Nuno P F Gonçalves

Marco Minella
email: marco.minella@unito.it

Gilles Mailhot

Marcello Brigante

Alessandra Bianco Prevot
email: alessandra.biancoprevot@unito.it

Photo-activation of persulfate and hydrogen peroxide by humic acid coated magnetic particles for Bisphenol A degradation

Keywords:

Magnetic particles (MPs) coated with humic acid (HA) prepared under anoxic atmosphere were tested as heterogeneous photo-Fenton catalyst for the activation of hydrogen peroxide (H 2 O 2 ) and persulfate (S 2 O 8 2-) using Bisphenol A (BPA) as a model pollutant. The role of HA coating, pH value and H 2 O 2 /S 2 O 8 2-concentration were investigated. A positive contribution of HA coating on H 2 O 2 and S 2 O 8 2-activation was found. The highest BPA degradation rates were achieved at acidic conditions (pH 3) with both H 2 O 2 and S 2 O 8 2-, however persulfate

showed a significant efficiency even at pH 6, interesting feature in the light of decreasing the wastewater treatment costs. By the addition of selective quenching agents, % OH and SO 4 %were identified as the main reactive species involved in the BPA abatement. An important contribution of the S 2 O 8 2-photolysis on the overall BPA transformation was highlighted. The reuse of the catalyst was investigated and similar efficiency using H 2 O 2 and S 2 O 8 2-activation was observed until the third catalytic cycle. Experiments carried out using real wastewater samples, showed a good, even if less efficient compared to pure water, BPA removal.

Introduction

Among the advanced oxidation processes (AOPs), Fenton, photo-Fenton and Fenton-like processes have been attracting wide attention to degrade organic pollutants in water [1][2][3] due to their ability to generate highly reactive species able to abate biorecalcitrant species [4][5][START_REF] Enami | Proc. Natl. Acad. Sci[END_REF]. In the "classic" thermal Fenton process, the highly reactive species, mainly hydroxyl radical ( % OH), but also superoxidized iron species [START_REF] Minero | [END_REF] are generated by the reaction between Fe(II) ions and hydrogen peroxide. H 2 O 2 forms an hydrated Fe(II)-H 2 O 2 complex through the ligand exchange of a water molecule located in the first ligand sphere, then an inner-sphere two-electron-transfer gives an intermediate Fe(IV) complex which reacts with a molecule of water giving Fe 3+ and % OH [8].The reaction rate strongly depends on the pH with a maximum at pH 3, subsequently the treated solution has to be neutralized before its releasing, with sludge formation and increase of the overall cost of the process [9]. During the process the formation of hypervalent iron species (Fe 4+ , as well as Fe 5+ and Fe 6+ ) has been proven by stopped-flow experiments and UV/Vis spectroscopy [START_REF] Wink | Proc. Natl. Acad. Sci[END_REF]. The Fe 3+ can be reduced to Fe 2+ closing the cycle and making the process catalytic by H 2 O 2 (with slow kinetics) or through an external energy input (e.g. photo-reduction of Fe 3+ complexes in the so-called photo-Fenton process). A way to avoid the formation of useless and costly to be dismissed iron sludge at the end of the process is using iron-based catalyst easily to be recovered. In heterogeneous Fenton and photo-Fenton reactions the H 2 O 2 is activated by iron supported in a solid matrix at acidic or even circumneutral pH. The catalyst recovery and the low amount of iron released in solution avoid the sludge formation and simplifies the process [START_REF] Usman | [END_REF]12].

Similarly to the % OH, the sulfate radical (SO 4

%-

) has a high oxidation potential (E 0 (SO 4 %-/SO 4 2-) = 2.43 V vs NHE [13], being investigated for oxidation of recalcitrant pollutants [14][15][16]. The activation of persulfate and peroxymonosulfate (S 2 O 8 2-/SO 5 2-) to generate SO 4 %-can be promoted by reactions of these precursors with iron both in the dark and under irradiation [17,18], in the presence and absence of ligands [19,20]. Additionally, the persulfate is described as less subject than % OH to constituents of the real water samples, presenting an higher selectivity toward some pollutants [17,21]. Considering the usual low cost and quite high reactivity of the most common proposed iron-based catalysts, the activation of H 2 O 2 and S 2 O 8 2-by these catalysts is raising growing interest in water treatment for emerging contaminant abatement [15,17]. The mechanism behind these heterogeneous processes has not been fully assessed yet, and it is worth to be studied in deeper details. Fe 3 O 4 magnetic particles (MPs) have been investigated as ironsource due to their Fenton catalytic activity, negligible toxicity, lowcost, facile recovery and recycling by means of a magnetic field [22]. Magnetite structure features a mixture of Fe(II)/Fe(III) oxide and it is well established the importance of Fe(II) ions for an active heterogeneous Fenton-like system [5,[START_REF] Enami | Proc. Natl. Acad. Sci[END_REF]23]. Bare Fe 3 O 4 MPs are susceptible to air oxidation and aggregation in aqueous systems. The stabilization with organic coating can reduce the oxidation of the magnetic phase (magnetite/maghemite) to hematite. Additionally, the introduction of humic(-like) substances, in both Fenton and photo-Fenton conditions appears to significantly enhance the pollutant degradation capacity [24,25], even though the effective role played by humic(-like) substances in the oxidation mechanism has not been already fully solved [26]. As an example, Aparicio and co-workers observed recently that core-shell magnetite-humic acids nanoparticles operate on the one hand as heterogeneous photosensitizers and on the other hand as catalysts in the photo-Fenton treatment for the oxidation of psychiatric drug carbamazepine. The MPs were excellent iron source promoting hydroxyl radical generation in the photo-Fenton process [27].

In this study, humic acid (HA) coated Fe The Sewage Treatment Plant Water (STPW) was collected from the outflow (immediately before the last disinfection and the water discharge) of a wastewater treatment plant on NW of Italy (January 15 th , 2019). Samples were used after a rough pre-filtration step, carried out through a grade qualitative filter paper (Whatman) removing large suspended solids and filtered using a hydrophilic 0.45 μm filter Sartolon Polyamide (Sartorius Biolab).

Preparation of MPs/HA

Humic coated magnetite particles were prepared by co-precipitation method under nitrogen atmosphere to avoid the Fe 2+ oxidation, as previously reported [28]. Briefly, 35 mL of a solution of FeCl 3 ⋅6H 2 O 0.68 M and FeSO 4 ⋅7H 2 O 0.43 M (molar ratio Fe(III)/Fe(II) = 1.5) were added to 65 mL of deoxygenated water at 90 °C, under vigorous mechanic stirring and N 2 continuous flow. Then 10 mL of ammonium hydroxide (25 %) and 50 mL of 0.5 wt.% HA solution was added rapidly and sequentially. The mixture was dried for 30 min at 90 °C, and then kept to cool to r.t. under continuous nitrogen flow. The obtained MPs (Fe 3 O 4 /0.5HA) were centrifuged and washed with 40 mL water five times. The MPs were then dried in a Tube Furnace under nitrogen flow at 80 °C for 15 h and manually crumbled. The bare Fe 3 O 4 MPs were prepared with the same procedure without the addition of HA solution.

The selected loading of HA demonstrated the best reactivity both in the dark and under irradiation. For the in-depth characterization of these MPs refer to [28].

Methods

Degradation experiments under UVA/UVB irradiation

Irradiations were performed in a cylindrical Pyrex reactor, placed in a rectangular box equipped on the top with four lamps (Sankio denki G15T8E) emitting mainly in the UVA and UVB region. The emission spectrum was recorded using an optical fiber coupled with a CCD spectrophotometer (Ocean Optics USD 2000) calibrated with a Deuterium Tungsten Halogen reference lamp. A total irradiance between 265 and 400 nm of 2150 μW cm -2 was measured (see Fig. S1).

