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Vapor growth of electrochromic thin films of transition metal oxides
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Journal of Crystal Growth 310 (2008) 2103–2109 www.elsevier.com/locate/jcrysgro
Vapor growth of electrochromic thin films of transition metal oxides K.A. Geshevaa,, T. Ivanovaa, B. Marsenb, G. Zolloc, M. Kalitzovad a
Central Laboratory of Solar Energy and New Energy Source, Bulgarian Academy of Sciences, Blvd. Tzarigradsko chausses 72, Sofia 1784, Bulgaria b Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA c Dipartimento di Energetica, Sapienza University of Rome, via A. Scarpa 14-16 00161 Rome, Italy d Institute of Solid State Physics, Bulgarian Academy of Sciences, Blvd. Tzarigradsko chausses 72, Sofia 1784, Bulgaria Available online 23 December 2007
Abstract Mixed oxide films of transition metals gain considerable much attention due to their interesting optoelectronic properties. The low temperature chemical vapor growth processing of thin films of mixed W and Mo oxides is presented. The investigation is related to optimization of films structure and the related optoelectronic properties in dependence on the chemical vapor deposition (CVD) process parameters. Their electrochromic behavior and photoelectrode properties were studied. r 2008 Elsevier B.V. All rights reserved. PACS: 68.47.Gh; 42.70.a; 81.15.Gh; 78.20.Jq Keywords: A1. X-ray diffraction; A3. Metalorganic chemical vapor deposition; B1. Oxides; B2. Electrochromic properties
1. Introduction Thin films of transition metals are known as electrochromic materials, capable of changing their transmittance under a small voltage applied across them [1,2]. Electrochromic devices are a multi-layer coating on conductive glass, consisting of working electrode (electrochromic WO3, for instance), a purely ionic electrolyte, and a counter electrode layer, all those placed between transparent conductive layers (e.g. SnO2:Sb). When voltage is applied to transparent conductors, ions are inserted (coloring) or extracted (bleaching) from the electrochromic layer, resulting in a modulation of optical properties. Electrochromic devices (‘‘smart windows’’) with their tunable transmittance for visible and infrared radiation will play an important role for energy efficient architectural and automotive glazing in the future [3,4]. Other interesting application of transition metal oxide films is their usage in photoelectrochemical (PEC) system for producing hydrogen directly from water using sunlight as the energy source. Some suitable materials for these Corresponding author.
E-mail address: [email protected] (K.A. Gesheva). 0022-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.12.034
purposes are semiconductors that can convert light into electrons, which are then used to split water into hydrogen and oxygen. This idea of splitting water by using a semiconductor photoelectrode was reported for the first time in 1972 by Fujishima and Honda [5]. A lot of effort was devoted to find efficient transition metal oxide photoelectrodes as they seem to be the only materials stable against photocorrosion. But their major disadvantage is the large bandgap, resulting in a poor visible light absorption. TiO2 becomes as one of the most promising candidates due to the fact that it is widely available and with high chemical stability. Its anatase form reveals favorable energetic positions of the band edges with respect to the water reduction and oxidation potentials, which is an advantage over other metal oxides. The visible light absorption can be improved by introducing additional energy levels in the bandgap [6]. On the other hand, WO3 films have demonstrated good photoactive properties. Tungsten oxide films have recently been employed in multi-junction water-splitting devices [7,8] for the purpose of solar-powered hydrogen production. In this application, photoanodes of tungsten oxide facilitate the oxygen evolution reaction while hydrogen is evolved at a suitable counter electrode.
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Based on this and on the fact that our Mo/W mixed oxide films with their similarity in structure and composition to WO3 approach its electrochemical properties, in this study for the first time to our knowledge, APCVD mixed oxide films based on W and Mo were subjected as photoelectrode to water splitting experiments. As a continuation of our work on these materials, the present paper deals with low temperature chemical vapor growth of mixed W and Mo oxides together with single MoO3 and WO3. This investigation is related to optimization of films structure and the vibrational properties depending on the chemical vapor deposition (CVD) process parameters in case of thicker films employed. Their electrochromic behavior and photoelectrode properties were studied. 2. Experimental procedure Mixed oxide films based on Mo and W were successfully obtained by the atmospheric pressure CVD method [9]. The CVD equipment consists of a horizontal quartz reactor with cold walls, and heating is facilitated by a high frequency generator (HFG). A graphite susceptor covered with an SiC coating, where the substrates are placed, is inserted in the CVD reactor. The substrate temperature used was 200 1C, a relatively low temperature for metal oxide deposition. The precursor was a physical mixture of molybdenum and tungsten carbonyls in ratio 1:4 in favor of W(CO)6. The sublimation temperature was 90 1C. The carbonyl vapors were carried by Ar flow to the CVD reactor as this Ar flow rate is related to the flow rate of oxygen (the reactive gas, entering in the reactor from a separate line) with a specific gas flow ratio, which is kept 1:32. The deposition time was 70 min and the film thickness was 600 nm. The substrates used were Si wafers (for IR studies), glass and conductive glass substrates (Donnelly type), covered by an SnO2:Sb film with 8 O/cm sheet resistance. X-ray diffraction (XRD) data were obtained on a Scintag Pad-V diffractometer with a Cu Ka source, at 2y scanning speed of 1 or 2 deg/min. The Cu Ka source settings were 45 kV and 40 mA and the detector used was liquid-nitrogen cooled Ge detector. Reflection high energy electron diffraction (RHEED) patterns have been recorded in an AEI EM6G electron microscope equipped with a suitable high precision diffraction stage located at the bottom of the column; the electron beam acceleration voltage was 60 kV. In order to study the surface of the deposited film, the diffraction patterns have been recorded with the electron beam glazing the surface at the chosen zone axis; if the sample is slightly tilted about the axis belonging to the sample surface that is perpendicular to the beam, the diffraction patterns originates from a thicker surface layer that is probed by the beam in these conditions. Each of the RHEED pattern from the studied oxides has been calibrated comparing it to the one obtained from the relevant Si (1 0 0) substrate recorded at (0 1 1) zone axis.
