DOI: 10.5061/dryad.h70rxwdhs
title: Nanoporous copper and copper oxide with hollow microsphere structure prepared by polystyrene template-assisted synthesis

description: The use of porous metals with highly porous nano- or submicro-structured
 walls for energetic materials applications has been considered in recent
 years. The aim of this work is to construct a stable porous structure for
 synthetize of copper azide primary explosive. For this purpose, hollow
 copper and copper oxide microspheres were successfully prepared by
 electroless plating using polystyrene (PS) templates with diameter of 529
 nm. SEM and XRD analysis were conducted to characterize the microstructure
 and composition of the products, and ICP was used to measure the variation
 of Cu2+ concentration during plating. Results show that the amount of
 copper deposit increased with increasing pH value. By adding enough NaOH
 solution during plating, copper ions could be all reduced while the
 microstructure of copper coatings destroyed. The addition of stabilizers
 had a negative effect on PS/Cu core-shell structures. Hollow copper
 microspheres with shell thickness of 100 nm were obtained by sintering
 PS/Cu core-shell microspheres in N2 while hollow copper oxide microspheres
 were obtained in air.
description: <b>Methods</b> PS microspheres are synthesized by soap-free emulsion polymerization using a mixed solution of deionized water and methanol as a dispersant, potassium persulfate as an initiator, and styrene as a monomer. The specific steps are as follows: 40 mL deionized water, 7 mL methanol, 0.08 g KPS and 5mL styrene are added in round-bottom flask fitted with two necks, and N<sub>2</sub> is introduced into the bottle to purge O<sub>2</sub> out for about 10 min. The flask is closed with silicone-greased stoppers and clamps, and then reacts in a 75 °C water bath at 500 rpm for 12 h. PS microspheres are obtained by washing the polymerized product and then drying in an vacuum oven at 50 °C for 24 h. The preprocessing of PS microspheres is given in Reference 18. The electroless plating copper bath is consisted of CuSO<sub>4</sub>·5H<sub>2</sub>O (0.1 mol×L<sup>-1</sup>), Na<sub>2</sub>EDTA (0.12 mol×L<sup>-1</sup>), pH buffer (NaOH) and 2, 2-bipyridine (0.01g·L<sup>-1</sup>), potassium ferrocyanide (0.01 g·L<sup>-1</sup>) and methanol (50.00 mL·L<sup>-1</sup>)). The preprocessed PS was immersed into a freshly prepared copper bath, and dipped reductant HCHO (15.00 mL·L<sup>-1</sup>) under ultrasonic to start reaction. Then, the PS/Cu composite was obtained by centrifugation and rinsed with deionized water for three times and dried in vacuum oven at 50 °C. Finally, nanoporous copper and copper oxide microspheres are obtained by sintering PS/Cu core-shell at 400 °C for 1 h in the N<sub>2</sub> and in the air with the heating rate of 5 °C·min<sup>-1</sup>, respectively. <b>Characterization</b> Particle size and the particle size distribution of PS microspheres are performed by laser particle size analyzer (Mastersizer 2000, UK). The thermal performances of PS microspheres and PS/Cu composites are measured by thermogravimetric analyzer (TG/DTA6300, Japan) at a heating rate of 10 °C·min<sup>-1</sup> from 25 °C to 600 °C. The IR spectra of PS microspheres are collected by infrared spectrometer (TENSOR 27, Germany). The Cu<sup>2+</sup> concentration is measured by inductively coupled plasma optical emission spectrometer (ICP-OES, PE OPTIMA 7000DV, USA). The microstructure of samples is observed by scanning electron microscopy (SEM, S4800, Japan). X-ray diffraction (XRD) analysis is performed on a diffractometer (Rigaku, Japan) with Cu Kα radiation in the range from 5° to 80°.
description: Figure 1
 is the particle size distribution and TG-DTG curves of PS. The PS diameter
 ranges from 100 nm to 1 μm, and the peak particle value is 529 nm which
 could be considered as the average diameter. The gentle peak distributed
 from 2 μm to 11 μm is caused by the agglomeration of PS due to its
 sub-micrometer diameter. The TG-DTG curves in N<sub>2</sub>
 give the thermal performance of the prepared PS, as can be seen in Figure
 1b. PS starts decomposing at about 320 °C and completes until 450 °C.
