Electrochromic W1–x–yTixMoyO3 Thin Films Made by Sputter

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Electrochromic W1−x−yTixMoyO3 Thin Films Made by Sputter Deposition: Large Optical Modulation, Good Cycling Durability, and Approximate Color Neutrality M. A. Arvizu,* G. A. Niklasson, and C. G. Granqvist Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden S Supporting Information *

ABSTRACT: Tungsten oxide thin films are used in electrochromic devices such as variable-transmittance “smart windows” for energy efficient buildings with good indoor comfort. Two long-standing issues for WO3 thin films are their limited durability under electrochemical cycling and their blue color in transmission. Here, we show that both of these problems can be significantly alleviated by additions of titanium and molybdenum. We found that ∼300 nm-thick films of sputter deposited W1−x−yTixMoyO3 are able to combine a midluminous transmittance modulation of ∼70% with good color neutrality and durability under extended electrochemical cycling. The Ti content should be ∼10 at. % in order to achieve durability without impairing transmittance modulation significantly, and the Mo content preferably should be no larger than 6 at. % in order to maintain durability. Hence, our results give clear guidelines for making three-component mixed-oxide thin films that are suitable for electrochromic “smart windows”.

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Thin films of tungsten oxide (WO3) are of special relevance in electrochromics. This is the material where electrochromism was first discovered11 and, as mentioned above, it is the material of choice for developing applications such as “smart windows”. However, WO3 films need to be improved in order to enhance their performance and, in particular, the durability under longterm insertion/extraction of ions and electrons and the color properties need careful attention, as considered next. Durability is of vital importance for “smart windows” and most other EC-based technologies. An addition of Ti to WO3 can boost the durability under ion insertion/extraction as discovered many years ago by Hashimoto and Matsuoka12,13 in studies of thin films prepared by coevaporation of WO3 and TiO2 powders. Their results were verified by Göttsche et al.14 for thin films made by sputter deposition and sol−gel technology, and subsequent supportive work has included EC thin films prepared by reactive DC as well as midfrequency magnetron sputtering,15,16 spin-coating,17 and electrodeposition.18 Ti additions to EC MoO3 thin films have a similar beneficial effect.19,20 We note that degraded EC films of WO3 and TiO2 can be rejuvenated repeatedly by electrochemical treatment, both galvanostatically21−24 and potentiostatically,25 but it is presently unclear how these procedures can be implemented for entire EC devices, which means that the need to enhance the durability of individual EC films remains urgent. Turning now to color, it is widely known that EC thin films of WO3 have a characteristic blue appearance in transmission

INTRODUCTION Electrochromic (EC) materials are able to change their optical properties persistently and reversibly under the action of an electric field.1,2 The optical modulation is caused by joint insertion/extraction of ions and electrons and depends on the polarity of the field. Materials coloring upon charge insertion/ extraction are called “cathodic”/“anodic”, respectively. Electrochromism exists in both inorganic and organic materials, each class having specific pros and cons with regard to applications. Among the inorganic EC materials, the most widely studied ones are a number of transition metal oxides with those based on W, Ti, Mo, and Nb having cathodic coloration and oxides of Ni and Ir being anodic.1,2 Thin films of EC oxides can be used in various types of optical technology, such as in “smart windows”3 capable of providing energy efficiency in buildings by offsetting the need for cooling4,5 due to excessive inflow of solar energy, at the same time as indoor comfort can be enhanced by preventing dazzling light.6,7 The EC oxides can be used also in several other “green” nanotechnologies.8 A “smart window” embodies a multilayer structure with principle kinship to an electrical thin-film battery. A centrally positioned electrolyte is surrounded by an EC film and a thinfilm counter-electrode capable of joint insertion/extraction of ions and preferably having EC properties complementary to those of the first EC film. A combination of a cathodic W-oxidebased film and an anodic Ni-oxide-based film has particularly favorable properties9 and is used together with inorganic or polymer-based electrolytes in commercial “smart windows”.3 The three-layer stack is located between transparent electrical conductors which typically are based on wide-bandgap heavily doped oxide semiconductors.3,10 © 2017 American Chemical Society

Received: December 8, 2016 Revised: February 22, 2017 Published: February 23, 2017 2246

