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Single-crystalline silver nanowire arrays directly synthesized onto substrates by template-assisted chemical wetting
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Single-crystalline silver nanowire arrays directly synthesized onto substrates by template-assisted chemical wetting Chenghao Zhang , Chun Li , Xiaoqing Si , Zongjing He , Junlei Qi , Jicai Feng , Jian Cao PII: DOI: Reference:
S2589-1529(19)30325-4 https://doi.org/10.1016/j.mtla.2019.100529 MTLA 100529
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Materialia
Received date: Accepted date:
1 August 2019 11 November 2019
Please cite this article as: Chenghao Zhang , Chun Li , Xiaoqing Si , Zongjing He , Junlei Qi , Jicai Feng , Jian Cao , Single-crystalline silver nanowire arrays directly synthesized onto substrates by template-assisted chemical wetting, Materialia (2019), doi: https://doi.org/10.1016/j.mtla.2019.100529
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Single-crystalline silver nanowire arrays directly synthesized onto substrates by template-assisted chemical wetting Chenghao Zhang, Chun Li, Xiaoqing Si, Zongjing He, Junlei Qi, Jicai Feng, Jian Cao* State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
Abstract: Silver (Ag) nanowire arrays have attracted extensive research attention for their outstanding electrochemical detection sensitivity and optical properties. The most frequently used technology for fabricating Ag nanowire arrays is electrodeposition within the pores of anodic aluminium oxide (AAO) templates. In this paper, single-crystalline Ag nanowire arrays were grown by the capillary wetting of molten Ag inside the nanopores of AAO templates at a temperature of 970°C. To improve the infiltration height for the molten Ag in the capillaries of the AAO templates, copper oxide (CuO) was added as a supplement because it can improve the wetting behaviour between molten Ag and AAO templates. After the Ag nanowire arrays were embedded within the AAO pores, a novel hydrothermal etching method was proposed to remove the AAO templates because AAO showed a phase transition, which made it difficult to remove the AAO after heating at 970°C. To test the properties of the obtained Ag nanowire arrays, the surface-enhanced Raman scattering performance was also studied. Keywords:
Anodic
aluminium
oxide;
*Corresponding author. E-mail address: [email protected] (Jian Cao)
Chemical
1
wetting;
Ag
nanowire
array
1. Introduction One-dimensional nanowires have attracted tremendous interest in many fields because of their novel properties [1, 2]. In particular, Ag nanowires are widely used in numerous areas such as electrochemistry [3] and optics [4] due to their high electrochemical detection sensitivity, excellent conductivity, thermal conductivity and high specific surface area [5-7]. The methods used for preparing Ag nanowires have a significant influence on the applications for different fields [8-10]. One of the most commonly used approaches to fabricate Ag and other nanowire arrays is based on using AAO as a template [11, 12], which contains a large number of macropores with a uniform diameter and depth that is fabricated by anodic oxidization of aluminium foil [13]. Porous AAO can be used as a template for fabricating 1-D nanowires (NWs) and other nanomaterials for applications of modern nanomaterials and nanodevices [14-16]. Single-crystal Ag nanowire arrays have been fabricated by utilizing AAO templates combined with electrodeposition [17]. To fabricate nanowires using AAO templates via electrodeposition, a thin Au film is normally sputtered onto one side of the template and used as an electrode for subsequent electrodeposition [18, 19]. Some researchers have attempted to fabricate an AAO layer directly onto substrates by sputtering an Al layer onto the surface of a substrate followed by anodization [20]. In addition to the electrodeposition method, physical deposition has also been adopted for fabricating metal nano-arrays by evaporating metal onto AAO templates in a vacuum chamber [21]. Luan et al. [22] fabricated vertically standing large-scale Ag nanowire arrays using a special moulding method under pressure (500 MPa) for realization the deformation of Ag. Another commonly used template-assisted method for fabricating microstructures and nanostructures including nanowire arrays is capillary wetting infiltration, which involves
melting the original materials and then allowing them to infiltrate into the templates automatically [23, 24]. For example, Sohinia [25] proposed a facile technique for fabricating Nylon-11 nanowire arrays based on using a capillary template infiltration method within AAO templates. With this method, molten materials can facilely infiltrate into AAO templates and form nanowire arrays. However, to date, few methods for fabricating Ag nanowire arrays based on capillary wetting have been reported. In this study, we report a novel and facile preparation technique for growing singlecrystalline Ag nanowire arrays using a chemical capillary infiltration method within the nanopores of AAO templates, using Ag and CuO as original infiltration materials. Meanwhile, Ag-CuO can form a bonding joint with the target substrates so that the nanowire arrays can be brazed with the target substrates. For removing AAO templates via a phase transition, a hydrothermal etching method with NaOH aqueous solution was carried out in a microform high-pressure autoclave. Finally, the morphology of single-crystalline Ag nanowire arrays and their surface-enhanced Raman scattering (SERS) applications are systematically characterized. 2. Experimental section 2.1 Materials AAO templates with a nominal pore diameter of 40-60 nm were purchased from Shangmu Technology Company. Sodium hydroxide (NaOH), phosphoric acid (H3PO4) and deionized water were purchased from Baida Chemical Reagent Company. Ag powder and CuO powder were purchased from Beijing Dk Nano technology Co. Ltd. Beijing, China. Ag powder and CuO power were milled for 2 h using QM-SB planetary ball mill to prepare Ag-CuO powder mixtures. In the powder mixtures, the mole percentage of CuO ranged from 1% to 16%. Then, the powder mixtures were pressed into sheets using a powder-tableting machine (PT15T, LEAO, China).
