Ibrahim Koita

Xiaoyan Li

Luiz Tizei

Jean-Denis Blazit

Nathalie Brun

Etienne Janod

Julien Tranchant

Benoît Corraze

Laurent Cario

Mapping metal/insulator nanodomains switching in V 2 O 3 by variable-temperature electron spectromicroscopy investigations

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Mapping metal/insulator nanodomains switching in V 2 O 3 by variable-temperature electron spectromicroscopy investigations

Ibrahim Koita 1 , Xiaoyan Li 2 , Luiz H. G. Tizei 3 , Jean-Denis Blazit 1 , Nathalie Brun 1 , Etienne Janod 4 , Julien Tranchant 4 , Benoît Corraze 4 , Laurent Cario 4 , Marcel Tencé 5 , Odile Stéphan 1 and Laura Bocher 1 1 Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405, Orsay, France, United States, 2 Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405, Orsay, France, Ilede-France, United States, 3 Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405, Orsay, France, France, 4 Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 Rue de la Houssinière, 44322 Nantes, France, United States, 5 Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405, Orsay, France, ORSAY, France Taming abrupt phase transitions existing in transition metal oxides is considered as a promizing approach for the development of advanced information processing and storage systems. V2O3 is a prototypical system exhibiting metal-to-insulator transitions (MITs) that can be activated under controlled external stimuli such as temperature (T), pressure, or chemical doping [1,2,3] . A precise and detailed understanding of these electronic transitions in Mott insulators and strongly correlated systems such as (V1-xCrx)2O3 is essential to further enhance their potential capabilities for non-volatile memories [4] and neuromorphic applications [5] .

The (V1-xCrx)2O3's phase diagram, established by the McWhan's pioneer studies [1] based on X-ray diffraction and transport measurements (Fig. 1a), contains two insulating (AFI and PI) and one metallic (PM) phases. Two types of MITs, both first order transitions, have been evidenced among the different structural, magnetic and electronic states probed at the macroscopic scale. The PI ↔ PM "pure" Mott transition can be achieved either by a low-level Cr doping (x ≥ 1.2%) or by applying pressure and involves a substantial volume change (≈ 1.2 %) at the transition without crystallographic symmetry breaking. Conversely, the PM ↔ AFI transition at ca. 160 K in pure V2O3 presents a symmetry breaking from rhombohedral to monoclinic, associated with (i) a magnetic transition to an antiferromagnetic ordered state, (ii) a large volume change (+1.4%) and (iii) a MIT yielding a resistivity change of 7 orders of magnitude. This T-driven MIT has been extensively studied at the macroscopic scale [6] and remains still a perfect arena to probe in situ the V2O3 structural and electronic evolutions at the finest scale.

More recently, the T-induced MIT has been mapped in situ by scanning photoemission spectroscopy [7] , X-ray linear dichroism associated to PEEM [8] and nano-IR [9] revealing the microscopic coexistence of insulator/metallic (I/M) domains, but with 25 nm spatial resolution at best. Beyond this, micro-RX experiments associated with nano-IR mapping have investigated the competitive mechanisms between both structural and electronic phases. A clear shift between the rhombohedral/monoclinic and metal/insulator temperature transitions could be underlined and the presence of metallic monoclinic domains could be further inferred [9] . All these results drawn a more complex scenario than McWhan suggested first at the macroscopic scale.

Understanding the V2O3 electronic phase separation and its local mechanisms governing the the I/M domains dynamics across the MITs remains of key interests. Advanced monochromated electron spectromicroscopes emerged these last years as real game-changers for nanomaterials characterization. The possibility of probing spectroscopic signatures with an ultra-high EELS resolution, i.e. down to few meV, extends a larger range of relevant electronic excitations (from IR to soft X-ray) available at the subnm scale and below [10] . These last instrumental capabilities can be further associated with in situ (variable-T and electrical biasing) options [11] providing a unique tool capable to give a direct, simultaneous and local access to structural, optical and electronic information, hence opening an entirely new avenue for exploring phase transitions at the finest scale. Here it will enable us to correlate the formation, nucleation, and dissolution of I/M nanodomains in V2O3 under variable-T conditions by investigating the electronic and structural competitions taking place at boundaries as thin as an atomic column. Variable-T UHR-STEM/EELS experiments were performed on the NION CHROMATEM 200 MC equipped of a double-tilt HennyZ cryo-holder using MEMS to vary continuously the temperature conditions from 120 K (Fig. 1b).

First, low-T thermal cycling across the electronic transition were monitored for probing and mapping the PM and AFI spectroscopic signatures along the [11-20] zone axis (Fig. 1c). Initially, Abe et al. performed low-loss and core-loss EELS at specific temperatures evidencing (i) a sharp spectral feature at ca. 1 eV in the PM state attributed to an interband plasmon that vanishes in the AFI phase and (ii) an energy shift of ca. 500meV on the O-K edge between PM and AFI states associated with a decrease in intensity in AFI related to the lower V3d -O2p hybridization [12] . Here we investigated the EELS low-loss signatures upon cooling and warming enabling us to highlight the temperature range of both electronic phases (Fig. 2a) and a 15K hysteresis between the MIT and IMT. We further mapped the low-loss PM and AFI signatures at the sub-nm scale upon thermal cycling confirming the coexistence of I/M nanodomains limited here by a defined domain wall (Fig. 2b). K-means clustering analyses [13,14] were applied on the low-loss hyperspectral data confirming an electronic homogeneity of each PM/AFI domains (Fig. 2c and2d). Remarkably, periodic inhomogeneities of the HAADF intensity contrast can be observed solely in the corresponding AFI domain. These peculiar HAADF features were already characterized as twinned monoclinic domains observed in the AFI phase by Van Landuyt et al. [15] and Ayroles et al. [16] . Nanodiffraction 4D STEM is currently under investigations to identify the local distribution of the monoclinic/rhombohedral phases upon thermal cycling. All these combined results will enable us to correlate precisely the structural and electronic domains formation mechanisms and clarify the formation dynamics of the I/M domain wall.

  The authors acknowledge funding from the EDPIF, the National Agency for Research under the JCJC program IMPULSE No. ANR-19-CE42-0001 and the program of future investment TEMPOS-CHROMATEM with the Reference No. ANR-10-EQPX-50, and the European Union's Horizon 2020 research and innovation program under grant agreement No 823717 (ESTEEM3).

Figure 1 .

 1 Figure 1. a. Phase diagram of (V1-xMx)2O3 (M for Cr or Ti) function of temperature, pressure and chemical doping adapted from McWhan et al. pioneer works [1,2,3] . b. SEM image of the heating/biasing MEMS used for the variable-T experiments with the enlarged view of the V2O3 thin lamella. c. STEM-HAADF image of V2O3 oriented in the [11-20] zone axis along with the structural model and the FFT in inset

Figure 2 .

 2 Figure 2. a. Low-loss EELS spectra (sum of 2000 spectra acquired in 100 ms each) probed every 1 K upon cooling and warming across the PM/AFI transition. The sharp feature at 1.1 eV marked with * is observed solely in the PM phase. For clarity, EELS spectra are shifted vertically with respect to each other. b. STEM-HAADF image acquired at 147 K, the related hyperspectral data were acquired in the region of interest (black square). c. Low-loss EELS spectra extracted from I/M domains in the region of interest. d. Map of the coexisting I/M domains acquired in the region of interest at 147 K and obtained by K-means clustering method


Hocine Khelfane

Caroline Andreazza-Vignolle

Aline Y Ramos

José Penuelas

Thierry Sauvage

Pascal Andreazza
email: pascal.andreazza@univ-orleans.fr

Out-of-equilibrium supported Pt-Co core-shell nanoparticles stabilized by kinetic trapping at room temperature

Keywords: XX, XX, XX No code given

The chemical stability of supported CoPt nanoparticles in out-of-equilibrium core-shell configurations was investigated mainly by anomalous grazing incidence small angle x-ray scattering (AGISAXS) in association with combined transmission electron microscopy and x-ray absorption spectroscopy. CoPt nanoparticles were prepared at room temperature by ultrahigh vacuum atom beam deposition using two different routes: simultaneous deposition of the two metals (CoPt) or sequential deposition. In this last case, Co deposition on a Pt-core (Pt@Co) and the reverse configuration (Co@Pt) are explored. In the Pt@Co case, our experimental analysis of 2.5nm particles show the stability of a Pt rich-core (80% Pt) surrounded by a two-monolayers-thick Co shell. In the reverse case, the core-shell structure is also stabilized, while the codeposited sample leads to an alloyed structure. These results suggest that the growth kinetics can trap the thermodynamically non-favorable core-shell structure even for this system which has a high alloying tendency. Besides the lack of atom mobility at room temperature, this stabilization maybe associated with core strain effects. Post thermal treatment of core-shell samples induces a structural transition from the core-shell configuration to the equilibrium alloyed configuration. This study demonstrates that the element-selective scattering technique, AGISAXS is highly efficient for the extraction of chemical segregation information from multi-component supported nanoparticles, such as core-shell structures, up to ultimate small sizes.

Introduction

Bimetallic nanoparticles (NPs) have received considerable attention because besides the size reduction effect, the addition of a second metal provides a method to control their properties and functionalities. [1][2][3] In magnetic data storage applications, most of 3d-based NPs exhibit a decrease of magnetization and magnetic anisotropy due to surface contamination (poisoning, oxidation, ligands). At the same time, in heterogeneous catalysis, a decrease of catalytic activity can be due to inappropriate arrangement of atoms in the particles (poisoning, segregation). In both cases, using core-shell nanoparticles can be a great solution depending on the nature of the shell. In fact, core-shell NPs present even more attractive features than a simple protective or stabilizing function, due to the unusual electronic or magnetic properties of shells alone or combined a Present address: Laboratoire de Physique Théorique, Université A. Mira, 06000 Bejaia, Algérie with the properties of the core. [1,4] However, these properties are depended on kinetically-and thermodynamicallycontrolled growth mechanisms which influence the stability of the core-shell structure. These different mechanisms depend on intrinsic parameters of metals such as binding strengths, surface energies, atomic radii, as well as extrinsic aspects related to preparation methods or experimental conditions, such as the nature of substrates when supported or the nature of matrix when embedded, environment, temperature and deposition rate, that contribute to the formation of many types of nanoalloy structures.

