ORIGINAL PAPER

Lattice polarization effects in electrochromic switching in WO3−xfilms studied by pulse-nanogravimetric technique

Maria Hepel & Lumbini Indee Dela-Moss & Haley Redmond

Received: 2 June 2013 /Revised: 4 August 2013 /Accepted: 7 August 2013 /Published online: 24 August 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract The interactions of various types of cations with thetungsten trioxide lattice have been investigated to evaluatepossible intercalation of these cations and the occurrence oflattice polarization leading to the near-surface structural latticedamage. The interactions of cations, such as the large mono-valent cations (K+, Et4N

+, CtMe3N+ cations), transition metal

dications (Ni2+), heavy metal ions (Cd2+), and representativelanthanides (La3+) and actinides (Th4+), in competition withintercalation of H+ ions have been investigated using pulse-nanogravimetric technique. The effects of these cations inelectrochromic processes of WO3 proceeding during cathodicreduction have been assessed. For all of the metal ions studied,a large increase in the apparent mass uptake (up to eightfold)in comparison to pure H+ ion ingress was observed upon thefilm coloration induced by a cathodic potential pulse. Theexperiments indicate that the apparent mass gains, althoughlarge, are insufficient to confirm predominant contribution ofmetal ions in the ion transport along the channels in WO3

lattice. The lower decoloration rate in the case of Ni2+ ions, incomparison to H+ and other transition metal cations (Cd2+),has been attributed to a slow dissociation of Ni2+ ions fromlattice-bound oxygen atoms. For et4N

+ cation, which is toolarge to enter channels in WO3, a dissociative reduction of theWO3 and severe lattice damage was observed. Among themetal ions investigated, only K+ ions have been found tocause a dissociative reduction of WO3 and near-surface latticedamage. Strong lattice polarization effects and irreversiblebinding have been found for La3+ and Th4+ cations.

Keywords Intercalation .WO3 electrochromic films .WO3

lattice polarization . Dissociative reduction . Heavymetalcation insertion .WO3 interactionwith La(III) and Th(IV)

Introduction

Transition metal oxides, and among them the tungsten trioxide,have been extensively studied in view of their interesting semi-conducting, optical, and catalytic properties. The promising ap-plications of tungsten trioxide films span from electrochromicdisplay panels [1–6] and smart windows [7], through applica-tions in gas sensors [8], to solar energy conversion devices[9–14], catalysis [15–18], and photoanodes for environmentalpollutant degradation [19–21]. In this work, the interactions ofdifferent cations with WO3 lattice channels have been exploredin view of electrochromicWO3 utilization and the effects of theseinteractions on the induced lattice polarization and protectionagainst lattice damage have been elucidated.

The fundamental mechanism of WO3 electrochromism isbased on simultaneous injection of electrons and small cations(H+, Li+) [22, 23], with concomitant reduction of W6+ to W5+

and W4+ in WO3 lattice. The cation insertion process(intercalation) induces substantial changes in electronic andoptical properties ofWO3 [24, 25] manifested by intense coloralteration. Thus, in the general reaction:

WO3-x + yM+ + ye- MyWO3-x

the color of WO3 film changes from transparent colorless todeep blue which is attributed to the formation of small po-larons [26]. For an efficient electrochromic process, the size ofan intercalating cation should be sufficiently small [27–32] toavoid detectable lattice polarization and irreversible latticeexpansion that may result in lattice damage and even destruc-tion of the film [6, 33, 34]. The use of Li+ ions in electro-chromic devices may serve as an example of successful

Dedicated to the memory of Professor Vladimir Sergeevich Bagotsky

M. Hepel (*) : L. I. Dela-Moss :H. RedmondDepartment of Chemistry, State University of New York at Potsdam,Potsdam, NY 13676, USAe-mail: hepelmr@potsdam.eduURL: www2.potsdam.edu/hepelmr

J Solid State Electrochem (2014) 18:1251–1260DOI 10.1007/s10008-013-2219-8

application of tungsten trioxide for electronic displays. On theother hand, when small cations are not available, we havefound that the reduction results in the decomposition of aWO3 lattice [6] instead of the intercalation. Therefore, wehave postulated that an efficient intercalation shields thetungsten oxide films against lattice polarization and disso-ciative reduction [6].

In this work, we have further investigated the effects asso-ciated with interactions of various metal cations with WO3

lattice cages to explore the preventive measures against dis-sociative reduction of WO3 and to evaluate the intercalation/attachment of uncommon metal cations in the ion transportchannels in WO3 lattice. The WO3 nanoparticles with hexag-onal crystal structure and well-defined Raman vibrationmodes were used in this study.

