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microstructures with different morphologies
Colloids and Surfaces A: Physicochem. Eng. Aspects 349 (2009) 156–161
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and performance of 1D and 2D copper borate nano/microstructures with different morphologies Yunhui Zheng, Zichen Wang, Yumei Tian ∗ , Yuning Qu, Shengli Li, Dongmin An, Xue Chen, Shuang Guan College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 11 June 2009 Received in revised form 11 August 2009 Accepted 11 August 2009 Available online 19 August 2009 Keywords: Composite materials Nanostructures Copper borate Tribology
a b s t r a c t Copper borate particles are synthesized via one-step precipitation reaction in aqueous solution of sodium borate (Na2 B4 O7 • 10H2 O) and copper sulfate (CuSO4 • 5H2 O) with phosphate ester as the modifying agent. One-dimension (1D) and two-dimension (2D) copper borate nano/microstructures with different morphologies are well translated through thermally induced crystal growth processes with the special surfactant at different synthetic temperatures. The 2D copper borate particles are hydrophobic, and the friction coefficients of basic oil can be decreased with adding the hydrophobic copper borate. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The ability to control and manipulate the physical and chemical properties of materials as we desire is one of the challenging issues in chemistry and materials science. Modern chemists are now exploring ways to obtain such control at the nanometer scale. These novel phenomena are strongly related to two crucial nanoscale geometrical parameters: size and shape [1–5]. For example, it has been revealed both theoretically and experimentally that the electronic band of a crystal is gradually quantized as the crystal-size is reduced, resulting in an increase in the bandgap energy. Similarly, the shape of nanocrystals plays a crucial parameter in the determination of their properties. The shape of nanocrystals can be simply classified by their dimensionality [6,7]. During the last few years, researchers have been extensively studying efficient synthetic routes to well-defined nanocrystals with controlled size and shape. These methods include gas-phase syntheses utilizing vapor–liquid–solid (VLS) methods [8,9], chemical vapor deposition (CVD) [10,11], thermal evaporation [12,13] and liquid-phase colloidal syntheses in aqueous [14,15] or nonhydrolytic media [16,17]. In this paper, we report a general and highly effective synthetic route to well-defined nanocrystals with controlled size and shape. One-dimension (1D) and two-dimension (2D) copper borate
∗ Corresponding author. Tel.: +86 431 85155358; fax: +86 431 85155358. E-mail address: [email protected] (Y. Tian). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.08.012
nano/microstructures with different morphologies are well synthesized by using phosphate ester as the surfactant and controlling the reaction temperature. The better dispersible and hydrophobic products, with diameters of 500 nm and thicknesses of 100–200 nm are obtained through this method. The additive of these nanoellipsoidal inorganic particles into the basic oil, has greatly decreased the friction coefficient, and brought no toxicity to the environment, which is of great importance to the human. 2. Experimental 2.1. Materials All chemicals used in the synthetic procedures of this work were purchased from Beijing Chemicals Co. Ltd, which were of analytical purity and without any further treatments. Distilled water was applied for all synthesis and treatment processes. 2.2. Methods In a typical procedure, 50 mL of 0.1 mol·dm−3 Na2 B4 O7 • 10H2 O aqueous solution and 1 mL of 0.002 mol·dm−3 phosphate ester were loaded into a 500 mL three-neck round-bottom flask equipped with a thermometer and a mechanical stirrer. The mixture was kept at a certain temperature, to which 10 mL of 2 mol·dm−3 CuSO4 • 5H2 O aqueous solution was added over 0.5 h while being stirred [5]. The reaction was then continued at the same temperature for about 7 h. The precipitate was collected and washed several times with
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Fig. 1. SEM images of samples prepared for 7 h with phosphate ester at: (a) 10 ◦ C, (b) 30 ◦ C, (c) 50 ◦ C, (d) 70 ◦ C and (e) unmodified copper borate.
