Reversible Fe Magnetic Moment Switching in Catalytic Oxygen

Article pubs.acs.org/JPCC

Reversible Fe Magnetic Moment Switching in Catalytic Oxygen Reduction Reaction of Fe-Phthalocyanine Adsorbed on Ag(110) Juan Bartolomé,*,† Fernando Bartolomé,† Nicholas B. Brookes,‡ Francesco Sedona,§ Andrea Basagni,§ Daniel Forrer,§,∥ and Mauro Sambi§ †

Instituto de Ciencia de Materiales de Aragón and Departamento de Física de la Materia Condensada, Universidad de Zaragoza CSIC, Pedro Cerbuna 12, 50009 Zaragoza, Spain ‡ European Synchrotron Radiation Facility-ESRF, CS40220, F-38043 Grenoble Cedex 9, France § Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, 35131 Padova, Italy ∥ CNR-IENI, Via Marzolo 1, 35131 Padova, Italy S Supporting Information *

ABSTRACT: The consequences of the evolution of the active site in the oxygen reduction reaction catalyzed by iron phthalocyanine (FePc) submonolayers deposited on the Ag(110) surface are explored. The capacity of reversibly and sharply switching the magnetic state of FePc molecules by chemical means is conclusively evidenced by X-ray magnetic circular dichroism (XMCD) at the Fe L2,3 absorption edge. In the molecular oxygen-dosed phase, oxygen intercalates between the molecule and the surface, thereby switching the Fe magnetic moment from a nearly negligible (mexp Tot ≈ 0.26(1) μB) to an order of magnitude larger value (mexp ≈ 2.1(2) μ ). Moreover, the characteristic XMCD Tot B spectrum undergoes a crossover from metallic to oxidized Fe(III)-like line shape, in accordance with an oxygen-induced decoupling of the molecular electronic states from the underlying metal surface. The FePc acts as a reversible bifunctional chemical− magnetic switch at the atomic monolayer scale.

1. INTRODUCTION Molecular overlayers on ordered substrates are being applied in sensing elements,1 catalysis,2 molecular electronics,3 nonlinear optics,4 light-to-energy conversion,5,6 and spintronics.7 In this respect, bioinspired oxygen-binding metalated macrocycles such as iron-containing porphyrins (FePp) and phthalocyanines (FePc) are highly versatile because their properties remarkably fit the needs of many of the mentioned fields. In fact, they are potentially interesting in any domain concerned with reversible oxygen binding to heme-like systems, e.g., in the field of biological processes involved in respiration and photosynthesis, in catalysis of the oxygen reduction reaction (ORR) as substitutes of precious metals in the cathodic compartments of low-temperature fuel cells, and finallygiven the open shell electronic structure of the central metal ionin two-dimensional molecular arrays with adsorption-induced switchable molecular spins. Indeed, metal surface−molecule interactions modify the molecular orbitals (MO) and consequently the magnetic state of the molecule. The chemical reactivity of the molecular overlayer with other external atoms or small molecules may then be exploited to modify and in some cases to reproducibly control the spin density as well as the magnetization anisotropy of the surface-supported submonolayer (sub-ML) or monolayer (ML) systems.8,9 Doping of MPc’s with alkali metals may also modify the number of electrons in the 3d orbitals and the crystal field magnitude. As a consequence, the magnetic moment of Fe may © XXXX American Chemical Society

be strongly modified. For example, the magnetic moment of Lidoped FePc may increase by an order of magnitude with respect to the undoped case.10 In recent studies of FePc deposited on the Ag(110) surface, some of us11,12 have shown that the catalytic activity of the molecule−metal system in the ORR can be sharply and reproducibly switched by tuning the molecular coverage around the single ML: sub-ML phases are catalytically active, while the full ML undergoes a structural phase transition, which makes the system catalytically inert. The structure of the active site and its evolution with oxygen coordination was also determined at the single-molecule level (Figure 1). The most striking feature of the FePc/Ag(110) oxygendosed phase is selective and localized oxygen intercalation between FePc molecule and Ag, which is expected to effectively decouple the molecular electronic states from the metal surface ones, in turn dramatically affecting the Fe magnetization. Indeed, in previous works that have explored the magnetization switch by chemical means of Fe- and Co-containing surface coordination complexes, the coordination of small molecules (O2,8 CO,13 NO,9,13 NH314) to the spin-bearing metal centers always occurred from above (i.e., from the vacuum side), inducing only small perturbations in their vertical interaction Received: March 26, 2015 Revised: May 11, 2015

