Electrochemical Deposition of Manganese Oxides on Carbon Nanosheets

We have electrodeposited Mn-oxide films on Carbon nano-sheets (CNS), thin (< 10 nm) vertical standing graphitic sheets, having in mind potential energy storage applications of this high-surface-area composite structure.   We examined the deposition kinetics of Mn-oxides and physical-chemical properties of Mn-oxide/CNS composite films on substrates ranging from flat to high-aspect-ratio (HAR) Si pillars.  We have also explored the possibility to integrate this process into production line typical for microelectronics industry.

CNS was grown using RF-PECVD technique [1] onto both blanket 200 mm Si wafers coated with 70 nm thick PVD TiN, and wafer pieces with high-aspect-ratio Si pillars coated with 10 nm ALD TiN.  CNS was grown on TiN from CH₄/H₂mixture [1] to a height of approximately 1 µm.

Anodic electrodeposition of Mn-oxide, often also termed electrolytic manganese-dioxide (EMD), was performed on both blanket CNS/TiN/Si and patterned CNS/ALD TiN/Si samples using 0.3 M MnSO₄·xH₂O + 0.55 M H₂SO₄aqueous electrolyte.  Plating experiments were performed using a three-electrode clip-on Teflon cell connected to a computer controlled Autolab potentiostat PGSTAT100 (Metrohm), with Ag/AgCl as a reference electrode (RE) and Pt mesh as a counter electrode (CE).  Characterization of EMD/CNS films was performed using Scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy (RS), and X-ray photoelectron spectroscopy (XPS).

Figure 1 shows SEM images of as-grown CNS (a) and EMD coverage as a function of deposition current  (b and c).  EMD is plated galvanostatically, and potential-time responses were analyzed to help optimize deposition parameters for desired film thickness and structural properties.  The analysis was based on the theories of galvanostatic nucleation and growth [2], and proposed reaction mechanisms during galvanostatic deposition of EMD [3].  Further surface area enhancement was attempted through EMD/CNS deposition on HAR Si pillars coated with 10 nm ALD TiN.  Figure 2 shows HAR ALD TiN/Si  pillars before and after CNS and EMD deposition, respectively.  CNS coating was applied to various ALD TiN/Si structures having different aspect ratios and spacing between pillars, and then followed by EMD deposition.

A typical Raman spectrum of our EMD/CNS structure, shown in Figure 3, contained broad features in the range 450 to 730 cm^-1 where characteristic EMD peaks are expected [4 and references therein], and showed characteristic CNS peaks in 1000 to 3500 cm^-1range [5]. 

Thanks to its sensitivity to local structure/symmetry Raman spectroscopy proved to be a useful tool for characterization of disordered and defective EMD.  We were able to detect EMD structural changes upon annealing at various temperatures and lithiation in anhydrous propylene carbonate solvent with 1 M LiClO₄, and also monitor the effects of plating and post-plating processing on CNS.

References:

[1] D. J. Cott, M. Verheijen, O. Richard, I. Radu, S. De Gendt, S. van Elshocht, and P. M. Vereecken, Carbon, 58(2013), 59.

[2] A. Milchev and M.I. Montenegro, J. Electroanal. Chem., 333 (1992) 93.

[3] A. D. Cross, A. Morel, T. F. Hollenkamp, and S. W. Donnea, J. Electrochem. Soc., 158(2011), A1160.

[4] R. Baddour-Hadjean and J.-P. Pereira-Ramos, Chem. Rev., 110, (2010), 1278.

[5] S. Kurita, A. Yoshimura, H. Kawamoto, T. Uchida, K. Kojima, M. Tachibanaa, P. Molina-Morales and H. Nakai,   Journal of Applied Physics, 97(2005), 104320.

Figure Captions:

Figure 1.  SEM images of (a) as-received CNS, and Mn oxide anodically deposited on CNS at (b) 2.5  mA/cm², and (c) 0.25 mA/cm², from acidic sulfate solution.

Figure 2. (a) High aspect ratio ALD TiN/Si pillars after (b) CNS, and (c) EMD deposition.

Figure 3.  Raman spectrum of Mn oxides plated on CNS at 2.5 mA/cm² from acidic sulfate solution.  EMD stands for electrolytic manganese dioxide.  Characteristic CNS peaks are designated using capital letters.