Size Distribution Control of Copper Nanoparticles and Oxides: Effect of

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Size Distribution Control of Copper Nanoparticles and Oxides: Effect of Wet-Chemical Redox Cycling Chen Chen,† Siyi Cheng,† Tielin Shi,†,‡ Yan Zhong,† Yuanyuan Huang,† Junjie Li,† Guanglan Liao,†,‡ and Zirong Tang*,†,‡ †

State Key Laboratory of Digital Manufacturing Equipment and Technology and ‡Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, PR China

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 02/05/19. For personal use only.

S Supporting Information *

ABSTRACT: In this work, we studied the effect of liquid-phase redox cycling on the size of Cu nanoparticles and oxides. The mixed solution of sodium hydroxide and ammonium persulfate was applied as the oxidation system at room temperature, and ascorbic acid was used as reduction agent at 80 °C in the cycling process. It was found that pristine copper particles with average size of around 800 nm and wide distribution from 300 to 1300 nm could be turned into the resulting particles with the average size of around 162.3 nm with the distribution from 75 to 250 nm after 5 redox cycles. It was also observed that uniform copper oxide nanowires formed after 5 oxidation cycles could be easily reduced into fine copper nanoparticles. The critical tuning factors including the precursor size, morphology, defects, reaction time, and the way of adding oxidant were investigated. It was suggested that the synergetic driving effect of chemical reduction and nanostructure thermodynamic instability in solution accounted for the size reformation of the copper nanoparticles. This proposed method of size-shrinking could be developed as a general strategy for large-scale tuning the properties of copper nanoparticles for wide applications and extended to other metal particles as well.

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INTRODUCTION The large-scale synthesis of copper nanoparticles (Cu NPs) and oxides with well-controlled size and shape has been intensively pursued,1 not only for fundamental scientific interests due to their unique properties but also for a variety of promising applications such as catalysis,2−4 thermal management,5,6 flexible electronics,7 chemical sensors,8,9 energy storage,10,11 and so on.12,13 Cu NPs could also be an attractive replacement for silver and gold with the advantages of the elemental abundance and relatively low cost. The properties of Cu NPs depend strongly on their dimensions. For example, compared to that of their bulk counterpart, the sintering temperature of Cu NPs of around 100 nm easily decreases to 250 °C14−17 due to high surface area and small internal volume, which appeals special interest such as low-cost conductive ink in the rapidly developing field of printable electronics. A wealth of preparation methods have been developed for synthesizing Cu NPs, including gas-phase physical processes © XXXX American Chemical Society

(wire explosion, magnetic sputtering, laser ablation, atomization)18−20 and liquid-phase or solid-phase chemical processes.21−24 Through proper selection of copper precursor and reduction system, Cu nanostructures with controlled size and morphology could be synthesized for target applications.25−27 In general, the physical method is a good strategy to rapidly synthesize massive copper particles with larger dimensions mainly in the submicron range, while the liquidphase reduction method is favorable for synthesizing much finer Cu NPs. Although great efforts have been made to control the size of Cu NPs through adding dispersing and capping agents to avoid agglomeration and oxidation, the liquid-phase methods for large-scale production are still in urgent need.1,4 Recently, it was reported that CuO and Cu microparticles could be mutually converted upon redox cycling in oxidation/ Received: November 6, 2018

