Published: March 07, 2011

r 2011 American Chemical Society 5265 dx.doi.org/10.1021/jp109327x | J. Phys. Chem. C 2011, 115, 5265–5270

ARTICLE

pubs.acs.org/JPCC

Electrochemical Behavior andMagnetic Properties of VanadiumOxideNanotubesAndreia I. Popa,*,† Evgenia Vavilova,†,‡ Christine T€aschner,† Vladislav Kataev,† Bernd B€uchner,† andR€udiger Klingeler†,§

†Leibniz Institute for Solid State and Materials Research, IFW Dresden, 01171 Dresden, Germany‡Zavoisky Physical-Technical Institute of RAS - 420029, Kazan, Russia§Kirchhoff Institute for Physics, University of Heidelberg, INF 227, 69120 Heidelberg, Germany

1. INTRODUCTION

In recent years, it turned out that nanoscaled materials canyield improved performance when used in electrodes of energyconversion devices, for example, batteries. For example, electro-des made of nanoparticles of transition metal oxides such asCoO, NiO, CuO, or FeO exhibit electrochemical capacities of700mAh/gwith 100% capacity retention for up to 100 cycles andhigh recharging rates.1

The particular advantage of nanoparticle-based electrodesincludes shorter lithium diffusion lengths and higher surface areathan in micrometer electrode materials that leads to performanceimprovement of lithium-ion batteries.2,3

Among nanostructured materials those with tubular morphol-ogy are of large interest due to size-induced properties andcomplex nanoscale architecture.4-6 In this context, self-orga-nized oxide nanostructures provide a promising approach. In-norganic nanostructures such as TiS2, SnO2, or MoS2-xIy with x≈ 0 and y ≈ 0.3 have been studied in terms of batteryapplication.7,8 TiO2 represents nowadays a powerful candidatefor this application as well.9 In this paper, we focus on self-assembled vanadium oxide (VOx) nanostructures synthesizedfor the first time by Spahr et al.10 In general, VOx can be found inmany forms and can have many and very different chemical orphysical properties, in particular when different oxidation statesof the vanadium ions are realized. The nanotubular modificationthat is addressed here can be synthesized only when organicspacer molecules are applied. Such spacer molecules can be forexample primary amines or diamines, helping to retain thenanotube shape of the VOx layers. To be specific, VOx nanotubescan have diameters of 15-150 nm, tunnel openings between 5and 50 nm, and lengths up to 15 μm. The nanotubes are formed

by rolled VOx layers that show similarities with BaV7O16 3 nH2O,where n = 4.4. In this material, V7O16 layers are stacked along onedirection with Ba2þ ions and H2O molecules located in betweenthe layers. Analogously, the VOx nanotubes are formed by similarlayers with the above-mentioned organic spacers placed inbetween. There can be 2-30 layers of VOx forming eithertubular or spiral structures.10-12 The organic molecules do notonly affect the morphology of the nanotubes but also theirelectronic properties since the oxidation state of the vanadiumions is influenced.13 In addition, the amines embedded betweencrystalline VOx layers can be exchanged by various metal cations,such as alkaline, alkaline-earth, or transition metals, withoutdestroying the nanotubular shape but influencing their electro-chemical properties.10,14-16

To assess possible applications of this material in lithiumbattery technology,17 several electrochemical studies have beenperformed explicitly on ion-exchanged VOx-NT. For example,Mn0.1VO2.5 3 nH2O was reversibly intercalated/deintercalatedwith Li and the material shows a specific capacity of 140 mAh/g.15Mo-doped tubes have an initial specific capacity of 200mAh/g that drops to half after 50 cycles.18 Naþ, Caþ, and Kþ were aswell successfully embedded between VOx layers. All of thesehave shown good electrochemical properties with averagedspecific capacities around 150 mAh/g over 100 cycles.16 Thestructural defects are also relevant in electrochemical propertiesof such materials. It has been shown that defect rich VOxnanorolls exhibit superior electrochemical properties compared