distilled water to remove the by-products, and then dried at 60 ◦ C for 8 h to obtain the final copper borate powders. 2.3. Characterizations The morphology and the size of the samples were examined using a Hitachi scanning electron microscope (SEM) with a fieldemission-scanning electron microscope (JEOL-6700F) and a Hitachi H-800 transmission electron microscope (TEM), at an accelerator voltage of 200 kV. The powders were dispersed in absolute ethanol and ultrasonicated before SEM and TEM characterization. The structure and the composition of products were analyzed by X-ray powder diffraction (XRD) (SHIMADZU XRD-6000 diffractometer employing Ni-filtered Cu K␣ radiation, at a scanning rate of 6◦ min−1 with 2Â ranging from 10◦ to 60◦ ). Water contact angle of samples was measured by using a FTÅ 200 (USA) contact angle analyzer. The powder sample was first pressed into a wafer under 10 MPa pressure and then was measured by sessile drop method. This method offered twofold advantages: it required a very small amount of liquid and a scant solid surface.
IR spectroscopy of the samples as powder-pressed KBr pellets was examined in the wave number range from 4000 to 500 cm−1 at a resolution of 4 cm−1 using a JIR-5500 (JEOL) spectrophotometer at room temperature. The friction coefficients of the base oil with hydrophobic copper borate nanoparticles, in comparison to the base oil and the base oil with unmodified copper borate powder were performed in the laboratory atmosphere (humidity 50%) using a screen display terminal face friction and wear tester (MMU-10G, made by Jinan Shijin Group Co. Ltd., China). The friction tests were conducted at a rotating speed of 600 rpm and under a constant load of 600 N, for the test duration of 30 min [18]. 3. Results and discussion 3.1. SEM micrograph of the powders It could be clearly observed that the better dispersible 1D nanowhiskers had been obtained with diameters of 100 nm, lengths of about 2 m, and the aspect ratios close to 20 when the temperature had reached 10 ◦ C (Fig. 1a). Then the temperature had
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Fig. 2. TEM of the prepared copper borate with phosphate ester for 7 h at: (a) 10 ◦ C, (b) 30 ◦ C and (c) 70 ◦ C (inset the SAED).
been enhanced to 30 ◦ C, and the shape of the products was absolutely spindle with diameters of 100–200 nm, lengths of about 1 m (Fig. 1b). When the reaction temperature had been increased to 50 ◦ C, the powders consisted of a few ellipsoids and some spindly particles (Fig. 1c). Finally, it could be clearly observed that the products were absolute ellipsoids with about 500 nm in diameters and 100–200 nm in thickness (Fig. 1d), when the reaction temperature was at 70 ◦ C for 7 h with phosphate ester as the modifying agent. Moreover, it could be clearly observed that unmodified copper borates had rulelessly grown when the reaction temperature was at 70 ◦ C for 7 h without phosphate ester (Fig. 1e). 3.2. TEM micrograph of the products Fig. 2 shows the internal framework of nanowhiskers (Fig. 2a), spindles (Fig. 2b) and ellipsoids (Fig. 2c) of copper borate, respectively, which are consistent with the results of SEM. The lattice in an electron diffraction pattern appeared due to the diffracted electron beam from a set of lattice planes in the crystallites present in the sample satisfying the Bragg diffraction condition. Using this selected area electron diffraction (SAED) technique, One-dimension (1D) and two-dimension (2D) copper borate nano/microstructures were revealed to be perfectly single crystalline.
3.4. Contact angle of powders The contact water angle is widely used as a criterion for evaluating the surface hydrophobicity. In order to study the surface characteristics, the copper borate powder was analyzed to measure the relative contact angle. Fig. 4 presents the changes of the contact angle containing the samples which were prepared at different synthetic temperatures. When the synthetic temperature had been increased from 30 ◦ C to 70 ◦ C (Fig. 4a–c), the wettability of samples was decreased. The contact angle of the sample, which synthesized at 30 ◦ C for 7 h was 24.41◦ (Fig. 4a). Then, the contact angle of the synthesized sample synthesized at 50 ◦ C for 7 h was 96.60◦ (Fig. 4b). When the synthetic temperature was increased to 70 ◦ C for 7 h, the contact angle was enhanced to 111.58◦ (Fig. 4c). From the results, we could deduce that the contact angle had been increased along with the synthetic temperature increasing.