A

DOI: 10.1021/acs.inorgchem.8b03125 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry reduction gas, and the average grain size of CuO decreased gradually from initial 722 to 365 nm through 5 cycles.28 However, ultrafine Cu2O and Cu NPs of around 2 nm could also be interconverted in colloidal solution by extended redox cycling reaction, and the morphology remained stable even after 10 cycles.29 These results suggest that the size of large Cu NPs can be reduced, while the size of ultrasmall Cu NPs remains stable with ligand through redox cycling. The structural transformations were also reported for other single metal and binary alloys during oxidation and reduction cycling systems. It was observed that the initial Fe and FeNi NPs were both converted into nanowires after one cycle and gradually into nanopores through 5 cycles.30 The surface energies of transition metal have strong influences on their redox equilibria and phase stability,31 which is attributed to the morphology and size evolution of nanostructures. Inspired by these discoveries of particle size and morphology evolution during redox cycling, we propose a new strategy for size shrinking of large copper particles, which could be easily obtained from mass production with microscale size via a physical process. The redox cycling process was developed with the proper design of the oxidation and reduction liquid system. In this work, the oxidation system is designed with the mixed solution of NaOH and (NH4)2S2O4, and the reduction system is the ethylene glycol solution containing ascorbic acid. Ascorbic acid is a weak reducing agent with weak reaction driving force and can avoid aggregation.32 The advantage of this chemical redox method in the liquid phase is that abundant submicron particles can be transformed into NPs by simple operations. Here we demonstrate that the size distribution of copper particles changes from 300 to 1300 nm to 75−250 nm after 5 redox cycles, and the contributing factors in the evolution process are explored in detail.

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Figure 1. Overview of redox cycling. (a) Schematic diagram of redox cycling process. (b) Size evolution diagram of large Cu particles to smaller NPs through progressive redox cycling. (c) Morphology evolution progress of oxide product during redox cycling. copper oxides from nanosheets to nanowires through successive oxidation cycling is illustrated in Figure 1c. Materials Characterization. The morphology and structure of the obtained samples were examined by scanning electron microscopy (SEM, FEI Nova NanoSEM 450), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) on a FEI Tecnai G2 STWIN microscope, X-ray diffraction (XRD) with radiation from a Cu target (Kα, λ = 0.154 nm), X-ray photoelectron spectrometer (XPS) on a VG MultiLab 2000 system with a monochromatic Al Kα X-ray source (Thermo VG Scientific).

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RESULTS AND DISCUSSION Oxidation Cycles. For better understanding the consequence of redox cycling on Cu particles and their oxides, here we started with the oxidation cycles. Pristine Cu particles were oxidized at the first cycle, and the resulting morphology and structure images are displayed in Figure 2. Figure 2a shows the morphology of copper oxide after a 10 min reaction, where sparse nanowires are observed from the microparticle surface. Figure 2b shows the morphology of oxides after a 30 min reaction, where well-defined flower-like microstructures are observed. And the TEM image reveals that the detailed morphology of the flower-like oxide is assembled from densely compacted nanosheets as “petals” (Figure 2c). Obviously, the nanosheets were formed from nanowire growth, which has also been reported in the previous literature.34 The detailed microstructure of the obtained oxides was further investigated by HRTEM (Figure 2d). Three lattice fringes are marked by white lines and arrows, and the corresponding lattice spacings are 0.811, 2.324, and 2.532 Å, respectively, which are in good agreement with the d value of the (111) facet of the Cu2O and the (111) and (002) facets of the CuO crystal. In the liquidoxidation system, Cu particles are oxidized as the following equations.35,36

EXPERIMENTAL SECTION

Chemicals. All chemicals including sodium hydroxide (NaOH), ammonium persulfate ((NH4)2S2O8), ascorbic acid (AA), ethylene glycol (EG), and Cu particles ranging from 300 to 1300 nm were obtained from Aladdin Reagent Co. Deionized water was used throughout all experimental processes. All other chemicals used in this investigation were of analytical grade. Oxidation of Cu NPs. Prior to oxidation, the Cu particles (0.5 g) were washed and rinsed several times with deionized water containing several drops of AA and ethanol. 0.0075 M NaOH and 0.0003 M (NH4)2S2O8 were both added into 100 mL of deionized water and stirred with a magnetic rod until dissolved completely. Then the solution was transferred to the PVC infusion package, dropped into the beaker containing Cu particles, and stirred for 30 min at room temperature (slow addition oxidation). After that, the precipitates were collected by centrifugation at 10000 rpm. The products obtained were dried at 60 °C and stored in vacuum. Reduction of Copper Oxides. 3 g of AA was dissolved in 100 mL of EG under ultrasound treatment for 10 min. The obtained copper oxide was added into the solution and stirred for 2 h at 80 °C in an oil bath. The precipitates in the beaker were collected by centrifugation at 6500 rpm. The obtained products were dried at 60 °C and stored in vacuum. Redox Cycling of Cu/Cu Oxides. As oxidation and reduction reaction can be easily carried out respectively, the platform of redox cycling process could be set up.33 The schematic diagram of redox cycling process of Cu/Cu oxide is shown in Figure 1a. Each cycle contains one oxidation process followed by a reduction process. For convenience, the ith reduction product is termed Re(i), and the ith oxidation product is termed Ox(i). In Figure 1b, the shrinking trend of the size and size distribution of Cu particles is illustrated in the progress of the reduction cycling. The morphology evolution of