2.2 Fabrication of a Ag nanowire-array A schematic for the Ag nanowire arrays growth process via capillary wetting within an AAO template is displayed in Fig. 1. First, Ag-CuO powder mixtures were placed between an αalumina ceramic substrate and AAO template. Upon heating at 970°C for 30 min in air, the Ag-CuO power mixtures melted and infiltrated into the nanopores of the AAO template. After the temperature was decreased to room temperature, Ag-CuO was embedded within the AAO template. Then, the sample was processed by hydrothermal etching with 3 M NaOH aqueous solution in a microform high-pressure autoclave at 250°C for 60 minutes. Finally, the sample was cleaned with NaOH aqueous solution, H3PO4 aqueous solution, deionized water and then alcohol. 2.3 Characterization The microstructure of the as-prepared Ag nanowire arrays was observed by scanning electron microscopy (SEM, HELIOS NanoLab 600i, FEI, America) and transmission electron microscopy (TEM, Talos f200x, FEI, America). The crystal type and phase composition were characterized by X-ray diffraction (XRD, Empyrean Bruker D8 diffractometer) using Cu Kα radiation (λ = 0.15405 nm). DSC (STA449F3(-QMS403D-Is 50), Netzsch, German) was applied to detect the phase transition temperature for the AAO templates. Raman and SERS experiments were performed using a confocal microscope-based Raman spectrometer (LabRAM XploRA) with a laser excitation at a wavelength of 532 nm (1.5 mW), using Rhodamine-6G (R6G) as a probe molecule. Before Raman spectral examination, substrates with Ag nanowire arrays were immersed into varying concentrations of Rhodamine-6G (R6G) aqueous solution for 2 h at room temperature. 3. Results and discussion 3.1 Growth mechanism for the Ag nanowire arrays
First, it is required to discuss the possibility to fabricate Ag nanowire arrays by capillary wetting between AAO and molten Ag-CuO. The theoretically maximum height at which molten Ag can reach within the nanopores of AAO is analysed by Jurin’s law [26] as follows:
h
2 cos gr
(1)
where h is the maximum height, γ is the surface energy of molten Ag, θ is the contact angle of the meniscus at the nanopore walls, ρ is the density of molten Ag, g is the gravitational acceleration and r is the radius of the AAO nanopores. The chemical component of the AAO templates is Al2O3. The contact angle of pure molten Ag on the surface of Al2O3 is greater than 90° [27], which means the calculated maximum height is negative. This reveals that pure molten Ag is not able to infiltrate into AAO nanopores by capillary wetting. Interestingly, the contact angle of molten Ag on the surface of Al2O3 will decrease when CuO is added because the supplementary CuO can decrease the interfacial energy between molten Ag and Al2O3 [28]. The equilibrium contact angle between molten Ag and AAO templates is analysed using the expression [29]:
cos sg ls
(2)
where γsg is the solid surface energy and γls is the solid-liquid interfacial energy. Due to the supplementary CuO, a decrease in γls can lead to a decrease in θ, which means better capillary infiltration of molten Ag within the AAO nanopores. Thus, a mixture of Ag and CuO is a good choice for fabricating Ag nanowire arrays by capillary wetting. Furthermore, Ag-CuO can form a bonding joint with the target substrates so that the nanowire arrays can be brazed with the target substrates. 3.2 Interfacial analysis of nanowire arrays embedded into the AAO template
The morphology and crystal phase of the AAO templates is shown in Fig. 2. Fig. 2a shows that numerous nanopores are uniformly distributed in the AAO. The cross-section image of the AAO templates in Fig. 2b shows a thickness of approximately 65 μm. According to the XRD result for the AAO template shown in Fig. 2c, only several halo amorphous peaks can be observed. After the molten Ag-CuO infiltrated into the nanopores of the AAO templates, the sample was cut and polished to characterize the interface. Fig. 3a presents the cross-sectional SEM image of the AAO/Ag-4%CuO interface following heating at 970°C for 30 min in air. The AAO layer is well bonded to the Ag layer and the interface between AAO and Ag layer is flat. In addition, a large number of short white lines can be observed at the bottom of the AAO layer. From the EDS analysis shown in Fig. 3b corresponding to the boxed region A in Fig. 3a, it can be determined that elemental Al and O make a major contribution to the content and that the content of elemental Ag and Cu is 8.99% and 2.99%, respectively, which proves that Ag-CuO infiltrates into the AAO template. Fig. 3c illustrates the cross-sectional line scan for the Ag element and Al element corresponding to the yellow vertical line in Fig. 3a. In Fig. 3c, the black and red lines show the Al and Ag element distributions, respectively. A transitional region exists for which the content of elemental Al decreases while the content of elemental Ag increases, corresponding to the region of white lines in Fig. 3a, which illustrates the infiltration of the Ag element. For a more detailed analysis of Ag-CuO infiltration within AAO templates, the sample interface observed in Fig. 3a was cut by a focused ion beam (FIB) and then analysed by TEM. Fig. 4 shows a HAADF-STEM image of the interface and its corresponding elemental distribution maps. In Fig. 4a, black strips and white strips are parallel and alternately arranged in one direction. A wavy-like interface between the black and white strips can be clearly distinguished, agreeing with the surface morphology of the AAO inner walls. From
the elemental distribution maps in Fig. 4b)-d), it can be observed that the major element in the white strips is Ag, confirming that Ag infiltrates into the nanopores of the AAO templates. In addition, the major elements in the black strips are Al and O, confirming that the AAO template maintains its original morphology and chemical composition, Al2O3. Fig. 5a displays a bright-field TEM image corresponding to the STEM image in Fig. 4a. The highlighted circle area in Fig. 5a was further analysed by a high-resolution transmission electron microscope (HRTEM), as shown in Fig. 5b, in which the distance of the crystal planes was found to be 0.143 nm, agreeing with the Ag lattice parameters. The insert image shows the corresponding fast Fourier transform (FFT) pattern, which corresponds to Ag (220) from the [111] direction. Fig. 5c shows another HRTEM image of the highlighted area in Fig. 5a after changing the position and direction of the sample. Through a similar analysis process, Ag with a zone axis of [112] can be distinguished clearly. Therefore, the HRTEM image proves that Ag infiltrates into the nanopores of AAO and forms single crystal Ag nanowires embedded into the pores. 3.3 Effect of CuO content on the height of the nanowire arrays CuO was added to improve the capillary wetting between molten Ag and AAO templates. The effect of CuO content on the infiltration height of molten Ag in the capillaries of the AAO templates was studied. Fig. 6 presents cross-sectional SEM images of the nanowires embedded into AAO using Ag-CuO powders with varying CuO content. The heating temperature and holding time were 970°C and 30 min, respectively. Fig. 6a shows a crosssectional SEM image of the sample without CuO addition, in which white lines do not appear in the AAO part. This outcome confirms that pure molten Ag is not able to infiltrate into the AAO nanopores, in agreement with the theoretical calculation result in Section 3.1. As shown in Fig. 6b-6f, the height of the white lines increases gradually when the content of CuO increases from 1% to 16%, indicating that the height of the Ag nanowires increases as CuO
