needs to be explored for further research. Combining the report about the silver nanostructures synthesized using Cu2 O and Cu as reductants [13], we propose a possible early growth process of the redox reaction of Ag ions and Cu microcubes on ITO glasses. Chen et al. discussed the detailed formation process of Ag nanowires synthesized by small Cu2 O NPs [13]. In that report, they stated that the Ag ions could decompose Cu2 O into smaller NPs, which were absorbed on the faces of Ag sequentially to induce the formation of silver nanowires when adequate Ag ions existed. Although the sub-micron cubes glued on the substrate were hard to be broken into small NPs rapidly like Cu2 O spheres dispersed in solution, we could clearly see rough silver dots or even nanowires on the surface of cubes and NPs far from them (Fig. 4a) [16]. Small nanospheres are not sufficient for the anisotropic growth, the agglomerates evolved from aggregation and coalescence of nanospheres are crucial for anisotropic growth, such as nanowires [43] (Fig. 4b) and nanosheets (Fig. 4c). We speculate that all these agglomerates about 100 nm in the figures were the growth sites of silver nanostructures if they were weak etched.
It is relevant between internal structure of a seed and shape taken by a resulted nanocrystal. Nanostructures can grow from agglomerates with capping agents (for anisotropic structures) or without (single-crystals, generally) [43,46]. It is well known that increasing or lowering lattice plane energy by adjusting surfactant can promote the anisotropic growth, but how it takes place without surfactant in our experiment is interesting. Liu stated that the short-circuited nanobatteries formed between the copper nano/microparticles and protruding silver “seeds” initiated the galvanic displacement. Therefore, in our opinion, it is logical to believe that the nanobatteries can play the similar action as the surfactant which increases the energy of some planes by transfer electrons for further anisotropic growth of silver nanostructures. So long nanorods formed on the cubes while short nanorods or big NPs formed far away (Fig. 4a and b) [27]. Besides the original cubes, the decomposed parts of the reductants can also act as short-circuited nanobatteries together with silver on their surface. These two are the main possessions for forming silver seeds for further growth of nanostructures, but sometimes the former one may not always support the further anisotropic growth of silver (such as the third kind cube introduced below). As declared in Liu’s report, the Gibbs free energy of the electrochemical cell potential and the Nernst equation are important parameters in determining thermodynamic balance of the galvanic displacement reaction and further the structures of silver (Eqs. (1)–(3)) [27]. G = −nFE E = E0 −
RT a ln Red nF aOx
Ox + ne− = Red
(1) (2)
(3)
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Fig. 3. XRD patterns of the products after dealing the coppers with different concentration of AgNO3. (a) 0.2 mM, (b) 0.5 mM, (c) 0.8 mM, (d) 1.0 mM, the nanosheets were partial oxidized.
In consideration of the XRD results, we think that the detailed process of Eq. (3) could be as follows (Eqs. (4) and (5)). Ag+ + Cu = Ag+Cu+ +
+
(4) 2+
Ag + Cu = Ag+Cu
(5)
The two steps Eqs. (4) and (5) are the possible reaction process initially, and did not discuss in his report. So the possible process could be as follows, in the beginning, the surface of the Cu was oxidized to form Cu2 O NPs, the NPs can either be decomposed by Ag ions and enter into the AgNO3 solution or react with Ag ions on the surface of the cubes immediately (Fig. 4d). Taking nanosheets as example, we used three different kinds of cubes as reductant to explore their influence on the generation of agglomerates and the formation of nanosheets. Here three kinds of cubes are shown below. The original cubes are growing point of the small flakes or agglomerates for nanosheets. The component of the contact point should be Cu2 O considering Eq. (4). Holes may exist in it by etching of Ag+ [47]. The probable process of Ag growth is associated with the nanobatteries reaction, cation exchange reaction, and reduction with photogenerated electrons [47–49]. From SEM and TEM images (Fig. 5a and b), we can find fractured cubes without some surfaces (both A), which is attributed to decomposition and reaction with Ag+ near it. It seems that these cubes are most likely exhausted inside-out, due to silver sprout formed earlier on the cubes.
The puppet cubes near the original ones are reductants supplier. In most cases, large sheets are found on the cracked cube with a puppet cube near them (Fig. 5b (B)). Along the black arrow, color is deepening if ignoring the trace. It is likely that the cube was consumed via the trace. We believe that the puppet cubes participate in the reaction, because they act as center of the semicircular Ag. Faces of those puppet cubes are generally smooth with few Ag NPs (Fig. 5c, and the possible covered cube in Fig. 5a (B)). The traces parallel to the edge of cube are the layered growth peripheries of the cube [39]. When nanosheets from original cubes extend to the trace, the puppet cubs are inclined to provide electrons to the silver nanosheets edge through the trace due to the battery cell action [27]. The independent cubes act only as the provider of small Cu2 O NPs and are covered by many Ag NPs which fail to develop into nanosheets (Fig. 5d). In general, these rough cubes stay away from nanosheets, until the nanosheets extend on them at the later period (Fig. 5a (C)). The feature of these cubes is that the NPs on microcubes are hemispherical, which means anisotropic growth cannot take place there. We speculate that small Cu2 O NPs can hardly escape out so that NPs cannot reach the hemispheres to form agglomerates. What’s more, the reductive NPs formed near these cubes are apt to add to the existing nanosheets which have higher energy, thought silver nanosheets are further. On the other hand, the nanobatteries are not all reason for arousing the anisotropic growth.