Experimental

Chemicals

High-purity tungsten metal powder (99.95 %) with 1 μmparticle diameter was purchased from Alfa Aesar (Ward Hill,MA, USA). The concentrated (30 %) hydrogen peroxide(H2O2) and tetraethyl ammonium chloride (Et4NCl, TEAC)were obtained from Sigma (St. Louis, MO, USA). Hexadecyltrimethyl ammonium bromide (CtMe3NBr, CTAB) wasobtained from Alfa Aesar. All other reagents including saltsof Ni(II), Cd(II), La(III), and Th(IV) were of analytical gradepurity and were used without further purification.

Apparatus

The electrochemical setup employed for voltammetric andnanogravimetric measurements consisted of an ElchemaPotentiostat/Galvanostat, Model PS-205B, an electrochemicalquartz crystal nanobalance, Model EQCN-700 (Elchema,USA), and a Data Logger and Experiment Control System,Model DAQ-716v, operating under Voltscan 5.0 data acquisi-tion and processing software. A double-junction saturated(KCl) Ag/AgCl electrode was used as the reference electrodeand Pt wire as the counter electrode. Mirror polished gold-coated quartz crystal piezoresonators (QC-10Au-PB) withresonant frequency of 9.975 MHz, used as the substrates forworking electrodes, were obtained from Elchema. The geo-metrical surface area of the working electrode was 0.1963 cm2

and the roughness factor RAu=1.3. The EQCN procedure andinterpretation were described previously [35–37]. The appar-ent mass changes Δm were calculated from the fundamentalfrequency shift Δf using the equation: Δm =−0.8673 Δfwhich is based on Sauerbrey theory [35, 38, 39]. The apparentmass change includes changes in the filmmass and effects duesurface stress [1]. The Raman spectra were recorded using a

Nicolet DXR Raman Microscope (Thermo Fisher Scientific,Waltham, MA, USA). The samples of WO3 for Raman spec-troscopic measurements were prepared from a WO3 platingsolution after catalytic decomposition of H2O2 ligand anddeposition on a smooth gold electrode on a quartz crystalpiezoresonator wafer. Raman measurements were performedin a closed chamber using stabilized 633 nmHe–Ne laser with8 mW power, focused onto a 0.8-μm diameter spot, andmeasured in the spectral range of 500–3,500 cm−1.

Procedures

Tungsten trioxide films were synthesized by the electrochem-ical deposition procedure described earlier [19, 40]. Briefly,the electroplating solution for tungsten trioxide depositionwas prepared by dissolving a metallic tungsten powder (1 g)in concentrated (30 %) H2O2 (5 mL). After the exothermicreaction has ended, the solution was diluted with 2-propanol(20 mL) and distilled water (45 mL). The tungsten dissolutionproceeds with the formation of complexes of W6+ with H2O2,viz. WO3(H2O2)H2O, equivalent to pertungstic acid with for-mula: H2WO5·H2O. The obtained solution was slightly acid-ified with diluted H2SO4 solution (avoiding precipitation ofWO3) and part of the excess of H2O2 was catalyticallydecomposed over a Pt black mesh. The WO3 films wereprepared by potentiostatic electrodeposition from this platingbath on highly-oriented pyrolytic graphite (HOPG) substratesand on an Au-coated AT-cut quartz crystal (QC)piezoresonator wafers for pulse-nanogravimetric experiments.Films were quickly and thoroughly washed with distilledwater and 1 mM H2SO4 solution. The WO3−x films thusobtained were characterized using atomic force microscope(AFM) and Raman spectroscopy (Fig. 1).

The AFM images presented in Fig. 1a, obtained for WO3−x

films deposited at E =−900 mV vs. Ag/AgCl reference, indi-cate that a single nanoparticle layer of WO3 has been formedwith nanoparticle mean diameter of 28±5 nm. These films aretoo thin to provide sufficient contrast in electrochromicswitching. Therefore, the depositions of WO3−x films forpulse-nanogravimetric experiments were carried out at a muchlower cathodic overpotential that favors growth over the nu-cleation process. The AFM image in Fig. 1b presents such afilm obtained at E =−450 mV on a QC/Au substrate. Thenanoparticle diameter is typically 400±75 nm. The Ramanspectra of as-synthesized WO3 films show Raman modes atwave numbers 940, 801, and 687 cm−1. As we have shownearlier [41], the Raman spectra are very sensitive to structuralmodifications of the material used for measurements. WO3

films obtained in our syntheses are hexagonal [42], and theirRaman spectra differ from those for monoclinic m -WO3

which show two Raman vibration modes at 809 and718 cm−1 [43]. The m -WO3 films were not used in ourexperiments because of low intercalation rate.