content increases. This outcome occurs because the increasing CuO content can improve the wetting behaviour between the molten Ag and AAO template [28], leading to an increase in the infiltration height in the capillaries of the AAO template. However, cracks can be observed in the AAO template when the CuO content reaches 16%, as shown in Fig. 6f. The white Ag within the cracks shows that the cracking processes occur when Ag-CuO is in liquid form during the high-temperature stage, which means there are no restrictions or stress between the AAO and molten Ag-CuO. It is known that Al2O3 can react with CuO to give CuAlO2 or CuAl2O4 [30]. Thus, the possible reason for the formation of cracks is that a part of the AAO reacts with the CuO to form copper aluminium oxide when the molten Ag-CuO infiltrates into the nanopores of the AAO template. The copper aluminium oxide has a different density with the remaining AAO, leading to a volume change and non-uniform internal stress in the AAO template. Therefore, cracks occur when the stress exceeds the strength limit. 3.4 AAO templates removed by hydrothermal etching To obtain released Ag nanowire arrays, AAO templates are required to be removed after the Ag nanowire arrays are embedded within the AAO templates. AAO templates are currently removed by using phosphoric acid or NaOH aqueous solution under atmospheric pressure. However, AAO templates undergo a phase transition after heating, and it becomes difficult to remove such templates once heated to a temperature of over 800 °C [31, 32]. To analyse the phase transition for the AAO templates, a DSC test was carried out from room temperature to 1100°C, as shown in Fig. 7a. In the DSC result, there is a turning point at a temperature of 893°C, which means that a phase transition occurs before heating to 970°C. The XRD result for AAO after heating at 970°C for 30 min is shown in Fig. 7b. Comparing this result with the XRD result shown in Fig. 2c, several peaks corresponding to η-Al2O3 [33-
35] are observed to appear, which means AAO shows a phase transition from amorphous Al2O3 to η-Al2O3 after heating. For a more detailed analysis of the AAO phase transition, selected-area electron diffraction (SAED) was used to characterize the lattice structure. The patterns taken from the AAO after heating are shown in Fig. 7c. The diffraction spots confirm that after heating the AAO is polycrystalline. These spots can be divided into several layers from the inside to the outside, and each layer corresponds to an interplanar distance. Through a comparison of the measurement results (R1=0.4516 nm, R2=0.2758 nm, R3=0.2348 nm, R4=0.1984 nm, R5=0.1643 nm, R6= 0.1514 nm, R7=0.1392 nm) with the data in PDF#770396, these layers are found to agree with the lattice parameters for η-Al2O3. A HF aqueous solution can be applied to remove an AAO template with a phase transition when AAO is used for the fabrication of carbon-based materials because carbon-based materials can resist the corrosion of HF aqueous solution [36, 37]. However, the use of HF aqueous solution is not feasible in the AAO-assisted fabrication of metal nanomaterials because HF aqueous solution is strongly corrosive, which means that it will damage metal nanomaterials including Ag nanowires. To the best of our knowledge, without silver damage, no available methods for removing AAO templates with a phase transition have been proposed. Therefore, a new method with stronger etching properties without Ag damage is required to be developed. It is known that if an alkaline aqueous solution is heated in a sealed container, the air-pressure can be improved to raise the boiling temperature and enhance the corrosive properties. Thus, we proposed a hydrothermal etching method that used NaOH aqueous solution to remove AAO with a phase transition, which was carried out in a microform high-pressure autoclave at 250°C for 60 minutes. Afterward, the sample was cleaned with NaOH aqueous solution, H3PO4 aqueous solution, deionized water and then alcohol.
Fig. 8 displays the Ag nanowire arrays obtained after removing the AAO templates. Fig. 8a, Fig. 8b and Fig. 8c show the Ag nanowire arrays fabricated with Ag- 1%CuO, Ag4%CuO and Ag-CuO 8%, respectively. It is clear that the length of the Ag nanowires fabricated by the molten Ag-CuO capillaries correspondingly increase with a gradual increase in CuO content, which agrees with the results in Fig. 6. In addition, the aggregation phenomenon becomes gradually more obvious. High aspect ratio metal nanowire arrays normally aggregate to form bundles or disordered mats after the AAO templates are removed [38]. This aggregation phenomenon can contribute to the forces between two nanowires, including short-range van der Waals forces and the tension arising from perturbations in the drying process [39, 40]. When the sum of several forces overwhelms the maximum force that the nanowires can withstand, an agglomeration effect will occur. If the van der Waals force is neglected, the force between two nanowires [41, 42] can be expressed as: Fst 2R 2 cos2
1 d(4 R d )
(3)
where γ is the liquid-air surface tension, R is the radius of the nanowires, d is the separation distance between nanowires, and θ is the meniscus contact angle between the liquid and nanowires. The relationship between the tension force and the deflection of the nanowires is:
Felas
3 ER 4 4 L3
(4)
where E, L and δ are the Young’s modulus and the length and deflection of the Ag nanowires, respectively. When the deflection of the nanowires is larger than the distance between two nanowires, they will bend and contact. The detailed analysing process is shown in supplemental document. An increase in length L will lead to a decrease in Felas . Thus, aggregation phenomena become increasingly obvious, as shown from Fig. 8a to Fig. 8c. Fig.
