Article pubs.acs.org/JPCC
Effect of Deposition Conditions on the Electrochromic Properties of Nanostructured Thin Films of Ammonium Intercalated Vanadium Pentoxide Xerogel Metodija Najdoski,*,† Violeta Koleva,‡ and Aksu Samet†
J. Phys. Chem. C 2014.118:9636-9646. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/23/19. For personal use only.
†
Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University, POB 162, Arhimedova 3, 1000 Skopje, Republic of Macedonia ‡ Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bldg. 11, 1113 Sofia, Bulgaria ABSTRACT: We have examined the influence of the deposition temperature and deposition time on the electrochromic activity of nanostructured (NH4)0.3V2O5·1.25H2O thin films prepared by the newly developed chemical bath deposition method. The film structure is characterized by XRD powder diffraction, IR spectroscopy, and TG-DTA techniques. SEM and AFM observations evidence that the common ribbon-like structure is modified by the different deposition conditions mainly with respect to the particle sizes, ribbons orientation, and films porosity. The electrochemical and electrochromic properties of two series of thin films prepared at 50 and 85 °C for different deposition time are studied. All films exhibit two-step electrochromism with color changes yellow/green and green/blue, related to the stepwise reduction/oxidation processes between V(V) and V(IV) sites. The electrochromic changes are stable after prolonged film storage and for at least a hundred cycles. The optical properties of (NH4)0.3V2O5·1.25H2O thin films are found to depend significantly on the deposition conditions as the higher deposition temperature is favorable in providing higher values of the transmittance variance. The best result achieved for the transmittance variance of 55% at 400 and 900 nm clearly demonstrates the potential of ammonium intercalated vanadium oxide xerogel thin films for commercial application in electrochromic devices.
1. INTRODUCTION Vanadium pentoxide xerogels, of the general formula V2O5· nH2O, have been known more than a century as very reactive materials with rich intercalation chemistry, electronic and ionic conductivity, and electrochromic behavior.1−3 During the past 20 years, a number of papers report attractive results about industrial applications of hydrated vanadium pentoxides.4 Because of the high lithium intercalation capacity (theoretically up to 2 equiv of lithium per vanadium), the thin films of V2O5· nH2O gels have been used as antistatic coatings,4 humidity sensors,4 cathodes in lithium ion batteries,5,6 supercapacitors,7 electrodes in electrochromic devices,4,8,9 and electrochromic mirrors.9 The electrochromism in the V2O5·nH2O xerogels is related to the reversible reduction/oxidation of V(V) to V(IV) or to a lower valence vanadium state depending on the applied voltage which gives rise to several colors like yellow, green, blue, and violet.4,10 In this regard, the V2O5·nH2O xerogels are very promising materials, since they have the potential to broaden the color palette displayed by inorganic electrochromics and thus to extend the range of their functions. Research shows that the electrochromic performance of materials, including V2O5·nH2O, is significantly influenced by the deposition parameters:11 method of deposition, kind of precursors used, experimental conditions such as precursor concentration, temperature, pressure, rate of deposition, etc. The deposition parameters strongly affect the microscopic film © 2014 American Chemical Society
characteristics such as structure, crystallinity, and morphology, all being directly responsible for the electrochromic characteristics. The knowledge of the relationship between deposition parameters, film characteristics, and optical properties is an important key for control and optimization of the film electrochromic properties. Various methods are reported for the preparation of V2O5· nH2O xerogels thin films like electrodeposition,12 radio frequency sputtering,13 spin or dip coating,8,9 and quenching method.14 All these methods offer different advantages depending on the application of interest, but all of them need of specific equipment (vacuum system, spin or dip coater, etc.) and special conditions (pressure, gas flow control, high temperature). The most often used chemical method that allows formation of both monodispersed powders and thin films of V2O5·nH2O xerogels is the sol−gel synthesis.3 It is developed by three main synthetic routes:3 acidification of NaVO3 using ion-exchange and polymerization of the resultant HVO3 in water; hydrolysis and condensation of vanadium alkoxides; reaction between H2O2 and V2O5 powder. The ionexchange process, however, is not suitable for large-scale production, while the alkoxide route is rather delicate because Received: December 28, 2013 Revised: April 10, 2014 Published: April 14, 2014 9636
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various techniques. Because of that, we believe that the results of the present work are valuable in terms of optimizing the deposition procedure and understanding the deposition mechanism of vanadium pentoxide xerogels by the CBD method. Moreover, the new knowledge obtained is a contribution to the elucidation of the fundamental relationship: preparation method−microscopic properties−macroscopic properties.