Among nanoalloys, CoPt nanostructures are very interesting systems because they offer the possibility to tune the magnetic properties by changing the composition and the chemical order. [5] In particular, the Pt neighborhood around Co atoms can induce an enhancement of the magnetic moment and anisotropy. [6,7] However, at nanosize, due to the presence of numerous atomic configurations or defects (partial alloying, oxidation, surface disorder, strain, etc), magnetic properties can greatly differ from the expected performance. [8][9][10][11] Recent electrocatalysis studies suggest that Pt alloyed with Co, may not only help to reduce the loading of Pt, but also allow the individual functional components to work synergistically to enhance the catalytic performance toward the improvement of the oxygen reduction reaction. [12,13] In addition, the coreshell configuration can induce core-to-shell strain effect which is extensively used in (electro)catalysis to promote the shell metal performance. [14] These behaviors motivate the development of core-shell CoPt-based particles for three distinct features [15][16][17]; i) the core-shell structure is able to maintain favorable properties of the core (magnetism in particular) while protecting the nanoparticles from oxidation thanks to the shell; ii) the shell can enhance the properties of the core (magnetic, catalytic); iii) the core only stabilizes the structure of the particle whereas the useful property results from the shell. However, producing and stabilizing a CoPt core-shell structure is not an easy task, and few researchs examine these mechanisms. [15,18] Core-shell nanoparticles are produced mainly by wet chemical methods, [15,[22][23][24] for which the core-shell structure is mostly induced or stabilized by an environment effect (synthesis media, ligands). In fact, the synthesis of a core of one metal surrounded by a thin shell of another metal can be achieved via two main fabrication strategies [1,4,5]: the first one is to take advantage of the difference of surface energy and atomic size of the two metals in order to promote the segregation of one metal; the second one is to trap the core-shell arrangement in a metastable state by a kinetic or environment effect. The former case is difficult to achieve in the Co-Pt system due to the strong tendency of CoPt alloying [19,20,10] although several theoretical works based on DFT calculations [8,21] show a tendency of Pt segregation at the surface of CoPt NPs. In the latter, the metastable state is trapped by sequential preparation: one type of metal is often first nucleated (or deposited) and forms the core, while the second metal is grown in a second step as a shell (surrounding the nucleated core).

Thus, the goal of this work was to control the formation and the stability of core-shell NPs to provide a chemically-or magnetically-active coating around a metallic core. The objective was to investigate the stability of CoPt NPs, prepared at room temperature by a physical method and different routes, and to study the consequence of thermal treatments (annealing) which can induce a higher mobility of species (atoms and sometimes particles). The sequential deposition (Co deposited on the Pt-core named Pt@Co and the reverse configuration Co@Pt) and the co-deposition (Co and Pt together named CoPt) were compared. Firstly, we will focus our attention on the element-selective morphological and structural characterizations of the stability of a thermodynamically nonfavorable Co-Pt atom arrangement: Pt@Co. Secondly, this structural configuration will be compared with those of CoPt and Co@Pt routes. The study after thermal treatments will allow the question of stability of core-shell or alloyed structure to be addressed. Several techniques were combined to provide a comprehensive description of NPs, difficult to achieve without complementarity and crossfertilization effects between techniques. We have performed a two absorption edge (Co K and Pt L3) analysis of XANES (x-ray absorption near edge structure) measurements supplemented with a EXAFS (extended x-ray absorption fine structure) [25] signal interpretation to obtain the crosschecked local structure around each of the two elements. Transmission electron microscopy (TEM) has given the actual size distribution and morphology of the NPs. The largest experimental piece of information was obtained by anomalous grazing incidence small angle x-ray scattering (Anomalous GISAXS, i.e. AGISAXS). Small-Angle X-ray Scattering (SAXS) is a well-established technique giving information on spatial correlation and morphological features of nanometric or submicrometric objects. [START_REF] Andreazza | Nanoalloys: Synthesis, Structure and Properties[END_REF] The scattering intensity collected close to the X-ray incidence is directly related to large-scale inhomogeneities in the electron density of the probed material. In nanoalloys, the scattering contributions of heterogeneities in particles (phase separation, surface segregation) can be disentangled using the anomalous effect taking advantage of the variation of the scattering factor close to an absorption edge of a given element. [START_REF] Simon | Resonant Anomalous X-ray Scattering[END_REF][START_REF] Goerigk | [END_REF] For nano-objects supported on a dense substrate, limiting the probed depth, the combination of grazing incidence small-angle X-ray scattering [START_REF] Andreazza | Nanoalloys: Synthesis, Structure and Properties[END_REF]29,30] with the anomalous technique (AGISAXS) provides a powerful method to extract the scattering data from multi-component supported NPs, such as core-shell structures. [31,32] Detecting chemical order or demixing in nanoalloys, experimentally is a non-trivial issue. Consequently the development and combination of relevant methods to determine the size, shape, structural and chemical arrangements, and spatial organization of NPs assemblies are still key challenges. This paper presents an application of such combined approach of CoPt nanoalloy phase separation, comparing co-deposited and sequentially-deposited NPs in the 1-4nm size range.

Experimental

Particle preparation

Co and Pt atoms were deposited at room temperature, in a UHV chamber using electron beam evaporation, leading to the formation of 1-4nm NPs . The base pressure was typically a few 10 -10 mbar to avoid any contamination during the deposition. Preliminarily degassed at 600˚C, Si with its SiO 2 native layer coated with an amorphous carbon (a-C) layer was chosen as substrate to limit the clustersubstrate interactions [20] during the metal condensation. Consequently, the NPs were randomly oriented on the substrate. Co and Pt deposition rates were calibrated to control the amount of the deposited metal and the average composition during co-deposition or sequential deposition. These rates (0.2 -0.6 × 10 15 atoms/cm 2 /hour) were selected as low as possible in order to induce an atomic rearrangement at room temperature during the growth (self-annealing effect) within the bimetallic NPs. [33] After deposition, the supported NPs were covered by an amorphous alumina cover layer for AGISAXS and XAS experiments, to avoid Co oxidation, as demonstrated in previous studies [37]. This protected layer is deposited in situ under UHV just after metallic nanoparticle deposition. To confirm this assumption, the chemical state of each covered sample was checked by x-ray photoelectron spectroscopy (XPS). Uncovered samples were used for TEM observations and to post-check the composition of the NP assembly by an average method, Rutherford backscattering spectroscopy (RBS) and a local method, energy dispersive X-ray spectroscopy (EDX). In all cases, the local composition on a few particles obtained in STEM-EDX mode are the same as the average composition, especially for as-grown nanoparticles, with a width of compositional distribution of less than 5%.

Analysis methods

The scattering experiments were performed at the SWING beamline at the Synchrotron Facility SOLEIL (France) using a two-dimensional AVIEX CCD camera. The sampledetector distance was set to 1200mm in order to optimize the q range of the data collection with respect to the particle size. The incidence angles, α i = 0.45˚and 0.60ů sed for Pt L3 and Co K absorption edges respectively, for all samples are significantly larger than the critical angles of the Al 2 O 3 coverlayer and the a-C underlayer to allow the scattering of CoPt NPs. GISAXS 2D pattern were recorded in an adapted framework, as a function of the in-plane q y and out-of-plane q z components respectively, parallel and perpendicular to the substrate. [38,39] The in-plane intensity informs about the particle correlations (in-plane patterning) and the size distribution while outof-plane intensity contains only shape information. The detailed experimental procedure for data collection and data handling is described in ref. [31,32]. The small angle scattering was measured at six energies just below the Co K and the Pt L3 edges, corresponding to a scattering factor variation of 3.4 and 6.6 electrons respectively. The analysis methods were of two types, referred to "direct method" and "differential method". [32,40,[START_REF] Goerigk | [END_REF] In the direct method, the intensities I(q, E) measured at different energies were fitted directly with calculated models taking into account the variation of the scattering factor with energy, extracting the morphological parameters. In the differential method, the partial structure factors were first extracted from the difference between the intensities measured at different energies with various levels of approximation. [41] At this stage a qualitative interpretation of the data is possible, prior to model fitting. This allows the selection of relevant morphological models. Our AGISAXS data at both edges were analyzed by the combination of the two methods.

Fig. 1 displays the 2D GISAXS pattern collected at an energy far from the Pt edge. The 2D patterns reveal the in-plane and out-of-plane organization and shape of the NP assembly. In the direct method, the form factor (corresponding to a truncated sphere shape with a size dispersion) and the structure factor were simultaneously fitted with calculated scattering profiles from at least two experimental cross sections (intensity cuts), in the q y and q z directions (Fig. 1b-c). [39,42,[START_REF] Andreazza | Nanoalloys: Synthesis, Structure and Properties[END_REF] In addition, XAS spectra at the Co K (7709 eV) and Pt L3-edges (11564 eV) of supported CoPt particles and Co3Pt bulk material were recorded at the European Synchrotron Radiation Facility ESRF (BM30 FAME station, Grenoble, France). [34] Data were collected at 4K and at RT over an extended energy range: 11450 to 12200 keV for Pt, and 7600 to 8500 eV for Co, using a 30-element fluorescence detector. XANES was interpreted in comparison with litterature results on CoPt nanosized granular films of differents compositions [47,11] and pure Co, Pt and Co3Pt bulk reference samples. EXAFS measurements were performed only on two samples with a methodology explained in the electronic supplementary material. [44,20] up to an average diameter of 1.2nm, then Co atoms were deposited on the Pt cores. While the NP average size increased during Co deposition, the density of clusters remained constant close to 1 × 10 13 NPs/cm 2 (Fig. 2a andb). Then, static coalescence between NPs occurs (Fig. 2c) and the density drops progressively down to 0.8 × 10 13 NPs/cm 2 . During this second stage, the elementary neighboring NPs coalesce. This phenomenon is still limited to the amount deposited (5.5 × 10 15 atoms/cm 2 ). From these TEM images, the diameter distribution of elementary particles [45,20] can be fitted with a Gaussian function allowing the determination of the mean NP diameter D and the width of size distributions, which remained narrow with a constant relative geometrical standard deviation σ(D)/D around 0.3. These results show that no nucleation of Co NP occurs outside Pt core sites. This behavior is due to the high Pt particle density, the nucleation sites on the substrate being already saturated by Pt atoms. All Co atoms are deposited on Pt cores. The initial particles remain almost individual until a composition of PtCo3 (Fig. 2b-c). At this stage, the open issue is: what is the atom arrangement in these particles: an out-of equilibrium core-shell configuration or an alloyed state? To achieve a clear evidence of the core-shell nanostructure formation of these Pt@Co samples, x-ray absorption spectroscopic analysis was also performed, with a strictly parallel procedure, for co-deposited samples showing similar average composition (PtCo3) and particle size distribution (centered around 2-2.5nm). Normalized XANES spectra at the Co K edge for Co-Pt NPs are shown in Fig. 3a, along with the Co3Pt alloyed bulk spectrum, plotted for reference. Changes in the intensity and shape of the edge shoulder and the white line (denoted as feature A and B, respectively, in Fig. 3a) are appreciable and reflect changes in the electronic structure of the Co and Pt atoms. The shoulder A is assigned to transitions from the 1s levels to the unoccupied states mainly originating from the 4p orbitals. In bulk systems, this feature varies upon alloying of Co and Pt, reflecting a population rearrangement between the electronic levels. A drop of the A shoulder is expected to be concomitant of a lowering of the 4p band at Co sites with increasing number of Pt neighbors (see the inset of Fig. 3a). Such a drop was already observed in previous work [47]. The band structure effect for bulk alloy compounds is observed as a split of the white line B, while the nanoparticle samples show a broad white line.