Received: September 29, 2010Revised: February 1, 2011

ABSTRACT: The electrochemical behavior and magnetic properties of electro-chemically lithium-doped mixed valence vanadium oxide nanotubes (VOx-NTs)have been studied. The materials were synthesized hydrothermally. Primary amines(CH3-(CH2)11-NH2) were used as spacer molecules aiding to retain thenanotubular shape of the VOx layers. Cyclic voltammograms and potentiostaticmeasurements indicate that the electrochemical behavior of this material is fairlysensitive to the preparation conditions. Specific capacity values for this material arehigher than similar template VOx-NTs. Lithium doping level of x = 0.1 in twodifferent VOx-NTs gives rise to room-temperature ferromagnetism in LixVOx-NTs.

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to well-ordered material with specific capacities of approximately300 mAh/g.19 These values already approach the theoreticalcapacity for exchanged nanotubes that amounts to about 370mAh/g when all vanadium is reduced to V3þ. This extreme caseis however unlikely because total reduction of V5þ to V3þ wouldinduce strong structural deformations (collapsing) of the VOx-NT structure.20

Besides studying the applicability of VOx-NTs as electrodematerials in lithium ion batteries, their electronic properties havebeen studied both in the pristine material11,21,22 and after lithiumdoping.21,23,24 By doping the VOx-NTs with either electrons21,24

or holes,21 a nonlinear field dependence of the magnetization isobserved at room temperature. This surprising ferromagnetic-like response renders VOx-NTs a novel type of self-assemblednanoscaled magnets.

In this work, both the electrochemical performance and theeffect of Li-intercalation on the magnetic properties of hydro-thermally synthesized VOx-NTs have been studied. For thispurpose, cyclic voltammetry and potentiostatic measurementshave been performed. Using these electrochemical methods, ithas been found that the electrochemical properties of similarlysynthesized nanotubes differ from each other. Most probablythese differences arise from influence of amines used duringsynthesis of the nanotubes. In addition, the effect of lithiumdoping on the magnetic properties of VOx-NTs has been studiedafter electrochemically intercalating different amounts ofLithium into VOx-NTs.

2. EXPERIMENTAL SECTION

2.1. Synthesis. Vanadium oxide nanotubes were synthesizedusing the hydrothermal procedure similar to that described inrefs 13 and 21. A solution of vanadium (V5þ) triisopropoxide[VO(iOPr)3] and a primary amine (CH3-(CH2)11-NH2) in amolar ratio of 2:1 in absolute ethanol (3 mL per g of[VO(iOPr)3]) was stirred under nitrogen atmosphere for 1 h.This resulted in a light-yellow solution of alkoxide-amine adduct.This was hydrolyzed with water (5 mL per g of [VO(iOPr)3])while stirring. The adduct was continually stirred and aged atroom temperature for ∼4 days, turning dark orange. Thehydrothermal reaction of this composite in an autoclave at180 �C for 8 days yielded a black powder of VOx-NTs thatwas then washed with ethanol and hexane and dried at 85 �C invacuum for∼30 h. The VOx-NTs have been characterized usingscanning electron microscopy (SEM) in a FEI Nova-Nanosem-200 and transmission electron microscopy (TEM) in FEITecnai F30.2.2. Electrochemical Measurements. Electrode materials

have been prepared from a mixture of VOx-NTs, 10 wt % ofCarbon SP (Timcal, LTd, Bodio, Switzerland), and 5% of PVDFbinder (Solexis, Tavaux Cedex, France). Approximately 10 mg ofmixture pressed in pellet was placed in two-electrode Swagelok-like cells. The cells were assembled inside the glovebox under Ar-atmosphere (O2/H2O < 2 ppm). In the two-electrode cell,lithium metal was used as negative electrode and VOx-NTs aspositive electrode. The two parts were separated by two sheets ofWhatman borosilicate glass fiber, soaked in electrolyte to main-tain good ionic conductivity and well-physical separation of thetwo electrodes. The electrolyte used was 1 M solution of LiPF6salt in ethylene carbonate (EC)/dimethyl carbonate (DMC)mixture 1:1 in weight (Ferro Corporation, Ohio, U.S.A.). Cyclicvoltammetry and potentiostatic measurements were performed