The experiments were performed at room temperature using a circulation cooling system. 50 mL of 100 mg/L MPs suspension and 20 μM BPA at adjusted pH with H 2 SO 4 were sonicated for 20 min, then the H 2 O 2 or Na 2 S 2 O 8 solutions were added. The suspension was magnetically stirred and placed under irradiation. At defined time interval 3 mL were sampled and the reaction immediately quenched with 200 μL of methanol. The samples were filtered using a 0.45 μm filter before the analysis.

The catalyst reusability experiments were performed by recovering the catalyst after 3 h under irradiation using a magnet. The recovered catalyst was 3 times washed and then dried at room temperature before being used in the next run.

The experiments with STPW were performed on BPA-spiked solutions. STPW samples had 14.25 mg C L -1 of total organic carbon (TOC), 34.16 mg C L -1 of inorganic carbon (IC), 7.74 mg N L -1 of total nitrogen (TN) and pH value of 7.3.

Analysis

The concentration of BPA was measured through an ultra-performance liquid chromatography (UPLC) AQUITY (Waters, USA) equipped with photodiode array detector and by using a C18 AQUITY UPHLC BEH column (2.1 × 100 mm, 1.7 μm). The flow rate was 0.3 mL min -1 and the mobile phase was a mixture water methanol (60/40, v/v). The BPA concentration was quantified at 220 nm.

TOC, IC and TN were measured using a Shimadzu TOC-5000 analyzer (catalytic oxidation on Pt at 680 °C).

The determination of Fe released in solution was evaluated by a spectrophotometric procedure as reported in the Supplementary material (hereafter SM).

Results and discussion

H 2 O 2 and S 2 O 8 2-activation

In order to evaluate the role of HA coating on the H 2 O 2 and S 2 O 8 2activation during the degradation of BPA, experiments were run by comparing the BPA degradation in the presence of both the hybrid MPs and the bare magnetite at different pH values. In all cases the BPA disappearance followed a pseudo-first order kinetics with an exponential decay (C/C 0 = exp(-k×t), where C and C 0 are the BPA concentration at the irradiation time t and before irradiation, k is the pseudo-first order kinetic constant). Fig. 1a shows the BPA disappearance in the presence of H 2 O 2 . It was observed an enhancement of the activity promoted by the HA coating for all pH. In detail, at pH 3 the complete abatement of BPA was observed after 2 h in the presence of MPs/HA (k = 0.034 ± 0.003 min -1 ), while with the bare Fe 3 O 4 only 80 % abatement was achieved (k = 0.015 ± 0.001 min -1 ), in agreement with results previously described [28]. A comparable tendency was observed in the presence of persulfate, as can be observed in Fig. 1b (e.g. = 2.344 ± 0.096). In the presence of H 2 O 2 the BPA abatement was considerably slower for pH higher than 4, while the effect on the persulfate system was less significant. In fact, even at pH 6 after 2 h under irradiation, 80 % of BPA degradation was achieved 2 2 = 2.47 ± 0.08). This indicates that the investigated hybrid catalyst did not show any pure photocatalytic properties, while its activity was strictly related to its activation ability towards H 2 O 2 and S 2 O 8 2-, as shown in Fig. S2.

It is well established that the pH adjustment and its subsequent neutralization, before water discharge, is responsible for an important part of the cost on the Fenton-like processes [29]. Keep the system working at circumneutral pH can be determining for the process costeffectiveness allowing the feasibility for its application as water treatment. Considering this and in the light of possible in real scenario applications, the further persulfate experiments were performed at pH 6 while in the case of hydrogen peroxide at pH 3 to keep the system active (at pH 6 the H 2 O 2 activation by Fe 3 O 4 /0.5HA was negligible both in the dark and under irradiation, see Fig. 1a).

Contribution of homogeneous catalysis

It has been reported that the contribution of homogenous (photo)-Fenton promoted by Fe(II) and Fe(III) ions released in solution by magnetite-based materials can have a not negligible effect in the abatement of organic substrates [30]. In this work the influence of irradiation on the iron release (concentration and redox speciation) was evaluated by determining the concentration of iron in solution, keeping the solution in the dark or under irradiation at pH 3, 4 and 6. Fig. 2 shows the Fe(II) and Fe(III) concentration spectrophotometrically determined. At time zero (after the sonication period) it was observed a higher amount of Fe(II) with respect to Fe(III) in agreement with the absence of H 2 O 2 -able to oxidize Fe(II) to Fe(III) -and the higher solubility of Fe(II) with respect to Fe(III) (pKs Fe(OH)3 = 38.8, pKs Fe(OH)2 = 15.1 [START_REF]Quantitative Chemical Analysis[END_REF]). In the dark it was not observed any increase of the released Fe(II) over time, while under irradiation the release of Fe (II) and Fe(III) (less marked, and only at pH 3) was more obvious. The photo-dissolution of the magnetite-based materials under irradiation has been largely reported as a consequence of the formation of surface complexes which may trap e CB -and (or) h VB + efficiently, thus enhancing the photodissolution. Spinel type oxides (as the catalyst here tested), such as γ-Fe 2 O 3 (maghemite) or Fe 3 O 4 , (magnetite) are known to be more easily photocorroded than hematite [START_REF] Litter | [END_REF]. The solubility decrease of iron (hydr)oxides can explain the lower Fe(II) and Fe(III) ions concentration observed for the systems at higher pH in the absence of effective ligands for both Fe(II) and Fe(III).

The mechanism of the photodissolution of the catalyst is ultimately quite complex and the data here reported can only partially clarify it. The organic coating has an active role in the process especially because the carboxylic moieties at the surface are effective ligands for Fe +2 and Fe +3 . Moreover, these iron complexes at the surface are not only redox active species able to react with hydrogen peroxide or persulfate, but they are also photoactive species. Note that, the spectrophotometric method used to determine the iron concentration in solution was carried out on filtered solutions and consequently is silent on the concentration and speciation of the iron complexed by organic ligands at the surface of the catalyst.

The contribution of the iron released in solution on the initial BPA degradation was evaluated as follows: BPA and H 2 O 2 /S 2 O 8 2-were added to the supernatant solution after removing the heterogeneous catalyst by filtration. To evaluate the effect of the iron photo-dissolution, degradation experiments were performed with supernatant at time zero, but also with the solution after 2 h under irradiation. Fig. 3 shows the BPA degradation on supernatant at pH 3 after adding H 2 O 2 (Fig. 3a) and on supernatant at pH 6 after S 2 O 8 2-addition (Fig. 3b). For both systems it was observed the BPA degradation in the homogenous phase.

The addition of H 2 O 2 to the pH 3 supernatant obtained after 2 h under irradiation allows to achieve a higher BPA degradation comparing with the corresponding supernatant at t 0 . The higher concentration of iron resulted from the photo-dissolution of the catalyst. This tendency was not observed with persulfate. In fact, the supernatant at t 0 allowed to obtain higher BPA degradation than the supernatant after 2 h under irradiation. The low and stable over the time concentration of iron observed at pH 6 (Fig. 2), suggested an equal production of reactive species during the experiments with the two supernatants. The decreased rate of transformation observed with the supernatants obtained after 2 h of irradiation, can be explained considering the quenching effect of the HA that were partially released in solution from the organic coating, as previously reported with similar materials [33,34]. This was sustained by the negative effect observed on the BPA degradation when the direct photolysis of persulfate was carried out in solution containing HA (0.4 mg/L -the same HA amount added with 100 mg/L of catalystconsidering the HA loading equal 0.4 % w/w) which acted as efficient scavengers of reactive species (see Fig. 3).