Let us recall that the Co K-edge white line (labelled peak B1-B2) reflects the 4p empty states of Co near the Fermi energy, while the pre-edge feature (labelled peak A) arises from transitions to more localized unoccupied 3d states, influenced by Co 4p-3d hybridization. The intensity of peaks B1-B2 depends on both the number of available 4p states, and the structure of the system. The shape of the white line of NP samples is less structured than the Co3Pt alloyed bulk sample but closer to the codeposited sample than the Pt@Co sample. In addition, the reduction of the intensity of shoulder A in the CoPt samples with respect to that of a pure Co sample reveals a delocalization of the Co 3d empty states, and is a proof of Co 4p-3d/Pt 5d hybridization. [11] In our samples, it is a first signature of the low amount of Pt atoms in the neighborhood of Co in Pt@Co samples. As at the Co Kedge, normalized XANES spectra at the Pt L3-edge for the Co-Pt samples are shown in Fig. 3b), along with that of a Co3Pt alloyed bulk for comparison. Since Pt is a 5d metal, the L3 absorption edge appears for transitions from 2p to both 5d3/2 and 5d5/2 bands. In general, the white line feature of the Pt L3 in the Co-Pt samples is reduced as the amount of Pt in the alloyed sample decreases. [11] Fig. 3b does not show variations in the XANES signal between codeposited and sequential samples, no significante difference in the nature of the neighborhood of Pt is evidenced.

Structural parameters for the nearest-neighbor N N atoms, obtained from the fit of the experimental spectra (as shown in the supplementary material), are shown in Table I. These data were obtained from the simultaneous first-shell fitting of the Co and Pt edges. The uncertainties on structural parameters were estimated from the standard deviations of the experimental data. Let us first examine the results for the samples obtained by codeposition. The interatomic distances between the Co-Co and Pt-Pt nearest neighbor were 2.55 and 2.70 Å, respectively and 2.63 Å between the Co-Pt NN atoms which is consistent with the values measured by several authors [46,47] for alloyed CoPt nanoparticles. Moreover, the fact that R CoCo , R CoP t and R P tP t are different [46,48] also implies that the particles are either strained with significant relaxation as the result of size reduction, or partially chemically ordered. [47] Interestingly, the mean-square deviation parameter σ 2 is unusually large (close to 1 × 10 -2 Å2 ) for all bonds. This is the signature of a significant dispersion of the NN distances. More trivially, the resulting Co and Pt first shell coordination numbers N tot = 8.1 ± 0.5 and 8.6 ± 1, respectively are less than the bulk fcc value of 12, owing to the small size of NPs and the relatively large number of surface atoms with small coordination numbers. Considering the Co and Pt neighbors, Table 1 shows that: i) the total coordination numbers for Pt and Co nearestneighbors are similar, ii) the number of Pt or Co neighbors surrounding a given emitter are nearly identical, i.e. the number of Co scatterers and Pt scatterers around Co (or Pt) are nearly the same. These two considerations are in agreement with a local structure of co-deposited particles in a mixed state, and consistent with an alloyed model (Fig. 4a).

Turning now to the sequentially deposited particles, the local structure appears different. Concerning the number of nearest neighbors, Table 1 shows that they are not similar. This behavior can be interpreted as a phase separation between Co and Pt atoms. Moreover, the first shell coordination numbers for Pt and Co are different: N tot = 7.4 ± 1 and 11.9 ± 1 for Co and Pt atoms respectively. This significant NN difference reflects a segregation of Co and Pt species, consistent with the hypothesis of a Pt-rich core (maximum coordination value) surrounded by a Co-rich shell (low value due to surface atoms). These EXAFS data in terms of number of Pt-Co Co-Co and Pt-Pt bonds are in accordance with those expected for the core-shell model structures (Fig. 4) and are very different from the PtCo alloy one. Similar EXAFS results from both absorption edges were found in the case of core-shell particles prepared by complex sequential chemical methods: in dendrimer-encapsulated Pd-Pt and Pd-Cu [49] or oxidized Pt-Ru particles. [50] The Co-Co and Pt-Pt homo-atomic bond lengths (2.53 and 2.74 Å, respectively), spread apart more largely than for the codeposited sample and get closer to the values found in the bulk monometallic phases (average values for the bulk face-centered cubic (fcc) phase of Co and Pt of 2.51 and 2.77 Å, respectively [5]). It is worth noting that the R Co-Co distances are larger while the R P t-P t distances are shorter compared to the bulk values. Although contracted bonds are commonly observed in metallic nanoparticles due to their lower coordination environment, [51,52] it appears that for these samples, the Pt-Pt distances were not fully relaxed with the Co deposition. Interestingly, the σ 2 parameter remains large for both Co-Co and Co-Pt bonds, but decreases for Pt-Pt bonds (0.5 × 10 -2 Å2 ), indicating that the core has a higher degree of order. These results reveal the stabilization of the core-shell structure which may be related to the core strain effect.

Further evidences of the core -shell morphology of the sequentially grown NPs can be obtained more directly from differential analysis of AGISAXS measurements. Small angle scattering is collected at two energies near the Pt L3-absorption edge and two energies near the Co K-absorption edge ((E 2 ) and (E 1 ), near and far from the edge respectively). The form factor of a heterogeneous particle with a core diameter D c and a shell thickness ∆R is dependent on electron densities of the core and of the shell (for the CoPt system, the Pt scattering factor is three times larger than the Co one). Thus, the variation of intensity between two energies at the Co edge is characteristic of the Co part of the particle and similarly, of the Pt part at the Pt edge. Fig. 5 shows the out-of-plane scattering intensity from the entire particle and the difference ∆I = I(E 1 )-I(E 2 ) for sequentially deposited Pt@Co particles, where E 1 and E 2 are the energies 11461 and 11560 eV near the Pt L3 edge and 7606 and 7705 eV near the Co K edge, respectively. The I and ∆I are significantly different at the Pt-edge (Fig. 5(a)), which reflects the size difference between the entire particle (Co and Pt parts) and the Pt part. We did not observe the same effect at the Co K edge (Fig. 5(b)). These two behaviors can be explained by the presence of Co in the entire particle, while the Pt is confined in the core. However, due to the high difference between atomic scattering power of Co and Pt, at the Pt edge, the homogeneous part (Pt-Pt) is dominant in the differential intensity, while at the Co edge, the Co-Pt and Co-Co terms are equivalent. Consequently, a model of a pure Co-shell/Pt-core is also compatible, which would give also a small variation of intensity at the shell edge (Co) as observed here.

Quantitative morphological parameters were obtained from the direct method. The GISAXS intensity were fitted in the distorded wave Born approximation (DWBA) framework using IsGISAXS software. [42] The output morphological parameters (inset in Fig. 6) were the NP diameter D, height H and interparticle distance Λ, and especially in the case of core-shell structure, the core diameter D c and the shell thickness ∆R = (D -D c )/2. [35,36,32] The best results were obtained with a spherical shape truncated by the substrate surface with an aspect ratio H/D < 1. A narrow Gaussian size distribution with relative geometrical standard deviation close to σ(D)/D = 0.3 was used for each simulation. Several form factors were used: homogeneous alloyed and totally or partially Co-Pt separated core-shell NPs. The best fit corresponds to a core-shell configuration in agreement with the results suggested by the differential method. The fitting results shown in Fig. 6, gave D = 2.8nm, H = 2.3nm and ∆R = 0.35nm. These results correspond to a model of a Pt-rich core (80% Pt atoms) covered by a shell of about two Co monolayers. The atom number ratio between Pt and Co calculated with this model is in agreement with the ratio obtained by the RBS method over the whole sample or by EDX at the particle scale (N P t /N Co = 3).

The combination of AGISAXS and EXAFS analysis, shows that for sequentially deposited Pt@Co NPs, the growth kinetic traps a core-shell structure, with a strained Pt core, even if these two metals have a strong tendency to form an alloy.

Annealed Pt@Co and reverse Co@Pt deposition

In this section, we report the AGISAXS investigation on the morphology changes induced by annealing at low temperature (500 • C) of Pt@Co NPs obtained by sequential deposition. In a similar procedure, the small angle scattering was measured at the vicinity of the Co K edge and the Pt L3 edge as shown in Figure 7. Represented in the "differential mode", the 1D AGISAXS intensities of the annealed particles (5 × 10 15 atoms/cm 2 , 3.5nm) reveal a similar behavior in the I and ∆I profiles at the Pt-edge (Fig. 7) and at Co-edge (not shown). These results are interpreted as a mixing between Co and Pt atoms in the entire particle. For randomly distributed Co and Pt atoms in the particle, all partial scattering factors (Pt-Pt, Co-Pt and Co-Co) should be homothetic and consequently, the intensity profiles exhibit the same shape. This behavior confirms a chemical transition from the core-shell to the alloyed structure when the temperature (500 • C) induces enough atom mobility to achieve the equilibrium alloyed structure. Similar measurements for CoPt co-deposited particles sample (6 × 10 15 atoms/cm 2 , 2.2nm) show also homothetic profiles (only Pt edge results are shown in Fig. 7(b)) which confirm the alloying of bimetallic CoPt particles without segregation, as was already demonstrated. [20,33] We have therefore clear evidence of a transition to the alloyed configuration by annealing. The occurence of such transition strengthens certainty about the initial core-shell configuration.

