Applied Surface Science 565 (2021) 150288 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article Oxidation processes at the surface of BaTiO3 thin films under environmental conditions Irena Spasojevic a, b, Guillaume Sauthier a, José Manuel Caicedo a, Albert Verdaguer c, Neus Domingo a, * a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain b Department of Chemistry, Universitat Autónoma de Barcelona, Bellaterra, 08193 Barcelona, Spain c Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra, 08193 Barcelona, Spain A R T I C L E I N F O A B S T R A C T Keywords: Dissociation and adsorption of water on ferroelectric oxide surfaces play important role in the processes of XPS screening and switching dynamics of ferroelectric polarization, as well as in catalytic processes which can be BaTiO3 thin films additionally coupled with light, temperature or vibration stimuli. In this work, we present XPS study of ferro­ Ferroelectric materials electric BaTiO3 thin films and determine the entanglement between surface chemistry, polarization direction and BaO2 Surface oxidation stability, by observing changes upon time exposure to environmental conditions, heating in O2 atmospher­ Water splitting e and irradiation in vacuum. We devote special attention to Ba 3d spectral region and identify two different oxidation states of O atoms in the compounds of Ba. While this second specie was generally attributed to Ba in surface compounds where it has different oxygen coordination than in the bulk, based on the XPS results of oxygen annealed thin films, we demonstrate that this so far neglected component, corresponds to barium peroxide (BaO2) and identify it as important active specie for the study of screening mechanisms closely related with catalytic activity present in this ferroelectric material. Finally, we report on chemically assisted polarization switching in thin films induced by heating in vacuum or exposure to X-Ray radiation due to the formation of positive surface electric field created by oxygen or electron vacancies, respectively. 1. Introduction termination of the oxide layer [12-15], whereby water molecules can adsorb as a whole (molecular adsorption) or dissociate resulting in the As one of the commonly found surface adsorbates on all ambient- formation of hydroxyl ions (OH− ) and protons (H+) building strong exposed surfaces, water influences all surface related processes such as chemical bonds with the metal ions of the substrate [16]. It has been catalytic reactions, adhesion [1], friction [2] etc. Particular attention demonstrated that BaO termination of barium titante (BaTiO3) single has been devoted to the investigation of adsorbed water and its effects crystal prompts dissociative adsorption of the first water layer, while in on ferroelectric oxide materials, where due to its dipolar character and the case of TiO2 termination dissociative and molecular adsorption ionic conductivity, it has important role in external screening of detri­ might occur [14]. mental depolarization fields [3] and ferroelectric phase stabilization [4]. Other less investigated, yet very important, are the electrocatalytical As a consequence, ferroelectric phase stability [4] and even switching properties of ferroelectric materials, due to the native presence of sur­ dynamics [5,6] are becoming inevitably coupled to their surface face electric fields or when coupled with some external stimuli such as chemical composition. However, the relationship between water and light, change of temperature or vibrations [17-20]. Driven by the need of ferroelectric properties is bidirectional: the different polarization states depolarization fields screening, surface charges can accommodate or will influence the affinity and sticking coefficients of chemical species on supply negative/positive charges and therefore actively take part in oppositely polarized ferroelectric substrates, as demonstrated from surface chemical reactions. On this way, by changing the polarization Near-ambient pressure X-Ray photoelectron spectroscopy (NAP-XPS) [7- one can create oxidative or reducing ferroelectric surfaces on demand 9] and desorption studies of water and short-chain alcohols [10,11]. [21-23]. Furthermore, water adsorption is strongly influenced by the In order to fully understand complex processes happening at the * Corresponding author. E-mail address: neus.domingo@icn2.cat (N. Domingo). https://doi.org/10.1016/j.apsusc.2021.150288 Received 17 