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DOI: 10.1021/acs.jpcc.5b02916 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

The FePc (Alfa Aesar GmbH, 95% purity) was dosed in vacuum with a resistively heated quartz crucible in UHV, Dr. Eberl’s NTEZ low-temperature evaporator for organics. The Ag substrate was held at room temperature (RT) during deposition. The amount of FePc deposited was calibrated in deposition time, and the quality of the deposited layer was checked in situ by scanning tunneling microscopy (STM). The central Fe atom of each molecule can be easily seen as a central bright spot (see Figure 1). This is a consequence of the prevalent localization of the lowest unoccupied molecular orbital (LUMO) states of the FePc molecule on Fe. Because the high density (HD) full 1 ML is catalytically inactive,12 the preparation of the low density (LD) sub-ML sample needs to be finely controlled by checking the structure of the overlayer by LEED measurements and confirmed by STM immediately prior to the insertion of the sample into the X-ray chamber. The FePc adsorption by Ag is very robust, as proven by the very small number of FePc molecules that have detached from the substrate after 23 h of irradiation at the Xray beamline under ultrahigh vacuum. In a second step, the LD sample was oxygen-dosed (OD) in vacuum at a pressure of 10−6 mbar through a leak valve until saturation. The third step consists in reverting the OD sample to the oxygen-free phase state by annealing for 60 min at 370 K. The oxygen atoms remain bound to the Ag substrate, albeit in different positions. The resulting sample, denoted as submonolayer annealed, ANN, is not identical to LD because it shows a larger density of defects in the FePc layer (Figure 1a). 2.2. X-ray Absortion, X-ray Linear Polarized Absortion, and X-ray Magnetic Circular Dichroism. The X-ray linear polarized absorption (XLPA) and XMCD experiments were performed at the ID08 beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble. The Ag single crystal was oriented with the Ag[11̅0] direction perpendicular to the beam plane of incidence. It was fixed to a Ta sample-holder to avoid any contamination in the XAS caused by stray X-rays scattered by any external object containing Fe. A spherical grating monochromator was used. The energy resolution at the Fe L2,3 energy region is about ΔE/ E = 5 × 10−4. The detection technique was total electron yield. In both experiments a monochromatic X-ray beam impinges the sample with an incidence angle γ with respect to the normal to the substrate (and the molecule plane), within the range 0° < γ < 70°. In the XLPA experiment, the polarization of the electric field of the incoming X-ray beam is denoted as horizontal (H) and vertical (V) for parallel and perpendicular polarization to the storage ring electron orbit plane, respectively. In XMCD, the incoming light is right- and left-circularly polarized. The XMCD experiment was performed with μ0H = 4 T applied magnetic field, which guarantees that the sample is magnetically saturated.17 To avoid experimental artifacts, the four combinations of field direction and circular polarization are recorded for each angle. The XMCD signal is the difference between absorption spectra recorded with the helicity antiparallel and parallel to the field. In all cases, the polarization rate was well above 99%. All experiments were carried out at T = 6 K. No radiation damage of the sample was detected. The signal contribution of the FePc layer, superimposed to the much more intense background due to the Ag substrate, amounts to ca. 1% of the total signal. The contribution from the Ag substrate is caused by MIII edge EXAFS oscillations in the

Figure 1. (a) STM images of the self-organized FePc layer on Ag(110): LD Phase (left panel), the bright centers correspond to FePc molecules adsorbed to the Ag substrate in on-top positions; OD phase (right panel), after O2 dosing for 90 s at p = 10−6 mbar, the dimmed spots correspond to oxygen-bonded iron in FePc; ANN (left panel), annealed phase. (b) LD (left panel) and OD (right panel) phases: observed and calculated images of the single FePc molecule. (c) Balland-stick models of the top and side views of FePc adsorbed on the Ag(110) surface in the (left panel) LD and (right panel) OD phases. For each phase, the NN coordination scheme of the Fe atom is reported: square planar in LD and nearly prismatic in OD. Color code: Fe, orange; N, blue; C, black; H, white; O, red; Ag, gray.

with the underlying surface. As far as intercalated oxygen is concerned, studies to date regarding its effect on the magnetization of FePc have been performed only by depositing the molecules on an oxygen-predosed metallic surface.15,16 This has prompted us to explore the change of the magnetic properties of the FePc molecule deposited on Ag upon dosing with oxygen in the ORR and its reversibility upon annealing. This goal is achieved by the application of X-ray circular magnetic dichroism (XMCD), an element-selective spectroscopy sensitive to sub-ML coverage. The spectra were recorded in the incoming photon energy range 695 < Eph < 745 eV that covers the Fe L2,3 edges, corresponding to the 2p → 3d and the 2p → 4s dipolar transitions.