Figure 8 shows the out-of-plane total scattering intensity and the difference ∆I for sequentially deposited particles in a reverse configuration, Co then Pt with a CoPt3 composition. The I and ∆I are significantly different at the Co-edge (Fig. 8(b)), which reflects the size difference between the entire particle (Co and Pt parts) and the Co part. These results are similar to those described in the Pt@Co configuration. Indeed, if an effect is observed at the core edge, no effect is observed at the shell edge (Pt-edge) (Fig. 8a). These results confirm that the growth kinetics or/and the core structure trap the core-shell structure for sequentially deposited Co-core and Pt-shell at room temperature in an out-of equilibrium configuration. The fitting of these scattering intensity profile gives D = 2.3nm, H = 1.8nm and ∆R = 0.4nm. It corresponds to an average shell thickness of about 2 monolayers of Pt. In the case of core-shell particles, it is worth noting that it is difficult to identify the chemical configuration if the shell thickness is below 2 monolayers, even for two metals with high scattering contrast.

In the previous case of Pt@Co, one argument to explain the stabilization of the Pt-core Co-shell configuration could be an effect of the alumina coverlayer. For Co@Pt configuration, this reason is also relevant. However, the most likely reason appears to be the kinetic trapping which is consistent with the mechanism of limited diffusion observed at low temperature in alloyed CoPt nanoparticle. Indeed, even if the CoPt chemically L1 0 ordered phase is thermodynamically stable at room temperature, [19] CoPt NPs grow generally in the metastable face-centered-cubic (fcc) chemically disordered phase A1. Annealing is then required to promote atomic diffusion and to reach a chemically ordered state. [10,39,5] Among the origins of this behavior, the major obstacles to the order are the tendency of CoPt NPs to grow with multiple twinning, surface segregation (Pt-rich surface layer) or internal strains. [21,53,54] Among these, the two latter effects may also be a source of stability for our core-shell NPs.

Conclusion

We have reported here a comprehensive architectural evaluation of the supported Co@Pt and Pt@Co core-shell alloyed nanoparticles that provides spatially and elementary resolved structural information on NPs with size lower than 4 nm. This comparative study illustrates how anomalous grazing incidence small-angle X-ray scattering (AG-ISAXS), combined with transmission electron microscopy/ energy dispersive spectroscopy (TEM/EDX) analyses provide complementary information used to construct a detailed description of the core-shell and alloy nanostructures. The X-ray absorption spectroscopy measurements are in agreement with the AGISAXS results. We can clearly differentiate NPs of the same size in alloyed or core shell structures with two-monolayers-thick shells. AGISAXS allowed a very detailed view for the two main samples in opposite configurations: Co@Pt sequentially grown nanoparticles led to a Co-core surrounded by a Pt-shell, and Pt@Co sequentially grown nanoparticles led to a non trivial result with the reverse configuration of a Pt-rich core surrounded by a Co shell with a lattice contraction of the core relative to the bulk metal. The former case is in agreement with results on alloyed bulk system surfaces or on larger NPs, [15,22] showing that a Pt top-layer may be stabilized. In the latter case, the strained structure present in Pt-core at this size range may contribute to this tendency to kinetically stabilize the core-shell configuration.

These results allow us to claim that the lack of atom mobility at room temperature is the main origin of the stabilization of core-shell structures for NPs obtained by sequential deposition, even for the CoPt system which has a high alloying tendency. The mechanism of kinetic trapping poses an interesting theoretical question. Further investigations of the growth or rather the stability and consenquently ageing of metal nanoalloys is an open direction of research, already explored theoretically in several systems [55][56][57][58][59] but more rarely experimentally due to the need to carry out time resolved investigations. Finite-size phenomena of strain-dependent atom rearrangement is expected to be of major importance for the relative stability of the different cluster structures. Recent in situ and real-time investigations of the structural and morphological evolution during the sequential deposition in ultra high vacuum confirmed the interest of these interpretations and methods. [41,60] 5 Authors contributions All the authors were involved in the preparation of the manuscript. All the authors have read and approved the final manuscript.

Acknowledgements

We would like to thank our former and present Ph.D. students especially, N. Bouet, for their fruitful preliminary contributions to these works. The authors are grateful to Christine Mottet (CINaM, Marseille, France) for enthusiastic and helpful discussions. Finally, the authors acknowledge ESRF (Grenoble, France) and SOLEIL (Gifsur-Yvette, France) synchrotron facilities for synchrotron radiation access. We would like to thank Jean-Louis Hazemann and Javier Perez for assistance in using FAME and SWING beamlines, respectively. RBS experiment was done at CEMHTI, Orléans, France. In addition, some of the author would like to acknowledge support from the International Research Network IRN Nanoalloys (CNRS) to facilitate and to coordinate the exchanges on the nanoalloy subject.

Electronic supplementary material

The supplementary material gives the details of the EX-AFS methodology used in the study for the sequentially deposited Pt@Co3 and PtCo3 codeposited samples. In addition, anomalous X-ray scattering methodology for spectra analysis is explained.

Fig. 1 .

 1 Fig. 1. (a) Two-dimensional GISAXS patterns of the Pt-Co sample measured near the Pt L3 edge (intensity in log scale). (b)qy and (c) qz one-dimensional cross-sections (qy and qz cuts in (a)) measured at different energies show the anomalous intensity variation in linear scale.

Fig. 2 .

 2 Fig. 2. TEM micrographs and size distribution histograms of Co-Pt NPs prepared at 300K by sequential deposition: (a) initial Pt core only (1.3 × 10 15 atoms/cm 2 and 1.2 nm mean size), (b) Pt core then Co (4.0 × 10 15 atoms/cm 2 and 1.5 nm mean size) and (c) Pt core then Co (5.5 × 10 15 atoms/cm 2 and about 2 nm across).

Fig. 3 .

 3 Fig. 3. XANES spectra (a) at the Co K edge and (b) at the Pt L3 edge for a sequentially deposited Pt@Co3 (light-orange line) sample, a codeposited PtCo3 sample (dark-violet line) indicating the A, B1, B2 and C features. The XANES spectrum of a Co3Pt alloyed bulk material is shown for comparison (pink dashed line). The inset shows an enlargement of the A shoulder for the two NP samples.

Fig. 4 .

 4 Fig. 4. Configurations of the CoPt3 nanoparticle models (a) alloyed and (b) Pt core-Co shell. The Pt and Co atoms are represented by green large spheres and black small spheres, respectively.

Fig. 5 .

 5 Fig. 5. GISAXS one-dimensional qz cuts of Pt@Co sequentially deposited particles in log scale: (a) at Pt L3 edge, (b) at Co K edge. Differential intensity ∆I = I(E1) -I(E2) (symbols) and total intensity I(E1) (solid line) are shown for each graphs. I(E1) and I(E2) are the energies 11461 and 11560 eV and 7606 and 7705 eV at the Pt L3 edge and at Co K edge, respectively. ∆I is normalized with respect to I(E2) to allow a direct comparison.

Fig. 6 .

 6 Fig. 6. GISAXS measured (symbols) and simulated (solid line) (a) qy in-plane cross-section and (b) qz out-of-plane crosssection of Pt@Co sequentially deposited particle sample. For a better understanding, diameter D, height H and interparticle distance Λ, and the core diameter Dc in a core-shell structure are shown in the inset.

Fig. 7 .

 7 Fig. 7. GISAXS one-dimensional qz cuts at the Pt L3 edge of : (a) Pt@Co sequentially deposited particles annealed at 500 • C, (b) PtCo codeposited particles. Normalized differential intensity ∆I = I(E1) -I(E2) (symbols) and total intensity I(E1) (solid line) are shown for each graphs in log scale.

Fig. 8 .

 8 Fig. 8. GISAXS one-dimensional qz cuts of reverse Co@Pt sequentially deposited particles : (a) at Pt L3 edge, (b) at Co K edge. Differential intensity ∆I = I(E1) -I(E2) (symbols) and total intensity I(E1) (solid line) are shown for each graphs. I(E1) and I(E2) are at the energies 11461 and 11560 eV, and 7606 and 7705 eV at the Pt L3 edge and at Co K edge, respectively. ∆I is normalized with respect to I(E2) to allow a direct comparison.

Table 1 .

 1 Number of nearest-neighbors N N (with a standard deviation of 0.5) and interatomic distances R around Co and Pt atoms for (s-index) sequential and (c-index) codeposited NPs, (R in Å and σ 2 in Å2 with uncertainities of 0.02 Å and 0.002 Å2 respectively) deduced from EXAFS fits.

	edge bond	N Ns Rs	σ 2	N Nc Rc	σ 2
	Co	Co-Co 5.2	2.53 0.008 4.7	2.55 0.010
		Co-Pt 2.2	2.59 0.009 3.4	2.63 0.007
	Pt	Pt-Co 5.1	2.59 0.008 4.0	2.63 0.011
		Pt-Pt	6.8	2.74 0.005 4.6	2.70 0.009
	3 Results and discussion
	3.1 Sequential Pt@Co deposition
	Figure 2 shows TEM images illustrating the morphological
	evolution of supported NPs prepared by sequential depo-
	sition at room temperature: Pt NPs were obtained first by
	a Volmer-Weber nucleation mode

Electronic supplementary material

Out-of-equilibrium Pt-Co core-shell supported nanoparticles stabilized by kinetic trapping at room temperature Hocine Khelfane 1 , Caroline Andreazza-Vignolle 1 , Aline Y. Ramos2 , José Penuelas 3 , Thierry Sauvage 4 and Pascal Andreazza 1 1 Interfaces, Confinement, Matériaux et Nanostructures, ICMN, Université d'Orléans, CNRS, 1B, rue de la Férollerie, F-45071 Orléans Cedex, France, e-mail: Pascal.Andreazza@univ-orleans.fr