March 2021; Received in revised form 10 May 2021; Accepted 31 May 2021 Available online 3 June 2021 0169-4332/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). I. Spasojevic et al. Applied Surface Science 565 (2021) 150288 Fig. 1. Results of X-ray diffraction analysis of SrRuO3/BaTiO3 bilayers grown on SrTiO3 substrate. (a) 2θ/ω coupled scan around the (0 0 2) reflection of the SrTiO3 substrate, SrRuO3 buffer electrode and BaTiO3 thin film, (b) X-Ray reflectivity measurements and (c) X-ray reciprocal space map (XRSM) around (-1 0 3) Bragg peak, showing that SrRuO3 and BaTiO3 layers are coherent and fully strained to SrTiO3 substrate. ferroelectric surfaces and take advantage of them, it is necessary to oxygen pressure pO2 = 100 mTorr and at deposition temperatures of identify all the chemical species and their behavior in different experi­ 635 ◦ C and 700 ◦ C, respectively. After the deposition sample was cooled mental conditions and time frames. By means of powerful tools such as down slowly to the room temperature with the rate of 3 ◦ C/min in NAP-XPS, the presence of species such as molecular water and hydroxyl saturated O2 atmosphere (pO2 = 104 mTorr) in order get rid of any (OH) groups has been well established [12]. Of particular interest is the present oxygen vacancies. detection of surface oxygen species such as metal-peroxo complexes that can appear as intermediate species directly associated with oxidative 2.2. Structural characterization water splitting reactions [12,24,25]. However, species related with the presence of surface Ba-related compounds have been scarcely studied Structural properties of as grown BaTiO3 thin films were studied by [26] and in most of the cases they show inconsistent results. X-Ray diffraction. Coupled 2θ/ω scans around (0 0 2) reflection, X-Ray In this report, we present an XPS study of epitaxial and monodomain reflectivity (XRR) and X-Ray reciprocal space map (XRSM) measure­ BaTiO3 thin films exposed to ambient conditions. Beside the expected ments were done on PANalytical X’Pert Pro diffractometer. species, such as molecular water, hydroxyl (OH) groups and different carbon-related species (COx) we report on the existence of additional 2.3. Atomic Force Microscopy (AFM) analysis specie, observable as a high energy component in Ba 3d spectral region. Up to know, assignment of this peak is controversial and still under AFM imaging was performed on an MFP-3D AFM (Asylum Research). debate: it was generally attributed to Ba in surface compounds in which In all the experiments, PPP-EFM tips (Nanosensors) with a stiffness Ba has different oxygen coordination than in the bulk [27,28], but oc­ constant k = 2.8 N/m and coated with Ptlr5 were used. Topography of casionally it has also been inconsistently assigned to BaCO3 species the samples was imaged in tapping mode, and ferroelectric properties [29,30]. In order to delve into its origin, we studied the correlation of were probed by Piezoresponse Force Microscopy (PFM) in DART mode this peak with all the contributions on the O 1s and C 1s regions, after and Switching Spectroscopy PFM (SS-PFM). annealing the thin films in oxygen. Further on, by combined use of XPS and Piezoresponse Force Microscopy (PFM) we studied effect of pro­ longed atmosphere exposure on oppositely polarized BaTiO3 thin films 2.4. XPS measurements and observe striking differences in surface chemical composition and hydrophilic character of up- and down- polarized surfaces, which we XPS measurements were performed at room temperature with a interpret in the light of enhanced oxidizing activity of down-polarized SPECS PHOIBOS 150 hemispherical analyzer (SPECS GmbH, Berlin, BaTiO3 thin films. Finally, we report on the observed polarization Germany) with a base pressure of 5x10− 10 mbar using monochromatic switching in thin films under different environmental conditions. We Al K-alpha radiation (1486.74 eV) as excitation source operated at 300 observe that after heating in vacuum or exposure to X-Ray radiation the W. The energy resolution as measured by the FWHM of the Ag 3d5/2 films spontaneously switch the ferroelectric polarization in both cases peak for a sputtered silver foil was 0.62 eV. Some of the samples were from up to down-polarized state. We will explain these dynamics in annealed in a separate chamber in O2 atmosphere and then transferred terms of both electrical and chemical modifications of the surface, back to the XPS chamber by vacuum suitcase