DOI: 10.1021/acs.chemmater.6b05198 Chem. Mater. 2017, 29, 2246−2253

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Chemistry of Materials

represented as W1−x−yTixMoyOz, with specific data for x, y, and z reported in Table S1. It is seen that the Mo content was dependent on PMo and lay in the 0.02 ≤ y ≤ 0.22 range. The amount of Ti, on the other hand, remained almost constant regardless of PMo and depended on which of the W−Ti targets was used; specifically, the Ti content was 0.10 ≤ x ≤ 0.12 for the W(95 wt %) + Ti(5 wt %) target and 0.17 ≤ x ≤ 0.20 for the W(90 wt %) + Ti(10 wt %) target. The oxygen content lay consistently in the 3.0 ≤ z ≤ 3.1 interval. Accuracies were ±0.01 for both x and y and ±0.1 for z. The data on x are consistent with those observed in our prior work on EC films of W1−xTixO316 and W1−x−qTixNiqO3.41,42 The fact that z was always close to 3 allows us to label the films simply as W1−x−yTixMoyO3, as is done henceforth. Structural characterization of as-deposited and annealed films was performed with XRD using a Siemens D5000 instrument working with Cu Kα radiation. Figure 1 reports characteristic data for thin films of

due to a broad optical absorption band centered at a wavelength (λ) of ∼850 nm,1,11 i.e., in the near-infrared, and extending into the luminous range at 400 < λ < 700 nm. A bluish hue is often undesirable in buildings-related applications, but the optical properties can be improved by intermixing with another oxide. Pioneering work toward color-neutral EC thin films was performed by Faughnan and Crandall26 who showed that the peak of the coloration band in W1−yMoyO3 thin films, deposited by coevaporation of WO3 and MoO3 powders, was shifted toward shorter wavelengths so as to achieve a better match to the response of the human eye and hence a more neutral color. Analogous results were reported by Yamada and Kitao.27 A number of supporting studies of EC W1−yMoyO3 thin films have been reported subsequently on thin films prepared by various techniques including thermal evaporation,28 reactive DC magnetron cosputtering,29 reactive sputtering assisted by a selective sublimation processing technique,30 chemical vapor deposition,31,32 spray pyrolysis,33 electrochemical deposition from a metal peroxide bath,34 and atmospheric pressure plasma jet deposition.35 Virtually complete color neutrality was obtained for WO3−MoO3− V2O5 thin films deposited by evaporation, as shown in early work by Sato and Seino.36 The improved color performance of WO3−MoO3-based thin films allows them to be used in EC devices with transparent counter-electrodes, for example, based on Ce oxide,37,38 but these properties are of interest also for Nioxide-based counter-electrodes with some brownish−yellowish anodic coloration that in itself is able to provide color matching to some degree.39 In the present work, we demonstrate, by significantly extending earlier research surveyed above, that additions of Ti to enhance durability of EC WO3 thin films can be combined with additions of Mo to improve color neutrality, the net result being thin films with superior eletrochromism.

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EXPERIMENTAL SECTION

Thin films of W1−x−yTixMoyO3 were prepared by reactive DC magnetron cosputtering in a deposition system based on a Balzers UTT 400 unit. Targets were 5 cm diameter plates of pure Mo and of alloys comprising W(95 wt %) + Ti(5 wt %) and W(90 wt %) + Ti(10 wt %) (Plasmaterials), all with 99.99% purity. The sputter system was evacuated to 2 × 10−4 mTorr. Argon and oxygen, both of 99.997% purity, were then introduced through mass-flow-controlled gas inlets. The O2/Ar ratio was 0.30, and the pressure was ∼30 mTorr. The discharge power for the W−Ti alloy target was set at 225 W while the power PMo on the Mo target was varied in the 75−250-W-range in order to prepare films with different Mo concentrations. Substrates for optical and electrochemical measurements were unheated 5 × 5 cm2 glass plates precoated with transparent and electrically conducting layers of In2O3:Sn (ITO) having a sheet resistance of 40 Ω. Films for compositional analysis by Rutherford Backscattering Spectrometry (RBS) were deposited onto glassy carbon plates. The target−substrate separation was 13 cm. The film thickness was 300 ± 20 nm as determined by surface profilometry using a Bruker DektakXT instrument. Postdeposition annealing of some films, solely for analyses by X-ray diffractometry (XRD), was performed by heating in air at 400 °C for 1 h using a conventional tube-furnace. Earlier work of ours has employed the same deposition parameters to prepare oxide films based on WO3, MoO3, W1−xTixO3, and W1−yMoyO3.16,29 Elemental characterization of the thin films was performed with RBS using 2 MeV 4He ions backscattered at an angle of 170°. This technique has high sensitivity for heavy elements such as W, Ti, and Mo. The RBS results were fitted to a model of the film−substrate system by use of the SIMNRA simulation program.40 Typical RBS spectra are shown in Figure S1 and demonstrate accurate fits between experimental and modeled data. Film compositions were first