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Fig. 4. Representative SEM image (a) of sphere Ag NPs at 0.2 mM AgNO3 , the big NPs and nanorod were formed after mild aggregation and coalescence on the top right corner, (b) short Ag nanowire on the cube and shorter rod started form agglomerates at 0.5 mM AgNO3 , both the cubes and the agglomerates can induce the anisotropic growth, (c) nanoplate growth from the agglomerates, (d) the possible growth formation of the nanoplates, agglomerates on microcubes and assembled by escaped reductants NPs.
Fig. 5. (a) Typical SEM images of the three cubes for nanosheets, the copper were prepared by electrodeposition containing 0.001 M CuSO4 and 1 mM 1-propanol, the potential at -0.3 V vs. Ag|AgCl sustained for 450 s. TEM image of (b) the nanoplates with an original cube (A) and a puppet cube (B), (c) a small puppet cube with light traces, and a small NPs, (d) the corner of an independent cube with many silver hemispheres on the surface.
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Fig. 6. SEM images of the (a) NPs prepared by 0.2 mM AgNO3 and (b) netty silver prepared by 0.5 mM AgNO3 for 40 min, TEM images of the OA (c) by big NPs and (d) short nanorod for netty silver.
3.4. Oriented attachment supported structures evolution Here we introduce the growth mode of large nanosheets which were not involved in Liu’s paper. It has been proven that nanoplates and nanobelts were generated when AgNO3 concentration is higher than 10 mM [27], while rough surface of microspheres affected our observation and judge about subtle change on the surface of copper. Thus copper cubes with smooth surface were obtained by low-level voltage electrodeposition; and lower concentrations of AgNO3 solution were used in our experiments. It is obvious that concentration of Ag ions influences the amount and size of reductant in turn the size and the shapes of Ag nanostructures. Under lowest concentration of AgNO3 (0.2 mM), reductive species were released into the solution, Ag ions around reacted with them, resulting in scattered Ag NPs. Fig. 6a is the SEM image of the as-prepared products at this concentration. As the number of decomposed NPs is enhanced with increasing concentration of silver ions (0.5 mM), stack and connection are inevitable to gain insight into the mechanism of conversion from the initial large Ag NPs to the final Ag nanonets product. Fig. 6b shows the SEM images of one sight of the final morphology seen at this stage. 3D nets consist of short nanorods and NPs, some cubes are enfolded by nets (A), others were vacant and remained the silver skeleton around them (B). TEM analysis of the initial and intermediate stage of 3D network was made, as presented in Fig. 6c and d. Fig. 6c shows the earlier stage of nets, with many rods composed of NPs by OA, and we can still find joints of NPs (arrows). As it is shown in Fig. 6d, the short nanorods connected with each other by OA too. The narrow areas of the short rods are
the possible grain boundaries (arrows), so OA occurred easier with smaller NPs at higher concentration of AgNO3 . There exists a transition from nets to nanosheets. In the condition of big reductants and lack of Ag ions, wirelike structure is of priority, that is to say, not all corners were satisfied with the attachment conditions. As shown in Fig. 7a, snakelike nanowires got together into plane, and many nanowires are stacking on them. It is indicated that these long nanowires are trying to form nanosheets by OA [50], acting as there were a mysterious force pressing the former 3D nets into 2D planes. We consider that prior formation of nanowires by large NPs elevated the difficulty for nanoplates, and that is to say the smaller nanostructures are easier for nanosheets. Further, Fig. 7b shows the layered nanosheets assembled by the attachment of small nanoplates while the silver is relatively small. The arrows show the attachment of nanoflakes. These attachments are imperfect, where holes and faults exist universally in the sheets. Because of the higher energy of faults, reductants give priority to migrate to those edges, resulting in formation of other nanosheets on the original one after Ag ions are reduced. All the nanosheets above are easy to curve and fold, which are distinguishable to flat plates due to the fast growth process. When the concentration of Ag ions is appropriate, NPs and 2D OA both existed in the process of the formation of nanosheets. As shown in Fig. 7c, a black reductant NPs adsorbed prior on to the edges to eliminate high-energy faces, then Ag ions are reduced around the NPs, and connected with the nanosheets. Meanwhile, the black dots in the nanosheets are the remains of reductants, which will act as short-circuited nanobatteries with Ag around them to improve further growth of nanoplates until exhausted. The light holes in the plates are the vacancies of depleted ones. Further
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Fig. 7. TEM image of (a) 2D netty packaged by snakelike naowires and (b) huge layered nanosheets by OA, (c) attachment of a reductive NP on the edge of nanoplate, (d) further insight of the holes in the nanoplates, (e) huge nanosheets by imperfect 2D OA and (f) its corner insight.
observation of the holes from TEM image reveals that reductants are less than 10 nm (Fig. 7d). Fig. 7e and f is the TEM images of huge nanosheet and its one corner at higher resolution respectively. The arrows in Fig. 7e show distinct gaps of imperfect attachment. Two parts of huge nanosheets folded inward indicate an unfinished combination of nanoplates, in other word; the boundaries of nanoplates still exist. Small silver nanoplates are attaching on the edges of huge nanosheet (white arrows). Insight view of the edge is showed in Fig. 7f, from which we can see two potential triangles (A and B) involved in this part, small gap and contact boundary are clearly distinguishable between the potential internal triangular nanoplates, and two stairs (arrows) on the right side of triangle were developing inside out to widen it.
