Journal ofMaterials Chemistry A

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aInstitute of Materials Research and Enginee

Research (A*STAR), 3 Research Link, 117602

edu.sgbJurong Consultants Pte Ltd, 8 Jurong Town

† These authors contributed equally to th

Cite this: J. Mater. Chem. A, 2014, 2,3417

Received 6th November 2013Accepted 6th December 2013

DOI: 10.1039/c3ta14550f

www.rsc.org/MaterialsA

This journal is © The Royal Society of C

Optimized production of copper nanostructureswith high yields for efficient use as thermalconductivity-enhancing PCM dopant

Thammanoon Sreethawong,†a Kwok Wei Shah,†a Shuang-Yuan Zhang,a Enyi Ye,a

Suo Hon Lim,a Uma Maheswaran,b Whey Ying Maob and Ming-Yong Han*a

Copper nanostructures with a high yield are synthesized by a controlled disproportionation of CuCl

in oleylamine reaction medium without the involvement of strong reducing agents adopted in

conventional approaches. The highest copper yield (50%), a maximum theoretical value, is obtained

by optimizing both the initial amount of CuCl added to the reaction medium and the reaction

temperature. A potential use of the copper nanostructures in greatly enhancing thermal conductivity

of a hydrated CaCl2$6H2O salt phase change material (PCM) is further demonstrated. A high thermal

conductivity enhancement of the PCM (>50%) is achieved by doping a small amount (<0.2 wt%) of

the copper nanostructures. The great enhancement with the use of a very small amount of the

copper nanostructures makes the doping cost-effective for practical thermal energy storage

applications.

Introduction

Phase change materials (PCMs) have been extensively inves-tigated for thermal energy storage (e.g., waste heat recoveryand solar thermal energy utilization) due to their high latentheat of fusion upon melting and solidifying cycles, and PCM-based thermal energy storage can be achieved isothermallyduring the phase change.1–4 Nowadays, PCMs are mainly usedfor human-associated activities, particularly for the heatingand cooling of “green” buildings.3,5–9 Their use usuallyencounters the inherent problem of low thermal conductivity,which consequently hinders the thermal energy transfer anddrastically reduces their heat absorption/release capability. Anumber of strategies have been developed to maximize thethermal energy storage capacity of PCMs through improvingtheir thermal energy transfer properties using thermallyconductive materials, such as impregnating PCMs in matrices(graphite), incorporating metal frameworks (copper andaluminum) in PCMs, and dispersing particles (copper, copperoxide, graphite, etc.) in PCMs.10–21 Among these, copperparticles have been considered to be a more suitable thermallyconductive material as a PCM dopant due to their highthermal conductivity and low cost.18,20,22–24 Microsizedcopper particles are commonly produced by atomization,

ring, Agency for Science, Technology and

, Singapore. E-mail: my-han@imre.a-star.

Hall Road, 609434, Singapore

is work.

hemistry 2014

electrolysis, hydrometallurgy, or solid state reduction.25,26

Their surface is easily oxidized upon air exposure, resulting inan ineffective improvement of thermal conductivity of PCMsaer doping due to their greatly reduced thermal conductivityas compared with the metallic state.27–29 Currently, there isa demand to develop efficient surface-protection methodsfrom oxidation.

Nanosized copper particles (nanoparticles) are usuallyproduced by a wet-chemistry approach in the presence ofsuitable ligands/stabilizers, which can strongly bind to thecopper surface during their preparation. The surface-coatedcopper nanoparticles have high resistance to oxidation, whichprovides better functionality as a PCM dopant for enhancingthermal conductivity. As demonstrated in this work, surface-coated copper nanostructures are synthesized in a controlledmanner by disproportionation of CuCl in oleylamine as areaction medium at high temperatures. The copper yield wasmaximized (increased by �147%) by using an optimized rangeof initial amounts of CuCl (4.8–6.6 g in 36 mL medium) at areaction temperature of 270 �C. This method does not involvestrong reducing agents, and the resulting copper product isvery stable when stored in air or organic solvents due to thesurface coordination with oleylamine.30 The surface-protectedcopper nanostructures exhibited a very high capability forenhancing the thermal conductivity of a hydrated CaCl2$6H2Osalt PCM (i.e., �52% enhancement using 0.17 wt% coppernanostructures). The great enhancement with the use of asmall doping content and its promising scalable synthesisare very important in PCM-based thermal energy storageapplications.

