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Nano-sized amorphous carbon covered surface formed by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs)
Accepted Manuscript Full Length Article Nano-sized amorphous carbon covered surface formed by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs) Xinjian Wang, Junyan Liu, Lijun Yang, Yu he, Yang Wang PII: DOI: Reference:
S0169-4332(18)31071-7 https://doi.org/10.1016/j.apsusc.2018.04.109 APSUSC 39108
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
3 December 2017 28 February 2018 11 April 2018
Please cite this article as: X. Wang, J. Liu, L. Yang, Y. he, Y. Wang, Nano-sized amorphous carbon covered surface formed by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs), Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.04.109
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nano-sized amorphous carbon covered surface formed by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs) Xinjian Wang a,b, Junyan Liu a,b,*, Lijun Yang a,b, Yu he a,b, Yang Wang a, b,* a
State Key Laboratory of Robotics and System (HIT), Harbin, 150001, P. R. China School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, 150001, P. R. China *Author to whom correspondence should be addressed. E- mail: [email protected] or [email protected] b
Abstract In this paper, a nano-sized amorphous carbon covered surface was fabricated by an additive manufacturing process of selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs) in ambient condition. This technique synthesizes pure Cu by chemical reduction route using an organic solvent during laser melting. Additionally, the polymer in the solvent was decomposed by the thermal energy, which resulted in the formation of nano-sized amorphous carbon. Consequently, two layers of copper and carbon were fabricated on the substrate of stainless steel which was illustrated by the cross-section SEM. The upper layer of carbon possesses super-hydrophobic and high-light absorptance properties. The reaction products were characterized by the analyze of XRD, XPS and EDS. The contact angle and dynamic behavior of water droplet on the fabricated surface were carried out to demonstrate the super-hydrophobicity. The study provides an easy and flexible method for the fabrication of the functional surface by laser sintering in air ambient. Keywords:
copper,
super-hydrophobic
amorphous
carbon,
nano-particles,
selective
laser
melting,
1. Introduction Additive manufacturing (or 3D printing), a highly flexible manufacturing technique, is widely studied recently.[1, 2] Using a controlled laser, metallic additive manufacturing sintered the micro particles to form the specific structures. However, the metallic additive manufacturing process often requires vacuum or air protection environment in order to avoid the oxidization of the metallic powder, which increases the cost and equipment complexity. Recently, the technique of photolithography-free electrode fabrication has been developed based on sintering the metal nanoparticle inks, such as gold (Au), silver (Ag) and copper (Cu), in air ambient.[3-5] The copper nanoparticle (Cu NP) or copper oxide (CuO) has been increasingly considered as a potential replacement ink material for the expensive noble metal NP ink.[6, 7] The technique uses a laser beam to sinter the Cu NP with the alcohol solvents and polyvinylpyrrolidone (PVP) in air ambient. The hydroxyl in the hydrate solvent can reduce the CuO into Cu, which guarantee the fabrication process in air ambient. Based on the mechanism of photolithography-free electrode fabrication, we have developed a method of selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs) to fabricate the metallic powders in air ambient. By the method of SLM-IP Cu NPs, we have fabricated friction-reducing surface texture on the substrate of stainless steel layer by layer.[8] Additionally, during the research, we found a nano-sized amorphous carbon covered surface could be formed with the specific process parameters, which possess the properties of hydrophobicity and high light aborptance. It is known that the surface should possess a micro/nano hierarchical structure and a low surface energy in order to achieve the super-hydrophobicity.[9-11] Numerous methods have been developed to fabricate the super-hydrophobic surface structure on metal, like wet
chemical etching[12], electrochemical deposition[13], vapor deposition[14], self-assembly monolayers(SAM)[15] and etc.. Large and non-uniformed surface could hardly manufactured. Thus, we assume that SLM-IP Cu NPs could provide a flexible and controllable method for fabricating hydrophobic surface. In this paper, a nano-sized amorphous carbon covered surface was produced by the method of SLM-IP Cu NPs. Under the irradiation of laser, solvent with metallic nanoparticles were sintered. XRD, XPS and SEM analyses were conducted to investigate the chemical composition and morphological structure of the surface. The contact angle (CA), sliding angle (SA) and dynamic behavior of water droplet were measured to verify the super-hydrophobic property. And the light aborptance was also tested. 2. Material and methods 2.1 Materials All the reagents were analytical grade and deionized water was used throughout the experiments. Cu NPs with an average diameter of 70~100 nm (Guangzhou Hong Wu Material Technology Co., Ltd., China) were used in this study. Polyvinylpyrrolidone (PVP, MW 10000) were adopted to stabilize the NPs in the solvent mixture. ANSI 304 stainless steel plate with a thickness of 2 mm (TISCO, Taiyuan Iron & Steel Co., Ltd.) were cleaned three times in an ultrasonic bath with an ethanol/water (50:50) solution for 5 min before use as the substrates. 2.2 Preparation of super-hydrophobic surface by SLM-IP Cu NPs An organic solvent of PVP (20.2wt%) and ethylene glycol (36.5wt%) was made to prevent the agglomeration and oxidation of Cu during SLM. The Cu NPs (43.3 wt%) were dispersed in the organic solvent and stirred with an ultrasonic wave for 25 min.
