The effect of oxygen vacancies on water wettability of transition metal based SrTiO

Understanding the structural, physical and chemical properties of the surface and interfaces of di ﬀ erent metal-oxides and their possible applications in photo-catalysis and biology is a very important emerging research ﬁ eld. Motivated in this direction, this article would enable understanding of how di ﬀ erent ﬂ uids, particularly water, interact with oxide surfaces. We have studied the water contact angle of 3d transition metal oxide thin ﬁ lms of SrTiO 3 , and of 4f rare-earth oxide thin ﬁ lms of Lu 2 O 3 . These metal oxides were grown using pulsed laser deposition and they are atomically ﬂ at and with known orientation and explicitly characterized for their structure and composition. Further study was done on the e ﬀ ects of oxygen vacancies on the water contact angle of the 3d and 4f oxides. For 3d SrTiO 3 oxide with oxygen vacancies, we have observed an increase in hydroxylation with consequent increase of wettability which is in line with the previous reports whereas an interesting opposite trend was seen in the case of rare-earth Lu 2 O 3 oxide. Density functional theory simulations of water interaction on the above mentioned systems have also been presented to further substantiate our experimental ﬁ ndings.


Introduction
Metal oxides comprise a diverse class of materials with intriguing properties and vital technological utility. Controlling and/or altering the interface between liquid ('water' specically) and metal oxide surfaces, which is termed as "wetting", is of immense scientic interest. Understanding the underlying phenomena of wetting at the molecular level in such material systems is crucial for numerous technological processes. Among these metal oxides, reversible tunability of water wetting has been extensively studied in systems such as TiO 2 (ref. 1) and ZnO. [2][3][4][5] Reversible wettability and enhancement of hydrophilicity in metal oxide systems have been obtained through ultra violet (UV) illumination, 5,6 visible light, 7 electro-wetting, 8 structural modications, 9,10 thermal treatment 11 and laser irradiation. 12 Detailed studies have been done to understand the underlying mechanism for enhancement in hydrophilicity in transition metal oxides for e.g. ZnO and TiO 2 and the plausible reasons that have been reported for this consequence are removal of organic contaminants, 13 transition of crystal structure, 14 increase of surface roughness, 15 change of surface chemical compositions/hydroxyl content, oxygen vacancies and Ti 3+ or Zn + defect sites. 16 In particular, modications in wetting through changes in the electronic structure of transition metal oxides due to defect formation have gained attention over the past several years. 1,3,4,12,17 Such modications have been demonstrated through UV irradiation and photo-induced hydrophilicity has been observed for TiO 2 , SnO 2 , ZnO, WO 3 , V 2 O 5 . [18][19][20][21][22] The dissociation of water molecules into H and OH at vacancies sites of TiO 2 have been imaged through non-contact AFM and STM. [23][24][25][26] SrTiO 3 whose electronic structure resembles TiO 2 does not exhibit photoinduced hydrophilicity under UV light, clearly indicates that wetting properties depend intimately on surface electronic structure, and not solely stoichiometry or surface roughness. 27 The increase in interactions of water with the surface defect sites have been well documented in the case of TiO 2 and ZnO but the effect of oxygen vacancies on the 4f element based metal oxides remains largely unexplored except a recent study on CeO 2 . 28 In this article we have studied the wettability of pulsed laser deposition (PLD) grown rare-earth Lu 2 O 3 oxide, with oxygen vacancies and have also investigated the effect of vacancy healing in such systems. In order to understand the underlying mechanism and also to make a comparison, commercially available SrTiO 3 substrates were studied by creating oxygen vacancies through thermal annealing. An opposite trend (in terms of how wettability varies with oxygen vacancy) has been observed in the case of these 3d and 4f oxides and this is in accordance with the density functional theory (DFT) calculations performed for such systems.

Experimental and computational methods
The macro scale contact angle measurements were carried out using the video based fully automated Data Physics optical contact angle microliter dosing system (OCA 40 Micro). Di water drops (1 ml per drop) with known surface tension (72.80 N m À1 ) were dispensed using Teon coated motor driven syringe. The contact angles were measured at ambient conditions and a video was recorded (72 frames per second) for every dispensed solvent droplet. Any dynamic changes to the droplet on the surface can be precisely observed through this method. The contact angle was estimated using the sessile drop technique by measuring the angle between the tangent lines along solidliquid interface and liquid-vapour interface of the liquid contour as shown in Fig. 1. A contact angle of 0 and 180 correspond to complete wetting and non-wetting respectively. Surfaces exhibiting contact angles below 90 are called hydrophilic and those above 90 are called hydrophobic.