J. Mater. Chem. A, 2014, 2, 3417–3423 | 3417

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ExperimentalChemicals

Copper(I) chloride (CuCl, 99%, Sigma-Aldrich), oleylamine(C18H35NH2, 70%, Sigma-Aldrich), hexane (Primechem Chem-icals), and hydrated CaCl2$6H2O salt PCM (savE® HS 29, PlussPolymers) were used as received without further purication.

Synthesis of copper nanostructures

Typically, oleylamine (36 mL) was mechanically stirred andheated to 80 �C in a three-neck ask, followed by degassing withnitrogen under vacuum for 20 min. For a typical synthesis, apre-determined amount of CuCl was added to the degassedoleylamine at 80 �C, followed by rapidly heating to 270 �C (themixed solution becomes transparent and yellow when thetemperature goes up to �140 �C due to the complete formationof the Cu+–oleylamine complex). Aer maintaining at 270 �C for1 h to produce reddish colloidal copper nanostructures, thereaction system was allowed to cool to room temperature. Thecopper nanostructures were separated from the reactionmedium by centrifugation. The collected copper nanostructureswere puried by washing with hexane via redispersion–centri-fugation three times. In comparison, the synthesis of coppernanostructures was also performed at 200 �C for 1 h using 6 mLor 36 mL oleylamine.

Characterization

Bright-eld transmission electron microscopy (TEM) images ofcopper nanostructures were recorded using a JEOL 2100microscope operated at an accelerating voltage of 200 kV. Inpreparing the samples, a drop of hexane solution containingcopper nanostructures was placed on the surface of a laceyformvar/carbon 300-mesh copper grid, and the grid was allowedto dry in air. Powder X-ray diffraction (XRD) patterns of coppernanostructures were recorded by a Bruker GADDS D8 Discoverdiffractometer using CuKa radiation (1.5418 A) at a workingvoltage and current of 40 kV and 40 mA, respectively. Fouriertransform infrared spectroscopy (FTIR) spectra of the fresh andused oleylamine reaction media, as well as that of the synthe-sized copper nanostructures, were recorded using a PerkinElmer Spectrum 2000 FTIR spectrophotometer.

Thermal diffusivities of the copper nanostructures-dopedPCM samples were measured using a NETZSCH LFA 447NanoFlash™ diffusivity apparatus at room temperature with aspecial aluminum/stainless steel liquid sample holder. Thethermal diffusivities of the samples were measured ve times,and the standard deviation of the ve measurements was lessthan 2.5%. Aerward, the thermal conductivity of each samplewas calculated by using l(T) ¼ r(T)cp(T)a(T), where l is thermalconductivity (W m�1 K�1) of the sample, r is the bulk density (gcm�3) of the sample determined from its volume andmass, cp isthe specic heat capacity (J g�1 K�1) of the sample determinedfrom the known standard values (i.e., 2.261 and 0.389 J g�1 K�1

for the PCM and copper, respectively), a is thermal diffusivity(mm2 s�1) of the sample (averaged value from the vemeasurements for each sample), and T is temperature. In

3418 | J. Mater. Chem. A, 2014, 2, 3417–3423

preparing the samples, the synthesized copper nanostructureswere highly dispersed in the hydrated CaCl2$6H2O salt PCMabove the phase change temperature under ultrasonicationwith the aid of a small amount of ethanol (i.e., 2 wt%) as adispersion-facilitating agent.