1252 J Solid State Electrochem (2014) 18:1251–1260

The molecular dynamics and quantum mechanical calcu-lations of electronic structure for WO3, EtN

+, and CtMe3N+

were performed using density functional theory (DFT) withB3LYP functional and 6-311G* basis set and semiempiricalmethod PM3, embedded inWavefunction Spartan 12 [44, 45].The electron densities d are expressed in atomic units, au−3,where 1 au=0.52916 Å and 1 au−3=6.7491 Å−3.

Results and discussion

Theory

The frequency shift Δf measured by the electrochemicalquartz crystal nanobalance instrument consists of several com-ponents [35]:

Δf ¼ Δ fm þΔ fs þΔ fv þΔ fx ð2Þ

where Δfm, Δf s, and Δfv are the frequency shifts due to thechange of film mass, stress imposed on the crystal, and visco-elastic properties of solution, respectively. The main contribu-tions to the measured Δf signal have been Δfm+Δf s, whilethe contributions of Δfv and other influencing factors Δfxhave been minimized by maintaining constant temperature,pressure, and solution viscosity. The separation of terms

Δfm+Δf s is difficult when stress change is nonnegligible [1,46, 47] and more than one kind of species contribute to themass change. Fortunately, for WO3 films, it has been foundthat the stress-related termΔf s increases with increasingΔfmduring the intercalation process so they do not cancel eachother. Moreover, the two terms can be accurately determinedfor hydrogen intercalation using the isotopic method [1] withD2O solution replacing H2O which increases the true massresponse by a factor of 2, while the stress-related contributionremains unchanged for the intercalation of D+ and H+. Inaddition to that, for the low-stress WO3 films that we havedeveloped recently [1],Δfm andΔf s are approximately equalfor hydrogen ion intercalation so thatΔfm≈Δf s and thusΔf ≈2Δfm. Therefore, for each new WO3 film, we can determinethe true mass response and the stress-related response forhydrogen ion intercalation. In the following experiments, thecontribution of metal ions in the intercalation process is esti-mated not to exceed 2–10 %, the remaining contributionascribed to hydrogen ions. Hence, the overwhelming part ofthe stress effects is generated by hydrogen ions rather than bymetal ions. On the other hand, stress increases in proportion tothe partial flux of ions and their size but is invariant with themass of ions. This means that heavy metal ions contributestrongly to the Δfm term due to the large molar mass differ-ential with respect to hydrogen, while the Δf s term increasesapproximately proportionally to the total flux of ions, popu-lated mostly by hydrogen ions. The molar flux of cations

Fig. 1 a , b AFM images of athin film of WO3 nanoparticleselectrodeposited on a HOPGsubstrate at E=−900 mVand bQC/Au piezoelectrode at E=450 mV, with scratched surface ina to show thickness of thedeposit; c Raman spectrum ofWO3 nanoparticles synthesizedby hydrogen peroxide complexprocedure; and d expanded viewof the low frequency region of theRaman spectrum; excitation: He–Ne laser, λex=633 nm, power,8 mW

J Solid State Electrochem (2014) 18:1251–1260 1253

ψ =(1/S )(∂n /∂t ), where S is the surface area, n is the numberof moles of cations, and t is the time, can be expressed by:

ψ ¼ ψH þ ψM ¼ 1

FS

∂qH∂t

� �þ 1

zFS

∂qM∂t

� �ð3Þ

Here, the indices H and M stand for the hydrogen andmetal ions, respectively, q is the charge consumed in thereduction (oxidation ) process, z is the ionic charge(valency) of a metal ion, and F is the Faraday constant (F =96,485 C/val). After introducing the competitive contributionof metal ions x to the molar flux, one obtains:

ψ ¼ 1−xð ÞFS

∂q∂t

� �þ x

zFS

∂q∂t

� �

¼ 1

FS1−xþ x

z

� � ∂q∂t

� �ð4Þ

The true mass change of the film is then given by:

Δmtrue ¼ MHΔqHF

þMMΔqMzF

¼ 1−xð ÞMHΔq

Fþ xMH

Δq

zF

¼ 1þ xMM

z− 1

� �� �Δq

Fð5Þ

and the mass-to-charge ratio by:

∂mtrue

∂q

� �¼ 1

F1þ x

MM

z− 1

� �� �ð6Þ

assuming that x is approximately constant during the interca-lation experiment. However, the true mass change is notdirectly accessible because of the contribution of stress-related term Δf s in the measured frequency shift Δf .

