- Email: [email protected]
Electroless copper coating of cenospheres using silver nitrate activator
November 2002
Materials Letters 57 (2002) 151 – 156 www.elsevier.com/locate/matlet
Electroless copper coating of cenospheres using silver nitrate activator S. Shukla, S. Seal *, Z. Rahaman, K. Scammon Advanced Materials Processing and Analysis Center (AMPAC) and Mechanical, Materials, and Aerospace Engineering Department (MMAE), Eng. 381, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32816, USA Received 2 November 2001; accepted 12 November 2001
Abstract Electroless Cu-coating of fly ash cenosphere particles is demonstrated in the present investigation. The electroless Cucoating process is modified by replacing the conventional PdCl2 activator with AgNO3 activator to reduce the overall cost of the coating process. The cenosphere particles are characterized by scanning electron microscope, energy dispersive spectroscopy, auger electron spectroscopy (AES), X-ray diffraction analysis, and focussed ion beam (FIB) microscopy during and after the coating process. Relatively uniform and continuous coating thickness of f 350 nm is obtained under the given coating conditions. The possible mechanism of electroless Cu-coating of cenosphere particles utilizing AgNO3 activator is suggested. Cu-coated fly ash particles find applications in manufacturing conducting polymers for EMI-shielding applications. D 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.Pq Keywords: Electroless coating; Copper; Fly ash cenospheres; Silver nitrate; Activation; Catalyst
1. Introduction Fly ash cenospheres are primarily a by-product in power generation plants. Research is in progress to effectively use this by-product to produce new usable and profitable materials as they pose major disposal and environmental problems [1]. Fly ash is a nonconducting ceramic; however, electrical conductivity may be imparted to these particles by depositing pure metals such as Cu and Ag on their surface. Due to
*
Corresponding author. Tel.: +1-407-823-5277; fax: +1-407823-0208. E-mail address: [email protected] (S. Seal).
their low density, Cu- and Ag-coated fly ash cenospheres find applications as conducting fillers, which can be used in polymer matrices to manufacture conducting polymers. Cu- and Ag-coating of cenosphere particles using various activators is currently under development in our lab. Recently, we reported electroless Cu- and Ag-coating of cenosphere particles [1,2], which are useful in manufacturing conductive polymer for EMI-shielding application by dispersing these fillers in a selected polymer matrix [3]. In these investigations, however, the electroless Cu- and Agcoating is obtained using conventional sensitization and activation steps involving costlier activator viz. palladium chloride (PdCl2). As a result, the overall cost of the electroless process for coating Cu and Ag on cenosphere particle surface is still a major problem
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 7 2 2 - X
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S. Shukla et al. / Materials Letters 57 (2002) 151–156
in commercialization. In our view, there is no open literature regarding the electroless metal coating of cenosphere particles using cost-effective chemicals. From this viewpoint, the objective of the present article is to demonstrate the electroless Cu-coating of cenosphere particles by modifying the electroless Cu-coating process reported recently [1]. In this report, we replace the costlier PdCl2 activator by cheaper AgNO3 activator without affecting the properties of Cu-coating.
