A simple room temperature fast reduction technique for preparation of a copper nanosheet powder

Abstract

In this study, we demonstrate the synthesis of stable large copper nanosheets using fast reduction at room temperature. Nanosheets are synthesized by reducing Cu(OH)₄²⁻ with hydrazine hydrate in the presence of CTAB at room temperature. The size of the nanosheet is critically dependent on the concentration of NaOH and reducing agent, and an optimum concentration range has been determined experimentally. The large nanosheets may be centrifuged and dried into a powder and re-dispersed without any apparent size or shape change. Additional impurities (nano-rods, nano-particles) produced in the sub-optimal process is readily removed by the precipitation–re-dispersion process. While re-dispersing in chloroform, it was observed that the smaller nanosheets assembled like a pack of cards due to a high centrifugal field to form large nanosheets.

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Introduction

Apart from nanoparticles, nanorods and sheets find applications in nanodevices,¹ TCO films,² conducting ink,³ lithium ion batteries⁴ and many other areas. Copper is one of the important nanomaterials on which research is rather limited. The main reason probably is the low oxidation resistance of copper, which requires more rigorous synthesis procedures. In this study, we focus on room temperature synthesis of copper nanosheet powders. This powder can be re-dispersed into a variety of solvents to produce a stable oxidation resistant colloid.

One of the most prominent nanosheet material is graphene/graphene oxide, which has numerous applications.^5–7 Metal oxide nanosheets also have favourable electronic properties that find applications in many areas ranging from electronics to energy storage.⁸ Two-dimensional nanostructures such as nanosheets are mainly prepared by hydrothermal methods. Tang et al. (2012) synthesized graphene oxide nanosheets by a hydrothermal method using glucose as a reagent. Dubal et al. (2010) prepared copper oxide multilayer nanosheets for use in super capacitors using a chemical bath deposition method.⁹ Hu et al. (2010) presented a simple hydrothermal method for the synthesis of magnetic metal (Mn, Co, Fe and Ni) oxide nanosheets by refluxing the metal salt solution at 190 °C for different periods.¹⁰

Metallic nanosheets also possess unique physical and chemical properties due to large exposure of surface atoms.¹¹ For example, Huang and co-workers prepared ultrathin nanosheets of palladium of 1.8 nm thickness that finds application in photothermal therapy.¹² Gold nanosheets with controlled thickness are also successfully produced by Niu and co-workers.¹³ Duan et al. (2014) synthesized ultrathin rhodium nanosheets, which show efficient catalytic activities. Recently, Dang et al. (2014) prepared copper nanosheets using a hydrothermal method wherein PVP is used as a growth directing agent and applied those nanosheets in the formation of conductive inks.¹⁴ Hydrothermal methods generally involve prolonged heating of the reaction mixture.

While nanosheets are produced mostly by hydrothermal methods, nanorods of a variety of materials are being prepared by a large number of techniques, including hydrothermal,^15,16 sol–gel,¹⁷ seed mediated growth,¹⁸ vacuum vapor deposition,¹⁹ microwave assisted polyol synthesis,²⁰ thermal decomposition,²¹ electrochemical methods,²² and photochemical methods.²³ Zhu et al. (2010)²⁴ produced silver nanorods by ultrasonic irradiation of the aqueous solution of sliver nitrate, methenamine (HMTA) and polyvinyl pyrrolidone (PVP) for a period of 60 min. Yu et al. (1997)²⁵ synthesized gold nanorods by an electrochemical method at an electrolysis time of 30 min and a temperature of 38 °C. Zhang et al. (2006)²⁶ prepared copper nanorods by a hydrothermal method at a temperature of 120 °C for 4 hours. Liu and Bando (2003)²⁷ synthesized copper nanorods by a vacuum vapor deposition method within the TEM column. Panigrahi et al. (2006)²⁸ prepared copper nanorods using glucose as reducing agent at 80 °C.

From the foregoing discussion, it is clear that elongated nanostructures (nanorods/nanosheets) are synthesized without using any strong reducing agents. The reaction also involves heating the reaction mixture for a prolonged period. One of the very few exceptions is the method by Cao et al.²⁹ wherein a strong reducing agent was used at room temperature for synthesis of copper nanorods. Synthesis of copper nanosheets using a simple reduction of metal salts in the presence of a surfactant at room temperature has not been previously reported. In this study, we present a method to produce large copper nanosheets at room temperature using a simple fast reduction.