3.5. Raman Silver nanostructure is a well known SERS substrate for sensing. The SERS sensitivity is influenced by the shape and size of the nanostructure, so we used RB (10−5 M) as target molecule to investigate the SERS enhancement of all the Ag nanostructures (Fig. 8). The Raman bands at 218 and 635 cm−1 are associated with Cu2 O [51], and other bands at 618, 1199, 1288, 1363, 1508, 1564, 1596, and 1647 cm−1 , belong to the SERS spectra of RB absorbed on Ag/Cu2 O [16]. It is well known that the SERS signal of pure Ag is not very intense as a weak electromagnetic field it created, and Ag/Cu2 O heterostructures can induce a larger electromagnetic field so that they are able to excite a more intense LSPR. As shown in Fig. 8, nanoparticles (a) had lowest enhancement, so we believe
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[7]
[8]
[9]
[10]
[11] [12] [13]
Fig. 8. Raman spectra of RB absorbed on Ag with AgNO3 concentration of (a) 0.2 mM, (b) 0.5 mM, (c) 0.8 mM, (d) 1 mM.
these nanoparticles away from cubes have few reductants inside. When the concentration of AgNO3 is higher, reductant were dispersive or even insert into nanoplate (unspent reductants particles in nanoplates) to enhance the contact surface with Ag, and the irregular shapes of Ag, can stimulate the electromagnetic enhancement [13]. 4. Conclusions We presented a facile and effective route for the shapecontrolled synthesis of Ag nanostructures by using the electrodeposited Cu nanocubes as reducing and template guiding agent. The concentration and distribution of Ag ions influenced the final shapes of products. The concentration distribution of the Ag ions around Cu cubes determined the sizes and amount of the reductants released from the cubes. These adjustable reductants combined with various concentrations of Ag ions determined the final shapes of silver products. Small reductive NPs were found to be absorbed on the edges of nanoplates and then reduced the Ag ion around them to accelerate the growth of plates. Cuprous oxide is found to participate in the reaction detected by XRD diffraction. The OA as well as OR mechanism are used to illustrate the growth process of these nanostructures. Dislocation loop emission is found during the formation of nanosheets.
[14]
[15] [16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
Acknowledgments [26]
This research work was financially supported by the National Natural Science Foundation of China (51272183 and 51572202) and the Nanotechnology Program of Suzhou (ZXG201438).
[27]
[28]
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of silver nanostructures by simple redox under electrodeposited copper microcubes and the orient attachment growth of 2D silver Guoxing Wu, Sanjun Yang, Qiming Liu ∗ School of Physics and Technology, Key Lab of Artificial Micro- and Nanostructures of Ministry of Education, Wuhan University, Wuhan 430072, PR China
a r t i c l e
i n f o
Article history: Received 28 June 2015 Received in revised form 2 September 2015 Accepted 5 September 2015 Available online 8 September 2015 Keywords: Copper microcubes Silver nanostructures 2D growth Oriented attachment
a b s t r a c t Copper microcubes about 500 nm were electrodeposited on ITO glasses. Silver nanoparticles, netty consist of short nanorods or nanowires, nanosheets with thickness about 40 nm, were successfully obtained by immersing ITOs with different concentration of AgNO3 solution. XRD, SEM and TEM were applied to characterize the products. Silver ions were initially reduced on the surface of copper, and then gradually decomposed the copper. Cuprous oxide intermediate was found to participate in the redox reaction. Both agglomerates on the cubes and escaped reductant nanoparticles act as the positions for anisotropic growth. Based on the experimental results, the roles of three kinds of cubes are discussed in preparing the nanosheets before proposing the possible growth process. Oriented attachment influenced and controlled the final shapes, such as layered nanonets, nanoplates and nanosheets. Big nanoparticles were inclined to link as nanowires, netty and even 2D porous structure consisted of ‘nanosnakes’, small reductant nanoparticles with silver around absorbed on the edge of silver nanoplate and further accelerated the extension of nanoplate until worked out, holes on the nanoplate confirmed that the adsorbed matters could be reductant nanoparticles. Reductant nanoparticles can also be exhausted before silver nanoparticles and nanoflakes absorbed on the growth positions. © 2015 Published by Elsevier B.V.