The Fourier transform infrared (FTIR) spectroscopy was performed in the spectral region of 350–1600 cm1 by Shimadzu FTIR spectrophotometer IRPrestige-21. The studied samples were deposited on Si substrates and bare Si wafer was used as background. Photoelectrodes for water-splitting experiments were prepared by soldering leads to the uncoated SnO2:Sb bottom contact on the substrate edges, and insulating those leads with epoxy. The finished photoelectrodes were subjected to current-vs.-potential scans in a three-electrode setup, using a platinum sheet counter electrode and a saturated calomel reference electrode, in 0.33 M H3PO4 electrolyte. Current-vs.-potential scans were recorded under chopped simulated AM1.5 Global light produced by an Oriel 1-kW solar simulator. Cyclic voltammetric experiments were performed in a standard three-electrode arrangement [9]. The cell used Pt as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode. The electrodes were immersed in the various electrolytes as 0.3 mol/l LiClO4 and 1 mol/l LiClO4 dissolved in propylene carbonate (PC). The proton mechanism of EC process was studied by using acid electrolytes such as 1 M H2SO4+glycerine or 0.5 M H3PO4 and 1 M H3PO4 solutions. The glycerine is added to the acid electrolytes to prevent films from deterioration as WO3 and MoO3 are known to be unstable in acidic solutions [10]. The color change is detected by optical system, attached to the voltammetry setup. The current density vs. voltage curves (voltammograms) were registered between 1 and +1.5 V at different scanning rates ranging from 5 to100 mV/s. The color efficiency CE and the optical modulations were estimated from the voltammetric experimental data. 3. Results and discussions The XRD spectra of as-deposited and annealed at 500 1C films are presented on Fig. 1, manifesting a broad peak (2y ¼ 20–301) an evidence for the amorphous structure of the mixed MoO3–WO3 films in as-deposited state, where only XRD lines due to SnO2 conducting layer is detected. The annealing at 500 1C leads to crystallized structure of the film. The XRD peaks of tin oxide (at 2y ¼ 26.61, 37.91, 51.51 and 61.51) are also manifested in the spectrum of the annealed film (Fig. 1), meanwhile new intensive and sharp XRD peaks are observed, attributed to molybdenum or tungsten oxides. It was determined that most XRD lines can be attributed to WO3 monoclinic phase (ICSD 17003). The presence of a crystalline orthorhombic MoO3 phase is possible since its prominent reflections at 2y ¼ 34.31, 35.71, 41.81, 50.11 and 56.11 (JCPDS 05 0508) show overlap with the present pattern of annealed films. The XRD study shows that the MoO3–WO3 films are amorphous in as-deposited state and crystallize after annealing at 500 1C, as the W oxide crystallizes and Mo atoms partially substitute the tungsten ones. Regarding the
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Fig. 1. XRD spectra of as-deposited and annealed at 500 1C mixed APCVD MoO3–WO3 films deposited on conductive glass substrate.
electrochromic properties, a disordered film structure favors the performance. These results are not very well coincided with previous Raman measurements, where the crystallized fractions were Mo oxides. These discrepancies can be arisen from different film growth on Si and on conductive glass substrates, and on the other hand although other technological conditions are the same, the XRD samples were obtained for longer deposition time and are respectively thicker. In thin film technology, greater thickness favors crystallization during film growth [11]. Using the Scherer’s equation [12], the crystallites size was estimated from the XRD peaks. The crystallites size values determined are in the range of 14.58–47.02 nm. As it can be seen from the obtained values the sizes vary significantly and are quite small. There we must add, that CVD MoO3 film, prepared by similar process and annealed at 400 1C shows average crystallite size of 34.87 nm, determined by XRD data. Previous AFM study [9] revealed that the CVD MoO3, WO3 and MoO3–WO3 films possess grained structure, but the grains are inhomogeneous and they look like agglomerates or clusters formations. For example, the as-deposited mixed oxide revealed grains with average size of 327 nm, but they possess inner structure. The AFM image of mixed oxide annealed at 400 1C shows cluster grains with sizes around 153 nm. The wide variety of grain size was also observed by SEM microscopy of CVD MoO3–WO3 film, deposited on conductive glass and annealed at 500 1C and even in this micrograph it can be seen that the grains are also clusters. In this sense, the small crystallites size estimated from XRD analysis does not contradict with AFM and SEM results. The RHEED study, presented below reveals again that the mixed oxide micrograph is similar to tungsten oxides, confirming that the crystallized phases are predominantly tungsten ones. The CVD MoO3, WO3 and MoO3–WO3 films for RHEED measurements were deposited on Si wafers at similar technological parameters. In Fig. 2 are reported the RHEED patterns obtained from MoO3 films deposited at 150 1C (a) and 200 1C (b) and annealed at
500 1C. All the other deposition parameters, such as the sublimator temperature, the deposition time, etc. have been kept fixed so that the crystallographic differences must be attributed to the deposition temperature only. As evidenced in Fig. 2a, the MoO3 film obtained at 150 1C is amorphous, whereas the deposition at 200 1C results in polycrystalline samples. The diffraction rings detected in this last case, better evidenced with the drawing in Fig 2(b), results sometimes broad and not well-defined, indicating that the experimental pattern is most probably made of different very close diffraction rings. The measured interplanar distances corresponding to the brightest rings are reported in Table 1 proving that the experimental diffraction pattern is fully compatible with the known MoO3 orthorhombic phase. Although unexpected, quite interesting are the results for the samples that were supposed to be WO3 and Mo/W oxide mixtures deposited at 200 1C and annealed at 500 1C (all the other deposition parameters were the same as the above discussed MoO3 films) shown in Figs. 3(a) and (b), respectively. Also in these cases, the obtained diffraction patterns are typical for polycrystalline samples. Thus the deposition process results in very similar diffraction patterns of nominally different samples meaning that, most probably, films of identical compositions have been deposited. This agrees well with previous Auger analysis [9], revealing that the composition of the mixed oxide films is predominantly WO3 and Mo content is very low (below 6%). The interplanar distances related to the well-evidenced rings of the pattern are listed in the following Table 2. Unfortunately, the indexing of these patterns is not straightforward because the measured inter-planar distances do not match completely with the known data of the monoclinic/triclinic phases of WO3: for instance, triclinic WO3 does not include strong lines for dp1.6, whereas monoclinic WO3 should have strong lines at 3.3, 2.15/2.17 and 1.82 that do not appear in the experimental pattern. On the other hand the monoclinic ground state phase of