The solvent of Cu NPs was irradiated by a fiber laser (wave length of 1070nm, spot size of 17 μm). The samples were prepared with a laser power of 12 W, velocity of 10 mm/s and scan space of 0.02 mm. The detailed description of SLM-IP Cu NPs and experiment setup was published in our previous work[8]. 2.4 Instrument and characterization The morphology of the samples was analyzed by a scanning electron microscope (SEM, ZEISS Merlin Compact). X-ray photoelectron spectroscopy (XPS) was recorded by PHI5700. Contact angles (CA) were measured at ambient temperature (OCA20, Dataphysics Instruments GmbH, Filderstadt). A droplet (3 μL) of DI water was carefully dropped onto the surface of the sample and three times of CA values were collected. The dynamic behavior of the water droplet dropped on the fabricated surface was determined by a high speed video camera (Vision Research Phantom v710). The aborptance was tested by Hitachi U-2000. 3. Results and Discussion 3.1 Mechanism of sintering the ink based Cu NP Cu NPs were dispersed homogeneously in the solvent of ethylene glycol supported by the long chain of the polymer. And the solvent mixture is slot-died onto the substrate of stainless steel evenly. The Cu NPs were heated under the irradiation of laser, which leads to a series of chemical reactions in the solvent with the surrounded materials. The fabrication process is assumed to be based on the mechanism of acid reduction.[16, 17] The Cu NPs beneath the liquid surface will be sintered into large solid under the protection of liquid that isolated the metallic NPs from air. Response to [1.1(a)]: Although process was executed under the protection of ethylene glycol and PVP, a small proportion of copper would inevitably be oxidized, as a consequence of the ethylene glycol evaporation.[3] Furthermore, hydroxide of
the alcohol solvent and lactam ring of PVP would react under the thermal energy.[17, 18] The reaction would generate amorphous carbon, methylene gas, methylamine gas and acetic acid. The acetic acid is decomposed to formic acid that reduces the copper oxide to copper. Response to [1.1(a)]: After the sintering, the residues like amorphous carbon, copper oxide and unsintered Cu NPs would be washed off in the rinsing procedure. Only a small proportion of copper oxide would left with the pure copper solid. The process is illustrated in Figure 1. During the fabrication process, amorphous carbon was formed due to the decomposition of PVP. We found that the fabricated top layer is evenly covered by the nano-sized amorphous carbon. Thus, the surface layer of carbon possesses a super-hydrophobic property.