without exposure to the induced by processes such as the formation of positive surface electric atmosphere. The fitting of the XPS spectra was performed by CasaXPS fields created by oxygen or electron vacancies, respectively. software after Shirley subtraction of the background with a combination of Gaussian/Lorentzian functions in a ratio 70:30 for all peaks. Decon­ 2. Experimental details volution of the spectra in the O 1s, C 1s, Ba 3d5/2 and Ti 2p regions is described in detail in the text. In order to compare spectra obtained from 2.1. SrRuO3 (SRO)/BaTiO3 (BTO) thin film preparation different samples, O 1s spectra were normalized by the intensity of the bulk oxide peak, while for Ba 3d5/2 and C 1s spectra, the BaO 3d5/2 and Epitaxial ferroelectric SrRuO3/BaTiO3 bilayers were grown by Ti 2p peak intensities were used. Pulsed Laser Deposition (PLD) technique on SrTiO3 (STO) substrates. A KrF excimer laser (λ = 248 nm, fluence 5.4 J/cm2, 2 Hz repetition rate) 3. Results and discussion was used to ablate stoichiometric ceramic targets of SrRrO3 and BaTiO3. As received (1 0 0) oriented SrTiO3 single crystals (CrysTec GmbH) were 3.1. Structural, ferroelectric and surface characterization of SrRuO3/ used as substrates in all depositions. Target-substrate distance was fixed BaTiO3 thin films at 8 cm. Prior to the growth of BaTiO3 layer, we firstly grow SrRuO3 layer which serves as a bottom electrode for PFM measurements. Both Structural properties of as grown BaTiO3 thin films were studied by SrRuO3 and BaTiO3 layer were deposited in O2 atmosphere with partial X-Ray diffraction in 2 theta/omega (2θ/ω) geometry, whereby ω and 2θ denote incident and diffracted angle, respectively. High resolution 2θ/ω 2 I. Spasojevic et al. Applied Surface Science 565 (2021) 150288 Fig. 2. (a) AFM topography of a 5 x 5 µm2 area showing the atomic steps of a perfectly layered sam­ ple. (b) PFM Phase image of the as-grown up polarized area, where a squared pattern of 3.5 × 3.5 µm2 has been switched to down polarization (purple area) by applying a positive voltage of Vtip = 8 V and switched back to up polarization with a reversed voltage of Vtip = − 8 V (yellow squared inset). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 3. Decomposition of XPS spectra at Ba 3d (a), Ti 2p (b), C 1s (c) and O 1s (d) regions of as-grown up-polarized BaTiO3 thin film after being exposed to the air during 20 min, revealing contributions from undissociated water molecules, hydroxyl (OH) groups, peroxide species (Surface O−2 ) and different carbon-related species (COx) (see detailed explanation in the text). coupled scan recorded around the (0 0 2) reflection in the 390 – 490 (-1 0 3) SrTiO3 Bragg reflection. From this measurement is evident that range shows three peaks belonging to tetragonal structure of BaTiO3 and SrRuO3 and BaTiO3 layers are coherent and fully strained to the single orthorhombic structure of SrRuO3 thin films, as well as a peak that stems crystal SrTiO3 substrate (Fig. 1c). The in-plane lattice parameter of both from SrTiO3 substrate (Fig. 1a). Out-of-plane lattice parameters have layers is 3.905 Å and it is equal to the in-plane lattice parameter of values of cSRO = 3.955 Å for SrRuO3 and cBTO = 4.149 Å for BaTiO3 thin SrTiO3 substrate. c/a ratio for BaTiO3 thin film is 1.06, therefore film, respectively. These values are in agreement with the values re­ showing c-orientation of the BaTiO3 film. The topography of the as- ported in literature for the fully strained SrRuO3/BaTiO3 bilayers grown grown thin film was imaged by AFM tapping mode (Fig. 2a). On the on SrTiO3 (1 0 0) substrates [31]. The results from reflectivity mea­ surface of the film some atomic step-terrace structures are visible, which surements (Fig. 1b) were analyzed by X’Pert Reflectivity Software is characteristic of layer-by-layer growth, but also some holes. Root- (Malvern-Panalytical) and common Fourier method from which we mean square surface roughness (Rrms) is about 0.4 nm. obtained a thickness of 33 nm for the SrRuO3 and 21 nm for the BaTiO3 The PFM phase image of as-grown and electrically written ferro­ layer, respectively. electric domains is presented in Fig. 2b. In order to check the as-grown In order to investigate the strain state of epitaxial SrRuO3/BaTiO3 polarization state, we electrically write two squares of opposite polari­ bilayers we have recorded X-Ray reciprocal space map (XRSM) around zation by applying a DC voltage to the tip of Vtip = + 8 V (forcing the 3 I. Spasojevic et al. Applied Surface Science 565 (2021) 150288 polarization down) and Vtip = – 8 V (forcing the polarization to switch up) while scanning in contact over an area of 3.5 x 3.5 μm2 and 1 x 1 μm2 respectively. Since the phase contrasts of the inner square polarized with – 8 V and that of background are the same, we can conclude that the as- grown BaTiO3 thin film is out-of-plane up-polarized, as expected for the BaTiO3 films grown under compressive strain on this electrode [31]. Finally, we study the pristine-like surface composition of an as- grown sample (Sample A), transferred to XPS chamber within 20 min after growth in order to minimize time of exposure to ambient condi­ tions. Fig. 3 shows the XPS spectra of an up-polarized BaTiO3 thin film at the Ba 3d (a), Ti 2p (b), C 1s (c) and O 1s (d) regions. The Ti 2p spectrum has been deconvoluted into the characteristic spin–orbit split doublet. The Ti 2p3/2 peak has a main component at 458.8 eV due to Ti4+ valence state. The presence of possible surface oxygen vacancies can be deter­ mined by existence of shoulder in the low BE side of the Ti4+ 2p3/2 peak which would correspond to reduced Ti3+. The absence of a Ti3+ peak is in line with the absence of oxygen vacancies close to the surface, within the limit of detection of our XPS equipment. The complex structure of the O 1s spectrum shown in Fig. 3d was fitted using the model explained in detail in reference [12]. Briefly, the lowest binding energy (BE) peak at 529.7 eV belongs to the oxide peak, while the higher BE peaks can be related to different adsorbates species present on the surface. Peak at ΔBE = + 0.95 eV has been associated to different types of hydroxyl OH groups on the surface bound to the metal, such as Ti- or Ba-bonded terminal OH groups (Ti–OH, Ba–OH) and bridging hydroxyl groups (Ox-H). The peak at ΔBE = + 1.8 eV from the main oxide peak has been associated to different carbon-related species COx such as carbonates CO2− 3 , C– O bonds, ester (C–(C– O)–OR) or – – carboxylic acid (C–(C– – O)–OH) compounds. Following peak at ΔBE = + 2.75 eV, here named as “surface O−2 peak”, corresponds to chem­ isorbed oxygen species such as Ti-peroxo and -superperoxo complexes. It is worth to mention here that this specie is structurally similar to the oxygen in metal peroxides such as BaO2. Finally, a peak at ΔBE = + 4 eV, clearly observed in AP-XPS measurements in water atmosphere [7,12], has been usually associated with physisorbed molecular water bounded to other water molecules. However, C-O bonds from ester or carboxylic acid compounds can also contribute to the same peak as molecular water. From a depth profile analysis done by comparing the spectra taken at 0 deg with the one taken at 60 deg, and in agreement with the previously reported model [12], the OH species seem to be the ones close to the surface, followed by the carbonate and peroxide species, and all of them laying below the molecular water layer. Finally, the he C 1s region has been fitted with four well-resolved peaks as shown in Fig. 3c. The peak at lowest binding energy of 284.8 eV belongs to C atoms attached to another C or H atom (C–C(H)). Components at ΔBE = + 1.4 eV and ΔBE = + 2.6 eV from the adven­ titious carbon peak are associated with C atom attached by single (C–O) or double bond (C– – O) to O atom. The peak at ΔBE = + 3.65 eV can be related with C atom in carbonate (CO2− 3 ), ester (C–(C– O)–OR) or – carboxylic acid (C–(C– – O)–OH) compounds [32]. Finally, the peak at highest binding energy ΔBE = + 4.5 eV that is observed only in some cases has been related with the presence of bicarbonates (HCO−3 ). Spectrum in the Ba 3d region exhibits the typical 3d3/2 and 3d5/2 doublet, which can be further resolved in two different contributions. Fig. 4. Decomposition of XPS spectra at the Ba 3d (a), C 1s (b) and O 1s (c) The peak at the lowest BE of 779.0 eV (for the Ba 3d5/2) belongs to Ba regions, taken in vacuum conditions at RT and with an incident angle of 60◦ atoms in the oxide lattice. A second peak is observed at a higher BE, i.e. (increased surface sensitivity) of an up-polarized BaTiO3 sample. The spectra ΔBE = + 1.5 eV shifted from the main peak. By comparing the spectra are taken before (full orange lines) and after (blue dotted lines) annealing the taken at 0 deg with the one taken at 60 deg, with higher surface sensi­ sample at 250 ◦ C in saturated O2 atmosphere. . Ba 3d5/2 spectra show that tivity, we