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Abstract
Optical and mechanical properties of size-controlled Cu particles (1.5 µm, 500 nm and 50 nm) fabricated by one-step electrodeposition were studied. First, surface morphology and composition were characterized by SEM and EDS, with crystal structure by TEM, SAED and XRD. Antioxidant ability of 50nm was verified by TGA. In the simple and novel synthesis process, Cu particles of 1.5 µm with polyhedron morphology were firstly synthesized. The increase of current density and addition of potassium ferrocyanide trihydrate played key roles in the grain refinement to 500 nm and 50 nm, respectively. Then, particular focus was given to the improvement of optical and mechanical properties with size reduction, by SERS, UV-Vis and nanoindentation. These properties were gradually enhanced with the decrease of particle size, and Cu particles of 50 nm show the best performance.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, Cu nanoparticles have become attractive materials and been found widespread application for electrical, optical, catalytic, mechanical applications [1–5]. Their unusual properties like optical and mechanical performance often differ significantly from the bulk materials, depending on the size, morphology, dispersibility and uniformity. Plentiful studies have been carried out for the synthesis of Cu particles with improved properties, one of the key aspect is controlling of smaller particle size [6–9].
In addition to other routes such as physical vapour deposition, chemical vapour deposition, reverse micelle and chemical reduction, electrodeposition is by far accepted as the most popular methods due to its low-costs, convenience, low toxic, highly production rates, easy to industrialize and mass-produce [10–13]. More importantly, it is a feasible technique to produce Cu particles with a range of sizes, from micron to nanometer dimensions for different requirements for size-effected performance [7,14,15]. Through varying the electrolysis conditions (voltage, current density, pH, temperature, composition and concentration of electrolyte), it is possible to control the size of products by 1∼2 orders of magnitude. Nevertheless, the main drawback of electrodeposition, at present, is typically leading deposits to a wide size distribution, especially in nano-size, most of the reported synthesis produce particles of large polydispersity (≥20%) [16–18]. However, a narrow size distribution is a key parameter to obtain reproducible and controllable optical, chemical and physical properties of products. Therefore, how to realize delicate control over particle size is one of the hottest research areas [19]. Indeed, another drawback is the enhanced tendency to oxidize under ambient conditions as size is reduced, bring challenge of stabilization during particle refinement [20,21]. The study on the synthesis of Cu particles of controlled size is thus less developed than Ag or Au, many groups have worked on the stabilization of Cu particles against oxidation but sometimes to the detriment of the size and shape control [22,23].
As is known, SERS phenomenon can be easily observed from the surface of Au, Ag and Cu [24,25]. The signal is influenced by several significant parameters, such as size or shape of substrate [26,27]. It is important to develop smaller size in order to maximize their SERS signals. Researchers have less research on Cu materials because their chemical properties are unstable and SERS signal is weak compared to Au and Ag [26,27]. However, due to the low cost and abundant resources, they still have great application prospects especially in exploring the size-dependent SERS enhancement [28,29]. On the other hand, the wavelength of UV-Vis absorption spectroscopy or Light extinction spectrometry also have strong relationship with the size of particles [13,30,31]. Inappropriate size or aggregation would seriously affect the LSPR mode and greatly weaken the intensity of peaks. It can be identified that only particles with good dispersion and uniform size can create the intense LSPR peak, which plays a key role in several technological fields such as chemical and biological sensing, ultrasensitive biosensing and nanophotonics [30,32,33]. Although some studies have been performed on influence of different size or shape of metal particles on them, there has been very little research simultaneously on synthesizing Cu particles for a definitive understanding. In many reports, the mechanical properties of particles also have a strong correlation with size, usually the smaller the better, based on a uniform particle size distribution [34–36]. Therefore, it will be very interesting to conduct size-controlled synthesis of Cu particles by electrodeposition while retaining high stability and see how their properties being increased by decreasing particle size [13].
In view of aforementioned, the goal of this work is to synthesize Cu particles with controlled size, uniform distribution, less liable to oxidation, by one-step electrodeposition method and study their size-effected optical and mechanical properties. Different electrodeposition conditions were investigated in order to deposit stable Cu particles of 1.5 µm, 500 nm and 50 nm. These particles were characterized by SEM, EDS, TEM, SADE and XRD for surface morphology, composition and crystal structure analysis. Antioxidant ability was verified by TGA. Spectroscopic techniques (SERS and UV-Vis) and nanoindentation were used for a quantitative examination of the relationship between different size, showing potential applications in control of optical and mechanical performance.
2. Experimental
2.1 Reagents and materials
Potassium ferrocyanide trihydrate (99%) was obtained from J&K Scientific Ltd.. R6G (≥99.5%), CuSO4·5H2O, anhydrous ethanol, and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co. Ltd., China. All reagents were used as received without further purification. Fresh solutions were prepared before experiments to avoid their deterioration with time.