1. Introduction Morphology-specific nanostructures have gained increasing attention because of their potential applications in the fields of catalysis, delivery vehicles, photonic materials, optics and sensors, and so on. Silver, as one of the most widely studied metal, has received great attention for their strong plasmon absorption in the NIR region and SERS substrates. Experimental studies have proven that the pointed nanostructures are critical to attain highly Ramanenhanced substrates, due to the possession of abundant ‘hot sites’ which could enhance the electromagnetic field (EM) near the laserirradiated noble metal particle surfaces [1]. Ag nanocrystals can take most of the shapes that have been observed of any other facecentered cubic (fcc) metals [2]. Many methods have been developed to prepare nanostructures such as surfactant-mediated synthesis [3–6], self-organized on highly oriented pyrolytic graphite (HOPG), template-assisted synthesis [7], seeded growth synthesis [8,9],
∗ Corresponding author. E-mail address: [email protected] (Q. Liu). http://dx.doi.org/10.1016/j.apsusc.2015.09.055 0169-4332/© 2015 Published by Elsevier B.V.
photochemical synthesis [10–12], hydrothermal methods [13], and polyol synthesis [14]. Cu2 O is a weak inorganic reducing agent and has been used as a template agent for preparing different rare metals in various shapes, such as Au, Pt, Pd, and especially Ag which nanostructures synthesis are epidemic recently [1,15–18]. Wang et al. prepared nanosheets-assembled silver hollow microcubes with strong highly sensitive surface enhanced Raman scattering (SERS) using cubic Cu2 O template-assisted method. Yang et al., presented Cu2 O/Ag composite nanoframes with hollow truncated cube Cu2 O nanoframes synthesized hydrothermally as template [17]. In order to discuss SERS properties of silver nanostructures, Yang prepared Ag nanoparticles (NPs) on different lattice planes and interfaces of octahedron and truncated octahedron [16]. Spherical NPs is another template for preparing various Ag nanomaterials, Chen et al., reported a rapid synthesis of silver nanowires and network structures using cuprous oxide nanospheres as reducing agent and growth substrate or template [13]. They found that the SERS intensity of the network silver reached maximum value in the shape of spherical, short nanorods and dendrites, for which the explanation was ascribed to the electromagnetic (EM) enhancement on the “rough surface”. Similar to the Cu2 O, copper is another reducing
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agent but not widely used because of their fast oxidation when and after manufacture. Silver films with rough face, made up of dendrite coral-like or linked NPs, were generally used to cover on copper bulk and alloy by facile galvanic replacement reaction to discuss the water-repellent property [19,20] and enhanced antibacterial activity [21]. It is not precise to study the detailed reaction on bulk copper due to its rough surface, thus the preparation of smooth nano- or micro-copper seems to be necessary and of priority. A number of methods have been reported for fabricating the copper nanostructures, such as polyol reduction [22], chemical [23], UVlight [24], ␥ irradiation [25], sonoelectrochemical, and so on [26]. They either need higher energy or explosive, contaminative organic additives. Chen [13] and Liu [27] used Cu2 O and commercial Cu nanospheres as reductants respectively to prepare silver nanostructures are two typical synthetic examples to discuss the redox characteristic. Chen thought that the nanonets were formed due to the (Ostwald ripening) OR after silver ions were reduced by small Cu2 O NPs decomposed from nanospheres, and Liu proposed that the nanobatteries initiated the galvanic displacement, and then silver were reduced on the top of protruding silver “seeds” on their surface, but it was still unclear for its zonal growth. Although the growth mechanisms of the nanobelts obtained by Cu spherical NPs were proposed in Liu’s report [27], and he also elaborated the galvanic displacement reaction between silver ions and copper nanospheres, but he did not mention the Cu2 O in the reaction, and growth of the huge nanosheets was also not mentioned. Two-dimension (2D) template-directed strategy, surfactantmethods, CO or halide ion confined growth, seeded growth, hydrothermal and solvothermal methods, photochemical synthesis, are the most popular methods for preparing 2D nanosheets. Oriented attachment (OA) plays significant roles in tailoring materials with controllable sizes and morphologies. Disordered building blocks spontaneously organized with each other by relatively weak interactions (e.g. van der Waals, hydrogen bonds, p–p, and so on), have long been used for designing a series of ordered structure, and many inorganic and organic materials have been successfully obtained by this self-assembly method, such as CdTe, Co9 S8 . Different from the self-assembly method, OA requires small inorganic nanoparticles as building blocks to connect with each other in a confined 2D space or NPs nearby to share a common crystallographic face to minimize their high surface energy and meet a thermodynamic balance for the formation of thick 2D