8d shows a magnified SEM image of Fig. 8b, in which the surfaces of the Ag nanowires are observed to be worm-like. The Ag nanowire arrays shown in Fig. 8b were scraped with a blade and then transferred to the surface of a copper mesh for further TEM observation. The TEM image in Fig. 8e shows that the diameter of the Ag nanowire is approximately 50 nm, which agrees with the diameter of the nanopores in the AAO templates. From the TEM image, the Ag nanowire has an obvious worm-like surface, which is determined by the surface of the AAO nanopore inner walls, as shown in Fig. 4a and Fig. 5a. Ag nanowires and other metal nanomaterials synthesized by an electrodeposition method assisted by an AAO template also show a similar worm-like surface [17]. The SAED pattern shown in Fig. 8f corresponds to Ag (220) from the [111] direction, which agrees with the pattern in Fig. 5b, confirming that single-crystalline Ag nanowire arrays were successfully synthesized with this method.
3.5 SERS properties of the Ag nanowire arrays Fig. 9a displays the SERS spectra measured for R6G molecules at randomly selected positions on the surfaces of Ag nanowire arrays fabricated with varying CuO content. As a reference, the Raman spectra for a flat Ag foil immersed in 10-7 M R6G aqueous solution were first detected; however, the signature bands for R6G were not clear. Obvious peaks appear in the spectra measured for the surfaces with Ag nanowire arrays. It is evident that the Ag nanowire arrays fabricated with Ag-1%CuO exhibit the highest enhancement efficiency. When the content of CuO increases, the intensities of the SERS signal peaks for the corresponding Ag nanowire arrays become weaker. This outcome can be attributed to the fact that the increasing CuO content gives rise to longer Ag nanowires, leading to increased aggregation, which probably decreases the Raman signal intensities. Additionally, residual CuO may be another affecting factor contributing to the weaker SERS properties. The SERS performance for varying R6G concentrations present on Ag nanowire arrays fabricated with Ag-1%CuO is shown in Fig. 9b. Raman peaks can be clearly distinguished when the concentration of the R6G aqueous solution is 10-11M, which proves that the Ag nanowire arrays fabricated by capillary wetting can be applied as high sensitivity substrates for SERSbased measurements. Conclusions A novel method was successfully developed for fabricating single-crystalline Ag nanowire arrays based on the use of an AAO template and chemical capillary wetting between molten Ag-CuO and AAO. The interface for Ag infiltration into AAO templates was carefully characterized. A mixture of Ag and CuO can infiltrate into the nanopores of AAO templates based on capillary wetting at 970 °C. SEM and TEM analysis revealed that Ag infiltrated into the nanopores of AAO templates and formed embedded single-crystalline Ag nanowires. The 12
effect of CuO content on the capillary wetting and infiltrating height was systematically investigated. Ag nanowire arrays with different lengths were successfully obtained through changing the ratio of CuO. The addition of CuO was proved to improve the wetting behaviour between the molten Ag and AAO templates, leading to Ag infiltration. As the CuO content increases, the infiltration height increases. When the CuO content reaches 16%, cracks occur in the AAO, which reveals that the addition of CuO is required to be maintained in a certain range. Due to the phase transition in AAO during heat processing, a novel hydrothermal etching method with high pressure was proposed. Use of a NaOH aqueous solution at 250°C in a microform high-pressure autoclave was proved to be effective for removing AAO without destroying the Ag nanowire arrays. Single-crystalline Ag nanowire arrays fabricated with this method show potential for use as substrates for SERS measurements. After the Ag nanowire arrays were immersed into a R6G solution with concentrations ranging from 10-7 M to 10-11M, characteristic peaks due to R6G were clearly observed. Conflicts of interest There are no relevant conflicts to declare. Acknowledgement The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China under Grant Nos. 51622503, 51805114 and U1737205. References [1] C. R. Martin, Science 266 (1994) 1961-1966. [2] M. Park, W. Kim, B. Hwang, S. M. Han, Scripta Mater. 161 (2019) 70-73. [3] Z. Qiao, X. Yang, S. Yang, L. Zhang, B. Cao, Chem. Commun., 52 (2016) 7998-8001.
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Figure 1 Scheme of the growth procedure of Ag nanowire arrays within AAO template.
Figure 2 Characterization of the AAO template. (a) SEM image of AAO pores, (b) optical microscope image of AAO cross-section and (c) XRD result of AAO.
Figure 3. Characterization of the AAO/Ag-CuO interface. (a) BSE image of crossing section, (b) EDS result of region A in Fig.3a and (c) EDS line scan analysis.
Figure 4. STEM observation and elemental distribution maps of nanowire arrays. (a) STEM image, (b) Ag element, (c) Al element and (d) O element.
Figure 5. TEM observation of nanowire arrays embedded within an AAO template. (a) Bright-field TEM image, (b) HRTEM image and its FFT from [111 ] direction and (c) HRTEM image and its FFT from [112] direction.
Figure 6. SEM images of nanowire-arrays embedded in AAO at different CuO content. (a) Ag, (b) Ag-1%CuO, (c) Ag-2%CuO, (d) Ag-4%CuO, (e) Ag-8%CuO and (f) Ag-16%CuO.
Figure 7. Characterization of the AAO after heating. (a) DSC of AAO from 20°C to 1100°C, (b) XRD analysis of AAO after heating and (c) SAED patterns of AAO after heating.
Figure 8. Characterization of the Ag nanowire arrays. (a), (b), (c) SEM of Ag nanowire arrays with Ag-1%CuO, Ag-4%CuO and Ag-8%CuO, respectively, (d) magnified image of Fig.8b, (e) TEM of an Ag nanowire and (f) SAED of an Ag nanowire.
Figure 9. SERS spectra of R6G adsorbed. (a) On different Ag nanowire arrays with 10 -7M R6G and (b) with different concentrations of R6G.
