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Investigation of the surface composition of electrodeposited black chromium by X-ray photoelectron spectroscopy
Applied Surface Science 324 (2015) 837–841
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Investigation of the surface composition of electrodeposited black chromium by X-ray photoelectron spectroscopy ˇ sunien ¯ ˙ V. Jasulaitiene, ˙ I. Jureviˇciut ¯ e˙ S. Surviliene˙ ∗ , A. Ceˇ e, Center for Physical Sciences and Technology, A. Gostauto 9, 01108 Vilnius, Lithuania
a r t i c l e
i n f o
Article history: Received 10 June 2014 Received in revised form 30 October 2014 Accepted 10 November 2014 Available online 18 November 2014 Keywords: Black chromium Zinc oxide XPS spectra Chemical composition
a b s t r a c t The paper reviews black chromium electrodeposited from a trivalent chromium bath containing ZnO as a second main component. The chemical compositions of the top layers of the black chromium coatings were studied by the X-ray photoelectron spectroscopy method. The surface of black chromium was found to be almost entirely covered with organic substances. To gain information on the state of each element in the deposit bulk, the layer-by-layer etching of the black chromium surface with argon gas was used. Analysis of XPS spectra has shown that the top layers of black chromium without zinc are composed of various Cr(III) components, organic substances and metallic Cr, whereas metallic Cr is almost absent in black chromium containing some amount of Zn(II) compounds. The ratios of metal/oxide phases were found to be 10/27 and 2/28 for black chromium without and with zinc, respectively. It has been determined that owing to the presence of ZnO in the Cr(III) bath, the percentage of metallic chromium is substantially reduced in black chromium which is quite important for good solar selective characteristics of the coating. The results confirm some of earlier observations and provide new information on the composition of the near-surface layers. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Black coatings are widely used in solar collectors due to their high absorptance. Black deposits containing chromium are used under the name “black chromium” in solar collector manufacture. Traditionally, the black chromium coatings were deposited on metallic substrates (Ni, Cu, steel) from a hexavalent chromium bath. The data on electrodeposition and some properties of the black chromium coatings have been reported in several publications [1–5]. According to literature data the black chromium film is composed of metallic and oxide phases of chromium. In this case the top layer is composed mainly of oxides, whereas the substrate/coating interface is enriched with metallic chromium. The earlier sources [6,7] suggested that a metal was incorporated into the oxide film as fine separate grains forming light absorption centers owing to which the deposits may appear black. Further research of black chromium electrodeposition from the Cr(VI) electrolyte revealed that nitrates play a major role in the formation of black chromium acting on both the electrochemical
∗ Corresponding author. Tel.: +370 67617825. ˙ E-mail address: [email protected] (S. Surviliene). http://dx.doi.org/10.1016/j.apsusc.2014.11.052 0169-4332/© 2014 Elsevier B.V. All rights reserved.
nucleation and growth mechanisms and precisely they are responsible for the final chemical composition and micro-structural characteristics [8]. It was found that the black-Cr film is made of Cr2 O3 , unlike the white chromium film, which was mainly composed of Cr(OH)3 with a small amount of metallic Cr. The experimental evidence points to the fact that the presence of nitrates are also necessary to obtain black chromium from the trivalent chromium bath, thus attesting to the above-mentioned view. Electrodeposition of black chromium from trivalent chromium bath is important for the practical use. It is known [9] that black chromium may be deposited from Cr(III) baths containing Co(II) or Fe(II) as a second main component. Co-deposition of nickel and cobalt made it possible to increase the thermal resistance of black chromium coating [10]. It was shown in our previous work [11] that high-quality black chromium coatings with regard to their optical characteristics may be obtained from a Cr(III) electrolyte containing ZnO as a second main component. These investigations are being continued with the purpose to clarify the role played by ZnO during black chromium plating. To answer the question on the role of ZnO in this process, it is essential to obtain more detailed information on the composition of the black chromium coatings formed. The aim of this work was to investigate the chemical compositions of the top layers of black coatings formed in Cr(III) baths without and
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Table 1 The base composition of the black Cr(III) bath. Component
g l−1
CrCl3 ·6H2 O NH2 CH2 COOH H3 BO3 NaCl NaNO3 ZnO pH 1.2 Temperature 20 ◦ C ic = 0.4 A cm−2
250 18.75 30.0 60.0 3.0 5.0
with ZnO and to compare the distribution of elements from surface to depth in these coatings. 2. Experimental Black chromium coatings were electrodeposited on steel containing 99.4% of iron (Steel-3). Before electrodeposition the substrate was mechanically polished with further electropolishing. The steel substrate was electropolished in the commercial solution containing 27.5 g l−1 sodium carbonate, 17.5 g l−1 sodium tripolyphosphate, 2.5 g l−1 sodium silicate, 2.5 g l−1 synthanol DS10 (CAS No 12679-83-3) and 50 g l−1 sodium hydrate at a current density of 0.05 A cm−2 for 5 min at 50 ◦ C, using the substrate as an anode and a Ti plate as a cathode. After that samples were washed with distilled water, soaked in a hot 20 vol.% HCl solution, washed again with distilled water and then immediately placed in the plating bath. A bath of a volume of 1 l with two vertical Pt anodes and a cathode between them was maintained at a constant temperature. The cathode (substrate) and anode were disposed within the bath at a distance of 25 mm. The current density during plating was 0.4 A cm−2 and the temperature was 20 ◦ C. As it was mentioned before [11], initially the chemical composition of the plating bath (Table 1) and working conditions were designed taking into account the degree of blackness, homogeneity and adhesion of the coating. The use of room temperature (18–25 ◦ C) in black chromium plating is a matter of common knowledge. It is needed to prevent the loss in both quality and blackness of the deposit. It was found experimentally that the percentage of zinc in black chromium little by little increases with current density and the ratio Cr/Zn slightly decreases, whereas this ratio increases with temperature (about 0.08 per 1 ◦ C). Taking into account the results of corrosion tests and solar selective characteristics of the black chromium coatings [11], the optimum plating conditions (ic = 0.4 A cm−2 and t = 20 ◦ C) were selected as a compromise between the efficiency and deposit quality. Elemental analysis of coatings and the valence state of elements were studied using XPS. The spectra were recorded with a Vacuum Generator (VG) ESCALAB MK II spectrometer. The nonmonochromatic Al K˛ X-ray radiation (hv = 1486.6 eV) was used for excitation. The Al twin anode was powered at 14 kV and 20 mA. The photoelectron take-off angle was 45◦ with respect to the sample surface normal and spectra of Cr2p, Zn2p, O1s, C1s and N1s were taken at the constant analyzer energy mode (20 eV pass energy). The base pressure was kept below 5 × 10−8 Torr in the working chamber. The spectrometer was calibrated in reference to Ag3d5/2 at 368.0 ± 0.1 eV and Au4f7/2 at 83.8 ± 0.1 eV. XPS depth profiling was performed in the preparation chamber where the argon gas pressure was maintained at 6 × 10−5 Torr with 50 l/s pumping speed at the gauge. Every time after sputtering the specimen was carried immediately to the analyzer chamber to avoid the rest gas adsorption. The quantitative elemental analysis was performed by determining peak areas and taking into account empirical sensitivity factors for each element [12,13]. A standard program was used
Fig. 1. XPS depth profiles of Cr2p3 spectra recorded before and after sputtering top layers of black chromium deposited on the steel substrate from the Cr(III) bath (a) and Cr(III) + ZnO bath (b).
