The image shown in Figure 2 is the catalyzed decomposition of a 30% aqueous solution of H₂O₂ monopropellant flowing in a microchannel coated with RuO₂ NRs. The chemical activity, observed as the production of bubbles along the vertical chamber walls, was visualized using a Vision Research Phantom High-Speed Camera operating at a frame rate of 6300 FPS. In this particular image greater activity is occurring on the right side of the chamber wall. The actual physical system finds the microchannel lying flat with the liquid H₂O₂ solution pumped from bottom to top in the image. The average rate of bubble formation along the low nanorod density side of the channel was 907 bubbles per second uniformly distributed along the channel wall, compared to 580 bubbles per seconds at widely-spaced formation sites on the high nanorod density side.

It was initially assumed that the greater decomposition activity would occur on the wall with the highest density of NRs, due to catalytic activity. Upon a review of SEM images of the channel walls, the opposite was true, there was greater relative activity on the side of the channel that was decorated with the lower density of RuO₂ NRs. Heterogeneous reaction kinetics predicts that the higher density of reaction sites provided by the RuO₂ NRs should produce the greater chemical reactivity. Observation of a higher activity on the surface decorated with a lower density of RuO₂ NRs suggests that something else is controlling the reaction rate, which is inversely dependent on nanorod density.

It is proposed that the reason for the lower chemical activity on the left side of the chamber image is a result from the inability of the H₂O₂ liquid to wet on the catalyst surface. The rate of chemical activity is observed as the production rate of bubbles along the catalyst wall. The production rate of bubbles is dramatically higher on the chamber wall that is decorated with the lower density of nanorods. A notable component of this observation is that the production rate of bubbles on the channel wall with

the higher nanorod density was actually limited to only a few sites on the channel wall. It is proposed that H₂O₂ decomposition is only occurring in regions on the channel wall where the density of NRs is low due to a defect, thus allowing wetting of the catalyst surface. The H₂O₂ liquid is repelled from the RuO₂ NR coated surface, similar to the way that a water droplet is repelled from a rose pedal or blade of grass (i.e., the Lotus Effect). Thus, the catalytic surface here displays a greater level of hydrophobicity; the degree of hydrophobicity may be quantified by measuring the contact angle of the droplet on the surface.

The phenomenon of wetting on surfaces covered with nanostructured materials has been the subject of considerable investigation for many years [4] - [13] . Suppose that the contact angle is defined as the angle formed by the droplet at the intersection of the three different phases; air, liquid and solid. A theoretical prediction for the contact angle obtained for nanomaterials similar to that used here can be found in the pioneering work by Casie and Baxter [16] . The contact angle is defined to be:

where = roughness of the wetted surface.

= area fraction of the liquid-air interface.

= equilibrium contact angle on solid phase.

= equilibrium contact angle on air phase.

To further explore the nature of the unexpected chemical activity of the H₂O₂ solution droplet in contact with the RuO₂ NR catalyst coating, this wetting phenomenon was simulated with a droplet of liquid water, placed onto the RuO₂ NR surface. Our laboratory had in its possession a collection of RuO₂ NR samples which represented a range of different NR surface densities and sizes. The different samples were obtained by growing the NR samples for different lengths of time [14] . A collection of SEM images of different RuO₂ NR samples is shown in Figure 3.

The application of a water droplet to the RuO₂ NR samples shown in Figure 3 is shown in Figure 4. The contact angle was measured as shown in Figure 5. The RuO₂ NR coatings increase in both surface density of nanorods and size, principally girth of the NRs, with length of deposition time. The water droplet fails to wet on the RuO₂ NR coated surface as the NR surface density and NR girth increases (see Figure 6).

In this portion of the investigation, conductive substrates (Si, Ti and Al/Si) coated with thin films of RuO₂ NRs were used as electrodes in the electrolysis of aqueous solutions of H₂SO₄. Our laboratory has previously reported on the investigation of the use of NR coated electrodes for use as cathodes in the electrolysis of both basic and acidic aqueous solutions. A more complete description of these electrolysis experiments can be found in previously published work by Cross et al. [3] [17] . The nanomaterial coated electrode surface is similar to the RuO₂ nanorods coated material whose SEM image was shown in Figure 1(a) [14] . The RuO₂ material is an electrical conductor and can be treated

electrically as part of the electrode [15] . As stated above the increase in resulting interfacial area has been measured in previous experiments to be on the order of 10X greater [3] .