2. Experimental 2.1. Materials Fly ash cenosphere particles used in the present investigation have a low density of < 0.7 g/cm3. The raw material is supplied by Energy Strategy, USA. The particle size distribution varies from 70 to 280 Am with a mean particle diameter of 115 Am [1]. 2.2. Chemicals The chemicals used for the electroless Cu-coating of cenosphere particles include SnCl2 (anhydrous min. 99%), NaOH (99.3%), NaKC4H4O6 (min. 99%), CuSO45H2O (approx. 99%), HCHO (37 wt.%) (from Sigma, USA), HCl (37 wt.%) (from Aldrich, USA), AgNO3 (99.9+%, ACS reagent), and NH4OH (28.0 – 30.0% NH3, ACS reagent) (from Alfa Aesar, USA). 2.3. Procedure The general sequential steps involved in the present electroless Cu-coating process are as follows. A total of 5 g of as-received cenosphere particles are first stirred in an acidic SnCl2 bath, containing 10 g/l of SnCl2 and 40 ml/l of concentrated HCl acid, for 2 h (sensitization step). The sensitized particles are filtered off, then transferred to the homogeneous aqueous bath containing AgNO3 (9 g/l) and aq. NH3 acid (9 ml/l) and stirred in this bath for 2 h (activation step). After stirring, the activated particles are filtered off and washed thoroughly with deionized water. The particles are then transferred to a coating bath involving 12 g/l CuSO4, 10 g/l NaOH, 50 g/l NaKT, and 12 ml/l of HCHO for actual Cu-deposition, and stirred for 30 min or until the
blue color of the solution disappears. The Cu-coated particles are then filtered off, washed with deionized water, and dried in a vacuum oven at 110 jC for 1 h. 2.4. Characterization A JEOL-SEM 6400 is used to analyze the surface morphology and the size distribution of uncoated and Cu-coated particles, while EDX analysis is performed to understand their chemical constituents. To avoid any charging, the as-received fly ash cenosphere particles are coated with thin layer of gold using a sputter coater (K350, Emitech, Ashford, Kent, England) for SEM analysis. The structure analysis of Cucoated fly ash cenosphere particle surface is carried out (within 2-h range of 10 –80j) using Rigaku X-ray Diffractometer (XRD) utilizing CuKa X-radiation of ˚ . Auger electron spectroscopic wavelength 1.54 A (AES) analysis is conducted, at a base pressure of 10 10 Torr, within the K.E. range of 110 – 700 eV (beam voltage of 3 kV, eV/step 1 eV, time/step 50 ms), for the activated and washed cenosphere particles. Focussed ion beam (FIB) (FIB 200 TEM, FEI, Hillsboro, OR) analysis is performed on one of the Cucoated cenosphere particle to measure the Cu-coating thickness. The detail procedure for the Cu-coating thickness measurement via FIB analysis is described elsewhere [1]. In short, the thickness measurement using FIB technique is accomplished via milling a hole through one of the Cu-coated cenosphere particle surface. During the thickness measurement, the sample stage is tilted by 45j; hence, the measured thickness is corrected by dividing it by a factor of cos 45j for obtaining true coating thickness.
3. Results and discussion 3.1. Coating morphology and chemistry The SEM analysis of uncoated fly ash cenosphere particles, as reported in our previous work [1], showed that they have spherical surface morphology. The chemical constituents of these particles as revealed by EDX analysis showed that these particles are mainly composed of mixture of oxides such as SiO2, Al2O3, and Fe2O3, while other oxides of trace elements such as K, Ca, Mg, and Ti were also present [1].
S. Shukla et al. / Materials Letters 57 (2002) 151–156
Typical SEM micrographs of one of the Cu-coated cenosphere particles at low and high magnifications are presented in Fig. 1(a) and (b), respectively, while EDX analysis is presented in Fig. 1(c). In Fig. 1(a), cenosphere particles appear to be coated uniformly using the modified electroless coating process. Further, the coating appears to be made up of nanoparticles having average size of f 200 –300 nm (Fig. 1(b)). The EDX analysis of one of the Cu-coated cenosphere particles (Fig. 1(c)) shows prominent
153
presence of Cu within the coating. This indicates successful Cu-coating of cenosphere particles using the modified electroless coating process utilizing AgNO3 activator. The strong oxygen peak, which was observed for uncoated cenosphere particles [1], is absent in Fig. 1(c), suggesting minimum copper oxide formation within the bulk of the coating and the deposition of pure metallic Cu0 only. No impurities are observed in the Cu-coating, within the resolution limit of EDX, which further indicates a high purity
Fig. 1. Typical SEM micrographs of Cu-coated fly ash cenosphere particles at (a) low and (b) high magnifications. (c) EDX analysis of Cucoated fly ash cenosphere particles.