for data processing (XPS spectra were treated by Shirley-type background subtraction and fitted with mixed Gaussian–Lorentzian functions). 3. Results and discussion XPS analysis was performed to determine the chemical composition and valence states of elements in the top layers of the black chromium coatings. Analogous studies were performed earlier [14,15] for light chromium deposited from Cr(III) electrolytes containing Co(II) or Ni(II) as a second main component, which demonstrated the presence of thick oxide films (about 20 nm) on the surface and both carbide and metallic chromium in the bulk of the deposits. Thus, the XPS studies have shown that in the bulk of light chromium deposit metallic chromium accounts for about 50 at%. It should be mentioned that light chromium exhibited certain features that distinguished it from black chromium obtained in the proposed Cr(III) electrolytes. It has been shown before [11] that the use of ZnO in black chromium plating allows us to improve both corrosion and optical properties of the black chromium coating. To elucidate the role of ZnO in this process, in particular, its impact on some characteristics of black chromium, it was essential to obtain more detailed information on the composition of top layers of the black chromium coatings. The XPS spectra recorded from surface to depth of the coatings (6–120 nm) indicated the presence of Cr2p3, O1s, C1s, N1s peaks and in addition Zn2p3 peaks for the coatings deposited from the bath containing a ZnO additive. The chemical states of elements were identified by comparison of the photoelectron binding energies (BE) obtained with those in literatures [12,13]. A full width at half maximum (FWHM) of Cr2p3 peak suggests the presence of more than one component in the spectra presented in Fig. 1a and b. The BE values of the dominant peaks are marked by the vertical dash lines in the pictures. It is perfectly obvious from Fig. 1a, that black chromium deposited from the Cr(III) bath without ZnO additive contains some amount of
S. Surviliene˙ et al. / Applied Surface Science 324 (2015) 837–841
839
Fig. 2. Deconvoluted Cr2p3 spectra for black chromium without zinc recorded after sputtering of top layers: 6 nm (a), 15 nm (b), 45 nm (c), and 120 nm (d).
metallic chromium even in the outer layer of the coating. In reality, these spectra are deconvoluted into three components (Fig. 2). The shift in peak positions suggests a change in the chemical state of chromium [16,17]. The data in Table 2 show positions of the Cr2p3, Zn2p3, O1s and C1s peaks in the deconvoluted spectra for black chromium without zinc (sample “a”) and with zinc (sample “b”), which allow elucidating the chemical state of atoms. It is seen that the peak at BE = 574.1–574.5 eV, which is characteristic of metallic Cr0 [18], exists in all depth levels of sample “a”, whereas in sample “b” it emerges only after sputtering of top layers (about 45 nm). The other peak (BE = 575.0–576.6 eV), which suggests the presence of Cr2 O3 , CrOx , emerges after sputtering top thin layer of about 6 nm in both “a” and “b” samples. The presence of hydrated oxides
and hydroxides (BE = 577.5 ± 0.4 eV) in both samples may be related to the alkalinity of the near cathode layer owing to the increasing hydrogen evolution during electrolysis. The peaks at BE ∼580 eV are attributable to the metal-organic compounds which dominate on the surface, though a few of them were detected even at a depth of 120 nm. It is known [19] that inorganic species in Cr(III) complexes may be replaced by the organic ligands (L) forming various organo-chromium complexes. In the electrolytes used the substitution of each water molecule by the ligand results in the formation of [Cr(Gly)(H2 O)4 ]2+ , [Cr(Gly)2 (H2 O)2 ]+ and [Cr(Gly)(H2 O)3 (OH)]+ complexes, where (Gly) is NH2 CH2 COO− [20]. The increase in pH of the near cathode layer during electrolysis causes precipitation of the inert [Cr(Gly)(H2 O)3 (OH)]+ complex [21], which may be
Table 2 Binding energy of 2p electrons (for chromium and zinc) and 1s electrons (for oxygen and carbon) in the deconvoluted spectra recorded before and after sputtering of top layers for black chromium with no zinc (sample “a”) and with zinc (sample “b”). Depth (nm)
Sample “a”
Sample “b”
Cr2p3 (eV)
O1s (eV)
C1s (eV)
Cr2p3 (eV)
O1s (eV)
Zn2p3 (eV)
C1s (eV)
0
574.2 ± 0.1 577.3 ± 0.1 579.2 ± 0.1
531.0 ± 0.1 532.8 ± 0.1 534.4 ± 0.1
285.4 ± 0.1 286.7 ± 0.1 289.4 ± 0.1
577.3 ± 0.1 579.0 ± 0.1
532.8 ± 0.1 534.2 ± 0.1 535.4 ± 0.1
1022.6 ± 0.1 1023.2 ± 0.1
286.0 ± 0.1 287.6 ± 0.1 289.9 ± 0.1
6
574.5 ± 0.1 576.7 ± 0.1 578.7 ± 0.1
530.4 ± 0.1 531.9 ± 0.1 533.9 ± 0.1
284.7 ± 0.1 286.0 ± 0.1 288.8 ± 0.1
575.8 ± 0.1 577.3 ± 0.1 578.2 ± 0.1