have extracted a depth profile and observed that this second annealing causes an increment of the component at + 1.5 eV from BaO peak, specie is much more superficial than the main Ba peak. Hitherto, the which is in direct relation with the increment of surface O−2 peak at + 2.75 eV from main oxide peak in O 1s region and therefore can be related with the assignment of this peak has been controversial and remains still under presence of BaO2. (For interpretation of the references to colour in this figure debate: it is generally attributed to Ba in surface compounds in which Ba legend, the reader is referred to the web version of this article.) has different oxygen coordination than in the bulk, such as barium carbonate BaCO3 or barium hydroxide Ba(OH)2. However, these find­ ings are not justified, since the peak at ≈ + 1.5 eV from BaO is observed in many samples and in a lot of cases exists even when carbonate species 4 I. Spasojevic et al. Applied Surface Science 565 (2021) 150288 Fig. 5. Schematic representation of polarization switching from up to down state, caused by formation of positive electric field Ev associated with (a) creation of surface oxygen vacancies O+v due to vacuum annealing or (b) creation of electron vacancies due to X-rays photoelectronic excitations, leading to positive sur­ face charging. cannot be detected by XPS (i.e. in C 1s or O 1s spectra) nor by IR mea­ the surface, and contains some degree of contamination in the form of surements at the surface [16,26,28,33]. Moreover, the oxidation envi­ carbonates. ronment of Ba in BaCO3 molecules should be similar to that of the bulk, so any contribution should be located close to or overlapping with the 3.2. Effect of annealing in O2 atmosphere on surface chemistry main oxide peak. Nonetheless, this peak has also been observed for BaTiO3 single crystals cleaved in vacuum conditions, where no external After the XPS characterization showed in Fig. 3, the sample was molecules were available to interact with the surface [34]. Therefore, removed from XPS chamber and placed in an annealing chamber. the component at higher BE cannot be univocally related to the presence Sample was annealed at 250 ◦ C in saturated O2 atmosphere (P = 104 of BaCO3 species. On another hand, the presence of previously proposed mTorr) and afterwards cooled down slowly with a rate of 3 ◦ C/min. Ba(OH)2 [28] is improbable in the case of samples exposed to ambient, After annealing treatment, sample was transferred to XPS chamber since CO2 from the atmosphere would react instantaneously with Ba without breaking the vacuum by using a suitcase. Fig. 4 shows the Ba (OH)2 forming BaCO3 following the reaction: Ba(OH)2 + CO2 → BaCO3 3d5/2 (a), C 1s (b) and O 1s (c) XPS spectra of the BaTiO3 thin film + H2O [26]. Furthermore, in literature one can see that the high energy measured before and after the annealing treatment. By comparing the Ba 3d5/2 peak is always followed by the presence of another peak in the obtained Ba 3d5/2 spectrum after annealing with initial spectrum of this O1s spectrum at ΔBE = + 2.8 eV (± 0.1 eV) from the main oxide peak, sample, it can be seen that the concentration of BaO2 has strongly which in the corresponding literature is arbitrary associated to surface increased. The spectra at the C 1s region shows that annealing treatment OH groups or carbonate species [20,28,30,35]. In our previous studies of diminishes the concentration of advantageous carbon as well as C– –O oxide perovskite surfaces in water vapor atmosphere, we demonstrated 2− bonds, while the peak corresponding to the (CO3 )/(C– (C– O)– R) – – – that surface OH peak is placed rather closely to the oxide peak, giving almost doesn’t suffer any change. This can be due to the simultaneous that its intensity was uniformly increasing as water was dosed into the effect of CO2− 3 species decrement and (C–(C– O)– R) species incre­ – – chamber [7]. Therefore, peak at ΔBE = + 2.8 eV from oxide cannot be ment. At the same time, COx peak in O1s region is decreased following related to OH groups or carbonate species. the same trends observed for C– – O and (CO2− 3 )/(C– (C– O)– R) peaks in – – – In this work, we prove that the high energy component of Ba 3d5/2 2− C 1s region. Simultaneous decrement of CO3 species and increment of peak corresponds to BaO2. On one hand, the works [35,36] show that BaO2 can be explained by decomposition of carbonates at high tem­ the BE of BaO2 is expected to be shifted by ΔBE = 1.3–1.7 eV from the peratures on BaO and CO2 (BaCO3 → BaO + CO2), whereby free BaO BaO compound, in agreement with our observations. In addition, the terminations can be further oxidized to BaO2 and CO2 pumped away presence of the high energy peak in Ba 3d spectrum in our case is also from the XPS chamber. This oxidation also impacts as an increment of clearly associated with the presence of the peak at ΔBE = + 2.8 eV from the surface O−2 peak in the O 1s spectrum. The formation of BaO2 species oxide in the O1s spectrum, which we previously attributed to peroxide gives rise to the surface O−2 peak evidencing the interconnected behavior species [12] in agreement to what should be expected from BaO2. Sig­ of these two peaks (note that in this case, the rise of surface O−2 peak is nificant amount of BaO2 on our BaTiO3 surfaces might be related with not coming due to dissociative water surface oxidation reactions). This the sample preparation method: after the thin film growth at 700 ◦ C, the experiment confirms that the peak at ΔBE = + 1.5 eV from the main chamber is saturated with O2 while the temperature is ramped down peak in the Ba 3d region corresponds to the BaO2 species and is entan­ very slowly (3 ◦ C/min). These conditions of high O2 pressure and tem­ gled to the component at ΔBE = + 2.8 eV from the main peak in the O 1s. perature can foster the production of BaO2 by, for example, following Therefore, it can additionally be used to specifically follow this oxidative the reaction 2BaO + O2 → 2BaO2, and therefore this peak might not be reaction as a function of time and polarization. present on samples made by different procedures. Finally, taking into account clear evidence of a CO2− 3 peak in the C 1s 3.3. Environmental assisted polarization switching and O 1s regions, we cannot neglect the presence of BaCO3. Still, as mentioned before, the position of this peak in the Ba 3d spectrum can The polarization state of our as grown up-polarized samples is shown probably overlap that of the BaTiO3 position [37,38] from the bulk to be prone to reverse under specific environmental conditions and contribution making it undetectable within our energy resolution. under X-Ray exposure: when exposed to temperatures up to 200 ◦ C in So all in all, some few minutes of exposure to environmental con­ vacuum or when irradiated with X-Rays in the frame of an XPS experi­ ditions is enough for ferroelectric surfaces to react with water and hy­ ment in vacuum conditions, we observe a switching of the polarization drocarbon molecules from the atmosphere and create a layer of state in the whole area of the sample, as probed by PFM (images not adsorbates: after 20 min in ambient conditions, the surface of our as- shown here). grown up polarized BaTiO3 thin film contains hydroxyl groups ob­ This switching can be attributed generically to the creation of a tained after water splitting reactions, BaO2 and Ti-based peroxo com­ positive surface potential as shown in Fig. 5a,b by a) the formation of plexes probably obtained after further oxidation of hydroxyl groups on oxygen vacancies at the surface during vacuum annealing process or b) 5 I. Spasojevic et al. Applied Surface Science 565 (2021) 150288 Fig. 6. SS-PFM hysteresis loops of (a) as grown up-polarized BaTiO3 thin film, showing coercive voltage of Vc, up = 1.2 V with a slightly positive bias of Vbias, up = 0.4 V and (b) switched down-polarized BaTiO3 thin film obtained after vacuum annealing, with increased coercive voltage of Vc, down = 1.4 V and almost no bias. As a consequence of polarization switching, the internal bias disappears leading ultimately to a more robust polarization as observed by the increase of coercive voltage and absence of bias. the creation of electron vacancies during the XPS measurements. In both cases, this favors ferroelectric switching. Polarization switching might be an inherent part of XPS experiments when the size of X-Ray beam spot arriving to the sample is comparable to sample size, in a way that X-ray photons that are removing photoelec­ trons from the sample are leaving behind positively charged holes. These holes can be easily filled by negative charge carriers present in the highly conductive metallic samples, but not in the case of insulating samples such as ours. Negative charge needed for holes’ compensation might come from the bottom SrRuO3 electrode, however these electrons might recombine with positive charge carriers on their way to