New Insight into the Electrochromic Properties of Iron Oxides

Marco A. Garcia-Lobato, Arturo I. Martinez, Ramon A. Zarate, and Manuel Castro-Roman

Appl. Phys. Express 3 (2010) 115801

Reprinted from

# 2010 The Japan Society of Applied Physics

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New Insight into the Electrochromic Properties of Iron Oxides

Marco A. Garcia-Lobato, Arturo I. Martinez�, Ramon A. Zarate1, and Manuel Castro-Roman

Cinvestav, Unidad Saltillo, Carr. Saltillo-Monterrey Km. 13, Ramos Arizpe, Coah. 25900, Mexico1Departamento de Fısica, Facultad de Ciencias, Universidad Catolica del Norte, Casilla 1280, Antofagasta, Chile

Received September 16, 2010; accepted October 21, 2010; published online November 12, 2010

We report on the structural, optical and magnetic properties of iron oxide films that were electrochemically cycled in a LiOH aqueous solution. We

found that the electrochromic phenomenon is linked to the transformation of the film morphology; it goes from round-shaped particles to platy

morphology. Additionally, the following phenomena were observed: a gradual blue shift of the optical-absorption edge, an increase of the

saturation magnetization and the appearance of new Raman bands. The change of these properties helped us to understand the coloration

mechanism for electrochromism in iron oxides. # 2010 The Japan Society of Applied Physics

DOI: 10.1143/APEX.3.115801

Iron oxides have been widely used from prehistoric tohigh-tech settings.1,2) They have been the subject of nu-merous scientific investigations; despite this, the electro-

chromic properties have received little attention.3,4) Sincethe first published studies that describe the electrochromicproperties of iron oxides, some discrepancies can be found.Early reports describe that iron oxides exhibit a bleached stateat cathodic potentials or an anodic-like coloration.5–8) Thesestudies employed iron electrodes oxidized electrochemicallyin an aqueous solution of OH� ions; subsequently, the thickoxide layers were cycled displaying electrochromic activity.These works do not describe precisely what the initial ironoxide present in the films is; another point is that the preparedelectrochromic films on iron rods were not able to developpractical applications.5–8) Afterward, iron oxide films pre-pared by sol–gel also exhibited bleached states at cathodicpotentials;9,10) these papers concluded that only the �-Fe2O3

phase displays electrochromism, while the �-Fe2O3 phase ischromogenically inert.9) Furthermore, other works reportedthat iron oxide films immersed in organic solvents get dark atcathodic potentials with a little optical modulation when Liþ

ions are inserted.11) In contrast, higher optical modulation hasbeen reported for amorphous and �-Fe2O3 films.12,13) Includ-ing a double electrochromic behavior was described later foriron oxide films with sulfate residues.14) All these discrepan-cies can be caused by the presence of different phases of theiron oxide films and the electrolytes/solvents used in theinsertion/extraction electrochromic cycles. Currently, as wasalso mentioned by Granqvist, the experimental basis is toosmall to allow a meaningful discussion of the mechanismfor the electrochromism in iron oxides.3) In order to depictthe coloration mechanism of iron oxides, this letter relatesthe optical, structural and magnetic changes with the colormodulation of iron oxide films.

Iron oxide films were prepared on SnO2:F (FTO) sub-strates (5�/square) by the spray pyrolysis technique. Forthe spraying process, a 1M aqueous solution of FeCl3 wasatomized through a pneumatic nebulizer operated at 2 bar ofcompressed dried air. The depositions were carried out insidea gas extraction chamber at 200 �C. The substrate temperaturewas regulated with a temperature controller and was keptconstant during the deposition time by pausing the sprayingprocess for intervals of 10 s. In order to decrease the residualstress of the film/substrate system, the coated substrates wereslowly cooled until reaching ambient temperature.

In-situ optical changes of iron oxide films during theelectrochemical cycles were registered in an Ocean OpticsUSB4000-VIS-NIR spectrometer at wavelengths from 350to 930 nm. The intercalation/deintercalation of charge wascarried out through cyclic voltammetry and chronoampero-metry in a Pine Wavenow potentiostate; the electrolyte usedwas LiOH dissolved in water. For the spectro-electrochemi-cal experiments, an iron oxide film with a thickness of150� 20 nm, the Ag/AgCl reference electrode and a Pt-wire(counter electrode) were immersed in a quartz cell contain-ing the electrolyte. The structures of as-prepared, bleachedand colored films were determined using micro-Ramanspectroscopy in a Witec CRC200 (laser � ¼ 514:5 nm). Inorder to minimize the structural changes promoted by thelaser, a light intensity as low as 0.7mW was used for theRaman characterization.15) The morphology of the as-prepared and cycled films was determined by atomic forcemicroscopy (AFM). The thickness of the iron oxide filmswas measured using a Vecco Dektak-8 Stylus profiler. Themagnetic properties were measured at room temperature inan alternating gradient magnetometer AGM Micromag 2900by Princeton measurements.