Identification of reactive species

The reaction of H 2 O 2 and S 2 O 8 2-with iron (in solution and/or at the catalyst surface, in the dark or under irradiation) can promote the formation of both SO 4 %-and % OH according to the following reactions:

Fe 2+ + S 2 O 8 2-→ Fe 3+ + SO 4 2-+ SO 4 % - (1)
SO 4 %-+ OH -→ SO 4 2-+ % OH (relevant only under basic conditions)

(2)

SO 4 %-+ H 2 O → HSO 4 -+ % OH (3) Fe 2+ + H 2 O 2 → Fe 3+ + OH -+ % OH (4) Fe 3+ + H 2 O 2 → Fe 2+ + HO 2 % + H + (5) Fe(OH) 2+ + hν → Fe 2+ + % OH (6)
Both % OH and SO 4 %-can react not only with the contaminants, but also with the oxidants (Eqs. 7 and 9) which, at high concentration, are able to scavenge efficiently the reactive species depressing the overall reaction rate

SO 4 %-+ S 2 O 8 2-→ S 2 O 8 %-+ SO 4 2- ( 7
)
% OH + S 2 O 8 2-→ S 2 O 8 %-+ OH - (8)
% OH + H 2 O 2 → H 2 O + HO 2 % (9)
Furthermore, under UVA/UVB irradiation the overlap between the emission spectrum of the adopted lamps and the absorption spectra of H 2 O 2 and S 2 O 8 2-can promote the photolysis of both the oxidants [35].

→ S O 2 S O UVA UVB 2 8 2- / 4 •- ( 10
) → H O 2O H UVA UVB 2 2 / • (11)
To understand the role of the generated reactive species involved in the investigated degradation processes, BPA photodegradation was carried out in the presence of radical scavengers (see Fig. S3). Isopropanol was used due to the high reactivity with hydroxyl radical ( = × -k Ms 1. 9 10 ) [37], the BPA degradation was carried out also in the presence of t-butanol that is a more selective quenching of 4 10

HO isop •, 9 1
% OH ( = × - -- k M s 3.1 10
HO t but •, 9 1 1 ), than of SO 4 %- ( = × - -- - k M s 8.
SO t but , 5 1 1 4 •
) [36,37]. Fig. S3 shows an only limited inhibition of the BPA degradation in the presence of t-butanol demonstrating that the degradation is mainly promoted by SO 4 % -.

Effect of H 2 O 2 and S 2 O 8 2-concentration

To elucidate the effect of H 2 O 2 concentration on the catalytic system, experiments were performed by adding different H 2 O 2 concentration (0.5, 1, 2, 3, 5 and 10 mM) at pH 3 (Fig. 5a) and pH 4 (Fig. 5b) in the presence of 100 mg/L of catalyst. The inset of Fig. 4a shows a quite complex behavior for the BPA initial transformation rate at pH 3, with an increment of the BPA degradation rate up to 1 mM, then a sharp decrease and for [H 2 O 2 ] > 3 mM a further increase. While at pH 4 the highest degradation was achieved at 2 mM, but similar rates were observed for higher concentrations (see inset of Fig. 5a). This tendency can be only partially explained by the scavenging effect of H 2 O 2 (Eq. 9) as often reported in the literature [38,39], because the decrease of the initial rate as a consequence of the increment of the oxidant concentration can justify a bell-profile and not a profile with a maximum and a minimum as that observed at pH 3. At high concentration of H 2 O 2 a further process different to the photo-Fenton one (e.g. the direct photolysis of H 2 O 2 which is a first order process with respect to hydrogen peroxide, and consequently increased linearly with the increase of the H 2 O 2 concentration) might sustain the overall transformation compensating the scavenge of the % OH, formed through the reactions 4 and 7, by H 2 O 2 .

The BPA degradation was performed as well at different persulfate The recorded profiles are the results of a network of reactions composing the Fenton heterogeneous process (activated by H 2 O 2 of S 2 O 8 2-) which complexity has been already evidenced in some previously published articles [40][41][42][43][44]; for instance among the experimental parameters also the wavelength range of the irradiation showed to play a not negligible role. Note that too high concentrations of S 2 O 8 2-must be avoided because the higher is the concentration of persulfate, the higher is the concentration of sulfate (the last product of the persulfate activated processes) in the final treated water. As an example, the Italian legislation imposed for the discharge of water in surface bodies a concentration of SO 4 2-< 1000 mg/L (10.4 mM) [45]. This value might be easily exceeded by treating the wastewater with persulfate at concentration higher than 5 mM (in this case the water should be furtherly treated after the heterogeneous photo-Fenton process to remove the excess of sulfate before the discharge). Carrying out the photo-Fenton process with H 2 O 2 the same problem might be met. It is true that if H 2 O 2 reacts completely the final products are environmental benign, but the need to carry out the photo-Fenton process at pH 3 and the successive phase of neutralization give very salted water that must be further treated before the discharge.

Catalyst recover and reuse

The catalyst reusability was tested by magnetically recovering the catalyst at the end of the BPA degradation. The catalyst was washed with water and dried before use for the next run. Fig. S4 shows the BPA abatement in 3 consecutive runs in the presence of H 2 O 2 at pH 3 and S 2 O 8 2-at pH 6. At pH 6 in the presence of S 2 O 8 2-no decrease of activity was observed, while at pH 3 in the presence of H 2 O 2 a very slight decrease on BPA abatement was observed. In both cases the tested catalyst did not show a significant deactivation/poisoning with the use.

Effect of water matrix composition

The order kinetics. The initial rate in real matrix was higher than in ultrapure water, but after 30 min the process slowed down giving after 3 h a lower degradation than in ultrapure water. A hypothesis is that the presence of organic species able to act as iron ligands gave an initial higher concentration of iron in solution able to react with H 2 O 2 , then the higher concentration of organic/inorganic scavengers partially block the reaction. With persulfate, the BPA degradation followed in both cases a pseudo-first order kinetics, but due to the high concentration of organic and inorganic scavengers [46,47] ) [35].

Conclusions

Coating F 3 O 4 with HA efficiently induced higher H 2 O 2 and persulfate activation for the BPA removal at different pH, comparing with the pristine magnetite. The Fe 3 O 4 /0.5HA showed high capability to activate H 2 O 2 at pH 3 with a substantial decrease of activity at pH higher than 4, while in the presence of persulfate even at pH 6 the system was still able to efficiently remove BPA. Both with H 2 O 2 and S 2 O 8 2-, the heterogeneous photo-Fenton process here investigated was operational in real wastewater, even if with lower kinetics than in ultrapure water.

The main reactive species involved in the process catalyzed by

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  using S 2 O 8 2-(despite ∼15 % with H 2 O 2 ). The initial rates recorded in all the experiments of Fig. 1 are reported in

Fig. 1 .Fig. 2 .

 12 Fig. 1. BPA photo-degradation with 1 mM H 2 O 2 (a), and 1 mM of S 2 O 8 2-(b) in the presence of bare Fe 3 O 4 or Fe 3 O 4 /0.5HA (100 mg/L) at different pH. C 0 BPA

Fig. 3 .

 3 Fig. 3. BPA photodegradation in supernatant solutions obtained at time zero (T 0 ) and after 2 h of irradiation in the presence of 100 mg/L Fe 3 O 4 /0.5HA with H 2 O 2 1 mM at pH 3(a) and S 2 O 8 2-1 mM at pH 6 (b); BPA degradation with S 2 O 8 2-1 mM and HA 0.4 mg/L).

Fig. 4 .

 4 Fig. 4. Degradation profiles of BPA under irradiation with increasing concentration of initial H 2 O 2 in the presence of Fe 3 O 4 /HA (100 mg/L) at pH 3 (a) and pH 4 (b). Inset of figure (a): initial BPA degradation rate as a function of the H 2 O 2 concentration.

4 %-

 4 Fe 3 O 4 /0.5HA were % OH (in the presence of H 2 O 2 ) and SO (in the presence of S 2 O 82-).The recovery experiments highlighted no significant loss of activity with the reuse of the material both with H 2 O 2 at pH 3 and S 2 O 8 2-at pH6.With H 2 O 2 (at pH 3) the photo-Fenton process in the presence of Fe 3 O 4 /0.5HA gave always better results than the "pure" photolysis of hydrogen peroxide, while with S 2 O 8 2-the presence of the catalyst gave a positive effect in comparison with the persulfate photolysis at [have to be avoided to prevent the production of final water with a sulfate concentration exceeding the law limits). At the same time, the photo-Fenton process at pH 3 can give after the final neutralization very salted water that must be further treated before the discharge. CRediT authorship contribution statement Nuno P.F Gonçalves: Investigation, Writing -original draft. Marco Minella: Conceptualization, Writing -review & editing, Supervision. Gilles Mailhot: Writing -review & editing. Marcello Brigante: Supervision, Writing -review & editing. Alessandra Bianco Prevot: Conceptualization, Writing -review & editing, Funding acquisition.