Figure 1. X-ray diffractograms for ∼300 nm-thick films of the shown compositions studied in the as-deposited state and after annealing at 400 °C for 1 h. The (hkl) indices correspond to either the monoclinic phase of WO3 or the orthorhombic α-phase of MoO3. Arrows indicate diffraction peaks due to ITO. Note that the vertical scales are different for the two sets of data. W0.70Mo0.30O3 and W0.70Ti0.10Mo0.20O3 in the as-deposited state and after annealing. All as-prepared samples showed nothing but diffraction features due to ITO (in agreement with JCPDS−ICDD card number 88-0773) and hence were X-ray amorphous. The samples with and without Ti behaved very differently under heat treatment: the W1−yMoyO3 film crystallized into a mixed phase with monoclinic WO3 structure (JCPDS−ICDD card number 83-0950) and orthorhombic MoO3 structure, known as α-phase (JCPDS−ICDD card number 05-0508), whereas the W1−x−yTixMoyO3 film remained amorphous. All of the W1−yMoyO3 films exhibited some crystallization after annealing, and all of the W1−x−yTixMoyO3 films remained 2247

DOI: 10.1021/acs.chemmater.6b05198 Chem. Mater. 2017, 29, 2246−2253

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Chemistry of Materials amorphous after annealing. The crystallization-impeding effect of Ti is consistent with previous results for W1−xTixO3 films.16,17,43,44 The electrochemical behavior of the W1−x−yTixMoyO3 thin films was studied with cyclic voltammetry (CV) by using a Solartron 1286 Electrochemical Interface and a three-electrode electrochemical cell. The electrolyte was 1 M LiClO4 in propylene carbonate, and lithium foils served as reference and counter electrodes. All measurements were performed inside an argon-filled glovebox with water content less than 0.6 ppm. The voltage sweep range was 1.7−4.0 vs. Li/Li+ for durability studies, and the voltage sweep rate was 10 mV/s. Optical measurements were carried out using two techniques: Durability data were recorded in situ, with samples immersed in the electrolyte, at a midluminous wavelength of 550 nm, using an Ocean Optics fiber-optic instrument. The electrochemical cell was positioned between a tungsten halogen lamp and the detector, and the 100% level was taken as the transmittance measured before the sample was put into the electrolyte. We also made ex situ spectrophotometric measurements of normal transmittance T(λ) and near-normal reflectance R(λ) in the 300 < λ < 2500 nm range, encompassing solar radiation, by use of a PerkinElmer Lambda 900 spectrophotometer equipped with an integrating sphere accessory. The latter recordings were performed for a voltage sweep range of 2.0−4.0 vs. Li/ Li+ on films that had undergone 20 CV cycles ending with films in a fully colored state; the samples were subsequently taken out of the glovebox, rinsed in 2-propanol to remove electrolyte residues, and blown dry with nitrogen gas. We verified that the in situ and ex situ procedures gave similar results by making consecutive recordings in the 350 < λ < 800 nm range with the fiber-optical instrument and with the spectrophotometer; data are reported in Figure 2. Color specifications for the films were obtained from T(λ), specifically using the CIE 1931 Colorimetric System and assuming a daylight illuminant (D65).45 This system describes color by two parameters x and y located inside an area bounded by a curve, representing the spectrally pure colors, and where the end points of the luminous range are connected by a straight line.

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Figure 2. In situ and ex situ spectral optical transmittance for fully colored ∼300 nm-thick EC films of the shown compositions. The films have undergone pretreatment comprising 20 CV cycles at 2.0−4.0 V vs. Li/Li+ at 10 mV/s. The ripples in the in situ optical data at λ < 450 nm are due to low signal intensity connected with absorption in the glass substrate.

RESULTS AND DISCUSSION We first consider the durability of W1−x−yTixMoyO3 thin films under harsh CV cycling in the voltage range of 1.7−4.0 V vs. Li/Li+. Figure 3 demonstrates that some irreversible decrease of charge capacity takes place during the initial CV cycling, but this effect is much smaller for CV cycle numbers in the range of 20−80. Analogous films of WO3, investigated in an earlier work of ours,16 degraded heavily under the same type of treatment as will be elucidated shortly. Published data on EC W1−yMoyO3 thin films also showed rapid degradation under harsh CV cycling.29 The most salient result from Figure 3 is that the addition of Ti to W1−yMoyO3 thin films provides significantly enhanced durability under extended CV cycling within a voltage interval that otherwise leads to rapid deterioration of the electrochemical performance. Inspection of the CV data also reveals an unambiguous detrimental influence on the durability of increased amounts of Mo, the effect being most noticeable between the first and subsequent CV cycles. More detailed information on the roles of Ti and Mo additions to WO3 with regard to the electrochemical properties under CV cycling was obtained by plotting the evolution of the inserted (Qinserted < 0) and extracted (Qextracted > 0) charge densities as well as the irreversibility of the ion insertion/ extraction process. The irreversibility was expressed as the sum of the charge densities for insertion and extraction for successive CV cycles, given by δQ = Q inserted + Q extracted