Graphical Abstract
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Single-crystalline silver nanowire arrays directly synthesized onto substrates by template-assisted chemical wetting Chenghao Zhang , Chun Li , Xiaoqing Si , Zongjing He , Junlei Qi , Jicai Feng , Jian Cao PII: DOI: Reference:
S2589-1529(19)30325-4 https://doi.org/10.1016/j.mtla.2019.100529 MTLA 100529
To appear in:
Materialia
Received date: Accepted date:
1 August 2019 11 November 2019
Please cite this article as: Chenghao Zhang , Chun Li , Xiaoqing Si , Zongjing He , Junlei Qi , Jicai Feng , Jian Cao , Single-crystalline silver nanowire arrays directly synthesized onto substrates by template-assisted chemical wetting, Materialia (2019), doi: https://doi.org/10.1016/j.mtla.2019.100529
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Single-crystalline silver nanowire arrays directly synthesized onto substrates by template-assisted chemical wetting Chenghao Zhang, Chun Li, Xiaoqing Si, Zongjing He, Junlei Qi, Jicai Feng, Jian Cao* State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
Abstract: Silver (Ag) nanowire arrays have attracted extensive research attention for their outstanding electrochemical detection sensitivity and optical properties. The most frequently used technology for fabricating Ag nanowire arrays is electrodeposition within the pores of anodic aluminium oxide (AAO) templates. In this paper, single-crystalline Ag nanowire arrays were grown by the capillary wetting of molten Ag inside the nanopores of AAO templates at a temperature of 970°C. To improve the infiltration height for the molten Ag in the capillaries of the AAO templates, copper oxide (CuO) was added as a supplement because it can improve the wetting behaviour between molten Ag and AAO templates. After the Ag nanowire arrays were embedded within the AAO pores, a novel hydrothermal etching method was proposed to remove the AAO templates because AAO showed a phase transition, which made it difficult to remove the AAO after heating at 970°C. To test the properties of the obtained Ag nanowire arrays, the surface-enhanced Raman scattering performance was also studied. Keywords:
Anodic
aluminium
oxide;
*Corresponding author. E-mail address: [email protected] (Jian Cao)
Chemical
1
wetting;
Ag
nanowire
array
1. Introduction One-dimensional nanowires have attracted tremendous interest in many fields because of their novel properties [1, 2]. In particular, Ag nanowires are widely used in numerous areas such as electrochemistry [3] and optics [4] due to their high electrochemical detection sensitivity, excellent conductivity, thermal conductivity and high specific surface area [5-7]. The methods used for preparing Ag nanowires have a significant influence on the applications for different fields [8-10]. One of the most commonly used approaches to fabricate Ag and other nanowire arrays is based on using AAO as a template [11, 12], which contains a large number of macropores with a uniform diameter and depth that is fabricated by anodic oxidization of aluminium foil [13]. Porous AAO can be used as a template for fabricating 1-D nanowires (NWs) and other nanomaterials for applications of modern nanomaterials and nanodevices [14-16]. Single-crystal Ag nanowire arrays have been fabricated by utilizing AAO templates combined with electrodeposition [17]. To fabricate nanowires using AAO templates via electrodeposition, a thin Au film is normally sputtered onto one side of the template and used as an electrode for subsequent electrodeposition [18, 19]. Some researchers have attempted to fabricate an AAO layer directly onto substrates by sputtering an Al layer onto the surface of a substrate followed by anodization [20]. In addition to the electrodeposition method, physical deposition has also been adopted for fabricating metal nano-arrays by evaporating metal onto AAO templates in a vacuum chamber [21]. Luan et al. [22] fabricated vertically standing large-scale Ag nanowire arrays using a special moulding method under pressure (500 MPa) for realization the deformation of Ag. Another commonly used template-assisted method for fabricating microstructures and nanostructures including nanowire arrays is capillary wetting infiltration, which involves
melting the original materials and then allowing them to infiltrate into the templates automatically [23, 24]. For example, Sohinia [25] proposed a facile technique for fabricating Nylon-11 nanowire arrays based on using a capillary template infiltration method within AAO templates. With this method, molten materials can facilely infiltrate into AAO templates and form nanowire arrays. However, to date, few methods for fabricating Ag nanowire arrays based on capillary wetting have been reported. In this study, we report a novel and facile preparation technique for growing singlecrystalline Ag nanowire arrays using a chemical capillary infiltration method within the nanopores of AAO templates, using Ag and CuO as original infiltration materials. Meanwhile, Ag-CuO can form a bonding joint with the target substrates so that the nanowire arrays can be brazed with the target substrates. For removing AAO templates via a phase transition, a hydrothermal etching method with NaOH aqueous solution was carried out in a microform high-pressure autoclave. Finally, the morphology of single-crystalline Ag nanowire arrays and their surface-enhanced Raman scattering (SERS) applications are systematically characterized. 2. Experimental section 2.1 Materials AAO templates with a nominal pore diameter of 40-60 nm were purchased from Shangmu Technology Company. Sodium hydroxide (NaOH), phosphoric acid (H3PO4) and deionized water were purchased from Baida Chemical Reagent Company. Ag powder and CuO powder were purchased from Beijing Dk Nano technology Co. Ltd. Beijing, China. Ag powder and CuO power were milled for 2 h using QM-SB planetary ball mill to prepare Ag-CuO powder mixtures. In the powder mixtures, the mole percentage of CuO ranged from 1% to 16%. Then, the powder mixtures were pressed into sheets using a powder-tableting machine (PT15T, LEAO, China).