crystals or nanonets [28,29]. Photoinduced conversion of silver nanospheres to nanoplates was firstly reported early this century [30]. Later, a series of silver nanostructures such as nanorices [31], nanowires [32], dendritic silver [33], nanosheets and so on [34], were synthesized utilizing OA method, including huge [35] and ultrathin nanosheets [36] and films [37] of other inorganic materials. In view of these concerns, the development of an environmenttemperature, simple solution-based method for large-scale preparation of silver hierarchical structure remains attractive. The use of Cu or Cu2 O as reductants is considerable. Electrodeposition offers a promising way to prepare the inorganic metallic oxide and metals because it is low-cost, simple, and environmental friendly [38]. Well-defined Cu or Cu2 O nanostructures such as nanocubes, dendritic, flower-like, NPs have been prepared through the electrochemical deposition method [39–41]. In fact, Cu2 O/Cu core–shell nanocubes, cubooctahedrons, nanospheres can also be obtained by electrodeposition [42]. Considering the simplicity and efficiency of this approach, in this paper, we prepared silver nanostructures (especially, 2D nanoplates and nanosheets) using Cu cubes without any added surfactant molecule capping agent and toxic reagent. The process of their growth was also discussed. The experimental results revealed that the growth of silver NPs, nanosheets and nanonets emitted from the surface of the cubes was mainly
influenced by the concentration distribution of Ag ions near the substrate. A probable growth mechanism of the nanosheets and nanonets was presented. Three kinds of the cubes played roles in preparing Ag nanosheets are also proposed. OA influenced and controlled the final shapes, the 2D OA was observed during the nanosheets’ growth [34]. 2. Experimental 2.1. Materials and preparation All the chemical reagents were obtained from the Sinopharm Chemical Reagent Co., Ltd. and used without further purification. ITO (ITO-p001) glasses were purchased from Zhuhai Kaivo Optoelectronic Technology Co. Ltd. The working area was about 8 mm × 10 mm. Sheet resistance was less than 10 /䊐. A typical three-electrode cell containing an Ag|AgCl (3 M KCl) reference electrode (RE) and a platinum counter sheet electrode (CE, 2 cm × 2 cm) was used for electrochemical measurements and Cu cubes electrodeposition. The CE electrode was cleaned ultrasonically for 2 min before the deposition experiments. The working electrode (WE) and ITO were cleaned ultrasonically by acetone (>99.5%), ethanol and deionized water for 5 min, and then dried under 50 ◦ C for several minutes in the oven sequentially. In a typical synthesis, 20 ml electrolyte was prepared with 10 mM CuSO4 , 1 mM KCl, 1.5 ml npropanol (>99%) and deionized water without any other additives. The constant voltage was −0.3 V vs. Ag|AgCl was sustained for 450 s. Cu was treated with different concentrations of AgNO3 (>99.8%, 4 ml) for 40 min if it was without other states. 2.2. Characterization The results of XRD were collected on a Bruker D8 advance X-ray ˚ The acceleratdiffractometer by Cu K␣ radiation ( = 1.5406 A). ing voltage and current were 40 kV and 40 mA, respectively. The results were obtained at a step of 4◦ /min and the range (2) was from 30◦ to 80◦ . SEM images were obtained using a FEI Sirion FEG operating at 20 kV; TEM images were acquired from a JEM-2100 microscope (200 kV). TEM images were acquired from a JEM-2100 microscope (200 kV). The Raman scattering spectra of all the samples were acquired using a micro-Raman system (Lab RAM HR800). Ar laser (488.0 nm) was used as the excitation source with a power kept at 5.6 mW. The prepared composite substrates were immersed in Rhodamine B (RB) solutions for 10 h. The concentration of the RB was 10−5 M. The substrates were douched several times with deionized water and dried before Raman measurement. 3. Results and discussion 3.1. Preparation of Cu microcubes Fig. 1a shows a typical SEM image of the as-prepared copper microcubes by electrodeposition, the insert at the top-right corner is the insight view of the cubes. From the XRD pattern of the Cu cubes (Fig. 1b), the diffraction peaks at 2 = 43.3◦ , 50.4◦ are corresponding to the {1 1 1}, {2 0 0} crystal planes of face-centered cubic (fcc) Cu, respectively. It is different from the inside-out seed growth in hydrothermal synthesis where (2 0 0) diffraction is the strongest peak [43], the microcubes tended to show (1 1 1) diffraction as the strongest peak while other peaks were weak or even covered by the peaks of ITO, due to their preferential orientation with {1 1 1} planes parallel to the substrate and low coverage rate [44]. Fig. 1c shows the typical TEM image of the silver nanoplate sprouting from the agglomeration. Some raised parts were found on the edge of the nanoplate, and one edge is folded inward. The selected area electron
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Fig. 1. (a) SEM and (b) XRD patterns of copper microcubes prepared by electrodeposition containing 0.010 M CuSO4 and 1 mM 1-propanol, (c) typical TEM image of silver nanoplate made by using the copper as reductant (d) selected-area electron diffraction pattern image of the single nanoplate.
