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

Structural changes occurring during reduction of thin (ca. 10 nm) MnO2 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 MnO2 films is comparable to that for thicker layers as well as for MnO2 battery electrodes. The MnO2 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 D2O for H2O 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 MnO2 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.