Results and discussion

Disproportionation of CuCl in oleylamine as a reactionmedium, i.e., the transformation of monovalent copper ions(Cu+) to metallic copper (Cu0) and divalent copper ions (Cu2+), isused for the synthesis of the copper nanostructures, as shown inFig. 1a. The yield and morphology of resulting copper nano-structures are strongly inuenced by the initial amount of CuCladded and reaction temperature (Fig. 1b–d, insets showing thepowders of reddish copper nanostructures aer purication).Experimentally, 6 mL oleylamine was heated to 80 �C and 0.2 gCuCl was then added, followed by heating up quickly to 200 �C.Aer 1 h disproportionation of 0.2 g CuCl in 6 mL oleylamine at200 �C, a lower copper yield of 37% (Fig. 1b) was obtained ascompared with the maximum copper yield of 50% for completedisproportionation. The copper product was observed to have auniform nanowire morphology (�50 nm in diameter and >10mm in length), revealing one-dimensional anisotropic growth ina controlled manner. The XRD pattern of the copper product(Fig. 1b) shows the face-centered cubic copper phase, as seenfrom the (111), (200), and (220) crystalline peaks (JCPDS no. 03-1018). No copper oxide peaks were observed, revealing the highpurity of the copper nanostructures.

To maintain the initial concentration of CuCl at 0.1 g per 3mL medium, both the amount of CuCl and volume of oleyl-amine were simultaneously increased 6 times to 1.2 g and 36mL, respectively. For the larger scale synthesis, the copper yieldwas signicantly reduced to 10% aer 1 h of reaction at 200 �C(Fig. 1c), which is much lower than 37% for the 0.2 g/6 mLsystem. The time required to heat the 1.2 g/36 mL system from80 to 200 �C is �12 min. Aer maintaining at 200 �C for �10–15min, reddish colloidal copper was formed. In comparison, it tooka shorter amount of time (�6 min) to heat the 0.2 g/6 mL systemfrom 80 to 200 �C, and the induction time for the dispropor-tionation of CuCl in oleylamine to form colloidal copper was alsomuch shorter (�1–2 min). The faster nucleation and growth forthe 0.2 g/6 mL system than the 1.2 g/36 mL system can producecolloidal copper quickly, resulting in uniform copper nanowires.For the 1.2 g/36 mL system, initial nucleation is slower when thetemperature goes up to 200 �C. Due to its slow disproportion-ation, even aer a certain period of reaction time, the concen-tration of CuCl precursor is still high enough for slowernucleation to continuously take place, resulting in non-uniformcopper nanowires accompanied by smaller nanoparticles(Fig. 1c). As a consequence, the slower nucleation and growth ledto the production of copper nanostructures with a lower yield.The resulting copper nanostructures from this system also havehigh purity, as revealed by the XRD pattern in Fig. 1c.

To accelerate the disproportionation of CuCl for the 1.2 g/36mL system, the reaction temperature was increased from 200 to270 �C. As a result, the copper yield greatly increased three-fold

This journal is © The Royal Society of Chemistry 2014

Fig. 1 (a) Reaction showing the disproportionation of Cu+ precursor to copper nanostructures in oleylamine reaction medium. The maximumyield of copper nanostructures from the disproportionation is 50%. (b) Yield, morphology, and XRD pattern of copper nanostructures synthesizedat 200 �C using an initial amount of CuCl of 0.2 g and 6 mL medium. (c) Yield, morphology, and XRD pattern of copper nanostructuressynthesized at 200 �C using an initial amount of CuCl of 1.2 g and 36 mL medium. (d) Yield, morphology, and XRD pattern of copper nano-structures synthesized at 270 �C using an initial amount of CuCl of 1.2 g and 36 mL medium. For all the synthesis conditions, the initialconcentration of CuCl precursor and reaction time were kept constant at 0.1 g per 3 mL medium and 1 h, respectively. The insets in (b–d) showthe physical appearance of the reddish copper nanostructure products after purification.