Δf ¼ −C f ΔmH þΔmMð Þ þΔ f s

¼ −C fΔq

F1þ x

MM

z− 1

� �� �þΔ f s ð7Þ

The apparent mass changeΔm* obtained from the measuredvalues ofΔf includes the stress-related term (ms

*) as well:

Δm� ¼ −1

C fΔf ¼ ΔmþΔm�

s ð8Þ

which is given by:

Δm�s ¼ −

1

C fΔ f s ¼ aMH

ΔqHF

ð9Þ

with a ≈1 for low-stress WO3 films for pure H+ intercalation.Hence, by combining equations for m true and m s

*, the equa-tion for the apparent mass change is obtained in the form:

Δm� ¼ Δq

F1þ aþ x

MM

z−1−a

� �� �≅Δq

F2þ x

MM

z−1−a

� �� �

ð10Þ

and for constant a and x :

∂m�

∂q

� �¼ 1

F1þ aþ x

MM

z−1−a

� �� �≈1

F2þ x

MM

z−2

� �� �

ð11Þ

In view of the above, the effect of metal ion intercalation/attachment can be followed by monitoring the apparent masschanges or directly measured frequency shifts. The secondterm in the square bracket expression for apparent masschange, viz. x (MM/z −1−a ), is directly responsible for theapparent mass gains observed in the presence of metal ions.

Reversible electrochromic switching with WO3−x films

The intercalation of small cations, like H+ or Li+, into a WO3

lattice channel results in optical density changes manifestedby a transition from transparent colorless to dark blue appear-ance. The insertion of cations can be followed by monitoringthe EQCN frequency shift since the apparent mass of anelectrochromic film increases in proportion to the mass ofintercalated cations. However, due to the compressive stress[1, 46, 47] generated by WO3 lattice expansion, an additionalfrequency shift is observed in EQCN measurements. Bothfrequency shifts (i.e., the component due to mass loading andthe component due to compressive stress) increase with increas-ing degree of intercalation [1]. Typical EQCN frequency shiftand current transients recorded for a WO3−x film during poten-tial switching between E1=+0.5 Vand E2=-0.5 V vs. Ag/AgClreference in a standard 10 mMH2SO4 solution are presented inFig. 2.

The reaction equation describing the overall electroreduc-tion of WO3−x that involves an intercalation of cations M

+ canbe represented by:

WO3-x + yM+ + ye- MyWO3-x

where x is the nonstoichiometric coefficient which is gener-ally small (0≤x ≤0.3). Here, the reduction of the net valencestate of tungsten atoms from W+6−2x to W6−2x−y is compen-sated by intercalation of M+ cations to satisfy the film elec-troneutrality. The estimated concentration of color centers gen-erated during a cathodic potential pulse, determined by numer-ical integration of the coloration current (Q was typically from

1254 J Solid State Electrochem (2014) 18:1251–1260

1 to 3 mC) and assumed hexagonal close packing arrangementof spherical WO3−x nanoparticles with diameter 400 nm, isfrom 1.31×1021 to 3.95×1021 cm−3, corresponding to the uni-valent cation content y from 0.07 to 0.21.

Monocation competition

The ingress rate of monocations injected into a WO3 lattice isexpected to decrease with increasing ionic radius ofdesolvated cations. The penetration depth of larger cationsmay also be considerably reduced, even down to the surfacemonolayer of atoms inWO3 in some cases. Below, we presentdata obtained in measurements of competitive intercalation oftwo cations with different sizes: H+ (diameter 3.5 Å in theform of H3O

+ or nil in the form of protons) and K+ (diameter,2r =3.02 Å, desolvated). Both cations are smaller than thecavities in WO3 lattice (which are up to 5.2 Å). The experi-ments were carried out using pulse-nanogravimetric tech-nique. In these experiments, solutions with different

concentration ratios of H+ and K+ ions were prepared, andWO3 films were subjected to a cathodic reduction pulse, fromE1=+0.5 V vs. Ag/AgCl reference toE2=−0.5 V, followed byan anodic recovery step at E3=+0.5 V. The resonance fre-quency shifts recorded are presented in Fig. 3a.