2.2 Fabrication of a Ag nanowire-array A schematic for the Ag nanowire arrays growth process via capillary wetting within an AAO template is displayed in Fig. 1. First, Ag-CuO powder mixtures were placed between an αalumina ceramic substrate and AAO template. Upon heating at 970°C for 30 min in air, the Ag-CuO power mixtures melted and infiltrated into the nanopores of the AAO template. After the temperature was decreased to room temperature, Ag-CuO was embedded within the AAO template. Then, the sample was processed by hydrothermal etching with 3 M NaOH aqueous solution in a microform high-pressure autoclave at 250°C for 60 minutes. Finally, the sample was cleaned with NaOH aqueous solution, H3PO4 aqueous solution, deionized water and then alcohol. 2.3 Characterization The microstructure of the as-prepared Ag nanowire arrays was observed by scanning electron microscopy (SEM, HELIOS NanoLab 600i, FEI, America) and transmission electron microscopy (TEM, Talos f200x, FEI, America). The crystal type and phase composition were characterized by X-ray diffraction (XRD, Empyrean Bruker D8 diffractometer) using Cu Kα radiation (λ = 0.15405 nm). DSC (STA449F3(-QMS403D-Is 50), Netzsch, German) was applied to detect the phase transition temperature for the AAO templates. Raman and SERS experiments were performed using a confocal microscope-based Raman spectrometer (LabRAM XploRA) with a laser excitation at a wavelength of 532 nm (1.5 mW), using Rhodamine-6G (R6G) as a probe molecule. Before Raman spectral examination, substrates with Ag nanowire arrays were immersed into varying concentrations of Rhodamine-6G (R6G) aqueous solution for 2 h at room temperature. 3. Results and discussion 3.1 Growth mechanism for the Ag nanowire arrays
First, it is required to discuss the possibility to fabricate Ag nanowire arrays by capillary wetting between AAO and molten Ag-CuO. The theoretically maximum height at which molten Ag can reach within the nanopores of AAO is analysed by Jurin’s law [26] as follows:
h
2 cos gr
(1)
where h is the maximum height, γ is the surface energy of molten Ag, θ is the contact angle of the meniscus at the nanopore walls, ρ is the density of molten Ag, g is the gravitational acceleration and r is the radius of the AAO nanopores. The chemical component of the AAO templates is Al2O3. The contact angle of pure molten Ag on the surface of Al2O3 is greater than 90° [27], which means the calculated maximum height is negative. This reveals that pure molten Ag is not able to infiltrate into AAO nanopores by capillary wetting. Interestingly, the contact angle of molten Ag on the surface of Al2O3 will decrease when CuO is added because the supplementary CuO can decrease the interfacial energy between molten Ag and Al2O3 [28]. The equilibrium contact angle between molten Ag and AAO templates is analysed using the expression [29]:
cos sg ls
(2)
where γsg is the solid surface energy and γls is the solid-liquid interfacial energy. Due to the supplementary CuO, a decrease in γls can lead to a decrease in θ, which means better capillary infiltration of molten Ag within the AAO nanopores. Thus, a mixture of Ag and CuO is a good choice for fabricating Ag nanowire arrays by capillary wetting. Furthermore, Ag-CuO can form a bonding joint with the target substrates so that the nanowire arrays can be brazed with the target substrates. 3.2 Interfacial analysis of nanowire arrays embedded into the AAO template
The morphology and crystal phase of the AAO templates is shown in Fig. 2. Fig. 2a shows that numerous nanopores are uniformly distributed in the AAO. The cross-section image of the AAO templates in Fig. 2b shows a thickness of approximately 65 μm. According to the XRD result for the AAO template shown in Fig. 2c, only several halo amorphous peaks can be observed. After the molten Ag-CuO infiltrated into the nanopores of the AAO templates, the sample was cut and polished to characterize the interface. Fig. 3a presents the cross-sectional SEM image of the AAO/Ag-4%CuO interface following heating at 970°C for 30 min in air. The AAO layer is well bonded to the Ag layer and the interface between AAO and Ag layer is flat. In addition, a large number of short white lines can be observed at the bottom of the AAO layer. From the EDS analysis shown in Fig. 3b corresponding to the boxed region A in Fig. 3a, it can be determined that elemental Al and O make a major contribution to the content and that the content of elemental Ag and Cu is 8.99% and 2.99%, respectively, which proves that Ag-CuO infiltrates into the AAO template. Fig. 3c illustrates the cross-sectional line scan for the Ag element and Al element corresponding to the yellow vertical line in Fig. 3a. In Fig. 3c, the black and red lines show the Al and Ag element distributions, respectively. A transitional region exists for which the content of elemental Al decreases while the content of elemental Ag increases, corresponding to the region of white lines in Fig. 3a, which illustrates the infiltration of the Ag element. For a more detailed analysis of Ag-CuO infiltration within AAO templates, the sample interface observed in Fig. 3a was cut by a focused ion beam (FIB) and then analysed by TEM. Fig. 4 shows a HAADF-STEM image of the interface and its corresponding elemental distribution maps. In Fig. 4a, black strips and white strips are parallel and alternately arranged in one direction. A wavy-like interface between the black and white strips can be clearly distinguished, agreeing with the surface morphology of the AAO inner walls. From
the elemental distribution maps in Fig. 4b)-d), it can be observed that the major element in the white strips is Ag, confirming that Ag infiltrates into the nanopores of the AAO templates. In addition, the major elements in the black strips are Al and O, confirming that the AAO template maintains its original morphology and chemical composition, Al2O3. Fig. 5a displays a bright-field TEM image corresponding to the STEM image in Fig. 4a. The highlighted circle area in Fig. 5a was further analysed by a high-resolution transmission electron microscope (HRTEM), as shown in Fig. 5b, in which the distance of the crystal planes was found to be 0.143 nm, agreeing with the Ag lattice parameters. The insert image shows the corresponding fast Fourier transform (FFT) pattern, which corresponds to Ag (220) from the [111] direction. Fig. 5c shows another HRTEM image of the highlighted area in Fig. 5a after changing the position and direction of the sample. Through a similar analysis process, Ag with a zone axis of [112] can be distinguished clearly. Therefore, the HRTEM image proves that Ag infiltrates into the nanopores of AAO and forms single crystal Ag nanowires embedded into the pores. 3.3 Effect of CuO content on the height of the nanowire arrays CuO was added to improve the capillary wetting between molten Ag and AAO templates. The effect of CuO content on the infiltration height of molten Ag in the capillaries of the AAO templates was studied. Fig. 6 presents cross-sectional SEM images of the nanowires embedded into AAO using Ag-CuO powders with varying CuO content. The heating temperature and holding time were 970°C and 30 min, respectively. Fig. 6a shows a crosssectional SEM image of the sample without CuO addition, in which white lines do not appear in the AAO part. This outcome confirms that pure molten Ag is not able to infiltrate into the AAO nanopores, in agreement with the theoretical calculation result in Section 3.1. As shown in Fig. 6b-6f, the height of the white lines increases gradually when the content of CuO increases from 1% to 16%, indicating that the height of the Ag nanowires increases as CuO