Extended X-ray Absorption Fine Structure Methodology

Our methodology to analyse the XAS data followed the typical procedure used in EXAFS many papers, as in ref. [1]. The XAS spectra at the Co K (7709 eV) and Pt L3-edges (11564 eV) of supported CoPt particles and Co3Pt bulk material were recorded at the European Synchrotron Radiation Facility ESRF (BM30 FAME station, Grenoble, France). Data were collected at 4K and at RT over an extended energy range: 11450 to 12200 keV for Pt, and 7600 to 8500 eV for Co, using a 30-element fluorescence detector. Co and Pt foils were used as references. In order to obtain the local environment around the Pt and Co atoms, simulation and fitting of the EXAFS experimental data were performed with IFEFFIT [2,3]. The kweighted x-ray absorption fine structure function were Fourier transformed using Kaiser-Bessel window and subsequently fitted in R space (radial interatomic distance). Theoretical paths were calculated with the FEFF code using muffin tin potentials and the Hedin-Lunqvist approximation for the energy-dependent part [4]. Figure S1 shows Fourier-transformed EXAFS modulus (a) at the Co K edge and (b) at the Pt L3 edge. From this procedure, analysis have been carried out simultaneously for the data collected at the two edges (multi-edge analysis). Such procedure allows an accurate determination of the first-shell Co-Co, Co-Pt and Pt-Pt coordination numbers N (scattered atom number), atomic distances R and mean-square deviation in R, σ 2 . To avoid misinterpretation, many constraints have been applied: RPt-Co (Pt-edge)= RCo-Pt (Co-edge) and σ 2 Pt-Co = σ 2 Co-Pt, and Ntot value (i.e. the average coordination number) chosen in a range in accordance with the particle size distribution deduced from TEM or GISAXS [5,6,7]. In addition, the number of neighbours was checked quantitatively to be consistent with the core size, the shell thickness, the size of choose model for each element type (Co and Pt). The compositional distribution (typically of 5%, deduced from STEM-EDX measurements on our samples prepared by UHV deposition due a low atoms mobility at RT and a high density of particles on the substrate) is enough narrow to agree to only one nanoparticle model in each sample. In addition, at each analysis, Co-O contributions have also been tested and appeared negligible.

Anomalous X-ray scattering Methodology

The detailed experimental procedure for data collection and data handling is described in ref. [8,9]. The small angle scattering was measured for at six energies just below the Co K and the Pt L3 edges, corresponding to a scattering factor variation of 3.4 and 6.6 electrons respectively. The analysis methods were within two types, referred as "direct method" and "differential method" [9,10] The GISAXS intensity of the assembly of supported particles can be expressed in the local monodisperse approximation with a form factor of nanoparticles (depending on the particle shape function) calculated in the DWBA framework. NPs morphological parameters (radius R, height H and inter-particle distance) have been extracted from GISAXS experimental data by comparison with simulations using IsGISAXS software [11]. The best results are obtained with a spherical shape that can be truncated by the substrate surface taking an aspect ratio H/2R < 1. A narrow Gaussian size distribution with relative geometrical standard deviation close to σ(R) / R = 0.2 according to preliminar TEM observations is used for each simulation [typically as in ref . 12]. Based on the qualitative results of the differential method, several form factors were used: homogeneous truncated sphere for alloyed and pure particles and core-shell truncated sphere for sequentially deposited particles. -In the direct method, the intensities I(q,E) measured at different energies were fitted directly with calculated models taking into account the variation of the scattering factor with energy, extracting the morphological parameters. For example, Figure S2a shows the weak variation in the simulated profiles in the case of alloyed particles and core-shell particles of a same size. Only the Yoneda peak (the strong enhancement of the intensity originates from the interference of incoming and reflected beam) is significantly different, which is the signature of the nature of the particle external surface (Co for core Pt-shell Co and Pt-Co mixed for alloyed). -In the differential method, the partial structure factors were first extracted by difference between the intensities measured at different energies with various levels of approximation [9]. At this stage a qualitative interpretation of the data is possible, prior to model fitting. For example, Figure S2b shows the high difference of the GISAXS profile for Pt@Co core-shell and Pt core alone particles of same core-size distribution. In this case at the Pt-edge of the core, the intensity coming from the Pt core alone reflects the lower size effect with respect to size of the entire particle (Co shell and Pt core). In addition, the AGISAXS data analysed at both edges, enriches the quality of the interpretation, with the combination of the two methods. profile is given for comparison. This case corresponds to a typical experimental sample obtained sequentially, Pt then Co, and the final combined analysis gives R=1.4nm, H= 2.3nm for the entire particle and ΔR= 0.35nm for the Co shell thickness. These results agree perfectly with a model of a Pt supported particle covered by a shell of two Co monolayers excepted at the substrate-Pt core interface.

Rutherford Backscattering method

Rutherford backscattering spectroscopy (RBS) was performed using α particles of 2 MeV extracted from a Pelletron accelerator in the CEMHTI laboratory (Orléans, France). The scattering geometry is : scattering angle = 165°, exit angle 15°, incident angle 0°. The detector size is 25 mm2 and sample to detector distance is 90 mm. Typical RBS spectra of Pt-Co nanoparticles deposited on a-C/SiO2/Si(100) are shown in Fig. S3. The left side of the spectrum, at low backscattering energies, corresponds to Si and O atoms, The area under the Pt and Co peak in the high energy side of the spectrum is directly related to the number of platinum atoms in the porous substrate. The RBS spectra were fitted using the SIMNRA software, to estimate the number of deposited Co and Pt atoms and their concentration [14,15].


Changjie Shi

Yong Wang

Kai Zhang
email: kevin_zhangk@126.com

Eric Lichtfouse
email: eric.lichtfouse@inrae.fr

Cong Li
email: congil@aliyun.com

Yunshu Zhang
email: zhangys@usst.edu.cn

Fe-biochar as a safe and efficient catalyst to activate peracetic acid for the removal of the acid orange dye from water

Keywords:

The mechanism of the Fe-biochar/PAA process was investigated by removing acid orange.

• CH 3 C(O)O⋅ and CH 3 C(O)OO⋅ act as the primary radicals. • There may also be non-radical effects in the process of acid orange removal. • Fe-biochar has a high recycling rate in the Fe-biochar/PAA process.

Introduction

Peracetic acid (PAA) is a common oxidant with low toxicity generating few by-products, and has a high redox potential of 1.06-1.96 V. PAA has been extensively used in medical and health, agricultural construction, food production, beverage processing, and the textile industry [START_REF] Monarca | Mutagenicity and disinfection by-products in surface drinking water disinfected with peracetic acid[END_REF][START_REF] Henao | Disinfection by-products formation and ecotoxicological effects of effluents treated with peracetic acid: a review[END_REF][START_REF] Jo | Effect of a peroxyacetic acid mixture as green chemical on rice bacterial and fungal pathogens[END_REF][START_REF] Silva | Peracetic acid: structural elucidation for applications in wastewater treatment[END_REF][START_REF] Tang | Removal of thiophene sulfur model compound for coal by microwave with peroxyacetic acid[END_REF]. PAA spontaneously decomposes into reactive oxygen species at pH below 8.2, thus removing pollutants [START_REF] Coyle | Peracetic acid as an alternative disinfection technology for wet weather flows[END_REF]. Therefore, PAA is considered as an alternative to chlorinated disinfectants. Nonetheless, PAA alone cannot remove most refractory organic pollutants. To solve this issue, the low peroxy bond energy, of 38 kcal mol -1 , of PAA can be activated by catalysis to generate highly reactive radicals such as HO⋅ and R-O⋅ [START_REF] Rokhina | Free radical reaction pathway, thermochemistry of peracetic acid homolysis, and its application for phenol degradation: spectroscopic study and quantum chemistry calculations[END_REF]Rizzo et al., 2019;[START_REF] Ao | Peracetic acid-based advanced oxidation processes for decontamination and disinfection of water: a review[END_REF]. Such catalysis has been successfully applied to the removal of phenols [START_REF] Rokhina | Free radical reaction pathway, thermochemistry of peracetic acid homolysis, and its application for phenol degradation: spectroscopic study and quantum chemistry calculations[END_REF][START_REF] Ghanbari | Acetaminophen degradation by a synergistic peracetic acid/UVC-LED/Fe(II) advanced oxidation process: kinetic assessment, process feasibility and mechanistic considerations[END_REF][START_REF] Zhao | New insights into the activation of peracetic acid by Co(II): role of Co(II)-Peracetic acid complex as the dominant intermediate oxidant[END_REF], dyes [START_REF] Zhou | Activated carbon fibers as an effective metal-free catalyst for peracetic acid activation: implications for the removal of organic pollutants[END_REF][START_REF] Abd-Elhamid | Fabrication and characterization of a novel (GO/ PAA/PAM) nanocomposite as effective adsorbent for cationic dyes[END_REF][START_REF] Yuan | Peracetic acid enhanced electrochemical advanced oxidation for organic pollutant elimination[END_REF], and antibiotics [START_REF] Wu | Degradation of organic compounds by peracetic acid activated with Co3O4: a novel advanced oxidation process and organic radical contribution[END_REF][START_REF] Chen | New insight into the mechanism of electroassisted pyrite minerals activation of peroxymonosulfate: synergistic effects, activation sites and electron transfer[END_REF][START_REF] Liu | Novel nonradical oxidation of sulfonamide antibiotics with Co(II)-Doped g-C3N4activated peracetic acid: role of high-valent cobalt-oxo species[END_REF] using various methods.