  Table 1 of SM. No significant BPA degradation was observed under direct photolysis or in the presence of Fe 3 O 4 /0.5HA under irradiation. On the contrary, both the photolysis of persulfate and H 2 O 2 in the absence of the catalyst allowed a partial degradation of BPA, faster with S 2 O 8

			2-
	than with H 2 O 2 (	k k S O H O 2 8 2	-

  1 )[36]. With H 2 O 2 we observed an almost

	complete inhibition of BPA degradation ( k k isop H O H O 2 2 2 2 = 35.0 ± 0.12) in-dicating that the degradation is mainly due to the % OH radicals.
		The BPA degradation is strongly inhibited by isopropanol also in the
	case of S 2 O 8 2-( are able to	--2 = 11.06 ± 0.09). However, since isopropanol 2 2 8 S O S O 2 8 isop k k quench both % OH and SO 4 -% radicals
	( k	SO 4 •	-,	isop	=	8.6	×	--Ms 7 10 1 1

  -1 dissolved inorganic carbon, pH = 7.6 and 0.14 μS cm -1 conductivity. It is quite difficult on the basis of the composition of the water only, give more insights into the role that the specific components of the matrix have on modifying the rate of BPA transformation. Nevertheless, it is possible to hypothesize that the presence of hydroxyl and sulfate radical scavengers can limit the overall degradation. For instance, it is well know that organic matter is an important radical scavenger[35]. Moreover, the presence of inorganic carbon can lead to the formation of carbonate radicals (CO 3 %-), less reactive towards BPA than % OH and SO 4

														in the STPW the
	rate was roughly 4 times lower ( sample used to simulate a real-word scenario had 2.7 mg C L -1 dissolved ---k k S O 2 2 8 Milli Q S O 2 = 4.43 ± 0.10). The STPW 2 8 STPW
	organic carbon, 41.1 mg C L % -as previously reported
	(	k	HO BPA , •	=	8.6	×	--Ms 9 10 1 1	;	k	-SO BPA , 4 •	=	4.7	×	--Ms 9 10 1 1	;	k	-CO BPA , 3 •
				=	9.2	×	--M s 7 10 1 1

Acknowledgements

This work is part of a project that has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 765860. Authors also acknowledge the financial support for experiments provided by the Region Council of Auvergne, from the "Féderation des Recherches en Environnement" through the CPER "Environment" founded by the Region Auvergne, the French government, FEDER from the European Community and from CAP 20e25 I-site project.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.12.028.


E Martin

F Roulland

S Grenier

F Appert

J Juraszek

M Trassin

C Bouillet

E Chikoidze

C Arnold

B Berini

Y Dumont

S Colis

S Barre

G Versini

D Preziosi

C Leuvrey

N Blanc

N Boudet

G Pourroy

N Viart

C Lefèvre

Non-auxetic/auxetic transitions inducing modifications of the magnetic anisotropy in CoFe 2 O 4 thin films

An auxetic behaviour is evidenced in CoFe2O4 thin films grown by pulsed laser deposition on (100) MgO substrates under various O2/N2 pressures. This rare behaviour for an intrinsic material is observed for intermediate oxidation conditions, in between two non auxetic domains delimited by high (>0.05 mbar) and low (<0.03 mbar) O2/N2 deposition pressures. Combining X-ray resonant diffraction and Mössbauer spectroscopy, we experimentally prove that the auxetic behaviour is related to the presence of cobalt ions in the tetrahedral sites. The impact of the structural modifications caused by the various oxidation conditions on the electronic and magnetic properties are studied for the various oxidation domains. Variations as important as a transition from a p-type semiconducting to an insulating behaviour, or from an in-plane to an out-of-plane magnetization are observed when spanning from the lowest (0.01 mbar) to the highest (1 mbar) studied oxidation pressures.

Introduction

Auxetics are peculiar materials that have a negative Poisson's ratio and expand (resp. shrink) under uniaxial tension (resp. compression) in the direction transverse to the load [1]. Such materials have gained considerable interest from the scientific community over the last decade thanks to their potential applications as stents [2], orthopedic bone plates [3] or skin grafts [4] in medicine, as energy absorbers in sports [5] or crashworthiness situations [START_REF] Zhang | Dynamic crushing responses of bioinspired re-entrant auxetic honeycombs under in-plane impact loading[END_REF][START_REF] Guo | Deformation behaviors and energy absorption of auxetic lattice cylindrical structures under axial crushing load[END_REF], as high sensitivity sensors [START_REF] Taherkhani | Highly sensitive, piezoresistive, silicone/carbon fiber-based auxetic sensor for low strain values[END_REF][START_REF] Rong | Electric field modulation in the auxetic effect of BP-analog monolayer As and Sb by firstprinciples calculations[END_REF], or as highly tunable molecular sieves [START_REF] Li | Negative and near-zero Poisson's ratios in 2D graphene/MoS2 and graphene/h-BN heterostructures[END_REF]. The archetype of the auxetic structure is the re-entrant honeycomb network, thanks to its peculiarly designed porosity [START_REF] Zhang | Dynamic crushing responses of bioinspired re-entrant auxetic honeycombs under in-plane impact loading[END_REF][START_REF] Mir | Review of Mechanics and Applications of Auxetic Structures[END_REF][START_REF] Wei | A novel star auxetic honeycomb with enhanced in-plane crushing strength[END_REF][START_REF] Hu | Thermal shock resistance behavior of auxetic ceramic honeycombs with a central crack or an edge crack[END_REF]. Most of the known auxetic materials are artificially designed metamaterials [START_REF] Gatt | Hierarchical Auxetic Mechanical Metamaterials[END_REF] or composites which can be patterned through 3d printing or weaving [3,[START_REF] Agnelli | Design and testing of 3D-printed micro-architectured polymer materials exhibiting a negative Poisson's ratio[END_REF][START_REF] Quan | 3d printed continuous fiber reinforced composite auxetic honeycomb structures[END_REF][START_REF] Chen | Structural design and characterization of highly elastic woven fabric containing helical auxetic yarns[END_REF][START_REF] Kamrul | Deformation behavior of auxetic woven fabric based on reentrant hexagonal geometry in different tensile directions[END_REF]. Few materials are intrinsically auxetic at the molecular level, because the structure has to feature interior empty spaces to behave in an auxetic way [START_REF] Han | Two nature-mimicking auxetic materials with potential for high energy absorption[END_REF][START_REF] Yeganeh-Haeri | Elasticity of α-Cristobalite: A Silicon Dioxide with a Negative Poisson's Ratio[END_REF][START_REF] Huang | Negative Poisson's Ratio in Modern Functional Materials[END_REF][START_REF] Gatt | A force-field based analysis of the deformation mechanism in α-cristobalite[END_REF]. Furthermore, as shown in the recent review of R. Peng et al., 2D auxetic materials are rather rare and the huge majority are non-oxides [START_REF] Peng | Two-dimensional materials with intrinsic auxeticity: progress and perspectives[END_REF]. Recently, such intriguing auxetic behaviours have been observed in perovskites [START_REF] Chen | Modulation of Abnormal Poisson's Ratios and Electronic Properties in Mixed-Valence Perovskite Manganite Films[END_REF]or spinels thin films. Hoppe et al. have for example observed the coexistence of enhanced saturation magnetization with an auxetic behaviour in NiFe2O4 ultrathin films [START_REF] Hoppe | Enhanced ferrimagnetism in auxetic NiFe 2 O 4 in the crossover to the ultrathin-film limit[END_REF]. Epitaxial CoFe2O4 (CFO) thin films grown on SrTiO3 (STO) substrates displayed an auxetic behaviour accounted for by the hinge-like 3D honeycomb network formed by the oxygen sublattice [START_REF] Valant | Molecular Auxetic Behavior of Epitaxial Co-Ferrite Spinel Thin Film[END_REF]. This observation was however questioned by Foerster et al. who observed no auxetic behaviour, but simply an anomalously low Poisson's ratio (ν) for thin films of thicknesses ranging from 4 up to 150 nm, grown on STO or on MgAl2O4 (MAO) [START_REF] Foerster | The Poisson Ratio in CoFe 2 O 4 Spinel Thin Films[END_REF]. This subject has received intensive consideration because the elastic properties are in close connection with the electronic and magnetic properties and the source of new opportunities to access to new materials properties. CFO is a wellknown magnetic material involved in spintronic devices with a high Néel temperature (T N = 793K) [START_REF] Teillet | Magnetic structure of CoFe 2 O 4[END_REF], an important saturation magnetization (Ms = 200 emu/cm 3 ), a large magnetic anisotropy (K = 200 kJ/m 3 ) [START_REF] Valenzuela | Magnetic Ceramics[END_REF], and good corrosion resistance and mechanical stability [START_REF] He | Reduced graphene oxide-CoFe2O4 composites for supercapacitor electrode[END_REF][START_REF] Wang | One-step accurate synthesis of shell controllable CoFe 2 O 4 hollow microspheres as high-performance electrode materials in supercapacitor[END_REF]. Recently, Ferreiro-Vilas et al. reported tuning the Poisson's ratio in CFO through the deposition method (pulsed laser deposition (PLD) vs. polymer assisted deposition (PAD)) [START_REF] Ferreiro-Vila | Apparent auxetic to non-auxetic crossover driven by Co 2+ redistribution in CoFe 2 O 4 thin films[END_REF]. While the samples grown by PLD show positive ν values, samples grown by PAD have negative Poisson's ratios. The authors proposed a theoretical model stating that the auxetic behaviour of CFO is related to the distribution of Co 2+ ions among the octahedral (Oh) and tetrahedral (Td) sites of the spinel structure, as a result of its accommodation of the epitaxial stress.