Another important aspect explored in this study is the phase transfer of copper nanosheets. Phase transfer of a nanosheet has not been reported so far. This is challenging due to its high aspect ratio and higher surfactant loading. Recently, Liu et al. (2013)³⁰ successfully transferred PEGylated gold nanorods to chloroform by a facile mechanical phase transfer approach. Goulet et al. (2012)³¹ transferred CTAB-stabilized Au, citrate-stabilized Ag and PVP-stabilized Ag nanoparticles and nanorods to toluene by ligand exchange using a precipitation and redispersion technique. In this study, we use the precipitation and redispersion technique to transfer copper nanosheets to various solvents such as chloroform, dimethylsulfoxide (DMSO), ethanol, methanol, hexane and toluene.

Experimental

Materials

Copper chloride dihydrate (CuCl₂·2H₂O) and dodecanethiol (DDT) were purchased from SD Fine Chemicals, India. Copper sulphate pentahydrate (CuSO₄·5H₂O), copper nitrate trihydrate (Cu(NO₃)₂·3H₂O), sodium borohydride, sodium hydroxide (NaOH), hydrazine hydrate (N₂H₂·2H₂O), dimethylsulfoxide (DMSO), and chloroform were purchased from Merck Chemicals, India. Cetyltrimethylammonium bromide (CTAB) was obtained from SRL Chemicals, India. Sodium dodecylbenzenesulfonate (SDBS) was purchased from LOBA Chemicals, India. Ethanol (AR 99.9%) was received from Jiangsu Huaxi International, China. All the chemicals were used as received without any purification.

Deoxygenation of solvents

Oxygen concentration in the solvents was reduced by deoxygenating the solvents by inert gas (argon/nitrogen) purging in a conical flask for about 30 minutes prior to synthesis. After deoxygenating, the dissolved oxygen level in the solvents was found to be ∼1 mg L⁻¹, as measured by a dissolved oxygen meter (HACH: HQ30d).

Synthesis of copper nanosheets in aqueous phase

Copper nanosheets in the aqueous phase were synthesized by modifying the protocol published by Cao et al. (2003). First, 1.5 mmol of copper chloride dihydrate (CuCl₂·2H₂O) was added to 100 ml of 1.5 M NaOH solution followed by stirring for about 10 minutes. After complete dissolution of copper chloride into NaOH solution, 6 mmol of cetyltrimethylammonium bromide (CTAB) surfactant was added and stirred for 20–30 minutes till the complete dissolution of the surfactant. Then, the abovementioned solution was mixed with 0.25 ml of 80% hydrazine hydrate solution. After adding the hydrazine hydrate, the color of the solution immediately changes from dark blue to orange and then brick red within 1 minute, indicating the formation of copper nanosheets. When other salts and reducing agents are used, their concentration remains the same as that of the chloride salt.

Phase transfer of copper nanosheets

For phase transfer, 1 ml of aqueous copper nanosheets suspension was centrifuged at 12 000 rpm for 30 min to precipitate the nanosheets and then, 0.5 ml of chloroform containing dodecanethiol (10 μl) was added to the precipitate and sonicated for 1–2 minutes to redisperse the nanosheets into chloroform. It produced a brick red suspension. In some experiments, this centrifugation and redispersion step was repeated to observe the growth of nanosheets. Copper nanosheet dispersions in DMSO, ethanol, methanol, hexane and toluene were also prepared in the same manner.

Characterization

The absorption spectra of copper nanosheets was obtained using a UV-Vis spectrophotometer (Shimadzu UV-1800). The dimensions and morphology of copper nanosheets were investigated by a transmission electron microscope (Tecnai G2 20S Twin) with an accelerating voltage of 200 kV and scanning electron microscope (ZEISS EVO 60, MERLIN and JEOL). The thickness of the nanosheets was characterized using an atomic force microscope (AFM) (Agilent Technologies, Model-5500, USA). The oxidation pattern of copper nanosheets was studied using the X-ray diffraction technique (Model: X-ray Diffractometer Philips PW-17291710) and energy dispersive X-ray spectroscopy (EDAX).