 After decomposing, the weight of PS changes to 0 which means PS could be
 removed completely by sintering.   Figure 2 is
 the FT-IR spectrums of PS microspheres before and after roughness. The
 peak at 3440 cm<sup>-1 </sup>which corresponded to the
 stretching vibration of O-H is obviously strengthened and broadened. This
 result clearly indicates that the hydrophily of PS surface is improved by
 roughness process. Sensitization is to absorb sensitize ions onto
 substrate surface. Sn<sup>2+</sup> is the most used sensitize
 ions. Activation is a replacement reaction which reduces the activated ion
 onto substrate surface as catalytic activation center (usually using
 Pd).  
 Figure 3
 gives the microstructures of PS before and after preprocessing. The PS
 surface is smooth and clean (Figure 3a). After preprocessing, PS retains
 good sphere structure while the PS surface becomes rougher. And more
 importantly, it can be clearly seen that there are many nanoparticles
 absorbed on PS surface (Figure 3b). Those nanoparticles are considered as
 the catalytic activity centers (Pd) which are obtained by preprocessing
 process.  
 Figure 4
 is the SEM images of PS / Cu composite microspheres obtained by reaction
 for 60 min at the initial pH value of 12.35, 12.85, 13.35 and 13.85,
 respectively, and the addition amount of PS is 3
 g·L<sup>-1</sup>. It can be seen that with the increase of pH
 value, the reduced copper content on the PS surface increases
 significantly. When the pH value increases to 13.85, the agglomeration of
 copper particles is obvious. This is due to the high pH value which
 resulting in a rapid reaction. The deposited copper particles are close to
 100 nm when the pH value is small. The content of deposited copper also
 increases with the increased of pH value, and the copper particles grow up
 to about 100-200 nm. However, most of PS surface is not coated with copper
 due to the large specific surface area of PS and the relatively small
 amount of copper.  
 Figure
 5(a)gives the variations of Cu<sup>2+</sup> concentration in
 plating bath and pH value with deposition time, where the initial pH value
 is 12.85 and the PS loading amount is 3 g·L<sup>-1</sup>. It
 clearly displays that the change laws of Cu<sup>2+</sup>
 concentration coincide basically with those of pH value. The concentration
 of Cu<sup>2+</sup> and pH value decrease dramatically in first
 2 min and after that decrease slowly. HCHO is reductive only in alkaline
 solution and it reducibility increases with increasing pH value. In the
 first 2 min, the Cu<sup>2+</sup> reduction rate is very fast
 due to the relatively high pH value. The fast copper deposition consumes
 large amount of OH<sup>-</sup> and thus leads to the
 decreasing of pH value. The Cu<sup>2+</sup> reduction rate
 decreases dramatically due to the decrease of HCHO reducibility when pH
 value is reduced to about 9.5. The Cu<sup>2+</sup> reduction
 ratio is 34.94% with deposition time of 20 min. In order to maintain the
 reducibility of HCHO and thus improve the Cu<sup>2+</sup>
 reduction ratio, NaOH solution according to the stoichiometric coefficient
 (1 mol Cu<sup>2+</sup> corresponds to 4 mol NaOH) is added
 steady into the plating bath in first 20 min. And the variations of
 Cu<sup>2+</sup> concentration and pH value with deposition
 time are shown in Figure 5b. In first 2 min, the deposition reaction
 consumes large amounts of Cu<sup>2+</sup> and
 OH<sup>-</sup>, leading to rapid decreasing of pH value from
 12.5 to 10. After 2 min, pH value increases because the adding amount of
 NaOH is exceeds the consume amount. While Cu<sup>2+</sup>
 concentration is decreases to 0 at 13 min, this result indicates that the
 Cu<sup>2+</sup> reduction ratio improves to 100%. However,
 there are many red copper agglomerates floating at the plating bath
 surface during the later stage. This implies the adding of NaOH may
 disturb the growing trend of copper deposits. Figure 6 gives
 the microstructures of two PS/Cu composites samples obtained in above
 experiments. Due to insufficient copper deposits and large PS surface
 area, the deposited copper particles ranged from 50 nm to 100 nm are
 scattered sporadically on PS surface (Figure 6a). While adding NaOH
 solution during electroless plating, the amount of deposited copper
 increases obviously (Figure 6b). However, the subsequent
 Cu<sup>2+</sup> reduces on copper nanoparticles in priority
 and thus most PS surface is still uncovered. These results demonstrate
 that adding NaOH solution during electroless plating could improve
 Cu<sup>2+</sup> reduction but fail to obtain good PS/Cu
 core-shell structures.     