In this paper we explore the structural, electronic and magnetic behaviours in cobalt ferrite (CoFe2O4) spinel thin films according to the O2/N2 deposition pressure. We bring new information concerning this compound already very studied though and show that it is possible to modulate the mechanical behaviour of the films from auxetic to non-auxetic, the electronic properties from insulating to semiconducting, and the magnetic anisotropy form in-plane to out-of-plane, by tuning the oxidizing deposition atmosphere in a pulsed laser deposition experiment. We present here the experimental proof of the cationic redistribution in the spinel structure when the unusual auxetic behaviour is observed. While both the low and high pressure elaborated samples have a structure close to an inverse spinel, ca. ~1/4 of the tetrahedral sites are occupied by Co 2+ for samples elaborated in the intermediate pressure range, between 0.03 and 0.05 mbar.

Materials and methods

Sample preparation

CFO thin films of 60 nm in thickness were elaborated by pulsed laser deposition (PLD) using a KrF excimer laser with a wavelength of 248 nm, a repetition rate of 10 Hz and an energy density on the target of 1 J/cm 2 . The distance between the target and the substrate was fixed to 5 cm and the substrate temperature to 400°C. After deposition, the films were left to cool down to room temperature under the deposition pressure. The films were deposited on MgO (100) substrates (Crystal GmbH) from a ceramic CFO target in the range of [0.02 -1] mbar of O2:N2. CFO (a=8.39 Å) has a cell parameter approximately double that of MgO (a=4.21 Å), and the growth is expected to be cube-on-cube with a mismatch of 0.4 %. The CFO target was synthesized using a stoichiometric mixture of Fe2O3 (Strem Chemicals 99.8%) and Co3O4 (obtained in the laboratory by calcination of cobalt carbonate CoCO3, xH2O, Aldrich). The mixture of precursors was ball milled in a teflon jar using 1 mm diameter zircon balls in an NH3 aqueous solution. The powder was finally compacted into a 26 mm diameter pellet, and sintered in air, at 1200°C for 20 h in a platinum crucible.

Experimental methods

The Fe/Co = 2 ratio was checked by energy dispersive X-ray spectrometry (EDX). The structural (θ-2θ and reciprocal space mapping (RSM)) characterization of the thin films was carried out using a Rigaku Smart Lab diffractometer operating with a cupper rotating anode (Kα1 = 0.154056 nm).

The surface and the composition of the films were studied by scanning electron microscopy (SEM) using a JEOL 6700F microscope equipped with EDX analysis. The iron to cobalt ratio in all thin films was equal to 2. Atomic force microscopy (AFM) was performed in order to have a quantitative estimate of the roughness of the sample surface. Scans of 5x5 µm 2 were performed on different locations on each sample in order to check the sample homogeneity. Lamellae of the thin films cross sections were prepared by focused ion beam (FIB) micromachining. The observations were performed using a transmission electron microscope (TEM) JEOL 2100 FCs corrected for spherical aberration at the condenser level, with a 0.2 nn point to point resolution in TEM and 0.12 nm in scanning TEM mode (STEM). The electrical measurements were performed by using a KEITHLEY 4200-SCS semiconductor characterization system. Four electrical contacts in Ti/Au were RF sputtered on square shaped samples and two points I-V curves were recorded. A special option called "TORNADO" was developed by coupling a PPMS environment with a KEITHLEY 4200-SCS. The range of current was extended down to 10 -10 A, the voltage up to 200 V.

"TORNADO" gives the possibility to measure the resistivity, and the Hall Effect in the Van der Pauw configuration for samples with resistance up to 10 10 Ω. Magnetic isotherm hysteresis loops were recorded at room temperature with a SQUID apparatus operating up to 7 T. DANES (Diffraction Anomalous Near Edge Structure) measurements were performed at both the Fe and Co edges (7.11 and 7.71 keV, respectively). The synchrotron experiments were carried out on the collaborating research group (CRG) D2AM beamline at the European Synchrotron Radiation Facility. Samples were mounted on a seven-circle diffractometer equipped with various photodiode detectors, allowing recording the intensity of the incident beam I0, the intensities of the diffraction peaks and the fluorescence of the samples. The rotation matrix was preliminary determined with the help of different in-plane and out-of-plane reflections. Anomalous factors f' and f'' were extracted from the fluorescence data. Knowing f0, f' and f'' then allows the simulation of the scans for a given set of structural parameters. The refinement of the experimental spectra was performed by minimizing the sum of all reliability factors for all reflections and edges through a stochastic basinhopping algorithm (FitREXS program [START_REF] Lefevre | Determination of the cationic distribution in oxidic thin films by resonant X-ray diffraction: the magnetoelectric compound Ga 2-x Fe x O 3[END_REF]). Finally, 57 Fe Mössbauer spectra were measured using the conversion electron Mössbauer spectroscopy technique (CEMS) on ~100% 57 Fe isotope enriched CFO samples. The spectra were recorded under normal incidence using a gas-flow proportional counter mounted inside a closed cycle Janis cryostat. A mixture of He-5% CH4 gas was used at RT, whereas pure He gas was used at 80 K. The radioactive source was 57 Co in a Rh matrix displaced in constant acceleration mode, with an activity of about 1,8 GBq. The spectra were fitted with a least-square technique using the histogram method (MOSFIT program). Isomer shifts are given with respect to -Fe at 300 K.

