Hydrophobic and Metallophobic Surfaces: Highly Stable Non-wetting Inorganic Surfaces Based on Lanthanum Phosphate Nanorods

Metal oxides, in general, are known to exhibit significant wettability towards water molecules because of the high feasibility of synergetic hydrogen-bonding interactions possible at the solid-water interface. Here we show that the nano sized phosphates of rare earth materials (Rare Earth Phosphates, REPs), LaPO4 in particular, exhibit without any chemical modification, unique combination of intrinsic properties including remarkable hydrophobicity that could be retained even after exposure to extreme temperatures and harsh hydrothermal conditions. Transparent nanocoatings of LaPO4 as well as mixture of other REPs on glass surfaces are shown to display notable hydrophobicity with water contact angle (WCA) value of 120° while sintered and polished monoliths manifested WCA greater than 105°. Significantly, these materials in the form of coatings and monoliths also exhibit complete non-wettability and inertness towards molten metals like Ag, Zn, and Al well above their melting points. These properties, coupled with their excellent chemical and thermal stability, ease of processing, machinability and their versatile photo-physical and emission properties, render LaPO4 and other REP ceramics utility in diverse applications.

Chemical and physical changes of La 2 O 3 on atmospheric exposure(a) Graphical representation of increase in weight with time of the air-exposed powder compact along with its photographs taken at regular intervals (b) The La(OH) 3 structure (c) TG and DTA traces of the final powdery material confirming the de-hydroxylation steps of La(OH) 3

FT-IR Analysis
We have recorded FT-IR spectra of our LaPO 4 samples to confirm their chemical identity and purity and also the spectra of La 2 O 3 and La(OH) 3 to explain some anomalous observations we have found in the case of some of the rare-earth oxides which were projected recently (Reference 1 of the manuscript) for their super hydrophobicity. Given below are the FT-IR spectra of these three compounds (figure 3, S1). In the case of LaPO 4 we get absorption peaks at 1067, 986, 947, 923, 632, 608, 540 and 500 cm -1 . The spectra could be easily interpreted knowing the T d symmetry of PO 4 3unit present in LaPO 4  Surface energy, γ lmn was obtained using the difference in the Gibbs free energies of surface and that of corresponding bulk for unit surface area.
Gibbs free energy G is given by , where U is the internal energy of the system, p is the pressure and S is the entropy.
The difference in Gibbs free energies between the surface model and the bulk model of solid was calculated by the above method. We ignored zero-point energy and pV term as well as TS term because their differences in surface model and bulk model could be much smaller than the difference of internal energy. U is approximated by the total energy of the system. Therefore, Gibbs energy difference of the system becomes equal to the difference of the total energy, .
Hence, surface energy could be obtained using the difference in the total energy of slab model and that of corresponding bulk model per surface area.
First principles calculations in this paper were performed by the PAW method as implemented in VASP code. [2][3][4] The generalized gradient approximation with the exchangecorrelation functional proposed by PBE 5 was employed together with GGA+U approach 6, 7 in the simplified spherically averaged version. U eff = 7.5 eV is applied to the La 4f states to correct their position relative to La 5d levels. The plane-wave cutoff energy was 400 eV.
As a first step, the cell parameters of bulk model were calculated. For this, the k-point sampling condition ensured a good accuracy of total energies for each crystalline species within 1 meV/atom. All atomic positions and cell parameters were allowed to relax until their forces converged to be less than 0.02 eV/A to obtain structure for bulk.
Then, structural relaxation of slab model constructed based on optimized bulk model was performed only on the position of atoms.
.   The above image shows when the water droplets over surface coming contact with liquid nitrogen freeze over time. The droplets after reaching room temperature retain the shape showing the stability of the phosphate.

S5) Lanthanum Phosphate stability in chemical environments
In order to ascertain the stability and inertness of LaPO 4 for wide applications in diverse chemical environments, we checked the reactivity of LaPO 4 samples with various strong acids and bases. In a typical reaction a suspension of about 3 g of LaPO 4 powder was heated with conc. H 2 SO 4 for more about 2h and after cooling the reaction vessel the powder was filtered, dried and analysed using powder X-ray diffraction. Similar reactions were carried out on fresh samples of LaPO 4 using other acids like conc. HNO 3 and strong alkali like NaOH and the powder XRD were taken in all the cases to check their chemical identity and purity. In all the cases we found that even in powder form LaPO 4 was never reacting with strong acids and bases even in extreme conditions and the samples remain completely inert retaining 100% phase purity. The PXRD peaks of the treated samples were matching in all the cases to those of pure LaPO 4 . The dried samples obtained after the reactions also showed quantitative retrieval which further indicate the complete inertness of the material in those harsh chemical environments. .

S6) Thickness of the thin LaPO 4 film obtained
The SEM micrograph obtained for the LaPO 4 on glass surface shows the rod morphology of the phosphate and gives the average thickness of the coating as ~ 220 nm

S7) WCA measurement on uncoated and coated glass plates
The photographs show the wetting of the uncoated glass surface and LaPO 4 coated glass surface by water. Water spreads over the uncoated surface due to the very low WCA of 14°