The production rate of H₂ per unit area of a flat cathode surface is directly proportional to the current density supplied to the cathode. An example of a Tafel plot [3] [17] obtained for an electrolysis cell with the NR decorated cathode electrode, with the voltage applied between the cathode electrode and the Saturated Calomel Electrode (SCE)

reference, is shown in Figure 7. The anode in all cases was a piece of Pt wire and the cathode area was the macroscopic dimensions of the substrate piece. In this investigation the focus is on the overvoltage measured at the cathode electrode relative to the electrolyte. The cathode overvoltage (η_c) is defined as the experimentally applied voltage, relative to the electrolyte, measured with the SCE reference electrode, minus the theoretical voltage. The theoretical value is obtained from the Gibbs’ Free Energy for the reaction (2H⁺ + 2e⁻ → H₂(g), = 0.0 V), plus a correction supplied by the Nernst Equation for the non-standard electrolyte concentration (in this case V_Nernst = 0.0592 log[H⁺], [18] and a correction for the voltage drop across the SCE reference electrode (V_CRE = 0.2415 V) [19] .

The voltage required to induce current flow was lowest for the case of the cathode constructed of Pt and otherwise was found to vary with the material used to make the electrode.

The actual interfacial area of the cathode electrode surface should take into consideration the contribution of the nanorod coating. The total surface area for a nanorod coated electrode is calculated from the number density of nanorods per unit surface area times the surface area of an individual nanorod, with dimensions measured from an SEM image. A corrected current density was obtained from the measured current densities shown in Figure 7 and the corrected interfacial surface areas calculated from dimensions measured in Figure 3. Once corrected for surface area, the current density measured for the three nanorod coated samples merged into one, see Figure 8. This result suggests that the reason for the increase in the performance of the denser nanorod coated electrode is explained as the increased interfacial contact area. This observed increase in activity would not have been observed if surface wetting was limiting the electrochemical reaction at the higher nanorod densities.

The shape of an individual nanorod is rectangular with sharp edges and termination in a pyramid shape peak, as was shown in Figure 1(b). When electrically biased in free space, a high electric field region is formed around the exterior of the rectangular nanorod. In a water solution the surrounding medium is as a fluid with a finite relative permittivity that reduces the strength of this field. An interesting case exists in a liquid water bath during electrolysis, where the generation of bubbles presents the opportunity of an enhanced field strength in the vicinity of the nanorod electrode.

The electrolysis process involves the transfer of electrical charge from one chemical species to another [20] [21] . The electron in the vicinity of the reacting molecule can be modeled as a particle in a one dimensional potential well. When the potential well is placed in an electrical field, the energy required to escape the potential well is reduced to E_a − ΔE (see Figure 9). A nanorod coating on a biased electrode surface is similar to this simple model and can be used to reduce the energy required for electrolysis.

The wettability of the RuO₂ NR coated electrode material can be measured directly by observing the contact angle of a water droplet on the surface of the electrode material. The contact angle is inversely proportional to the degree of wetting that the water medium droplet makes with the nanorod coated surface. The contact angle should increase with the density of nanorods and/or the total interfacial area of the nanorod surface as predicted by the Casey-Baxter law for nanomaterial coated surfaces. The rate of the electrolysis reaction should be reduced by the reduction in wetting on the nanorod coated surface. This however is not what was observed, and the production rate of hydrogen increased with the surface density of RuO₂ nanorods, see Figure 10. One explanation for this observed behavior is that the electrical bias applied between the fluid droplet and nanorod surface influences the physical interaction of the two materials toward one another. It is proposed that the electrically biased surface electrostatically attracts the fluid to the nanorod coated electrode surface.

This was also observed independently in Figure 11 and Figure 12, where a droplet of biased and unbiased DI water and aqueous H₂SO₄ solution where placed side-by-side on a RuO₂ NR coated substrate surface for comparison. There is no obvious difference

between the wetting behavior of these two unbiased droplets, see Figure 11. In Figure 12, the same two droplets are biased positively relative to the nanorod surface. In the case of the dilute H₂SO₄ solution droplet (left), the droplet body is attracted to the nanorod coated surface and the contact angle is decreased indicating an increased level of wetting. It is also observed that the electrical bias has also initiated electrolysis at the nanorod coated surface. For the case of the biased DI water droplet (right), no noticeable change can be observed in the contact angle. This is to be expected as the conductivity of DI water is on the order of 10⁶Ω-cm, and no charge can be transferred to the droplet from the charging electrical probe.