The SrTiO 3 samples were commercially available high quality single crystal substrates from CrysTec GmbH. The Lu 2 O 3 thin lms were deposited on YSZ [1 0 0] substrates by pulsed laser deposition using a stoichiometric target of Lu 2 O 3 . YSZ [1 0 0] was chosen due to its similar cubic structure of intended Lu 2 O 3 [1 0 0] phase and similar lattice parameter of 5.14Å whereas Lu 2 O 3 [1 0 0] also has a lattice parameter of 11.03Å. Lu 2 O 3 target was synthesized by a conventional solid state reaction. The starting materials for a target were high purity (99.995% or higher) powders of Lu 2 O 3 . First, the powders were mixed for several hours manually using mortar and pestle. It was pressed into a 1 in. diameter die by hand press and then further pressed under 120 MPa for 30 min by cold isostatic pressure. It was then sintered at 1300 C inside the tube furnace for 15 h under the steady ow of oxygen gas. The sintered target were examined by X-ray diffraction using laboratory powder X-ray source.
The thin lms were deposited at a temperature of 850 C and at 10 mTorr oxygen partial pressure. Before growth chamber was pumped down to base pressure of z5 Â 10 À7 mbar. The 248 nm wavelength KrF laser was utilized to ablate the ceramic target. The incident laser energy density and the repetition rate were 1.5 J cm À2 and 5 Hz, respectively. The crystal structure and phase of the thin lms were examined by X-ray diffraction (Bruker D8 Advanced Thin Film XRD).
We employed density functional theory (DFT) calculations to gain insights into the properties of surface oxygen vacancies and their interaction with water. We have included a Hubbard U term on the Lu f states (U eff ¼ 5.4 eV) 29 and Ti d states (U eff ¼ 3.0 eV). 30 For SrTiO 3 , where formally the Ti electronic conguration is d 0 , we have veried that the reported energetics depend only slightly on the inclusion of the U term by comparing the results with pure DFT.
We considered the TiO 2 -terminated (001) surface of SrTiO 3 as it is expected to be the exposed surface of commercially available SrTiO 3 . For Lu 2 O 3 the [100] orientation of cubic bixbyite is polar, so we can expect some extent of reconstruction to compensate polarity. As a simplied model of a compensated nonpolar surface we have considered the (0001) surface of hexagonal Lu 2 O 3 . The surfaces were represented with slab of 10 and 12Å thickness for Lu 2 O 3 and SrTiO 3 , respectively, where for SrTiO 3 we used a symmetric non-stoichiometric slab. The concentration of oxygen vacancies and/or water molecules was set to 1/4 ML, by using a surface (2 Â 2) unit cell. The water adsorption energy was computed with respect to a water molecule in the gas phase: the total energies of an adsorbed water, the surface, and the gas phase water molecule, respectively. All atomic positions except the bottom oxide layer where fully relaxed until the forces on each atoms were below 0.01 eVÅ À1 . All calculations were performed within the PBE functional 31 as implemented in the VASP code. 32

Results and discussion
This study mainly reports morphological, structural and wettability behaviours of SrTiO 3 and Lu 2 O 3 sample surfaces before and aer annealing.
For SrTiO 3 , high quality single crystal substrates are bought from Crystec, GmbH, Germany and the as received substrate goes through a 6 hour annealing step at 900 C at a very low pressure of 10 À6 Torr. The experiment is performed inside a Pulsed Laser Deposition (PLD) chamber. This annealing step was done to create signicant amount of oxygen vacancies in the SrTiO 3 single crystal sample.
For Lu 2 O 3 , these REO thin lms were grown at low pressure (10 mTorr), so from the beginning they have signicant amount of oxygen vacancies in them. In this case the annealing process was signicantly different from the previous (SrTiO 3 ) one, here pristine Lu 2 O 3 thin-lms were annealed under atmospheric pressure at 900 C for 6 hours inside a PLD chamber. Here the annealing process actually reduces oxygen vacancies instead of creating ones.