the top layers of the sample, making this process highly ineffective. In that case uncompensated positive holes on the surface would create electric field Ev, opposing the as-grown polarization of the sample, which could Fig. 7. Decomposition of XPS spectra at the Ba 3d (a), C 1s (b) and O 1s (c) switch it down. These processes are schematically depicted in Fig. 5a,b. regions, taken in vacuum conditions at RT for down- (purple, full lines) and up- (yellow, dotted lines) polarized BaTiO3 samples after being exposed to ambient In a similar way, annealing in vacuum conditions up to 200 ◦ C can create conditions during a period of several months. Compared to the spectra taken on a positive surface voltage either by inducing new oxygen vacancies or by as grown up-polarized BaTiO3 shown in Fig. 3, long exposure to ambient con­ removing essential screening charges that can lead to ferroelectric po­ ditions leads to increment of adventitious carbon, carbonates and also surface larization switching. oxygen species dominated by Ti-peroxo complexes. Interestingly, this incre­ Fig. 6a,b show the PFM hysteresis cycles of as-grown up polarized ment is clearly more pronounced for down-polarized sample, which is repre­ and a switched sample after vacuum annealing. The as-grown sample sented in Table 1. (For interpretation of the references to colour in this figure shows a coercive voltage of Vc, up = 1.2 V with a slightly positive bias of legend, the reader is referred to the web version of this article.) Vbias, up = 0.4 V. This initial positive bias denotes the presence of a built- in down electric field on the as-grown sample that probably nucleates the as-grown up polarization state. This field can be related with the accumulation of positive charge at the interface between the film and 6 I. Spasojevic et al. Applied Surface Science 565 (2021) 150288 Table 1 Relative increase of the ratios of the species in O 1s, Ba 3d5/2 and C 1s regions of BaTiO3 thin films due to prolonged time exposure and as a function of polarization, when compared with an as-grown sample. The ratio of the oxygen species is taken by dividing the areas of the peaks with the area of the main oxide peak, corresponding to bulk BaO contribution; the ratio for Ba species is taken with respect to the main Ba peak, arising from the same contribution; and the ratios for carbon species are normalized again with respect to the main oxide peak area, taken as constant for all the samples. The first line shows the relative increase of species for an up polarized sample due to exposure to ambient conditions for three months, and the second line shows the relative increase in the species for a down polarized sample due to ambient exposure for four months, as compared to an as-grown sample. Sample O 1s Ba 3d C 1s 5/2 Exposure to - BaO2 2- P state OH COx O2 C-C C-O C=O CO3 ambient UP DOWN the bottom electrode that might be in the form of oxygen vacancies. are further oxidized to Ti –peroxo and –superperoxo complexes, which Instead, the switched sample shows an increase of stability as observed also results in increment of surface O−2 peak. Instead, Ba sites in BaO by a Vc, down = 1.4 V with almost no bias. Hysteresis loops obtained on terminated surfaces are not able to build stable metal-peroxo complexes switched samples after X-Ray exposure (not shown here) are showing like Ti atoms. In this case reaction with O2 from atmosphere is more the same trend of slight increase of coercive voltage and bias vanishing. probable to take place, giving rise to BaO2 and surface O−2 peaks but with Changes of ferroelectric hysteresis cycles can be explained as a conse­ much smaller reactivity rates. This reaction normally proceeds at quence of polarization switching, after which interfacial oxygen va­ increased temperatures as demonstrated earlier, however it has also cancies can move from the bottom interface to the surface where they been reported when electric potential of several hundreds of millivolts is get cancelled by reacting with available oxide species, leading ulti­ applied [39]. mately to a more robust polarization material as observed by the coer­ In comparison to other oxide materials, ferroelectrics have an addi­ cive voltage increase and the absence of bias in PFM hysteresis cycles. tional degree of freedom, i.e. polarization which can take different ori­ entations and therefore affect sign of the surface screening charge (i.e. 3.4. Effect of prolonged atmosphere exposure