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Fig. 2. Powder XRD pattern of Cu-coated fly ash cenosphere particles.
Cu-coating obtained using the modified electroless coating technique. Moreover, the substrate elements are also not observed in the EDX spectrum (Fig. 1(c)), suggesting deposition of relatively thick Cu-coating. Thus, SEM and EDX analyses of Cu-coated cenosphere particles qualitatively suggest pure, thick, and metallic Cu-coating developed on the cenosphere particle surface using AgNO3 activator.
identified by comparison with the PDF card (no. 040836) of JADE-SCAN software, revealing its FCC structure. As XRD signals arise from the bulk, it appears that pure metallic Cu0 exists within the bulk of the coating. This further suggests the successful deposition of pure metallic Cu0 on the cenosphere
3.2. Coating structure and its thickness In order to confirm the presence of pure metallic Cu0 within the bulk of the coating and to understand the relative orientation of Cu nanocrystallites within the coating, XRD analysis of the Cu-coated cenosphere particles is conducted. The obtained X-ray powder diffraction pattern is presented in Fig. 2. Interestingly, the X-ray diffraction pattern of the Cucoated cenosphere particles exhibits only Cu peaks. The amorphous background observed in Fig. 2 originates from the base material. In Fig. 3, major peaks (111)Cu, (200)Cu, and (220)Cu of pure metallic Cu0 are
Fig. 3. Typical FIB micrograph of Cu-coated fly ash cenosphere particle. (i) Cu-coated cenosphere particle surface, (ii) deposited Pt, (iii) Cu-coating, and (iv) cenosphere particle.
S. Shukla et al. / Materials Letters 57 (2002) 151–156
particle surface using the modified electroless coating technique utilizing AgNO3 activator. It is further observed from Fig. 2 that the Cu nanoparticles within the coating have a strong (111) orientation. It is well known that the specific free energy of Cu0 is minimum for (111) planes and is characteristic of FCC structures. The development of uniform and continuous nanocrystalline Cu-coating via formation of Cu nanoparticles having preferred crystallographic orientation appears to follow the sequential development of coating process reported earlier [1,2,4]. Typical FIB micrograph showing the cross-section a milled hole obtained over a Cu-coated cenosphere particle surface is presented in Fig. 3. Uniform, dense, and continuous Cu-coating is evident from Fig. 3. The average thickness of the Cu-coating can be directly measured from this figure and appears to be f 350 nm (this is a true thickness, which is obtained by dividing the measured thickness by the factor of cos 45j). Relatively uniform and continuous Cu-coating obtained in the present investigation suggests that AgNO3 is the cost-effective replacement as an activator for the conventional PdCl2 activator for the electroless Cu-coating of cenosphere particles. 3.3. Mechanism of electroless Cu-coating of fly ash cenosphere particles by utilizing AgNO3 activator We reported recently the mechanism of Cu- and Ag-coating of cenosphere particles utilizing conventional Pd0 catalyst [1,2]. Based on these earlier investigations and the data presented in this report, we now propose the mechanism of electroless Cu-coating of cenosphere particles utilizing AgNO3 activator and is described schematically in Fig. 4. It is observed in the previous investigation that after stirring the cenosphere particles in an acidic SnCl2 bath, Sn2 + ions get adsorbed on the particle surface forming a uniform layer [1]. These sensitized cenosphere particles are then stirred in the activation bath, which is prepared by dissolving AgNO3 in aqueous ammonical solution. This possibly results in the formation of [Ag(NH3)2] + ions in the activation bath [2,5], which get adsorbed on the cenosphere particle surface following the addition of the sensitized particles to the activation bath (Fig. 4(a)). Sn2 + ions present on the sensitized cenosphere particle surface (Fig. 4(a)), however, immediately reduces
155
Fig. 4. Mechanism of electroless Cu-coating of fly ash cenosphere particles utilizing AgNO3 as an activator. (a) I—Sn2 + and II— [Ag(NH3)2] + (in the activation bath), (b) III—Ag0 and IV—Sn2 + / Sn4 + species (after the activation and deionized washing), and (c) V—Cu0 (after the coating). The gray interior in the sketch represents fly ash cenosphere particle.