Results and discussion

Synthesis of copper nanosheets in aqueous phase

Fig. 1(a) shows the UV-Vis spectrum of the copper nanostructures produced by the present method. It is clear from the red shift in the UV-Vis spectrum that the nanostructures produced are elongated in nature. It shows a large peak shift of 30 nm from that of the copper nanoparticles^32,33 due to the two-dimensional structure of copper nanosheets. The peak shift depends on the aspect ratio of the copper nanostructures. The larger the aspect ratio, the higher the red shift, and a large sheet is expected.²⁶ The TEM of the nanostructure, as shown in Fig. 1(b), clearly demonstrates that the elongated structures are nanosheets of 1.5 μm in length and 0.5 μm in width. The thicknesses of these sheets are less than 50 nm, as judged by some sheets lying on their side (Fig. S1†).

Fig. 1 (a) UV-Vis spectrum and (b) TEM, (c) SEM and (d) HRTEM images of copper nanosheets in an aqueous phase.

The SEM of the nanosheets is shown in Fig. 1(c). The HRTEM image of the nanosheets is shown in Fig. 1(d). It can be seen that the uniform fringe structure spans a large distance, confirming that the nanosheets are produced mostly due to growth. To confirm the thickness of the nanosheets, atomic force microscopy (AFM) studies were performed. It can be seen (Fig. 2) that the nanosheets have an average thickness of around 25 nm, which is consistent with the thickness observed from TEM and SEM.

Fig. 2 AFM image and corresponding thickness profile of the large copper nanosheets in aqueous phase.

The concentrations of various chemical species in the synthesis protocol are the key parameters. The nanosheets are produced by tuning the hydrazine hydrate and NaOH concentrations in Cao's protocol for nanorods. We used lower concentrations of both hydrazine hydrate and NaOH to obtain the large nanosheets shown in Fig. 1(b). Changing the concentration deteriorates the nanosheets and beyond a limit, rods and particles are produced. For example, nanosheets may be obtained using a higher concentration of NaOH (3 M instead of 1.5 M) but the nanosheets are smaller in size, slightly irregular in shape and also contaminated by the presence of rods and particles, as shown in Fig. 3(a).

Fig. 3 TEM images of copper nanostructures in an aqueous phase synthesized by tuning the NaOH concentration (3 M) (a) before centrifugation and (b) after centrifugation. The (c) AFM image and (d) corresponding thickness profile of the as prepared small copper nanosheets in an aqueous phase.

However, the particles and rods can be separated from the nanosheets by centrifuging the sample at 10 000 rpm for 20 min. After centrifugation, the supernatant, which contains particles and small rods, can be discarded and the precipitate collected at the bottom can be dried to form a powder and can be redispersed in water. The TEM image of the redispersed sample, shown in Fig. 3(b), shows that the sample contains only sheets. The thickness of these nanosheets is very small (less than 10 nm), as shown in Fig. 3(c) and (d), compared to the nanosheets shown in Fig. 2.

Various other surfactants were also tried in the synthesis of copper nanosheets, wherein everything else remained the same and the surfactant concentration was varied. It was observed that non-ionic surfactants are ineffective. Anionic surfactant SDBS (4 mmol) could produce nanosheets (Fig. 4(a)) of smaller dimensions but the best result is obtained using CTAB, as shown before.

Fig. 4 (a) TEM of copper nanosheets synthesized using SDBS as a surfactant and (b) X-ray diffraction of copper nanosheets exposed to air for 1 day and 21 days.

One interesting feature of the synthesis protocol is that the nature of the salt does not affect the nanosheet formation. This is expected because the reacting species is the copper hydroxide intermediate, which was formed in all the cases tested. However, the reducing agent is a critically important component. We repeated the protocol using other metal precursors (CuSO₄·5H₂O and Cu(NO₃)₂·3H₂O) and sodium borohydride as the reducing agent while keeping the concentrations the same. It was observed that the nanosheets are formed for all metal precursors as long as hydrazine hydrate was used as the reducing agent, whereas large aggregates are formed instead of sheets when sodium borohydride is used as the reducing agent (Table S1†).