diffraction (SAED) pattern of the silver nanoplate is shown in Fig. 1d, the intense spots in the [1 1 1] zone axis are allowed {2 0 0} Bragg reflection and the diffraction pattern correspond to the normally forbidden 1/3 {4 2 2} Bragg reflection [27]. 3.2. The influence of the concentration of AgNO3 The concentration of AgNO3 plays a critical role in determining the shapes of Ag nanostructures. Morphology evolution of the silver nanostructures after treated with different concentration of AgNO3 is shown in Fig. 2. Firstly, we immersed the substrate into the 0.2 mM AgNO3 solution for 40 min. A great deal of Ag NPs about 80 nm was observed on the substrate and some on the surface of the cubes. It is obvious that something reductive have been diffused away from the cubes to solution, and then reacted with Ag+ , but whether small Cu2 O or Cu NPs is hard to verify. In consideration of the particle size on and away from the cubes and the distinct gaps on the corner of the cubes, we infer that the bigger ones may be on the surface of cubes originally and then fell off for some causes. It was also interesting to find few short silver nanorods in this sample (Fig. 2a), which was not mentioned in Liu’s report [27], and was similar to Yang’s report about the Ag/Cu2 O composite heterogeneous structure. So we surmise that the executive parts on the surface layer were Cu2 O, which is confirmed from the homologous XRD diffraction pattern in Fig. 3a. In electrodepositing copper nanostructure, small NPs of Cu are inclined to nucleate and grow
away from huge bulk Cu due to the Volmer–Weber island growth [39,45], so the high grain density around the cubes reflects the state of the outward propagation of decomposed NPs from the surface of the cubes. It is credible that the existence of smaller NPs outside the cubes are due to separation of reductant NPs from escaped Cu2 O bulk after a growth process. When the concentration of AgNO3 was increased to 0.5 mM (Fig. 2b), a number of clustered NPs were formed near the cracked cubes. These NPs mostly grew out from the rod-like silver, while NPs on the ITO and the clustered silver were fewer than the former one, indicating that the silver ions were prior to be reduced on the previous reduction sites when more silver ions were added. Further increasing the concentration of the AgNO3 to 0.8 mM, as shown in Fig. 2c, the NPs were almost formed on the whole cubes, and short silver nanowires extended from the clustered NPs. In fact, a few small nanoplates were also found in this experiment condition. When the concentration was 1 mM (Fig. 2d), the cubes were broken up into pieces, and nanosheets grew on them, the diagonal diameter of the nanosheets could reach 1 m. It was interesting that two nanoplates were connected with a narrow area (arrow). So the concentration of Ag+ ions determined the decomposition level of the cubes, which can control the sizes and amount of reductive in solution. We will show it below. Fig. 3 depicts the XRD patterns of the corresponding heterogeneous nanostructures on glass substrates referred above. These composite nanostructures were identified on the basis of the clearly distinguishable diffraction peaks at 2 = 38.2◦ , corresponding to
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Fig. 2. SEM images of the shapes evolution of heterostructures prepared by different concentration of AgNO3 , the gray surface indicates formation of Ag nanostructures. (a) 0.2 mM; (b) 0.5 mM; (c) 0.8 mM and (d) 1.0 mM.
the {1 1 1} crystal plane of face-centered cubic (fcc) Ag. Weak diffraction peak of Cu2 O and gradually decreased peak of Cu (1 1 1) were also found from Fig. 3, these indicated that Cu2 O had participated in the redox, it is very likely that silver nanostructures and copper were separated by Cu2 O layer until the copper cubes drained, or Cu2 O NPs were the reductants escaping from the cubes. The nanosheets were easy to oxidize when exposed in air (Fig. 3d).
3.3. Anisotropic growth from agglomerates Ignoring the final shapes of silver, we need to know what support their growth originally is. It is well known that anisotropic growth started on the polycrystalline seeds [2]. Therefore, the formation process of seeds needs to be explored for further research. Combining the report about the silver nanostructures synthesized using Cu2 O and Cu as reductants [13], we propose a possible early growth process of the redox reaction of Ag ions and Cu microcubes on ITO glasses. Chen et al. discussed the detailed formation process of Ag nanowires synthesized by small Cu2 O NPs [13]. In that report, they stated that the Ag ions could decompose Cu2 O into smaller NPs, which were absorbed on the faces of Ag sequentially to induce the formation of silver nanowires when adequate Ag ions existed. Although the sub-micron cubes glued on the substrate were hard to be broken into small NPs rapidly like Cu2 O spheres dispersed in solution, we could clearly see rough silver dots or even nanowires on the surface of cubes and NPs far from them (Fig. 4a) [16]. Small nanospheres are not sufficient for the anisotropic growth, the agglomerates evolved from aggregation and coalescence of nanospheres are crucial for anisotropic growth, such as nanowires [43] (Fig. 4b) and nanosheets (Fig. 4c). We speculate that all these agglomerates about 100 nm in the figures were the growth sites of silver nanostructures if they were weak etched.
It is relevant between internal structure of a seed and shape taken by a resulted nanocrystal. Nanostructures can grow from agglomerates with capping agents (for anisotropic structures) or without (single-crystals, generally) [43,46]. It is well known that increasing or lowering lattice plane energy by adjusting surfactant can promote the anisotropic growth, but how it takes place without surfactant in our experiment is interesting. Liu stated that the short-circuited nanobatteries formed between the copper nano/microparticles and protruding silver “seeds” initiated the galvanic displacement. Therefore, in our opinion, it is logical to believe that the nanobatteries can play the similar action as the surfactant which increases the energy of some planes by transfer electrons for further anisotropic growth of silver nanostructures. So long nanorods formed on the cubes while short nanorods or big NPs formed far away (Fig. 4a and b) [27]. Besides the original cubes, the decomposed parts of the reductants can also act as short-circuited nanobatteries together with silver on their surface. These two are the main possessions for forming silver seeds for further growth of nanostructures, but sometimes the former one may not always support the further anisotropic growth of silver (such as the third kind cube introduced below). As declared in Liu’s report, the Gibbs free energy of the electrochemical cell potential and the Nernst equation are important parameters in determining thermodynamic balance of the galvanic displacement reaction and further the structures of silver (Eqs. (1)–(3)) [27]. G = −nFE E = E0 −
RT a ln Red nF aOx
Ox + ne− = Red
(1) (2)
(3)
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Fig. 3. XRD patterns of the products after dealing the coppers with different concentration of AgNO3. (a) 0.2 mM, (b) 0.5 mM, (c) 0.8 mM, (d) 1.0 mM, the nanosheets were partial oxidized.