using a VMP multichannel controller (Princeton Applied Re-search, Illingen, Germany). The experiments were performed atfixed temperature of T = 25 �C.2.3. Magnetic Measurements. Electrochemical (potentiostatic)

techniques were used to obtain lithium doped VOx-NTs,i.e. LixVOx-NTs with the particular Li-content x = 0.05, 0.1,0.155, 0.2, and 0.4. The magnetic properties of both undopedand dopedVOx-NTswere studied. For this purpose, aMPMS-XL5SQUID magnetometer (Quantum Design, Darmstadt, Germany)was applied for investigations of the field dependence of themagnetization at constant temperatures. The 7Li NMR experi-ments were carried out with a Tecmag pulse solid-state NMRspectrometer in a field of 4 T.

3. RESULTS AND DISCUSSION

The morphology of the synthesized VOx-NTs was investi-gated by means of SEM and TEM. In Figure 1, the multilayertubular morphology is illustrated. In general, the tubes have innerdiameters between 20 and 30 nm and the outer diameters arevarying between 50 and 100 nm.3.1. Cyclic Voltammetry. In the following, electrochemical

studies of different samples (1-5) of VOx-NTs are described.Although the synthesis conditions of all five samples have beenvery similar, small deviations in the hydrothermal synthesisprocess yield subtle but visible differences in the electrochemicalbehavior.The cyclic voltammograms for the first cycle for VOx-NT (1)

are shown in Figure 2. The starting potential was around 3.2 V.From this value, the potential was cycled between 1.8 and 3.5 Vusing different scan rates. First, the cell was discharged (goingfrom 3.2 to 1.8 V) using scan rates of 0.2 and 0.05 mV/s,respectively. As expected, the overall behavior is similar for thetwo scan rates but the current is larger in case of the fastersweeping rate. Upon discharging, that is, Li-intercalation, tworeduction peaks indicating two different electron transfer reac-tions are encountered at 2.42 and 2.09 V, respectively. During thecell charging, one finds two peaks at 2.83 and 3.17 V. In general,the data shown in Figure 2 suggest that the dischargedmaterial inthe cathodic sweep was reversibly charged during the anodicsweep, or in other words that the electrons transfer process ischemically reversible. This electron transfer appears at a specificvoltage due to redox reaction when the reduction of V5þ to V4þ

takes place (or V4þ to V3þ). Note that at 2.73 V, the cathodicscan presents a shoulder, which is probably associated with theanodic peak at 2.64 V. For VOx-NT (1) the initial dischargecapacity is 140 mAh/g when the current was swept with a scanrate of 0.05 mV/s.Cyclic voltammograms for the materials VOx-NT (2), (3),

(4), and (5) performed with 0.02 mV/s are shown in Figure 3.The CVs are generally similar with reduction/oxidation peakpairs in the same potential region, but there are small differencesbetween the materials. For each batch, four consecutive CVcycles were performed (only the first circle is shown). For allexperiments, the cutoff potentials were 1.8 and 4 V, alwaysstarting with the discharge of the material. The starting potentialswere around (3.2 ( 0.2) V, indicating a stable behavior over themounting procedure of the electrode materials.In the following, subtle differences between the four materials

are discussed. For VOx-NT (2) upon discharge the electrontransfer due to reducing of V5þ to V4þ occurs at 2.83 V asdisplayed by the very pronounced reduction peak. Upon further