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Fig. 2. RHEED patterns obtained from CVD MoO3 films, at the temperature of 150 1C (a) and 200 1C (b). The two samples have been annealed at 500 1C for 1 h in air. Table 1 Indexing of the diffraction pattern shown in Fig. 1(b) for crystalline MoO3 film, prepared at the substrate temperature of 200 1C Ring
Measured distance (A˚)
MoO3 planes
Theoretical distance (A˚)
1 2 3 4 5 6 7 8
3.7 3.2 2.6 2.2/2.3 1.9/2.0 1.7/1.8 1.6 1.4
(1 1 0)–(0 4 0) (0 2 1) (0 4 1)–(1 1 1)–(1 0 1) (1 5 0)–(0 6 0)–(1 3 1) (2 1 0)–(2 0 0) (2 1 1)–(2 3 0)–(0 0 2) (0 8 1)–(1 7 1)–(0 4 2)–(1 1 2) (1 9 0)–(0 6 2)–(2 5 1)
3.46/3.81 3.26 2.53/2.7 2.27/2.33 1.96/1.98 1.73/1.85 1.56/1.66 1.47/1.47
tungsten dioxide that could be compatible with the shortest measured inter-planar distances (i.e. 1.2–1.6 A˚) does not have any reflection at 3.0 A˚. The presence of metallic Mo and W is most unlikely because in this case the pattern should contain strong lines close to d ¼ 2.2 A˚. After checking many other possible oxides mixtures, also taking into account rare phases, it can be supported the idea that the recorded diffraction pattern could be most probably interpreted in term of a triclinic WO3/monoclinic WO2 mixture that does not include the metallic phases and eventually contains monoclinic MoO2 (mainly for the sample that is made on mixed W and Mo oxides). Following this hypothesis the diffraction pattern can be interpreted as detailed in Table 2. But on the other side, the estimated d-spacings show values very close to 3.006, 2.527, 1.569 and 1.443 A˚, found in previous RHEED investigation [13] of CVD annealed MoO3 films, grown
on Si substrates. These values can be referred to orthorhombic molybdenum trioxide. Further study of the mixed film microstructure is necessary. This again confirms how large variety of microstructures of transition metal oxide could be generated employing the CVD vapor growth process. The vibrational properties of the mixed Mo/W oxide were characterized by FTIR spectroscopy. The IR frequencies reported in the literature vary depending on the size and morphology of the crystallites. Also, there can be an overlapping of different vibrational modes split by transverse optic and longitudinal optic effects [14]. The main region, where characteristic IR bands for all the transition metal oxides appear, is in the range of 200–1200 cm1. The lower wavenumbers beneath 400 cm1 are assigned to deformation bands. The stretching vibrations occur in the region of 1050–400 cm1. The as-deposited mixed film reveals (see Fig. 4) a number of absorption bands in the spectral range of 370–470 cm1, namely at 372.26, 391.55, 432.05 and 466.77 cm1. The main absorption band is very broad (covering 600– 850 cm1), suggesting contributions of different Mo–O and W–O bonds, it is centered at 646.15 cm1. The high temperature annealing results in shifting and splitting of this characteristic absorption band in two well-defined maxima at 725.23 and at 808.17 cm1, with a very weak shoulder near 660 cm1. Meanwhile, there are several IR bands at 366.48 cm1 and at 428.20, 470.63 and 497.63 cm1. It must be noted that there are no the absorption bands manifested in 920–990 cm1, which are due to the terminal bonds MoQO and are a sign for appearance of the orthorhombic crystalline MoO3 [15].
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Fig. 3. RHEED patterns obtained from CVD WO3 (a) and mixed Mo/W oxide (b) films, annealed at 500 1C for 1 h in air. Table 2 Interplanar distances and indexing measured from the RHEED patterns of Fig. 2a WO2
MoO2
Ring
Distance (A˚)
WO3
1 2 3
3.6 3.0 2.5/2.6
(0 0 2) (0 2 0) (1 1 2) (0 0 2)+similar
3.65/3.76 3.11/3.08 2.59/2.71
(0 1 1) – (0 2 0) (2¯ 1 1) (2¯ 0 2)
3.45 – 2.4/2.45
4 5 6
1.6/1.7 1.4/1.5 1.2
– – –
– – –
(2¯ 2 0) (1¯ 1 3) and similar (0 1 3) and similar –
1.7/1.73 1.54 –
The 646 cm1 band is associated with Mo–O–Mo bending vibration of the orthorhombic MoO3, but in this spectral range (632–670 cm1) the infrared active modes of W–O–W stretching vibrations for amorphous tungsten trioxide is also located, so they could participate as well. The IR bands located at 638 and 807 cm1 are supposed to be connected with the stretching W–O vibrations of the crystalline monoclinic structure of WO3 [16]. The absorption lines around 447 cm1 are typical for stretching modes of Mo–O of orthorhombic MoO3. The absorption band at 725 cm1, appearing in the IR spectrum of annealed film, is characteristic for the stretching mode of the monoclinic b-phase of MoO3 [17] or can be connected with WQO bending vibrations [18]. The absorption band at 807 cm1 observed for 200 1C film is attributed to ns(O–Mo–O) vibrations-characteristic for a-MoO3, and the same time this band at 807 cm1 is assigned to W–O stretching vibration. The FTIR spectra leads to conclusion that after annealing at 500 1C, there appear absorption bands that
(1¯ 1 1) – (2¯ 1 1) (2¯ 0 2) (1 1 1) (2 0 0) (3¯ 1 2) (2¯ 2 2) and similar (4¯ 0 2) (2 3 1)
3.42 – 2.4/2.44 1.7/1.72 1.4 1.21
can be related both to crystallized WO3 component and some to crystalline phases of orthorhombic, monoclinic MoO3. The employed CVD growth process leads to variety of films microstructure, resulting from changes in deposition process parameters. CVD MoO3–WO3 films were electrochemically characterized in order to investigate the electrochromic response. The films show deep coloration as a result of small ions (Li+ in many cases) intercalation. The expected reaction can be schematically written as Mox W1x O3 þ yLiþ þ ye ! Liy Mox W1x O3 :
(1)
The effect is reversible and after changing the voltage polarity (extracting Li+), the oxide films bleach and become transparent. Fig. 5 shows the electrochemical measurements performed in 1 M LiClO4 dissolved in PC electrolyte for different scan rates of as-deposited mixed Mo/W oxide film. It is observed that increasing the scan rate, the current rises significantly in comparison with the scan rates of
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Fig. 4. FTIR spectra of CVD MoO3–WO3 films, obtained at the substrate 1 temperature of 200 1C and gas flow ratio 32 in as-deposited state and after annealing at 500 1C for 1 h.