3.3. X-ray diffraction diagrams of copper borate The synthesized product was stable weak green powder at room temperature. Fig. 3 shows the XRD patterns of the 1D CuBO2 prepared at 10 ◦ C and 30 ◦ C for 7 h (Fig. 3a and b), the mixture of them prepared at 50 ◦ C for 7 h (Fig. 3c), and the 2D Cu(BO2 )2 prepared at 70 ◦ C for 7 h (Fig. 3d), which indicates the particles are all crystals. The diffraction data were in good agreement with JCPDS files No. 28–1256 (Fig. 3a and b) and No. 01–0472 (Fig. 3d). No characteristic peaks of impurities, other borates and unreacted compounds were observed. From Fig. 3a–d, we can observe the translation process of copper borate from 1D to 2D with the change of the reaction temperature.
Fig. 3. XRD of products obtained at: (a) 10 ◦ C, (b) 30 ◦ C, (c) 50 ◦ C and (d) 70 ◦ C for 7 h with phosphate ester.
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Fig. 4. Contact angle of samples prepared for 7 h at: (a) 30 ◦ C, (b) 50 ◦ C and (c) 70 ◦ C with phosphate ester.
3.5. FT-IR spectrum of composite powders FT-IR is used to determine which functional groups are presented in the samples. Fig. 5 shows the samples made without phosphate ester at 70 ◦ C (Fig. 5a), with phosphate ester at 10 ◦ C (Fig. 5b) and 70 ◦ C (Fig. 5c), and the pure phosphate ester (Fig. 5d) had different frameworks. There were –CH2 bands at 2846 and 2918 cm−1 in copper borate prepared at 70 ◦ C (Fig. 5c) and the pure phosphate ester (Fig. 5d). From the results, we could observe that the products prepared without phosphate ester at 70 ◦ C (Fig. 5a) and with phosphate ester at 10 ◦ C (Fig. 5b) had no redundant peaks formed. But Fig. 5c shows some dissimilarities with them, which contains some peaks in Fig. 5d. It could be deduced that the phosphate ester was not bonded to the surface of copper borate when
the synthetic temperature was at 10 ◦ C, but when the temperature was enhanced from 50 ◦ C to 70 ◦ C, the bond formed between phosphate ester and copper borate. 3.6. The mechanism of synthesis A balance between the kinetic and the thermodynamic growth regimes strongly governs the final shape of the nano/microcrystals. Isotropic growth of microcrystals is preferred under the thermodynamic growth regime that is characterized by a sufficient supply of thermal energy at higher temperature. In contrast, anisotropic growth along a specific direction is facilitated under a kinetic growth regime that is promoted at lower temperature [19]. In our experiments, the surface energy of the nano/microcrystals could be modulated by introducing surfactants which adsorbed onto surfaces of growing crystallites. When surfactants stabilized a certain surface by “selective adhesion”, the growth rate difference between different crystallographic directions could be accentuated. Selective adhesion of surfactants was critical in the synthesis of 2D copper borate, when the reaction was carried out at higher temperature. The growth rate was exponentially proportional to the surface energy under the kinetic growth process, and the energy difference between the higher energy surface and other lower energy surfaces could promote preferential growth along respectively. All samples obtained were elongated along their own crystallographically high energy directions when the reaction was carried out at lower temperature. The schematic illustration of possible growth mechanisms of copper borate nano/microstructures is shown in Fig. 6. 3.7. Friction coefficient of the copper borate
Fig. 5. FT-IR spectra of the powders prepared: (a) at 70 ◦ C without phosphate ester for 7 h, (b) at 10 ◦ C and (c) at 70 ◦ C with phosphate ester for 7 h, (d) pure phosphate ester.