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to 34% aer 1 h of reaction (Fig. 1d), showing the much fasterformation of copper at the elevated temperature. Experimen-tally, colloidal copper was formed obviously once the tempera-ture reached�210 �C (�10–15min induction time was observedabove, at 200 �C, for the obvious formation of copper). It tookanother �5 min to increase the temperature up to 270 �C. Thefaster disproportionation of CuCl at 270 �C can quickly producea higher concentration of Cu0 in the reaction system to formmuch thicker copper nanorods (Fig. 1d), which are achieved bynot only the anisotropic growth of copper nanostructures butalso the perpendicular growth for thickening them. The fast

This journal is © The Royal Society of Chemistry 2014

disproportionation of CuCl does not affect the formation of thepure copper product, as observed from the XRD pattern inFig. 1d that is similar to the one at 200 �C (Fig. 1c).

To improve and maximize the copper yield at 270 �C, theinitial amount of CuCl added to a controlled volume (36 mL) ofthe reaction medium was systematically increased from 1.2 g(Fig. 1d) to 2.4, 3.6, 4.2, and 4.8 g. Aer 1 h of reaction, thecorresponding yield was increased proportionally from 34% to38%, 47%, 49%, and 50%, respectively (Fig. 2). It was observedthat a maximum yield of 50% was achieved with the use of 4.8 gCuCl. With further increase in the initial amount of CuCl to 6.0

J. Mater. Chem. A, 2014, 2, 3417–3423 | 3419

Fig. 2 Yield of copper nanostructures synthesized at 270 �C usingincreasing initial amounts of CuCl precursor in the oleylamine reactionmedium. All the synthesis experiments were performed for 1 h in 36mLoleylamine.

Fig. 3 (a) FTIR spectra of oleylamine (i) before and (ii) after 1 h ofreaction at 270 �C using 4.8 g CuCl in 36 mL medium. (b and c)Magnified regions of (a) in the wavenumber ranges of 1750–1500 cm�1

and 2300–1800 cm�1, respectively.

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and 6.6 g, the maximum yield was retained. Aer the initialamount was continuously increased to 7.2, 7.7, and 8.0 g, lowercopper yields of 48%, 48%, and 46%, respectively, were ach-ieved aer 1 h of reaction, slightly less than 50%. When theinitial amount of CuCl precursor was further increased, thecopper yield was continuously decreased due to the presence ofmore insoluble precursor at the reaction temperature for theformation of copper nanostructures. The results reveal that theoptimized range for the initial amount of CuCl, 4.8–6.6 g in 36mL medium, leads to complete disproportionation in 1 h,providing the maximum copper yield at 270 �C. In comparison,there is an increase in copper yield of �147% when increasingthe amount of CuCl from 1.2 g in 36 mL to 4.8–6.6 g in 36 mL,revealing that the addition of an appropriate amount of CuCl tothe reaction medium is crucial for maximizing the copper yield.

With the increase in CuCl concentration from 1.2 to 4.8 g in36 mL oleylamine, the obvious formation of copper nano-structures was observed at a lower temperature. For the 1.2 g/36mL system, the color of the yellow reaction solution remainedunchanged until the temperature went up to �210 �C. Beyondthat, the solution became darker, indicating the formation ofcopper nanostructures. For the 4.8 g/36 mL system, the yellowreaction solution became darker quickly aer the temperaturereached �190 �C, indicating the faster nucleation to producemore nuclei for the growth of copper nanostructures with theconsumption of more CuCl. It is seen that the formation ofcopper was much faster for the 4.8 g/36 mL system than the 1.2g/36 mL system at a lower temperature. Also, the use of moreCuCl precursor (4.8–6.6 g in 36 mL) can drive the dispropor-tionation (Cu+ / Cu0 + Cu2+) in the right direction so as toachieve a faster and more complete reaction, leading to anincreased copper yield aer 1 h of reaction. For 7.2 and 7.7 g in36 mL, the fast formation of copper was also observed at arelatively low temperature (�180–185 �C). With these nearlysaturated and saturated concentrations, a slightly decreasedyield aer 1 h of reaction may arise from a slower diffusion ratein a more viscous system and the co-existence of some insoluble

3420 | J. Mater. Chem. A, 2014, 2, 3417–3423

precursor. With prolonging the reaction time, the copper yieldwas increased. With the change of the initial amount of CuCladded, the copper yield varied drastically but the products wereobserved to retain a similar morphology. Upon comparing the0.2 g/6 mL system (Fig. 1b) to the 7.7 g/36 mL system, thesynthesis was scaled up by >38 times based on the amount ofCuCl used. The results have demonstrated that this syntheticprocess is scalable with high yields. In addition to optimizingthe initial amount of CuCl added and reaction temperature,other parameters including reaction medium, reaction time,and reactor size/conguration/control are also very importantfor the promising mass production of copper nanostructureswith high yields.