Results and discussion

Structural, magnetic and electrical characterizations of the CFO thin films

The oxygen-pressure-dependence of the structural properties of CFO films grown on MgO (100) were investigated by X-ray diffraction experiments. θ-2θ scans with a scattering vector parallel to the normal of the surface were used to analyse the crystalline structure of the films (Fig. 1). All scans show the expected (002) and (004) reflections of the (100)-oriented MgO substrate, as well as two other peaks corresponding to the (004) and (008) reflections of CFO. Since no other diffraction peak is observed, we conclude to a (00l) oriented growth of the cobalt ferrite. (Fig. 1 and its top inset). The 2θ values corresponding to (004) and (008) CFO peaks increase when the pressure is increased, as shown in Fig. 1 for P=0.05, 0.5 and 1 mbar. Combination of both θ-2θ and RSM (reciprocal space mappings) measurements allow a complete determination of cell parameters as a function of the deposition pressure (Fig. 2). The in-plane lattice parameters were determined by RSM (Fig. 1). The zone was chosen in order to observe both the MgO (024) and CFO (048) nodes that present a four-fold symmetry. The proximity between the (024) node of the substrate and the (048) node of CFO allows an easy observation of the in-plane CFO lattice strain. The thin film and the MgO substrate display a perfect epitaxy at high pressure (Fig. 1 bottom inset), unlike the films grown at low pressure. The two in-plane cell parameters are identical for all deposition pressures and will be labeled a// in the following.

The variation of the out-of-plane lattice parameter, labeled a┴ in the following, is displayed in Fig.

2.

In the [0.05-1] mbar range, it takes lower values than the bulk one as already observed in other oxides, e.g. Y3Fe5O12 or Ba0.5Sr0.5TiO3 [START_REF] Dumont | Tuning magnetic properties with off-stoichiometry in oxide thin films: An experiment with yttrium iron garnet as a model system[END_REF][START_REF] Wang | Effects of oxygen pressure on lattice parameter, orientation, surface morphology and deposition rate of (Ba 0.02 Sr 0.98 )TiO 3 thin films grown on MgO substrate by pulsed laser deposition[END_REF]. For pressure below P=0.05 mbar (Fig. 2), the outof-plane parameter is higher than the bulk one and increases strongly up to 8.62 Å for P=0.01 mbar.

The cell parameters a// and a┴ are characterized by a monotonic but opposite evolution with pressure. The CFO unit cell undergoes a tetragonal distortion for all deposition pressures, except for the critical pressure Pc ~ 0.04 mbar, for which it is perfectly cubic. Above 0.05 mbar, the in-plane parameter a// is equal to 8.42 Å that is twice the lattice parameter of the substrate (aMgO = 4.21 Å).

The film perfectly matches onto the substrate as illustrated by the RSM in Fig. 1. The cell expands laterally due to the in-plane substrate induced tensile strain. For P ≤ 0.03 mbar, the films no longer adopt the parameters of the substrate and the ratio a┴/a// is greater than 1. Finally, in the intermediate pressure range [0.03-0.05 mbar], a┴ and a// are both higher than the bulk CFO lattice parameter, and despite the in-plane tensile strain of the films, the cell parameter expands in the out-of-plane direction. The variation of the films cell parameters with the deposition pressure thus allows to delimit three domains of pressures: a low pressure one, P ≤ 0.03 mbar, labelled LP in the following, an intermediate one, between 0.03 and 0.05 mbar, labelled IP and a high pressure one P ≥ 0.05 mbar, labelled HP. The Poisson's coefficient ν illustrates the unusual behaviour of IP samples. For thin films under 2D in-plane stress, ν is expressed by:

v = a bulk -a ⊥ 2 a ∥ -a ⊥ -a bulk
where abulk=8.392 Å (JCPDS 22-1086). While both LP and HP samples show a positive ν value, the IP sample shows a negative ν value characteristic of an auxetic behaviour (Table 1) [START_REF] Foerster | The Poisson Ratio in CoFe 2 O 4 Spinel Thin Films[END_REF][START_REF] Ferreiro-Vila | Apparent auxetic to non-auxetic crossover driven by Co 2+ redistribution in CoFe 2 O 4 thin films[END_REF].

Microscopy techniques provide insights on the surface topography and on the effect of the tensile strain on the CFO thin film structure. AFM observations of the surface are shown in Fig. 3. The root mean square (RMS) surface roughness of the 0.02 mbar sample (LP) is equal to 0.1 nm indicating a low roughness related to the small mismatch between MgO and CFO. The AFM images of the HP sample display fracture lines, possibly assigned to a relaxation phenomenon along the crystallographic planes. Since this sample shows a perfect matching with the substrate, it therefore endures a 0.4% tensile strain (aCFO bulk = 8.392 Å, aCFO films = 8.43 Å), and some local relaxation may occur periodically through the fracture lines. HRTEM observations of cross sections have pointed out dislocations within the thin film. Red circles indicate the observed dislocations on the Fourier transform of the HRTEM observation given in Fig. 4. While both IP and HP samples show a quite similar density of dislocation, the LP thin film exhibits a value two to three times higher, that could be ascribed to the high degree of distortion of the unit cell (Table 1). Some information on the magnetic properties of the CFO films may be determined from the room temperature hysteresis loops performed for the three pressure domains (Fig. 5). The LP samples show an in-plane easy magnetization axis, with coercive field values of about 3500 Oe and 300 Oe for the in-and out-of-plane measurements, respectively. The IP sample shows no preference for inor out-of-plane easy magnetization direction, with comparable coercive field values for the two configurations. Finally, the HP sample exhibits an out-of-plane easy magnetization axis. The anisotropy constants, i.e. the magnetoelastic anisotropy (Kme) arising from the coupling between the magnetic moments and the lattice and the shape anisotropy (Kshape) have been calculated (see Supplementary information = SI). According to Lisfi et al [START_REF] Lisfi | Reorientation of magnetic anisotropy in epitaxial cobalt ferrite thin films[END_REF], the contribution of the surface anisotropy (Ksurf) is negative, since the thicknesses are lower than 300 nm. All the anisotropy constants are negative for low pressure-prepared thin films (LP), leading to negative Ktot value (Table 1). High pressure-prepared samples (HP) have a positive magnetoelastic anisotropy (Kme).

Although the other contributions are negative, the total anisotropy is positive (see SI and Table 1).

These results are in agreement with the in-plane and out-of-plane easy magnetization direction deduced from the magnetic hysteresis loops for LP and HP samples respectively. Finally, the magnetoelastic constant of IP sample is positive and close to zero, much smaller than the HP sample in agreement with the quasi-overlay of out-of-plane and in-plane curves.

Room-temperature I-V curves obtained for characteristic samples of the three pressure domains are displayed in Fig. 6 and the deduced resistivity is given in Table 1. The HP sample shows a very high resistance, exceeding the range of measurable resistances, R> 10 10 Ω. The thermal evolution of resistivity indicates a semi-conducting behaviour for both IP and LP samples. Extraordinary Hall effect measurements recorded on the LP sample at 300 K and 60 kOe (not shown here) evidence positive charge carriers with concentration and mobility equal to p = 1.3×10 14 cm -3 and µ = 5 cm 2 /Vs, respectively. Such a p-type conductivity has already been reported for spinel-type thin films and can be ascribed to the presence of vacancies in the structure [START_REF] Katayama | p-Type Conductivity and Room-Temperature Ferrimagnetism in Spinel MoFe 2 O 4 Epitaxial Thin Film[END_REF][START_REF] Roy | Studies on Some Titanium-Substituted Fe 2 MoO 4 Spinel Oxides[END_REF][START_REF] Seki | Epitaxial thin films of p-type spinel ferrite grown by pulsed laser deposition[END_REF].