Results of friction tests of the base oil with different percentage of hydrophobic copper borate compared to the base oil and base oil with unmodified copper borate powders at a rotating speed of 600 rpm and under a constant load of 600 N for 30 min are shown in Fig. 7. It could be seen that the base oil with hydrophobic copper
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Fig. 8. A diagram of the antiwear mechanism.
Fig. 6. Schematic illustration of the possible growth mechanisms of copper borate nano/microstructures with different morphologies at the different reaction temperature (RT): (a) at 10 ◦ C, (b) at 30 ◦ C, (c) at 50 ◦ C and (d) at 70 ◦ C with phosphate ester for 7 h.
borates (Fig. 7c–g) gave a smaller and more stable friction coefficient than that of the base oil (Fig. 7a) and the base oil with the unmodified copper borate powders (Fig. 7b). The results showed that the friction coefficient of base oil was 0.116 (Fig. 7a), and other friction coefficients were 0.111 with 1% wt. of the unmodified copper borate (Fig. 7b), 0.087 with 10% wt. of hydrophobic copper borate (Fig. 7c), 0.078 with 5% wt. of the hydrophobic copper borate (Fig. 7d), 0.060 with 0.5% wt. of hydrophobic copper borate (Fig. 7e), 0.033 with 3% wt. of hydrophobic copper borate (Fig. 7f), and 0.012 with 1% wt. of hydrophobic copper borate (Fig. 7g). We could observe that the smallest friction coefficient of the mixture was 0.012, when the basic oil was mixed with the amount of 1% wt. hydrophobic copper borate, and the friction coefficient had not decreased with the increasing amount of the hydrophobic copper borate. 3.8. The mechanism of performance The reason why the values of the friction coefficient for the unmodified copper borate powders was greater than that for the hydrophobic copper borate, seemed to be due to the damming
actions of the pure copper borate powder accumulated and cohered in the front of the leading face of a steel board as its size was larger and no rules, as well as it was hardly dispersed in the base oil, so it could not easily penetrate into the interface of the base oil and showed the higher friction coefficient (Fig. 7b). The friction coefficient had not been decreased with the increasing amount of hydrophobic copper borate could be explained that overabundance of the copper borates assembled to produce the inner friction by themselves. The depressed friction coefficient of the base oil with the additive could be explained that at a given concentration, the hydrophobic copper borate more easily penetrated into the interface of the base oil, and the ellipsoidal products could turn the sliding friction into the rolling friction partly like “micro-axletree”. Moreover, particles of copper borate deposited or adsorbed on wear scar surface at first, and then formed an amorphous film due to the shearing effect [20]. This film could smooth wear scar like a fluid to decrease shearing stress. Therefore, the results showed a lower friction coefficient. A diagram of the antiwear mechanism is given in Fig. 8. 4. Conclusions One-dimension (1D) and two-dimension (2D) copper borate nano/microstructures with different morphologies were well translated through thermally induced crystal growth processes with a special surfactant at the different synthetic temperatures. We could change the 1D products to 2D products with the control of the synthetic temperature. Through our experiments, a general and highly effective synthetic route to well-defined nano/microcrystals with controlled size and shape had been achieved, consistent with the suggested formation mechanism. Furthermore, because of its hydrophobic property and morphology, the inorganic particles could be stably dispersed in the base oil and the friction coefficient of the base oil had been distinctly decreased by the additive of hydrophobic copper borate. The most important was that had brought no toxicity to the environment. Therefore it was of special use in economizing the energy sources and protecting the surroundings. References
Fig. 7. Effect of the base oil on friction coefficient: (a) without anything, (b) with 1% wt. of the unmodified copper borate, (c) with 10% wt. of the hydrophobic copper borate, (d) with 5% wt. of the hydrophobic copper borate, (e) with 0.5% wt. of the hydrophobic copper borate, (f) with 3% wt. of the hydrophobic copper borate and (g) with 1% wt. of the hydrophobic copper borate.