530.4 ± 0.1 532.3 ± 0.1 533.9 ± 0.1
1021.6 ± 0.1 1023.0 ± 0.1
283.4 ± 0.1 284.8 ± 0.1 286.3 ± 0.1
15
574.4 ± 0.1 576.3 ± 0.1 578.0 ± 0.1
530.6 ± 0.1 532.1 ± 0.1
284.3 ± 0.1 285.4 ± 0.1 287.6 ± 0.1
575.7 ± 0.1 577.0 ± 0.1 578.5 ± 0.1
529.9 ± 0.1 531.5 ± 0.1 533.3 ± 0.1
1021.7 ± 0.1 1023.5 ± 0.1
283.1 284.6 286.1 288.7
± ± ± ±
0.1 0.1 0.1 0.1
45
574.3 ± 0.1 576.0 ± 0.1 577.7 ± 0.1
530.6 ± 0.1 532.0 ± 0.1 533.7 ± 0.1
282.7 ± 0.1 284.5 ± 0.1 286.2 ± 0.1
574.3 576.0 577.2 578.8
± ± ± ±
0.1 0.1 0.1 0.1
530.3 ± 0.1 531.7 ± 0.1 533.6 ± 0.1
1021.9 ± 0.1 1023.3 ± 0.1
282.9 284.3 285.6 287.5
± ± ± ±
0.1 0.1 0.1 0.1
120
574.1 ± 0.1 575.9 ± 0.1 577.6 ± 0.1
530.6 ± 0.1 531.8 ± 0.1
282.5 ± 0.1 284.5 ± 0.1 286.6 ± 0.1
574.1 576.0 577.5 579.4
± ± ± ±
0.1 0.1 0.1 0.1
530.3 ± 0.1 531.7 ± 0.1 533.2 ± 0.1
1021.9 ± 0.1 1023.3 ± 0.1
283.1 ± 0.1 284.7 ± 0.1 286.7 ± 0.1
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Fig. 4. XPS depth profiles of N1s spectra for black chromium recorded before and after sputtering top layers.
Fig. 3. XPS depth profiles of O1s spectra for black chromium without zinc (a) and with zinc (b) recorded before and after sputtering top layers of black chromium deposited on the steel substrate.
captured by the growing black chromium deposit. The results obtained suggest that the surface of black chromium is almost entirely made up of such organo-chromium complexes. The oxygen spectra for both sample “a” and sample “b” recorded from surface to depth are presented in Fig. 3a and b. It is seen that the O1s peak is broad nearest to the surface and its position shifts toward lower binding energy into the depth. Table 2 shows the values of BE for every peak in the deconvoluted O1s spectra, which suggest the presence of several components. The peak at BE in the range 529.9–530.6 eV, which is assigned to oxygen in the oxide compounds (Cr2 O3 and ZnO), and the peak at BE = 531.6 ± 0.6 eV, which is associated with the OH− group from the coordination environment of Cr(III), were detected in all depths. The position of high energy peak (BE = 533.8 ± 0.4 eV) is associated with oxygen in organic fragments which, once involved in the coordination surrounding of Cr(III) ions, may be captured by the growing deposit, therefore, it is recognized even at a depth of about 120 nm. The O1s peak positions in the spectra recorded before and after argon sputtering show that the organic compounds dominate in the outer layer, whereas hydroxides and oxides of both chromium and zinc are the dominant components in the inner layers of the coating. The results suggest that the near-surface layer is rich in hydroxides, whereas oxides become dominant toward the substrate/coating interface. Deconvolution of the C1s spectra made it possible to reveal several components with carbon bonds. It is generally believed that the peak at BE >285.0 eV points to the presence of the specific functional organic groups, whereas a low energy peak (BE < 285.0 eV) is associated with carbon–metal interaction and points to the presence of coordination bonds between the carbon chains and chromium atoms [22]. The peaks at BE = 284.3–284.9 eV (Table 2) testify that the components with coordination bonds between the carbon chains and chromium atoms are present in the inner layers of both “a” and “b” samples. The components with BE = 286.1–286.9 eV, which may be assigned to the C N and C N bonds, are in both the
outer and inner layers of the black coatings, whereas the peaks at BE = 288–289 eV assigned to the C H and C C bonds in the organic compounds, present only in the outer layers of both coatings. The peak at BE of 287.5–288.0 eV can be assigned to the COH fragment and the highest energy peak (BE = 289.5–289.9 eV) assigned to the COOH group. Thus, the organic substances were found to be incorporated into the black chromium coatings and its amount decreases significantly from surface to depth. As for Zn2p3 spectra it is not easy to characterize the individual components, because BE of metallic Zn0 (BE = 1021.6 eV) coincides very closely with that of Zn(II) in ZnO (BE = 1021.2–1022.0). The peak at BE = 1022.7 is attributed to Zn(OH)2 . Weak zinc signals testify that black chromium under study is poor in zinc. Taking into account that during black chromium electrodeposition some of the cathodic films and other reduction products may be incorporated into the deposit [3], a weak nitrogen signals, which are located at about 400.2 ± 0.6 and 396.6–397.5 eV (Fig. 4), may be associated with the presence of amino-acid and/or its organometallic derivatives. It is not impossible that there are traces of Cr nitrides arising from the sputtering process. The peak positions at BE = 396.6 ± 0.2 eV and BE = 397.5 ± 0.2 eV may be assigned to CrN and Cr2 N, respectively. Taking into account that the reduction of Cr(III) to the metal results in deep reduction of the organic ligand from the inner coordination sphere of the chromium complex [23], the formation of Cr nitrides may occur during reduction of chromium from the chromium(III) complexes as well [14]. However, to reveal the traces of CrN or Cr2 N from the Cr2p3 spectra was impossible because BE of both CrN and Cr2 N is in the range 574.8–576.1 eV, which is in line with that of Cr2 O3 . Fig. 5a and b shows the results of quantitative XPS analysis estimated from the integrated areas under the C1s, O1s, Cr2p3 and Zn2p3 spectra for black chromium without zinc (a) and with zinc (b). It is seen that high carbon content, which is in the topmost layers of both “a” and “b” coatings, reduces dramatically to depth and after sputtering of about 120 nm it amounts to 4 at%. It is evident that the topmost layer of black chromium is almost entirely made up of organic substances and once adsorbed on the surface some of them can be trapped in the coating during electrolysis. Taking into account that black chromium containing some amount of zinc exhibited a better solar absorptance than black chromium without zinc [11] and considering that the solar absorptance of black chromium depends on the ratio Cr/Cr2 O3 [24], the ratios of metal/oxide phases in samples “a” and “b” need to be compared. As in the present study as-deposited black chromium coatings (without annealing) were used, which were found to have Cr2 O3 and other Cr(III) components as well, therefore, the ratios Cr/Cr(III) and Cr/Cr(III) + Zn(II) should be considered. Fig. 5a and b shows that the atomic percentage of Cr(III) increases continuously with depth throughout the film, whereas that of chrome metal increases only to depth about 40 nm and further remains almost unchanged up
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without and with ZnO have been examined. Analysis of XPS spectra has shown that the top layers of black chromium coatings without zinc are composed of various Cr(III) components, organic substances and metallic Cr, whereas metallic Cr is almost absent in black chromium containing some amount of Zn(II) compounds. The ratios Cr/Cr(III) and Cr/Cr(III) + Zn(II) were found to be 10/27 and 2/28 for black chromium without and with zinc, respectively. It has been determined, that owing to the presence of ZnO in the Cr(III) bath, the percentage of metallic chromium is substantially reduced in black chromium which is quite important for good solar selective characteristics of the coating. Acknowledgment The authors would like to thank Dr. Selskiene˙ A., Ph.D. for performing SEM experiments. References
Fig. 5. The percentage distribution of main elements obtained for the top layers of black chromium. The coatings were deposited on the steel substrate from the Cr(III) bath (a) and the Cr(III) + ZnO bath (b).
to a depth of 120 nm, as a result of which the ratios Cr/Cr(III) and Cr/Cr(III) + Zn(II) were found to be 10/27 and 2/28 for samples “a” and “b”, respectively. This analysis gives an insight into what happens with black chromium when ZnO is used as a second main component in the Cr(III) bath. It is apparent, that owing to the presence of ZnO in the Cr(III) bath, the percentage of metallic chromium is substantially reduced in black chromium which is quite important for good solar selective characteristics of the coating. 4. Conclusions The morphology and chemical composition of the top layers of black chromium coatings electrodeposited from the Cr(III) baths
[1] P.H. Holloway, K. Shanker, R.B. Pettit, R.R. Sowell, Thin Solid Films 72 (1980) 121–128. [2] K.D. Lee, W.Ch. Jung, J.H. Kim, Sol. Energy Mater. Sol. Cells 63 (2000) 125–137. [3] C. Anandan, V.K. William Grips, K.S. Rajam, V. Jayaram, P. Bera, Appl. Surf. Sci. 191 (2002) 254–260. [4] M. Aguilar, E. Barrera, M. Palomar-Pardave, L. Huerta, S. Muhl, J. Non-Cryst. Solids 329 (2003) 31–38. [5] M.R. Bayati, M.H. Shariat, K. Janghorban, Renew. Energy 30 (2005) 2163–2178. [6] S.R. Rajagopalan, K.S. Indira, J. Inst. Metals 93 (4) (1964) 129. [7] S.R. Rajagopalan, K.S. Indira, K.S.G. Doss, J. Electroanal. Chem. 10 (1965) 465–472. [8] M. Aguilar-Sánchez, M. Palomar-Pardavé, M. Romero-Romo, M.T. RamírezSilva, E. Barrera, B.R. Scharifker, Electrochemical nucleation and growth of black and white chromium deposits onto stainless steel surfaces, J. Electroanal. Chem. 647 (2010) 128–132. [9] T. Shikata, S. Veda, M. Inagaki, US Patent No. 4262060, 14 April 1981. [10] H. Gutwein, E. Erben, A. Muhlratzer, B. Cornils, B. Tihanyi, W. Dewin, US Patent No. 4473447, 25 September 1984. ˇ suniene, ˙ A. Ceˇ ˙ R. Juˇskenas, ˙ ˙ D. Buˇcinskiene, ˙ A. Selskiene, [11] S. Surviliene, ¯ e, ˙ Appl. Surf. Sci. 305 (2014) P. Kalinauskas, K. Juˇskeviˇcius, I. Jureviˇciut 492–497. [12] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Minneapolis, MN, 1978, pp. 190. [13] Alexander V. Naumkin, Anna Kraut-Vass, Stephen W. Gaarenstroom, Cedric J. Powell, NIST Standard Reference Database 20. Version 4.1 (Web Version), 2003. ˇ suniene, ˙ V. Jasulaitiene, ˙ A. Ceˇ ˙ A. Lisowska-Oleksiak, J. Solid State [14] S. Surviliene, Ionics 179 (2008) 222–227. ˇ suniene, ˙ A. Ceˇ ˙ V. Jasulaitiene, ˙ I. Jureviˇciut ¯ e, ˙ Appl. Surf. Sci. 258 [15] S. Surviliene, (2012) 9902–9906. [16] V. Maurice, W.P. Yang, P. Marcus, J. Electrochem. Soc. 141 (1994) 3016–3027. [17] T.P. Moffat, R.M. Latanision, J. Electrochem. Soc. 139 (1992) 1869–1879. [18] E. Desimoni, C. Malitesta, P.G. Zambonin, J.C. Riviere, Surf. Interface Anal. 13 (1988) 173–179. [19] Y.B. Song, D.-T. Chin, Electrochim. Acta 48 (4) (2002) 349–356. ˇ suniene, ˙ A. Ceˇ ˙ A. Selskis, R. Butkiene, ˙ Trans. IMF 91 (2013) 24–31. [20] S. Surviliene, [21] J. McDougall, M. El-Sharif, S. Ma, J. Appl. Electrochem. 28 (9) (1998) 929–934. [22] V.A. Safonov, L.N. Vykhodtseva, Y.M. Polukarov, O.V. Safonova, G. Smolentsev, M. Sikora, S.G. Eeckhout, P. Glatzel, J. Phys. Chem. B 110 (2006) 23192–23196. [23] L.N. Vykhodtseva, E.N. Lubnin, Yu.M. Polukarov, V.A. Safonov, Russ. J. Electrochem. 41 (2005) 919–924. [24] J.N. Sweet, R.B. Pettit, M.B. Chamberlain, Sol. Energy Mater. 10 (1984) 251–286.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Investigation of the surface composition of electrodeposited black chromium by X-ray photoelectron spectroscopy ˇ sunien ¯ ˙ V. Jasulaitiene, ˙ I. Jureviˇciut ¯ e˙ S. Surviliene˙ ∗ , A. Ceˇ e, Center for Physical Sciences and Technology, A. Gostauto 9, 01108 Vilnius, Lithuania
a r t i c l e
i n f o
Article history: Received 10 June 2014 Received in revised form 30 October 2014 Accepted 10 November 2014 Available online 18 November 2014 Keywords: Black chromium Zinc oxide XPS spectra Chemical composition
a b s t r a c t The paper reviews black chromium electrodeposited from a trivalent chromium bath containing ZnO as a second main component. The chemical compositions of the top layers of the black chromium coatings were studied by the X-ray photoelectron spectroscopy method. The surface of black chromium was found to be almost entirely covered with organic substances. To gain information on the state of each element in the deposit bulk, the layer-by-layer etching of the black chromium surface with argon gas was used. Analysis of XPS spectra has shown that the top layers of black chromium without zinc are composed of various Cr(III) components, organic substances and metallic Cr, whereas metallic Cr is almost absent in black chromium containing some amount of Zn(II) compounds. The ratios of metal/oxide phases were found to be 10/27 and 2/28 for black chromium without and with zinc, respectively. It has been determined that owing to the presence of ZnO in the Cr(III) bath, the percentage of metallic chromium is substantially reduced in black chromium which is quite important for good solar selective characteristics of the coating. The results confirm some of earlier observations and provide new information on the composition of the near-surface layers. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Black coatings are widely used in solar collectors due to their high absorptance. Black deposits containing chromium are used under the name “black chromium” in solar collector manufacture. Traditionally, the black chromium coatings were deposited on metallic substrates (Ni, Cu, steel) from a hexavalent chromium bath. The data on electrodeposition and some properties of the black chromium coatings have been reported in several publications [1–5]. According to literature data the black chromium film is composed of metallic and oxide phases of chromium. In this case the top layer is composed mainly of oxides, whereas the substrate/coating interface is enriched with metallic chromium. The earlier sources [6,7] suggested that a metal was incorporated into the oxide film as fine separate grains forming light absorption centers owing to which the deposits may appear black. Further research of black chromium electrodeposition from the Cr(VI) electrolyte revealed that nitrates play a major role in the formation of black chromium acting on both the electrochemical
∗ Corresponding author. Tel.: +370 67617825. ˙ E-mail address: [email protected] (S. Surviliene). http://dx.doi.org/10.1016/j.apsusc.2014.11.052 0169-4332/© 2014 Elsevier B.V. All rights reserved.
nucleation and growth mechanisms and precisely they are responsible for the final chemical composition and micro-structural characteristics [8]. It was found that the black-Cr film is made of Cr2 O3 , unlike the white chromium film, which was mainly composed of Cr(OH)3 with a small amount of metallic Cr. The experimental evidence points to the fact that the presence of nitrates are also necessary to obtain black chromium from the trivalent chromium bath, thus attesting to the above-mentioned view. Electrodeposition of black chromium from trivalent chromium bath is important for the practical use. It is known [9] that black chromium may be deposited from Cr(III) baths containing Co(II) or Fe(II) as a second main component. Co-deposition of nickel and cobalt made it possible to increase the thermal resistance of black chromium coating [10]. It was shown in our previous work [11] that high-quality black chromium coatings with regard to their optical characteristics may be obtained from a Cr(III) electrolyte containing ZnO as a second main component. These investigations are being continued with the purpose to clarify the role played by ZnO during black chromium plating. To answer the question on the role of ZnO in this process, it is essential to obtain more detailed information on the composition of the black chromium coatings formed. The aim of this work was to investigate the chemical compositions of the top layers of black coatings formed in Cr(III) baths without and
838
S. Surviliene˙ et al. / Applied Surface Science 324 (2015) 837–841
Table 1 The base composition of the black Cr(III) bath. Component
g l−1
CrCl3 ·6H2 O NH2 CH2 COOH H3 BO3 NaCl NaNO3 ZnO pH 1.2 Temperature 20 ◦ C ic = 0.4 A cm−2
250 18.75 30.0 60.0 3.0 5.0
with ZnO and to compare the distribution of elements from surface to depth in these coatings. 2. Experimental Black chromium coatings were electrodeposited on steel containing 99.4% of iron (Steel-3). Before electrodeposition the substrate was mechanically polished with further electropolishing. The steel substrate was electropolished in the commercial solution containing 27.5 g l−1 sodium carbonate, 17.5 g l−1 sodium tripolyphosphate, 2.5 g l−1 sodium silicate, 2.5 g l−1 synthanol DS10 (CAS No 12679-83-3) and 50 g l−1 sodium hydrate at a current density of 0.05 A cm−2 for 5 min at 50 ◦ C, using the substrate as an anode and a Ti plate as a cathode. After that samples were washed with distilled water, soaked in a hot 20 vol.% HCl solution, washed again with distilled water and then immediately placed in the plating bath. A bath of a volume of 1 l with two vertical Pt anodes and a cathode between them was maintained at a constant temperature. The cathode (substrate) and anode were disposed within the bath at a distance of 25 mm. The current density during plating was 0.4 A cm−2 and the temperature was 20 ◦ C. As it was mentioned before [11], initially the chemical composition of the plating bath (Table 1) and working conditions were designed taking into account the degree of blackness, homogeneity and adhesion of the coating. The use of room temperature (18–25 ◦ C) in black chromium plating is a matter of common knowledge. It is needed to prevent the loss in both quality and blackness of the deposit. It was found experimentally that the percentage of zinc in black chromium little by little increases with current density and the ratio Cr/Zn slightly decreases, whereas this ratio increases with temperature (about 0.08 per 1 ◦ C). Taking into account the results of corrosion tests and solar selective characteristics of the black chromium coatings [11], the optimum plating conditions (ic = 0.4 A cm−2 and t = 20 ◦ C) were selected as a compromise between the efficiency and deposit quality. Elemental analysis of coatings and the valence state of elements were studied using XPS. The spectra were recorded with a Vacuum Generator (VG) ESCALAB MK II spectrometer. The nonmonochromatic Al K˛ X-ray radiation (hv = 1486.6 eV) was used for excitation. The Al twin anode was powered at 14 kV and 20 mA. The photoelectron take-off angle was 45◦ with respect to the sample surface normal and spectra of Cr2p, Zn2p, O1s, C1s and N1s were taken at the constant analyzer energy mode (20 eV pass energy). The base pressure was kept below 5 × 10−8 Torr in the working chamber. The spectrometer was calibrated in reference to Ag3d5/2 at 368.0 ± 0.1 eV and Au4f7/2 at 83.8 ± 0.1 eV. XPS depth profiling was performed in the preparation chamber where the argon gas pressure was maintained at 6 × 10−5 Torr with 50 l/s pumping speed at the gauge. Every time after sputtering the specimen was carried immediately to the analyzer chamber to avoid the rest gas adsorption. The quantitative elemental analysis was performed by determining peak areas and taking into account empirical sensitivity factors for each element [12,13]. A standard program was used
Fig. 1. XPS depth profiles of Cr2p3 spectra recorded before and after sputtering top layers of black chromium deposited on the steel substrate from the Cr(III) bath (a) and Cr(III) + ZnO bath (b).
for data processing (XPS spectra were treated by Shirley-type background subtraction and fitted with mixed Gaussian–Lorentzian functions). 3. Results and discussion XPS analysis was performed to determine the chemical composition and valence states of elements in the top layers of the black chromium coatings. Analogous studies were performed earlier [14,15] for light chromium deposited from Cr(III) electrolytes containing Co(II) or Ni(II) as a second main component, which demonstrated the presence of thick oxide films (about 20 nm) on the surface and both carbide and metallic chromium in the bulk of the deposits. Thus, the XPS studies have shown that in the bulk of light chromium deposit metallic chromium accounts for about 50 at%. It should be mentioned that light chromium exhibited certain features that distinguished it from black chromium obtained in the proposed Cr(III) electrolytes. It has been shown before [11] that the use of ZnO in black chromium plating allows us to improve both corrosion and optical properties of the black chromium coating. To elucidate the role of ZnO in this process, in particular, its impact on some characteristics of black chromium, it was essential to obtain more detailed information on the composition of top layers of the black chromium coatings. The XPS spectra recorded from surface to depth of the coatings (6–120 nm) indicated the presence of Cr2p3, O1s, C1s, N1s peaks and in addition Zn2p3 peaks for the coatings deposited from the bath containing a ZnO additive. The chemical states of elements were identified by comparison of the photoelectron binding energies (BE) obtained with those in literatures [12,13]. A full width at half maximum (FWHM) of Cr2p3 peak suggests the presence of more than one component in the spectra presented in Fig. 1a and b. The BE values of the dominant peaks are marked by the vertical dash lines in the pictures. It is perfectly obvious from Fig. 1a, that black chromium deposited from the Cr(III) bath without ZnO additive contains some amount of
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Fig. 2. Deconvoluted Cr2p3 spectra for black chromium without zinc recorded after sputtering of top layers: 6 nm (a), 15 nm (b), 45 nm (c), and 120 nm (d).
metallic chromium even in the outer layer of the coating. In reality, these spectra are deconvoluted into three components (Fig. 2). The shift in peak positions suggests a change in the chemical state of chromium [16,17]. The data in Table 2 show positions of the Cr2p3, Zn2p3, O1s and C1s peaks in the deconvoluted spectra for black chromium without zinc (sample “a”) and with zinc (sample “b”), which allow elucidating the chemical state of atoms. It is seen that the peak at BE = 574.1–574.5 eV, which is characteristic of metallic Cr0 [18], exists in all depth levels of sample “a”, whereas in sample “b” it emerges only after sputtering of top layers (about 45 nm). The other peak (BE = 575.0–576.6 eV), which suggests the presence of Cr2 O3 , CrOx , emerges after sputtering top thin layer of about 6 nm in both “a” and “b” samples. The presence of hydrated oxides
and hydroxides (BE = 577.5 ± 0.4 eV) in both samples may be related to the alkalinity of the near cathode layer owing to the increasing hydrogen evolution during electrolysis. The peaks at BE ∼580 eV are attributable to the metal-organic compounds which dominate on the surface, though a few of them were detected even at a depth of 120 nm. It is known [19] that inorganic species in Cr(III) complexes may be replaced by the organic ligands (L) forming various organo-chromium complexes. In the electrolytes used the substitution of each water molecule by the ligand results in the formation of [Cr(Gly)(H2 O)4 ]2+ , [Cr(Gly)2 (H2 O)2 ]+ and [Cr(Gly)(H2 O)3 (OH)]+ complexes, where (Gly) is NH2 CH2 COO− [20]. The increase in pH of the near cathode layer during electrolysis causes precipitation of the inert [Cr(Gly)(H2 O)3 (OH)]+ complex [21], which may be
Table 2 Binding energy of 2p electrons (for chromium and zinc) and 1s electrons (for oxygen and carbon) in the deconvoluted spectra recorded before and after sputtering of top layers for black chromium with no zinc (sample “a”) and with zinc (sample “b”). Depth (nm)
Sample “a”
Sample “b”
Cr2p3 (eV)
O1s (eV)
C1s (eV)
Cr2p3 (eV)
O1s (eV)
Zn2p3 (eV)
C1s (eV)
0
574.2 ± 0.1 577.3 ± 0.1 579.2 ± 0.1
531.0 ± 0.1 532.8 ± 0.1 534.4 ± 0.1
285.4 ± 0.1 286.7 ± 0.1 289.4 ± 0.1
577.3 ± 0.1 579.0 ± 0.1
532.8 ± 0.1 534.2 ± 0.1 535.4 ± 0.1
1022.6 ± 0.1 1023.2 ± 0.1
286.0 ± 0.1 287.6 ± 0.1 289.9 ± 0.1
6
574.5 ± 0.1 576.7 ± 0.1 578.7 ± 0.1
530.4 ± 0.1 531.9 ± 0.1 533.9 ± 0.1
284.7 ± 0.1 286.0 ± 0.1 288.8 ± 0.1
575.8 ± 0.1 577.3 ± 0.1 578.2 ± 0.1
530.4 ± 0.1 532.3 ± 0.1 533.9 ± 0.1
1021.6 ± 0.1 1023.0 ± 0.1
283.4 ± 0.1 284.8 ± 0.1 286.3 ± 0.1
15
574.4 ± 0.1 576.3 ± 0.1 578.0 ± 0.1
530.6 ± 0.1 532.1 ± 0.1
284.3 ± 0.1 285.4 ± 0.1 287.6 ± 0.1
575.7 ± 0.1 577.0 ± 0.1 578.5 ± 0.1
529.9 ± 0.1 531.5 ± 0.1 533.3 ± 0.1
1021.7 ± 0.1 1023.5 ± 0.1
283.1 284.6 286.1 288.7
± ± ± ±
0.1 0.1 0.1 0.1
45
574.3 ± 0.1 576.0 ± 0.1 577.7 ± 0.1
530.6 ± 0.1 532.0 ± 0.1 533.7 ± 0.1
282.7 ± 0.1 284.5 ± 0.1 286.2 ± 0.1
574.3 576.0 577.2 578.8
± ± ± ±
0.1 0.1 0.1 0.1
530.3 ± 0.1 531.7 ± 0.1 533.6 ± 0.1
1021.9 ± 0.1 1023.3 ± 0.1
282.9 284.3 285.6 287.5
± ± ± ±
0.1 0.1 0.1 0.1
120
574.1 ± 0.1 575.9 ± 0.1 577.6 ± 0.1
530.6 ± 0.1 531.8 ± 0.1
282.5 ± 0.1 284.5 ± 0.1 286.6 ± 0.1
574.1 576.0 577.5 579.4
± ± ± ±
0.1 0.1 0.1 0.1
530.3 ± 0.1 531.7 ± 0.1 533.2 ± 0.1
1021.9 ± 0.1 1023.3 ± 0.1
283.1 ± 0.1 284.7 ± 0.1 286.7 ± 0.1
840
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Fig. 4. XPS depth profiles of N1s spectra for black chromium recorded before and after sputtering top layers.