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cycling, this value slightly shifts to higher potentials. Thecorresponding oxidation peak is situated at 2.94 V. Both peaksexhibit shoulders which are most probably related and disappearupon cycling. The peaks are broader in VOx-NT (3) but at verysimilar potentials: the reduction peak is centered on 2.8 V(anodic peak at 2.99 V) and shifts to 2.82 V during furthercycling. The current peak intensities vary more over cycling thanthose in VOx-NT (2). Again, the shoulder at 2.57 V completelydisappears already in the second cycle of this experiment andboth the reduction and oxidation peaks narrow upon cycling. ForVOx-NT (4) and (5), the first reduction occurs at a lowerpotential than in the other materials, that is, at 2.56 and 2.62V, respectively. Here, there are stronger effects of cycling thatyield a shift to 2.82 V (2.9 V) in the fourth cycle. The oxidationpeaks are found at 2.96 and 3.1 V. Note, that the oxidation peak isnarrower than the cathodic one. The peak around 2 V isattributed to possible reduction of V4þ to V3þ, as also valid forVOx-NT (3).Figure 4 illustrates a resume of the specific capacity values for

VOx-NTs studied in this work taken from CV experiments. Thehighest initial discharge capacity is found for VOx-NT (3) and(5), which displays values of 145 mAh/g. The specific capacitiesdecrease upon cycling, for example, to 114 mAh/g in batch (3).In contrast, VOx-NT (4) has 123 mAh/g initial dischargecapacity that increases to 153 mAh/g in the second cycle.The electrochemical behavior presented in Figure 2 (for VOx-

NT (1)) and Figure 3 (for VOx-NT (2-5)) differs, although allmaterials have been synthesized in a very similar way. In additionto the effects of the synthesis procedure, aging plays an importantrole in VOx-NTs since the nanoscaled morphology implies

chemical reactivity. Moisture, that is, water is known to reduce25

the V5þ to V4þ, and therefore in the studied materials the mixedvanadium valence is possibly different from the value of 4.4þ

suggested in refs 11 and 22. It is also possible that if extra waterenters in the VOx-NTs, the available space for Li intercalationdecreases, controlling their electrochemical properties.Also different amounts of the amines influences the mixed

valence of the V in the VOx layers thereby causing possibledifferences between similarly prepared materials. In addition, thesurfactant can reduce the available surface area for redox reac-tions and can also form aggregates that would block the Li ionsaccess to intercalation sites in the VOx layers. These effects canbe smaller if the nanotubes have cracks in the walls. This atomicscale disorder would give a higher accessibility for Li ions andthus an increased specific capacity. Different sizes of the nano-tubes can give different Li diffusion distances. Short distancesgive high surface area and this lowers the Liþ insertion ratedensity in the intercalation process, which in turn, postpones thedecreasing of the capacity.In the following, the differences in the electrochemical behavior

of the materials will be discussed. The CV of VOx-NT (1)presented in Figure 2 is similar to that of template free VOx-NTshown in ref 10, raising the question of amines role in this material.In general, the amines are expected to cause a partial passivation ofthe nanotubes.10 However, the electron transfer reactions presentin VOx-NT (1) occur at similar potential as inCa-exchanged VOx-NT20 indicating that in the first case the amines did not signifi-cantly avert the lithium intercalation process.

Figure 3. Cyclic voltammograms for VOx-NT (2), (3), (4), and (5)performed with a scan rate of 0.02 mV/s. The arrows indicate thedischarge and charge of the material.

Figure 1. Tubular morphology of VOx-NTs proven by SEM (left) and TEM (right) studies.

Figure 2. Cyclic voltammogram for VOx-NT (1) performed with scanrate of 0.2 and 0.05 mV/s, respectively. The arrows indicate thedischarging and charging of the material.