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and performance of 1D and 2D copper borate nano/microstructures with different morphologies Yunhui Zheng, Zichen Wang, Yumei Tian ∗ , Yuning Qu, Shengli Li, Dongmin An, Xue Chen, Shuang Guan College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 11 June 2009 Received in revised form 11 August 2009 Accepted 11 August 2009 Available online 19 August 2009 Keywords: Composite materials Nanostructures Copper borate Tribology
a b s t r a c t Copper borate particles are synthesized via one-step precipitation reaction in aqueous solution of sodium borate (Na2 B4 O7 • 10H2 O) and copper sulfate (CuSO4 • 5H2 O) with phosphate ester as the modifying agent. One-dimension (1D) and two-dimension (2D) copper borate nano/microstructures with different morphologies are well translated through thermally induced crystal growth processes with the special surfactant at different synthetic temperatures. The 2D copper borate particles are hydrophobic, and the friction coefficients of basic oil can be decreased with adding the hydrophobic copper borate. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The ability to control and manipulate the physical and chemical properties of materials as we desire is one of the challenging issues in chemistry and materials science. Modern chemists are now exploring ways to obtain such control at the nanometer scale. These novel phenomena are strongly related to two crucial nanoscale geometrical parameters: size and shape [1–5]. For example, it has been revealed both theoretically and experimentally that the electronic band of a crystal is gradually quantized as the crystal-size is reduced, resulting in an increase in the bandgap energy. Similarly, the shape of nanocrystals plays a crucial parameter in the determination of their properties. The shape of nanocrystals can be simply classified by their dimensionality [6,7]. During the last few years, researchers have been extensively studying efficient synthetic routes to well-defined nanocrystals with controlled size and shape. These methods include gas-phase syntheses utilizing vapor–liquid–solid (VLS) methods [8,9], chemical vapor deposition (CVD) [10,11], thermal evaporation [12,13] and liquid-phase colloidal syntheses in aqueous [14,15] or nonhydrolytic media [16,17]. In this paper, we report a general and highly effective synthetic route to well-defined nanocrystals with controlled size and shape. One-dimension (1D) and two-dimension (2D) copper borate
∗ Corresponding author. Tel.: +86 431 85155358; fax: +86 431 85155358. E-mail address: [email protected] (Y. Tian). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.08.012
nano/microstructures with different morphologies are well synthesized by using phosphate ester as the surfactant and controlling the reaction temperature. The better dispersible and hydrophobic products, with diameters of 500 nm and thicknesses of 100–200 nm are obtained through this method. The additive of these nanoellipsoidal inorganic particles into the basic oil, has greatly decreased the friction coefficient, and brought no toxicity to the environment, which is of great importance to the human. 2. Experimental 2.1. Materials All chemicals used in the synthetic procedures of this work were purchased from Beijing Chemicals Co. Ltd, which were of analytical purity and without any further treatments. Distilled water was applied for all synthesis and treatment processes. 2.2. Methods In a typical procedure, 50 mL of 0.1 mol·dm−3 Na2 B4 O7 • 10H2 O aqueous solution and 1 mL of 0.002 mol·dm−3 phosphate ester were loaded into a 500 mL three-neck round-bottom flask equipped with a thermometer and a mechanical stirrer. The mixture was kept at a certain temperature, to which 10 mL of 2 mol·dm−3 CuSO4 • 5H2 O aqueous solution was added over 0.5 h while being stirred [5]. The reaction was then continued at the same temperature for about 7 h. The precipitate was collected and washed several times with
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Fig. 1. SEM images of samples prepared for 7 h with phosphate ester at: (a) 10 ◦ C, (b) 30 ◦ C, (c) 50 ◦ C, (d) 70 ◦ C and (e) unmodified copper borate.