Fig. 5. Cyclic voltammetry curves of CVD as-deposited MoO3–WO3 films, measured at three different scan rates.
20 and 2 mV/s. The higher currents can cause destruction of the electrochromic films; so it must be avoided. The shapes of the registered voltammograms and the peaks positions differ for the various rates although generally the curves show no significant features and peaks, suggesting an amorphous structure. The peaks due to the injection of Li+ ions are not well defined, but the corresponding peaks caused by ion extraction are located at 0.37, 0.62 and 0.75 V for the scan rates 50, 20 and 10 mV/s, respectively. Table 3 presents the electrochromic characteristics of asdeposited MoO3–WO3 films. It can clearly be seen that scan rate strongly influences on the transferred charge, optical modulation and color efficiency. This dependence was found out for the spectral range 500–750 nm. As it can be seen, the highest values are obtained for the scan rate of
10 mV/s for these CVD oxide films. Decreasing the scan rates up to 5 or 2 mV/s leads to bigger charges and worse optical color changes resulting in lower value of the optical modulation and color efficiency. The obtained values of the electrochromic parameters, at 10 mV/s scan rate are very high and are even higher than those of pure WO3 films. So far, WO3 is the most studied electrochromic material and finds some practical applications. It must be noted, that the pure tungsten oxide film, obtained by similar CVD processing possesses color efficiency around 83.7 cm2/C and optical modulation 39% at the wavelength 550 nm. In the same time, CVD MoO3 film shows smaller value of 39 cm2/C. This reveals that the mixed oxide films show superior electrochromic properties than the pure oxides. The cited color efficiency values for MoO3 films are 41 cm2/C [14] or even smaller than 12–16 cm2/C [19], meanwhile for WO3 films these values vary in the range of 25–62 cm2/C [20] and Patil and Patil [21] reported color efficiency for their mixed Mo–W oxide films 63 cm2/C. The electrochromic study manifest that mixing the two transition metal oxides leads to better electrochromic performance and the color efficiency values considerably exceed those of pure molybdenum or tungsten oxide films. The good electrochromic characteristics may be due to the disordered structure of CVD obtained films. PEC testing revealed low photocurrents on the asdeposited samples, but significant photocurrent in the annealed sample. Current-vs.-potential scans for both samples are shown in Fig. 5. The onset of a dark current is visible at 1.6 V vs. SCE. The maximum photocurrent of the asdeposited sample is 4 mA/cm2 (at 1.8 V vs. SCE) while that of the annealed sample is 56 mA/cm2 (at 1.5 V vs. SCE) (Fig. 6). The PEC data suggest that the carrier transport properties are positively influenced by the annealing. While light is absorbed in both as-deposited and annealed samples, only in the annealed one carrier separation occurs at a significant level. The current films are optimized for the electrochromic application, and therefore, too thin for efficient solar energy conversion. However, the observation of photocurrents in the present annealed films suggests that, with some optimization, AP-CVD films may become a viable synthesis route for water-splitting WO3 photoelectrodes. In the case of sputtered WO3 films, the microstructure has been found to be a key to photoelectrode performance [8]. Due to superior carrier transport properties, largegrained polycrystalline films are better photoanodes than small-grain films or amorphous films. Avenues exist to produce large-grain polycrystalline films by AP-CVD and post-processing. It is possible that a different set of AP-CVD fabrication parameters has to be employed to optimize the material for either the electrochromic application or for photoelectrolysis. 4. Conclusions In conclusion, the research revealed that atmospheric pressure CVD technology is a suitable deposition method
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Table 3 Electrochemical characteristics of as-deposited MoO3–WO3 films, derived from the voltammetric data Scan rate (mV/s)
I anodic (mA)
I cathodic (mA)
DQ (mC)
DT (550 nm)
CE (cm2/C) (550 nm)
50 20 10 5 2
1.22 0.64 0.36 0.09 0.08
1.68 0.76 1.08 0.63 0.39
19.18 20.91 30.64 33.54 65.90
21.10 29.96 45.21 49.59 21.20
43.49 53.97 196.46 88.69 43.49
The measurements were carried out in electrolyte 1 M LiClO4+PC.
Research Program Project DO1-377/NTE-04/16.06.2006, module 2.