content increases. This outcome occurs because the increasing CuO content can improve the wetting behaviour between the molten Ag and AAO template [28], leading to an increase in the infiltration height in the capillaries of the AAO template. However, cracks can be observed in the AAO template when the CuO content reaches 16%, as shown in Fig. 6f. The white Ag within the cracks shows that the cracking processes occur when Ag-CuO is in liquid form during the high-temperature stage, which means there are no restrictions or stress between the AAO and molten Ag-CuO. It is known that Al2O3 can react with CuO to give CuAlO2 or CuAl2O4 [30]. Thus, the possible reason for the formation of cracks is that a part of the AAO reacts with the CuO to form copper aluminium oxide when the molten Ag-CuO infiltrates into the nanopores of the AAO template. The copper aluminium oxide has a different density with the remaining AAO, leading to a volume change and non-uniform internal stress in the AAO template. Therefore, cracks occur when the stress exceeds the strength limit. 3.4 AAO templates removed by hydrothermal etching To obtain released Ag nanowire arrays, AAO templates are required to be removed after the Ag nanowire arrays are embedded within the AAO templates. AAO templates are currently removed by using phosphoric acid or NaOH aqueous solution under atmospheric pressure. However, AAO templates undergo a phase transition after heating, and it becomes difficult to remove such templates once heated to a temperature of over 800 °C [31, 32]. To analyse the phase transition for the AAO templates, a DSC test was carried out from room temperature to 1100°C, as shown in Fig. 7a. In the DSC result, there is a turning point at a temperature of 893°C, which means that a phase transition occurs before heating to 970°C. The XRD result for AAO after heating at 970°C for 30 min is shown in Fig. 7b. Comparing this result with the XRD result shown in Fig. 2c, several peaks corresponding to η-Al2O3 [33-
35] are observed to appear, which means AAO shows a phase transition from amorphous Al2O3 to η-Al2O3 after heating. For a more detailed analysis of the AAO phase transition, selected-area electron diffraction (SAED) was used to characterize the lattice structure. The patterns taken from the AAO after heating are shown in Fig. 7c. The diffraction spots confirm that after heating the AAO is polycrystalline. These spots can be divided into several layers from the inside to the outside, and each layer corresponds to an interplanar distance. Through a comparison of the measurement results (R1=0.4516 nm, R2=0.2758 nm, R3=0.2348 nm, R4=0.1984 nm, R5=0.1643 nm, R6= 0.1514 nm, R7=0.1392 nm) with the data in PDF#770396, these layers are found to agree with the lattice parameters for η-Al2O3. A HF aqueous solution can be applied to remove an AAO template with a phase transition when AAO is used for the fabrication of carbon-based materials because carbon-based materials can resist the corrosion of HF aqueous solution [36, 37]. However, the use of HF aqueous solution is not feasible in the AAO-assisted fabrication of metal nanomaterials because HF aqueous solution is strongly corrosive, which means that it will damage metal nanomaterials including Ag nanowires. To the best of our knowledge, without silver damage, no available methods for removing AAO templates with a phase transition have been proposed. Therefore, a new method with stronger etching properties without Ag damage is required to be developed. It is known that if an alkaline aqueous solution is heated in a sealed container, the air-pressure can be improved to raise the boiling temperature and enhance the corrosive properties. Thus, we proposed a hydrothermal etching method that used NaOH aqueous solution to remove AAO with a phase transition, which was carried out in a microform high-pressure autoclave at 250°C for 60 minutes. Afterward, the sample was cleaned with NaOH aqueous solution, H3PO4 aqueous solution, deionized water and then alcohol.
Fig. 8 displays the Ag nanowire arrays obtained after removing the AAO templates. Fig. 8a, Fig. 8b and Fig. 8c show the Ag nanowire arrays fabricated with Ag- 1%CuO, Ag4%CuO and Ag-CuO 8%, respectively. It is clear that the length of the Ag nanowires fabricated by the molten Ag-CuO capillaries correspondingly increase with a gradual increase in CuO content, which agrees with the results in Fig. 6. In addition, the aggregation phenomenon becomes gradually more obvious. High aspect ratio metal nanowire arrays normally aggregate to form bundles or disordered mats after the AAO templates are removed [38]. This aggregation phenomenon can contribute to the forces between two nanowires, including short-range van der Waals forces and the tension arising from perturbations in the drying process [39, 40]. When the sum of several forces overwhelms the maximum force that the nanowires can withstand, an agglomeration effect will occur. If the van der Waals force is neglected, the force between two nanowires [41, 42] can be expressed as: Fst 2R 2 cos2
1 d(4 R d )
(3)
where γ is the liquid-air surface tension, R is the radius of the nanowires, d is the separation distance between nanowires, and θ is the meniscus contact angle between the liquid and nanowires. The relationship between the tension force and the deflection of the nanowires is:
Felas
3 ER 4 4 L3
(4)
where E, L and δ are the Young’s modulus and the length and deflection of the Ag nanowires, respectively. When the deflection of the nanowires is larger than the distance between two nanowires, they will bend and contact. The detailed analysing process is shown in supplemental document. An increase in length L will lead to a decrease in Felas . Thus, aggregation phenomena become increasingly obvious, as shown from Fig. 8a to Fig. 8c. Fig.
