Figure 1(a–c) shows the XRD plots of sputter deposited Y₂O₃, thermally oxidized Y-Y₂O₃ and Cd-CdO template assisted Y₂O₃ coatings. Reactively sputtered coatings (Figure 1(a)) showed reflections corresponding to (211), (222) and (620) of cubic Y₂O₃ (JCPDS card no. 025–1200). Figure 1(b) shows XRD pattern for thermally oxidized Y-Y₂O₃ coating. Two peaks are observed at 2θ = 30.7° and 55.3°, which correspond to (222) and (620) planes of cubic Y₂O₃,respectively (JCPDS card no. 025–1200). The peak centered at 2θ = 30.7° corresponds to (002) plane of hexagonal yttrium (JCPDS card no. 12–702). The presence of diffraction peaks from Y and Y₂O₃ show that only the top surface layer of the coating is oxidized. The XRD plot for Cd-CdO template assisted Y₂O₃ coating is shown in Figure 1(c). The peak observed at 2θ = 55.3° corresponds to the (620) plane of Y₂O₃. Two additional peaks are also observed at 2θ = 32.9° and 38.1°, which are attributed to the (111) and (200) planes of cubic CdO, respectively (JCPDS card no. 5–0640). The peak centered at 2θ = 38.1° corresponds to (101) plane of metallic hexagonal Cd (JCPDS card no. 5–0674). The diffraction peaks for Cd and CdO are from the template. Here we observed diffraction peaks for both Cd and CdO, which indicates that only the uppermost surface layer of the coating is oxidized, as the Cd-CdO template is prepared by sputtering followed by thermal oxidation. The average grain size of Y₂O₃ coatings was calculated from the prominent peaks using Scherrer’s formula. The average grain sizes for sputtered deposited Y₂O₃, thermally oxidized Y-Y₂O₃ and template assisted Y₂O₃ coatings were 15.0 nm, 15.5 nm and 19.5 nm, respectively.

The chemical structure of the coating was studied by micro-Raman spectroscopy (Figure 2). Yttrium sesquioxide crystallizes in the cubic system and is a body centered cubic with space group Ia3 (Z = 16). The structure is related to the structure of fluorite, with each yttrium ion located at the center of the cube from which two of the eight neighboring oxygens of the fluoride have been removed. As the structure is body centered, the unit cell contains the primitive cell twice. The latter cells, containing eight formula units, were used for the theoretical numbering of vibration. The irreducible representations for optical and acoustical modes are [40,41]:

Г_op = 4 A_g + 4 E_g+ 14 F_g+ 5 A_2u + 5 E_u+ 16 F_u

Г_ac = Fu,

where A_g, E_g and F_g are Raman active, F_u is infrared (IR) active and A_2u and E_u are inactive. Twenty two Raman lines of A_g, E_g and F_g modes and sixteen F_u IR bonds are then predicted. Figure 2(a–c) shows the Raman spectra of Samples 1–3. The frequencies of various Raman bands were determined using deconvolution of the Raman data by Gaussian fit as shown in Figure 2. The assignment of the Raman spectrum of cubic Y₂O₃ for Samples 1–3 is given in Table 1. The Raman spectrum of Sample 1 (Figure 2(a)) showed six peaks at 151.9, 187.3, 328.9, 399.7, 470.6 and 559.1 cm⁻¹ [40,41]. The peaks observed at 187.3, 328.9 and 559.1 cm⁻¹ are attributed to the F_g+ E_g mode of Y₂O₃ [40,41]. The peak centered at 399.7 cm⁻¹ corresponds to F_g and at 151.9, 470.6 cm⁻¹ correspond to F_g + A_g modes of Y₂O₃[40,41]. For Sample 2, the Raman peaks observed at 134.2 and 399.7 cm⁻¹ are attributed to F_g, 187.3, 328.9 and 559.0 cm⁻¹ are attributed to F_g + E_g, and 470.6 cm⁻¹ is attributed to F_g + A_g modes of cubic Y₂O₃ (Figure 2(b)) [40,41]. The Raman spectrum of template assisted Y₂O₃ coating (Sample 3) is shown in Figure 2(c). In addition to the above modes we observed a few additional peaks at 115.6, 440.0 and 559.8 cm⁻¹. The peaks centered at 115.6 and 396.9 cm⁻¹ are attributed to F_g + A_g and F_g modes of Y₂O₃, respectively. The peaks observed at 440.0 and 559.8 cm⁻¹ correspond to F_g+ E_g mode of Y₂O₃ [40,41]. For Sample 3 an additional peak is observed at 259.0 cm⁻¹, which corresponds to CdO. This peak is from the Cd-CdO template.

Sputter deposited Y₂O₃, thermally oxidized Y-Y₂O₃ and template assisted Y₂O₃ coatings show different surface morphologies. The surface morphologies of Samples 1–3 were examined by FESEM and are shown in Figure 3(a–f) at two different magnifications. The water contact angles for Samples 1, 2 and 3 were 99°, 117° and 155°, respectively as shown in the insets in Figure 3(a–c). The dynamic contact angle measurements for Sample 3 showed an advancing water contact angle of 154° and a receding water contact angle of 144° with a contact angle hysteresis of 8° (data not shown). The relatively low contact angle of the sputter deposited Y₂O₃ and thermally oxidized Y-Y₂O₃ coatings compared to the template assisted Y₂O₃ coating is attributed to a densely packed nano-grain like microstructure without any void space (shown in Figure 3(a,b) and Figure 3(d,e) at lower and higher magnifications, respectively). The sputter deposited and thermally oxidized Y₂O₃ coatings show hydrophobicity with an ultralow surface roughness (discussed later). It is well known that, improving the surface roughness is a crucial factor for the fabrication of a superhydrophobic surface [42]. To improve the surface roughness a new process was applied. The Cd-CdO template was prepared with a high average surface roughness (R_a = approximately 178 nm, data not presented) and shows superhydrophobic nature (water contact angle > 150°). A thin film of Y₂O₃ is deposited on the Cd-CdO template which enhanced the surface roughness, resulting in the superhydrophobicity. The surface morphology of the template assisted Y₂O₃ coating is shown in Figure 3(c,f) at low and high magnifications, respectively.