2.2 Electrodeposition of Cu particles
The synthesis process took place in a one-compartment, two-electrode square container (5 cm × 5 cm × 5cm). Highly purified Cu discs (99.99 wt %), with a diameter of 10 mm and 3 mm thick, were used as cathodic substrates. Commercially pure steel discs with the same size was positioned at a distance of 5 mm from the upper surface of the substrates as anode. The same size of cathode and anode could ensure a steady electric distribution, furtherly a more uniform particle size. Two kinds of electrolytes were used to complete the preparation of Cu particles. Electrolyte solution A (for 1.5 µm and 500 nm) was prepared with CuSO4•5 H2O (100 g/L) in deionized water. The 1.5 µm particles were obtained at a constant potential of 5 V, current density of 0.1A/cm2 after 10 min in solution A. Tunning the current density to 1A/cm2 but other conditions remain unchanged, the 500 nm particles were generated. Electrolyte solution B was composed of solution A and potassium ferrocyanide (30 mg/L). The 50 nm particles were observed when the reaction time was reduced to 5min in Electrolyte solution B.
Prior to electrodeposition, both sides of discs were mechanically polished by silicon carbide sheets (from #400 to #7000) to obtain a mirror finish, and then ultrasonically cleaned in hydrochloric acid and deionized water to remove dust or greasy medium that accumulated on the surface. After electrodeposition, the substrate covered with particles was washed with deionized water and anhydrous ethanol successively for cleaning, then wiped with lint free paper, dried in the air for SEM characterization. For other characterizations, Cu particles were peeled from the substrates and dispersed in ethanol as suspension. For better reproducibility and comparability, all the samples were prepared in short time prior to the characterization and detection.
2.3 Morphology and composition characterizations
Scanning electron microscopy (SEM) was used for characterizing the morphology, size, and distribution of Cu particles. Simultaneously, Energy dispersive X-ray spectroscopy (EDS) patterns for elemental composition. The images and patterns were obtained by MIRA 3 LMH (TESCAN Brno, s.r.o) microscope operating at 5 kV and 20kV. Samples were prepared by mounting the deposited substrates directly on the holder with carbon tape.
Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) were used for characterizing the structure. All images were obtained by JEOL 2010 plus transmission electron microscope operating at an acceleration voltage of 200 kV, and recorded by a Gatan 1k × 1k slow scan CCD camera. Samples were prepared by casting a drop of the suspension further grind with a mortar on a 200 mesh carbon-coated Cu grid before drying them in vacuum. The suspension, composed of milled Cu particles and anhydrous ethanol, was sonicated for 5 min in deionized water.
For X-ray diffraction (XRD) measurements, XRD patterns were recorded using a Bruker D8 X-ray diffractometer (Cu Kα source) with a 2θ range of 20° to 80°. After centrifugation, Cu particles obtained from the precipitate of the as-prepared suspension were coated on a glass slide to form films prior to the vacuum dry.
Thermal analysis (TGA) was carried out on Mettler-Toledo TGA2 / DSC3 by heating 10 mg of Cu particles to 100 °C for 10 min in a flow of air (25 mL min-1) to remove moisture, followed by increasing the temperature to 600 °C at a rate of 10 °C min-1.
2.4 Optical and mechanical properties characterizations
Rhodamine 6G (R6G) molecules were used for the study of Cu particles in Surface-enhanced Raman scattering (SERS). SERS spectra were recorded using a confocal Raman spectrometer (Renishaw inVia) coupled to Zeiss microscope with a 50× objective in backscattering geometry. The 532 nm laser was used as the excitation source. The backscattered signals from 11 mg pure R6G and 1 mg Cu particles (1.5 µm, 500 nm, 50 nm, respectively) mixed with 10 mg R6G were collected in the range of 300 cm−1 -1800 cm−1. UV-visible absorption spectra (UV-vis) were carried out with a UV-Vis spectrophotometer (UV-2550, Shimadzu), at room temperature, between 400 nm and 800 nm, using 1-mm-pathlength quartz cuvettes. The tested Cu particles were separated from the substrates, dispersed in anhydrous ethanol, and then subjected to ultrasonic treatment for 30 minutes. Nanoindentation was conducted on TI 950 Tribolndenter with 30 µN of loading force and 30 s of loading time. Samples were prepared by mounting the deposited substrates directly on the holder with crystal glue.