References
Fig. 6. Photocurrent-vs-potential scan of WO3 film electrodes in 0.33 M phosphoric acid under chopped simulated 1-sun AM1.5G illumination.
for preparation of mixed molybdenum/tungsten oxide films with excellent electrochromic performance. The characterized structure of MoO3–WO3 films is found to be a mixture of predominantly W crystalline oxide phases and some MoO3. The mixed structure of Mo/W-based oxide films favors the electrochromic effect, resulting in high color efficiency and optical modulation values approaching the electrochromic properties of the most studied WO3. When employed as water-splitting photoanode, modest photocurrents were observed in the present annealed films. With some optimization of the transport properties, AP-CVD mixed oxide films may become viable water-splitting photoelectrodes. Acknowledgment The authors acknowledge the financial support of the Ministry of Education and Science under the National
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Journal of Crystal Growth 310 (2008) 2103–2109 www.elsevier.com/locate/jcrysgro
Vapor growth of electrochromic thin films of transition metal oxides K.A. Geshevaa,, T. Ivanovaa, B. Marsenb, G. Zolloc, M. Kalitzovad a
Central Laboratory of Solar Energy and New Energy Source, Bulgarian Academy of Sciences, Blvd. Tzarigradsko chausses 72, Sofia 1784, Bulgaria b Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA c Dipartimento di Energetica, Sapienza University of Rome, via A. Scarpa 14-16 00161 Rome, Italy d Institute of Solid State Physics, Bulgarian Academy of Sciences, Blvd. Tzarigradsko chausses 72, Sofia 1784, Bulgaria Available online 23 December 2007
Abstract Mixed oxide films of transition metals gain considerable much attention due to their interesting optoelectronic properties. The low temperature chemical vapor growth processing of thin films of mixed W and Mo oxides is presented. The investigation is related to optimization of films structure and the related optoelectronic properties in dependence on the chemical vapor deposition (CVD) process parameters. Their electrochromic behavior and photoelectrode properties were studied. r 2008 Elsevier B.V. All rights reserved. PACS: 68.47.Gh; 42.70.a; 81.15.Gh; 78.20.Jq Keywords: A1. X-ray diffraction; A3. Metalorganic chemical vapor deposition; B1. Oxides; B2. Electrochromic properties
1. Introduction Thin films of transition metals are known as electrochromic materials, capable of changing their transmittance under a small voltage applied across them [1,2]. Electrochromic devices are a multi-layer coating on conductive glass, consisting of working electrode (electrochromic WO3, for instance), a purely ionic electrolyte, and a counter electrode layer, all those placed between transparent conductive layers (e.g. SnO2:Sb). When voltage is applied to transparent conductors, ions are inserted (coloring) or extracted (bleaching) from the electrochromic layer, resulting in a modulation of optical properties. Electrochromic devices (‘‘smart windows’’) with their tunable transmittance for visible and infrared radiation will play an important role for energy efficient architectural and automotive glazing in the future [3,4]. Other interesting application of transition metal oxide films is their usage in photoelectrochemical (PEC) system for producing hydrogen directly from water using sunlight as the energy source. Some suitable materials for these Corresponding author.
E-mail address: [email protected] (K.A. Gesheva). 0022-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.12.034
purposes are semiconductors that can convert light into electrons, which are then used to split water into hydrogen and oxygen. This idea of splitting water by using a semiconductor photoelectrode was reported for the first time in 1972 by Fujishima and Honda [5]. A lot of effort was devoted to find efficient transition metal oxide photoelectrodes as they seem to be the only materials stable against photocorrosion. But their major disadvantage is the large bandgap, resulting in a poor visible light absorption. TiO2 becomes as one of the most promising candidates due to the fact that it is widely available and with high chemical stability. Its anatase form reveals favorable energetic positions of the band edges with respect to the water reduction and oxidation potentials, which is an advantage over other metal oxides. The visible light absorption can be improved by introducing additional energy levels in the bandgap [6]. On the other hand, WO3 films have demonstrated good photoactive properties. Tungsten oxide films have recently been employed in multi-junction water-splitting devices [7,8] for the purpose of solar-powered hydrogen production. In this application, photoanodes of tungsten oxide facilitate the oxygen evolution reaction while hydrogen is evolved at a suitable counter electrode.
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Based on this and on the fact that our Mo/W mixed oxide films with their similarity in structure and composition to WO3 approach its electrochemical properties, in this study for the first time to our knowledge, APCVD mixed oxide films based on W and Mo were subjected as photoelectrode to water splitting experiments. As a continuation of our work on these materials, the present paper deals with low temperature chemical vapor growth of mixed W and Mo oxides together with single MoO3 and WO3. This investigation is related to optimization of films structure and the vibrational properties depending on the chemical vapor deposition (CVD) process parameters in case of thicker films employed. Their electrochromic behavior and photoelectrode properties were studied. 2. Experimental procedure Mixed oxide films based on Mo and W were successfully obtained by the atmospheric pressure CVD method [9]. The CVD equipment consists of a horizontal quartz reactor with cold walls, and heating is facilitated by a high frequency generator (HFG). A graphite susceptor covered with an SiC coating, where the substrates are placed, is inserted in the CVD reactor. The substrate temperature used was 200 1C, a relatively low temperature for metal oxide deposition. The precursor was a physical mixture of molybdenum and tungsten carbonyls in ratio 1:4 in favor of W(CO)6. The sublimation temperature was 90 1C. The carbonyl vapors were carried by Ar flow to the CVD reactor as this Ar flow rate is related to the flow rate of oxygen (the reactive gas, entering in the reactor from a separate line) with a specific gas flow ratio, which is kept 1:32. The deposition time was 70 min and the film thickness was 600 nm. The substrates used were Si wafers (for IR studies), glass and conductive glass substrates (Donnelly type), covered by an SnO2:Sb film with 8 O/cm sheet resistance. X-ray diffraction (XRD) data were obtained on a Scintag Pad-V diffractometer with a Cu Ka source, at 2y scanning speed of 1 or 2 deg/min. The Cu Ka source settings were 45 kV and 40 mA and the detector used was liquid-nitrogen cooled Ge detector. Reflection high energy electron diffraction (RHEED) patterns have been recorded in an AEI EM6G electron microscope equipped with a suitable high precision diffraction stage located at the bottom of the column; the electron beam acceleration voltage was 60 kV. In order to study the surface of the deposited film, the diffraction patterns have been recorded with the electron beam glazing the surface at the chosen zone axis; if the sample is slightly tilted about the axis belonging to the sample surface that is perpendicular to the beam, the diffraction patterns originates from a thicker surface layer that is probed by the beam in these conditions. Each of the RHEED pattern from the studied oxides has been calibrated comparing it to the one obtained from the relevant Si (1 0 0) substrate recorded at (0 1 1) zone axis.