of its dependence on many parameters like temperature, pH, reactant concentration, and high reactivity of the alkoxide.3 Another mild chemical approach for thin films manufacturing is the chemical bath deposition (CBD). The CBD method is based on precipitation reactions at low precipitation rate in the presence of a substrate. This deposition technique is very simple, low cost, and applicable to large area film preparation and allows deposition at low temperatures on both sides of odd shape substrates, including polymer substrates. These advantages make the CBD method very attractive, and it has been successfully used to obtain thin films of sulfide, selenide, and oxide semiconductors.15,16 The efficiency of the CBD method for the formation of a highly porous structure of the films, and hence much better electrochromism and electrochemical reversibility compared to the sol−gel and electrodeposition methods, has been clearly demonstrated in the case of NiO films.17 So far, there are no reports on the application of the CBD method for preparation of thin films of V2O5·nH2O xerogels. The benefits of the CBD method provoked us to develop a simple chemical bath procedure for deposition of nanostructured thin films of V2O5·nH2O xerogels. Our team has experience with the CBD technique and promising results on electrochromic thin films of sodium vanadium bronzes18 and manganese oxides19,20 have been recently reported. An important issue of the method that had to be addressed was its ability to produce nanostructured films. In the past years, the design in the nanoscale region has gained increasing importance since it opens new opportunities to improve material performance (fast insertion/extraction kinetics, enhanced durability, etc.). Various forms of hydrated vanadium pentoxides have been obtained: nanoribbons,21 nanorods,22 nanobelts,23 nanofibers,24 nanorolls,1 and nanotubes,1,4 which display interesting properties for different applications.4,25,26 In this work we have reported a new chemical bath deposition method for preparation of ammonium intercalated vanadium pentoxide xerogel with a composition (NH4)0.30V2O5·1.25H2O. In principle, the method is very simple and is based on the acidification of ammonium metavanadate solution by acetic acid at temperatures between 50 and 85 °C. It is very important that the deposition parameters like vanadium concentration and temperature are not chosen at random, but considering our experience on the composition of the precipitates and the precipitation rate in the NH4VO3−CH3COOH system. Research is focused on the effect of the deposition parameters like temperature and deposition time on the electrochromic properties of (NH4)0.30V2O5·1.25H2O with a view to giving new insight into the relationship between the rate of deposition (governed by temperature), the films thickness, and the optical spectra. We have provided evidence that the deposition rate affects the morphological and microstructure features of the films which are then responsible (at least partly) for the difference in the optical spectra. The proper selection of the deposition parameters enables to obtain two series of thin films with a thickness varying in a wide range from 100 to 1400 nm and thus to find out the optimum film thickness providing the highest value of the transmittance variance at rather different wavelengths, 400 and 900 nm, without change of the chemical composition of the films. It is worth noting that most of the studies on the electrochromic thin films, including vanadium pentoxide xerogel films, concern one or few films having close thickness that are further subjected to investigations with
2. EXPERIMENTAL SECTION The thin films are prepared on commercially available glass substrates of SnO2:F (FTO) which possess high optical transparency of 80% in the visible spectrum and electrical resistance of 10−20 Ω/cm2. Before deposition, the substrates are cleaned in the sequence: with detergent, alkaline solution, hydrochloric acid, hexane, acetone and finally rinsed with deionized water and dried in air. The thin films of (NH4)0.3V2O5·1.25H2O are prepared by a chemical bath deposition method. The chemical bath solution is prepared from 0.5 g of NH4VO3 and 45 cm3 of deionized water (0.045 M) in a beaker with a volume of 100 cm3 at 40 °C. The pH value of the prepared solution is 5. After filtration that prevents undesired begging of precipitation, 50 cm3 of glacial acetic acid is added to the filtrate, and the deposition solution is ready. The clean and dry substrates are placed vertically supported to the wall of the beaker with nonconductive side facing the wall. The solution is then heated up to 50 or 85 °C (deposition temperature) and stirred continuously on a magnetic stirrer. Hereafter, the precipitate and films prepared at 50 °C will be denoted as VA0550, and accordingly these prepared at 85 °C will be denoted as VA0585. The beginning of the deposition reaction is observed as an appearance of orange opaque state of the solution, which occurs when the solution temperature reaches to around 45 °C. However, the deposition time starts to be measured from the moment when the temperature of the chemical bath reaches to 50 °C for VA0550 and 85 °C for VA0585 films. The operating temperature is maintained up to the moment when the substrates with thin films are removed from the chemical bath, so-called deposition time. Then, the thin films are washed with ethanol and dried at ambient conditions. By varying the deposition time (from 10 to 50 min at 50 °C and from 3 to 50 min at 85 °C), we have prepared two series of VA0550 and VA0585 thin films. It is very important that the higher temperature determines a higher deposition rate without change of the chemical composition of the xerogel formed. Because