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As described above for batches (2-5), there are variations ofthe anodic and cathodic peak positions. It is straightforward toassign the differences to structural deformation, aging of thematerial, or crystallinity loss during cycling. A resistance of thereduced VOx layers to readopt their original structure may alsogenerate differences.20 Themost obvious difference in the CVs ofthe materials is the presence of additional clear features around 2V in the cathodic scans of VOx-NT (3) and (5) and at 3.7 V in theanodic scan (5). The cathodic peaks might indicate the secondreduction process V4þ to V3þ of the materials, although usuallypotentials lower than 2 V are needed for this electron transfer.16

Peak broadening in the second cycle compared to the first one, asmentioned for VOx-NT (3), can be explained by structuralrearrangements of the electrochemical active VOx layers afterthe first cycle. Clear modifications of an I-V curve in the secondcycle compared to the first one suggest that irreversible trans-formations occur during the initial Li insertion process. In thefollowing cycles, neither substantial change in the peak potentialvalue nor in the curve shape is present, indicating quasi redox-reversible intercalation process for these cycles.Another interesting feature is the observation of a very broad,

nearly plateaulike reduction peak in VOx-NT (5). One mightspeculate that in this region the broadening and overlapping ofthe reduction peak itself and the shoulder seen in VOx-NT (2)and (3) yield the plateaulike behavior. Broadening is a typicalresult of a more disordered structure. For example, in VOx-NT(3) the fact that the second cycle is different from the first oneindicates a structural rearrangement after the first cycle. This iseven more pronounced in VOx-NT (5). Here, already the peaksin the charging and discharging cycle exhibit different widths soLi-insertion and extraction is governed by different mechanism.The electrochemical behavior found for VOx-NTs (2-5) is

very similar to that described by Sun et al. for defect rich Naþ-exchanged VOx-nanorolls that have been synthesized by a similarmethod,19 indicating a possible amine deficiency in the first case.The hydrothermal step during the synthesis of the VOx-NTs iscrucial for the electrochemical properties of this material. It hasbeen shown that a larger amount of amines remained in betweenthe VOx layers after a Na-exchanged process induces differentelectrochemical behavior19 by blocking the Li ion access to sitesin the VOx layers and therefore affecting their electrochemicalproperties.The specific capacity behavior upon cycling displayed by the

data in Figure 4 is a feature also found in other VOx nanos-tructures (for example, in refs 10 and 19). It is typically caused byamorphization or other structural changes that appear overcycling. In the case of VOx-NTs at hand, Li might remainbetween the VOx layers so that it is not completely deintercalatedupon charging, thereby lowering the reversible capacity anddecreasing the number of available sites for Li ions.VOx-NTs studied in this work have slightly superior specific

capacity values than VOx-NTs with template found in theliterature. For VO2.45 (TEMP)0.34 (TEMP = C16H33NH2), theinitial specific discharge capacity amounts to 120 mAh/g. If thetemplates are removed, values of 180 mAh/g are found.10

Interestingly, the specific capacity values described in this workare comparable to those for different ion-exchanged VOx-NTs,although the synthesis methods are different.16 The higher valuesof specific discharge capacities for templated VOx-NTs in thepresent work indicates that the amines did not completely blockthe available sites for the Li intercalation/deintercalation process.

3.2. PotentiostaticMeasurements. Figure 5 shows potentio-static data on VOx-NT (3) where the potential of the electro-chemical cell was maintained constant until the current reached apreviously established limit (C/t rate). Subsequently, the poten-tial was decreased (when discharging) or increased (whencharging) using 5 mV steps. The rate used was C/100, that is,complete discharge in 100 h. The curves show slight changes ofthe slope at potential values of 2.4, 2.6, and 2.8 V. The absence ofan obvious plateau in these curves is the clear indication of amonophasic Li intercalation process, that is, there is no discretephase formation during cycling. Interestingly, the first threecycles of the potentiostatic experiment are different from thefollowing ones. This difference may arise from different inter-calation processes present in the first three cycles compared tothe later ones. A similar conclusion might be drawn fromdifferences between the first and the other cycles in CV experi-ments. One possible scenario is that in the beginning, Liþ ionsare inserted in a quite disordered way between the VOx-layers(valid for cycles 1 to 3). In a presumably second step of theintercalation process, the Liþ ions might then diffuse to findfavorable sites in the interlayer spaces of the structure, affectingthe VOx structure (valid from cycle 4 onward).