Cu + 4NaOH + (NH4)2 S2 O8 → Cu(OH)2 + 2Na 2SO4 + 2NH3 ↑ + H 2O

(1)

Δ Cu(OH)2 → CuO + H 2O

(2)

Cu 2 + + e− → Cu+

(3)

Cu+ + OH− → CuOH

(4)

2CuOH → Cu 2O + H 2O

(5)

NH4+

where the weak alkaline is a good complexing agent and widely used in the synthesis of copper hydroxide and copper oxide.37,38 When the surface of Cu particle is oxidized, the developed divalent copper ions diffuse into the solution and react with NH4+ to form Cu(NH4)42+ and then form Cu(OH)2 B

DOI: 10.1021/acs.inorgchem.8b03125 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) SEM images of oxides after 10 min oxidation. (b) SEM images of oxides after 30 min oxidation. (c) TEM images of oxides after 30 min oxidation. (d) The corresponding HRTEM images of oxides after 30 min oxidation.

Figure 3. SEM images of oxides (a) Ox(2), (b) Ox(3), (c) Ox(4), and (d) Ox(5).

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DOI: 10.1021/acs.inorgchem.8b03125 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. TEM images of oxides (a) Ox(2), (b) Ox(3), (c) Ox(4), and (d) Ox5). The insets are the HRTEM image and the corresponding fast Fourier transform pattern.

with OH−.39 Finally, copper oxide is formed by dehydration.40 Meanwhile, Cu+ is formed while Cu2+ accepts an electron. Subsequently, the precipitation of CuOH occurs with OH− and it is dehydrated into Cu2O.41 When copper particles were oxidized, the nuclei of copper hydroxide were initially formed on the particle surface. Since anions (O2−) spread more slowly than copper ions (Cu2+),42 the antennae structure was then formed (Figure 2a). In the end, the surrounding antennas underwent oriented attachment and were assembled together by static electricity and hydrogen bonding to form flower-like nanosheets43 (Figure 2b). The oxidation results of each following cycles were also presented. SEM images of Ox(2), Ox(3), Ox(4), and Ox(5) are shown in Figure 3. In Figure 3a, flower-like Ox(2) with the size of around 1 μm was formed. In Figure 3b, Ox(3) with both nanosheets and nanowires of about 2 μm long is observed. The Ox(4) with complete nanowires of around 4 μm long was obtained, as shown in Figure 3c. While nanowires with tens of micrometers in length and 30−40 nm in diameter are shown for Ox(5) in Figure 3d. Obviously, with the cycling process, the size of oxide in the axial direction increased significantly, from two-dimensional nanosheets to one-dimensional nanowires. Furthermore, it was also observed that the color of the reaction solution became darker quickly at the fifth oxidation compared with previous cycles, which demonstrated that the reactivity of reduced Cu NPs was greatly increased after a few cycles. The morphology of the oxides was further characterized by TEM, as shown in Figure 4. In Figure 4a, it is observed that the morphology of Ox(2) is nanosheet. The two insets are HRTEM and the fast Fourier transform (FFT) patterns, respectively. In the FFT patterns, three sets of diffraction spots could be indexed to strong reflections from the CuO (111), Cu2O (111), and (220). This means that the Ox(2) consists of