of that the thickness of the VA0550 series varies in close limits from 100 nm (10 min deposition time) to about 350−400 nm (50 min deposition time), while the thickness of VA0585 series varies in a wide range from 240 to 1400 nm (3 and 50 min deposition time, respectively). Accordingly, the same film thickness can be achieved for different deposition times depending on the temperature. For instance, a 200 nm thick film is prepared at 50 °C for a 30 min deposition, but only 3 min is enough at 85 °C to obtain a film having close thickness of 240 nm. Similarly, a 50 min deposition at 50 °C is approximately equivalent to 10 min deposition at 85 °C regarding a film thickness of about 400− 450 nm. The as-deposited thin films have light orange to dark orange color depending on the deposition time and temperature. The color of the reaction system at the beginning of the deposition 9637
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that share edges and corners to form long slabs (bilayers of single V2O5 layers) along the b-axis. This structure of the slabs is responsible for the formation of long nanoribbons in the xerogels observed by SEM.4,21,27 The space between the slabs in the xerogels is occupied by intercalated water molecules. The structural coherence is however limited to about 50 Å (close to the interslab separation), suggesting that the slabs are turbostratically disordered.20 Because of that, the typical XRD pattern of the xerogels is characterized by 3−5 broad peaks from the 00l series corresponding to the one-dimensional stacking of the layers perpendicular to the substrate. The absence of the 002 reflection is related to the double layer structure of the slabs.4,19,28 The basal distance between the layers depends on the amount of the water molecules, e.g., 11.55 Å for n ≈ 1.5−1.6 and 8.75 Å for n ≈ 0.5.18 For the as-prepared VA0550 and VA0585 gels the interlayer space deduced from the position of the first 001 peak (Figure 1) is d = 10.68 Å, which is consistent with the intercalation of a water content lower than 1.5. The XRD pattern of the as-deposited VA0550 film is also shown in Figure 1. As seen, it is dominated by the strong diffraction peaks due to the FTO substrate (PDF 46-1088), and even the first 001 peak is hardly visible (marked with an arrow). The same future is observed in the case of VA0585 film (not presented). In order to clarify the structure of the as-deposited films, further studies by IR spectroscopy are undertaken. In Figure 2, the IR spectra of the precipitates VA0550 and VA0585 (Figure 2a,c) are compared with the IR spectra of the
is orange but over time becomes red-brown at both reaction temperatures. All reagents used are analytical pure substances. The structure and composition of both thin films and precipitates of V2O5·nH2O xerogel were examined using a Rigaku Ultima IV X-ray diffractometer (Cu Kα radiation). The thermal studies (TG and DTA) of the precipitates from the chemical bath were carried by a LABSYS Evo apparatus (SETARAM) with a heating rate of 10 °C/min up to 500 °C in an air flow. Infrared spectra were recorded in KBr disks with a PerkinElmer System 2000 infrared interferometer. AFM investigations were made with NanoScopeV system (Veeco Instruments Inc.) in taping mode at room temperature. The morphology of the thin films was observed by scanning electron microscopy (JEOL JSM-5510). The film thickness of selected films was measured by an Alpha Step D-100 profilometer (measuring parameters: stylus force 5 mg, length 8 mm, range 10 μm, and speed 0.07 mm/s). The electrochemical properties were examined by cyclic voltammetry in conventional threeelectrode cell using a micro AUTOLAB II equipment (EcoChemie, Utrecht, Netherlands) in the potential range between −1 and +1 V. The prepared thin film is the working electrode, the reference electrode is Ag/AgCl (3 M KCl), and the auxiliary electrode is a platinum wire. In situ optical spectra of the thin films are recorded by a Varian Cary 50 Scan spectrophotometer in the range from 350 to 900 nm using a 1 M LiClO4 in propylene carbonate (PC) as electrolyte. In the electrochromic cell the voltage is varied from −2.5 to +2.5 V.
3. RESULTS AND DISCUSSION 3.1. Composition and Structure of VA0550 and VA0585. The XRD patterns of the precipitates obtained at 50 and 85 °C (VA0550 and VA0585, respectively) are shown in Figure 1. The two patterns are same implying the same
Figure 1. XRD patterns of the precipitates VA0550 and VA0585 and as-deposited VA0550 film. Figure 2. IR spectra of VA0550 precipitate (a), VA0550 film (b, red line), VA0585 precipitate (c), VA0585 film (d, blue line), and VA0175 precipitate (e).
composition of the precipitates. The patterns display three broad peaks of the characteristic 00l set (missing 002 peak) for V2O5·nH2O xerogels.4,18,19 The intensity of the first-order diffraction 001 is much larger than that of the other terms. The vanadium oxide xerogels exhibit a layered structure made of double V2O5 sheets stacked along the c-axis of a monoclinic unit cell.20 The double V2O5 layers are made up of square-pyramidal VO5 units (or strongly distorted VO6 units)
corresponding scraped films (Figure 2b,d). The comparison shows that the IR spectra of two precipitates are practically the same as well as the IR spectra of the films coincide with these of the precipitates within the limits of the experimental resolution. Thus, the IR spectroscopy confirms the same composition of 9638
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(or 1002/970 cm−1) and 736 cm−1 in (NH4)0.30V2O5·nH2O (Figure 2a,c). This phenomenon can be explained by bonding interactions between the NH4+ and V2O5 framework, most likely via Hbonds.30,33 Our observations are in full agreement with previous results concerning xerogels intercalated with different inorganic and organic species.31,33 The thermal behavior of the two xerogels is followed by TGDTA analyses (Figure 3). The comparison shows that the
the two precipitates and the respective films, i.e., the formation of vanadium pentoxide xerogel. The IR spectra of the prepared gels (Figure 2) display bands associated with three types of characteristic vibrations. The first type of vibrations is due to the V−O framework of the xerogels, the latter being characterized by three main vibrational bands around 1000, 760, and 500 cm−1.29−31 The observed doublet at 1003/974 cm−1 is attributed to the stretching vibrations of terminal VO groups (shorter V−O bond), the band at 736 cm−1 is due to asymmetric stretching vibrations of the bridged V−O−V units (longer V−O bonds), and the band at 536 cm−1 is assigned to the V−O−V symmetric stretch mixed with bending vanadium oxygen vibrations.29−31 The shoulder at 835 cm−1 which does not appear in pristine xerogels could arise also from asymmetric stretching vibrations of V−O−V units. The second type of characteristic vibrations refers to the water molecules, and their presence in the as-deposited gels is evident from the bands due to the OH stretching (3580 and 3430 cm−1) and bending vibrations (1617 cm−1). The band at 3580 cm−1 corresponds to the water molecules nearly free of hydrogen bonding, while the lower-frequency band at 3430 cm−1 is related to the water molecules hydrogen bonded with the oxygen of V2O5 or of other H2O molecules.30 The third spectral feature is the appearance of bands due to NH4+ ions32 at 3140, 3015 (shoulder), and 1400 cm−1 corresponding to N−H stretching (the first two bands) and H−N−H bending (the last band) vibrations. These data unequivocally evidence for the presence of the NH4+ ions in the xerogel framework in addition to the water molecules. Because of the layered structure, vanadium pentoxide gels are able to intercalate a wide variety of inorganic and organic guest species without change in the one-dimensional stacking of the layers.4,27,31 According to the elemental analysis, the content of ammonium ions in the two gels prepared at different temperatures is about 0.3 mol per V2O5; i.e., the prepared gels VA0550 and VA0585 can be described as (NH4)0.3V2O5· nH2O compositions. It should be mentioned that the gel prepared from the dilute NH4VO3 solution (0.009 M) at 75 °C have twice lower content of intercalated NH4+ ions (0.15 mol per V2O5). For a comparison, the IR spectrum of this gel (NH4)0.15V2O5·1.3H2O denoted as VA0175 is also presented in Figure 2e. The different content of the ammonium ions is clearly manifested in the IR spectra of the three gels (Figure 2a,c,e), especially in respect to the intensity of the band at 1400 cm−1 compared with that of the band around 1000 cm−1. For the two compositions (NH4)0.3V2O5·nH2O (VA0550 and VA0585) the two bands are of comparable intensity (Figure 2a,c), in contrast to (NH4)0.15V2O5·nH2O (VA0175) where the former band is twice less intense than the latter band (Figure 2e). This is in accordance with the 2-fold lower quantities of ammonium ions in VA0175 than that in VA0550 and VA0585. It is worth noting that the different content of intercalated ammonium ions reflects on the vibrations of the V2O5 framework of the xerogels. The IR spectroscopic analysis shows that in the case of comparatively low NH4+ content the positions of the three characteristic bands for pristine V2O5· nH2O (1015, 760, and 515 cm−1)6,30 are not affected by the intercalated ammonium ions: 1011, 766, and 526 cm−1 in (NH4)0.15V2O5·nH2O (Figure 2e). However, the increased ammonium content clearly causes a downshift of the two higher-frequency bands (weakening of the respective V−O bonds) and splitting into doublet of the first band: 1003/974
Figure 3. TG-DTA curves for VA0550 and VA0585.
courses of the two TG curves, and accordingly of the two DTA curves, are practically the same. The two TG curves display a continuous decrease in the mass in the temperature range from 40 to 365 °C, but two separate ranges could be outlined because of the difference in the slope in two sections from each TG curve: between 40 and 210 °C and between 210 and 365 °C. This separation in two steps is in accord with the observation of two endothermic effects on the DTA curves, at 115 and 278 °C. In the literature there are data for three18,30 and two steps.10 The latter endothermic effect is immediately followed by an exothermic effect at 350 °C due to the crystallization of orthorhombic V2O5 (PDF 41-1426). According to the TG curves, the total mass loss is practically same for the two gels: 13.50% for VA0550 and 13.40% for VA0585 equiv formally to 1.57 and 1.56 mol of H2O, respectively. For the intermediate state at 210 °C the mass loss is 7.9% (0.9 mol of H2O). Since the product of thermal heating of the gels is V2O5, it follows that the observed mass loss includes the release of both water molecules as a main component and ammonium ions as a minor component (0.3 mol), so that, more precisely, the water content is about 1.25 mol. Therefore, the compositions of the two gels are the same and can be expressed by the formula (NH4)0.3V2O5·1.25H2O. The lower water content in our gels than the usual (n ≈ 1.5−1.6) is in accord with the XRD data for a lower d-spacing between the layers (10.68 Å) than 11.55 Å. 3.2. Morphology of (NH4)0.3V2O5·1.25H2O Thin Films. The effect of the different deposition conditions on the film 9639
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Figure 4. SEM micrographs of (NH4)0.3V2O5·1.25H2O thin films prepared at different deposition conditions: (a, b) at 50 °C for 30 min, (c) at 85 °C for 3 min, and (d) at 85 °C for 40 min. Scale bars: (a) 0.5 μm, (b) 0.1 μm, (c) 0.5 μm, and (d) 10 μm.