For instance, ultraviolet-catalyzed PAA removal of pharmaceuticals has revealed the presence of HO⋅, CH 3 C(O)O⋅ and CH 3 C(O)-OO⋅ [START_REF] Cai | UV/Peracetic acid for degradation of pharmaceuticals and reactive species evaluation[END_REF]. Ultrasound-catalyzed PAA and NaCl allows efficient microbial decontamination [START_REF] Görgüç | Single and combined decontamination effects of power-ultrasound, peroxyacetic acid and sodium chloride sanitizing treatments on Escherichia coli, Bacillus cereus and Penicillium expansum inoculated dried figs[END_REF]. Thermal activation of PAA also generates abundant free radicals (Wang et al., 2020a). Application of these methods at large scale is nonetheless limited by the cost of external energy. Alternatively, PAA can be activated by transition metals such as Fe 2+ (Wang et al., 2020b(Wang et al., , 2021b)), Co 2+ [START_REF] Kim | Cobalt/peracetic acid: advanced oxidation of aromatic organic compounds by acetylperoxyl radicals[END_REF], Co 3 O 4 [START_REF] Wu | Degradation of organic compounds by peracetic acid activated with Co3O4: a novel advanced oxidation process and organic radical contribution[END_REF], CoFe 2 O 4 (Wang et al., 2021a), and Mn 2+ [START_REF] Rothbart | Mechanistic studies on the oxidative degradation of Orange II by peracetic acid catalyzed by simple manganese(II) salts. Tuning the lifetime of the catalyst[END_REF] to generate free radicals such as HO⋅ and R-O⋅, yet the recovery of those catalysts after reaction is difficult, thus inducing secondary pollution of natural waters. Safer energy-activated carbon materials can also be used to produce R-O⋅ and HO⋅ (Eqs. ( 1)-( 3)), though the catalytic power is usually lower than metal catalysis which produce abundant R-O⋅ with longer oxidation performance (Eqs. (4) and ( 5)). Therefore, research is actually focusing on anchoring metals on carbon materials to prevent metal leakage, to increase metal recyclability and to combine the catalytic effects of carbon and metals. In particular, cobalt salts has been anchored on carbon materials [START_REF] Dong | Lignin-derived biochar to support CoFe2O4: effective activation of peracetic acid for sulfamethoxazole degradation[END_REF]. Since cobalt is toxic and costly, here we hypothesized that anchoring iron salts on a carbon material would produce an efficient and safer catalysis. A one-step co-precipitation method has been studied to synthesize magnetic sludge -derived biochar and use it to activate persulfate to remove acid orange7 [START_REF] Wang | One-step preparation and application of magnetic sludge-derived biochar on acid orange 7 removal via both adsorption and persulfate based oxidation[END_REF]. However, relevant research on PAA activation has not yet appeared.

We report here the synthesis of Fe-biochar by simple pyrolysis, and the activation of PAA by Fe-biochar to remove the acid orange dye from water. We characterized the structure of Fe-biochar and we tested the effects of PAA concentration, pH, humic acids, chloride, bicarbonate on the reaction. The reaction mechanism was studied by radical quenching and electron paramagnetic resonance to identify radicals. The stability of the synthetic materials was evaluated. The study will provide reference for the development of activated PAA technology that can be practically applied to advanced treatment of drinking water or sewage.

CH 3 C(O)OOH̅̅→ energy CH 3 C(O)O ⋅ + HO⋅ (1) CH 3 C(O)OOH + HO⋅→CH 3 C(O)OO⋅ + H 2 O (2) CH 3 C(O)OOH + CH 3 C(O)O⋅→CH 3 C(O)OO⋅ + CH 3 C(O)OH (3) M n+ + CH 3 C(O)OOH → M (n+1)+ + CH 3 C(O)O ⋅ + OH - (4)
M (n+1)+ + CH 3 C(O)OOH → M n+ + CH 3 C(O)OO ⋅ + H + (5)

Materials and methods

Chemicals

The raw material of biochar is wheat straw provided by a farmland in Jining, Shandong. Acid orange and 15-20% PAA are from Aladdin Reagent Co, Shanghai. Supplementary Text S1 report the sources of other chemical reagents. All chemicals are of analytical grade. All solutions were prepared by deionized water.

Experimental procedures

Wheat straw was soaked 48 h in 0.2 mol/L NaOH for removing ash, wax and impurities on the surface of it. Then washed with deionized water until neutral, dried and grinded until it could pass through a 100mesh sieve, then dried and stored for later use. Straw powder (6 g every time) was dipped 6 h into the 30% Fe (NO 3 ) 3 ⋅9H 2 O, filtered and dried at 60 • C until constant weight. Then, the 2 g straw power was pyrolyzed 2 h in a tube furnace (700 • C) with nitrogen flow to produce Fe-biochar which was water-washed and 70% ethanol-washed (remove small molecular organics), dried, sealed and stored for later use.

All reactions were performed in 250 ml glass conical flasks with 400 rpm stirring at 23 ± 2 • C. PAA, H 2 O 2 and Fe-biochar or biochar were added to 200 ml of 50 mg/L acid orange solution, then 2 mL samples were taken out with a syringe at different times and quickly quenched in excess Na 2 S 2 O 3 solution. The pH of the initial solution was adjusted by adding 0.2 mol L -1 NaOH or 0.1 mol L -1 H 2 SO 4 to the solution. Free radical scavengers were ascorbic acid, tert-butyl alcohol (TBA), methanol and 2,4-hexadiene (2,4-HD). In the experiments of different influencing factors, the corresponding chemical agents were added before the reaction. As for the catalyst cycling tests, Fe-biochar was collected by suction filtration (0.45 μm) and washed with deionized water for three times, then dried under vacuum condition at 40 • C. PAA concentration was determined periodically. All experiments were at least repeated or performed multiple times.

Analytical methods

PAA concentration was measured by iodimetry [START_REF] Chen | Effects of water matrices on the degradation of naproxen by reactive radicals in the UV/peracetic acid process[END_REF]. The residual concentration of PAA during the experiment was determined by N, N-diethyl-p-phenylenediamine colorimetric method [START_REF] Cavallini | Comparison of methodologies for determination of residual peracetic acid in wastewater disinfection[END_REF]. The content of H 2 O 2 was determined by the titanium potassium oxalate method [START_REF] Yang | Enhanced Fe(III)-mediated Fenton oxidation of atrazine in the presence of functionalized multi-walled carbon nanotubes[END_REF]. pH was monitored by a pH meter PHSJ3 from Leici, Shanghai. The concentration of acid orange was analyzed by spectrophotometry. The free radicals were measured by electron paramagnetic resonance (EPR) analysis using 5,5-dimethyl2-pyrrolidone-N-oxyl (DMPO) and 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMP) as the trapping agent. The surface features and internal crystal structure of the samples were analyzed by the scanning electron microscope (SEM, ZEISS Sigma 500, Germany) and transmission electron microscope and high-resolution transmission electron microscope (TEM and HRTEM, FEI talos f200s, America). X-ray diffraction (XRD) patterns of samples were analyzed by X-ray diffractometer (Bruke D8 Advance, Germany) with Cu Kα radiation. N 2 adsorption-desorption curve was analyzed at 77 K (ASAP 2020, America) and the specific surface areas and bore diameter were measured by the standard Brunauer-Emmett-Teller (BET) method. The dissolved iron concentration was measured by ICP-OES (Optima8000, PerkinElmer, US).

Results and discussion

Characterization of Fe-biochar and biochar

We compared the structure of biochar and Fe-biochar by scanning electron microscopy (SEM, Fig. 1). At low magnification of 20 μm, biochar displays hollow cavities that should be suitable for loading Fe catalytic compounds (Fig. 1a). Both biochar and Fe-biochar has a similar layered structure of this scale (Fig. 1a,c). At high magnification of 2 μm, biochar has a smooth surface, whereas Fe-biochar displays numerous small granules, which likely result from the FeNO 3 ⋅9H 2 O treatment before biochar pyrolysis. Analysis of Fe-biochar by transmission electron microscopy (TEM) reveals dispersed granules at low magnification of 500 nm (Fig. 1e). Interplanar spacings between adjacent lattice fringes reveals the presence of Fe 2 O 3 (320) at 0.215 nm, Fe (110) at, 0.244 nm, Fe 3 O 4 (311) at 0.250 nm and Fe 3 C (221) at 0.304 nm (Fig. 1f).

Analysis by X-Ray diffraction confirms the presence of Fe at 44.5 • , Fe 2 O 3 at 38.7 • , Fe 3 O 4 at 35.6 • , and Fe 3 C at 51.9 • in Fe-biochar versus raw biochar (Fig. 1g). According to peak intensity, the relative abundance of Fe compounds is in decreasing order: Fe, Fe 3 O 4 , Fe 2 O 3 , and Fe 3 C. N 2 adsorption/desorption isotherms at high pressure, of P/P 0 higher than 0.5, reveal hysteresis loop for Fe-biochar (Fig. 1f). This isotherm type indicates a mesoporous structure [START_REF] Ahmad | Engineered biochar composites with zeolite, silica, and nano-zerovalent iron for the efficient scavenging of chlortetracycline from aqueous solutions[END_REF]. The specific surface area of Fe-biochar before the activation, of 128.36 cm 2 /g, is lower than that of biochar, of 146.15 cm 2 /g (Table S1). This might be due to the presence of Fe compounds in Fe-biochar pores, and to the stacking of biochar flakes around Fe compounds. The addition of iron also enlarged the pore volume from 0.06 to 0.29 cm 3 /g, and pore size from 2.5 to 8.0 nm of biochar (Table S1). Overall, our findings reveal the granulous structure of Fe-biochar and the presence of Fe, Fe 3 O 4 , Fe 2 O 3 , and Fe 3 C on Fe-biochar surface. At low relative pressure, it was similar to the type II curve, while at higher pressure (P/P 0 > 0.5), capillary condensation occurred in Fe-biochar with a hysteresis loop. This isotherm type corresponded to the mesoporous structure [START_REF] Ahmad | Engineered biochar composites with zeolite, silica, and nano-zerovalent iron for the efficient scavenging of chlortetracycline from aqueous solutions[END_REF]. In contrast, the curve change of biochar was weak, and the trend was closer to type II. It belongs to the microporous structure in terms of pore volume and pore size.

Removal of acid orange

We tested the removal of acid orange with PAA, biochar, biochar and PAA, Fe-biochar, Fe-biochar and H 2 O 2 , and Fe-biochar and PAA in distilled water (Fig. 2). Results show that PAA alone induces the lowest removal of 6.3% after 25 min of reaction. This suggests that PAA alone has little oxidizing power, and agrees with the fact that PAA reacts poorly with electron-withdrawing groups [START_REF] Kim | Reactivity of peracetic acid with organic compounds: a critical review[END_REF]. Biochar alone induces a slightly higher removal of 24.2%, which is likely explained mainly by adsorption of acid orange on biochar surface. A study also verified that biochar has a limited capacity and takes a long time to adsorb acid orange [START_REF] Huang | Adsorption of acid orange 7 in aqueous solution by biochar from peanut shell supported with clay mineral kaolinite[END_REF]. Biochar and PAA induce a slightly higher removal of 30.5%, which suggests an additive contribution of biochar adsorption and PAA oxidation. Remarkably, all materials containing iron display higher removals, of 45.5% for Fe-biochar, 49.7% for Fe-biochar and H 2 O 2 , and 93.3% for Fe-biochar and PAA. This indicates that a major part of acid orange removal is due to catalytic transformation induced by Fe species. Possible products include acid orange radicals and various removal products, which may be adsorbed on Fe-biochar or mineralized to CO 2 .