The phase purity of the nanosheets was investigated using powder XRD. First, copper nanostructures in the form of a powder were obtained by precipitating the aqueous phase nanosheets using centrifugation at 12 000 rpm for 30 min. Next, the precipitate was dried using an inert gas, followed by vacuum drying in a vacuum desiccator overnight. After complete drying of the precipitate, the nanosheets in the form of powder were collected and used for XRD characterization. The powder shows very little oxidation even though it was exposed to air for a day, as shown in Fig. 4(b). It takes almost a month (21 days) for significant oxidation to occur on the sample. This is in contrast to the case of copper nanoparticles, which get oxidized quickly. Hence, this nanostructure can readily be stored and transported.

Phase transfer of copper nanosheets

Next, we tried the phase transfer of copper nanosheets from the aqueous phase into an organic phase. Direct phase transfer of copper nanosheets to an organic solvent is very difficult due to their large size. There are a variety of phase transfer techniques available to transfer nanoparticles to an organic solvent,^34–36 but a much smaller number of techniques are available to transfer nanorods. To date, only a few instances are reported wherein gold nanorods of a low aspect ratio are phase transferred.^37,38 We tried to phase transfer the copper nanosheets by following the simple shaking of the aqueous-oil biphasic mixture with a hydrophobic ligand in the oil phase, but the nanosheets could not be transferred using this method and most of the nanosheets accumulate at the interface. Other phase transfer techniques such as ethanol mediated phase transfer³⁹ were also tried. Even this method failed to transfer the copper nanosheets to organic solvents.

Yang et al. (2005)³⁸ transferred gold nanorods from the aqueous phase to toluene by capping the gold nanorods with a water soluble thiol acid surfactant (mercaptosuccinic acid (MSA)) and using the electrostatic interaction between MSA and TOAB molecules in toluene. We also tried to transfer copper nanosheets using this method but failed. The only method left, therefore, is the direct precipitation and redispersion technique to transfer the nanosheets into various solvents.

The nanosheets obtained can be centrifuged, dried by nitrogen blowing and easily re-dispersed in water. The re-dispersed nanosheets in water are shown in Fig. 5(a), which are identical to those shown in Fig. 1(b). DMSO and chloroform are also successfully used as solvents for redispersing the nanosheets in the presence of dodecanethiol. The TEM of these re-dispersed samples are shown in Fig. 5(b) and (c). It can be seen that the nanosheets retain their shape and size after drying and re-dispersion. In addition to these solvents, the nanosheets could also be dispersed in ethanol and methanol containing dodecanethiol (SEM in ESI, Fig. S3†).

Fig. 5 TEM images of copper nanosheets redispersed in (a) water, (b) DMSO and (c) chloroform.

In all these cases, the colloid remains stable for weeks. However, when the nanosheets are dispersed in non-polar solvents with low polarity indices such as toluene and hexane (polarity indices 0.099 and 0.009, respectively), the colloid is very unstable. For toluene, the colloid crashes within 10 minutes, and for hexane it remains stable for up to an hour. It appears that the stability of the copper nanosheet colloid is strongly correlated with the polarity index of the solvent, as shown in Table S2.† Hence, the thiol coating on CTAB protected particles is possibly only partial. However, the thiol coating has a significant role in the stability and dispersibility as the dried nanosheet powder cannot form a stable colloid in any of the organic solvents without thiol. The nanosheets that were once dispersed in an organic solvent (i.e. with partial thiol coating) can be dispersed in an organic solvent without thiol but such samples show significantly lower stability.

Effect of cycles of centrifugation–re-dispersion

As shown previously, the impure samples of nanosheets prepared by sub-optimal concentrations can be purified by centrifugation. We also explored the scenario when such a centrifugation cycle is repeated many times. The results of centrifugation cycles repeated twice for samples dispersed in DMSO containing dodecanethiol are shown in Fig. 6. It can be seen that repeating the centrifugation twice does not have any effect on nanosheet size.

Fig. 6 TEM images of copper nanosheets in DMSO after (a) first round of precipitation and redispersion and (b) second round of precipitation and redispersion.

However, when two centrifugation steps were performed for particles dispersed in chloroform containing dodecanethiol, very large nanosheets formed, as shown in Fig. 7(b). SEM analysis also confirms that the powder contains larger nanosheets, as shown in Fig. 7(c), possibly formed by centrifugation induced oriented attachment. Centrifugation is known to induce aggregation among nanoparticles,⁴⁰ and this is the first report wherein it is shown that such aggregation occurs for nanosheets. It is important to note that simply storing the sample will not induce this oriented attachment (Fig. S4†), and centrifugation is needed for formation of large nanosheets.

Fig. 7 TEM images of (a) copper nanosheets after first round of precipitation and redispersion and (b) copper nanosheets after second round of precipitation and redispersion in chloroform. (c) SEM image of copper nanosheets produced after second round of precipitation and redispersion in chloroform (original particles are shown in Fig. 3(a)). (d) HRTEM image of copper nanosheets after second round of precipitation and redispersion in chloroform. (e) AFM image and (f) corresponding thickness profile of copper nanosheets in chloroform after second round of precipitation and redispersion in chloroform.

In this case, the nanosheets aggregate like a pack of cards to form larger sheets, as evident from the fringes in two directions shown in the HRTEM of the samples (Fig. 7(d)), which indicate that the larger sheets are grown mostly due to assembly of smaller sheets by oriented attachment.¹⁴ This is further confirmed by the increase in thickness of the sheets due to aggregation. The primary sheets formed by the sub-optimal process are very thin, around 6 nm (Fig. 3(c)), but the aggregated nanosheets have a thickness of 40 nm, as seen from Fig. 7(e).

The aggregation driven growth of the nanosheets is clearly related to their stabilization through polarity of the solvent. Two rounds of centrifugation are not enough for aggregate formation in DMSO but enough for assembly of sheets in chloroform. However, it is possible that the nanosheets dispersed in DMSO will aggregate after more centrifugation cycles. This is found to be the case, as shown in Fig. S5,† wherein a few large nanosheets can be seen to be forming after 4 cycles of centrifugation in DMSO. The number of such structures increases after eight centrifugation cycles (Fig. 8), but the process is far from completion. This is in stark contrast with the case of chloroform, wherein the process is complete after the second cycle and the nanosheet does not grow further, even after quadrupling the number of centrifugation cycles. Fig. 8 also shows how oriented attachment of sheets leads to large nanosheet formation.

Fig. 8 TEM image of copper nanosheets in DMSO after 8 cycles of precipitation and redispersion.

Conclusions

In summary, a simple chemical reduction method has been employed successfully for the preparation of copper nanosheets in an aqueous phase at room temperature. Cao's method was successfully modified to obtain the copper nanosheets. NaOH and hydrazine hydrate concentrations play a crucial role in the synthesis process. Copper nanosheets are highly resistant to oxidation, even exposed to air for one month. Small copper nanosheets are also prepared successfully using SDBS as the surfactant.

Copper nanosheets are also successfully transferred to other solvents, such as chloroform, dimethylsulfoxide (DMSO), ethanol, methanol and hexane, by the precipitation and redispersion technique with the help of dodecanethiol. The large nanosheets retain their size and shape when redispersed in water, DMSO, ethanol and chloroform. However, smaller nanosheets prepared by increased NaOH concentration grow in size after repeated cycles of precipitation–redispersion using centrifugation. Such growth is complete after two cycles of centrifugation–re-dispersion in chloroform and remains incomplete for more stable suspension in DMSO after many cycles. Such growth has been attributed to the oriented attachment of smaller nanosheets during centrifugation.

Acknowledgements

We acknowledge financial support from the SRIC (ISIRD Grant), IIT Kharagpur. We thank Mr Sudipto Mondal, Central Research Facility, IIT Kharagpur for helping us in obtaining TEM images. We thank Mr Chandrakanth, Department of Chemistry, IIT Kharagpur for performing X-ray diffraction measurements.

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Footnote

† Electronic supplementary information (ESI) available: TEM, SEM and AFM. See DOI: 10.1039/c5ra24635k

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