Synthesis, characterizations and hydrophobicity of micro/nano scaled heptadecafluorononanoic acid decorated copper nanoparticle

1 Department of Mechanical Engineering, National Taiwan University, Taipei-10617, Taiwan 2 Thermal division, Advanced Vital Component Company, Taiwan *Corresponding author. Email: phchen@ntu.edu.tw Synthesis, characterizations and hydrophobicity of micro/nano scaled heptadecafluorononanoic acid decorated copper nanoparticle Hung-Hsia Chen1, R. Anbarasan1, Long-Sheng Kuo1, Meng-Yu Tsai1, Ping-Hei Chen1, * and Kuei-Feng Chiang2

Synthesis of nanosized material from the bulk resource material is a fascinating field of research because of its novelty and application in various science and engineering fields.
Various techniques are employed for the generation of nanosized material from its bulk one [1][2][3][4][5][6][7][8][9][10]. Generally, thermal treatments are employed for this purpose, unfortunately it requires costly instruments. In order to outwit this problem our research team has followed a conventional chemical method.
The advantages of the present investigations are 1)economically cheaper, 2)no use of expensive and hazardous chemicals and hence it is a pollution free method, 3)the resultant material has hydrophobic characters and 4) the simultaneous coating of hydrophobic material on the surface of the bulk resource material. A modified copper surface with super hydrophobic characteristics could be used either in vapor chamber or heat pipe that has been widely employed for heat spreading of electric devices.
Recently, Song et al [11] reported about the electroless replacement deposition method for the generation of super hydrophobic copper substrate. Super hydrobhopic surfaces based on copper hydroxide nano needles were reported by Wu and co-workers [12]. Wang and his research team [13] studied the super hydrophobic copper films. The super hydrophobic character was supported by water contact angle measurement reports. Super hydrophobic Cu(OH) 2 nano tubes from copper foil were reported in the literature [14]. Other authors also reported about the super hdyrophobic copper plate surface by different techniques [15][16][17]

Surface treatment and etching rate of the copper plate
The copper plate with the above mentioned dimensions   Figure 2 indicates the UV-Visible spectrum of Cu-HDFN.

UV-Visible spectroscopy
A negative absorbance peak was observed at 322 nm. This peak is quite natural and due to the n to Pi* transition [18] of the HDFNA decorated copper nano particle [19]. The hindered transition can be explained on the basis of highly negatively charged HDFNA decorated co-planarized copper nano particle.  Figure 3 indicates the surface morphology of Cu-HDFN. Figure 3a represents the plate like morphology. Figure 3b indicates the cauli flower like morphology. Flower like morphology is responsible for superhydrophobicity nature of a material [4]. The SEM results proved that the HDFNA decorated copper plate was slowly converted into super hydrophobic one due to the simultaneous coating of Cu-HDFN on the copper plate. Further, this can be confirmed with the contact angle measurement and will be discussed in the forth coming sessions. Figure 4 shows the topography of HDFNA coated copper nano particle. Figure 4a represents the agglomerated form of copper nano particle with the size of 400 nm. Figure 4b represents the sphere topography of copper nano particle with the size of 1065 nm. Figure 4c indicates the individual nano particle size of 20 nm with the distorted sphere morphology.

TEM analysis
The distortion is due to the adsorption of HDFNA on the surface of the copper nano particle. The TEM images concluded that during the synthesis of super hydrophobic Cu plate, nanosized copper was diffused into the reaction medium from the copper plate resource material and formed HDFNA coated copper nano particle and the same was coated on the surface of the resource material. This is an eco-friendly and economically cheaper method to produce the copper nano particle from the resource. Figure 5 shows the XPS of HDFNA decorated copper nano particle. It showed the binding energy of Cu2p 3/2 and Cu2p 1/2 at 933.08 and 950.2 eV [20] respectively. A signal at 933.08 eV confirmed the existence of Cu nano particle (zero oxidation state). Moreover, the % of copper was found to be 1.07%. The F1s level of F in HDFNA decorated copper nano particle was determined at 687.23 eV with 35.92% content.

XPS analysis
The C1s and O1s peaks of HDFNA are observed at 281.9 and 530.8 eV respectively. The XPS analysis concluded that the HDFNA is coated on the surface of the copper nano particle.
Again this supported the core-shell like structure of HDFNA decorated copper nano particle.

Determination of bulk etching rate
The bulk etching rate was determined from equation (1).
It was interesting to note that while increasing the etching time, the colorless reaction medium turns into pale green. This ultimately indicated that the concentration of the diffused copper nano particle was increased. The etching rate was determined from the plot drawn between the etching time and the difference in mass (see Fig. 6a [5]. Our report is in accordance with them. Figure 6b indicates the effect of etching time on the water contact angle, and discussed in the next session.

Contact angle measurement
The contact angle measurement confirmed the hydrophobicity nature of the HDFNA coated copper nano particle. After 5 days of etching, the CA was determined as 140.7 o . This proved the hydrophobic nature of the Cu-HDFN coated copper plate at the 5 th day. Figure 7 shows the picture of CA measured for the Cu-HDFN coated copper plate. Thus the contact angle co-supported the SEM morphology of HDFNA coated copper nano particle. While increasing the etching time, the water contact angle of copper plate was increased due to the chemical etching reaction. Figure 6b represents the effect of etching time on the water contact angle. Due to the surface etching reaction, the water contact angle was increased.

Conclusions
The important points are presented here as conclusions. 1) The HDFNA decorated copper nano particle was successfully declared that the HDFNA coated copper nano particle is a suitable candidate for the manufacture of copper nano particle with hydrophobic character. The above points inferred that our methodology yielded an economically cheaper and eco-friendly method with the nanosized copper particle.