The morphology of a thick film with a thickness of 1220 nm prepared at a prolonged deposition of 40 min at 85 °C is illustrated in Figure 4d. As seen, the film is very dense without any porosity. It is composed of ribbon-like units in the micrometer scale: around 1 μm wide and between 5 and 20 μm long. However, from the SEM image we are not able to specify whether these large ribbons are result of the growth or coalescence of primary smaller ribbon-like units during the prolonged deposition. The films under the SEM observations are studied by AFM as well (Figure 5). Part of the 2D surface topography of the (NH4)0.15V2O5·1.3H2O thin films is shown in Figure 5. In the cases of the two thinner films prepared at 50 °C for 30 min (Figure 5a) and at 85 °C for 3 min (Figure 5b), both substrate surfaces are entirely populated with spherical and slightly elongated grains, but their sizes are different in dependence on the deposition rates. Thus, at 50 °C the film displays grains with sizes between 70 and 200 nm with predominant larger grains, while the film prepared at 85 °C exhibits twice smaller grains with sizes between 50 and 100 nm. The smaller grains at higher temperature are formed as a result of the high nucleation rate. For both films the grains are tightly stuck each to other, and in the most cases they form elongated aggregates with length of about 250−500 nm for 50 °C and shorter aggregates of 50−300 nm for 85 °C. Moreover, the aggregates are randomly oriented on the substrate surfaces. It is very important that the size of the aggregates found by AFM agrees very well with the size of the ribbon-like units derived by the SEM images. This means that the ribbons observed by SEM are actually composed from primary nanosized particles, and this is particularly pronounced for the film obtained at 85 °C for 3 min where the particles have dimensions between 50 and 100
morphology is followed by SEM and AFM techniques. The thickness of the films subjected to SEM and AFM observations was measured as well. SEM images of VA0550 film prepared for 30 min deposition time having a thickness of 200 nm (Figure 4a,b) show that the surface of the substrate is comparatively well populated with the deposited material. The deposited material comprises a homogeneous structure from uniform elongated units with ribbon-like morphology. The ribbon-like units are of 40−50 nm wide and up to 500 nm (Figure 4b) long. It is also seen that the film is not entirely dense, and there are pores with diameters between 50 and 200 nm. This porosity is especially important for the electrochemical redox processes since it can ensure a better contact of the electrolyte with the deposited xerogel in depth of the film. The SEM image of the film prepared at 85 °C for 3 min deposition time with a thickness of 240 nm is depicted in Figure 4c. The comparison between the two films having close thicknesses (Figure 4a−c) but prepared with different deposition rate (a higher deposition rate at 85 °C) reveals that the overall morphology is preserved: randomly oriented ribbon-like units cover the substrate surfaces in the both cases. However, the size of the units appears to be dependent on the deposition rate, which particularly concerns the ribbon length. At higher deposition rate (85 °C) the ribbons are no longer than 250 nm (Figure 4c); i.e., the length is twice reduced as compared to the film obtained at lower deposition rate (50 °C) (Figure 4a,b). This can be related to the different deposition times: the longer deposition time provides more time for the ribbon growth. On the other hand, the smaller dimensions of the ribbons allows their more homogeneous distribution on the substrate surface, which could explain the lower porosity of the VA0585 film than of the VA0550 film (Figure 4a,c). 9640
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Figure 5. 2D image of surface morphology of (NH4)0.3V2O5·1.25H2O thin films prepared for different deposition times: (a) VA0550 film for 30 min deposition time; (b) VA0585 film for 3 min deposition time; (c) VA0585 film for 40 min deposition time.
nm. Both films are very rough and the amplitude of grains height is more than 1 μm (Figure 5a,b). The thick film (1220 nm) prepared at 85 °C for 40 min exhibits essentially different surface morphology (Figure 5c) as compared to the thinner ones (Figure 5b). It is seen that the whole surface is covered by thin rods that are about 100 nm wide and with varying length below and above 1 μm. The rods are tightly stuck each to other and strictly aligned parallel to each other in a preferred orientation, thus forming a nanostructured film. The rods appear to be monolithic, and individual grains inside the rods cannot be distinguished. The VA0585 film prepared for 40 min has lover roughness than the thinner ones, and the amplitude of grain height is about 500 nm. Considering the morphology features of the films prepared at the same temperature of 85 °C for a very short and for a long deposition time, we can speculate on the likely mechanism by which the nanograins are transformed into nanorods. If only processes of nucleation and growth are supposed, then
individual grains larger than these observed for 3 min should be expected for the long deposition time of 40 min. However, the absence of any grains and observation of long monolithic rods suggest that other processes play role in the nanorods formation. We assume that the transformation of the individual nanograins into the monolithic nanorods should involve to some extent a dissolution process of the smallest particles and a growth of the largest ones in a preferred orientation. 3.3. Electrochemical Properties. The electrochemical properties of (NH4)0.3V2O5·1.25H2O thin films are examined by cyclic voltammetry in 1 M LiClO4 in PC as electrolyte at a scanning rate of 10 mV/s. Figures 6 and 7 display cyclic voltammograms of selected films from the two series. The electrochemical experiments show that the lithium ions are reversibly extracted/inserted within the VA0550 and VA0585 thin films. We established that all thin films under investigations, in spite of their thickness and conditions of preparation exhibit two cathodic reduction peaks (Figures 6 and 7). However, regarding the reverse oxidation 9641
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anodic peaks of VA0550 are narrower than the respective peaks of VA0585 which indicates that the processes of Li insertion/ extraction are faster in the case of the film prepared at lower temperature. This behavior is opposite to the expectations considering the smaller sizes of the ribbons of VA0585 prepared for 3 min (i.e., larger active surface) than VA0550 prepared for 30 min according to SEM and AFM studies. On the other hand, however, VA0550 is characterized by higher porosity than VA0585 (Figure 4a,c), which is favorable for the electrochemical kinetics. Five voltammograms of two (NH4)0.3V2O5·1.25H2O thin films prepared at 85 °C with considerably different thicknesses (3 and 20 min deposition time, respectively) are shown in Figure 7. While the thinner film displays two cathodic and two anodic peaks at −0.15, −0.46 V/−0.24, 0.02 V, respectively, the thick film shows two cathodic at −0.21 and −0.50 V but one anodic peak at 0.12 V. It is essential that the latter peak has an intermediate position between the two anodic peaks observed at the thinner film. Because of that, we could suppose that this peak is a result of overlapping of two unresolved anodic peaks (analogous to these for the thinner film) probably related to some kinetic hindrances due to larger thickness. Referring to the literature,10,15,34 the cyclic voltammograms of vanadium oxide thin films exhibit one, two, or three pairs of oxidation/reduction peaks. The observation of one redox pair has been attributed to the amorphous nature of the films.15 Most often two redox pairs have been found in the CV curves of vanadium oxide(V) xerogels,31 and according to Benmoussa et al.34 the appearance of two redox pairs reflects the crystalline nature of the films. Most recently, Costa et al.10 have observed three redox pairs. The physical explanation of the two and three redox pairs is related to the stepwise reduction/oxidation processes and formation of different crystalline states LixV2O5.10,34−36 As reported in the concentration limits 0 < x < 1, lithium is reversibly inserted/extracted, giving rise to three distinct states (α, ε, δ phases), while at higher lithium content 1 < x ≤ 2 some structural reorganizations take place corresponding to partially irreversible phases.10,35,36 3.4. Optical Properties. The optical properties of two series of VA0550 and VA0585 films with different deposition time are studied in LiClO4/PC electrolyte in order to find out the most suitable deposition conditions, applied voltage, and response time for the optimal manifestation of electrochromism of (NH4)0.3V2O5·1.25H2O. Transmittance variance defined as a difference in the transmittance between the reduced (bleached) and oxidized (colored) state (ΔT = Treduced − Toxidized) at given wavelength is used to qualify the electrochromic properties of the films. The optical spectra in the range of 350−900 nm are recorded after application of voltages in a range of −2.5 to +2.5 V. When the (NH4)0.3V2O5·1.25H2O films are placed in the electrochromic cell and the reducing voltages from −1 to −2.5 V are applied, a noticeable change from light orange (colored state) to green and then to blue (bleached state) is observed. Reverse color changes occur at the oxidizing voltages from +1 to +2.5 V. The observed reversible color changes are typical for electrochromic vanadium pentoxide xerogels10 and are related to the transition between only two oxidation state: V(V) (yellow) and V(IV) (blue). The green color arises by mixing yellow and blue due to the presence of both vanadium ions. In order to determine the optimal voltage and response time required for the electrochromic reactions, we have performed some kinetic studies on VA0585 thin films (Figures 8 and 9).
Figure 6. Cyclic voltammograms (third scan) of thin films of VA0550 (200 nm) and VA0585 (240 nm) prepared for 30 and 3 min deposition time, respectively.
Figure 7. Five cyclic voltammograms of VA0585 thin films prepared for different deposition time: (a) 3 and (b) 20 min.
processes, two cases can be distinguished depending on the film thickness (Figure 7). Thus, the thinner films prepared for shorter deposition time at the two temperatures (to 30 min at 50 °C and to 10 min at 85 °C), i.e., having smaller thickness, exhibit two anodic peaks. Accordingly, the films prepared for longer deposition times, i.e., the thicker films, show only one anodic peak (Figure 7b). As an illustration, Figure 6 compares the cyclic voltammograms (third scan) of two thin films, VA0550 and VA0585, prepared for 30 and 3 min deposition time, respectively, that have close thicknesses of 200 and 240 nm. Both voltammograms exhibit two pears of redox peaks. The shape and peak potentials of these peaks are close enough to consider that the redox peaks reflect same electrochemical processes in the two films. During the cathodic potential sweep a reduction of V(V) to V(IV) takes place followed by Li insertion, while the opposite process of an oxidation of V(IV) back to V(V) accompanied by Li extraction occurs during the anodic sweep. However, the peak surface of VA0550 is smaller than that of VA0585, which reflects lower values of inserted/extracted charge (for example, inserted charge 6.25 mC/cm2 vs 8.30 mC/ cm2). This is in conformity with the smaller thickness of the VA0550 film. Moreover, the most intensive cathodic and 9642
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It is seen that the coloration response time (τc,90%) to obtain the orange color for VA0585 thin film prepared for 3 min deposition time is 20 s, while the bleaching response time τb,90% to obtain the blue color is 56 s (Figure 9a). The thicker VA0585 film (830 nm) exhibits longer coloration response times, τc,90% = 61 s, while the bleaching response time is shorter, τb,90% = 36 s (Figure 9b). From this kinetic study it follows that the switching time for the study of (NH4)0.3V2O5·1.25H2O thin films should be no less than 1 min at ±2.5 V. Since we have also prepared films for longer deposition time (films that are thicker than 830 nm), the next measurements on optical transmittance are performed at reasonable switching time of 3 min. In general, the difference in τc and τb is usual due to the different rate of the Li+ intercalation/deintercalation processes. In general, vanadium pentoxide xerogel thin films are characterized by longer response times than that for WO3, and for that reason they cannot be applied for display devices. Fortunately, most applications of electrochromic materials such as electrochromic windows require longer response times. In addition, the results obtained here for the response times at different wavelengths for thin films prepared for different deposition time allow designing thin films with tailored electrochromic properties for appropriate applications. Figures 10 and 11 show the optical transmittance spectra of two series of (NH4)0.3V2O5·1.25H2O thin films prepared at 50
Figure 8. Optical absorbance at 500 nm vs time at different voltages of VA0585 thin film prepared for 5 min deposition time.
Figure 9. Response time for VA0585 thin films prepared for (a) 3 min deposition time (240 nm) and (b) 20 min deposition time (830 nm).
Figure 8 illustrates the change of the absorbance at 500 nm for VA0585 thin film prepared for 5 min deposition time at four applied voltages between ±1 and ±2.5 V. Depending on the applied voltage the polarity is switched at different time (from 2 to 5 min): the higher the voltage is, the shorter the time. From the kinetic curves in Figure 8 it is seen that the increased voltage results in the increase in the both absorbance and rate of the redox reactions. The latter is obvious from the considerable reduction of the response time required for the redox reactions to take place on going from voltage of ±1 to ±2.5 V. Furthermore, data in Figure 8 clearly evidence that at the four applied voltages the reduction is significantly faster process than the oxidation. The response time (τ) is the time required for an electrochromic material to be changed from its bleached to its colored state or vice versa. Since there is no consistency in the determination of the response time, we have decided to express it as the time required to obtain 90%38 of the color at the electrode of interest. Figure 9 presents coloration/bleaching response times (τc and τb, respectively) obtained at 400 and 900 nm for two VA0585 thin films with considerably different thickness.
Figure 10. In situ optical transmittance spectra of (NH4)0.3V2O5· 1.25H2O thin films prepared at 50 °C for different deposition times: (a) 10, (b) 20, (c) 30, (d) 40, and (e) 50 min.
and 85 °C, respectively, for different deposition time. Data demonstrate that the optical spectra of the reduced and oxidized films are significantly different. It is seen (Figures 10 and 11) that the transmittance maxima of the reduced films are around 450−500 nm, while for the oxidized ones they are at 900 nm. In the region of 350−500 nm the reduced films display higher transmittance than the oxidized, while the opposite is true in the region of 500−900 nm. In general, with the increase in the deposition time the transmittance values at a given wavelength decrease for both states which is due to the higher 9643
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Table 1. Transmittance Variance (ΔT %) of VA0550 and VA0585 Thin Films of Different Deposition Time at Wavelengths of 400, 500, and 900 nm deposition time (min) wavelength (nm) 400 500 900 400 500 900
3 55 23 17
5
10
VA0585 33 3 28 29 27 37 VA0550 2 5 6
20
30
40
50
1 28 55
5 31 53
2 29 42
1 17 30
25 8 6
36 22 16
10 35 29
3 32 46
at 900 nm. Thus, at 400 nm the highest ΔT values of 55% for VA0585 and 36% for VA0550 are found for 3 and 30 min deposition time, respectively (thickness of 240 and 200 nm, respectively). At 900 nm the highest ΔT values of 56% and 46% are observed for VA0585 prepared for 20 min deposition time (830 nm thickness) and VA0550 prepared for 50 min deposition time, respectively. These findings clearly confirm the explicit effect of the higher deposition rate related to the higher nucleation rate which affects the morphological characteristics and film defect. For vanadium oxide nanorolls it has been shown that the defect-rich nanorolls display much larger specific capacity for reversible lithium intercalation compared to these without defects.39 The observed shift of the maximum ΔT values to higher wavelengths (from 400 to 900 nm) as the film thickness increases is a consequence of the combination of at least three optical effects: the red-shift of the absorption edge, overall decrease in the transmittance of both states, and notable decrease in the transmittance of the reduced films above 500 nm. These optical changes can also be related, at least partly, to some morphological and microstructure features of the films. Thus, the higher deposition rate at 85 °C in comparison to 50 °C leads to small nanoribbons as evidenced by SEM and AFM studies, and it surely would result in the creation of more defects, particularly for the short deposition times of 3 and 5 min. These two effects contribute to the high ΔT values at lower wavelengths: 400 and 500 nm. On the other hand, the growth of the grains (shown by SEM and AFM) and the decrease in the defects due to the processes of recrystallization and dissolution during the longer deposition times contribute to the optical changes leading to high ΔT values at 900 nm. The (NH4)0.3V2O5·1.25H2O thin films prepared by the CBD method are stable during the storage at ambient conditions, at least for a period of 10 months, which is evident from the insignificant difference in the optical spectra of a “fresh” and a “old” film (Figure 12a). We have also examined the optical changes during the reversible lithium insertion/extraction in the xerogel structure up to 100 cycles (Figure 12b). The results show that main changes in the optical spectra occur in the first 30 cycles where the decrease in ΔT is about 8% at 500 nm and 10% at 900 nm. Then, up to the 100th cycle the observed ΔT values slightly decrease within the limits of 2−4% at 500 nm and about 1% at 900 nm. Until now, to our knowledge, the best performance within vanadium oxides is reported by Scherer et al.40 for a mesoporous film exhibiting ΔT of 50% and high coloration efficiency and stability. The other reports give lower values for
Figure 11. Optical transmittance spectra recorded in situ of thin films with different deposition times prepared at 85 °C: (a) 3, (b) 5, (c) 10, (d) 20, (e) 30, (f) 40, and (g) 50 min.