8d shows a magnified SEM image of Fig. 8b, in which the surfaces of the Ag nanowires are observed to be worm-like. The Ag nanowire arrays shown in Fig. 8b were scraped with a blade and then transferred to the surface of a copper mesh for further TEM observation. The TEM image in Fig. 8e shows that the diameter of the Ag nanowire is approximately 50 nm, which agrees with the diameter of the nanopores in the AAO templates. From the TEM image, the Ag nanowire has an obvious worm-like surface, which is determined by the surface of the AAO nanopore inner walls, as shown in Fig. 4a and Fig. 5a. Ag nanowires and other metal nanomaterials synthesized by an electrodeposition method assisted by an AAO template also show a similar worm-like surface [17]. The SAED pattern shown in Fig. 8f corresponds to Ag (220) from the [111] direction, which agrees with the pattern in Fig. 5b, confirming that single-crystalline Ag nanowire arrays were successfully synthesized with this method.
3.5 SERS properties of the Ag nanowire arrays Fig. 9a displays the SERS spectra measured for R6G molecules at randomly selected positions on the surfaces of Ag nanowire arrays fabricated with varying CuO content. As a reference, the Raman spectra for a flat Ag foil immersed in 10-7 M R6G aqueous solution were first detected; however, the signature bands for R6G were not clear. Obvious peaks appear in the spectra measured for the surfaces with Ag nanowire arrays. It is evident that the Ag nanowire arrays fabricated with Ag-1%CuO exhibit the highest enhancement efficiency. When the content of CuO increases, the intensities of the SERS signal peaks for the corresponding Ag nanowire arrays become weaker. This outcome can be attributed to the fact that the increasing CuO content gives rise to longer Ag nanowires, leading to increased aggregation, which probably decreases the Raman signal intensities. Additionally, residual CuO may be another affecting factor contributing to the weaker SERS properties. The SERS performance for varying R6G concentrations present on Ag nanowire arrays fabricated with Ag-1%CuO is shown in Fig. 9b. Raman peaks can be clearly distinguished when the concentration of the R6G aqueous solution is 10-11M, which proves that the Ag nanowire arrays fabricated by capillary wetting can be applied as high sensitivity substrates for SERSbased measurements. Conclusions A novel method was successfully developed for fabricating single-crystalline Ag nanowire arrays based on the use of an AAO template and chemical capillary wetting between molten Ag-CuO and AAO. The interface for Ag infiltration into AAO templates was carefully characterized. A mixture of Ag and CuO can infiltrate into the nanopores of AAO templates based on capillary wetting at 970 °C. SEM and TEM analysis revealed that Ag infiltrated into the nanopores of AAO templates and formed embedded single-crystalline Ag nanowires. The 12
effect of CuO content on the capillary wetting and infiltrating height was systematically investigated. Ag nanowire arrays with different lengths were successfully obtained through changing the ratio of CuO. The addition of CuO was proved to improve the wetting behaviour between the molten Ag and AAO templates, leading to Ag infiltration. As the CuO content increases, the infiltration height increases. When the CuO content reaches 16%, cracks occur in the AAO, which reveals that the addition of CuO is required to be maintained in a certain range. Due to the phase transition in AAO during heat processing, a novel hydrothermal etching method with high pressure was proposed. Use of a NaOH aqueous solution at 250°C in a microform high-pressure autoclave was proved to be effective for removing AAO without destroying the Ag nanowire arrays. Single-crystalline Ag nanowire arrays fabricated with this method show potential for use as substrates for SERS measurements. After the Ag nanowire arrays were immersed into a R6G solution with concentrations ranging from 10-7 M to 10-11M, characteristic peaks due to R6G were clearly observed. Conflicts of interest There are no relevant conflicts to declare. Acknowledgement The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China under Grant Nos. 51622503, 51805114 and U1737205. References [1] C. R. Martin, Science 266 (1994) 1961-1966. [2] M. Park, W. Kim, B. Hwang, S. M. Han, Scripta Mater. 161 (2019) 70-73. [3] Z. Qiao, X. Yang, S. Yang, L. Zhang, B. Cao, Chem. Commun., 52 (2016) 7998-8001.
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Figure 1 Scheme of the growth procedure of Ag nanowire arrays within AAO template.
Figure 2 Characterization of the AAO template. (a) SEM image of AAO pores, (b) optical microscope image of AAO cross-section and (c) XRD result of AAO.
Figure 3. Characterization of the AAO/Ag-CuO interface. (a) BSE image of crossing section, (b) EDS result of region A in Fig.3a and (c) EDS line scan analysis.
Figure 4. STEM observation and elemental distribution maps of nanowire arrays. (a) STEM image, (b) Ag element, (c) Al element and (d) O element.
Figure 5. TEM observation of nanowire arrays embedded within an AAO template. (a) Bright-field TEM image, (b) HRTEM image and its FFT from [111 ] direction and (c) HRTEM image and its FFT from [112] direction.
Figure 6. SEM images of nanowire-arrays embedded in AAO at different CuO content. (a) Ag, (b) Ag-1%CuO, (c) Ag-2%CuO, (d) Ag-4%CuO, (e) Ag-8%CuO and (f) Ag-16%CuO.
Figure 7. Characterization of the AAO after heating. (a) DSC of AAO from 20°C to 1100°C, (b) XRD analysis of AAO after heating and (c) SAED patterns of AAO after heating.
Figure 8. Characterization of the Ag nanowire arrays. (a), (b), (c) SEM of Ag nanowire arrays with Ag-1%CuO, Ag-4%CuO and Ag-8%CuO, respectively, (d) magnified image of Fig.8b, (e) TEM of an Ag nanowire and (f) SAED of an Ag nanowire.
Figure 9. SERS spectra of R6G adsorbed. (a) On different Ag nanowire arrays with 10 -7M R6G and (b) with different concentrations of R6G.
Graphical Abstract
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: