EG-Assisted hand-in-hand growth of prism-like Cu₂O nanorods with high aspect ratios and their thermal conductive performance

Abstract

In this work, one-dimensional, monodispersed prism-like Cu₂O nanorods with a significantly high aspect ratio (∼100) have been successfully prepared by a simple hydrothermal synthesis with the assistance of ethylene glycol (EG). The obtained nanorods expose {100} faces and have a rectangular cross section. Their length, thickness and width are in the range of 50–100 μm, 0.5–1 μm and 0.5–1.5 μm, respectively. Pyrrole (Py) and EG play important roles in the formation of the prism-like nanorods. Py acts as a structure-directing reagent that makes the Cu₂O crystal grow along the [100] direction. EG acts as a “bridge” that controls the “hand-in-hand” growth of the intermediate Cu₂O nanowires and transforms them into prism-like nanorods. The products are first employed as thermal conductive nanofillers to improve the thermal conductivity of poly(vinylidene fluoride) (PVDF)-based polymer composites. The original thermal conductivity studies indicate that the rod-type structural nanomaterials are very efficient fillers for polymer composites. When they were embedded in the PVDF matrix at 30 wt%, the Cu₂O nanorods show a thermal conductivity enhancement (TCE) of 275% when compared with the pristine PVDF, which is 1.4 times higher than commercial Cu₂O (cubes) used as fillers in PVDF composites.

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Introduction

The fabrication of nano-materials has been one of the widely researched branches of nanoscience because of their excellent and unique properties.^1–3 To date, the application of nano-materials have been extended to almost all conceivable fields of science and technology, ranging from daily products to aerospace materials.^4,5 Continued progress of micro/nano-size devices with increasing power density in electronic chips required the development of high-effect thermal interface materials to resolve heat removal problems.^6,7 Nanofillers are undoubtedly one of the best materials to solve this problem.^8,9 As reported, potential nanofillers include alumina (Al₂O₃),¹⁰ zinc oxide (ZnO),¹¹ aluminum nitride (AlN)¹² and silver (Ag).¹³ As shown in the periodic table, copper and zinc are neighboring elements in the same period of the transition metals. Furthermore, silver and copper are in the same group. Thus, it would be expected that copper and its oxide could be used as efficient fillers in thermal conductive polymer composites.

Cuprous oxide (Cu₂O), an important metal oxide with a band gap of 2.0–2.2 eV, has potential applications in several fields such as solar cells,¹⁴ hydrogen energy conversion,¹⁵ catalysis,¹⁶ gas sensors,¹⁷ and electrode materials.¹⁸ Because of these extensive practical applications, various well-defined Cu₂O nanostructures have been synthesized in the past decades such as nanospheres,¹⁹ polyhedra,^20,21 and complex nanostructures.^22–24

It is well-known that the properties of nano-materials are highly size and shape dependent.²⁵ Recently, numerous investigations have shown that the thermal conductivity of nano-material based polymer composites depends not only on their sizes but also on their shapes.²⁶ The control of shape in addition to the size of the fillers with a high aspect ratio is of important for thermal management applications.²⁷ However, there are limited studies reporting the preparation of well-defined Cu₂O nanorods when compared with other 1D nanostructures. Chang et al. reported the preparation of Cu₂O nanorods with a diameter of 60 nm and length of 450 nm via electrodeposition onto a PAA template.²⁸ However, template methods always require a multi-step procedure, in addition, removal of the templates is inevitably deleterious to the pristine structure.²² The direct synthesis of Cu₂O nanorods still remains a significant challenge. A hydrothermal method is the simplest and probably the most feasible method to prepare 1D nanoscale materials. Because crystal growth is anisotropic, it is prone to grow slowly toward a 1D direction under hydrothermal conditions in the presence of a catalyst.²⁹ Therefore, it would be very interesting to synthesize Cu₂O nanorods by a hydrothermal method and investigate the thermal conductivity properties of the Cu₂O nanorods with a high aspect ratio, which have been seldom reported.

In the present work, we have synthesized prism-like Cu₂O nanorods through the reduction of (CuAc₂·H₂O) with Py as the reducing agent in a deionized water EG mixed solvent under hydrothermal conditions. The as-prepared prism-like nanorods were 50–100 μm in length, 0.5–1 μm in thickness, and 0.5–1.5 μm in width. A “hand-in-hand” growth mechanism was proposed to investigate the formation of the prism-like structure. Moreover, the performance characteristics of the nanorod structured material as thermal conductive nanofillers was first presented in PVDF-based thermal composites.

Experimental

Materials

Cupric acetate (CuAc₂·H₂O), N,N-dimethylformamide (DMF), pyrrole (Py), and commercial copper(I) oxide (Cu₂O) cubes (diameter 2.5 μm, purity 99%) were purchased from a commercial source (Aladdin) and used as received. Poly(vinylidene fluoride) (PVDF, Mw ∼534 000) powder was obtained from Sigma-Aldrich.

Synthesis of Cu₂O nanorods

First, CuAc₂·H₂O (0.16 mmol) was dissolved in a deionized water ethylene glycol mixed solvent (43 mL H₂O + 250 μL EG), and stirred for 10 min. Then, Py (22 μL) was added to the solution and stirred for another 10 min. All the solutions were kept in a water bath at 35 °C. Then, the reaction mixture was transferred to a 45 mL autoclave and maintained at 140 °C for 24 h. After naturally cooling to ambient temperature naturally, red-brown sediments were formed at the bottom of the autoclave. The product was centrifuged, washed with ethanol six times and dried at 60 °C.

Preparation of PVDF/Cu₂O composites

First, the Cu₂O nanorod sample was ground into a powder to make it easily dispersed in DMF. The fine powder thus obtained was used for subsequent fabrication. PVDF was dissolved in DMF (3 mL) and stirred for 60 min to form a stable solution. Second, a certain amount of Cu₂O fine powder (the total amount of Cu₂O and PVDF was 300 mg) was added to the PVDF/DMF solution, and the resulting mixture was sonicated for 180 min to form a relatively stable solution. Finally, a film was cast from the viscous PVDF/Cu₂O DMF solution onto a glass plate by a scraper. The film was dried in a vacuum oven at 60 °C for 8 h to slowly evaporate the solvent, and the PVDF/Cu₂O composite film was obtained with 3 cm × 6 cm in size and about 20 μm in thickness.

Characterization

The structure of the products was characterized by powder X-ray diffraction (XRD) using a Rigaku RotaflexDmax2200 diffractometer with Cu Kα radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) images of the samples were obtained using a Hitachi S-4800 with an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were recorded using a JEOL JEM-2100F with an accelerating voltage of 200 kV. The thermal conductivity of the composite film was evaluated by a Hot Disk Thermal Analyzer (TPS 2500S, Sweden) equipped with a 30 mm diameter Kapton sensor disk, which was setup following the transient hot-strip method. The technique was based on recording the transient temperature increase of a 25 μm-thick, 8 mm-wide, and 70 mm-long iron strip clamped between two halves of a sample and heated at a constant direct current in the temperature range of 20 °C to 700 °C.

Results and discussion

Characterization of prism-like Cu₂O nanorods

The crystallographic structure of the as-prepared products was confirmed by X-ray diffraction (XRD). As shown in Fig. 2d, all the diffraction peaks (2θ = 29.7°, 36.4°, 42.3°, 61.4° and 73.5°) could be well indexed to the cubic Cu₂O (JCPDS no. 05-0667). No other diffraction peaks from possible impurities, such as CuO or Cu, could be identified, indicating their purity.

The top-view scanning electron microscopy (SEM) image (Fig. 1a) indicated that the as-prepared products were a mass of straight prism-like nanorods and were uniform in size. The average size of the nanorods was measured to be 50–100 μm in length, 0.5–1 μm in thickness, and 0.5–1.5 μm in width. Thus, the aspect ratio of the Cu₂O nanorods was of the order of 50–100. By zooming onto an individual nanorod (Fig. 1b), it could be seen that the surface of the nanorod was not completely smooth but had some small bumps. Interestingly, unlike other nanorods, the cross section of the nanorods was rectangular rather than round (inset of Fig. 1b). The detailed structure and the growth direction of the nanorods were examined by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) analysis. Fig. 1c recorded at the end of an individual nanorod highlighted the rectangular feature of the cross section. The measured two vertical sets of lattice fringes in the high-resolution transmission electron microscopy (HRTEM) image (the bottom frame area in Fig. 1c) was ca. 0.298 nm, well-coincident with the (110) crystal plane of the cubic Cu₂O, demonstrating that the Face 2 in Fig. 1c were the {100} face. Fig. 1e recorded at the left frame area shows the distance between the fringes perpendicular to the wall was ca. 0.210 nm, which also agreed well with the (100) crystal plane of cubic Cu₂O, demonstrating that the Face 1 in Fig. 1c was also the{100} face. Therefore, the exposed faces of the prism-like nanorods were {100} faces. The SEAD pattern of the Cu₂O nanorods (Fig. 1f) exhibited as bright diffraction spots, revealing their single crystalline structure and indicated that the Cu₂O nanorods grow along the [100] direction.

Fig. 1 (a and b) SEM images of prism-like Cu₂O nanorods with different magnifications. Inset (b) SEM image of the cross section of an individual nanorod. (c) TEM image of the end of an individual nanorod. (d) HRTEM image of the bottom frame area in (c). (e) TEM image of the left frame area in (c). (f) Corresponding SAED pattern viewed along the [100] direction of the individual nanorod in (c).

To understand the formation mechanism of the prism-like Cu₂O nanorods, we performed XRD characterizations and extensive electron microscopy investigations on the samples collected at different stages. After 2 h, microspheres with average sizes of 1 μm dominated the products (Fig. 3a). The corresponding XRD pattern (Fig. 2a) of the black products consisted of four peaks at 2θ = 35.3°, 38.5°, 48.8° and 61.4°, which could be perfectly indexed to monoclinic CuO (JCPDS no. 05-0661). No other peaks from impurities, such as Cu₂O, could be identified. It suggests that CuO microspheres were formed at the initial stage. When the reaction had proceeded for 5 h, CuO microspheres dissolved to form CuO nanospheres with diameters in the range of 60–80 nm (Fig. 3b), and the corresponding HRTEM image (Fig. 3e recorded at the black frame area in Fig. 3b) verified that the distance between the fringes was ca. 0.253 nm, which corresponded to the spacing between the (002) lattice plane of CuO. In addition to the CuO nanospheres, nanowires with diameters in the range of 250–300 nm appeared (Fig. 3b). The HRTEM image of the nanowire (Fig. 3f recorded at the white frame area in Fig. 3b) shows that the distance between the fringes was ca. 0.295 nm, which corresponded to the spacing between the (110) lattice plane of Cu₂O. The corresponding XRD pattern (Fig. 2b) also demonstrated the formation of Cu₂O that significantly reduces the diffraction peaks from CuO with two new peaks emerging at 2θ = 36.4° and 42.3°, which agree well with the cubic Cu₂O (JCPDS no. 05-0667). When the reaction time reached 8 h, the amount of CuO nanospheres was significantly reduced, and the products mainly consisted of Cu₂O nanowires (Fig. 3c). The considerably intensified signals from Cu₂O in the XRD pattern (Fig. 2c) proved that Cu₂O was generated large quantities. Remarkably, these Cu₂O nanowires began to grow into nearly orderly prism-like wire bundles, which revealed a tendency to transform into prism-like nanorods. The inset of Fig. 3c clearly shows the joint between the two Cu₂O nanowires. The intermediate products obtained at 12 and 16 h clearly show the growth process from nanowires to nanorods (Fig. S1†). It should be pointed out that, as shown in the white frame area of Fig. S1b,† there were still some nanowires, which suggested that in the transformation process from nanowires to nanorods new nanowires were still being generated. Finally, when the hydrothermal treatment had proceeded for 24 h, perfect prism-like Cu₂O nanorods were formed (Fig. 3d). The corresponding XRD pattern (Fig. 2d) demonstrated that the diffraction peaks from Cu₂O dominated, while those from CuO disappeared. The obtained products were still prism-like Cu₂O nanorods if the reaction time was further extended to 48 h (Fig. S2†).

Fig. 2 XRD patterns of the intermediate products collected at (a) 2, (b) 5, (c) 8, and (d) 24 h.

Fig. 3 SEM images of the intermediate products collected at (a) 2, (b) 5, (c) 8, and (d) 24 h. (e) TEM image of the black frame area in (b). (f) TEM image of the white frame area in (b).

Combining the composition and structural characterization, the growth process of the prism-like Cu₂O nanorods can be divided into three stages. In the initial stage, uniform CuO microspheres formed through a two-step reaction process, as shown below:

Cu²⁺ + H₂O → Cu(OH)₂+ 2H⁺

Cu(OH)₂ = CuO + H₂O

Li et al. reported³⁰ that the organic monomer, such as Py, could act as a reducing agent during the hydrothermal process. However, because of the weak oxidizing ability of Cu²⁺, the reduction process of Py to reduce Cu²⁺ to Cu⁺ should be carried out at temperatures above 110 °C under hydrothermal conditions. In our case, the temperature inside the autoclave may not be high enough to reduce Cu²⁺ to Cu⁺ using Py within 2 h. Thus, only CuO was formed and aggregated as spherical shapes to lower the surface energy. In addition, it is well-known that Py can also act as a structure-directing reagent to make the crystal grow along a particular direction.³² Because of the rapid aggregation of CuO, Py did not function as a structure directing agent.

In the next stage, as the reaction proceeded for 5 h, with the temperature and pressure inside the autoclave further increased, the redox reactions between Py and CuO could occur, subsequently, CuO microspheres gradually decomposed to CuO nanospheres with diameters in the range of 60–80 nm (Fig. 2b).³¹ Meanwhile, because of the slow reduction process, Py would act as structure-directing reagent to induce the growth of nanowires (Fig. 2b). To confirm the role of Py, a control experiment was carried out in the absence of Py while maintaining the other reaction conditions unchanged (with only EG in the reaction solution). As shown in Fig. S3b,† only black products (CuO) with irregular sphere-like morphology were obtained. To further investigate the effect of EG, experiments were carried out without EG and Py in the reaction mixture. The obtained product was still black (CuO) with an irregular sphere-like morphology (Fig. S3a†), which suggested that the addition of EG did not have a significant effect on the formation of nanospheres.

In the last stage, with Cu₂O nanowires formed in large quantities, EG began to play an important role in the formation of prism-like Cu₂O nanorods. Control experiments demonstrated that only Cu₂O nanowires were obtained without EG. By contrast, when EG was introduced, the Cu₂O nanowires could grow into prism-like nanorods. In addition, the prism-like nanorods became increasingly apparent with an increase in EG. (Fig. S4†) As reported before,³³ EG could play a major role in the hydrothermal process as a co-surfactant in an aqueous system. EG has two hydroxyl groups (−OH) at both the ends when it was introduced to the reaction mixture. One end of the negatively charged ‘OH’ group could interact with the positively charged ‘Cu’ on the surface of the Cu₂O nanowires.³⁴ The other ‘OH’ group of EG could interact with the later formed Cu₂O nuclei from the reaction mixture. Thus, we speculated that EG may act as a “bridge” and control the Cu₂O nanowires to grow “hand-in-hand” into prism-like Cu₂O wire bundles. Following the Ostwald ripening process, the Cu₂O wire bundles gradually transformed into prism-like Cu₂O nanorods with exposed {100} faces, and is consistent with the equilibrium form of a crystal that tends to possess a minimum total surface energy.

We further investigated the effect of temperature on the final products. When the reaction temperature was decreased from 140 °C to 120 °C, the product mainly consisted of microspheres with average sizes of about 1–2 μm. In addition to microspheres, there was a small quantity of irregular nanorod structures. When the reaction temperature increased to 160 °C (as shown in Fig. S5b†), the Cu₂O nanorods assembled into hierarchical structures. Only when the reaction temperature was 140 °C, the prism-like Cu₂O nanorods were formed.

Based on the abovementioned analysis, we speculate that the formation of the prism-like Cu₂O nanorods occurred through a “hand-in-hand” growth process, and the probable growth route is illustrated in Scheme 1.

Scheme 1 Illustration of the formation of prism-like Cu₂O nanorods.

Thermal properties of the Cu₂O/PVDF composite

PVDF was selected as a matrix due to its good thermal stability and easy processing. The thermally conductive PVDF/Cu₂O composite films were typically prepared by solvent assistant mixing. A fine powder of Cu₂O nanorods was added to the PVDF/DMF solutions, followed by 180 min of sonication to form a relatively stable solution. The PVDF/Cu₂O composite film was obtained with a composite loading in the range of 10–30 wt%. The through-plane conductivity of the composite was measured with a hot disk thermal analyzer, which was operated on the transient hot-strip method. In this technique, the bulk through-plane thermal conductivity was calculated from the slope of the temperature versus time plot.³⁵

The thermal conductivities of the Cu₂O/PVDF composites are shown in Fig. 4. The room temperature thermal conductivity of the pure PVDF is about 0.12 W mK⁻¹. The use of both Cu₂O nanorods and commercial Cu₂O cubes as fillers in the Cu₂O/PVDF composites revealed a dramatic enhancement of thermal conductivity in comparison with the pure PVDF. When commercial Cu₂O cubes are used as fillers, the thermal conductivities of the PVDF-based composites show a slight increase, and a linear relationship is observed versus the loading, as shown in Fig. 4 (round line). However, for the prism-like Cu₂O nanorods, it should be noted that the improvement of thermal conductivity in the Cu₂O/PVDF nanocomposites is non-linear (See Fig. 4 square line). At low loading (below 10 wt%), Cu₂O nanofillers dispersed randomly in the PVDF matrix resulting in a slight increase in thermal conductivity. At a high filler loading, the thermal conductivity increased evidently with increasing Cu₂O content. When 30 wt% loading of the Cu₂O nanorods embedded in the PVDF matrix, the thermal conductivity of the composites is 2.7 times higher than that of the pristine PVDF.

Fig. 4 Thermal conductivities of the Cu₂O/PVDF composites as a function of weight fraction for Cu₂O nanorods and Cu₂O cubes.

This implies that efficient thermal transfer pathways start to form at a high fraction of nanorods because of rod-to-rod connection networks. It is believed that at Cu₂O nanorods fraction of higher than 30 wt%, a more effective improvement is expected. Fig. 5 shows the thermal conductivity enhancement (TCE, κ − κ₀/κ) versus weight loading for PVDF composites with commercial Cu₂O cubes and prism-like Cu₂O nanorod fillers. The thermal conductive performance of Cu₂O nanorods/PVDF composites is much better than that of Cu₂O cubes/PVDF composites. At 30 wt% filler loading, the TCE value of the Cu₂O nanorods-based composites is 5 times larger than those of Cu₂O cubes. The obvious difference between these two fillers was the aspect ratio of the particles. As recently reported, the control of shape in addition to the size of the fillers with high aspect ratios is of significant importance for the improvement of thermal property. These comparative results clearly confirm the improvement of thermal conductivity performance of composites with an increase in the aspect ratio. As shown in Fig. 1, the Cu₂O nanorods have a high aspect ratio and perfect prism-like nanostructure but commercial Cu₂O cubes are polyhedral in size of about 2.5 μm (Fig. S6†). After calculation, the aspect ratio of the Cu₂O nanorods is of the order of 50–100, which approaches the aspect ratio of the highest filler effect SWCNTs (100–1000). The contribution of the Cu₂O fillers to thermal conductivity increases as the aspect ratio increases, which is responsible for the improved thermal performance of Cu₂O in comparison with previous measurements on similar fillers.

Fig. 5 Comparison of thermal conductivity enhancement for Cu₂O nanorods and Cu₂O cubes in a PVDF matrix at different filler loadings.

In comparison with the research carried out on the thermal conductivity of inorganic/ceramic polymer composites, not much work has been done on 1D nanofiller-based polymer composites. Most of them are studied on nanoparticles, for example, AlN nanoparticles could give a TCE value of 87% with a 27.5 wt% loading of filler in epoxy composites,³⁶ and Al₂O₃ nanofillers reach 60% TCE with a 30 wt% loading in PVDF composites.³⁷ The Cu₂O cubes in this study can reach the same level of thermal improvement with the same filler loading. Among the few studies on 1D inorganic nanofillers, only nanowires and nanotubes have been reported such as ZnO nanowires and boron nitride nanotubes (BNNT).

To the best of our knowledge, there are still no thermal studies conducted on inorganic nanorod structures. The thermal conductive performance of Cu₂O nanorod based polymer composite is close to that published for ZnO nanowires based polymer composite.³⁸ BNNT exhibits much better thermal conductive performance because of its high intrinsic thermal conductivity with very low dielectric constants.⁹ However, BNNT is always synthesized with CVD, and it is hard to scale up the synthesis using a simple method. Further enhancement in the thermal conductivity of Cu₂O nanorod-based composites may be possible if the dielectric constants could be reduced. This might be achieved by introducing physical or chemical functionalities on the surface of the Cu₂O nanorods, which has worked for AlN and BNNT-based composites.^9,12 Therefore, these initial results and the advantage of Cu₂O nanorods will render Cu₂O-based composites as potentially thermally conductive materials for applications in electronic devices.

Conclusions

In conclusion, we have successfully suggested an effective hydrothermal method to prepare prism-like 1D Cu₂O nanorods with high aspect ratio (∼100). The “hand-in-hand” growth mechanism was proposed in the present work. EG acts as a “bridge” that controls the “hand-in-hand” growth of the intermediate Cu₂O nanowires and transforms them into the prism-like nanorods. When embedded in a PVDF matrix, the prism-like Cu₂O nanorods show a large thermal conductivity enhancement when compared with that of pristine PVDF. The significant suppression of the thermal conductivity of the 1D nanofiller-based composites was attributed to the presence of rod-to-rod connection networks. Further work on increasing filler loading and/or decreasing the dielectric constants of nanorods by chemical functionalization is envisaged to improve the thermal performance of the composites. Therefore, we demonstrate a rod-type material structure for the development of thermal interface materials.

Acknowledgements

The project was supported by the National Key Basic Research Program of China (2010CB934700), National Natural Science Foundation of China (51272012), Innovation Foundation of BUAA for PhD Graduates and Fundamental Research Funds for Central Universities.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03480e

‡ These authors contributed equally to this work.

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