CuO and Cu2O. In Figure 4b, nanosheets and nanowires coexist, which is consistent with the observation of the SEM image for Ox(3). As the inset shows, two sets of diffraction spots in the FFT patterns can be indexed to strong reflections from the CuO (111) and (002). This means that the Ox(3) consists of CuO only. In Figure 4c, the TEM image of nanowires is displayed for Ox(4). As the inset shows, three sets of diffraction spots were found in the FFT patterns, which could be indexed to strong reflections from the CuO (111) and (002), (11−3). In Figure 4d, TEM image of nanowires is also displayed for Ox(5). As the inset shows, we found that two sets of diffraction spots can be indexed to strong reflections from the CuO (111) and Cu(OH)2 (002) in the FFT patterns. The crystal phase of the obtained oxide was analyzed by powder XRD, as shown in Figure 5. All diffraction peaks in the pattern are consistent with CuO (JCPDS 481548), Cu2O (JCPDS 05-0667), Cu(OH)2 (JCPDS 35-0505), and Cu (JCPDS 04-0836). Figure 5a, b, c represent the XRD patterns of the Ox(1), Ox(3), and Ox(5), respectively. In Figure 5a, the major diffraction peaks at around 2θ values of 35.4°, 68.1° and 36.4°, 42.3° are well-indexed to the (002), (111) planes of CuO, and the (111), (200) planes of Cu2O, respectively. In Figure 5b, the major diffractions peaks are related to CuO (002), (111). In Figure 5c, the major diffractions peaks at around 2θ values of 35.4°, 68.1° and 34.1°, 38.1° are respectively indexed to the (002), (111) planes of CuO, and the (002), (041) planes of Cu(OH)2. After analysis according to the Scherrer equation, the average size of the NPs was decreased from 207 nm after one oxidation cycle, to 151 nm after three oxidation cycles and then 137.5 nm after five oxidation cycles. This was mainly due to the decreased size of Cu particles after each cycle. The fast kinetics of the redox reaction also played an important role in the decreased size.44 In particular, the change in size was caused by ion diffusion D

DOI: 10.1021/acs.inorgchem.8b03125 Inorg. Chem. XXXX, XXX, XXX−XXX

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dynamic equilibrium of dissolution, ionization, and subsequent reduction of copper oxide affects the copper particle size and morphology accordingly. AA is adsorbed onto the surface of copper particles by van der Waals force to prevent copper from oxidation. Furthermore, using ascorbic acid as the reducing agent and ethylene glycol as the protective agent, the method is mild, nontoxic, and cost-effective. Generally, copper oxide is reduced per the following equation:47 Cu 2 + + 2e− → Cu 0

(6)

The SEM images of original Cu particles and Re (1) are shown in Figure S1a, b, respectively. And the size distributions of the origin Cu particles and Re(1) are investigated in Figure S1c, d, respectively. A slight decrease in particle size was observed after one cycle. For comparison, the SEM images and size distribution of Re(2), Re(3), Re(4), and Re(5) are shown in Figure 7. The size distribution and average size of the original Cu particles, Re(1), Re(2), Re(3), Re(4), and Re(5) are fully detailed in Table 1. Overall, the size of Cu NPs decreased, and the size distribution was narrower with the process of cycling. The TEM images of Re(2), Re(3), Re(4), and Re(5) are shown in Figure 8. Polyhedron structures are observed for all the reductions, as shown in Figure 8a−d. The inset HRTEM images of Re(2) show the corresponding lattice spacings about 2.08 Å, which is in good agreement with the d value of the (111) facets of the Cu crystal. The lattice spacings of Re(3), Re(4), and Re(5) are the same as Re(2), matching (111) of Cu crystal. And this proves that all the reduction reactions were complete. These analyses showed the gradual transformation of Cu NPs size at each cycling. As we can see, the distribution of original Cu particles with microscale size was wide. And they became much more uniform after several redox cycles. It was striking that the size distribution of Cu NPs was gradually narrow down to 75−250 nm after 5 redox cycles. Tuning Mechanisms. In the liquid-phase redox process, numerous parameters affect the formation of final product, such as precursor, oxidant/reductant, and reaction environment, including solvent, dispersant, temperature, and time.48 The formation of nanostructures is a dynamic process controlled by both kinetics and thermodynamics.49 It has been widely reported that nanostructures are extremely unstable in many cases due to their high surface energy. Driven by thermodynamics, the morphological instability occurs, including Ostwald ripping, Rayleigh breakup, coalescence, and low-temperature sintering.50 The transforming process of the nanostructure is very fast due to the high activation energy for the diffusion of reactant atoms and ions. The large surface-to-volume ratios of a nanostructured material effectively reduce the kinetic barrier for diffusion.51 In this work, we studied five critical factors, including the size, morphology, defects, reaction time, and way of adding oxidant. In the oxidation cycling progress, we start with the effect of the size on the process. The size of precursor was decreased as shown in SEM images of original Cu particles and Re(1)− Re(4) in Figure S1 and Figure 7. By visual inspection and comparing with each previous cycle, the solution color changed faster from red to dark, showing that the reaction activity was increased significantly with the decreased size of Cu particles. As a result, the morphology of oxides has changed gradually from nanosheet to nanowire, showing that the oxide preferred oriented growth in the axial direction with the deceased size of precursor. With the decreased size of Cu