To rule out trivial (like hydrocarbon desorption) effect of annealing on water contact angle (WCA) we have waited long enough (more than 120 hours) before measuring the wettability data. However, before making any comparative study in the eld of water contact angle one must make sure that morphological and structural parameters are all similar in the samples before and aer annealing procedure. In this direction we have done Atomic Force Microscopy (AFM) and X-ray diffraction (XRD) study for quantication of the morphological and structural parameters, respectively.
In Fig. 2 we have shown the root mean square (rms) surface roughness value of the Lu 2 O 3 thin-lm and SrTiO 3 single crystal samples using AFM (Agilent -5500). Study of surface roughness is very important as it has been shown and well accepted that wettability is characterized by not only the chemical composition but also the roughness of the surface. 33 To rule out the effect of surface roughness we have shown the surface roughness value of pristine and annealed samples. In case of Lu 2 O 3 thin-lm samples the R rms values are very similar to each other, varying from 0.22 nm [ Fig. 2(a)] to 0.53 nm [ Fig. 2(b)] before and aer annealing respectively. Similarly the surface roughness of TiO 2 terminated SrTiO 3 single crystal varies from 0.15 nm [ Fig. 2(c)] to 3.01 nm [ Fig. 2(d)] before and aer annealing respectively. In TiO 2 terminated SrTiO 3 single crystal samples AFM images of the pristine samples [ Fig. 2(c)] clearly showing the atomic step-edges which is gone aer high pressure annealing [ Fig. 2(d)]. Value of all this surface roughness is very low, all of them are below 5 nm for both SrTiO 3 single crystal and Lu 2 O 3 thin-lm samples. This   amount of morphological change cannot explain the signicant change we observe in WCA value. XRD technique was used to study the effect of annealing on the structural properties of these thin-lms and substrates. PLD grown Lu 2 O 3 thin lms were studied using Bruker D8 Advanced Thin Film XRD. Fig. 3(a) shows the theta-2 theta scan of the pristine Lu 2 O 3 thin-lms grown at 800 C at 100 mTorr oxygen partial pressure and annealed Lu 2 O 3 thin-lms in red and blue lines respectively. As shown in the images all thin-lms are phase pure and Lu 2 O 3 lms grows along [1 0 0] direction. First XRD peak for Lu 2  In the SrTiO 3 case RBS technique could not be used to assess oxygen vacancy content as for oxides with more than two elements it is difficult to decisively nd out elemental composition at a very precise level, particularly when one element is low atomic number (oxygen) with very low Rutherford crosssection. As we have discussed above, the low oxygen partial pressure annealing should enhance amount of oxygen vacancies. To further detect the effect of annealing in the SrTiO 3 substrates photoluminescence (PL) was performed aer annealing at room temperature. Fig. 5 shows that the intensity of the broad PL peak (position around 475 nm) of SrTiO 3 substrate increases monotonically with increasing annealing time. It has been previously reported that broad PL emission of SrTiO 3 in blue range (between 450 to 500 nm) dramatically increase with increasing oxygen vacancy in the SrTiO 3 . 34-37 It has been theorized that oxygen vacancies in SrTiO 3 generate conduction carriers and stabilize a hole level in a self-trapped state and hence the doped conduction electrons and the ingap state produce a radiative process that results in blue-light emission. 34 Also here in Fig. 5 we nd a similar trend which clearly indicates that with increasing annealing time more oxygen vacancies are created in our SrTiO 3 substrates.  This journal is © The Royal Society of Chemistry 2016 Fig. 6 shows the contact angle measurement of Lu 2 O 3 thin-lms and SrTiO 3 single crystal before and aer annealing. As shown in Fig. 6(a) the as-grown Lu 2 O 3 thin-lms sample showing a moderately high water contact angle (70.6 ) before any annealing process, whereas following the annealing [ Fig. 6(c)] water contact angle comes out to be much lower (59.6 ). XPS spectra collected in the C 1s region (Fig. S2 †) show no signicant change upon annealing, suggesting that the change in contact angle is not due to the formation (or removal) of surface carbonates. A very similar behaviour of WCA upon annealing is also found on another REO thin-lm system of CeO 2 [see 'ESI' Fig. S3 for the details †].
Subsequently in the lower panel of the Fig. 6, i.e. in Fig. 6(c) and (d) we have reported the water contact angle of as-received and annealed SrTiO 3 single crystal samples respectively. The pristine SrTiO 3 single crystal samples shows high water contact angle (77.2 ) compared to the annealed thin-lms (48.1 ).