after less than 20 CV cycles for low to moderate Mo contents (y < 0.15), whereas a weak decline in δQ is still noticeable for high Mo contents (y > 0.15). No obvious influence of the Ti content is apparent, and data for x ≈ 0.1 (Figure 4a,b) and x ≈ 0.2 (Figure 4a′,b′) are similar. The fact that δQ is less than zero almost universally can be reconciled with irreversible Li+ ion incorporation, which is a well-known cause for degradation of EC WO3 thin films.21 We now turn to optical transmittance modulation upon charge exchange, defined by ΔT = Tbleached − Tcolored

(2)

Figure 5a,a′ displays data for W1−x−yTixMoyO3 thin films at λ = 550 nm for CV cycling in the voltage range of 1.7−4.0 V vs. Li/ Li+. Here, Tbleached and Tcolored pertain to fully bleached and colored films, respectively, corresponding to the end points of CV cycles such as those shown in Figure 3. Data for a WO3 film are included so as to provide a baseline; this film exhibits a rapid decline from an initial modulation span of ∼80% to a value as small as ∼20% after 80 CV cycles, which is in agreement with earlier observations.16 During the early CV cycling, the drop of ΔT is faster for the W1−x−yTixMoyO3 films than for the WO3 film. However, optical modulation is stabilized after ∼10 CV cycles and is ∼65% and ∼55% for films with Ti contents represented by x ≈ 0.1 and x ≈ 0.2,

(1)

As shown in Figure 4, the initial changes in Qinserted and Qextracted are followed by approximate reversibility (δQ ≈ 0) 2248

DOI: 10.1021/acs.chemmater.6b05198 Chem. Mater. 2017, 29, 2246−2253

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Figure 3. Cyclic voltammograms for ∼300 nm-thick W1−x−yTixMoyO3 films of the shown compositions. Data in the two columns refer to films obtained by sputter deposition from W−Ti targets of the indicated compositions. Measurements were performed after the reported numbers of CV cycles for a voltage sweep range of 1.7−4.0 V vs. Li/Li+ at 10 mV/s. Arrows indicate sweep direction.

Figure 4. Inserted and extracted charge densities (a and a′) and charge density difference between consecutive CV cycles (b and b′) for ∼300 nmthick W1−x−yTixMoyO3 films of the shown compositions. Data in the left-hand and right-hand pairs of panels refer to films obtained by sputter deposition from W−Ti targets of the stated compositions. Measurements were performed after the stated numbers of CV cycles for a voltage sweep range of 1.7−4.0 V vs. Li/Li+ at 10 mV/s. Insets display vertically expanded data on the charge density difference for CV cycle numbers of 20−80. Symbols denote data, and connecting straight lines were drawn for convenience.

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DOI: 10.1021/acs.chemmater.6b05198 Chem. Mater. 2017, 29, 2246−2253

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Figure 5. Optical transmittance modulation (a and a′) and coloration efficiency (b and b′) at a wavelength of 550 nm for ∼300 nm-thick W1−x−yTixMoyO3 films of the shown compositions. Data in the left-hand and right-hand pairs of panels refer to films obtained by sputter deposition from W−Ti targets of the stated compositions. Measurements were performed after the indicated numbers of CV cycles for a voltage sweep range of 1.7−4.0 V vs. Li/Li+ at 10 mV/s. Symbols denote data, and connecting straight lines were drawn for convenience.

Figure 6. Spectral optical absorption coefficient for ∼300 nm-thick W1−x−yTixMoyO3 films of the shown compositions. (a, a′) Data refer to films obtained by sputter deposition from W−Ti targets of the stated compositions. Measurements were performed after 20 CV cycles for a voltage sweep range of 2.0−4.0 V vs. Li/Li+ at 10 mV/s. The small irregularity in absorption coefficient at λ ≈ 855 nm is an experimental artifact; it does not affect assessments of the luminous performance of the films.