[Ag(NH3)2] + to Ag0 (Fig. 4(b)) according to the following reaction: 2þ 2½AgðNH3 Þ2 þ ðadÞ þ SnðadÞ
! 2Ag0ðadÞ þ Sn4þ ðadÞ þ 4NH3ðaqÞ
ð1Þ
Deposited Ag0 then acts as a catalyst for the subsequent Cu deposition in the electroless coating bath. This mechanism of activation, as discussed above, is based on the deposition of Ag0 catalyst on the cenosphere particle surface after the activation step, which is a key step in the entire electroless coating process. In order to confirm the presence of Ag0 catalyst on the cenosphere particle surface after the activation and the washing steps, AES analysis of the activated and washed cenosphere particles is conducted and the resulting spectrum is presented in Fig. 5. In the spectrum, various peaks related to C (KLL, 271 eV), O (KLL, 517 eV), Sn (MNN, 432 eV), Cl (LMM, 186 eV), and Ag (MNN, 362 eV) are
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4. Conclusions (1) Uniform and continuous metallic Cu-coating is obtained over fly ash cenospheres using the electroless coating technique utilizing AgNO3 activator. (2) AgNO3 activator is a cost-effective replacement for the conventional PdCl2 activator for the electroless Cu-coating of fly ash cenospheres. Fig. 5. Scanning auger analysis of activated and washed fly ash cenosphere particles.
identified by comparison with reference data [6]. The spectrum clearly shows the presence of Ag0 specie on the cenosphere particle surface, which indicates that the subsequent Cu0 deposition is catalyzed by the Ag0 specie in the modified electroless coating process utilizing AgNO3 activator. Moreover, no nitrogen is detected on the particle surface after the activation and washing steps (Fig. 5) which further supports the reduction of [Ag(NH3)2] + ions by Sn2 + to Ag0 according to the reaction presented in Eq. (1). As reported earlier, the potential activation sites for Cu deposition are generally covered with a sheath of Sn2 + or Sn4 + species after the activation and deionized washing of the base material [1,7]. In the present investigation, the presence of this sheath is possibly indicated by Sn peak observed in the AES spectrum (Fig. 5). This sheath (Fig. 4(b)) may hamper the catalytic activity of Ag0 sites and must be dissolved by the coating bath before the actual Cu deposition begins. Once the Cu deposition is initiated, the deposited Cu atoms themselves act as catalyst for further Cu deposition. Well-developed Cu-coating is then obtained after the completion of the electroless coating process (Fig. 4(c)). Thus, AgNO3 is successfully utilized in the present investigation to deposit metallic Cu0 on fly ash cenospheres and is a potential cost-effective replacement for the conventional PdCl2 activator.
Acknowledgements Authors take this opportunity to thank Energy Strategy Associates (USA) and Coal Resources (USA) for supplying two batches of fly ash cenosphere particles and for financial support. Authors are also thankful to Materials Characterization Facility, University of Central Florida.