Figure 1. Schematics of reduction and sintering processes If the super-hydrophobicity was resulted from the amorphous carbon produced by the decomposition of PVP, the existence of Cu NPs could be dislodged. Consequently, the solvent without Cu NPs was also tested under the irradiation of laser. But the super-hydrophobicity cannot be formed. Response to [1.1(b)]: According to the references[19, 20], the absorption coefficients of copper and ethylene glycol at the wavelength of 1070 nm are 8.59×105 cm-1 and 0.179 cm-1, respectively. The internal transmittance of ethylene glycol at the wavelength of 1070 nm is 83.6%. It can be inferred that ethylene glycol would be evaporated because of the high temperature on the NP surface. We assume that the existence of Cu NPs reduces the
reaction rate and provide the thermal energy, which resulted in the surface chemical material of carbon and the hierarchical hydrophobic structure. 3.2 Surface morphology Figure 2(a) is the TEM picture of the NPs which shows the size of the particle is around 70~100 nm. Surface morphology SEM of the sintered samples are shown in Figure 2(b) and (d) which illustrate the nano-sized structure of the amorphous carbon. Response to [1.2]: Figure 2(c) is the large SEM view of the prepared surface which shows that some particles splashed on the surface. The splash was resulted from the recoil pressure caused by the evaporation the ethylene glycol. Figure 2(e) and (d) are the pictures of the particle splashed on the surface. Figure 2(e) shows that the particle is supported by the nano-sized structure of amorphous carbon, which demonstrates the strength of the nano-sized structure. Additionally, the particle was homogeneously and tightly covered by the amorphous carbon as shown in Figure 2(d). It proves the decomposition mechanism by the thermal energy on the particle surface. The temperature increase of the metallic particle resulted in the formation of the carbon structure. The nano-size of the amorphous carbon is produced from the decomposition of the polymer. And the recoil pressure and mutual force due to the evaporation of hydrogen peroxide, carbon dioxide, methylene gas and methylamine gas leads to the formation of the hierarchical structure. The reaction rate contributes to the residue of amorphous carbon and the evaporation of liquid, otherwise the amorphous carbon would be evaporated or mixed with liquid.
Figure 2. (a) TEM picture of the Cu NPs, (b) (c) and (d) are the SEM pictures of the sample surface. (b) and (d) show the hierarchical structure of the sample surface. (c) illustrates the amorphous carbon formed on the particle surface.
Figure 3. Characterization of the fabricated surface. (a) XRD of the sample shows the surface contains the material of Cu and CuO. (b) EDS of the sample shows that the surface contains
three element of Cu, O and C respectively. (c) Wide XPS scan of the sample indicates that significant carbon and oxygen peak. Cu-related peak is laid in the dark region. (d) Cu-related XPS proves the consist of Cu and CuO. In order to analyze the composition of the sample, characterization tests were executed. Figure 3(a) is the X-ray diffraction (XRD) of the sample surface. The XRD peaks correspond to the FCC pure copper phase (43.2°, 50.4°, 74.1°, and 89.9°) and a small peak corresponds to the FCC copper oxide phase (36.63°) according to existing references (JCPDS No. 040836 and NO. 780428). Figure 3(b) is the Energy Dispersive Spectrometer (EDS) analysis result of the sample showing that the surface contains three element of Cu, O and C. Figure 3(c) is the wide XPS scan of the sample. The carbon peak indicates formation of the amorphous carbon. The oxygen peak implies the oxidization of copper. We identified the copper peak together with weak CuO peaks as shown in Figure 3(d). From the above characterizing analysis, it indicates the formation of the amorphous carbon and proves the reduction mechanism of copper sintering in air ambient. Additionally, the weak peak of XRD, EDS and XPS indicates the small proportion of CuO. The SEM of the cross section is shown in Figure 4(a) which illustrates the copper formation beneath the surface. The copper near the substrate is denser than the material near the surface. Response to [1.3]: We assume that the NPs are totally melted, because the size of copper film is much larger than the size of the original Cu particles (shown in Figure 4), and even the size of the big particle on the surface is much larger than 100nm (shown in Figure 2). The molten liquid will flow to the substrate and then solidified, influenced by the effect of natural convection and the evaporation of solvent.[21] Moreover, we assume that the low density (shown in Figure 4(a)) on the top is resulted from the evaporation of ethylene glycol
together with melt, solidification and chemical reactions. Element-map analysis results of the copper and iron are shown in Figure 4(b) and (c). Thus, two layer of copper and carbon were fabricated on the substrate of stainless steel.
Figure 4. (a) SEM of the cross section, (b) and (c) are the element map result of the sample.
Figure 5. Contact angle test result of the sample The contact angle (CA) was test and the result is shown in Figure 5. The fabricated surface expresses a good super-hydrophobicity with a CA as high as 160° and sliding angle as low as 3°. In order to specify the super-hydrophobic property, dynamic behavior of water droplet on the surface was also tested. It is believe that the water CA test is dependent on the method of drop shape analysis with the techniques employed.[22-24] The measurement may also be prone to investigator interpretation upon manual assignment of the angle. The dynamic behavior of the droplet is another convincing method of representing the super-hydrophobicity. The water droplet volume of 8 μL is dropped from a height of 20 mm according to the reference[24]. Figure 6 indicates that two bounces of the droplet is observed, which illustrates the super-hydrophobicity of the surface according to the definition in the
reference[24].