Fig. 3. XPS depth profiles of O1s spectra for black chromium without zinc (a) and with zinc (b) recorded before and after sputtering top layers of black chromium deposited on the steel substrate.
captured by the growing black chromium deposit. The results obtained suggest that the surface of black chromium is almost entirely made up of such organo-chromium complexes. The oxygen spectra for both sample “a” and sample “b” recorded from surface to depth are presented in Fig. 3a and b. It is seen that the O1s peak is broad nearest to the surface and its position shifts toward lower binding energy into the depth. Table 2 shows the values of BE for every peak in the deconvoluted O1s spectra, which suggest the presence of several components. The peak at BE in the range 529.9–530.6 eV, which is assigned to oxygen in the oxide compounds (Cr2 O3 and ZnO), and the peak at BE = 531.6 ± 0.6 eV, which is associated with the OH− group from the coordination environment of Cr(III), were detected in all depths. The position of high energy peak (BE = 533.8 ± 0.4 eV) is associated with oxygen in organic fragments which, once involved in the coordination surrounding of Cr(III) ions, may be captured by the growing deposit, therefore, it is recognized even at a depth of about 120 nm. The O1s peak positions in the spectra recorded before and after argon sputtering show that the organic compounds dominate in the outer layer, whereas hydroxides and oxides of both chromium and zinc are the dominant components in the inner layers of the coating. The results suggest that the near-surface layer is rich in hydroxides, whereas oxides become dominant toward the substrate/coating interface. Deconvolution of the C1s spectra made it possible to reveal several components with carbon bonds. It is generally believed that the peak at BE >285.0 eV points to the presence of the specific functional organic groups, whereas a low energy peak (BE < 285.0 eV) is associated with carbon–metal interaction and points to the presence of coordination bonds between the carbon chains and chromium atoms [22]. The peaks at BE = 284.3–284.9 eV (Table 2) testify that the components with coordination bonds between the carbon chains and chromium atoms are present in the inner layers of both “a” and “b” samples. The components with BE = 286.1–286.9 eV, which may be assigned to the C N and C N bonds, are in both the
outer and inner layers of the black coatings, whereas the peaks at BE = 288–289 eV assigned to the C H and C C bonds in the organic compounds, present only in the outer layers of both coatings. The peak at BE of 287.5–288.0 eV can be assigned to the COH fragment and the highest energy peak (BE = 289.5–289.9 eV) assigned to the COOH group. Thus, the organic substances were found to be incorporated into the black chromium coatings and its amount decreases significantly from surface to depth. As for Zn2p3 spectra it is not easy to characterize the individual components, because BE of metallic Zn0 (BE = 1021.6 eV) coincides very closely with that of Zn(II) in ZnO (BE = 1021.2–1022.0). The peak at BE = 1022.7 is attributed to Zn(OH)2 . Weak zinc signals testify that black chromium under study is poor in zinc. Taking into account that during black chromium electrodeposition some of the cathodic films and other reduction products may be incorporated into the deposit [3], a weak nitrogen signals, which are located at about 400.2 ± 0.6 and 396.6–397.5 eV (Fig. 4), may be associated with the presence of amino-acid and/or its organometallic derivatives. It is not impossible that there are traces of Cr nitrides arising from the sputtering process. The peak positions at BE = 396.6 ± 0.2 eV and BE = 397.5 ± 0.2 eV may be assigned to CrN and Cr2 N, respectively. Taking into account that the reduction of Cr(III) to the metal results in deep reduction of the organic ligand from the inner coordination sphere of the chromium complex [23], the formation of Cr nitrides may occur during reduction of chromium from the chromium(III) complexes as well [14]. However, to reveal the traces of CrN or Cr2 N from the Cr2p3 spectra was impossible because BE of both CrN and Cr2 N is in the range 574.8–576.1 eV, which is in line with that of Cr2 O3 . Fig. 5a and b shows the results of quantitative XPS analysis estimated from the integrated areas under the C1s, O1s, Cr2p3 and Zn2p3 spectra for black chromium without zinc (a) and with zinc (b). It is seen that high carbon content, which is in the topmost layers of both “a” and “b” coatings, reduces dramatically to depth and after sputtering of about 120 nm it amounts to 4 at%. It is evident that the topmost layer of black chromium is almost entirely made up of organic substances and once adsorbed on the surface some of them can be trapped in the coating during electrolysis. Taking into account that black chromium containing some amount of zinc exhibited a better solar absorptance than black chromium without zinc [11] and considering that the solar absorptance of black chromium depends on the ratio Cr/Cr2 O3 [24], the ratios of metal/oxide phases in samples “a” and “b” need to be compared. As in the present study as-deposited black chromium coatings (without annealing) were used, which were found to have Cr2 O3 and other Cr(III) components as well, therefore, the ratios Cr/Cr(III) and Cr/Cr(III) + Zn(II) should be considered. Fig. 5a and b shows that the atomic percentage of Cr(III) increases continuously with depth throughout the film, whereas that of chrome metal increases only to depth about 40 nm and further remains almost unchanged up
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without and with ZnO have been examined. Analysis of XPS spectra has shown that the top layers of black chromium coatings without zinc are composed of various Cr(III) components, organic substances and metallic Cr, whereas metallic Cr is almost absent in black chromium containing some amount of Zn(II) compounds. The ratios Cr/Cr(III) and Cr/Cr(III) + Zn(II) were found to be 10/27 and 2/28 for black chromium without and with zinc, respectively. It has been determined, that owing to the presence of ZnO in the Cr(III) bath, the percentage of metallic chromium is substantially reduced in black chromium which is quite important for good solar selective characteristics of the coating. Acknowledgment The authors would like to thank Dr. Selskiene˙ A., Ph.D. for performing SEM experiments. References
Fig. 5. The percentage distribution of main elements obtained for the top layers of black chromium. The coatings were deposited on the steel substrate from the Cr(III) bath (a) and the Cr(III) + ZnO bath (b).
to a depth of 120 nm, as a result of which the ratios Cr/Cr(III) and Cr/Cr(III) + Zn(II) were found to be 10/27 and 2/28 for samples “a” and “b”, respectively. This analysis gives an insight into what happens with black chromium when ZnO is used as a second main component in the Cr(III) bath. It is apparent, that owing to the presence of ZnO in the Cr(III) bath, the percentage of metallic chromium is substantially reduced in black chromium which is quite important for good solar selective characteristics of the coating. 4. Conclusions The morphology and chemical composition of the top layers of black chromium coatings electrodeposited from the Cr(III) baths
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