It is clear from the graph in Fig. 3a that during the cathodicpulse the apparent mass gain increases in proportion withincreasing concentration ratio of CK/CH. During the anodicbleaching pulse, the frequency shift drops very quickly (gener-ally, within 2 s) to a level below the initial level for all solutionscontaining K+ ions. It means that some damage to the surfacefilm structure must have occurred. We have observed a reduc-tive decomposition of WO3 films in solutions of large cationsnot able to intercalate into a WO3 interspace [6]. While the sizeof K+ ion is smaller than the c-channel diameter in WO3 lattice,any association of these cations with hydration water molecules(ca. 3 Å in size, each) would hinder the intercalation leading tothe lattice polarization and the observed lattice damage.

Also, it has to be noted that the bleaching stage in thepresence of K+ ions is unusually fast as compared to thecoloration stage. Since that fast diffusion of K+ ions fromthe bulk of WO3 particles to their surface upon the potentialpulse application to E3=+0.5 V is highly unlikely, it would bereasonable to consider a buildup of K+ ions in the near-surfacearea of WO3 particles rather than their deep penetration intothe bulk of the particles.

The maximum apparent mass change ratio for the compet-itive intercalation of K+ and H+, μexp=ΔmK*/ΔmH*=3.5,has been obtained from the data of Fig. 3a. The theoreticalmolar mass ratio is μK/H=MK/MH=39.1. Therefore, the ex-perimental ratio μ exp is much less than the theoretical molarmass ratio μK/H.

Pulse-nanogravimetric experiments for cathodization ofWO3 in the presence of large organic cations, Et4N

+ andCtMe3N

+, have also been performed. The obtained resultspresented in Fig. 3b show a large decrease of the apparentmass of the film during the cathodic pulse. This is in clearcontrast to the regular apparent mass increase induced by thecation intercalation. After the cathodization step, a severedamage to the WO3 lattice has been observed. No massrelaxation was observed during the anodic step. The filmdeterioration continues during the next cathodic pulses untilthe entireWO3 film is gone. Due to the large size of et4N

+ andCtMe3N

+ cations, the intercalation is not possible and thecathodization results in the dissociative reduction of WO3:

WO3−x þ 2ye−→ 1−yð ÞWO3− xþyð Þ= 1−yð Þ þ yWO42−

aqð Þ ð13Þ

Thus, large monocations which are not able to penetrateWO3 channels, such as Et4N

+ cations, cause a strong latticepolarization leading to substantial lattice damage. Smallermonocations which fit the channels when desolvated but

Fig. 2 Typical electrochromic characteristics obtained in a pulse-nanogravimetric experiment with potential steps between E1=+0.5 Vand E2=-0.5 V vs. Ag/AgCl: a current transient and b resonance fre-quency shift transient, for cathodic coloration and anodic bleaching steps,for a WO3−x film electrodeposited on a quartz crystal piezoresonator;frequency shift, −Δf , corresponds to the apparent mass increase Δmamplified by the effect of compressive stress

J Solid State Electrochem (2014) 18:1251–1260 1255

accumulate at the channel entrance when solvated, such as K+,cause some lattice polarization and detectable lattice damage.In mixed solutions, it is very likely that the H+ ions carry thecharge to the film bulk, while K+ ions accumulate in thechannel outlet area by electrostatic attraction causing latticepolarization. Excess of H+ ions prevents lattice damage bydecreasing the contribution of K+ ions in processes takingplace during the cathodization pulse.

Dication competitive ingress: case of Ni2+ and H+

It is interesting that the ionic diameter of desolvated Ni2+,which is small (2r =1.38Å), is not only smaller than that of K+

(2r =3.02 Å) but also smaller than that of Li+ (r =1.52 Å).With the hydration shell (2r =7.09 Å), it is unlikely to inter-calate WO3 lattice. The slow desolvation of hydrated Ni2+(aq)ions, which is well known, will surely affect the rate of thecoloration step. Nevertheless, there is likelihood that

desolvated Ni2+ cations may enter WO3 channels and betransported by diffusion to the bulk of WO3 particles. Theresults of pulse-nanogravimetric experiments carried out foran H+/Ni2+ system are illustrated in Fig. 3c.