Figure 6. Dynamic behavior of droplet dropped on the sample
Figure 7. Absorbance of the fabricated surface and the substrate Moreover, combining the material of amorphous carbon and the surface structure, the fabricated surface has a high absorptance. The light absorbance in the wavelength of 300 nm to 2000 nm was tested as shown in Figure 7. The average absorbance of the fabricated surface is much higher than the substrate of stainless steel, which indicated the role of the nano-sized carbon. Additionally, the absorbance peak at 562 nm was detected due to the existence of the Cu nanoparticles. The plasmon resonance of the Cu nanoparticles appeared at 562 nm according to the reference.[25] The small peaks around 800 nm were resulted from the changing of the light source which can be neglected. 4. Conclusions In this paper, we found that a nano-sized amorphous carbon covered surface could be fabricated based on the process of SLM-IP Cu NPs. Nano-sized structure of amorphous carbon was formed on the surface, due to the decomposition of the polymer PVP. The reaction
products were analyzed to prove the assumption of the fabrication mechanism. The fabricated surface expresses a good super-hydrophobicity with a CA as high as 160° and sliding angle as low as 3°. The dynamic behavior of water droplet on the fabricated surface also demonstrates the super-hydrophobicity of the surface. Moreover, the randomly distributed nano-sized amorphous carbon demonstrates a higher absorbance than the substrate of stainless steel. Thus, the method of SLM-IP Cu NPs provides an easy way to fabricate functional surface in air ambient. Acknowledgements This work was supported by the Foundation for Innovative Research Groups of the National Nature Science Foundation of China under Grant No.51521003, the Chinese National Natural Science Foundation under Contract No.61571153, No.51173034, Self-planned Task of State Key Laboratory of Robotics and System (HIT) and the Program of Introducing Talents of Discipline of Universities (grant No.B07108) for the research support.
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Highlights 1. An nano-sized amorphous carbon covered surface was fabricated by an additive manufacturing method of SLM-IP Cu NPs. 2. Two layers of copper and carbon were fabricated on the substrate of stainless steel. 3. Steady and dynamic behaviors of droplet were tested to illustrate the super-hydrophobicity.
S0169-4332(18)31071-7 https://doi.org/10.1016/j.apsusc.2018.04.109 APSUSC 39108
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
3 December 2017 28 February 2018 11 April 2018
Please cite this article as: X. Wang, J. Liu, L. Yang, Y. he, Y. Wang, Nano-sized amorphous carbon covered surface formed by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs), Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.04.109
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nano-sized amorphous carbon covered surface formed by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs) Xinjian Wang a,b, Junyan Liu a,b,*, Lijun Yang a,b, Yu he a,b, Yang Wang a, b,* a
State Key Laboratory of Robotics and System (HIT), Harbin, 150001, P. R. China School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, 150001, P. R. China *Author to whom correspondence should be addressed. E- mail: [email protected] or [email protected] b
Abstract In this paper, a nano-sized amorphous carbon covered surface was fabricated by an additive manufacturing process of selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs) in ambient condition. This technique synthesizes pure Cu by chemical reduction route using an organic solvent during laser melting. Additionally, the polymer in the solvent was decomposed by the thermal energy, which resulted in the formation of nano-sized amorphous carbon. Consequently, two layers of copper and carbon were fabricated on the substrate of stainless steel which was illustrated by the cross-section SEM. The upper layer of carbon possesses super-hydrophobic and high-light absorptance properties. The reaction products were characterized by the analyze of XRD, XPS and EDS. The contact angle and dynamic behavior of water droplet on the fabricated surface were carried out to demonstrate the super-hydrophobicity. The study provides an easy and flexible method for the fabrication of the functional surface by laser sintering in air ambient. Keywords:
copper,
super-hydrophobic
amorphous
carbon,
nano-particles,
selective
laser
melting,
1. Introduction Additive manufacturing (or 3D printing), a highly flexible manufacturing technique, is widely studied recently.[1, 2] Using a controlled laser, metallic additive manufacturing sintered the micro particles to form the specific structures. However, the metallic additive manufacturing process often requires vacuum or air protection environment in order to avoid the oxidization of the metallic powder, which increases the cost and equipment complexity. Recently, the technique of photolithography-free electrode fabrication has been developed based on sintering the metal nanoparticle inks, such as gold (Au), silver (Ag) and copper (Cu), in air ambient.[3-5] The copper nanoparticle (Cu NP) or copper oxide (CuO) has been increasingly considered as a potential replacement ink material for the expensive noble metal NP ink.