 Figure 7
 shows the morphology of PS / Cu composite microspheres when potassium
 ferrocyanide (0.01 g·L<sup>-1</sup>), 2, 2-bipyridine (0.01
 g·L<sup>-1</sup>) and 50.00
 mL·L<sup>−</sup><sup>1</sup> methanol is added to
 the plating solution and stabilizer is not added. It can be seen from
 Figure 7(a) that when stabilizer is added to the plating solution, a part
 of PS coating is complete, a small part of PS surface is not completely
 coated, and many copper particles are scattered. This is because the
 stabilizers inhibit the process of PS surface chemical plating to a
 certain extent and disturb the growth of copper, so the morphology of
 copper coating is damaged and the uniformity is poor. Figure 7(b) shows
 that the surface of PS microspheres is basically covered by copper coating
 with good uniformity without stabilizer. However, there are still a small
 amount of scattered copper particles, which are basically greater than 100
 nm, and the copper particles on the copper coating are less than 100 nm.
 Besides, the scattered copper particles are due to the fact that PS is not
 fully washed during the pretreatment, and the scattered Pd between PS as
 the active center will lead to the reduction of copper ions. The surface
 of PS microspheres is basically coated with copper coating and the
 uniformity is good, but there are still a small amount of scattered copper
 particles. The above results show that PS / Cu core-shell structure with
 complete morphology can be obtained without stabilizer in the plating
 solution. Figure 8 shows
 the TG curves of PS/Cu in N<sub>2</sub> and air atmosphere are
 presented in. It can be seen in Figure 8(a) that PS started thermal
 decomposing at about 320 ℃ and completed until 430◦C, and the mass loss
 ratio of PS/Cu in N<sub>2</sub> is 37.38%. The curve (b)
 indicates that Cu starts oxidation at 150 ℃ and the weight decreases due
 to the combusting or decomposing of PS when the temperature is further
 increased to 260 ℃. The thermal reaction complete at about 370 ℃, and the
 mass loss ratio of PS/Cu in air is 25.81%, which is consistent with the
 theoretical calculation.  
 Figure
 9(a) is the XRD pattern of the product obtained in
 N<sub>2</sub>, which closely matches Cu (JCPDS No. 04-0836)
 and no impurity peak is detected. The peaks at 43.31°, 50.46°and 74.13°
 are respectively corresponded to crystalline planes of (111), (200) and
 (220). Meanwhile, the diffraction peaks at 35.68°, 38.86° and 48.79° in
 Figure 9(b) corresponding to the reflections of (002), (111) and (-202)
 crystalline planes of copper oxide (JCPDS No. 45-0937), indicating that Cu
 is oxidized to CuO completely.   Figure 10
 gives the microstructures of the sintering products. As can be seen in
 Figure 10(a, b), Cu deposits maintain good sphere structure and form
 hollow Cu microspheres with shell thickness of about 100 nm after
 sintering. Cu nanoparticles are coarsening obviously compared to the
 microstructure before sintering (Figure 7b). The surface of Cu
 nanoparticles melts slightly and thus Cu particles connect to each other
 forming a strong hollow microsphere structure during sintering process.
 The glass transition temperature of PS is 80-10 °C and thus PS skeleton
 melts and collapses before Cu oxidation, leading shrink of microspheres.
 Figure 10(c, d) shows that the copper oxide still maintains a complete
 hollow spherical shell structure with thickness of about 200 nm, which is
 larger than copper microspheres (Figure 10a). This is due to the oxidation
 of copper, which makes the thickness increase and the adjacent microsphere
 also adheres together. The above results show that hollow copper
 microspheres can be obtained by sintering PS / Cu in nitrogen and copper
 oxide microspheres can be obtained by sintering in air.