Figure 1 shows the Raman spectra of as-deposited andcycled iron oxide films. It is observed that as-deposited filmsexhibit the typical hematite bands located at 226, 245, 292,411, 497, and 612 cm�1. After a procedure of intercalation/deintercalation of charge, the films transformed to anotherstructure, which was identified as �-FeOOH, see upperspectra of Fig. 1. Here, it is important to note that theRaman spectra was taken when the films were out of theelectrochemical cell, and the measurements revealed someimportant points for the electrochromic behavior of ironoxides. The Raman spectra of films extracted at cathodiccycles yield the same Raman spectra as films extracted atanodic cycles. This indicates the instability of the bleachedstate in open atmosphere, because the films transformsinstantaneously from transparent to yellow in color outsidethe electrochemical cell. The hematite to �-FeOOH trans-formation goes together with a surface morphology conver-sion from round-shaped particles to platy morphology asshown in the AFM images in Figs. 1(b) and 1(c). The platymorphology is typical of feroxyhyte (a low crystallized formof �-FeOOH) nanopowders, where the platelets are of around20–80 nm in length, with a thickness of 2–8 nm.16–18) In thiswork, the lengths of the �-FeOOH plates are around 300 nm,and the estimated thickness from a higher resolution imageis around 15 nm. The root mean square roughness for the�E-mail address: mtz.art@gmail.com

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hematite films surface is 104 nm, while for the �-FeOOHsurface is 161 nm.

Figure 2(a) shows the changes in the transmittance (T )spectra at different cycles of intercalation/deintercalation ofcharge; here, the films were electrochemically cycled from�1:1 to 0.2V (vs Ag/AgCl). Firstly, at early intercalationcycles, a double electrochromic behavior was observed [seeFig. 2(a)]. The double-like behavior is because at the firstcathodic cycle, there is an increase of T at short wavelengths(�) while a decrease is observed at long � . However, aftermore than 25 intercalation cycles (aged films), only theincrease of T is observed in the entire � range; this meansthat only the transparent state is observed at subsequentcathodic half-cycles. The transition from the transparentstate at the cathodic half-cycle to the colored state at theanodic one is clearly observed in Fig. 2(a); see the spectradenoted as aged film. In the T spectra of Fig. 2(a), a gradualblue shift of the absorption edge at anodic potentials can beappreciated when the cycle numbers increase. This suggeststhat the original hematite structure is not restored at theanodic half-cycle and that there is a transformation to anoxy-hydroxide (oxy-hydroxides exhibit their energy bandgap at lower � than hematite).8) The transformation fromhematite to an oxy-hydroxide (�-FeOOH) was also con-firmed by both Raman spectroscopy and AFM.

In a further analysis, the kinetics of the electrochromicphenomenon was studied when the iron oxide films weresubmitted to a constant potential for different times whileT was monitored. Figure 2(b) shows the results of theseexperiments when a film is submitted at a cathodic potentialof �1:1V in a T=Ti vs time graph, where Ti is the initialtransmittance of each state of the film. If T=Ti is smaller than1, the film gets less transparent at a given wavelength. Forclarity of the figure, the time axis of the first intercalationcycle is divided by 10; this means that the electrochromicprocess is 10 times slower than that observed in aged films.This indicates that at the first cycle, the intercalation ofcharge goes with an additional slower process. As AFMimages indicate, the process that goes along with intercala-tion of charge is the transformation of the morphology fromround-shaped nanoparticles of hematite to platy morphol-ogy. For � ¼ 400 nm in Fig. 2(b), it is observed that agradual increase of the transparency takes place for bothnew and aged films, while for � ¼ 750 nm, a slight decreaseis observed at short times, followed by a rise in thetransparency; again, the time scale of this phenomenon is 10times higher in new films. In new films, the slight darkishhue is observed for more than 100 s, while for aged films,this phenomenon is observed for only 10 s. The inset ofFig. 2(b) shows that the colored state is completed at almostthe same time (�20 s); these results reveal that the reversalswitching speed does not dependent on the cycle number.

The magnetic measurements also have valuable informa-tion regarding the phase transformations. Figure 3 shows the

(a)

(b) (c)

Fig. 1. Structural properties of iron oxide thin films. (a) Raman spectra

of iron oxide thin films before and after intercalation/deintercalation of

charge. AFM images for (b) as-deposited �-Fe2O3 and (c) �-FeOOH films.