[6, 7] The technique uses a laser beam to sinter the Cu NP with the alcohol solvents and polyvinylpyrrolidone (PVP) in air ambient. The hydroxyl in the hydrate solvent can reduce the CuO into Cu, which guarantee the fabrication process in air ambient. Based on the mechanism of photolithography-free electrode fabrication, we have developed a method of selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs) to fabricate the metallic powders in air ambient. By the method of SLM-IP Cu NPs, we have fabricated friction-reducing surface texture on the substrate of stainless steel layer by layer.[8] Additionally, during the research, we found a nano-sized amorphous carbon covered surface could be formed with the specific process parameters, which possess the properties of hydrophobicity and high light aborptance. It is known that the surface should possess a micro/nano hierarchical structure and a low surface energy in order to achieve the super-hydrophobicity.[9-11] Numerous methods have been developed to fabricate the super-hydrophobic surface structure on metal, like wet
chemical etching[12], electrochemical deposition[13], vapor deposition[14], self-assembly monolayers(SAM)[15] and etc.. Large and non-uniformed surface could hardly manufactured. Thus, we assume that SLM-IP Cu NPs could provide a flexible and controllable method for fabricating hydrophobic surface. In this paper, a nano-sized amorphous carbon covered surface was produced by the method of SLM-IP Cu NPs. Under the irradiation of laser, solvent with metallic nanoparticles were sintered. XRD, XPS and SEM analyses were conducted to investigate the chemical composition and morphological structure of the surface. The contact angle (CA), sliding angle (SA) and dynamic behavior of water droplet were measured to verify the super-hydrophobic property. And the light aborptance was also tested. 2. Material and methods 2.1 Materials All the reagents were analytical grade and deionized water was used throughout the experiments. Cu NPs with an average diameter of 70~100 nm (Guangzhou Hong Wu Material Technology Co., Ltd., China) were used in this study. Polyvinylpyrrolidone (PVP, MW 10000) were adopted to stabilize the NPs in the solvent mixture. ANSI 304 stainless steel plate with a thickness of 2 mm (TISCO, Taiyuan Iron & Steel Co., Ltd.) were cleaned three times in an ultrasonic bath with an ethanol/water (50:50) solution for 5 min before use as the substrates. 2.2 Preparation of super-hydrophobic surface by SLM-IP Cu NPs An organic solvent of PVP (20.2wt%) and ethylene glycol (36.5wt%) was made to prevent the agglomeration and oxidation of Cu during SLM. The Cu NPs (43.3 wt%) were dispersed in the organic solvent and stirred with an ultrasonic wave for 25 min.
The solvent of Cu NPs was irradiated by a fiber laser (wave length of 1070nm, spot size of 17 μm). The samples were prepared with a laser power of 12 W, velocity of 10 mm/s and scan space of 0.02 mm. The detailed description of SLM-IP Cu NPs and experiment setup was published in our previous work[8]. 2.4 Instrument and characterization The morphology of the samples was analyzed by a scanning electron microscope (SEM, ZEISS Merlin Compact). X-ray photoelectron spectroscopy (XPS) was recorded by PHI5700. Contact angles (CA) were measured at ambient temperature (OCA20, Dataphysics Instruments GmbH, Filderstadt). A droplet (3 μL) of DI water was carefully dropped onto the surface of the sample and three times of CA values were collected. The dynamic behavior of the water droplet dropped on the fabricated surface was determined by a high speed video camera (Vision Research Phantom v710). The aborptance was tested by Hitachi U-2000. 3. Results and Discussion 3.1 Mechanism of sintering the ink based Cu NP Cu NPs were dispersed homogeneously in the solvent of ethylene glycol supported by the long chain of the polymer. And the solvent mixture is slot-died onto the substrate of stainless steel evenly. The Cu NPs were heated under the irradiation of laser, which leads to a series of chemical reactions in the solvent with the surrounded materials. The fabrication process is assumed to be based on the mechanism of acid reduction.[16, 17] The Cu NPs beneath the liquid surface will be sintered into large solid under the protection of liquid that isolated the metallic NPs from air. Response to [1.1(a)]: Although process was executed under the protection of ethylene glycol and PVP, a small proportion of copper would inevitably be oxidized, as a consequence of the ethylene glycol evaporation.[3] Furthermore, hydroxide of
the alcohol solvent and lactam ring of PVP would react under the thermal energy.[17, 18] The reaction would generate amorphous carbon, methylene gas, methylamine gas and acetic acid. The acetic acid is decomposed to formic acid that reduces the copper oxide to copper. Response to [1.1(a)]: After the sintering, the residues like amorphous carbon, copper oxide and unsintered Cu NPs would be washed off in the rinsing procedure. Only a small proportion of copper oxide would left with the pure copper solid. The process is illustrated in Figure 1. During the fabrication process, amorphous carbon was formed due to the decomposition of PVP. We found that the fabricated top layer is evenly covered by the nano-sized amorphous carbon. Thus, the surface layer of carbon possesses a super-hydrophobic property.
