Electroinduced structural changes in manganese dioxide + manganese hydroxide films as characterized by real-time surface-enhanced Raman spectroscopy

Abstract

Structural changes occurring during reduction of thin (ca. 10 nm) MnO 2 films and oxidation of similar Mn(OH) 2 films electrodeposited onto electrochemically roughened gold have been examined using surface-enhanced Raman spectroscopy (SERS). Sequences of SER spectra were obtained during potential excursions under galvanostatic and linear-sweep voltammetric conditions by using multichannel Raman detection. The electrochemical redox behavior of the MnO 2 films is comparable to that for thicker layers as well as for MnO 2 battery electrodes. The MnO 2 film exhibits a broad asymmetric Raman band at 585 cm −1, due to Mn-O lattice vibrations. Upon initiating film reduction, this band narrows and a sharp feature at 670 cm −1 appears, also due to a metal-oxide vibration; neither band is shifted upon substituting D 2O for H 2O electrolyte. Further reduction yields a Mn-OH vibrational band (identified from the deuterium isotope shift) first at 415 cm −1, eventually being replaced by a broader Mn-OH feature at 470 cm −1. These latter features are consistent with the formation of MnOOH and Mn(OH) 2, respectively. The 670 cm −1 band is attributed tentatively to an intermediate Mn(IV)-O-Mn(III) species en route to MnOOH. Reoxidation of this film yields comparable electro-chemical and SERS responses to oxidation of Mn(OH) 2films. The 415 cm −1 Mn-OH feature reappears during Mn(OH) 2 oxidation and MnO 2 is eventually formed as evidenced by the appearance of the 585 cm −1 Mn-O band. However, the 670 cm −1 feature is not observed during film oxidation, an Mn-OH band at 500 cm −1 appearing in concert with the 415 cm −1 band instead. This is attributed to the formation of two distinct MnOOH phases during Mn(OH) 2oxidation. The virtues of SERS for elucidating redox-induced structural changes within oxide and other thin surface films are pointed out.