H 2 O 2 + Fe 2+ →⋅OH + Fe 3+ + OH - (6)
H 2 O 2 + Fe 2+ →H 2 O + Fe Ⅳ O 2+ (7) CH 3 C(O)OOH + Fe 2+ →CH 3 C(O)O⋅ + Fe 3+ + OH - (8) CH 3 C(O)OOH + Fe 2+ →CH 3 C(O)O -+ Fe 3+ + ⋅OH (9) CH 3 C(O)OOH + Fe 2+ →CH 3 C(O)OH + Fe Ⅳ O 2+ (10)
The slight removal improvement from 45.5% for Fe-biochar to 49.7% for Fe-biochar and H 2 O 2 results probably from direct H 2 O 2 oxidation and H 2 O 2 , activation by Fe (Eq. ( 6) and ( 7)). To further study the effect of H 2 O 2 , different concentrations of H 2 O 2 were added to the Fe-biochar/PAA/AOII process, as shown in Fig. S1. H 2 O 2 with higher concentration will inhibit the degradation effect of Fe-biochar/PAA. And we suspect that H 2 O 2 will quench the formed free radicals, which was also confirmed by Wang et al. (2020c). Therefore, the degradation of AOII was due to the activation of PAA by Fe-biochar. By contrast, the high removal improvement from 45.5% for Fe-biochar to 93.3% for Fe-biochar and PAA likely results from the strong activation of PAA by Fe (Eqs. ( 8)-( 10)) followed by removal of acid orange, which might partly favor adsorption of products on biochar. This finding agrees with the high reactivity of a mixture of Fe(II) and PAA [START_REF] Kim | Advanced oxidation process with peracetic acid and Fe(II) for contaminant degradation[END_REF]. The highest removal with to 93.3% for Fe-biochar and PAA is also supported by our results on the higher decomposition of PAA with Fe-biochar versus biochar and PAA alone (Fig. S2). Our further study of the optimal concentrations of Fe-biochar and PAA showed that a Fe-biochar dose of 0.3 g/L and a PAA concentration of 1.144 mM induced the highest removal of 99.8% (Text S2, Figs. S3 and4). Overall, anchoring Fe onto biochar induced a substantial increase of acid orange removal. This is explained by activation by Fe species anchored on the biochar, followed by removal of acid orange then possible adsorption of acid orange products on biochar.

Effect of pH

We studied the effect of pH on the removal of acid orange by Febiochar and PAA. Results show that acid orange removal in decreasing with pH, from 99.5% at pH 3-83.4% at pH 11 after 20 min of reaction (Fig. 3). The corresponding removal rates decrease from 0.2804 min -1 at pH 3 to 0.0519 min -1 at pH 11 (Fig. S5). After 20 min, curiously, the removal decreases at pH 9 and 11, which can first be partly ascribed to the reduction of actives species induced by the decomposition and hydrolysis of PAA (Eq. ( 11)-( 13)). Second, another explanation is the formation of peracetate radicals in alkaline conditions, which can in turn consume PAA (Eq. ( 14)), similarly to reports on the removal of naproxen with UV and PAA [START_REF] Zhang | Oxidation of β-lactam antibiotics by peracetic acid: reaction kinetics, product and pathway evaluation[END_REF][START_REF] Chen | Effects of water matrices on the degradation of naproxen by reactive radicals in the UV/peracetic acid process[END_REF]. Third, Fe species anchored on biochar are likely to form complexes in alkaline conditions, similarly to the formation of cobalt hydroxide complexes that inhibit reactions (Zhang et al., 2021b). Fourth, the Fenton reaction involving Fe 2+ activation of PAA works better under acidic conditions, as shown for the removal of methylene blue [START_REF] Yuan | Peracetic acid enhanced electrochemical advanced oxidation for organic pollutant elimination[END_REF]. Fifth, alkaline conditions should decrease the adsorption of acid orange onto Fe-biochar following the formation of more anions both on acid orange and on Fe-biochar, and in turn increase electrostatic repulsion. This would agree with the fact that the most efficient method to extract humic substances, e.g. from soils, involves alkaline extraction [START_REF] Mohinuzzaman | Insights into solubility of soil humic substances and their fluorescence characterisation in three characteristic soils[END_REF]. Overall, we found that acid orange removal decreases with pH and is possibly explained by removal of PAA, inactivation of Fe catalytic sites of Fe-biochar by formation of Fe hydroxydes and less adsorption of acid orange onto Fe-biochar due to formation of anionic groups. effect on the removal of acid orange under the Fe-biochar/PAA process. The decomposition of PAA was also investigated in the presence of humic acids in Fig. S6, which showed the amount of decomposition of PAA was significantly reduced, indicating that the addition of humic acids affected the effect of PAA. It was reported that humic acids can compete with acid orange and O 2 for electrons [START_REF] Chen | New insight into the mechanism of electroassisted pyrite minerals activation of peroxymonosulfate: synergistic effects, activation sites and electron transfer[END_REF] and inhibit the activation of PAA, the formation of free radicals and the scavenging of acid orange. In addition, humic acids can react with R-O⋅ with the rapid reaction rate constant of 1 × 10 4 L mg -1 s -1 [START_REF] Chen | Effects of water matrices on the degradation of naproxen by reactive radicals in the UV/peracetic acid process[END_REF]. The inhibitory effect of HCO 3 -was partly attributed to the formation of unreactive cobalt-bicarbonate complexes. The similar results were also found on the removal of SMX in the Co/PAA process (Wang et al., 2020c). Besides, HCO 3 -can react with radical species in Eq. ( 15), inhibiting the removal of acid orange. The result is consistent with the UV/PAA process [START_REF] Das Neves | Carbonate anion photolyzed by solar radiation or combined with peracetic acid to form reactive species for dye degradation[END_REF]. As shown in Fig. S6, the decomposition of PAA in the Fe-biochar/PAA process with HCO 3 -was inhibited, which indicated that the formation of unreactive iron-bicarbonate complexes. In Fig. 4c, the effect of Cl -on the removal of acid orange could be ignore. Cl -can react with radical species as Eq.( 16)-( 20). As shown in Fig. S6, the addition of Cl -had a negligible effect on the decomposition of PAA in the Fe-biochar/PAA process before 10 min, but in the end, it will affect the total amount of decomposition. Low concentration of Cl -is not sufficient to affect the Fe-biochar/PAA process and the reaction of high concentration of Cl -and radical species to form Cl⋅ will also accelerate the removal of acid orange [START_REF] Görgüç | Single and combined decontamination effects of power-ultrasound, peroxyacetic acid and sodium chloride sanitizing treatments on Escherichia coli, Bacillus cereus and Penicillium expansum inoculated dried figs[END_REF]. The intermediate concentration of Cl -inhibited the removal of acid orange to a certain extent due to the inhibition of R-O⋅ and HO⋅.

CH 3 C(O)OOH + H 2 O→CH 3 C(O)OO⋅ -+ H 3 O + (11) 2 CH 3 C(O)OOH → 2 CH 3 C(O)OH + O 2 (12) CH 3 C(O)OOH + H 2 O̅→ OH - CH 3 C(O)OH + H 2 O 2 (
HCO - 3 + HO⋅→CO ⋅- 3 + H 2 O ( 15
)
CH 3 CO 3 ⋅ + Cl -+ H + →Cl⋅ + CH 3 CO 3 H (16)
Cl -+ HO⋅→HClO -⋅ (17)

HClO -⋅ + H + →Cl⋅ + H 2 O (18) Cl -+ Cl⋅→Cl - 2 (19)
Cl ⋅ + Cl⋅→Cl 2 (20)

Reactive species accounting for acid orange removal in the Febiochar/PAA process

In Fig. 5a, ascorbic acid significantly decreases the removal rate of acid orange (38.1%) compared with without ascorbic acid (99.3%) in the same conditions. Ascorbic acid is a typical free radical scavenger, which was used to initially investigate whether the removal of acid orange in Fe-biochar/PAA process was caused by free radicals. Since the catalyst played a dominant role in the reaction, further research found that the addition of ascorbic acid did not affect the catalytic effect of Febiochar, which is consistent with the PAA and activated carbon fibers process results [START_REF] Zhou | Activated carbon fibers as an effective metal-free catalyst for peracetic acid activation: implications for the removal of organic pollutants[END_REF]. The above result indicated that there are active radicals in this process. According to the Eqs. ( 1)-( 3), ( 22) and ( 21) of PAA, it can be inferred that there are some radicals like singlet oxygen ( 1 O 2 ), HO⋅, CH 3 CO 2 ⋅, CH 3 CO 3 ⋅, ⋅CH 3 and CH 3 CO 2 ⋅ in Fe-biochar/PAA process. To further clarify the radical species, quenching experiments were performed by using different quenchers. Methanol can react with HO⋅ and R-O⋅ effectively (Zhang et al., 2021b). TBA has obvious quenching effect on HO⋅, but cannot react with R-O⋅ [START_REF] Sun | Rapid disinfection by peracetic acid combined with UV irradiation[END_REF][START_REF] Chen | Effects of water matrices on the degradation of naproxen by reactive radicals in the UV/peracetic acid process[END_REF][START_REF] Yan | Enhanced removal of oxytetracycline by UV-driven advanced oxidation with peracetic acid: insight into the degradation intermediates and N-nitrosodimethylamine formation potential[END_REF]. 2,4-HD was used as the scavenger for the R-O⋅ [START_REF] Kim | Cobalt/peracetic acid: advanced oxidation of aromatic organic compounds by acetylperoxyl radicals[END_REF]. As shown in Fig. 5b, addition of TBA only slightly slowed down the removal of acid orange. The result indicated that there are HO⋅ in this process, but not the main radical species, which was consistent with the results reported in the Co 3 O 4 /-PAA, CoFe 2 O 4 /PAA and UV/PAA processes (Wang et al., 2021a;Zhang et al., 2021a). In contrast, both methanol and 2,4 showed significant inhibitory effects on the removal of acid orange. After adding 1 mM methanol and 2,4-HD, the removal rate of acid orange decreased from 99.9% to 29.4% and 25.4%. The decomposition of PAA by 1 mM methanol and 2,4-HD was slightly (Fig. S6), which is not enough to explain the effect of decomposition of PAA. Furthermore, as shown in Fig. S7, excessive PAA did not affect the inhibition of acid orange removal by methanol and 2,4-HD, which indicated that the inhibition was not attributed to the decomposition of PAA but the radical species of HO⋅ and R-O⋅. Further result showed that 2,4-HD significantly inhibited the removal of acid orange, indicating that the major radical species were R-O⋅. Fig. 5b andc showed EPR spectra of DMPO and TEMPO adducts in the different processes. There were no obvious signals in PAA alone or Fe-biochar alone process whereas strong signals in Fe-biochar/PAA process, which was consistent with PAA/UVC-LED/Fe (II) process [START_REF] Ghanbari | Acetaminophen degradation by a synergistic peracetic acid/UVC-LED/Fe(II) advanced oxidation process: kinetic assessment, process feasibility and mechanistic considerations[END_REF]. As shown in Fig. 5c, HO⋅ signal (4-line signal, indicated by black stars in Fig. 5c) and carbon-centered radicals signal (6-line signal, indicated by red stars in Fig. 5c) were found which could be attributed to the oxidation of DMPO by CH 3 C(O)O⋅ and CH 3 C(O)OO⋅. In addition, 1 O 2 also be found in Fe-biochar/PAA process with a strong intensity signal after addition of TEMPO, suggesting that there is another possible cause for the formation of DMPOX. The DMPO was directly oxidize by 1 O 2 in non-radical pathways. As shown in Fig. 5, the signal intensity of 1 O 2 in Fe-biochar/PAA process was much stronger than that in PAA alone solution, suggesting that 1 O 2 was formed from the catalytic effect on the PAA rather than the self-decomposition of PAA alone. Combining the above radical species scavenging experimental results, it can be concluded that CH 3 C(O)O⋅ and CH 3 C(O)O⋅ are the main radical species in Fe-biochar/PAA process, but there may also be non-radical effects in the process of acid orange removal.