Figure 5. Potential versus specific capacity and the Li amount x inLixVOx-NT obtained from potentiostatic studies on VOx-NT (3). Thescans have been performed at rate C/100 (I = 0.027 mA). The arrowindicates numbering of cycles from 1 to 20.

Figure 4. Specific discharge capacity as obtained from theCVs shown inFigure 3.

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The data in Figure 5 again allow following the effect of cyclingon the specific discharge capacity. In the first cycle, it amounts to130 mAh/g, which corresponds to a Li-intercalation level of 0.66(cf. upper abscissa of Figure 5). This value decreases with cyclingand it reaches a roughly constant value after the tenth cycle. Thisdecrease can be the result of side reactions, or maybe Li ions sitesare blocked by amines. Note that due to a smaller potentialwindow used for the potentiostatic experiments, the specificcapacity is slightly lower than in the CVs. Using potentiostaticexperiments, it has been shown that the Li-intercalation processis a monophasic process with no drastic structural changes of thestudied material (see comments on Figure 5). The values of thespecific capacities found here are higher than those found forother template VOx-NTs presented in literature, but smaller thanfor template-free nanotubes. A previous study has shown that thestructure of mixed-valent vanadium oxide nanotubes is stableagainst electron transfer upon Li-doping.23 Generally, it is knownthat the nanotube morphology is preserved after cycling.17,26

3.3. Magnetic Properties. Using electrochemical methods asdescribed above, Li was intercalated in VOx-NTs in order tostudy the magnetic properties of LixVOx-NTs as a function ofelectronic doping for 0.05e xe 0.4. The magnetic properties ofundoped VOx-NTs have been already shown in detailelsewhere.22 In the pristine material, vanadium ions have a mixedvalence state, with an average valence count of ∼4.4þ (i.e., x ∼2.2 in VOx-NT).11 Hence, approximately 60% of the V sitesexhibit magnetic V4þ ions (3d1, S = 1/2) and 40% nonmagneticV5þ ions (3d0, S = 0). The magnetic properties reflect theappearance of individual spins, antiferromagnetic dimers, andtrimers.22 However, upon Li-doping the magnetic properties arecompletely changed, that is, the spin gap disappears in electro-chemically doped VOx-NTs and a ferromagnetic-like responseevolves at room temperature at the doping level x = 0.1.24

In general, by applying an electrical voltage, lithium is insertedin the crystal structure and the valence of the transitionmetal ion,that is, the number of electrons in the 3d shell of the V ions, isaltered. Doping with electrons is expected to reduce V5þ intoV4þ and therefore to increase the number of the magnetic sites,that is, V4þ (S = 1/2). An increase of the number of magneticsites is indeed observed with our experimental data. In addition,however, there is a clear evolution of a ferromagnetic-likeresponse upon doping visible in Figure 6. Here, the fielddependence of the magnetizationmeasured at room temperature

(T = 300 K) for LixVOx-NT is shown. For the undopedmaterial,the field dependence of themagnetization has a linear behavior asexpected for a paramagnet. Clear nonlinear field dependence inapplied fields up to 0.2 T appears at x = 0.05, gets mostpronounced at x = 0.1, and vanishes at higher doping levels.Our data hence provide clear evidence that Li-doping drasti-