as a function of polarization electric field). In that sense, screening charge act as an active layer, state on BaTiO3 thin films which is able to accommodate or supply negative charge and therefore take part in electrochemical reactions happening on the surface: down- Fig. 7 shows the effect of atmospheric exposure on BaTiO3 thin films polarized surface with positive surface electric field act as anode by surfaces as a function of their polarization state. Yellow lines show the being able to accept electrons produced in surface oxidation processes. XPS spectra at the Ba 3d5/2 (a), C 1s (b) and O 1s (c) regions of an as- In the case of BaTiO3, positive anodic potential on ferroelectric surface grown up polarized sample after three months of exposure to ambient might catalyze oxidation reaction in which oxygen atoms change conditions (sample B). Purple lines show the XPS spectra of sample A, valence state from − 2 in BaO to − 1 in BaO2. Besides, down-polarized taken about four months after the spectra shown in Fig. 3. After the first surfaces will also be able to accommodate the generated electrons XPS measurement we could see that polarization of sample A was formed by the oxidation of OH groups (i.e. loss of H atoms and electrons) switched, so that the sample has remained in a down polarized state for during the formation of Ti-peroxo and –superperoxo species. all this time. If we compare pristine BaTiO3 film (Fig. 3) with long-term Our results show that long exposure of BaTiO3 ferroelectric surfaces ambient exposed BaTiO3, apparent changes in the Ba 3d5/2, C 1s and O to ambient conditions during prolonged periods of time leads mainly to 1s regions can be appreciated for both polarization states. Table 1 shows an increment of Ti-peroxo species especially pronounced on the down- the change in the ratios of peaks as a function of time for the different polarized surfaces (as observed by a drastic increment of surface O−2 polarization states. For both samples, the amount of BaO2 on the surface peak), together with some increase of BaO2. Altogether, this turns down is increasing during the time, but the down polarized surface seems to be polarized surfaces into more hydrophilic ones, probably due to the more reactive. Increment of BaCO3 is also observable in the C 1s and O hydrogen bonds formation between water molecules and surface oxygen 1s region by following CO2− 3 and COx peaks, respectively, with higher species. This agrees with water adsorption study on PZT thin films [8], peaks for the down polarized surface. Surface O−2 peak in O 1s spectrum which shows that indeed down-polarized surfaces have more hydro­ shows the behavior of surface peroxide species. Here we see that during philic character as compared to up-polarized ones. It is to mention that the ambient exposure surface O−2 peak is increasing for both surfaces, probably, as also observed for PZT, once this adsorbates layer is formed being the negative polarized surface again the one showing higher upon time as a function of the polarization state, it cannot be reversibly reactivity. This peak should include the contribution from both, BaO2 switched and remains there as an imprint of the sample’s ferroelectric and Ti-peroxo complexes. In the case of complex water-assisted surface state history [8]. reactions, analysis of Ba 3d5/2 region can thus provide additional in­ formation regarding surface oxygen state, which is not always achiev­ 4. Conclusions able only from O 1s region. Since we can isolate the contribution from the increase of the BaO2 peak in the Ba 3d5/2 spectrum, this allows us to In this work, we have demonstrated that BaTiO3 thin film surfaces directly monitor the Ba driven electrochemical reactions as a function of are easily oxidized in different possible ways after exposure to ambient polarization. conditions, and that oxidative reactions strongly depend on the active Surface oxidation on the different metal active sites follows different surface sites involved. It is especially remarkable the formation of BaO2 reactions. Ti sites in TiO2 terminated surfaces are known to be very species happening in almost every BaTiO3 surface which was neglected reactive to water. They will mainly catalyze water splitting leading to up to now and has been demonstrated to be fostered by annealing pro­ strong surface hydroxylation. Furthermore, negative polarized surfaces cesses in O2 atmosphere. 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