Figure 5. XRD patterns of the obtained Cu oxide samples (a) Ox(1), (b) Ox(3), and (c) Ox(5).

during the exchange of oxygen vacancies. The redox process depended greatly on the size since it could be treated as creation and consumption of oxygen vacancies, respectively. Further confirming the formation of Cu oxides and its chemical states, XPS analysis is illustrated in Figure 6. The Cu

Figure 6. X-ray photoelectron spectra of the obtained Cu oxide samples (a) Ox(1), (b) Ox(3), and (c) Ox(5).

(2p) spectrum for Ox(1), Ox(3), and Ox(5) is shown in Figure 6a, b, c, respectively. It all confirmed the presence of Cu (2p3/2) at 933.7 eV, 932.7 eV and Cu (2p1/2) at 953.7 eV, 952.7 eV. The splitting between these two states was nearly 20 eV due to the formation of CuO. These satellite peaks at 940.5 and 943 eV confirmed the formation of CuO.45,46 In addition, the other peaks at 932.7 and 952.7 eV corresponded to the shakeup satellite peaks of Cu2O (2p3/2) and Cu2O (2p1/2) (Figure 6a). In the oxides of the first two cycles, Cu2O and CuO heterostructure was formed, indicating that the Cu particles were not oxidized completely in 30 min. But Cu2O was not observed in the following three cycles, which showed that the smaller Cu particles were completely oxidized. Reduction Cycles. AA with a conjugated structure could provide electrons to reduce the Cu2+ to Cu0, and it was oxidized from an enol to the adjacent diketone structure.26 The E

DOI: 10.1021/acs.inorgchem.8b03125 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. SEM images of the obtained Cu NPs samples (a) Re(2), (b) Re(3), (e) Re(4), and (f) Re(5). (c), (d), (g), and (h) show the size distributions of (a), (b), (e), and (f).

that of Re(2) from 200 to 1100 nm, the oxide morphologies were flower-like nanosheets and nanowires. We anticipate that other factors also help to enhance the reactivity of reduced Cu particle, where the reformed Cu particles went through periodically kinetic expansion (oxidation) and contraction (reduction) together with thermodynamically driven internal structure reorganization. It is expected that progressive cycling may generate more defects and smaller gain size compared with the previous cycling.53,54 All these factors may contribute to higher reactivity. In order to further understand the tuning mechanism and the effect of redox etching on defects accumulation in products, the detailed HRTEM images of the oxidation product from 100 nm Cu NPs and Ox(5) are respectively shown in Figure S3. In Figure S3a and b, two lattice fringes are all marked by white lines and arrows, with the lattice spacings of 2.324 Å and 2.532 Å, which correspond to the (111) and (002) facets of the CuO crystal, respectively. Comparing the two images, much more island-like structures are observed in the Figure S3b, and some are marked by white curves. The rich islands were formed due to etching in multiple redox reactions, indicating the accumulation of surface defects. These defects provide certain nucleation sites to form copper particles in the subsequent reduction process. The increase of nucleation sites leads to growth of more copper nanoparticles and the decrease of the size. Therefore, the accumulated defects in the redox cycling also contribute to the tuning process besides the precursor size and morphology.55−58

Table 1. Size Distribution of Re(1), Re(2), Re(3), Re(4), and Re(5) Origin Cu particles Re(1) Re(2) Re(3) Re(4) Re(5)

