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Charge-transfer band and SERS mechanism for the pyridine-Ag system
A135 Surface Science 182 (1987) 257-268 North-Holland, Amsterdam
257
AN ELLIPSOMETRIC STUDY ON THE EARLY STAGE OF PASSIVE FILM FORMATION ON NICKEL IN ACIDIC SOLUTIONS Yongku KANG
and Woon-kie PAIK
Department of Chemistry, Sogang University, Seoul 121, Korea Received 3 July 1986, accepted for publication 23 October 1986 Elfipsometric transient measurements were made on the nickel surface in acidic solutions. An automatic elhpsometer built for transient three-parameter elhpsometric measurement including reflectance recording was used to determine the optical constants and thickness of the passive film as functions of time. The real and imaginary parts of the refractive index showed some change before reaching steady values after the potential was changed. There was, however, no discontinuous or drastic variation of the optical constants even in the early stage of passive film growth, within a second when the thickness change was rapid. Experimental results pertaining to the question of film transformation are discussed.
Surface Science 182 (1987) 269-286 North-Holland, Amsterdam
269
CHARGE-TRANSFER BAND AND SERS MECHANISM FOR THE PYRIDINE-Ag Haruka YAMADA,
SYSTEM
Hisao NAGATA,
Kazuhiro TOBA and Yoshiko NAKAO
Department of Chemistry, Kwansei Gakuin University, Nishinomiya 662, Japan Received 18 September 1986; accepted for publication 25 October 1986 Transmission spectra in the ultraviolet and visible region as well as SERS spectra were measured for chemisorbed pyridine on vacuum evaporated (island) Ag films, and on annealed (heated) Ag films. Strong and clear SERS spectra were observed with an enhancement factor of 106 and were definitely assigned to the N-bonded pyridine. On adsorption of pyridine a new peak was detected at - 600 nm as the charge-transfer (CT) band for the pyridine-Ag interaction, which could neither be ascribed to ar~y impurity nor to aggregated Ag particles. The surface morphologies of the Ag films observed by a SEM (highest resohition 1.5 nm) did not show any appreciable changes on adsorption of pyridine, when the Ag substrates were properly prepared. The molar extinction coefficient of the CT band was 103 -10 a ~ tool- t cm- 1. On the annealed Ag films the CT band, which was well isolated from the Ag band, appeared on adsorption of pyridine and disappeared on desorption of pyridine, reversibly. The stronger the CT band, the stronger the SERS spectrum. The excitation profile of the Raman 1010 cm -1 band overlapped well with the CT band. The number of adsorbed pyridine molecules was estimated, from the ~r-~r* band of pyridine at 250 nm, to be (6-15)× 10 is molecules/cm2 corresponding to a coverage factor of 20-60. The contributions of the electromagnetic (EM) effect and the CT effect to the apparent SERS enhancement factor were estimated to be approximately 10-102 and 103, respectively. The reason why the CT band had not been detected as an optical absorption band was discussed. The CT band observed is compared with those reported by EELS or reflection measurements.
257
AN ELLIPSOMETRIC STUDY ON THE EARLY STAGE OF PASSIVE FILM FORMATION ON NICKEL IN ACIDIC SOLUTIONS Yongku KANG
and Woon-kie PAIK
Department of Chemistry, Sogang University, Seoul 121, Korea Received 3 July 1986, accepted for publication 23 October 1986 Elfipsometric transient measurements were made on the nickel surface in acidic solutions. An automatic elhpsometer built for transient three-parameter elhpsometric measurement including reflectance recording was used to determine the optical constants and thickness of the passive film as functions of time. The real and imaginary parts of the refractive index showed some change before reaching steady values after the potential was changed. There was, however, no discontinuous or drastic variation of the optical constants even in the early stage of passive film growth, within a second when the thickness change was rapid. Experimental results pertaining to the question of film transformation are discussed.
Surface Science 182 (1987) 269-286 North-Holland, Amsterdam
269
CHARGE-TRANSFER BAND AND SERS MECHANISM FOR THE PYRIDINE-Ag Haruka YAMADA,
SYSTEM
Hisao NAGATA,
Kazuhiro TOBA and Yoshiko NAKAO
Department of Chemistry, Kwansei Gakuin University, Nishinomiya 662, Japan Received 18 September 1986; accepted for publication 25 October 1986 Transmission spectra in the ultraviolet and visible region as well as SERS spectra were measured for chemisorbed pyridine on vacuum evaporated (island) Ag films, and on annealed (heated) Ag films. Strong and clear SERS spectra were observed with an enhancement factor of 106 and were definitely assigned to the N-bonded pyridine. On adsorption of pyridine a new peak was detected at - 600 nm as the charge-transfer (CT) band for the pyridine-Ag interaction, which could neither be ascribed to ar~y impurity nor to aggregated Ag particles. The surface morphologies of the Ag films observed by a SEM (highest resohition 1.5 nm) did not show any appreciable changes on adsorption of pyridine, when the Ag substrates were properly prepared. The molar extinction coefficient of the CT band was 103 -10 a ~ tool- t cm- 1. On the annealed Ag films the CT band, which was well isolated from the Ag band, appeared on adsorption of pyridine and disappeared on desorption of pyridine, reversibly. The stronger the CT band, the stronger the SERS spectrum. The excitation profile of the Raman 1010 cm -1 band overlapped well with the CT band. The number of adsorbed pyridine molecules was estimated, from the ~r-~r* band of pyridine at 250 nm, to be (6-15)× 10 is molecules/cm2 corresponding to a coverage factor of 20-60. The contributions of the electromagnetic (EM) effect and the CT effect to the apparent SERS enhancement factor were estimated to be approximately 10-102 and 103, respectively. The reason why the CT band had not been detected as an optical absorption band was discussed. The CT band observed is compared with those reported by EELS or reflection measurements.