The Fourier transform infrared (FTIR) spectroscopy was performed in the spectral region of 350–1600 cm1 by Shimadzu FTIR spectrophotometer IRPrestige-21. The studied samples were deposited on Si substrates and bare Si wafer was used as background. Photoelectrodes for water-splitting experiments were prepared by soldering leads to the uncoated SnO2:Sb bottom contact on the substrate edges, and insulating those leads with epoxy. The finished photoelectrodes were subjected to current-vs.-potential scans in a three-electrode setup, using a platinum sheet counter electrode and a saturated calomel reference electrode, in 0.33 M H3PO4 electrolyte. Current-vs.-potential scans were recorded under chopped simulated AM1.5 Global light produced by an Oriel 1-kW solar simulator. Cyclic voltammetric experiments were performed in a standard three-electrode arrangement [9]. The cell used Pt as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode. The electrodes were immersed in the various electrolytes as 0.3 mol/l LiClO4 and 1 mol/l LiClO4 dissolved in propylene carbonate (PC). The proton mechanism of EC process was studied by using acid electrolytes such as 1 M H2SO4+glycerine or 0.5 M H3PO4 and 1 M H3PO4 solutions. The glycerine is added to the acid electrolytes to prevent films from deterioration as WO3 and MoO3 are known to be unstable in acidic solutions [10]. The color change is detected by optical system, attached to the voltammetry setup. The current density vs. voltage curves (voltammograms) were registered between 1 and +1.5 V at different scanning rates ranging from 5 to100 mV/s. The color efficiency CE and the optical modulations were estimated from the voltammetric experimental data. 3. Results and discussions The XRD spectra of as-deposited and annealed at 500 1C films are presented on Fig. 1, manifesting a broad peak (2y ¼ 20–301) an evidence for the amorphous structure of the mixed MoO3–WO3 films in as-deposited state, where only XRD lines due to SnO2 conducting layer is detected. The annealing at 500 1C leads to crystallized structure of the film. The XRD peaks of tin oxide (at 2y ¼ 26.61, 37.91, 51.51 and 61.51) are also manifested in the spectrum of the annealed film (Fig. 1), meanwhile new intensive and sharp XRD peaks are observed, attributed to molybdenum or tungsten oxides. It was determined that most XRD lines can be attributed to WO3 monoclinic phase (ICSD 17003). The presence of a crystalline orthorhombic MoO3 phase is possible since its prominent reflections at 2y ¼ 34.31, 35.71, 41.81, 50.11 and 56.11 (JCPDS 05 0508) show overlap with the present pattern of annealed films. The XRD study shows that the MoO3–WO3 films are amorphous in as-deposited state and crystallize after annealing at 500 1C, as the W oxide crystallizes and Mo atoms partially substitute the tungsten ones. Regarding the
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Fig. 1. XRD spectra of as-deposited and annealed at 500 1C mixed APCVD MoO3–WO3 films deposited on conductive glass substrate.
electrochromic properties, a disordered film structure favors the performance. These results are not very well coincided with previous Raman measurements, where the crystallized fractions were Mo oxides. These discrepancies can be arisen from different film growth on Si and on conductive glass substrates, and on the other hand although other technological conditions are the same, the XRD samples were obtained for longer deposition time and are respectively thicker. In thin film technology, greater thickness favors crystallization during film growth [11]. Using the Scherer’s equation [12], the crystallites size was estimated from the XRD peaks. The crystallites size values determined are in the range of 14.58–47.02 nm. As it can be seen from the obtained values the sizes vary significantly and are quite small. There we must add, that CVD MoO3 film, prepared by similar process and annealed at 400 1C shows average crystallite size of 34.87 nm, determined by XRD data. Previous AFM study [9] revealed that the CVD MoO3, WO3 and MoO3–WO3 films possess grained structure, but the grains are inhomogeneous and they look like agglomerates or clusters formations. For example, the as-deposited mixed oxide revealed grains with average size of 327 nm, but they possess inner structure. The AFM image of mixed oxide annealed at 400 1C shows cluster grains with sizes around 153 nm. The wide variety of grain size was also observed by SEM microscopy of CVD MoO3–WO3 film, deposited on conductive glass and annealed at 500 1C and even in this micrograph it can be seen that the grains are also clusters. In this sense, the small crystallites size estimated from XRD analysis does not contradict with AFM and SEM results. The RHEED study, presented below reveals again that the mixed oxide micrograph is similar to tungsten oxides, confirming that the crystallized phases are predominantly tungsten ones. The CVD MoO3, WO3 and MoO3–WO3 films for RHEED measurements were deposited on Si wafers at similar technological parameters. In Fig. 2 are reported the RHEED patterns obtained from MoO3 films deposited at 150 1C (a) and 200 1C (b) and annealed at
500 1C. All the other deposition parameters, such as the sublimator temperature, the deposition time, etc. have been kept fixed so that the crystallographic differences must be attributed to the deposition temperature only. As evidenced in Fig. 2a, the MoO3 film obtained at 150 1C is amorphous, whereas the deposition at 200 1C results in polycrystalline samples. The diffraction rings detected in this last case, better evidenced with the drawing in Fig 2(b), results sometimes broad and not well-defined, indicating that the experimental pattern is most probably made of different very close diffraction rings. The measured interplanar distances corresponding to the brightest rings are reported in Table 1 proving that the experimental diffraction pattern is fully compatible with the known MoO3 orthorhombic phase. Although unexpected, quite interesting are the results for the samples that were supposed to be WO3 and Mo/W oxide mixtures deposited at 200 1C and annealed at 500 1C (all the other deposition parameters were the same as the above discussed MoO3 films) shown in Figs. 3(a) and (b), respectively. Also in these cases, the obtained diffraction patterns are typical for polycrystalline samples. Thus the deposition process results in very similar diffraction patterns of nominally different samples meaning that, most probably, films of identical compositions have been deposited. This agrees well with previous Auger analysis [9], revealing that the composition of the mixed oxide films is predominantly WO3 and Mo content is very low (below 6%). The interplanar distances related to the well-evidenced rings of the pattern are listed in the following Table 2. Unfortunately, the indexing of these patterns is not straightforward because the measured inter-planar distances do not match completely with the known data of the monoclinic/triclinic phases of WO3: for instance, triclinic WO3 does not include strong lines for dp1.6, whereas monoclinic WO3 should have strong lines at 3.3, 2.15/2.17 and 1.82 that do not appear in the experimental pattern. On the other hand the monoclinic ground state phase of