distilled water to remove the by-products, and then dried at 60 ◦ C for 8 h to obtain the final copper borate powders. 2.3. Characterizations The morphology and the size of the samples were examined using a Hitachi scanning electron microscope (SEM) with a fieldemission-scanning electron microscope (JEOL-6700F) and a Hitachi H-800 transmission electron microscope (TEM), at an accelerator voltage of 200 kV. The powders were dispersed in absolute ethanol and ultrasonicated before SEM and TEM characterization. The structure and the composition of products were analyzed by X-ray powder diffraction (XRD) (SHIMADZU XRD-6000 diffractometer employing Ni-filtered Cu K␣ radiation, at a scanning rate of 6◦ min−1 with 2Â ranging from 10◦ to 60◦ ). Water contact angle of samples was measured by using a FTÅ 200 (USA) contact angle analyzer. The powder sample was first pressed into a wafer under 10 MPa pressure and then was measured by sessile drop method. This method offered twofold advantages: it required a very small amount of liquid and a scant solid surface.
IR spectroscopy of the samples as powder-pressed KBr pellets was examined in the wave number range from 4000 to 500 cm−1 at a resolution of 4 cm−1 using a JIR-5500 (JEOL) spectrophotometer at room temperature. The friction coefficients of the base oil with hydrophobic copper borate nanoparticles, in comparison to the base oil and the base oil with unmodified copper borate powder were performed in the laboratory atmosphere (humidity 50%) using a screen display terminal face friction and wear tester (MMU-10G, made by Jinan Shijin Group Co. Ltd., China). The friction tests were conducted at a rotating speed of 600 rpm and under a constant load of 600 N, for the test duration of 30 min [18]. 3. Results and discussion 3.1. SEM micrograph of the powders It could be clearly observed that the better dispersible 1D nanowhiskers had been obtained with diameters of 100 nm, lengths of about 2 m, and the aspect ratios close to 20 when the temperature had reached 10 ◦ C (Fig. 1a). Then the temperature had
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Fig. 2. TEM of the prepared copper borate with phosphate ester for 7 h at: (a) 10 ◦ C, (b) 30 ◦ C and (c) 70 ◦ C (inset the SAED).
been enhanced to 30 ◦ C, and the shape of the products was absolutely spindle with diameters of 100–200 nm, lengths of about 1 m (Fig. 1b). When the reaction temperature had been increased to 50 ◦ C, the powders consisted of a few ellipsoids and some spindly particles (Fig. 1c). Finally, it could be clearly observed that the products were absolute ellipsoids with about 500 nm in diameters and 100–200 nm in thickness (Fig. 1d), when the reaction temperature was at 70 ◦ C for 7 h with phosphate ester as the modifying agent. Moreover, it could be clearly observed that unmodified copper borates had rulelessly grown when the reaction temperature was at 70 ◦ C for 7 h without phosphate ester (Fig. 1e). 3.2. TEM micrograph of the products Fig. 2 shows the internal framework of nanowhiskers (Fig. 2a), spindles (Fig. 2b) and ellipsoids (Fig. 2c) of copper borate, respectively, which are consistent with the results of SEM. The lattice in an electron diffraction pattern appeared due to the diffracted electron beam from a set of lattice planes in the crystallites present in the sample satisfying the Bragg diffraction condition. Using this selected area electron diffraction (SAED) technique, One-dimension (1D) and two-dimension (2D) copper borate nano/microstructures were revealed to be perfectly single crystalline.
3.4. Contact angle of powders The contact water angle is widely used as a criterion for evaluating the surface hydrophobicity. In order to study the surface characteristics, the copper borate powder was analyzed to measure the relative contact angle. Fig. 4 presents the changes of the contact angle containing the samples which were prepared at different synthetic temperatures. When the synthetic temperature had been increased from 30 ◦ C to 70 ◦ C (Fig. 4a–c), the wettability of samples was decreased. The contact angle of the sample, which synthesized at 30 ◦ C for 7 h was 24.41◦ (Fig. 4a). Then, the contact angle of the synthesized sample synthesized at 50 ◦ C for 7 h was 96.60◦ (Fig. 4b). When the synthetic temperature was increased to 70 ◦ C for 7 h, the contact angle was enhanced to 111.58◦ (Fig. 4c). From the results, we could deduce that the contact angle had been increased along with the synthetic temperature increasing.