To investigate the mechanism for the production of thecopper nanostructures, the FTIR spectra of oleylamine beforeand aer 1 h of reaction at 270 �C for the 4.8 g/36 mL systemwere recorded and are depicted for comparison in Fig. 3a. Thefresh oleylamine showed peaks at 722 cm�1 for the (–C–C–)n (n$ 4) bending band, 966 and 1070 cm�1 for the ]C–H bendingband, 1465 cm�1 for the –C–H bending band, 1580 cm�1 for the

This journal is © The Royal Society of Chemistry 2014

Fig. 5 Schematic diagram showing the formation of surface-coatedcopper nanostructures synthesized using oleylamine reactionmediumwith the simultaneous transformation of oleylamine to its corre-sponding imine and nitrile products (–R ¼ –C17H33).

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–N–H bending band, 2854 and 2924 cm�1 for the methyl –C–Hstretching band, 3005 cm�1 for the]C–H stretching band, and3300 cm�1 for the –N–H stretching band. Aer the reaction andproduct separation, the used oleylamine showed decreasedintensities for the –N–H stretching and –N–H bending peaks,and two new peaks were observed at 1634 cm�1 for the –C]Nstretching band and 2028 cm�1 for the –C^N stretching band(Fig. 3b and c). These results reveal the transformation ofoleylamine (–CH2–NH2) to its corresponding imine (–CH]NH)and ultimately to nitrile (–C^N) during the synthesis at hightemperature. The FTIR spectrum of the resulting copper nano-structures is shown in Fig. 4. The peaks at 3436 and 1636 cm�1

are ascribed to –O–H stretching and –O–H bending bands ofwater adsorbed on the sample. The other peaks correspondingto oleylamine-derived molecules (–N–H bending, –C]Nstretching, and –C^N stretching bands) are also observed,revealing that all of them concurrently exist on the surface ofthe copper nanostructures.

Based on the experimental results, a schematic diagram forthe production of copper nanostructures in oleylamine at 270�C is illustrated in Fig. 5. During the process of heating theCuCl/oleylamine mixture to 270 �C, a clear solution formsquickly once the temperature goes up to�140 �C, indicating thecomplete formation of the Cu+–oleylamine complex. The Cu+–oleylamine complex starts disproportionating to form colloidalcopper at �180–210 �C, depending on the initial concentrationof CuCl in the system. At 270 �C, the formation of coppernanostructures is caused by the accelerated disproportionation(Cu+ / Cu0 + Cu2+), resulting in the observed higher yield(Fig. 1d) than the reaction at 200 �C (Fig. 1c). As observed by theFTIR spectra of the used oleylamine (Fig. 3), the strong oxida-tion of oleylamine molecules at 270 �C produces oleylimine andoleylnitrile molecules, indicating that oleylamine as a reducingreagent can also participate in the reduction of Cu+ to Cu0 athigh temperature. These three compounds were also observedon the surface of the copper nanostructures (Fig. 4), revealingtheir surface-coating properties. The surface-coated copper

Fig. 4 FTIR spectrum of the copper nanostructures obtained after 1 hof reaction at 270 �C using 4.8 g CuCl in 36 mL medium.

This journal is © The Royal Society of Chemistry 2014

nanostructures have high resistance to oxidation. They possesshigh stability under ambient conditions, as observed from theirluster.