Figure 1. Schematics of reduction and sintering processes If the super-hydrophobicity was resulted from the amorphous carbon produced by the decomposition of PVP, the existence of Cu NPs could be dislodged. Consequently, the solvent without Cu NPs was also tested under the irradiation of laser. But the super-hydrophobicity cannot be formed. Response to [1.1(b)]: According to the references[19, 20], the absorption coefficients of copper and ethylene glycol at the wavelength of 1070 nm are 8.59×105 cm-1 and 0.179 cm-1, respectively. The internal transmittance of ethylene glycol at the wavelength of 1070 nm is 83.6%. It can be inferred that ethylene glycol would be evaporated because of the high temperature on the NP surface. We assume that the existence of Cu NPs reduces the
reaction rate and provide the thermal energy, which resulted in the surface chemical material of carbon and the hierarchical hydrophobic structure. 3.2 Surface morphology Figure 2(a) is the TEM picture of the NPs which shows the size of the particle is around 70~100 nm. Surface morphology SEM of the sintered samples are shown in Figure 2(b) and (d) which illustrate the nano-sized structure of the amorphous carbon. Response to [1.2]: Figure 2(c) is the large SEM view of the prepared surface which shows that some particles splashed on the surface. The splash was resulted from the recoil pressure caused by the evaporation the ethylene glycol. Figure 2(e) and (d) are the pictures of the particle splashed on the surface. Figure 2(e) shows that the particle is supported by the nano-sized structure of amorphous carbon, which demonstrates the strength of the nano-sized structure. Additionally, the particle was homogeneously and tightly covered by the amorphous carbon as shown in Figure 2(d). It proves the decomposition mechanism by the thermal energy on the particle surface. The temperature increase of the metallic particle resulted in the formation of the carbon structure. The nano-size of the amorphous carbon is produced from the decomposition of the polymer. And the recoil pressure and mutual force due to the evaporation of hydrogen peroxide, carbon dioxide, methylene gas and methylamine gas leads to the formation of the hierarchical structure. The reaction rate contributes to the residue of amorphous carbon and the evaporation of liquid, otherwise the amorphous carbon would be evaporated or mixed with liquid.
Figure 2. (a) TEM picture of the Cu NPs, (b) (c) and (d) are the SEM pictures of the sample surface. (b) and (d) show the hierarchical structure of the sample surface. (c) illustrates the amorphous carbon formed on the particle surface.