2. EXPERIMENTAL AND METHODS SECTION 2.1. Sample Preparation. FePc molecules were deposited on an Ag(110) single-crystal substrate, at sub-ML density, up to a maximum of 1 ML, in an ultrahigh vacuum (UHV) chamber (base pressure better than 10−10 mbar) connected to the X-ray absortion (XAS) and XMCD experimental chamber. The preparation method has followed the steps described in ref 12. The Ag crystal was cleaned by means of repeated cyclic bombardment with 1 kV Ar+ sputtering (1 h) and subsequent annealing at 890 K (35 min). Any trace of adsorbed oxygen was eliminated, and the surface quality and order was checked in situ by low-energy electron diffraction (LEED) measurement. B


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oxygen atoms. These O atoms reside on bridge sites between the Fe-bearing Ag atom and its Ag nearest neighbors along the close-packed [11̅0] row, i.e., the O atoms are interposed between the FePc molecule and the Ag surface (Figure 1c, right panel). Here, the NN local coordination of iron becomes approximately prismatic, i.e., more pronouncedly three-dimensional than in LD. The fraction of oxidized FePc amounts to about 70 ± 10% of the molecules on the surface. After annealing (ANN), oxygen atoms detach from Fe and diffuse into the subsurface layers of the Ag single crystal, while the FePc molecule largely recovers the geometric and electronic structure it had prior to oxidation (see Figure 1, left panel). The FePc-O2 cycle can be easily checked by STM. Indeed, the central Fe-centered density of states switches from bright (LD and ANN Figure 1, left panel) to dim (O2 uptake OD Figure 1, right panel) in the image because the LUMO states of the FePc molecule are modified upon oxygen uptake.12 Because successive cycles of oxygen dosing and annealing accumulate oxygen in the subsurface Ag layers, oxygen can be removed from the surface by low-energy ionized hydrogen bombardment, which fully closes the catalytic cycle.12 Several LD → OD → LD switches for individual FePc molecules as a function of oxygen dosage can be followed in real time by means of STM even prior to annealing, directly proving the catalytic activity of FePc even at RT (See Supplementary Movie S1 in ref 12). Annealing simply enhances the rate of OD decomposition and oxygen subsurface migration. As previously shown,12 the reduced oxygen that accumulates in the substrate topmost layers after a few oxygen dosing−annealing steps, which eventually limits the number of reversible cycles, can be effectively removed by low-energy hydrogen ion bombardment, thus restoring the interface to its initial conditions, both chemically and structurally. 3.1. XLPA Spectroscopy. We now explore the consequences of the LD → OD → ANN structural evolution of the Fe active center from the point of view of its magnetic properties. Figure 2 reports the XLPA at the Fe L3 edge along the catalytic cycle, i.e., for the LD → OD → ANN evolution of the catalytically active site. The XLPA spectrum at photon incident angle γ = 70° yields the information on in-plane (V) and out-of plane (H) empty electronic states around the Fe atoms (see inset) because the absorption intensity is directly proportional to the number of empty valence states in the direction of the incoming photon electric field E (X-ray “searchlight” effect24). XLPA spectra of the LD and ANN phase are superimposable, confirming that from the point of view of the Fe electronic configuration the catalytic cycle is perfectly reversible. For this reason, the same curve in each polarization has been used for both phases in Figure 2a. The polarization dependence is strongly dichroic, in particular the intense peak (B) at 709.75 eV in H polarization decreases strongly in the V configuration. This behavior, in qualitative similitude to previous assignments,17 allows us to identify this peak with the excitation of the Fe 2p3/2 electrons to the out-of-plane empty antibonding dz2(up) state hybridized with the N-s and N-pxy states associated with the NN nitrogen atoms. The strong linear dichroism of the features in XLPA spectra are fully consistent with the local square planar coordination of iron within the Pc macrocycle lying parallel to the Ag surface, perturbed by the symmetry lowering due to the presence of the substrate, as reported in Figure 1c, left panel. These features are very similar to the XLPA spectra found for FePc deposited on Ag(001).10