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Fig. 2. RHEED patterns obtained from CVD MoO3 films, at the temperature of 150 1C (a) and 200 1C (b). The two samples have been annealed at 500 1C for 1 h in air. Table 1 Indexing of the diffraction pattern shown in Fig. 1(b) for crystalline MoO3 film, prepared at the substrate temperature of 200 1C Ring
Measured distance (A˚)
MoO3 planes
Theoretical distance (A˚)
1 2 3 4 5 6 7 8
3.7 3.2 2.6 2.2/2.3 1.9/2.0 1.7/1.8 1.6 1.4
(1 1 0)–(0 4 0) (0 2 1) (0 4 1)–(1 1 1)–(1 0 1) (1 5 0)–(0 6 0)–(1 3 1) (2 1 0)–(2 0 0) (2 1 1)–(2 3 0)–(0 0 2) (0 8 1)–(1 7 1)–(0 4 2)–(1 1 2) (1 9 0)–(0 6 2)–(2 5 1)
3.46/3.81 3.26 2.53/2.7 2.27/2.33 1.96/1.98 1.73/1.85 1.56/1.66 1.47/1.47
tungsten dioxide that could be compatible with the shortest measured inter-planar distances (i.e. 1.2–1.6 A˚) does not have any reflection at 3.0 A˚. The presence of metallic Mo and W is most unlikely because in this case the pattern should contain strong lines close to d ¼ 2.2 A˚. After checking many other possible oxides mixtures, also taking into account rare phases, it can be supported the idea that the recorded diffraction pattern could be most probably interpreted in term of a triclinic WO3/monoclinic WO2 mixture that does not include the metallic phases and eventually contains monoclinic MoO2 (mainly for the sample that is made on mixed W and Mo oxides). Following this hypothesis the diffraction pattern can be interpreted as detailed in Table 2. But on the other side, the estimated d-spacings show values very close to 3.006, 2.527, 1.569 and 1.443 A˚, found in previous RHEED investigation [13] of CVD annealed MoO3 films, grown
on Si substrates. These values can be referred to orthorhombic molybdenum trioxide. Further study of the mixed film microstructure is necessary. This again confirms how large variety of microstructures of transition metal oxide could be generated employing the CVD vapor growth process. The vibrational properties of the mixed Mo/W oxide were characterized by FTIR spectroscopy. The IR frequencies reported in the literature vary depending on the size and morphology of the crystallites. Also, there can be an overlapping of different vibrational modes split by transverse optic and longitudinal optic effects [14]. The main region, where characteristic IR bands for all the transition metal oxides appear, is in the range of 200–1200 cm1. The lower wavenumbers beneath 400 cm1 are assigned to deformation bands. The stretching vibrations occur in the region of 1050–400 cm1. The as-deposited mixed film reveals (see Fig. 4) a number of absorption bands in the spectral range of 370–470 cm1, namely at 372.26, 391.55, 432.05 and 466.77 cm1. The main absorption band is very broad (covering 600– 850 cm1), suggesting contributions of different Mo–O and W–O bonds, it is centered at 646.15 cm1. The high temperature annealing results in shifting and splitting of this characteristic absorption band in two well-defined maxima at 725.23 and at 808.17 cm1, with a very weak shoulder near 660 cm1. Meanwhile, there are several IR bands at 366.48 cm1 and at 428.20, 470.63 and 497.63 cm1. It must be noted that there are no the absorption bands manifested in 920–990 cm1, which are due to the terminal bonds MoQO and are a sign for appearance of the orthorhombic crystalline MoO3 [15].
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Fig. 3. RHEED patterns obtained from CVD WO3 (a) and mixed Mo/W oxide (b) films, annealed at 500 1C for 1 h in air. Table 2 Interplanar distances and indexing measured from the RHEED patterns of Fig. 2a WO2
MoO2
Ring
Distance (A˚)
WO3
1 2 3
3.6 3.0 2.5/2.6
(0 0 2) (0 2 0) (1 1 2) (0 0 2)+similar
3.65/3.76 3.11/3.08 2.59/2.71
(0 1 1) – (0 2 0) (2¯ 1 1) (2¯ 0 2)
3.45 – 2.4/2.45
4 5 6
1.6/1.7 1.4/1.5 1.2
– – –
– – –
(2¯ 2 0) (1¯ 1 3) and similar (0 1 3) and similar –
1.7/1.73 1.54 –
The 646 cm1 band is associated with Mo–O–Mo bending vibration of the orthorhombic MoO3, but in this spectral range (632–670 cm1) the infrared active modes of W–O–W stretching vibrations for amorphous tungsten trioxide is also located, so they could participate as well. The IR bands located at 638 and 807 cm1 are supposed to be connected with the stretching W–O vibrations of the crystalline monoclinic structure of WO3 [16]. The absorption lines around 447 cm1 are typical for stretching modes of Mo–O of orthorhombic MoO3. The absorption band at 725 cm1, appearing in the IR spectrum of annealed film, is characteristic for the stretching mode of the monoclinic b-phase of MoO3 [17] or can be connected with WQO bending vibrations [18]. The absorption band at 807 cm1 observed for 200 1C film is attributed to ns(O–Mo–O) vibrations-characteristic for a-MoO3, and the same time this band at 807 cm1 is assigned to W–O stretching vibration. The FTIR spectra leads to conclusion that after annealing at 500 1C, there appear absorption bands that
(1¯ 1 1) – (2¯ 1 1) (2¯ 0 2) (1 1 1) (2 0 0) (3¯ 1 2) (2¯ 2 2) and similar (4¯ 0 2) (2 3 1)
3.42 – 2.4/2.44 1.7/1.72 1.4 1.21
can be related both to crystallized WO3 component and some to crystalline phases of orthorhombic, monoclinic MoO3. The employed CVD growth process leads to variety of films microstructure, resulting from changes in deposition process parameters. CVD MoO3–WO3 films were electrochemically characterized in order to investigate the electrochromic response. The films show deep coloration as a result of small ions (Li+ in many cases) intercalation. The expected reaction can be schematically written as Mox W1x O3 þ yLiþ þ ye ! Liy Mox W1x O3 :
(1)
The effect is reversible and after changing the voltage polarity (extracting Li+), the oxide films bleach and become transparent. Fig. 5 shows the electrochemical measurements performed in 1 M LiClO4 dissolved in PC electrolyte for different scan rates of as-deposited mixed Mo/W oxide film. It is observed that increasing the scan rate, the current rises significantly in comparison with the scan rates of
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Fig. 4. FTIR spectra of CVD MoO3–WO3 films, obtained at the substrate 1 temperature of 200 1C and gas flow ratio 32 in as-deposited state and after annealing at 500 1C for 1 h.