In order to further confirm the microstructure of these samples the FESEM data was also recorded at higher magnification, which showed a nanograin-like microstructure for sputtered deposited Y₂O₃ (Figure 3(d)), thermally oxidized Y-Y₂O₃ (Figure 3(e)) and non-uniform nanostructures for the template assisted Y₂O₃ coating (Figure 3(f)). The low resolution image (Figure 3(c)) shows the presence of solid components (i.e., whitish regions) and air pockets (i.e., darker regions). This combination of air gaps and solid regions behaves as the first or higher scale roughness of the coating as shown in the roughness profile presented later. Even though the air pockets are in sub-micron range, the roughness (that is, average height of hills and valleys) was in the nanometric scale. The high magnification (Figure 3(f)) showed the solid surface to consist of fused individual structures which generated a textured or a patterned surface. The textured surface acts as the second or lower scale roughness (believed to be a few tens of nanometers). This texturing gives rise to a multi-scale roughness in the Y₂O₃ coating deposited on the Cd-CdO template, which was responsible for the observed superhydrophobicity. According to the Cassie-Baxter model, the surface fraction of the solid (f₁) and air pockets (f₂) impacts the water contact angle for a composite surface which can be calculated by Equation 3. The air fractions for Samples 2 and 3 were 0.32 and 0.91, respectively (assuming Sample 1 with an average roughness 3 nm as a smooth surface and contact angle for Sample 1 = 99°). The surface roughness of as-deposited Y₂O₃ coatings was measured by AFM. Figure 4(a–c) shows the AFM images of sputter deposited Y₂O₃, thermally oxidized Y-Y₂O₃ coating and template assisted Y₂O₃ coating and the corresponding 2D roughness profiles are shown in Figure 5(a–c). The values of average surface roughness of sputter deposited Y₂O₃, thermally oxidized Y₂O₃ and template assisted Y₂O₃ coatings are: 3, 11 and 180 nm, respectively. The AFM data confirms the presence of nanoscale surface roughness. The presence of air-pockets in Sample 3 was confirmed by FESEM as shown in Figure 3(c), which contributes to the higher scale of roughness. The variation of the water contact angle with the template assisted Y₂O₃ coating thickness is shown in Figure 6. It is clearly seen that irrespective of the change in the thickness of the coating, the contact angle always remains greater than 150°. To cross check this observation, the surface roughness of the coatings was measured by AFM. The AFM images of the template assisted Y₂O₃ coating for different thicknesses are shown in Figure 7(a–c). The average surface roughness values for 30, 60 and 80 nm thickness of template assisted Y₂O₃ coatings are approximately 180, 185 and 188 nm, respectively. The Y₂O₃ coating follows the template/substrate morphology. It can be concluded that for coatings with different thicknesses of Y₂O₃, the change in the average surface roughness was negligible, which is responsible for superhydrophobicity for all thicknesses.

The contact angle and work of adhesion of the polar liquids for Samples 1–3 are shown in Figure 8(a,b), respectively as per the procedure described elsewhere [43]. Ideally the static CAs of the sample should be found experimentally using a series of probe liquids, with a balanced composition of polar and non-polar components. A suggested series of probe liquids that are easily available and cover the whole range of surface tension values are, polar protic–water, glycerol, formamide and aniline, and non-polar aprotic–diiodomethane, dodecane, hexadecane, ethylene glycol and benzene. In the present work we chose only three liquids for the initial estimate of the free surface energy of the samples. The dispersive and polar components of glycerol, formamide and water are reported in Reference 44. The static contact angles for Samples 1–3 for different probe liquids are presented in Table 2. Compared to non-polar liquids, the polar liquids interact differently with Y₂O₃ coatings because they exhibit a large dipole moment and have a strong tendency for hydrogen bonding. The contact angle of liquids (water, formamide and glycerol) for Samples 1–3 increased in the following order: θ_formamideθ_glycerolθ_water. The free energies of the liquid-solid, solid-vapor and liquid-vapor interfaces are dependent on the work of adhesion at the solid-liquid interface. The work of adhesion for Samples 1–3 is calculated, without taking into account the polar/apolar interactions by the Young-Dupre Equation [45]:

(4)

The work of adhesion of water for Samples 1–3 were 54, 43 and 7 mJ/m², respectively. It can be clearly seen that the work of adhesion for Sample 3 was very low compared to Samples 1 and 2. This is because the template assisted Y₂O₃ coating demonstrated a composite surface (Cassie Baxter state), which contains a void space filled with air combined with nonuniform nanostructures (shown in Figure 3(c)). This composite interface decreases the solid-liquid contact area and therefore decreases the work of adhesion. These results show that the presence of a micron scale void space combined with non-uniform nanostructures like morphology is responsible for superhydrophobicity in Sample 3.