References [1] S. Shukla, S. Seal, J. Akesson, R. Oder, R. Carter, Z. Rahman, Appl. Surf. Sci. 180 (2001) 35. [2] S. Shukla, S. Seal, D. Zhou, S. Schwarz, J. Nanosci. Nanotech. 1 (2001) 407. [3] S. Shukla, S. Seal, J. Akesson, R. Oder, R. Carter, (unpublished work). [4] C.G. Sonwane, L.G. Bhatagadde, J.R. Bellare, Q. Li, J. Mater. Sci. Lett. 18 (1999) 1205. [5] S. Shibata, K. Aoki, T. Yano, M. Yamane, J. Sol-Gel Sci. Technol. 11 (1998) 279. [6] L.E. Davis, N.C. MacDonald, P.W. Palmbug, G.E. Riach, R.E. Weber, Handbook of Auger Electron Spectroscopy: A Reference Handbook of Standard Spectra for Identification and Interpretation of AES Data, Perkin-Elmer, Minnesota, 1976. [7] B. Meenan, N. Brown, J. Wilson, Appl. Surf. Sci. 74 (1994) 221.
Materials Letters 57 (2002) 151 – 156 www.elsevier.com/locate/matlet
Electroless copper coating of cenospheres using silver nitrate activator S. Shukla, S. Seal *, Z. Rahaman, K. Scammon Advanced Materials Processing and Analysis Center (AMPAC) and Mechanical, Materials, and Aerospace Engineering Department (MMAE), Eng. 381, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32816, USA Received 2 November 2001; accepted 12 November 2001
Abstract Electroless Cu-coating of fly ash cenosphere particles is demonstrated in the present investigation. The electroless Cucoating process is modified by replacing the conventional PdCl2 activator with AgNO3 activator to reduce the overall cost of the coating process. The cenosphere particles are characterized by scanning electron microscope, energy dispersive spectroscopy, auger electron spectroscopy (AES), X-ray diffraction analysis, and focussed ion beam (FIB) microscopy during and after the coating process. Relatively uniform and continuous coating thickness of f 350 nm is obtained under the given coating conditions. The possible mechanism of electroless Cu-coating of cenosphere particles utilizing AgNO3 activator is suggested. Cu-coated fly ash particles find applications in manufacturing conducting polymers for EMI-shielding applications. D 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.Pq Keywords: Electroless coating; Copper; Fly ash cenospheres; Silver nitrate; Activation; Catalyst
1. Introduction Fly ash cenospheres are primarily a by-product in power generation plants. Research is in progress to effectively use this by-product to produce new usable and profitable materials as they pose major disposal and environmental problems [1]. Fly ash is a nonconducting ceramic; however, electrical conductivity may be imparted to these particles by depositing pure metals such as Cu and Ag on their surface. Due to
*
Corresponding author. Tel.: +1-407-823-5277; fax: +1-407823-0208. E-mail address: [email protected] (S. Seal).
their low density, Cu- and Ag-coated fly ash cenospheres find applications as conducting fillers, which can be used in polymer matrices to manufacture conducting polymers. Cu- and Ag-coating of cenosphere particles using various activators is currently under development in our lab. Recently, we reported electroless Cu- and Ag-coating of cenosphere particles [1,2], which are useful in manufacturing conductive polymer for EMI-shielding application by dispersing these fillers in a selected polymer matrix [3]. In these investigations, however, the electroless Cu- and Agcoating is obtained using conventional sensitization and activation steps involving costlier activator viz. palladium chloride (PdCl2). As a result, the overall cost of the electroless process for coating Cu and Ag on cenosphere particle surface is still a major problem
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 7 2 2 - X
152
S. Shukla et al. / Materials Letters 57 (2002) 151–156
in commercialization. In our view, there is no open literature regarding the electroless metal coating of cenosphere particles using cost-effective chemicals. From this viewpoint, the objective of the present article is to demonstrate the electroless Cu-coating of cenosphere particles by modifying the electroless Cu-coating process reported recently [1]. In this report, we replace the costlier PdCl2 activator by cheaper AgNO3 activator without affecting the properties of Cu-coating.