Fig. 5. Cyclic voltammetry curves of CVD as-deposited MoO3–WO3 films, measured at three different scan rates.
20 and 2 mV/s. The higher currents can cause destruction of the electrochromic films; so it must be avoided. The shapes of the registered voltammograms and the peaks positions differ for the various rates although generally the curves show no significant features and peaks, suggesting an amorphous structure. The peaks due to the injection of Li+ ions are not well defined, but the corresponding peaks caused by ion extraction are located at 0.37, 0.62 and 0.75 V for the scan rates 50, 20 and 10 mV/s, respectively. Table 3 presents the electrochromic characteristics of asdeposited MoO3–WO3 films. It can clearly be seen that scan rate strongly influences on the transferred charge, optical modulation and color efficiency. This dependence was found out for the spectral range 500–750 nm. As it can be seen, the highest values are obtained for the scan rate of
10 mV/s for these CVD oxide films. Decreasing the scan rates up to 5 or 2 mV/s leads to bigger charges and worse optical color changes resulting in lower value of the optical modulation and color efficiency. The obtained values of the electrochromic parameters, at 10 mV/s scan rate are very high and are even higher than those of pure WO3 films. So far, WO3 is the most studied electrochromic material and finds some practical applications. It must be noted, that the pure tungsten oxide film, obtained by similar CVD processing possesses color efficiency around 83.7 cm2/C and optical modulation 39% at the wavelength 550 nm. In the same time, CVD MoO3 film shows smaller value of 39 cm2/C. This reveals that the mixed oxide films show superior electrochromic properties than the pure oxides. The cited color efficiency values for MoO3 films are 41 cm2/C [14] or even smaller than 12–16 cm2/C [19], meanwhile for WO3 films these values vary in the range of 25–62 cm2/C [20] and Patil and Patil [21] reported color efficiency for their mixed Mo–W oxide films 63 cm2/C. The electrochromic study manifest that mixing the two transition metal oxides leads to better electrochromic performance and the color efficiency values considerably exceed those of pure molybdenum or tungsten oxide films. The good electrochromic characteristics may be due to the disordered structure of CVD obtained films. PEC testing revealed low photocurrents on the asdeposited samples, but significant photocurrent in the annealed sample. Current-vs.-potential scans for both samples are shown in Fig. 5. The onset of a dark current is visible at 1.6 V vs. SCE. The maximum photocurrent of the asdeposited sample is 4 mA/cm2 (at 1.8 V vs. SCE) while that of the annealed sample is 56 mA/cm2 (at 1.5 V vs. SCE) (Fig. 6). The PEC data suggest that the carrier transport properties are positively influenced by the annealing. While light is absorbed in both as-deposited and annealed samples, only in the annealed one carrier separation occurs at a significant level. The current films are optimized for the electrochromic application, and therefore, too thin for efficient solar energy conversion. However, the observation of photocurrents in the present annealed films suggests that, with some optimization, AP-CVD films may become a viable synthesis route for water-splitting WO3 photoelectrodes. In the case of sputtered WO3 films, the microstructure has been found to be a key to photoelectrode performance [8]. Due to superior carrier transport properties, largegrained polycrystalline films are better photoanodes than small-grain films or amorphous films. Avenues exist to produce large-grain polycrystalline films by AP-CVD and post-processing. It is possible that a different set of AP-CVD fabrication parameters has to be employed to optimize the material for either the electrochromic application or for photoelectrolysis. 4. Conclusions In conclusion, the research revealed that atmospheric pressure CVD technology is a suitable deposition method
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Table 3 Electrochemical characteristics of as-deposited MoO3–WO3 films, derived from the voltammetric data Scan rate (mV/s)
I anodic (mA)
I cathodic (mA)
DQ (mC)
DT (550 nm)
CE (cm2/C) (550 nm)
50 20 10 5 2
1.22 0.64 0.36 0.09 0.08
1.68 0.76 1.08 0.63 0.39
19.18 20.91 30.64 33.54 65.90
21.10 29.96 45.21 49.59 21.20
43.49 53.97 196.46 88.69 43.49
The measurements were carried out in electrolyte 1 M LiClO4+PC.
Research Program Project DO1-377/NTE-04/16.06.2006, module 2.
References
Fig. 6. Photocurrent-vs-potential scan of WO3 film electrodes in 0.33 M phosphoric acid under chopped simulated 1-sun AM1.5G illumination.
for preparation of mixed molybdenum/tungsten oxide films with excellent electrochromic performance. The characterized structure of MoO3–WO3 films is found to be a mixture of predominantly W crystalline oxide phases and some MoO3. The mixed structure of Mo/W-based oxide films favors the electrochromic effect, resulting in high color efficiency and optical modulation values approaching the electrochromic properties of the most studied WO3. When employed as water-splitting photoanode, modest photocurrents were observed in the present annealed films. With some optimization of the transport properties, AP-CVD mixed oxide films may become viable water-splitting photoelectrodes. Acknowledgment The authors acknowledge the financial support of the Ministry of Education and Science under the National
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