Ti content. The latter types of films display some minor recovery of ΔT beginning after ∼10 CV cycles.

respectively, as long as the amount of Mo remains moderately low, specifically y < 0.07 at low Ti content and y < 0.14 at high 2250

DOI: 10.1021/acs.chemmater.6b05198 Chem. Mater. 2017, 29, 2246−2253

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Figure 7. CIE 1931 chromaticity diagram with (x, y) coordinates for ∼300 nm-thick W1−x−yTixMoyO3 films of the shown compositions. (a, a′) Data refer to films obtained by sputter deposition from W−Ti targets of the stated compositions and were obtained from the same spectral transmittance data as those employed to derive spectral absorption coefficients reported in Figure 6.

∼70% at λ = 550 nm, which is ∼10% higher than for the latter sample.

Complementary data on coloration efficiency, defined as1 CE = ln(Tbleached /Tcolored)/ΔQ

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(3)

CONCLUSIONS This work has demonstrated that ∼300 nm-thick films of W1−x−yTixMoyO3 are able to show excellent electrochromic properties and combine a midluminous transmittance modulation of ∼70% with good color neutrality and durability under extended electrochemical cycling. The Ti content should not be excessive and ∼10 at. % is better than ∼20 at. %. The Mo content should be small, preferably no larger than 6 at. %, in order to maintain durability. Our results give clear guidelines for making three-component mixed electrochromic oxides with properties that are superior to those of commonly used single metal or two-component metal oxides. Our results are of considerable interest for developing electrochromic smart windows for energy efficient buildings with good indoor comfort.

where ΔQ is the charge exchange associated with ΔT, are reported in Figure 5b,b′. Clearly, it is desirable to have a large value of CE for applications such as “smart windows” since it leads to minimum power consumption for given transmittance modulation. Generally speaking, it is found that the CE drops during the initial voltammetric cycles but is then stabilized at a magnitude that is decreased as the Mo content is increased. The results are consistent with earlier data on EC W1−xTixO3 thin films.16 We now consider spectral optical data recorded ex situ in the wavelength interval of 300 < λ < 2500 nm for the voltage interval of 2.0−4.0 vs. Li/Li+. Results are given for the spectral absorption coefficient expressed by α(λ ) =

1 ⎛ 1 − R (λ ) ⎞ ln⎜ ⎟ d ⎝ T (λ ) ⎠

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(4)

ASSOCIATED CONTENT

S Supporting Information *

which is a good approximation for weakly absorbing films on glass substrates.46 Figure 6 shows α(λ) for films of WO3, MoO3, and W1−x−yTixMoyO3, where data on the former two films are included as baselines. The films with modest contents Mo, specifically y < 0.1, exhibit distinct near-infrared absorption bands, in agreement with literature data cited in the Introduction, and a bluish hue. Generally speaking, increasing values of x and y make the wavelength dependence of α(λ) less prominent in the luminous spectral range, which means that the visible appearance of these films approaches color neutrality. Quantitative color specifications are given in the chromaticity diagrams in Figure 7, and corresponding data on x and y are also reported in Table S2. Clearly the (x, y) coordinates for the WO3 film lie furthest from the region corresponding to color neutrality and indicate a distinctly bluish color, whereas coordinates for the MoO3 film are closer to this region. Samples of W0.70Ti0.10Mo0.20O3 and W0.61Ti0.17Mo0.22O3 show best color neutrality, but their electrochemical cycling durability is limited, as is apparent from Figure 5. However, samples of W0.82Ti0.12Mo0.06O3 and W0.76Ti0.19Mo0.03O3 display optical properties that are almost as favorable as those for the more Mo-rich films and were found to have good durability. The former of these samples has a large transmittance modulation of

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05198. Table of compositional data for W1−x−yTixMoyOz films; table of chromaticity coordinates (x, y); selected RBS spectra and corresponding simulation fitting for W1−x−yTixMoyOz films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: marvizu@fis.cinvestav.com.mx. ORCID

M. A. Arvizu: 0000-0002-9885-3161 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.A.A. thanks the Mexican Council for Science and Technology (CONACyT) for financial support to work at Uppsala University as a postdoctoral researcher. Complementary financing was received from the European Research 2251

DOI: 10.1021/acs.chemmater.6b05198 Chem. Mater. 2017, 29, 2246−2253

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Council under the European Community’s Seventh Framework Program (FP7/2007−2013)/ERC Grant Agreement No. 267234 (GRINDOOR). We are grateful to Annica Nilsson (Uppsala University) for assistance with the colorimetric analysis, to Kostadinka Gesheva (Bulgarian Academy of Sciences) and Ludvik Martinu (Polytechnique Montreal) for discussions, and to Daniel Primetzhofer and the staff of the Tandem Accelerator Laboratory at Uppsala University for support with RBS measurements.

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DOI: 10.1021/acs.chemmater.6b05198 Chem. Mater. 2017, 29, 2246−2253