3.3. X-ray diffraction diagrams of copper borate The synthesized product was stable weak green powder at room temperature. Fig. 3 shows the XRD patterns of the 1D CuBO2 prepared at 10 ◦ C and 30 ◦ C for 7 h (Fig. 3a and b), the mixture of them prepared at 50 ◦ C for 7 h (Fig. 3c), and the 2D Cu(BO2 )2 prepared at 70 ◦ C for 7 h (Fig. 3d), which indicates the particles are all crystals. The diffraction data were in good agreement with JCPDS files No. 28–1256 (Fig. 3a and b) and No. 01–0472 (Fig. 3d). No characteristic peaks of impurities, other borates and unreacted compounds were observed. From Fig. 3a–d, we can observe the translation process of copper borate from 1D to 2D with the change of the reaction temperature.
Fig. 3. XRD of products obtained at: (a) 10 ◦ C, (b) 30 ◦ C, (c) 50 ◦ C and (d) 70 ◦ C for 7 h with phosphate ester.
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Fig. 4. Contact angle of samples prepared for 7 h at: (a) 30 ◦ C, (b) 50 ◦ C and (c) 70 ◦ C with phosphate ester.
3.5. FT-IR spectrum of composite powders FT-IR is used to determine which functional groups are presented in the samples. Fig. 5 shows the samples made without phosphate ester at 70 ◦ C (Fig. 5a), with phosphate ester at 10 ◦ C (Fig. 5b) and 70 ◦ C (Fig. 5c), and the pure phosphate ester (Fig. 5d) had different frameworks. There were –CH2 bands at 2846 and 2918 cm−1 in copper borate prepared at 70 ◦ C (Fig. 5c) and the pure phosphate ester (Fig. 5d). From the results, we could observe that the products prepared without phosphate ester at 70 ◦ C (Fig. 5a) and with phosphate ester at 10 ◦ C (Fig. 5b) had no redundant peaks formed. But Fig. 5c shows some dissimilarities with them, which contains some peaks in Fig. 5d. It could be deduced that the phosphate ester was not bonded to the surface of copper borate when
the synthetic temperature was at 10 ◦ C, but when the temperature was enhanced from 50 ◦ C to 70 ◦ C, the bond formed between phosphate ester and copper borate. 3.6. The mechanism of synthesis A balance between the kinetic and the thermodynamic growth regimes strongly governs the final shape of the nano/microcrystals. Isotropic growth of microcrystals is preferred under the thermodynamic growth regime that is characterized by a sufficient supply of thermal energy at higher temperature. In contrast, anisotropic growth along a specific direction is facilitated under a kinetic growth regime that is promoted at lower temperature [19]. In our experiments, the surface energy of the nano/microcrystals could be modulated by introducing surfactants which adsorbed onto surfaces of growing crystallites. When surfactants stabilized a certain surface by “selective adhesion”, the growth rate difference between different crystallographic directions could be accentuated. Selective adhesion of surfactants was critical in the synthesis of 2D copper borate, when the reaction was carried out at higher temperature. The growth rate was exponentially proportional to the surface energy under the kinetic growth process, and the energy difference between the higher energy surface and other lower energy surfaces could promote preferential growth along respectively. All samples obtained were elongated along their own crystallographically high energy directions when the reaction was carried out at lower temperature. The schematic illustration of possible growth mechanisms of copper borate nano/microstructures is shown in Fig. 6. 3.7. Friction coefficient of the copper borate
Fig. 5. FT-IR spectra of the powders prepared: (a) at 70 ◦ C without phosphate ester for 7 h, (b) at 10 ◦ C and (c) at 70 ◦ C with phosphate ester for 7 h, (d) pure phosphate ester.