Resonant X-ray diffraction at Fe and Co edges

Resonant X-ray diffraction was performed at both the Fe and Co edges in order to determine the cationic occupation of the tetrahedral and octahedral sites. This technique has been shown to be very sensitive to the cationic distribution [START_REF] Lefevre | Determination of the cationic distribution in oxidic thin films by resonant X-ray diffraction: the magnetoelectric compound Ga 2-x Fe x O 3[END_REF]. Fig. 7 shows the simulated spectra for the 224 reflection of (Co x Fe 1-x )[Co 1-x Fe 1+ x ]O 4 as a function of the inversion parameter computed using the FitREXS software [START_REF] Lefevre | Determination of the cationic distribution in oxidic thin films by resonant X-ray diffraction: the magnetoelectric compound Ga 2-x Fe x O 3[END_REF]. The experimental data recorded on LP, IP and HP samples at the D2AM beamline are given in the insets for both Fe and Co edges. According to the simulated spectra, one can notice that the 224 node is very sensitive to the Td site. It is especially the case at the Co edge, for which no transition is expected in the case of purely inverse spinel structure (no Co in the Td sites). Conversely, simulations show that the transition at the Co edge is all the more pronounced when the cobalt content increases in the Td site. The experimental data of the three samples characteristic of the LP, IP and HP domains show a different behaviour at the Co edge.

While no transition is visible for the LP sample, a small transition is observed for the HP sample and an important one is evidenced for the IP sample. The lack of transition at the Co edge for LP sample clearly indicates that the Td site is only occupied by Fe atoms. For the two other samples, refinements of spectra recorded for different nodes of the reciprocal space, have been performed using the FitREXS software [START_REF] Lefevre | Determination of the cationic distribution in oxidic thin films by resonant X-ray diffraction: the magnetoelectric compound Ga 2-x Fe x O 3[END_REF]. Results are shown in supplementary information and Table S1.

The cobalt content of the Td site for the HP sample (0.07) is not zero but much lower than that of the IP sample (0.24). The structures of the IP and HP samples are therefore not perfect inverse spinels, and this is even more pronounced for the IP sample. This result brings additional experimental confirmation to the model exposed by Fereiro-Vila concerning the mechanism for auxetic behaviour in CFO thin films: the auxetic behaviour appears when the degree of inversion is not complete, i.e. when the structure is intermediate between the direct and the inverse spinel structures [START_REF] Ferreiro-Vila | Apparent auxetic to non-auxetic crossover driven by Co 2+ redistribution in CoFe 2 O 4 thin films[END_REF]. The critical cationic distribution above which the auxetic behaviour appears is in the range [0.07 -0.24].

Mössbauer spectrometry

The elongation of the CFO cell perpendicularly to the substrate observed for the low deposition pressures could have been assigned to the presence of Fe 2+ , stemming from the reduction of Fe 3+ under too mild oxidizing conditions. Goodenough has indeed pointed out the Jahn Teller effect that is possibly associated to Fe 2+ [START_REF] Goodenough | Jahn-Teller distortions induced by tetrahedral-site Fe 2+ ions[END_REF]. The Mössbauer spectrometry is the ideal method to evidence the valence of iron. Room temperature (RT) Mössbauer spectra of the LP and IP (as also representative of the HP samples) samples are depicted in Fig. 8 (a) and (b). All spectra exhibit a magnetic sextet with broad lines indicating a distribution of the Fe magnetic environments. They have been fitted with a distribution of hyperfine fields leading to average values of the isomer shift (δ) equal ) equal to 0.30(1) mm/s and 0.31(9) mm/s for LP and IP samples, respectively (Table 2). Those values unambiguously point out to the presence of only the high valence state of iron, Fe 3+ . At low temperature [Fig. 8(c)], the LP spectrum is resolved enough to take into consideration two contributions of refined values of δ) equal equal to 0.38(4) mm/s and 0.48(2) mm/s (Table 3). The refined hyperfine parameters indicate that 51,6 % of the probed iron is located in Td site. This value is close to what observed for the bulk CFO ( Fe Oh /Fe Td ≈1 ) [START_REF] Sawatzky | ossbauer Study of Several Ferrimagnetic Spinels[END_REF]. Another important feature that can be clearly seen on the RT Mössbauer spectra is the lines intensity ratio of the magnetic sextet that is different for LP and IP samples. Information about the direction of the magnetic moments can be obtained from the relative intensities of the Mössbauer absorption lines, which is given by 3:x:1:1:x:3 for pure magnetic hyperfine splitting, where x= 4 sin 2 ⟨ β⟩ 1+ cos 2 ⟨β⟩ and <β> is the average "cone-angle" between the incident -ray direction and the direction of the magnetic hyperfine field.

Complete in-plane spin orientation is indicated by ratios 3:4:1:1:4:3 or <β>=90°, whereas complete perpendicular orientation is present for ratios 3:0:1:1:0:3 or <β>=0°. For the LP sample, a value of <β> close to 90° is obtained from the fitting, confirming the in-plane magnetic anisotropy already deduced from the hysteresis loops measurements. A cone-angle value <β> = 48.9° is observed for the IP sample, evidencing the out-of-plane orientation of some of the Fe spins [Fig. 8(d)], in agreement with the hysteresis data.

Discussion about the chemical formula at low pressure

Assuming both cationic vacancies in the octahedral sites as encountered in maghemite (γ-Fe2O3) and anionic vacancies, the general formula of the CFO thin film is written

( Fe a Co 1-a ) Td [ Fe b Co c ⊡ 2-b-c ] Oh O 4 -δ ,
where a and b represent the iron content in tetrahedral and octahedral sites, respectively, c the cobalt content in octahedral sites and δ) equal the amount of anionic vacancies per formula unit. The EDX analysis showed that Fe to Co ratio is equal to

( Fe a Co 1-a ) [ Fe 0.94 a Co 1.97 a-1 ⊡ 3 -2.91 a ] O 4-δ
Taking into account the results of resonant X-ray diffraction at the Co edge of the (224) node (Fig. 7), i.e. the lack of transition at the Co edge for LP sample, we conclude that the Td site is fully occupied by Fe atoms. According to Prieto et al. [START_REF] Prieto | Highly oriented (111) CoO and Co 3 O 4 thin films grown by ion beam sputtering[END_REF], the presence of some Co +III within the structure can no longer be ruled out. Therefore, depending on the oxidation degree of Co atoms and taking into account the electroneutrality, the anionic vacancies content δ) equal ranges between 0.00 and 0.24. It results in two extreme cases: 1) no Co +III , which leads to δ) equal = 0.24 : The formula for the LP sample is in the domain bounded by these extreme values. The p-type semiconductor behavior can therefore be clearly explained by the presence of cationic and anionic vacancies. The presence of Fe 2+ being ruled out, the increase of the out-of-plane cell parameter observed for these LP samples however remains a puzzle. We are currently undertaking theoretical calculations to determine the optimal structure for CFO in the presence of such an amount of cationic (and anionic if no Co 3+ is present) vacancies.

(

Conclusions

We have studied the influence of the oxidizing O2:N2 pressure on the growth of pulsed laser deposited CFO thin films on MgO(100) substrates. Increasing this pressure from 0.01 mbar up to 1 mbar, we showed that the structural, magnetic and electrical properties of CFO thin films can significantly be varied, from a p-type semiconductor behaviour with in-plane magnetization to an insulating behaviour with out-of-plane magnetization. Another important result of this study is the demonstration of the possibility to stabilize an auxetic behaviour for the intrinsic spinel CFO compound. A decrease in the O2:N2 oxidizing pressure is shown to induce an increase of the out-ofplane cell parameter. This increase together with the tensile stress imposed to the films by the MgO (001) substrates tend to increase the volume of the CFO cell, until the relaxation of the substrateinduced stress when the films do not adopt the parameter imposed by the substrate any more. But before that, for intermediate pressures, we could observe a situation for which the cell parameters were increased in all three directions with respect to their bulk value. In these conditions, the CFO film was shown to have a negative Poisson's ratio, i.e. to be auxetic. We have experimentally determined the cationic distribution in our films by resonant X ray scattering and demonstrated that, as suggested by Fereiro-Vila et al. [START_REF] Ferreiro-Vila | Apparent auxetic to non-auxetic crossover driven by Co 2+ redistribution in CoFe 2 O 4 thin films[END_REF], the auxetic behaviour was related to a high occupation of the tetrahedral sites by cobalt ions (25 %). Such a high amount of Co 2+ Td is abnormal for the spinel structure of CFO, which should be inverse. We have even determined the limit quantity of Co 2+ Td below which CFO still adopts a non-auxetic behaviour.