As shown in this figure, the frequency shift (−Δf) associ-ated with the coloration process increases with increasingconcentration ratio of CNi/CH when CH is lower than ca.0.5 mM, but it is fully controlled by H+ ingress/egress forCH greater than 0.5 mM. In the transitional case, for CH=0.5 mM (curve 4), film instability is observed: the frequencyshift decreases from the level initially reached upon the po-tential step to E cat=−0.5 V due to the cation intercalation. Thedescending trend is maintained after the anodic potential pulseto E an=+0.5 V. Normally, at this potential, the cations de-intercalate, and the frequency shift relaxes to the initial level.However, in the case of curve 4, the frequency shift falls wellbelow the initial level which indicates film deterioration due tolattice polarization and reductive decomposition of WO3.

Fig. 3 Transients of the resonance frequency shift of a QC/Au/WO3

electrode in response to potential steps from E1=+0.5 V to E2=−0.5 Vand back to E1=+0.5 V, recorded for solutions with different cations: aconcentrations of KNO3 and H2SO4 [mM, mM]: (1) 0, 10, (2) 10, 10, (3)10, 1, (4) 10, 0.5, (5) 10, 0.2, (6) 10, 0.1, (7) 10, 0.05, and (8) 10, 0; bTEAC and CTAB: (1) 10 mM TEAC, (2) 10 mM TEAC+1 mM H2SO4,

and (3) 10 mM CTAB; c concentrations of NiSO4 and H2SO4 [mM,mM]: (1) 0, 10; (2) 10, 10; (3) 10, 1; (4) 10, 0.5; (5) 10, 0.2; (6) 10, 0.1; (7)10, 0.05; and (8) 10, 0; and d concentrations of CdSO4 and H2SO4 [mM,mM]: (1) 0, 10; (2) 10, 10; (3) 10, 1; (4) 10, 0.5; (5) 10, 0.2; (6) 10, 0.1; (7)10, 0.05; and (8) 10, 0. (Reproduced with permission from the Electro-chemical Society from ref [32])

1256 J Solid State Electrochem (2014) 18:1251–1260

It follows from the data of Fig. 3c that there are significantdifferences observed between curves 1–3 and curves 5–8.Looking at the competitive aspects of intercalation of H+

and Ni2+, one can realize that a predominant intercalation byH+ ions would lead to small frequency shift, whereas a pre-dominant intercalation by Ni+ ions would lead to large fre-quency shift due to the molar mass ratio μNi/H of Ni2+ to H+

given by: μNi/H=(MNi/2)/MH=29.35. The maximum experi-mental apparent mass ratio estimated from Fig. 3c betweencurve 8 and 1 is μexp=7.5. This is much less than the theoret-ical value of μNi/H, but it represents considerably higherdegree of Ni2+ contribution to the intercalation/attachmentprocess than it was observed for K+ ions. The deviation ofμ exp from μNi/H may be due to several factors, such as (a) theintercalation of Ni2+ is not complete in the time frame of thepulse-nanogravimetric experiment (i.e., the saturation of thefrequency shift has not been attained, e.g., curve 8) and (b) thehigher charge carried by Ni2+ cations may cause lattice polar-ization in WO3 which may lead to channel narrowing andreduced film capacity for intercalation.

It is also evident from Fig. 3c that the rate of intercalationdominated by H+ ions is much higher than that dominated byNi2+ ions. Hence, curves 1–3 show fast ingress and egress,and curves 5–8 show slow ingress and slow egress. Thecharacteristics for Ni2+ solutions are different than those forK+ solution in that they do not show overcompensated fre-quency shifts on in the reoxidation step (except for curve 4)and that the reoxidation process is slow.

Competitive interactions with heavy metal cations

Heavy metal ions have not been expected to show any ingressinto the WO3 lattice due to their large size. However, thedesolvated ionic diameters of some heavy metal cations arenot much larger than Li+ or Na+ due to strong electrostaticattraction of electrons by their highly-charged nuclei. Forinstance, the ionic diameter of Cd2+ cation (2r =1.90 Å) isclose to that of an Na+ cation (2r =2.04 Å), and it is smallerthan that of a K+ cation (2r =3.02 Å). Therefore, we haveinvestigated the charging/discharging characteristics of aWO3

film in the presence of Cd2+ and H+ cations using pulse-nanogravimetric method. The results obtained are illustratedin Fig. 3d.