(a)

(b)

Fig. 2. Optical properties of iron oxide films measured inside of the

electrochemical cell at different states of intercalation/deintercalation of

charge. (a) Transmittance (T ) spectra of an iron oxide thin film taken at

different numbers of intercalation/deintercalation cycles. (b) Evolution

of T (at fixed �) while a cathodic potential is applied (�1:1V) to the

electrochromic film at different numbers of intercalation/deintercalation

cycles. For clarity, in this graph the time axis of the first intercalation cycle

is divided by ten. The inset shows the evolution of T (at � ¼ 750 nm) while

an anodic potential is applied (0.2V).

M. A. Garcia-Lobato et al.Appl. Phys. Express 3 (2010) 115801

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changes that take place in the magnetic properties of ironoxide films at different electrochromic cycles. It was foundthat while the number of electrochromic cycles increases,the saturation magnetization increases and the coercivitydecreases. This indicates that a phase transformation takesplace, from a weakly ferromagnetic material with a highcoercive field to a more ferromagnetic material with a smallcoercivity. The magnetic characteristics of the first materialare typical of hematite, while ferromagnetic and smallcoercive fields are typically exhibited by the �-FeOOHphase, which is the unique ferromagnetic oxyhydroxide.1,16)

Based on a literature search regarding the properties ofiron oxides and hydroxides, the bleached state may beidentified as Fe(OH)2.

1,16) Fe(OH)2 is a white powderexhibiting a hexagonal structure, its band gap lies at theUV part of the spectrum, and it is unstable in air.1) Thetransformation of Fe(OH)2 to �-FeOOH is a topotacticredox reaction, where an electron and ion transference takesplace. In the reaction, each FeII(OH)6 octahedron becomesa FeIII(OOH)3 octahedron while the hexagonal structure isconserved.1,19,20) The platy morphology is typical of layeredstructures such as those shown in Fe(OH)2 and �-FeOOH.

1,20)

Then, for rapid color changes, the original hematite phase hasto be transformed into a layered structure such as Fe(OH)2at cathodic potential (�1:1V); if the potential is inverted(0.2V), �-FeOOH results. This means that hematite is out ofthe electrochromic cycle; this conclusion can be extended toother ferric oxide phases, because for rapid color changes,a topotactic transformation has to take place.3) Maghemitecan also be excluded from the reversible electrochromiccycling because it is cubic or tetragonal,1,16) and no topotactictransformation can take place. This is in contrast to theconclusion that maghemite exhibits electrochromis.9) Hema-tite, which is away from the cyclic electrochromic mechan-ism can be obtained again by annealing the electrochromic�-FeOOH films at 200 �C. Additionally, in an interestingmass-balance diagram of the topotactic transformation ofFe(OH)2 to �-FeOOH, magnetite is reported as an inter-mediate state;19) this could explain the appearance of theslight darkish hue observed for short times before completelyreaching the transparent state. The mechanism that includesthe topotactic and non-topotactic transformations is schema-tized in Fig. 4. The scheme shows that different iron oxidesparticipate in the electrochromic reverse cycle and hematite

only contributes as a starting phase but can be obtained againby annealing the cycled films.

In summary, we have described in this letter the exper-imental basis that allows a meaningful discussion of themechanism for the electrochromism in iron oxides. Contraryto other works that conclude that a dehydrated iron oxidescan exhibit cyclic electrochromic properties, in this letterwe show that dehydrated iron oxide is only a starting phasefor the transformation to a Fe(OH)2 layered phase and thatsubsequent reversible cyclic transformations are the typicaltopotactic reactions that have been observed in iron oxides.

Acknowledgments This work was supported by the initiative of

multidisciplinary projects of Cinvestav. The support of Conacyt is also

recognized. We greatly thank M. A. Gatica and N. Pariona-Mendoza for

fruitful discussion. M.A.G.L. thanks the doctoral scholarship sponsored by

Conacyt. R.A.Z. thanks to Fundacion Andes under the project C-13876.

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Fig. 3. Changes in the magnetic properties that take place during

electrochromic cycling.

Fig. 4. Topotactic (filled lines) and non-topotactic (dashed lines) trans-

formations in iron oxides. The reversible reactions indicate that they

participate in the coloration mechanism for electrochromism in iron oxides.

M. A. Garcia-Lobato et al.Appl. Phys. Express 3 (2010) 115801

115801-3 # 2010 The Japan Society of Applied Physics