Figure 3. Characterization of the fabricated surface. (a) XRD of the sample shows the surface contains the material of Cu and CuO. (b) EDS of the sample shows that the surface contains
three element of Cu, O and C respectively. (c) Wide XPS scan of the sample indicates that significant carbon and oxygen peak. Cu-related peak is laid in the dark region. (d) Cu-related XPS proves the consist of Cu and CuO. In order to analyze the composition of the sample, characterization tests were executed. Figure 3(a) is the X-ray diffraction (XRD) of the sample surface. The XRD peaks correspond to the FCC pure copper phase (43.2°, 50.4°, 74.1°, and 89.9°) and a small peak corresponds to the FCC copper oxide phase (36.63°) according to existing references (JCPDS No. 040836 and NO. 780428). Figure 3(b) is the Energy Dispersive Spectrometer (EDS) analysis result of the sample showing that the surface contains three element of Cu, O and C. Figure 3(c) is the wide XPS scan of the sample. The carbon peak indicates formation of the amorphous carbon. The oxygen peak implies the oxidization of copper. We identified the copper peak together with weak CuO peaks as shown in Figure 3(d). From the above characterizing analysis, it indicates the formation of the amorphous carbon and proves the reduction mechanism of copper sintering in air ambient. Additionally, the weak peak of XRD, EDS and XPS indicates the small proportion of CuO. The SEM of the cross section is shown in Figure 4(a) which illustrates the copper formation beneath the surface. The copper near the substrate is denser than the material near the surface. Response to [1.3]: We assume that the NPs are totally melted, because the size of copper film is much larger than the size of the original Cu particles (shown in Figure 4), and even the size of the big particle on the surface is much larger than 100nm (shown in Figure 2). The molten liquid will flow to the substrate and then solidified, influenced by the effect of natural convection and the evaporation of solvent.[21] Moreover, we assume that the low density (shown in Figure 4(a)) on the top is resulted from the evaporation of ethylene glycol
together with melt, solidification and chemical reactions. Element-map analysis results of the copper and iron are shown in Figure 4(b) and (c). Thus, two layer of copper and carbon were fabricated on the substrate of stainless steel.
Figure 4. (a) SEM of the cross section, (b) and (c) are the element map result of the sample.
Figure 5. Contact angle test result of the sample The contact angle (CA) was test and the result is shown in Figure 5. The fabricated surface expresses a good super-hydrophobicity with a CA as high as 160° and sliding angle as low as 3°. In order to specify the super-hydrophobic property, dynamic behavior of water droplet on the surface was also tested. It is believe that the water CA test is dependent on the method of drop shape analysis with the techniques employed.[22-24] The measurement may also be prone to investigator interpretation upon manual assignment of the angle. The dynamic behavior of the droplet is another convincing method of representing the super-hydrophobicity. The water droplet volume of 8 μL is dropped from a height of 20 mm according to the reference[24]. Figure 6 indicates that two bounces of the droplet is observed, which illustrates the super-hydrophobicity of the surface according to the definition in the
reference[24].
Figure 6. Dynamic behavior of droplet dropped on the sample
Figure 7. Absorbance of the fabricated surface and the substrate Moreover, combining the material of amorphous carbon and the surface structure, the fabricated surface has a high absorptance. The light absorbance in the wavelength of 300 nm to 2000 nm was tested as shown in Figure 7. The average absorbance of the fabricated surface is much higher than the substrate of stainless steel, which indicated the role of the nano-sized carbon. Additionally, the absorbance peak at 562 nm was detected due to the existence of the Cu nanoparticles. The plasmon resonance of the Cu nanoparticles appeared at 562 nm according to the reference.[25] The small peaks around 800 nm were resulted from the changing of the light source which can be neglected. 4. Conclusions In this paper, we found that a nano-sized amorphous carbon covered surface could be fabricated based on the process of SLM-IP Cu NPs. Nano-sized structure of amorphous carbon was formed on the surface, due to the decomposition of the polymer PVP. The reaction
products were analyzed to prove the assumption of the fabrication mechanism. The fabricated surface expresses a good super-hydrophobicity with a CA as high as 160° and sliding angle as low as 3°. The dynamic behavior of water droplet on the fabricated surface also demonstrates the super-hydrophobicity of the surface. Moreover, the randomly distributed nano-sized amorphous carbon demonstrates a higher absorbance than the substrate of stainless steel. Thus, the method of SLM-IP Cu NPs provides an easy way to fabricate functional surface in air ambient. Acknowledgements This work was supported by the Foundation for Innovative Research Groups of the National Nature Science Foundation of China under Grant No.51521003, the Chinese National Natural Science Foundation under Contract No.61571153, No.51173034, Self-planned Task of State Key Laboratory of Robotics and System (HIT) and the Program of Introducing Talents of Discipline of Universities (grant No.B07108) for the research support.
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Highlights 1. An nano-sized amorphous carbon covered surface was fabricated by an additive manufacturing method of SLM-IP Cu NPs. 2. Two layers of copper and carbon were fabricated on the substrate of stainless steel. 3. Steady and dynamic behaviors of droplet were tested to illustrate the super-hydrophobicity.