In consideration of the XRD results, we think that the detailed process of Eq. (3) could be as follows (Eqs. (4) and (5)). Ag+ + Cu = Ag+Cu+ +
+
(4) 2+
Ag + Cu = Ag+Cu
(5)
The two steps Eqs. (4) and (5) are the possible reaction process initially, and did not discuss in his report. So the possible process could be as follows, in the beginning, the surface of the Cu was oxidized to form Cu2 O NPs, the NPs can either be decomposed by Ag ions and enter into the AgNO3 solution or react with Ag ions on the surface of the cubes immediately (Fig. 4d). Taking nanosheets as example, we used three different kinds of cubes as reductant to explore their influence on the generation of agglomerates and the formation of nanosheets. Here three kinds of cubes are shown below. The original cubes are growing point of the small flakes or agglomerates for nanosheets. The component of the contact point should be Cu2 O considering Eq. (4). Holes may exist in it by etching of Ag+ [47]. The probable process of Ag growth is associated with the nanobatteries reaction, cation exchange reaction, and reduction with photogenerated electrons [47–49]. From SEM and TEM images (Fig. 5a and b), we can find fractured cubes without some surfaces (both A), which is attributed to decomposition and reaction with Ag+ near it. It seems that these cubes are most likely exhausted inside-out, due to silver sprout formed earlier on the cubes.
The puppet cubes near the original ones are reductants supplier. In most cases, large sheets are found on the cracked cube with a puppet cube near them (Fig. 5b (B)). Along the black arrow, color is deepening if ignoring the trace. It is likely that the cube was consumed via the trace. We believe that the puppet cubes participate in the reaction, because they act as center of the semicircular Ag. Faces of those puppet cubes are generally smooth with few Ag NPs (Fig. 5c, and the possible covered cube in Fig. 5a (B)). The traces parallel to the edge of cube are the layered growth peripheries of the cube [39]. When nanosheets from original cubes extend to the trace, the puppet cubs are inclined to provide electrons to the silver nanosheets edge through the trace due to the battery cell action [27]. The independent cubes act only as the provider of small Cu2 O NPs and are covered by many Ag NPs which fail to develop into nanosheets (Fig. 5d). In general, these rough cubes stay away from nanosheets, until the nanosheets extend on them at the later period (Fig. 5a (C)). The feature of these cubes is that the NPs on microcubes are hemispherical, which means anisotropic growth cannot take place there. We speculate that small Cu2 O NPs can hardly escape out so that NPs cannot reach the hemispheres to form agglomerates. What’s more, the reductive NPs formed near these cubes are apt to add to the existing nanosheets which have higher energy, thought silver nanosheets are further. On the other hand, the nanobatteries are not all reason for arousing the anisotropic growth.
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Fig. 4. Representative SEM image (a) of sphere Ag NPs at 0.2 mM AgNO3 , the big NPs and nanorod were formed after mild aggregation and coalescence on the top right corner, (b) short Ag nanowire on the cube and shorter rod started form agglomerates at 0.5 mM AgNO3 , both the cubes and the agglomerates can induce the anisotropic growth, (c) nanoplate growth from the agglomerates, (d) the possible growth formation of the nanoplates, agglomerates on microcubes and assembled by escaped reductants NPs.
Fig. 5. (a) Typical SEM images of the three cubes for nanosheets, the copper were prepared by electrodeposition containing 0.001 M CuSO4 and 1 mM 1-propanol, the potential at -0.3 V vs. Ag|AgCl sustained for 450 s. TEM image of (b) the nanoplates with an original cube (A) and a puppet cube (B), (c) a small puppet cube with light traces, and a small NPs, (d) the corner of an independent cube with many silver hemispheres on the surface.
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Fig. 6. SEM images of the (a) NPs prepared by 0.2 mM AgNO3 and (b) netty silver prepared by 0.5 mM AgNO3 for 40 min, TEM images of the OA (c) by big NPs and (d) short nanorod for netty silver.