CH 3 C(O)O ⋅ →⋅CH 3 + CO 2 (21) ⋅CH 3 + O 2 →CH 3 OO⋅ (22)

Reusability of Fe-biochar for the removal of acid orange

The reuse rate of the catalyst is an important index to evaluate the performance of the catalyst. As shown in Fig. 6, after the catalyst was recycled for 5 times, the removal rate was still as high as 92.8%, which

Conclusion

In this study, the Fe-biochar catalyst was successfully prepared by a one-step method, and was applied to PAA activation for the first time. The results showed that the Fe-biochar/PAA process degraded acid orange rapidly and had a good effect, which was better than that of metalfree biochar/PAA process. Affected by the metals and the properties of PAA, this process had better removal effect under acidic and neutral conditions. The inhibition on the removal of acid orange was enhanced with the increase of the concentration of humic acids and H 2 CO 3 -in water, but the effect of Cl -was negligible. The process produced mainly R-O⋅, HO⋅ and 1 O 2 , and quenching experiments suggested that R-O⋅ played a dominant role in the removal process, but there may also be non-radical effects in the process of acid orange removal. Compared with the effective metal catalysis, this process can effectively prevent secondary pollution caused by metal leakage, and the results showed that the iron content in the solution after 25 min of reaction was far lower than the drinking water standard requirements. The Fe-biochar/ PAA process was selective for the removal of different pollutants. In addition, the high recycling rate of Fe-biochar also indicated the practicality of the Fe-biochar/PAA process in selectively degrading pollutants. The first proposal of Fe-biochar/PAA process and its remarkable removal effect provide a reference for drinking water or sewage treatment.

was only about 8% less than the first removal rate. The results showed that the use of farmland waste wheat straw combined with iron to prepare iron-loaded biomass carbon can play a significant role in the catalysis of PAA. The XRD patterns of Fe-biochar before and after reaction were shown in Fig. S8, Fe-biochar still appeared characteristic peaks at 44.5 • , 38.7 • , 35.6 • , 51.9 • and 56 • , indicating that Fe-biochar activation of PAA had little effect on its own crystal form, and also reflected the good stability. Additionally, as shown in Fig. S9, the Febiochar/PAA process is selective to degrade pollution, which is attributed to the high selectivity of R-O⋅. The results also showed that the process had the best removal effect on the bisphenol A of the four pollution, which was owing to the acetyloxy radical showed high reactivity to electron-rich functional groups that contained in these compounds [START_REF] Kim | Cobalt/peracetic acid: advanced oxidation of aromatic organic compounds by acetylperoxyl radicals[END_REF]. In addition, the concentration of the leached iron also was investigated in this research. In this research, the concentration of the leached iron was merely 0.065 mg L -1 after 25min, which is lower than the health reference level of 0.3 mg L -1 in drinking water. Hence, the heterogeneous Fe-biochar/PAA process has great prospects for utilization, and would be economical and without secondary pollution in wastewater/drinking water treatment.

Fig. 1 .

 1 Fig. 1. The SEM images of (a) biochar (2 μm), (b) biochar (20 μm), (c) Fe-biochar (2 μm), (d) Fe-biochar (20 μm), (e) TEM image and (f) HRTEM image of Fe-biochar, (g) N 2 adsorption-desorption isotherm curves of biochar and Fe-biochar, (h) XRD patterns of biochar and Fe-biochar.

  Fig. 2. Removal of acid orange under different processes. [acid orange] 0 = 0.143 mM, [PAA] 0 = [H 2 O 2 ] 0 = 1.144 mM, [catalysts] 0 = 0.3 g/L, pH = 7.0.

Fig. 3 .

 3 Fig. 3. (a) Effect of initial pH on the removal of acid orange in the Fe-biochar/ PAA process. [acid orange] 0 = 0.143 mM, [PAA] 0 = 1.144 mM, Fe-biochar 0 = 0.3 g/L. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4 .

 4 Fig. 4. Effect of(a) humic acids, (b)HCO 3 -and (c) Cl -on the removal of acid orange in the Fe-biochar/PAA process. [acid orange] 0 = 0.143 mM, [PAA] 0 = 1.144 mM, Fe-biochar 0 = 0.3 g/L, pH 7.0. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5 .Fig. 6 .

 56 Fig. 5. (a) Effect of different free radical scavengers on the removal of acid orange in the Fe-biochar/PAA process, experimental and simulated EPR spectra of spintrap adducts in the Fe-biochar/PAA process: (b) DMPO, (c) TEMPO. [acid orange] 0 = 0.143 mM, [PAA] 0 = 1.144 mM, Fe-biochar 0 = 0.3 g/L, pH 7.0. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Acknowledgments

This work is supported by Natural Science Foundation of Shanghai (No.20ZR1438200) and the National Natural Science Foundation of China(No. 51778565).

Data availability

Data will be made available on request.

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.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.chemosphere.2022.135686.

Supporting Information for

Fe-biochar as a safe and efficient catalyst to activate peracetic acid for the removal of the acid orange dye from water Text S1. Sources of the chemical reagents FeNO3•9H2O, H2O2 (30% w/w), methanol, NaCl, 70% alcohol, Na2HCO3, Humic acid (HA), KI, Na2S2O3, N,Ndiethyl-p-phenylenediamine (DPD), H2SO4, NaOH, Ascorbic acid (AA), Tertiary Butyl Alcohol (TBA), methanol and 2,4-hexadiene (2,4-HD), sulfamethoxazole (SMX), bisphenol A (BPA) and perfluorooctanoic acid (PFOA) were purchased from Aladdin (Shanghai, China), 5,5-dimethyl2-pyrrolidone-N-oxyl (DMPO) and 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMP) were purchased from Enzo Biochem (Inc., New York, USA).

Text S2. Effects of peracetic acid and Fe-biochar concentrations

Effect of PAA dose on the acid orange removal in the Fe-biochar/PAA process was investigated in Fig. S2. Compared with the process of Fe-biochar alone (removal rate=44.5%), the removal rate of acid orange was increased to 82.7%-99.9% in the Fe-biochar/PAA process with the concentration of PAA changed, indicating that PAA play an important role in the process of Fe-biochar/PAA process removal. As shown in Fig. S2a, when the dose of PAA increased from 0.143 to 1.144mM, the removal rate also increased. Higher concentrations of PAA will provide more reactive species for degrading acid orange. But when the concentration of PAA was higher than 1.144mM, the result was opposite. The reaction rate constant had the same changes (Table .S2). In this research, PAA concentration of 1.144mM had the largest reaction rate constant (kobs=0.2574 min -1 ). When the concentration of PAA is too high, the excess PAA will inhibit the production of reactive species [START_REF] Dong | Lignin-derived biochar to support CoFe2O4: effective activation of peracetic acid for sulfamethoxazole degradation[END_REF]. In addition, as the PAA concentration increases, the concentration of coexisting H2O2 also increases in the reaction solution [START_REF] Wu | Degradation of organic compounds by peracetic acid activated with Co3O4: a novel advanced oxidation process and organic radical contribution[END_REF]. Fig. S3 showed the effect on removal of acid orange when the H2O2 concentration in the process was increased. The result suggested that excess H2O2 will inhibit the generation of R-O• (mainly CH3C(O)OO• and CH3C(O)O•) (Eq. S1 and S2) and then affect the removal of acid orange. Active radicals are mainly generated by PAA, while excess H2O2 inhibits the decomposition of PAA in the reaction solution [START_REF] Chen | Effects of water matrices on the degradation of naproxen by reactive radicals in the UV/peracetic acid process[END_REF].

The effect of Fe-biochar concentration on the removal of acid orange in Fe-biochar/PAA process was investigated. In Fig. S2b, the concentration of acid orange hardly changed in the absence of Fe-biochar process and the removal of AOII by PAA alone can be negligible. However, the concentration of acid orange decreased significantly after adding Fe-biochar to the reaction process. The experimental result showed that the removal rates of acid orange increased with increment of Fe-biochar dosages, indicating PAA required Fe-biochar as a catalyst to promote the generation of active radicals (Rokhina et al., 2013). In addition, more Fe-biochar will accelerate the removal of acid orange from Table .S3. The inconspicuous removal of acid orange when the dosage of Fe-biochar was higher than 0.3g/L was likely owing to the dosages of 0.3g/L were enough to accelerate PAA generate radicals. The results indicated that when the initial concentration of Fe-biochar is 0.3g/L under this experimental condition, acid orange has been degraded completely in 25 min, that is, the removal rate reaches 99.8%.