2. Experimental 2.1. Materials Fly ash cenosphere particles used in the present investigation have a low density of < 0.7 g/cm3. The raw material is supplied by Energy Strategy, USA. The particle size distribution varies from 70 to 280 Am with a mean particle diameter of 115 Am [1]. 2.2. Chemicals The chemicals used for the electroless Cu-coating of cenosphere particles include SnCl2 (anhydrous min. 99%), NaOH (99.3%), NaKC4H4O6 (min. 99%), CuSO45H2O (approx. 99%), HCHO (37 wt.%) (from Sigma, USA), HCl (37 wt.%) (from Aldrich, USA), AgNO3 (99.9+%, ACS reagent), and NH4OH (28.0 – 30.0% NH3, ACS reagent) (from Alfa Aesar, USA). 2.3. Procedure The general sequential steps involved in the present electroless Cu-coating process are as follows. A total of 5 g of as-received cenosphere particles are first stirred in an acidic SnCl2 bath, containing 10 g/l of SnCl2 and 40 ml/l of concentrated HCl acid, for 2 h (sensitization step). The sensitized particles are filtered off, then transferred to the homogeneous aqueous bath containing AgNO3 (9 g/l) and aq. NH3 acid (9 ml/l) and stirred in this bath for 2 h (activation step). After stirring, the activated particles are filtered off and washed thoroughly with deionized water. The particles are then transferred to a coating bath involving 12 g/l CuSO4, 10 g/l NaOH, 50 g/l NaKT, and 12 ml/l of HCHO for actual Cu-deposition, and stirred for 30 min or until the
blue color of the solution disappears. The Cu-coated particles are then filtered off, washed with deionized water, and dried in a vacuum oven at 110 jC for 1 h. 2.4. Characterization A JEOL-SEM 6400 is used to analyze the surface morphology and the size distribution of uncoated and Cu-coated particles, while EDX analysis is performed to understand their chemical constituents. To avoid any charging, the as-received fly ash cenosphere particles are coated with thin layer of gold using a sputter coater (K350, Emitech, Ashford, Kent, England) for SEM analysis. The structure analysis of Cucoated fly ash cenosphere particle surface is carried out (within 2-h range of 10 –80j) using Rigaku X-ray Diffractometer (XRD) utilizing CuKa X-radiation of ˚ . Auger electron spectroscopic wavelength 1.54 A (AES) analysis is conducted, at a base pressure of 10 10 Torr, within the K.E. range of 110 – 700 eV (beam voltage of 3 kV, eV/step 1 eV, time/step 50 ms), for the activated and washed cenosphere particles. Focussed ion beam (FIB) (FIB 200 TEM, FEI, Hillsboro, OR) analysis is performed on one of the Cucoated cenosphere particle to measure the Cu-coating thickness. The detail procedure for the Cu-coating thickness measurement via FIB analysis is described elsewhere [1]. In short, the thickness measurement using FIB technique is accomplished via milling a hole through one of the Cu-coated cenosphere particle surface. During the thickness measurement, the sample stage is tilted by 45j; hence, the measured thickness is corrected by dividing it by a factor of cos 45j for obtaining true coating thickness.
3. Results and discussion 3.1. Coating morphology and chemistry The SEM analysis of uncoated fly ash cenosphere particles, as reported in our previous work [1], showed that they have spherical surface morphology. The chemical constituents of these particles as revealed by EDX analysis showed that these particles are mainly composed of mixture of oxides such as SiO2, Al2O3, and Fe2O3, while other oxides of trace elements such as K, Ca, Mg, and Ti were also present [1].
S. Shukla et al. / Materials Letters 57 (2002) 151–156
Typical SEM micrographs of one of the Cu-coated cenosphere particles at low and high magnifications are presented in Fig. 1(a) and (b), respectively, while EDX analysis is presented in Fig. 1(c). In Fig. 1(a), cenosphere particles appear to be coated uniformly using the modified electroless coating process. Further, the coating appears to be made up of nanoparticles having average size of f 200 –300 nm (Fig. 1(b)). The EDX analysis of one of the Cu-coated cenosphere particles (Fig. 1(c)) shows prominent
153
presence of Cu within the coating. This indicates successful Cu-coating of cenosphere particles using the modified electroless coating process utilizing AgNO3 activator. The strong oxygen peak, which was observed for uncoated cenosphere particles [1], is absent in Fig. 1(c), suggesting minimum copper oxide formation within the bulk of the coating and the deposition of pure metallic Cu0 only. No impurities are observed in the Cu-coating, within the resolution limit of EDX, which further indicates a high purity
Fig. 1. Typical SEM micrographs of Cu-coated fly ash cenosphere particles at (a) low and (b) high magnifications. (c) EDX analysis of Cucoated fly ash cenosphere particles.
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Fig. 2. Powder XRD pattern of Cu-coated fly ash cenosphere particles.
Cu-coating obtained using the modified electroless coating technique. Moreover, the substrate elements are also not observed in the EDX spectrum (Fig. 1(c)), suggesting deposition of relatively thick Cu-coating. Thus, SEM and EDX analyses of Cu-coated cenosphere particles qualitatively suggest pure, thick, and metallic Cu-coating developed on the cenosphere particle surface using AgNO3 activator.
identified by comparison with the PDF card (no. 040836) of JADE-SCAN software, revealing its FCC structure. As XRD signals arise from the bulk, it appears that pure metallic Cu0 exists within the bulk of the coating. This further suggests the successful deposition of pure metallic Cu0 on the cenosphere
3.2. Coating structure and its thickness In order to confirm the presence of pure metallic Cu0 within the bulk of the coating and to understand the relative orientation of Cu nanocrystallites within the coating, XRD analysis of the Cu-coated cenosphere particles is conducted. The obtained X-ray powder diffraction pattern is presented in Fig. 2. Interestingly, the X-ray diffraction pattern of the Cucoated cenosphere particles exhibits only Cu peaks. The amorphous background observed in Fig. 2 originates from the base material. In Fig. 3, major peaks (111)Cu, (200)Cu, and (220)Cu of pure metallic Cu0 are
Fig. 3. Typical FIB micrograph of Cu-coated fly ash cenosphere particle. (i) Cu-coated cenosphere particle surface, (ii) deposited Pt, (iii) Cu-coating, and (iv) cenosphere particle.
S. Shukla et al. / Materials Letters 57 (2002) 151–156
particle surface using the modified electroless coating technique utilizing AgNO3 activator. It is further observed from Fig. 2 that the Cu nanoparticles within the coating have a strong (111) orientation. It is well known that the specific free energy of Cu0 is minimum for (111) planes and is characteristic of FCC structures. The development of uniform and continuous nanocrystalline Cu-coating via formation of Cu nanoparticles having preferred crystallographic orientation appears to follow the sequential development of coating process reported earlier [1,2,4]. Typical FIB micrograph showing the cross-section a milled hole obtained over a Cu-coated cenosphere particle surface is presented in Fig. 3. Uniform, dense, and continuous Cu-coating is evident from Fig. 3. The average thickness of the Cu-coating can be directly measured from this figure and appears to be f 350 nm (this is a true thickness, which is obtained by dividing the measured thickness by the factor of cos 45j). Relatively uniform and continuous Cu-coating obtained in the present investigation suggests that AgNO3 is the cost-effective replacement as an activator for the conventional PdCl2 activator for the electroless Cu-coating of cenosphere particles. 3.3. Mechanism of electroless Cu-coating of fly ash cenosphere particles by utilizing AgNO3 activator We reported recently the mechanism of Cu- and Ag-coating of cenosphere particles utilizing conventional Pd0 catalyst [1,2]. Based on these earlier investigations and the data presented in this report, we now propose the mechanism of electroless Cu-coating of cenosphere particles utilizing AgNO3 activator and is described schematically in Fig. 4. It is observed in the previous investigation that after stirring the cenosphere particles in an acidic SnCl2 bath, Sn2 + ions get adsorbed on the particle surface forming a uniform layer [1]. These sensitized cenosphere particles are then stirred in the activation bath, which is prepared by dissolving AgNO3 in aqueous ammonical solution. This possibly results in the formation of [Ag(NH3)2] + ions in the activation bath [2,5], which get adsorbed on the cenosphere particle surface following the addition of the sensitized particles to the activation bath (Fig. 4(a)). Sn2 + ions present on the sensitized cenosphere particle surface (Fig. 4(a)), however, immediately reduces
155
Fig. 4. Mechanism of electroless Cu-coating of fly ash cenosphere particles utilizing AgNO3 as an activator. (a) I—Sn2 + and II— [Ag(NH3)2] + (in the activation bath), (b) III—Ag0 and IV—Sn2 + / Sn4 + species (after the activation and deionized washing), and (c) V—Cu0 (after the coating). The gray interior in the sketch represents fly ash cenosphere particle.
[Ag(NH3)2] + to Ag0 (Fig. 4(b)) according to the following reaction: 2þ 2½AgðNH3 Þ2 þ ðadÞ þ SnðadÞ
! 2Ag0ðadÞ þ Sn4þ ðadÞ þ 4NH3ðaqÞ
ð1Þ
Deposited Ag0 then acts as a catalyst for the subsequent Cu deposition in the electroless coating bath. This mechanism of activation, as discussed above, is based on the deposition of Ag0 catalyst on the cenosphere particle surface after the activation step, which is a key step in the entire electroless coating process. In order to confirm the presence of Ag0 catalyst on the cenosphere particle surface after the activation and the washing steps, AES analysis of the activated and washed cenosphere particles is conducted and the resulting spectrum is presented in Fig. 5. In the spectrum, various peaks related to C (KLL, 271 eV), O (KLL, 517 eV), Sn (MNN, 432 eV), Cl (LMM, 186 eV), and Ag (MNN, 362 eV) are
156
S. Shukla et al. / Materials Letters 57 (2002) 151–156
4. Conclusions (1) Uniform and continuous metallic Cu-coating is obtained over fly ash cenospheres using the electroless coating technique utilizing AgNO3 activator. (2) AgNO3 activator is a cost-effective replacement for the conventional PdCl2 activator for the electroless Cu-coating of fly ash cenospheres. Fig. 5. Scanning auger analysis of activated and washed fly ash cenosphere particles.
identified by comparison with reference data [6]. The spectrum clearly shows the presence of Ag0 specie on the cenosphere particle surface, which indicates that the subsequent Cu0 deposition is catalyzed by the Ag0 specie in the modified electroless coating process utilizing AgNO3 activator. Moreover, no nitrogen is detected on the particle surface after the activation and washing steps (Fig. 5) which further supports the reduction of [Ag(NH3)2] + ions by Sn2 + to Ag0 according to the reaction presented in Eq. (1). As reported earlier, the potential activation sites for Cu deposition are generally covered with a sheath of Sn2 + or Sn4 + species after the activation and deionized washing of the base material [1,7]. In the present investigation, the presence of this sheath is possibly indicated by Sn peak observed in the AES spectrum (Fig. 5). This sheath (Fig. 4(b)) may hamper the catalytic activity of Ag0 sites and must be dissolved by the coating bath before the actual Cu deposition begins. Once the Cu deposition is initiated, the deposited Cu atoms themselves act as catalyst for further Cu deposition. Well-developed Cu-coating is then obtained after the completion of the electroless coating process (Fig. 4(c)). Thus, AgNO3 is successfully utilized in the present investigation to deposit metallic Cu0 on fly ash cenospheres and is a potential cost-effective replacement for the conventional PdCl2 activator.
Acknowledgements Authors take this opportunity to thank Energy Strategy Associates (USA) and Coal Resources (USA) for supplying two batches of fly ash cenosphere particles and for financial support. Authors are also thankful to Materials Characterization Facility, University of Central Florida.
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