Results of friction tests of the base oil with different percentage of hydrophobic copper borate compared to the base oil and base oil with unmodified copper borate powders at a rotating speed of 600 rpm and under a constant load of 600 N for 30 min are shown in Fig. 7. It could be seen that the base oil with hydrophobic copper
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Fig. 8. A diagram of the antiwear mechanism.
Fig. 6. Schematic illustration of the possible growth mechanisms of copper borate nano/microstructures with different morphologies at the different reaction temperature (RT): (a) at 10 ◦ C, (b) at 30 ◦ C, (c) at 50 ◦ C and (d) at 70 ◦ C with phosphate ester for 7 h.
borates (Fig. 7c–g) gave a smaller and more stable friction coefficient than that of the base oil (Fig. 7a) and the base oil with the unmodified copper borate powders (Fig. 7b). The results showed that the friction coefficient of base oil was 0.116 (Fig. 7a), and other friction coefficients were 0.111 with 1% wt. of the unmodified copper borate (Fig. 7b), 0.087 with 10% wt. of hydrophobic copper borate (Fig. 7c), 0.078 with 5% wt. of the hydrophobic copper borate (Fig. 7d), 0.060 with 0.5% wt. of hydrophobic copper borate (Fig. 7e), 0.033 with 3% wt. of hydrophobic copper borate (Fig. 7f), and 0.012 with 1% wt. of hydrophobic copper borate (Fig. 7g). We could observe that the smallest friction coefficient of the mixture was 0.012, when the basic oil was mixed with the amount of 1% wt. hydrophobic copper borate, and the friction coefficient had not decreased with the increasing amount of the hydrophobic copper borate. 3.8. The mechanism of performance The reason why the values of the friction coefficient for the unmodified copper borate powders was greater than that for the hydrophobic copper borate, seemed to be due to the damming
actions of the pure copper borate powder accumulated and cohered in the front of the leading face of a steel board as its size was larger and no rules, as well as it was hardly dispersed in the base oil, so it could not easily penetrate into the interface of the base oil and showed the higher friction coefficient (Fig. 7b). The friction coefficient had not been decreased with the increasing amount of hydrophobic copper borate could be explained that overabundance of the copper borates assembled to produce the inner friction by themselves. The depressed friction coefficient of the base oil with the additive could be explained that at a given concentration, the hydrophobic copper borate more easily penetrated into the interface of the base oil, and the ellipsoidal products could turn the sliding friction into the rolling friction partly like “micro-axletree”. Moreover, particles of copper borate deposited or adsorbed on wear scar surface at first, and then formed an amorphous film due to the shearing effect [20]. This film could smooth wear scar like a fluid to decrease shearing stress. Therefore, the results showed a lower friction coefficient. A diagram of the antiwear mechanism is given in Fig. 8. 4. Conclusions One-dimension (1D) and two-dimension (2D) copper borate nano/microstructures with different morphologies were well translated through thermally induced crystal growth processes with a special surfactant at the different synthetic temperatures. We could change the 1D products to 2D products with the control of the synthetic temperature. Through our experiments, a general and highly effective synthetic route to well-defined nano/microcrystals with controlled size and shape had been achieved, consistent with the suggested formation mechanism. Furthermore, because of its hydrophobic property and morphology, the inorganic particles could be stably dispersed in the base oil and the friction coefficient of the base oil had been distinctly decreased by the additive of hydrophobic copper borate. The most important was that had brought no toxicity to the environment. Therefore it was of special use in economizing the energy sources and protecting the surroundings. References
Fig. 7. Effect of the base oil on friction coefficient: (a) without anything, (b) with 1% wt. of the unmodified copper borate, (c) with 10% wt. of the hydrophobic copper borate, (d) with 5% wt. of the hydrophobic copper borate, (e) with 0.5% wt. of the hydrophobic copper borate, (f) with 3% wt. of the hydrophobic copper borate and (g) with 1% wt. of the hydrophobic copper borate.
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