scanned photon energy range. An XAS experiment on the virgin Ag substrate, cleaned by Ar etching, was performed at the same experimental conditions to verify that no contamination from spurious Fe existed. After the Ag background signal was subtracted, the Fe L2,3 edge XAS and XLPA contributions were extracted and normalized to the atomic continuum signal at E > 730 eV. 2.3. Density Functional Theory Calculations. The density functional theory (DFT) and DFT+U calculations were performed on FePc adsorbed on Ag(110) before and after the oxygen dosage, using the Quantum-ESPRESSO suite of codes.18 At variance with previously reported calculations,12 we exploited the full periodicity of the self-assemblies by inserting the system in the proper c(10 × 4) cell. The Ag surface was simulated by means of an asymmetric slab model of five atomic layers, where the bottommost two were kept frozen during geometry optimizations. Atomic coordinates were optimized until convergence thresholds of 10−3 Ry on energy and 10−4 Ry/bohr on forces were met. Wave functions were expanded on a plane wave basis set, with cut-offs on wave functions and augmentation charge set at 27 and 250 Ry, respectively, while the core−valence interactions were described through ultrasoft pseudopotentials.19,20 In plain DFT calculations, we used the PBE implementation of the exchange-correlation density functional,21 which was augmented by a Hubbard U correction, as implemented in quantum-ESPRESSO,22 in the case of DFT +U calculations. Hubbard U was applied to the Fe site only, and its effective value of 3.9 eV was taken from the literature.23 In both the DFT and the DFT+U cases, a Γ-point integration scheme was used during the relaxation runs; electronic structures were then refined by means of single-point calculations on a regular 2 × 2 k-point mesh. Atomic charges were computed within the Löwdin scheme to render deeper details on electron distribution around the Fe atom. The density difference was calculated by using the FePc molecule and the oxidized surface as fragments, both kept in their final geometries, i.e., the geometry they have in the LD phase.

3. RESULTS The catalytically active FePc sub-ML film of interest in this work displays two low-density superstructures simultaneously: a c(10 × 4) phase, previously denoted as R1_LD (Figure 1a, and Supporting Information Figure S1b) and a p(10 × 4) phase (Figure S1b), denoted as R2_LD.12 In R1_LD, Fe sits on top of an Ag surface atom, while in R2_LD, Fe bridges two nearby Ag atoms along the [11̅0] close-packed row (Figure S1b). The two phases also differ in the local azimuthal orientation of individual FePc units. However, the surface density (0.42 nm−2) and the reactivity toward oxygen are the same for both phases. Also, the iron square planar nearest neighbor (NN) coordination, perturbed by the presence of the substrate, is similar for the two phases (Figure 1c, left panel).12 For these reasons, in this work we shall mostly refer collectively to both R1 and R2 phases as LD, with no loss of detail. Where relevant, comments on particular features distinguishing the two phases will be made. The exposure to molecular oxygen of LD produces a single, well-defined oxygen-binding intermediate phase. Its most stable structure, compatible with experimental STM and spectroscopic evidence (XAS, XPS), is described by the so-called FePc/(η2-O2)/Ag(110) complex, denoted as oxygen-dosed (OD) (Figure 1, right panels), wherein the Fe atom at the center of the FePc molecule is placed on top of a surface Ag atom and is pulled toward the Ag surface by two C


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Figure 2. Normalized XLPA of the phases: (a) LD and ANN and (b) OD samples. Inset: scheme of the XLPA experiment. The incoming beam at grazing incidence is shown with the photon electric field, E, in the H and V polarization orientations. The features at A−E are explained in the Supporting Information, section S1. Vertical dashed lines help to follow the evolution of the peaks.

In contrast, the XLPA spectra of the OD phase are radically different from any of the other phases (Figure 2b), with a single intense peak D at 711.3 eV and with strongly suppressed dichroism with respect to the LD and ANN phases. The presence of unreacted FePc molecules (typically 30 ± 10% of the overall amount) can explain the residual presence of peaks A and B in the V and H polarizations, respectively. Such pronounced differences between the LD and OD XLPA spectra can be caused only by the rearrangement of the charges in the FePc−substrate complex. Indeed, the oxygen atoms in the OD environment that generate the local prismatic coordination modify the crystal field splitting of the Fe 3d states (Figure 1c, right). Overall, the XLPA analysis confirms the reversibility of the catalytic cycle, and the dramatic dichroism change has a direct correspondence with the geometry modification of the Fe NN coordination, previously determined by STM and XPD measurements and simulated by DFT.12 A more detailed tentative assignment of the different XLPA peaks to the available empty states is reported in the Supporting Information, section S1. 3.2. X-ray Absorption Spectroscopy. The XAS spectra of LD and ANN phases are practically identical, as shown in panels a and c of Figure 3, respectively. The peaks observed coincide in energy with the A, B, C, D, and E measured in the XLPA measurements shown in Figure 2. This confirms that the application of the 4 T magnetic field does not modify the electronic state of Fe in the molecule. Peak B, not visible at normal incidence, appears clearly as the incident angle increases, as expected for a peak associated with the excitation to the dz2(up) state. Very similar spectra were found for FePc deposited on a Ag(001) single crystal.10 In contrast, the XAS spectra of OD measured with circularly polarized light are nearly independent of incident angle (Figure 3b). They feature a prepeak, shoulder, high main peak, and

Figure 3. Normalized XAS of the LD, OD, and ANN phases at the Fe L3 edge. (a) LD phase. (b) OD sample. In the inset, a scheme of the circularly polarized incoming beam at incident angle γ is shown. (c) ANN samples. The features at A−E are explained in the Supporting Information, section S1. Dashed lines help to follow the evolution of the peaks. In the three experiments, T = 6 K and μ0H = 4 T.

shoulder at 709.06 (A), 710 (B), 711.33 (D), and 712 eV (E), respectively. Thus, the shape and evolution with incident angle of this phase is completely different from that of either the LD or ANN phases. The XAS spectrum resembles qualitatively that of an Fe(III) ion in an octahedral site.25 3.3. XMCD Spectroscopy. Figure 4a reports XMCD spectra at the Fe L2,3 edges measured at γ = 0° along the catalytic cycle, i.e., for the LD, OD, and ANN phases. A qualitative analysis of the spectra evidences a strong difference in magnetic behavior between the nonoxidized LD and ANN configurations and the oxidized OD phase. In the first two cases the spectra show just a minimum at the L3 edge with no fine structure and a nearly zero signal at the L2 edge (Figure 4a, inset). They are very similar in shape to those deriving from Fe on a metallic substrate.26 In contrast, the OD XMCD spectrum at γ = 0° has a 10-fold larger intensity and a very different shape: at the L3 edge, the spectrum has three main features with negative (F), positive (G), and negative (H) intensity as a function of increasing energy, which is characteristic of Fe in an oxidic (nonmetallic) environment subject to a crystal field.27,28 The XMCD at the L2 edge shows two maxima, also resembling those characteristic of Fe(III) compounds in Td or Oh symmetry.29 To confirm the emerging picture, Figure 4a also includes the result of multiplet calculations of the XMCD spectrum performed for an Fe(III) ion with 10Dq = 1.8 eV, Ds = Dt = 0 eV, in D4h symmetry under an applied field normal to the D


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Figure 4. Normalized XMCD spectra of the LD, OD, and ANN samples, at the Fe L2,3 edge (insets: zoomed XMCD spectra of the LD (blue) and ANN (black) samples). Incident angle: (a) γ = 0°; (b) γ = 55°. Note the quantitative and qualitative differences of the OD with respect to the other two phases. (a) (black thin line) Simulation of the XMCD spectrum of Fe(III) in tetragonal symmetry, calculated with the multiplets code CTM4XAS and parameters given in ref 30.

Figure 5. mL(γ)/nh and mSeff(γ)/nh as determined with the optical sum rules for dipolar excitations for (a) LD and (b) OD phases. (The lines show the fits to eqs 2 and 3 in Supporting Information, section 2.)

components of the orbital moment perpendicular (z) and parallel (xy) to the FePc molecular plane (Table 1). The experimental uncertainty is rather large because of the smallness of the XMCD signal. However, if one looks at the fitting results reported in Table 1, some conclusions are reached: both phases display planar anisotropy, with easy angular and spin magnetization directions in the xy plane, i.e., parallel to the Ag surface. In particular, for the LD phase the orbital magnetic moment has no perpendicular component. Moreover, for both phases the values of the spin magnetic moment are greater than the orbital magnetic moment by about one order of magnitude. Regarding the differences induced by oxygen dosing, as already observed from XMCD spectra in Figure 4, the magnetization value for the OD phase is distinctly larger than the value registered for the LD phases: more specifically the isotropic mS/nh undergoes a 5-fold increase, from 7.1 × 10−2 μB/hole to 3.7 × 10−1μB/hole. In addition, because at most 70% of the FePc molecules on the surface react with oxygen in the actual experimental conditions, the magnetic moment of the OD phase should be corrected by taking into account that 30% of the signal originates from nonoxidized FePc molecules, which means that the mS/nh value for the LD-subtracted (i.e., “pure”) OD phase is at least 5.0 × 10−1 μB/hole.

molecular plane,30 for qualitative comparison. The characteristic double minima in L3 and double maxima in L2 are clearly reproduced. In summary, through oxygen dosing and annealing, we are able to switch the magnetization of FePc on and off, with a signal change of one order of magnitude between the two states. XMCD spectra have been registered at different γ angles (Figure 4b reports the spectra at the magic angle γ = 54.7°. The contribution to the XMCD signal from the magnetic moment m⃗ of a molecule subjected to an applied field is proportional to the projection of m⃗ in the direction of the field (in this case parallel to the X-ray beam direction). The optical sum rules for dipolar excitations31,32 have been applied to obtain the orbital magnetic moment mL(γ)/nh and the effective spin moment mSeff(γ)/nh, per 3d valence hole, where nh is the number of holes in the 3d band. These values obtained at various incident angles have been reported in Figure 5 for the as-deposited LD and the oxidized OD phases. One should note that m⃗ Seff = m⃗ S − 7m⃗ T, where mS is the isotropic spin moment and mT is the intra-atomic dipolar component. In fact, the mT component is quite significant because it dominates the dependence of mSeff on the angle of incidence. In the present experiments, the mT component has been determined from the fit to the angle-dependent expression of mSeff(γ) (see Supporting Information, section S2). Moreover, because the isotropic spin moment component, mS, can be obtained from the measurements at the magic angle by the relation mSeff (γ = 54.7°)/nh = mS /nh,33 we have also measured the XMCD at this particular angle to obtain the purely spin component. Subsequently, the mT contribution has been subtracted to mSeff to obtain the purely spin moment component. The fitting of the experimental curves in Figure 5 allows us to determine the isotropic spin component, mS, and the directional

4. DISCUSSION To obtain the actual magnetization values and to afford a comparison with other systems, nh has been estimated from ab initio DFT calculations including a Hubbard term (DFT+U)22 on FePc adsorbed on Ag(110) before and after the oxygen 3d 3d 3d dosage as nh = 10 − (n3d e ↑ + ne ↓), where ne ↑ and ne ↓ are the number of 3d electrons in the majority spin-up and minority spin-down states, respectively (for technical details, see Supporting Information, section S3). Moreover, DFT calcu3d lations yield the magnetization of the 3d band mDFT Tot = (ne ↑ 3d −ne ↓)μB. Table 2 reports the calculated nh for the two phases, the values of mL and mS obtained from the experimental data E


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The Journal of Physical Chemistry C Table 1. Magnetic Moment Parameters (in μB/hole)a mLz/nh LD OD ODb ANN

0 1.2 × 10−2 1.7(1) × 10−2 4.0 × 10−3

thin film sub-ML Au a

mLxy/nh 6.5 2.2 2.8(2) 1.5

× × × ×

mTz/nh

mS/nh −3

10 10−2 10−2 10−2

7.1 3.7 5.0(3) 7.1

× × × ×

−2

10 10−1 10−1 10−2

4.3 2.9 3.9(2) 5.9

× × × ×

ref.