It can be seen from Fig. 3d that there is a gradual increase inthe frequency shift (apparent mass change) with the increasingconcentration ratio of CCd/CH. The exception is curve 8 for a10 mM Cd2+ (with no acid added), which shows a lowerfrequency shift than in the case of a 10 mM Cd2++0.1 mM H+ solution (curve 7). The frequency shift decaysquickly upon film reoxidation to a lower level than that beforethe film reduction. This behavior clearly indicates reductivefilm damage. The lattice polarization and film damage are notsurprising as they are associated with the absence of acidic

protection and relatively large size of Cd2+ ion. Remarkably,no film damage has been encountered for all other solutionssince uponWO3 reoxidation, the frequency shift relaxed backto the initial level. It is worth noting that the rate of decoloringstage for Cd2+ ions is very fast, similar to the case of K+ ions.

The analysis of the maximum apparent mass ratio observedfor predominant interactions of Cd2+ ions and H+ ions withWO3 lattice has revealed that μ exp=8.6 (from the data ofcurves 7 and 1). The theoretical molar mass ratio μCd/H forCd2+ to H+ is given by μCd/H=(MCd/2)/MH=112.4/2=56.1.Hence, μexp≪μCd/H.

Interactions of small tri- and tetravalent cations with WO3

The high molar mass elements of the lanthanide and actinideseries forming Me3+ and Me4+ cations have relatively smallionic radii, and hence, they are suitable for testing their inter-actions with WO3 lattice. It is also interesting how the highcharge of these cations contributes to the lattice polarizationeffects in WO3. In the following, we have examined La3+ andTh4+ cations using pulse-nanogravimetric technique.

In order to evaluate fitting of desolvated La3+ and Th4+

cations in WO3 channels, we have compared the ionic diam-eters of La3+ and Th4+ with channel diameter and with alkalimetal cations. Thus, the ionic diameters of La3+ and Th4+ are2.32 and 2.10 Å, respectively, and they are only slightly largerthan that for Na+ (2r =2.04 Å) but considerably smaller thanK+ (2r =3.02 Å). We have investigated the charging/discharging characteristics of an electrochromic WO3 film inthe presence of these two representative lanthanides and acti-nides using similar approach as described in previous sections.The recorded pulse-nanogravimetric responses of a WO3-coated piezoelectrode using solutions with varying concentra-tion ratios of La3+/H+ are presented in Fig. 4 and those forTh4+/H+ are presented in Fig. 5.

The responses for La3+ cations show substantial resonancefrequency shifts upon the cathodic pulse application. Therecorded frequency shifts increase with increasing La3+/H+

concentration ratio. In order to evaluate the competitivenessof H+ and La3+, we have analyzed the maximum apparent massratio observed for predominant interactions of La3+ and H+

ions. The theoretical molar mass ratio μLa/H for La3+ to H+:μLa/H=(MLa/3)/MH=138.9/3=46.3. The maximum apparentmass ratio estimated from Fig. 4 between curve 7 and 1 isμexp=9.9. This is the highest number yet, though, still muchless than the theoretical value of μLa/H. Consequently, the La

3+

cations cannot satisfy the requirement of charge balance, andthe H+ ions must remain the major intercalating ions.

The responses for Th4+ cations presented in Fig. 5 showmuch smaller resonance frequency shifts upon the cathodicpulse application as compared to other metal cations examinedso far. It is very likely that these strongly polarizing cations donot desolvate easily and, together with the hydration water, their

J Solid State Electrochem (2014) 18:1251–1260 1257

size exceeds the channel diameter of WO3. The fact that thefrequency shifts (−Δf) actually increases, instead of decreasingas in the case of et4N

+ cations, means that Th4+ cations bind tothe surface of WO3 nanoparticles. Once in the bound(adsorbed) state, Th4+ cations will strongly polarizeWO3 latticebecause of their very high positive charge. The interaction ofTh4+ with negatively charged O sites of WO3 lattice wouldlikely diminish the mobility of desolvated Th4+ cations andhinder their movement into the bulk WO3 phase. For the sakeof comparison with other metal ions, we have calculated the

maximum apparent mass gain expected for Th4+ cations if theywould be the sole charge carriers. The theoretical molar massratio μTh/H for Th4+ to H+: μTh/H=(MTh/4)/MH=232/4=58.The experimental value is barely around 2, as follows fromthe maximum frequency shift ratio estimated from Fig. 5 be-tween curve 5 and 1: μexp=2.0. This is much less than thetheoretical value of μTh/H. Therefore, the Th

4+ cations cannotsatisfy the requirement of charge balance and the H+ ions mustremain the major intercalating ions.

