2348 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 8 Sang-Min Lee et al. http://dx.doi.org/10.5012/bkcs.2013.34.8.2348 Electrochemical Multi-Coloration of Molybdenum Oxide Bronzes Sang-Min Lee, Viswanathan S. Saji, and Chi-Woo Lee * Department of Advanced Materials Chemistry, Korea University, Sejong 339-700, Korea. * E-mail: cwlee@korea.ac.kr Received March 18, 2013, Accepted May 14, 2013 We report a simple electrochemical approach in fabricating multiple colored molybdenum (Mo) oxide bronzes on the surface of a Mo-quartz electrode. A three step electrochemical batch process consisting of linear sweep voltammetry and anodic oxidation followed by cathodic reduction in neutral K2SO4 electrolyte at different end potentials, viz. -0.62, -0.80 and -1.60 V (vs. Hg/HgSO4) yielded red, blue and yellow colored bronzes. The samples produced were analyzed by XRD, EDS, and SIMS. The color variation was suggested to be associated with the cations intercalation into the oxide formed and the simultaneous structural changes that occurred during the cathodic reduction in neutral aqueous medium. Key Words : Mo oxide bronze, Bronzes, Electrochemistry, EQCM Introduction Molybdenum (Mo) and its oxides have unique structural and chemical properties and are widely investigated in the areas of catalysis, sensors, capacitors, lithium-ion batteries, solar cells, photochromism and electrochromism. 1-11 The electrochromism in Mo oxides and the formation of Mo oxide bronzes have been successfully rationalized by the theory of the simultaneous ingression of cations and elec- trons. The MoO 6 octahedra, with sharing the edges and corners having zigzag chains and unique layers provide the open channels for the intercalation. 1,2 The injected electrons were trapped by some Mo 6+ , forming Mo 5+ , and the coloration was attributed to the intervalence charge-transfer transition between Mo 6+ and the electroreduced Mo 5+ . 8-11 MoO3 + x (A + + e - ) = Ax Mox V Mo1-x VI O3 (1) where A stands for H + , NH4 + or alkali, alkaline earth, or rare earth metal ion. Previously we reviewed fundamental electrochemistry of Mo and its oxides, 8 reported the electrochemical formation of surface Mo blue 12 in acid solution and the anionic dependence of redox poperties. 13 In this work, we wish to report our findings on the electrochemical multi-coloration of surface Mo oxide bronzes electrochemically derived from Mo in neutral aqueous solutions. To the best of our know- ledge this is the first report on the fabrication of red, blue, and yellow colored bronzes on Mo in aqueous solution by an electrochemical strategy. The protocol includes a batch process consisting of (1) linear sweep voltammetry (LSV-1) from -0.70 to -0.10 V, (2) chronoamperometry (CA) at -0.10 V for 60 s and (3) linear sweep voltammetry (LSV-2) from -0.10 V to different end potentials (ranging from -0.40 to -1.80 V). LSV-1 and CA were performed to make the electrode’s surface active with the formation of additional oxides/hydroxides. LSV-2 was performed to reach the desir- ed coloration. To define the electrochemical conditions of preparing optimum Mo oxide bronzes, electrochemical quartz crystal microbalance (EQCM) technique was hired as the observed changes in current and frequency are so sensitive that the effects of the minute changes in the experimental variables of potential, sweep rate, and electrolysis time on the color of the resulting Mo bronzes can be readily detected and adjusted. Experimental Mo quartz QCM working electrodes consisted of Mo sputtered onto Ti/crystal at 9 MHz AT-cut quartz. The elec- trolyte was 0.5 M K2SO4. Experiments were conducted at ambient laboratory conditions. A quartz crystal microbalance (QCM 922, Princeton Applied Research) combined with a potentiostat (Versastat 3, Princeton Applied Research) was employed for electrochemical studies. The counter and reference electrodes used were Pt sheet and Hg/HgSO4 (Koslow Instruments, USA), respectively. Electrochemical experiments were started after a waiting time of 5 min (open circuit potential ~ -0.69 to -0.66 V). The electrode was subjected to LSV-1 from -0.70 to -0.10 V followed by CA at -0.10 V for 60 s. The cathodic reduc- tion was facilitated by LSV-2, where the electrode was polarized from -0.10 V to different end potentials; ranging from -0.40 to -1.80 V. LSV-1, CA and LSV-2 were per- formed as a batch process. The electrochemical multi- coloration reported in this work was not exactly reproducible when one of the three steps was omitted. Similar multi- coloration was observed when the solution was carefully replaced during the step of CA. The color changes were not completely reversible upon potential reversal. The scan rate used was 20 mV/s. After the experiment, the electrode was taken out and repeatedly rinsed in water and dried in flowing N 2 . AR grade (Sigma Aldrich) chemicals and double di- stilled water was used. Experiments were also conducted in 0.5 M Li 2 SO 4 . Phase structure of the Mo quartz electrodes before and after the electrochemical experiments were analyzed by X-ray diffraction (XRD, Rigagu D/max-2200)