film absorption associated with the larger film thickness. However, the decrease in the transmittance is more pronounced for the reduced films, especially in the region above 500 nm, so that the optical changes of the reduced films give the major contribution to the overall changes of ΔT. It is also important that the fundamental absorption edge for both reduced and oxidized films exhibits a red-shift (to lower energy) with the increase in the film thickness. The red-shift of the absorption edge reflects a decrease in the optical band gap, and it has been attributed to the increase in the grain size and effective decrease in the imperfections at grain-boundary regions.38 On the other hand, the increasing grain size makes the films surface rough as evidenced by AFM. This results in the increased light scattering losses at the interface and accounts for the observed decrease in the transmittance with increasing grain size.38 Table 1 summarizes data for ΔT at three wavelengths (400, 500, and 900 nm) for the two series of (NH4)0.3V2O5·1.25H2O thin films prepared at 50 and 85 °C for different deposition times. The variation in the ΔT values clearly demonstrates the effect of different deposition conditions on the electrochromic properties. It is also seen that in the most cases the relationship between ΔT and the deposition time for the two series is not linear, but it passes through a maximum. However, the highest ΔT value at given wavelength is achieved for different deposition times. Data show that the shorter deposition time is favorable for achievement of high ΔT values at 400 nm, while the longer deposition time is beneficial for the high ΔT values 9644
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yellow/green and green/blue. Our findings clearly show that the optical spectra of the films are significantly influenced by the deposition temperature and deposition time. The increased film thickness is found to lead mainly to a red-shift of the absorption edge in the optical spectra of both reduced and oxidized films (decrease in the optical band gap), overall decrease in the transmittance for both states, and significant decrease in the transmittance of the reduced films above 500 nm. It is established that the shorter deposition time i.e. smaller film thickness is favorable for providing high ΔT values at 400 nm, while the longer deposition time, i.e., large film thickness, is beneficial for the high ΔT values at 900 nm. This provides an opportunity to broaden the range of functions of the electrochromic material at a constant chemical composition. The explicit effect of the higher deposition rate to achieve high ΔT values is demonstrated, and it is attributed, at least partly, to the morphological and microstructure features of the films. The excellent achievements in ΔT of around 55% are a manifestation for the potential of ammonium intercalated vanadium oxide xerogel thin films for their commercial application in different electrochromic devices.
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Figure 12. Optical transmittance spectra of reduced and oxidized states: (a) “fresh” VA0550 thin film prepared for 40 min deposition time and “old” film (in 10 months); (b) VA0585 thin film prepared for 7 min deposition time after 1, 30, 50, and 100 cycles.
Corresponding Author
*Tel +389 78570700; fax +389 23226865; e-mail metonajd@ pmf.ukim.mk (M.N.).
ΔT: 40%, 27%, 25%, and 20%. This comparison shows the capability of the developed CBD method to produce electrochromic thin films with promising electrochromic properties. The results here obtained are highly valuable and will serve as a base for further investigations in different aspects. Regarding the optical properties and electrochromic performance, some studies on the determination of absorption coefficients, refractive indices, optical band gap, coloration efficiency, and prolonged cycling stability are currently in progress. To get more insights into film microstructure defects, film porosity, and the mechanism of the lithium insertion/extraction, some studies by TEM, BET surface area measurements, IR, and Raman spectroscopy will be undertaken as well. 10,41
42
34,43
AUTHOR INFORMATION
44
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Alexander von Humboldt Stiftung for providing the electrochemical equipment without which the present study would not have been possible.
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REFERENCES
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4. CONCLUSIONS The newly developed chemical bath deposition method is successfully used for the preparation of nanostructured thin films of ammonium intercalated vanadium oxide xerogel with a composition (NH4)0.30V2O5·1.25H2O. The method is based on the acidification of 0.045 M solution of NH4VO3 with acetic acid at 50 and 85 °C. The prepared films are stable during the storage at ambient conditions. The composition and structure of the thin films are examined by X-ray powder diffraction, IR spectroscopy, and TG-DTA analyses. The influence of the deposition parameters like temperature and deposition time on the film morphology is examined by SEM and AFM techniques. It is found that the overall morphology comprises elongated ribbon-like units composed by nanoparticles, but some characteristics like particle sizes, ribbons orientation, and porosity are modified in dependence on the deposition conditions. The electrochemical properties and electrochromic activity of two series of thin films prepared at 50 and 85 °C for different deposition time have been studied in 1 M LiClO4 in propylene carbonate as an electrolyte. The cyclic voltammograms display two relatively stable redox pairs which correspond to the two-step electrochromism with color changes 9645
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