10−3 10−2 10−2 10−3

this this this this

1.1 × 10−1

2.0 × 10−1

2.4 × 10−1

2.7 × 10−2

17

1.1 × 10−2

3.4 × 10−2

3.7 × 10−2

2.9 × 10−2

34

work work work work

Estimated error of ±5%. After correcting for the presence of 30 ± 10% LD phase. b

Table 2. Estimated Magnetic Moments Compared to Simulationsa nh = 10 − neDFT R1_LD R1_ODc TF17,b

3.4 4.1 2.67

mLxy

mexp Tot

mS −2

2.2 × 10 1.1 × 10−1 5.3 × 10−1

α-Fe2O3 (Fe(III))25,d

−1

2.4 × 10 2.0 6.4 × 10−1

mDFT Tot −1

2.6 × 10 2.1 1.2

2.0 3.9 6.9 × 10−1

4.222(9)

a

nh is the number of holes in the 3d band (calculated by DFT+U). mLxy and mS (μB) are calculated from the experimental data included in Table 1, xy DFT 3d 3d multiplied by nh calculated with the DFT+U method. mexp Tot = mL + mS (in μB) is the maximum Fe moment (3d states); mTot = (ne ↑ − ne ↓)μB (calculated by DFT+U). bTF: thin film. cCorrected of LD contribution. dAs deduced from neutron diffraction.

reported in Figure 5, along with the calculated mDFT Tot . Data for a FePc thin film are included for comparison.17 In qualitative agreement with experiments, DFT+U calculations predict a large increase in the magnetic moment on the Fe atom. Indeed, the spin polarization on the Fe 3d-band changes from mDFT Tot = 2.0 μB in the R1_LD to 3.9 μB in the OD phase (see Table 2). When the occupation of Fe d orbitals is considered, it appears that the spin moment increase is due both to a decrease in the population of minority spin (S−) and to an increase in the population of majority spin (S+) states. Indeed, on passing from the LD phase to the OD phase, the S+ Fe 3d orbitals acquire 0.6 electrons, whereas the S− ones lose 1.3 electrons (see Supporting Information, Table S1). As expected, the interaction with oxygen causes the Fe atom to acquire a larger positive charge, suggesting an increase in the oxidation state of Fe. Accordingly, each O atom carries a net charge of 0.5 e−. We infer from the density difference map shown in Figure 6 that

parallel to the molecule is an order of magnitude smaller than the predicted value (Table 2). In XMCD measurements performed on a FePc monolayer on Au(110), a similarly reduced Fe magnetic moment in the molecular plane has been reported, (mSeff + mL)// = 0.23 μB,35 and tentatively explained34,36 as caused by charge transfer between the d6 Fe configuration and the metal substrate through its dz2 orbital, assuming a weak mixing with the d7 Fe configuration. In addition, similar to our theoretical results, the magnetic moment residing at the Fe atom mFe = 2.14 μB, calculated by means of DFT calculations with the generalized gradient approximation including a Hubbard term (GGA+U), yields also a too large value compared to the experiment.37 It is interesting to note that in a film of 0.5 ML FePc deposited on a ferromagnetic substrate, such as Co(001), where FePc is ferromagnetically coupled to the Co substrate, the magnetic moment of Fe, (mSeff + mL)// = 0.56 μB (S = 1 state), is also smaller than the bulk value.35,38 However, in the Co substrate case the total moment of the Fe ion in FePc is not quenched so strongly as when deposited on the nonmagnetic Ag or Au substrate. When the Co substrate is previously oxidated, the magnetic moment of the deposited FePc is coupled antiferromagnetically to the susbstrate, but its moment remains in the S = 1 state.16 In this case, DFT calculations (GGA+U) also lead to predicted values of mFe which are too high with respect to the experimental values.39 At any rate, one concludes, on one hand, that the degree of hybridization with the substrate depends strongly on the substrate metal and, on the other hand, that DFT methods predict only qualitatively the observed magnetic moments of Fe in the FePc molecule. Most importantly, mSexp = 2.0 μB is nearly one order of magnitude larger in the OD case with respect to the asdeposited LD phase. This is the most relevant result of this work because it corresponds to the sharpest chemically driven spin switch of surface-supported molecular magnets reported to date. It can be concluded that the intercalation of oxygen between the molecule and the substrate breaks the strong hybridization between the Fe 3d and the Ag metallic states, so

Figure 6. Charge density difference map in the O−Fe−O plane. Blue (red) regions represent electron depletion (accumulation).