As copper has a high thermal conductivity, the as-synthe-sized copper nanostructures were then applied for enhancingthe thermal conductivity of a PCM, which usually encounters aninherent problem of low thermal conductivity. The thermalconductivity enhancement capability of the hydratedCaCl2$6H2O salt was tested with different copper contents inthe range of 0.02–0.17 wt%. The thermal conductivity results ofthe PCM are represented by its percentage enhancement aercopper doping as compared with that of the pure PCM. Asshown in Fig. 6a, the thermal conductivity enhancement of thePCM signicantly increased from 0 to �22%, �43%, and �52%with increasing the doping content from 0 to 0.02, 0.08, and0.17 wt%, respectively. This great thermal conductivityenhancement (�52%), achieved by doping the PCM with a verysmall amount of copper (0.17 wt%), demonstrated a highthermal conductivity-enhancing capability of the copper nano-structures synthesized in the present work. The greatenhancement with the use of a very small amount of the coppernanostructures makes the doping cost-effective for practicalthermal energy storage applications.

The signicant increase in thermal conductivity by dopingcopper nanostructures into the PCM is schematically illustratedin Fig. 6b, which shows a rapid transfer and dissipation of heatin copper nanostructure-doped PCM from le to right (atemperature gradient in Fig. 6b). When the heat source issupplied at one side of the doped PCM, the highly thermalconductive copper nanostructures can accumulate heat effi-ciently so as to heat up their surrounding PCM rapidly. Incomparison, the pure PCM is heated up very slowly in theabsence of copper nanostructures. The Murshed–Leong–Yangmathematical model31 was applied to understand the increasein the thermal conductivity of the PCM by doping with coppernanostructures. It can be expressed as

km ¼ kf

�1þ 0:27as

4=3

�ks

kf� 1

���1þ

�0:52as

1� as1=3

��ks

kf� 1

��

1þ as4=3

�ks

kf� 1

��0:52as

1� as1=3

þ 0:27as1=3 þ 0:27

�8>><>>:

9>>=>>;;

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Fig. 6 (a) Thermal conductivity enhancement of the hydratedCaCl2$6H2O salt PCM as a function of copper nanostructure content.(b) Schematic diagrams of heat transfer/dissipation in the purehydrated CaCl2$6H2O salt PCM and copper nanostructure-dopedhydrated CaCl2$6H2O salt PCM after heating.

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where km is thermal conductivity of the copper nanostructure-doped PCM, kf is the thermal conductivity of the PCM (i.e., 0.5W m�1 K�1), ks is the thermal conductivity of the coppernanostructures (i.e., 2971 W m�1 K�1), and as is the copperparticle volume fraction. The model also predicts a similarincrease in thermal conductivity at low copper content whilethere is a moderate deviation of the predicted value at highercopper content. For instance, there is only �2% deviation fromthe experimental value at 0.08 wt% copper content, while thereis �22% deviation from the experimental value at 0.17 wt%copper content. In order to reduce the deviation at higherdoping content, the size, shape and geometry of the dopedparticles (e.g., elongated copper nanostructures) need to beconsidered to amend this model, which was previously devel-oped based on spherical particles.

Conclusions

The disproportionation of CuCl in oleylamine reaction mediumis used to synthesize copper nanostructures with a high yield.

3422 | J. Mater. Chem. A, 2014, 2, 3417–3423

Optimization of the initial amount of CuCl added to thereaction medium and the reaction temperature results in acopper yield of 50%, a maximum theoretical value. Thesynthesized copper nanostructures are very attractive forthermal energy transfer applications. As demonstrated in thiswork, a small doping content of copper nanostructures (0.17wt%) results in a very high capability for enhancing thethermal conductivity (�52% enhancement) of the hydratedCaCl2$6H2O salt PCM. The increased thermal conductivity ofthe PCM is benecial for PCM-related thermal energy storagepurposes, especially for “green” building materials. Thecopper nanostructures are a potential candidate to be used asan efficient thermal conductivity-enhancing PCM dopant forpractical applications.

Acknowledgements

This work was nancially supported by A*STAR's Science andEngineering Research Council (SERC), Ministry of NationalDevelopment (MND), and Institute of Materials Research andEngineering, Singapore.

Notes and references

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