3. Results and discussions
3.1 Size and morphology analysis
The synthetic process was in Cu-based electrolyte containing CuSO4·5H2O (100 g/L) at an applied potential of 18 V and current density of 0.1A/cm2 for 10 min, the related processing conditions are given in the Experimental Section. Figure 1(a) and (b) show a uniform distribution and octahedral Cu crystals with regular shape and an average size of ∼1.5 µm. All the polyhedral particles have smooth surfaces, clear edges and corners. Particle diameters was reduced to ∼500 nm with the current density increased to 0.5A/cm2, as shown in Fig. 1(c) and (d). There was broader size distribution of particles compared to that under the current density of 0.1A/cm2, but the regular octahedral shape of Cu crystal was maintained. This analysis suggests that there are some effects from current density on particle size dispersion. Furtherly, when potassium ferrocyanide was added into the basic electrolyte solution as a grain refiner, an obvious grain refinement (∼50 nm) was observed (see Fig. 1(e) and (f)), which was consistent with the grain refinement rule of potassium ferrocyanide reported by Wenbo Lou et al [37]. As evident in the images, the size distribution restored to a narrow level (<10%), but it was difficult to find regular structure of polyhedral crystals. It can be explained by the fact that small-sized crystals have not grown into polyhedrons with regular shapes. At the same time, EDS patterns in Fig. 1(a), (c) and (e) furtherly demonstrate the clear crystal structure of the synthesized Cu particles.
Fig. 1. SEM images of Cu particles with different sizes: 1.5 µm (a-b), 500 nm (c-d), and 50 nm (e-f), the illustrations in the upper right corner are the EDS patterns.
3.2 Structure and stability analysis
Cu particles were grinded and deposited on ultra-thin carbon film for TEM characterizations to study crystalline structure. Figure 2(a), (c) and (e) show the clear morphology of pieces of milled Cu particles (1.5 µm, 500 nm and 50 nm). The typical HRTEM image of the particles are shown in Fig. 2(b), (d) and (f), as noted, the fringe spacing of 2.088 Å, 1.808 Å and 1.2780 Å can be indexed to the lattice plane (111), (200) and (220) of face-centered cubic (FCC) Cu (JCPDS No. 65-9743), respectively. The inset SAED patterns can be indexed to plane of FCC structure of FCC Cu (JCPDS No. 65-9743). At the same time, some dislocations and other defects can also be seen in the figures, which is common phenomena in the electrodeposition process.
Fig. 2. TEM characterizations of Cu particles with different sizes: 1.5 µm (a-b), 500 nm (c-d) and 50 nm (e-f), insets of panel b, d and f show the corresponding selected area electron diffraction (SAED) patterns obtained from corresponding area.
Cu particles of three particular sizes were further characterized by XRD (Fig. 3(a)), with scanning range from 20° to 80°. There were three characteristic peaks at 43.2°, 50.3° and 74.1° marked with indices of (111), (200), and (220) crystal planes of FCC Cu (JCPDS No. 65-9743). As previously stated, these results are well consistent with those reported previously for metallic Cu by other methods [38–40]. The sharp and strong peaks indicate high crystallinity in the synthesized Cu particles. The intensity ratios of the (2 0 0) to (1 1 1) diffraction peaks suggest that all these three kinds of particles are dominated by (111) crystal plane orientation and have higher hardness and corrosion resistance. Furthermore, no other impurities such as Cu2O, CuO or Cu(OH)2 were detected, confirming the high purity of Cu particles obtained by this synthetic method.
Fig. 3. X-ray diffractograms of Cu particles with different sizes: 1.5 µm, 500 nm and 50 nm (a) and TGA analysis of 50nm (b).
The stability of 50nm Cu particles aganist oxidation was also examined through thermal analysis which was performed under a flow of air up to 600 °C. The analysis result shows that Cu particles are stable even if reduced to a range of tens of nanometers in air up to 200 °C, above which there is a stepwise weight gain (Fig. 3(b)) and appeared similar to the work of M. Ibrahim Dar et al [8]. The total weight gain observed is 23%, which results from the formation of CuO and is in accordance with the theoretical value of 25% [38].
3.3 SERS study
Typically, SERS can be easily observed with coinage metals such as Au, Ag and Cu. These materials exhibit localized surface plasmon resonance (LSPR) bands in the visible region due to excitation of the conduction electrons after irradiation with light. Since Cu particles in this work provide many sharp edges and corners, which can confine EM fields and generate strong near-fields concentrated at vertices, it is expected to have visible enhancement in SERS. At the same time, R6G has been widely applied in SERS research owing to its mature vibration property [41–46], and was selected as probe molecules to prove SERS performance.