Size disribution (nm)

Average size (nm)

300−1300 200−1200 200−1100 100−900 100−600 75−250

718.1 691.6 625 470.7 378.2 162.3

particles after each cycle, the reaction of Cu(NH4)42+ with OH− was greatly increased due to higher reactivity. The growth rate of nanowire was also enhanced with increased length.52 To verify the size effect, another experiment was designed. Tiny Cu NPs with the size of around 100 nm (commercially purchased) were applied as the precursor to implement a single oxidation process. The SEM images of Cu NPs and the oxidation production are shown in Figure S2. The size distribution of smaller Cu NPs is between 40 and 140 nm, and the average diameter is 89.39 nm, as shown in Figure S2b. It revealed that the morphology of the Cu oxide was nanowire, confirming that the precursor size was critical for the morphology of the oxide. However, if only from the size-related reactivity to explain the observed phenomena, it is not convincing since the Cu particles were in a wide distribution. For example, the size distribution of Re(4) was from 200 to 600 nm, compared to F

DOI: 10.1021/acs.inorgchem.8b03125 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. TEM images of the obtained Cu NPs samples (a) Re(2), (b) Re(3), (e) Re(4), and (f) Re(5). And the insets are the corresponding HRTEM.

In the reduction process, we also studied the effects of the precursor morphology and the reaction time in the reduction of Cu NPs. As shown in Figure 3 and Figure 7, the different morphology of the precursor had a huge impact on the size and size distribution. The size of Cu NPs reduced by CuO nanowires with narrower size distribution will be smaller than that of Cu NPs reduced by CuO nanosheets. To understand the reduction process from CuO nanowires to Cu NPs, we analyzed the reduction intermediate state of Ox(5) with reaction time. We monitored both the solution and the precursor at the reaction time of 1 h. As Figure 9a shows, Cu nanocrystals of 2−4 nm reduced from the precursor and diffused into the solution are detected. The structure of Cu nanocrystals is characterized by HRTEM shown in Figure 9b. Two lattice fringes are marked by white lines and arrows, and the corresponding lattice spacings are 2.324 and 2.532 Å, respectively, which are in good agreement with the d value of (111) and (002) facets of the CuO crystal. Meanwhile, the remaining CuO precursors were transformed into Cu NPs, shown in Figure 9c, which are arranged orderly and compactly in curved line. The size distribution is around 30−130 nm with the average size of 80.5 nm, as shown in Figure 9d. These observations of the reduction results of 2 h shown in Figure 7 suggest that the reduction process could be generally divided into 3 stages with time. In the initial stage, the surface of CuO precursor was reduced and Cu nanocrystals were generated and grown,59 and Cu nanocrystals were formed. It is reported that the nucleation and growth of Cu NPs are supposed to proceed with the reduction of Cu2+ ion to Cu, and quickly reach the saturation point.48 In the second stage,

Figure 9. (a) TEM image of Cu nanocrystals diffused in the solution reduced by CuO nanowires. (b) HRTEM of Cu nanocrystal. (c) SEM images of Cu NPs when the reaction time reached 1 h. (d) Size distribution of c.

oxygen vacancies were formed in the remaining precursor by reducing agents, which generated strong Rayleigh instability in the nanowires.60 The nanowires were deformed, fragmented G