3.4. Oriented attachment supported structures evolution Here we introduce the growth mode of large nanosheets which were not involved in Liu’s paper. It has been proven that nanoplates and nanobelts were generated when AgNO3 concentration is higher than 10 mM [27], while rough surface of microspheres affected our observation and judge about subtle change on the surface of copper. Thus copper cubes with smooth surface were obtained by low-level voltage electrodeposition; and lower concentrations of AgNO3 solution were used in our experiments. It is obvious that concentration of Ag ions influences the amount and size of reductant in turn the size and the shapes of Ag nanostructures. Under lowest concentration of AgNO3 (0.2 mM), reductive species were released into the solution, Ag ions around reacted with them, resulting in scattered Ag NPs. Fig. 6a is the SEM image of the as-prepared products at this concentration. As the number of decomposed NPs is enhanced with increasing concentration of silver ions (0.5 mM), stack and connection are inevitable to gain insight into the mechanism of conversion from the initial large Ag NPs to the final Ag nanonets product. Fig. 6b shows the SEM images of one sight of the final morphology seen at this stage. 3D nets consist of short nanorods and NPs, some cubes are enfolded by nets (A), others were vacant and remained the silver skeleton around them (B). TEM analysis of the initial and intermediate stage of 3D network was made, as presented in Fig. 6c and d. Fig. 6c shows the earlier stage of nets, with many rods composed of NPs by OA, and we can still find joints of NPs (arrows). As it is shown in Fig. 6d, the short nanorods connected with each other by OA too. The narrow areas of the short rods are
the possible grain boundaries (arrows), so OA occurred easier with smaller NPs at higher concentration of AgNO3 . There exists a transition from nets to nanosheets. In the condition of big reductants and lack of Ag ions, wirelike structure is of priority, that is to say, not all corners were satisfied with the attachment conditions. As shown in Fig. 7a, snakelike nanowires got together into plane, and many nanowires are stacking on them. It is indicated that these long nanowires are trying to form nanosheets by OA [50], acting as there were a mysterious force pressing the former 3D nets into 2D planes. We consider that prior formation of nanowires by large NPs elevated the difficulty for nanoplates, and that is to say the smaller nanostructures are easier for nanosheets. Further, Fig. 7b shows the layered nanosheets assembled by the attachment of small nanoplates while the silver is relatively small. The arrows show the attachment of nanoflakes. These attachments are imperfect, where holes and faults exist universally in the sheets. Because of the higher energy of faults, reductants give priority to migrate to those edges, resulting in formation of other nanosheets on the original one after Ag ions are reduced. All the nanosheets above are easy to curve and fold, which are distinguishable to flat plates due to the fast growth process. When the concentration of Ag ions is appropriate, NPs and 2D OA both existed in the process of the formation of nanosheets. As shown in Fig. 7c, a black reductant NPs adsorbed prior on to the edges to eliminate high-energy faces, then Ag ions are reduced around the NPs, and connected with the nanosheets. Meanwhile, the black dots in the nanosheets are the remains of reductants, which will act as short-circuited nanobatteries with Ag around them to improve further growth of nanoplates until exhausted. The light holes in the plates are the vacancies of depleted ones. Further
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Fig. 7. TEM image of (a) 2D netty packaged by snakelike naowires and (b) huge layered nanosheets by OA, (c) attachment of a reductive NP on the edge of nanoplate, (d) further insight of the holes in the nanoplates, (e) huge nanosheets by imperfect 2D OA and (f) its corner insight.
observation of the holes from TEM image reveals that reductants are less than 10 nm (Fig. 7d). Fig. 7e and f is the TEM images of huge nanosheet and its one corner at higher resolution respectively. The arrows in Fig. 7e show distinct gaps of imperfect attachment. Two parts of huge nanosheets folded inward indicate an unfinished combination of nanoplates, in other word; the boundaries of nanoplates still exist. Small silver nanoplates are attaching on the edges of huge nanosheet (white arrows). Insight view of the edge is showed in Fig. 7f, from which we can see two potential triangles (A and B) involved in this part, small gap and contact boundary are clearly distinguishable between the potential internal triangular nanoplates, and two stairs (arrows) on the right side of triangle were developing inside out to widen it.
3.5. Raman Silver nanostructure is a well known SERS substrate for sensing. The SERS sensitivity is influenced by the shape and size of the nanostructure, so we used RB (10−5 M) as target molecule to investigate the SERS enhancement of all the Ag nanostructures (Fig. 8). The Raman bands at 218 and 635 cm−1 are associated with Cu2 O [51], and other bands at 618, 1199, 1288, 1363, 1508, 1564, 1596, and 1647 cm−1 , belong to the SERS spectra of RB absorbed on Ag/Cu2 O [16]. It is well known that the SERS signal of pure Ag is not very intense as a weak electromagnetic field it created, and Ag/Cu2 O heterostructures can induce a larger electromagnetic field so that they are able to excite a more intense LSPR. As shown in Fig. 8, nanoparticles (a) had lowest enhancement, so we believe
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[8]
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Fig. 8. Raman spectra of RB absorbed on Ag with AgNO3 concentration of (a) 0.2 mM, (b) 0.5 mM, (c) 0.8 mM, (d) 1 mM.
these nanoparticles away from cubes have few reductants inside. When the concentration of AgNO3 is higher, reductant were dispersive or even insert into nanoplate (unspent reductants particles in nanoplates) to enhance the contact surface with Ag, and the irregular shapes of Ag, can stimulate the electromagnetic enhancement [13]. 4. Conclusions We presented a facile and effective route for the shapecontrolled synthesis of Ag nanostructures by using the electrodeposited Cu nanocubes as reducing and template guiding agent. The concentration and distribution of Ag ions influenced the final shapes of products. The concentration distribution of the Ag ions around Cu cubes determined the sizes and amount of the reductants released from the cubes. These adjustable reductants combined with various concentrations of Ag ions determined the final shapes of silver products. Small reductive NPs were found to be absorbed on the edges of nanoplates and then reduced the Ag ion around them to accelerate the growth of plates. Cuprous oxide is found to participate in the reaction detected by XRD diffraction. The OA as well as OR mechanism are used to illustrate the growth process of these nanostructures. Dislocation loop emission is found during the formation of nanosheets.
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Acknowledgments [26]
This research work was financially supported by the National Natural Science Foundation of China (51272183 and 51572202) and the Nanotechnology Program of Suzhou (ZXG201438).
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