It is also characteristic for pulse-nanogravimetric responsesof WO3 films to La3+ and Th4+ cations that in the anodic de-intercalation steps, there is no complete apparent mass recov-ery observed. This behavior is certainly attributed to theirstrong interactions with the lattice.

Assessment of the interactions of various cations with latticechannels in WO3 nanoparticles

The interactions of metal ions with WO3 lattice result inchanges of electronic, optical, and structural propertiesof WO3. The water molecules that may be involved inthe reactivity at the surface and/or participate in thecharge transport through the channels in WO3 may playan important role in the electrochromic behavior ofWO3 films.

The slow dehydration, which is well known for Ni2+(aq),may impede the effective intercalation of this small cation.

Fig. 5 Transients of the resonance frequency shift of a QC/Au/WO3

electrode in response to potential steps from E1=+0.5 V to E2=−0.5 Vand back to E1=+0.5 V, recorded for solutions with different concentra-tions of Th(NO3)4 and H2SO4 [mM, mM]: (1) 0, 10; (2) 10, 0.2; (3) 10,0.1; (4) 10, 0.05; and (5) 10, 0

Fig. 4 Transients of the resonance frequency shift of a QC/Au/WO3

electrode in response to potential steps from E1=+0.5 V to E2=−0.5 Vand back to E1=+0.5 V, recorded for solutions with different concentra-tions of LaAc3 and H2SO4 [mM, mM]: (1) 10, 10; (2) 10, 1; (3) 10, 0.5;(4) 10, 0.2; (5) 10, 0.1; (6) 10, 0.05; and (7) 10, 0

Fig. 6 Structural features of tungsten trioxide channels: ring of WO6

octahedra joined via the O tips showing a cross section of a channel alongthe c-axis with diameter a =0.52 nm; electrostatic potential mapped onthe electron density surface: color coded from high potential—blue, tolow potential—red, ρ=0.05 au−3

1258 J Solid State Electrochem (2014) 18:1251–1260

The interesting feature of the interactions of metal cationswith WO3 channel walls is that they induce the WO3 latticepolarization by attaching to negatively charged oxygen sites atthe junctions between WO6 octahedra (Fig. 6). This attach-ment induces distortions in the lattice and influences furthermass transport along the channels. The lattice polarization ismost strongly expressed for La3+ and Th4+. The moleculardynamic simulations and quantum mechanical calculations ofelectronic structures indicate that metal ions are not preferen-tially located at the center of cavities in WO3 but closer tooxygen atoms of WO6 octahedra which confirm the hypoth-esis about lattice polarization.

Conclusions

The interactions of WO3 lattice with large monovalentcations (K+, Et4N

+, CtMe2N+), transition metal dications

(Ni2+), heavy metal ions (Cd2+), as well as representativecations of lanthanides and actinides (La3+ and Th4+) havebeen investigated. These interactions differ considerablyamong the cations studied. Very large monocations(Et4N

+, CtMe3N+), which are unable to fit into the

WO3 channels, cannot intercalate and do not protectWO3 lattice from reductive damage. The largest of themetal ions studied, K+ cation, intercalates the least, al-though the observed apparent mass gain is large. Theheavy metal cation, Cd2+, shows a large mass gain overH+ and does not cause any damage to the lattice down tothe acid concentration of 0.05 mM. The case of Ni2+ isinteresting since this cation is actually smaller than Li+

and generates a large apparent mass gain of 7.4, stillmuch less than expected theoretically from molar massratio of Ni2+/H+ although much higher than that observedfor K+. The dynamics of the intercalation process forNi2+ is slower than that for other metal ions which canbe attributed to the slow dehydration of this cation. Theslow decoloration step is likely due to the slow disasso-ciation of Ni2+ ions from oxygen atoms in the walls ofWO3 channels at the junctions of WO6 octahedra and theslow hydration process. The pulse-nanogravimetric tech-nique has been found very sensitive in detecting smalllattice damage occurring in the coloration step whichremains unnoticed in the chronoamperometric character-istics. It also shows clearly that there is no damage toWO3 when it is anodically protected. During the cathodicreduction, the efficient intercalation protects the oxideagainst reductive damage. Highly charged cations (La3+,Th4+) cause a lattice polarization, leading to the latticedamage and deterioration of the film, and interact strong-ly with oxygen sites at the junctions of WO6 octahedraresulting in only partial de-intercalation in the anodicpotential pulse.

Acknowledgments This work was supported by the National ScienceFoundation grants no. CCLI-0126402 and 0941364.

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