DOI: 10.1021/acs.inorgchem.8b03125 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry into smaller units, and reduced to Cu NPs.61 In the third stage, those small Cu NPs coalesced and ripened to more uniform and dispersed NPs. In general, with the synergetic effect of nanostructure instability and reduction, CuO nanowires were transformed into Cu NPs. Meanwhile, the reactivity of nanowire precursor was higher compared with other precursor morphologies. Therefore, the optimal control of reaction time leads to smaller Cu NPs. As for the effect of oxidation addition rate, we designed a comparative experiment. Instead of slowly dropping the mixed solution containing NaOH and (NH4)2S2O8 into the beaker (slow addition oxidation process), we added all the solution into the beaker immediately and stirred for 30 min (fast addition oxidation process) and repeated redox cycling. The obtained oxides were termed as Ox′(i) for the ith oxidation cycle and Re′(i) for the ith reduction cycle. The SEM images of Ox′(1), Ox′(2), Ox′(3), Ox′(4), and Ox′(5) are shown in Figure S4. In Figure S4a, many NPs and nanosheets were formed. Compared with Ox(1), it seemed that some copper NPs had not been oxidized yet. In Figure S4b, nanosheets and nanoparticles covered with layers of nanosheets are observed. Nanowires and numerable NPs are shown in Figure S4c. The length of Ox′(3) nanowires is about 2 μm. All nanowires occur in Ox′(4) (Figure S 4d). This means that Cu NPs are completely oxidized. Two micrometer long nanowires are shown in Figure S4e. Obviously, the morphology of oxides follows the general evolution trends from nanosheets to nanowires. However, the evolution rate of the fast addition oxidation process was much slower than that of the slow addition oxidation process. Since the molar ratio of oxidant to Cu particles, in the fast addition oxidation process, was much higher than that in the slow addition oxidation process, more oxide islands grown and attached on the surface of the copper particles may result in passivation, which made it more difficult for the further oxidation. The SEM images of Re′(1), Re′(2), Re′(3), Re′(4), and Re′(5) are shown in Figure S5. And the size and distribution of Re′(1), Re′(2), Re′(3), Re′(4), and Re′(5) are shown in Table S1. Through further redox cycling, we found the size of Re′(5) was even smaller and more uniform. The trend of size shrinking was the same as that of previous observations of slow addition cycles. However, since the morphology evolution of oxides from nanosheet to nanowire was slower, the shrinking rate of copper NPs was also slower. Therefore, the control of the oxidation and reduction system was also very critical to minimize the cycling steps for the target size of Cu NPs. Overall, the proposed strategy to shrink the size of Cu particles was successful. However, much work needs to be done to optimize the reaction system and the process. The method is also a scalable route to synthesis a large quantity of uniform copper oxide nanowires from simple cycling oxidation of large copper particles. And it is promising for application in a variety of fields, such as catalysis, energy storage, and sensors. The synthetic procedures developed in the present study also offer several important advantageous features over the conventional methods for the synthesis of metal nanocrystals. First, a large quantity of NPs can be easily obtained when the reactors are placed in parallel. Second, the synthetic process is environmentally friendly and economical, because there is nothing like toxic and expensive reagents such as metal chlorides. Third, the cycling process may help to tune the internal structure of copper NPs, which is desired for many applications such as high-performance catalysts. Fourth, the

synthetic method is a generalized process which may be used to synthesize other kinds of metal NPs.

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CONCLUSIONS Liquid-phase redox cycling has great impacts on the size modulation of Cu NPs and oxides. A new and scalable method is proposed to achieve the control of size and morphology of Cu NPs and their oxides. The copper size distribution was tuned from 300−1300 nm to 75−250 nm. And through optimizing the fifth reduction reaction, the size distribution can be further reduced to 30−130 nm. We have studied five critical process factors including the size, morphology, defects, reaction time and the way of adding oxidants in the designed reaction system. Through process optimization, it could be developed as a general strategy for large-scale tuning the properties of Cu NPs and extended to other metal particles as well. Moreover, large-scale CuO nanowires are successfully synthesized through redox cycling. The simple preparation method of Cu/CuO nanostructures offers a distinct advantage for a range of applications such as catalysis, sensing, optoelectronics, and photonics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03125. SEM and TEM images; the data of the size distribution (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Author Address: 1037 Luoyu Road, Wuhan 430074, PR China. E-mail address: [email protected] (Z. Tang). ORCID

Guanglan Liao: 0000-0002-1849-5473 Zirong Tang: 0000-0001-8535-5852 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China with Project No. 2015CB057205, the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT13017), and the National Science Foundation of China (No. 51775218). We would like to thank the Analytical and Testing Center of Huazhong University of Science and Technology for the field emission scanning electron microscopy (FESEM) testing and Xia Chen engineer in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support in TEM test.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03125 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03125 Inorg. Chem. XXXX, XXX, XXX−XXX