An another remarkable result is the evidence of anionic and possible cationic vacancies at low pressures resulting to unexpected magnetic and electrical behaviours. The LP sample is characterized by a symmetry breaking which is not induced by the substrate. The crystal structure solution and the mechanism of distortion along the c-axis is the key point for the understanding of its properties. Theoretical developments are now in progress to better understand the involved mechanism. The inset displays the thermal variation of resistivity. Tables Table 1. Characteristic parameters of CFO thin films deposited at P = 0.02 mbar, P = 0.04 mbar and P = 0.5 mbar: out of plane (a┴) and in-plane cell parameters (a//), ratio of the cell parameters (a┴ / a//), density of dislocation deduced from TEM observation, Poisson's ratio ν, cobalt content in in tetrahedral site (Co)Td deduced from resonant X-ray diffraction spectra, anisotropy constants Ktot (see SI), resistivity at room temperature, carrier concentration and mobility.

Figure captions

Low

Fig. 1 .Fig. 2 .

 12 Fig. 1. Out-of-plane XRD pattern recorded in the 36°-46° 2θ range of CoFe2O4 (CFO) thin films elaborated at P = 0.05 mbar (blue), P = 0.5 mbar (red) and P = 1 mbar (green), respectively. Insets are (top): a magnitude around 2θ = 94°, and (bottom) the reciprocal space mapping (rsm) of the (048) node of CFO at P = 1 mbar.

Fig. 3 .Fig. 4 .

 34 Fig. 3. AFM images of 5x5 µm² areas of CoFe2O4 thin films elaborated at P = 0.02 mbar (LP), P = 0.04 mbar (IP) and P = 0.5 mbar (HP)

Fig. 5 .Fig. 6 .

 56 Fig. 5. Room temperature magnetic hysteresis loops measured with a applied field of 70 kOe, for samples deposited at 0.02 mbar, 0.04 mbar and 0.5 mbar.

Fig. 7 .

 7 Fig. 7. Simulated resonant X-ray scattering ( Co x Fe 1-x ) [ Co 1-x Fe 1+ x ] O 4 thin films as a function of the cobalt content (x) in Td site for the 224 node computed with the FitREXS software[21]. The experimental spectra recorded at the D2AM beamline of ESRF on low pressure (LP), intermediate (IP) and high pressure (HP) samples at both Fe and Co edges are displayed in the insets.

Fig. 8 .

 8 Fig. 8. (a) Room temperature Mössbauer spectra of CFO thin film deposited at P = 0.02 mbar and (b) at P = 0.04 mbar. (c) CEMS spectra at 80K of CFO thin film deposited at P = 0.02 mbar and (d) sketch of magnetic anisotropy deduced from the fit of the spectrum.

Table 2 .

 2 Hyperfine refined parameters deduced from the fit of the CEMS spectra at room temperature of CFO thin film deposited at P = 0.02 mbar (LP) and P = 0.04 mbar (IP).

		pressure (LP)	Intermediate pressure	High pressure (HP)
			(IP)
	P(mbar)	0.02	0.04	0.5
	a // (Å)	8.234(4)	8.446(1)	8.439(3)
	a ┴ (Å)	8.604(2)	8.419(1)	8.325(1)
	a ┴ / a //	1.045(1)	0.997(1)	0.986(1)
	number of dislocation	0.06	0.02	0.03
	per nm 2
	Poisson's ratio ν	0.40(1)	-0.34(2)	0.42(2)
	Cobalt content in	0.00	0.24	0.07
	tetrahedral site (Co) Td
	K tot (J/m 3 )	~ -59.5	~ 0	~ 15
	Resistivity at 300 K	9.5 x 10 3	1.9 x 10 4	n/a
	(Ω.cm)
	Carrier concentration	p = 1.3 x 10 14	n/a	n/a
	Mobility (cm 2 /Vs)	5	n/a	n/a

Table 3 .

 3 Hyperfine refined parameters deduced from the fit of the CEMS spectra at 80K of CFO thin film deposited at P = 0.02 mbar (LP).

	Site	<δ) equal > (mm.s -1 )	<B hf > (T)	<β> (°)	Occupation (%)
	T d	0.38(4)	50.3(1)	72(1)	51
	O h	0.48(2)	53.6(1)	72(1)

Acknowledgments

This work was done with the financial support from the CNRS, the Ministère de l'Enseignement Supérieur et de la Recherche, and the laboratory of excellence Nanostructures in Interaction with their Environment (LabEx NIE 11-LABX-0058-NIE) and from Ile de France region equipment funding ("NOVATECS" C'Nano IdF Project No. IF-08-1453/R). We would like also to acknowledge support from Region of Normandy and the European Regional Development Fund of Normandy (ERDF) in the frame of the MAGMA project. We thank the IPCMS X-ray diffraction and MEB-CRO platforms.

Supplementary information 1. Calculation of the anisotropy constants

The total anisotropy (K tot ) of strained thin film can be written as the sum of the several contributions. The main one is the magnetoelastic anisotropy (K me ) which describes the coupling between the magnetic moments and the lattice. The other terms can be considered as correcting factors. The shape anisotropy (K shape ) is correlated to the demagnetizing factor which depends on the geometry of the system. Finally, the surface anisotropy (K S ) takes into account the symmetry breaking and thus deflects the moment close to the surface. The value of this last term is inversely proportional to the thickness of the samples. The total anisotropy (K tot ) can be thus written as:

where t c is the critical thickness of the film (see text).

• Due to the tetragonal distortion, the magnetoelastic constant can be formulated as [1][2][3]: dynes/cm 2 , C 12 = 1.06×10 12 dynes/cm 2 . Results of computation are given in Table S1.

• Shape anisotropy is simplified for the thin films due to the fact that d 33 =1 and 0 elsewhere in the demagnetizing tensor leading toK shape =-2 π M 2 where M is the magnetization [5]. Results of computation are given in Table S1.

• Lisfi et al. have estimated that the critical thickness t c for CFO for the spin-reorientation transition is around 300 nm [1]. The thicknesses of our samples are smaller (~60 nm) such that the first term 2 K S /t c is negative for all our samples. Since IP sample has both out-of-plane and in-plane curves which are almost superimposed, one can assume that K tot ≈ 0 involving that any external magnetic field affects the easy direction of magnetization. It results to 2 K S t c =-K me -K shape for this sample leading to 2K s /t ~ -4.5 J/m 3 . Both LP and HP samples have same thickness as IP one. One can thus conclude that 2K s /t c should be the same for all samples. Results of computation are given in Table S1.

Table S1

Low K tot (J/m 3 ) ~ -59.5 ~ 0 ~ 15 Computation of the magnetoelastic anisotropy (K me ), the shape anisotropy (K shape ), the surface anisotropy (2K s /t) and the total anisotropy (K tot ) of samples characteristic of the three evidenced pressure domains.

Resonant X-ray diffraction refinement of the of both IP and HP samples at both Co and Fe edges

The synchrotron experiments were carried out on the Collaborating research group (CRG) D2AM beamline at the European Synchrotron Radiation Facility. Samples were mounted on a seven-circle diffractometer equipped with various photodiode detectors, allowing recording of the intensity of the incident beam I 0 , the intensities of the diffraction peaks and the fluorescence of the samples. The rotation matrix was preliminary determined with the help of different in-plane and out-of-plane reflections.

Anomalous factors were first computed using the fluorescence data. The data are first corrected for both background and air absorption. f''(E) is then extracted by using a step function to match the Sasaki values. f'(E) is then obtained using the Kramers-Kronig transform of f''(E). Knowing f 0 , f' and f'' allows thus computation the experimental scans for a given set of structural parameters. The refinement of the different spectra was performed by minimizing the sum of all reliability factors for each nodes and edges through a stochastic basin-hopping algorithm (FitREXS program) (Figures S1 andS2).