Figure 4 presents the Raman spectra of pure R6G powder and the same weight of R6G mixing with different Cu particles (1.5 µm, 500 nm and 50 nm, respectively) under 532 nm close to the plasmonic peak. We selected some typical fingerprint vibrational bands of R6G that are significantly obvious and consistent with those reported in other literatures. The characteristic peaks of 613 cm−1, 775 cm−1 and 1183 cm−1 belonged to C-C-C ring in-plane bending, C-H out-of-plane bending and C-H in-plane bending, respectively. In addition, 1310 cm−1, 1361 cm−1, 1505 cm−1, 1573 cm−1 and 1648 cm−1 were associated with the stretching modes of fragrant C-C in-plane [47]. These peaks described above are illustrated by the dashed lines, all the signals except 1310 above of different samples have been enhanced, but there are also some obvious differences in enhancement effect of different particle size. The sample mixed with 50 nm has the highest degree of enhancement, and 1.5 µm has the lowest enhancement effect. By quantitatively comparing the intensities at 613 cm-1, the Raman enhancement ratio is about 1.5, 2, 4 between pure R6G and mixing with 1.5 µm, 500 nm and 50 nm, respectively, and agreeing with 1-2 orders proposed by Zhong-Qun Tian [33]. To varying degrees, the enhancement effect of other peaks is increased by a factor of about 2∼4 due to the higher surface-to-volume ratio as particle size reduced. Interestingly, as the size reduced from 1.5 µm to 500 nm, a clear peak of 1310 appears, and when further to 50 nm, a clear peak of 1410 also appears. This new signal is enhanced when particle size was reduced to 50 nm and no other change of the position of any peaks is observed. This implied that 50 nm is a more suitable size for Cu particles for SERS chemical applications. In general, the enhancement is determined by the wavelength of the exciting light, composition, temperature, surface morphology and so on [47,48]. Given that the only difference among these samples is particle size (and as indicated earlier), we conclude that particle size plays a key role in the improvement of Raman signal. The larger SERS signal at smaller sizes demonstrates the importance of controlling particle size of Cu in optical applications. And it is important to remark that these spectrograms possess the same features than those reported [48,49].
Fig. 4. Raman spectra of pure R6G powder and R6G mixed with dispersed Cu particles with different sizes: 1.5 µm, 500 nm and 50 nm.
3.4 UV-Vis study
Optical absorption of Cu particles dispersed in anhydrous ethanol was investigated via UV-vis spectroscopy at room temperature under ambient conditions. In doing so, Fig. 5 shows the fitted peaks of Cu particles at different particle sizes. The 1.5 µm, 500 nm and 50 nm Cu particles have well-defined LSPR extinction bands and maximum absorbance near 650 nm, 620 nm, and 600 nm, respectively. These characteristic absorption peaks confirm the presence of Cu particles and have red-shift and higher intensity as the particle size decreases, just as the same phenomenon already discussed by Chenhuinan Wei et al [38]. The 1.5 µm and 50 nm have highest and lowest peak intensity, respectively. At the same time, the 500 nm has a much broader absorption peak when well dispersed in solution, which indicates a broad size distribution of Cu particles without any aggregation and keeps consistent with the broad size distribution analyzed from Fig. 1(c) and (d). In addition, since Cu particles were monodisperse, the coupling effect is not considered. The peak width is sensitive to the size distribution of Cu particles, corresponding quite well to that reported by Taekyung Yu in Au nanoparticles [44]. Furthermore, there exists no observable absorption tail at 800 nm, indicating the absence of the CuO phase in the samples. Thus, size-controlled synthesis of Cu particles with narrow size distribution would be possible with this approach.
3.5 Mechanical study
Mechanical properties of Cu particles have the same size-effect law as optical performance. As shown in Fig. 6, Cu particles of different sizes were subjected to the same loading force (30 µN), and unloaded after maintaining a consistent loading time of 30 s. Finally, the 50 nm leave the shallowest indenter pit of less than 2 nm, while 6 nm and 12 nm for 500 nm and 1.5 µm, respectively. Results indicate that 50 nm has the highest hardness while the 1.5 µm the lowest.
4. Conclusion
This paper presented a simple and efficient electrochemical formation of well-antioxidant Cu particles with an average size of 1.5 µm, 500 nm and 50 nm, by altering the current density and adding additives. The synthesized Cu particles were characterized by SEM, EDS, TEM, SEAD, XRD and TGA for size, morphology, structure and stability analysis, which conformed that particles had uniform size distribution and good crystallinity, especially the 50 nm. More importantly, a similar relationship between optical and mechanical properties on the size effect was discovered by SERS, UV-Vis and nanoindentation. It is concluded that size has a visible impact on the peak of the Raman spectrum and the UV absorption spectrum of Cu particles, and the 50 nm has the best performance. Meanwhile, the position of UV-Vis peak also shifts to left in small degree with decreasing size and narrower size distribution brings sharper peaks. This work opens the door to the size-controlled synthesis of Cu particles from 1.5 µm to 50 nm, not only improved optical and mechanical properties by size refinement but also good understanding of size effect.
Funding
National Natural Science Foundation of China (51727901, 51801138).
Acknowledgements
The authors acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 51801138 and 51727901).
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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