the Fe−O interaction is mostly electrostatic in nature. Indeed, electron density depletion is clearly visible around the Fe atom, while the O atoms acquire electrons, with only a small charge buildup along the Fe−O bond axis. However, the comparison of experimental and DFTcalculated magnetic moments shows that the maximum total moment, mexp Tot = 0.26(1) μB, of LD with the applied field F


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The Journal of Physical Chemistry C that FePc is effectively decoupled from the underlying metal states. To compare the magnetic moment values with a well-known reference Fe(III) system, in Table 2 we have included an entry for α-Fe2O3 (hematite), which has all Fe atoms in octahedral sites with Fe(III) valency and high spin, S = 5/2. Its magnetic moment, as determined from neutron diffraction, is mS = 4.222(9) μB at 10 K,25 which is much higher than in the OD phase. Another possibility is that Fe(III) in FePc/Ag is in the intermediate spin state S = 3/2, as is likely to happen in Fe porphyrin deposited on an oxygen monolayer that covers a metallic (Ni) substrate.40 Therefore, the magnetic moment of OD (mexp Tot = 2.1(2) μB), even after correction for the presence of LD, is about 50% lower than expected for a fully transformed high-spin Fe(III) atom (mS = 5 μB) but is close to the case of intermediate spin Fe(III) (mS = 3 μB).

The present work shows that the FePc/Ag(110) interface acts as a reversible bifunctional chemical−magnetic switch at the atomic monolayer scale.

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ASSOCIATED CONTENT

S Supporting Information *

The empty electronic states of the RD, LD, and ANN phases and XLPA spectra (section S1); dependence of the XMCD on incidence angle (section S2); DFT and DFT+U population anlysis (section S3). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02916.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34 976 761218. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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5. CONCLUSIONS We have explored the oxygen-induced magnetic moment switching of FePc molecules involved in the catalysis of the ORR when deposited on Ag(110). As expected, the sub-ML FePc low-density phases show a metallic-type XMCD spectral line shape and very small net moment. Curiously, the normal contribution to the orbital magnetic moment is zero, i.e., it has only an in-plane component. Upon O2 dosing, the system evolves to the OD, FePc/(η2O2)/Ag(110) phase, whose XMCD line shape changes abruptly to become similar to the one expected from an Fe(III) ion. In both phases magnetic planar anisotropy is observed. The total fraction of oxidized molecules amounts to about 70% of the deposited FePc molecules. The estimation of the spin moment (mS = 2.0(2) μB) suggests that Fe(III) is in an intermediate spin state, S = 3/2, caused by the ligand field of the N4−O2−Ag C2v coordination. The previously discussed structural reversibility of the molecule−substrate catalytic system involved in the ORR process12 shows up nicely also in the behavior of the Fe 3d electronic states because the XAS and XMCD spectra of the annealed phase ANN are nearly identical to those of the LD starting phases. However, because upon annealing oxygen diffuses into the Ag substrate, the process is not fully cyclic unless oxygen is removed by supplying low-energy ionized hydrogen.12 The switching of the Fe magnetic moment that accompanies the ORR process is also reversible. We measured an increase in the Fe magnetic moment by about an order of magnitude when oxygen is dosed on LD to produce the OD phase. Oxygen binding breaks the Fe 3d−Ag 5s electron hybridization, and the FePc magnetic behavior in the OD phase becomes similar to that observed for Fe porphyrin submonolayers deposited on oxygen adsorbed on a Ni surface.40 Finally, strong indication of a crossover from metallic Fe to Fe(III) localized magnetic moment evolves from the analysis of the XLPA and XMCD line shapes of the OD sample at normal incidence. Moreover, a comparison to multiplet calculations for an Fe(III) ion and DFT+U calculations to predict the magnetic moments of the oxidized FePc/Ag system, clearly indicate that electron depletion of the Fe ion and negative charge uptake at the O atoms has taken place. Therefore, the observed extraordinary effect can be explained as caused by an electron transfer from Fe toward the O atoms.

ACKNOWLEDGMENTS Financial support from the projects MINECO MAT2011/ 23791, MAT2014-53921-R, and DGA IMANA E34, as well as MIUR project PRIN 2010BNZ3F2 “DESCARTES” and Progetti di Ricerca di Ateneo, (CPDA118475/11) are acknowledged here. The ESRF data correspond to the CH3747 experiment.

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Reversible Fe Magnetic Moment Switching in Catalytic Oxygen

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