cally changes the magnetic interaction between the vanadiummagnetic moments. While the pristine material is governed byantiferromagnetic dimers and trimers, room temperature ferro-magnetism is present in Li0.1VOx-NT. The detailed analysis ofESR, μSR, and NMR enabled us to conclude that the ferromag-netic-like response in Li0.1VOx-NT is associated with the forma-tion of small ferromagnetic clusters.24 Interestingly, the data inFigure 6 imply a particular role for the doping level x = 0.1 whereadditional ferromagnetic-like contribution is most pronounced.A quantitative analysis of the nonlinear feature in M (B) is doneby subtracting the linear high-magnetic field dependence of themagnetization that allows extracting the saturation moment ofthe ferromagnetic-like response. The result of this analysis isshown in Figure 7. The data clearly show the particularly largeeffect at x = 0.1 as well as a strongly nonmonotonous dopingdependence. The particular role of this doping level is corrobo-rated by the shift of the 7Li- NMR line (Figure 7). The mainfactor affecting a monotonic growth of the NMR frequency ofthis line upon Li doping is the hyperfine interaction with thevanadium d-electrons. The increase of the Li doping providesadditional electrons to the matrix thus, progressively reducingthe number of nonmagnetic V5þ (4d0, S = 0) ions and increasingthe number of magnetic V4þ (4d1, S = 1/2) ions containing oneelectron on the d-shell. Apparently, the origin of the sharp lineshift for the Li0.1VOx-NT sample is different. 51V NMR (notshown here) does not indicate any anomalies that could berelated to the excess of the amount of the magnetic V4þ sites atthis particular doping level. Therefore, the 7Li NMR line shift canbe only due to the internal magnetic field at the position of thelithium nuclei.24 Thus an “extra” displacement of the lithium lineof the Li0.1VOx-NT sample to a higher frequency (Figure 7)gives evidence for the appearance of the static (at least on theNMR time scale) ferromagnetically ordered areas around the Lisites that create an internal field on all Li nuclei in the sample.The unexpected occurrence of room-temperature ferromag-

netism upon electrochemical lithium doping of VOx-NTs hasbeen described in more detail in ref 24 for LixVOx-NTs (1). Forboth LixVOx-NTs (1) and LixVOx-NTs (3), it appears that the

Figure 6. Field dependence of the magnetization of LixVOx-NT (3) at300 K for x = 0, 0.05, 0.1, 0.155, 0.2 ,and 0.4.

Figure 7. Doping dependence of the saturation magnetization (left-hand side) and of the 7Li- NMR line shift (right-hand side) for LixVOx-NTs (3).

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Li concentration x = 0.1 represents the optimal doping level forthe room temperature ferromagnetism hinting at a delicatebalance between electronic and structural factors. Although ourstudies exhibit differences in the electrochemical response, theevolution of HTFM appears robust against small deviations, thatis, in morphology. High-temperature ferromagnetism attributedto ferromagnetic clusters formation has been also found innanoscale diluted magnetic semiconductors (DMS)27,28 andascribed to collective polaronic effects.29,30 As discussed in ref24, a polaronic scenario may be relevant for VOx-NTs as wellsuggesting that an appropriate tuning of the electronic, structural,and magnetic properties of transition metal oxide nanostructuresby lithium intercalation may indeed result in high-temperatureferromagnetism.

4. CONCLUSIONS

In the present work, mixed valence VOx-NTs have beencharacterized both by electrochemical and magnetic methods.It is concluded that the electrochemical behavior of the VOx-NTs is very sensitive to the preparation conditions. Differentsurfactant content and/or amount of water could influence theelectrochemical properties of VOx-NTs. Different nanotubesizes, as well as aging of the material can be another source fordifferences in electrochemical behavior of similarly preparedVOx-NTs. The electrochemical properties of the studied com-pounds are superior to other template-VOx-NTs. Using electro-chemical techniques, the charge carrier density in VOx-NT canbe modified and controlled via lithium intercalation into thestructure. Room-temperature ferromagnetism has been foundfor the optimal Li doping level Li0.1VOx-NTs in two differentbatches, although being distinct from the electrochemical pointof view.

’AUTHOR INFORMATION

Corresponding Author*E-mail: a.popa@ifw-dresden.de.

’ACKNOWLEDGMENT

Collaboration and discussions with C. Masquelier and M.Morcrette are gratefully acknowledged. Work has been sup-ported by the Deutsche Forschungsgemeinschaft (KL 1824/2and BU-887/13-1) by BMBF within the initiative LIB 2015(Nachwuchsgruppe 03SF0340) and by the Russian Foundationfor Basic Research (08-02-91952-NNIO-a).

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