Here we nd an interesting contradictory behaviour in REO Lu 2 O 3 (and CeO 2 ) thin lm and SrTiO 3 single crystal samples: for SrTiO 3 , oxygen vacancy promotes wettability, while for Lu 2 O 3 (and CeO 2 ) it is the opposite directionoxygen vacancy hinders wettability.
Finally to understand the contradictory behaviour of water contact angle with the creation of oxygen vacancies in SrTiO 3 and Lu 2 O 3 systems, we have used Density Functional Theory (DFT) calculations. Various congurations have been considered for the adsorption of water on the oxide surface in presence of an oxygen vacancy. In the case of the TiO 2 -terminated SrTiO 3 (001) surface we found that in the most stable conguration water adsorbs dissociatively at the vacancy site with an adsorption energy of À2.30 eV, to be compared to À0.74 and À0.98 eV of molecular and dissociative adsorption on the clean surface. In the dissociated conguration the oxygen of water heals the vacancy and the two hydrogen atoms form hydroxyl groups with surface oxygen atoms, Fig. 7(d). A similar mechanism has been already reported for TiO 2 and other oxides as discussed in the introduction. The polar hydroxyls groups created upon contact of the defective surface with water can form oxygen bonds with the subsequent water layers increasing the surface wettability. 38 Conversely, on the hexagonal Lu 2 O 3 surface healing of oxygen vacancies with water is not energetically favourable and the most stable conguration consists of molecular water adsorbed on a Lu site adjacent to the vacancy ( Fig. 6(b)), resulting in an adsorption energy of À0.74 eV (compared to À0.37 eV of the clean surface). This adsorption mode  does not create hydroxyls groups and thus does not enhance surface wetting in agreement with the contact angle measurements. The increased wetting observed experimentally for the pristine sample, Fig. 5, can be ascribed to OH species present on this sample, likely at low coordinated sites, which desorbs (being oxidized to leave a lled, unprotonated oxygen site only, desorbing as water) upon annealing. This interpretation is consistent with O 1s data, Fig. S1, † which show a higher OH signal for the pristine sample.
The difference between the two materials can be linked to the different nature of the oxygen vacancy on the two oxides. In the case of SrTiO 3 the vacancy formation reduces the Ti ions from +4 to +3 oxidation state (with a concentration of two Ti 3+ per vacancy site), Fig. 7(c). When water is adsorbed the two extra-electrons remain in the Ti 3d states and the concentration of Ti 3+ is maintained. This similarity between the electronic properties of O-decient and hydroxylated surface has been already observed for TiO 2 . 39 In the case of Lu 2 O 3 , the Lu f states are not accessible for reduction and the two electrons le in the material when a neutral oxygen vacancy is formed stay localized at the vacancy site Fig. 7(a). This state corresponds to a defect state in the band gap. When water is introduced, the adsorption at the vacancy site is hindered as the extra electrons cannot be accommodated at the metal sites (as the Lu states are too high in energy). As a consequence, the molecule stays at a metal site near the vacancy where it can benet from electrostatic interaction with the trapped electrons.
We also note a correlation between the increased wetting at defective SrTiO 3 which is more conductive, compared to Lu 2 O 3 which maintains its insulating nature.

Conclusion
In this article, we have reported the contradictory behaviour of water contact angle with oxygen defect creation in 3d transition metal based oxide SrTiO 3 and 4f rare-earth based Lu 2 O 3 systems. In case of SrTiO 3 we nd that oxygen vacancy promotes wettability, where as in case of Lu 2 O 3 we nd the effect of oxygen is exactly opposite, i.e. that oxygen vacancy inhibits wettability. Post growth annealing at either high or low oxygen partial pressure was used to remove or create oxygen vacancy in both Lu 2 O 3 and SrTiO 3 samples respectively. We have used XRD and AFM measurement to rule out any possible role of structural and surface morphological change (with annealing) that can affect water contact angle. Further study was done using RBS and room temperature PL technique to demonstrate the change of oxygen vacancy level before and aer annealing. Finally DFT calculations was used to conclude that in the case of SrTiO 3Àx we expect an increase of hydroxylation compared to stoichiometric SrTiO 3 with a consequent increase of wettability, which effect is not present for Lu 2 O 3 .

Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the nal version of the manuscript.