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Investigation of Combinatorial TiO2-MoO3 Mixed Layers to Enhance the Coloration Efficiency

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03 July 2024

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04 July 2024

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Abstract
Electrochromic measurements have been performed throughout the full composition range of reactive magnetron-sputtered mixed Titanium oxide and Molybdenum oxide (TiO2-MoO3) to search for enhanced Coloration Efficiency (CE). Optical parameters and composition have been determined and mapped by using Spectroscopic Ellipsometry (SE). Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDS). EDS has been used to check the SE results as it was used in our earlier paper [N. T. Ismaeel et al, Materials 2023, 16(12), 4204] for (TiO2-SnO2). Ti and Mo targets were put separately from each other, and the Indium-Tin-Oxide (ITO) covered glass and Si-probes on a glass substrate (30 cm × 30 cm) were moved under the two separated targets (Ti and Mo) in a reactive Argon-Oxygen (Ar-O2) gas mixture. By using this combinatorial process, all the compositions (from 0 to 100%) were achieved in the same sputtering chamber after one sputtering process. The CE (the change of light transmission for the unit electric charge) of mixed metal oxides (TiO2-MoO3) that are deposited by reactive sputtering has been studied in an electrochemical cell. This paper aims to assess the results of investigations of such materials showing the enhanced electrochromic behavior compared to the pure materials and to enhance the Coloration Efficiency.
Keywords: 
Subject: Chemistry and Materials Science  -   Surfaces, Coatings and Films

1. Introduction

Metal oxides are widely studied with respect to their electrochromic behavior for applications such as display devices and smart windows. To decrease the absorbed heat in buildings, electrochromic films have been used as smart windows. Electrochromic materials have been applied in energy-effective glazing, automobile sunroofs, smart windows, and mirrors. The structure of a smart window contains an electrochromic material layer (usually metal oxide) sandwiched between a transparent conductive layer and some solid electrolyte. Recently, energy efficiency has been affected and focused on energy solution strategies for utilizing this important field.
The electrochromic process is based on a reversible redox process and characterized by the coloration efficiency (CE). The energy-saving windows (smart window), energy storage systems: such as electro-chromic (EC), photochromic and thermochromic have been designed on the same mechanism and both have sandwich device structures, and it was based on the elec-trochemical reaction of the electrode materials.
Transition metal (Titanium, Tungsten, Nickel, Vanadium, Molybdenum and others) oxide films are the most interesting and most widely studied materials for this purpose. Conventional thin film preparation methods include for instance: chemical methods (spin coating, sol–gel deposition, chemical bath deposition, Langmuir–Blodgett technique, etc.), chemical and physical vapor deposition, electrochemical methods (anodization, plating), see ref. [1] and references therein.
The Ti oxide (TiO2) was extensively studied prepared by the different methods: sputtering [2,3], chemical vapor deposition [4] and spray pyrolysis [5,6], various wet chemical techniques [7,8], and anodization [9,10,11].
The most widely studied EC oxide is Tungsten oxide (WO3), and films of this material have been prepared by several different methods. Examples for physical vapor deposition: thermal evaporation [12,13,14,15,16], sputtering [17,18], pulsed laser deposition [19,20] and spray pyrolysis [21].
Electrochromic Mo oxide (MoO3) shows similar behaviour to W oxide, and widespread studies used films prepared by evaporation [22,23], chemical vapor deposition [24], and wet chemical techniques [25,26].
Regarding V-pentoxide-based materials with intermediate one of the EC properties as CE, we note films prepared by vacuum evaporation [27], sputter deposition [28], spray pyrolysis [29], chemical techniques [30,31,32,33], electrodeposition [34,35] and inkjet printing [36].
Nickel oxide films (NiO) were rather considered in hydrous nickel oxide form as EC material [37,38].
Nevertheless, relatively few publications studied the possible advantages (higher coloration efficiency) of the mixtures of different metal-oxides as electrochromic material. The electrochromic effectiveness (the change of light absorption for the same electric charge) can be higher in mixed metal-oxide layers.
Earlier, we performed experiments with mixed metal-oxides and found positive effect in electrochromic behavior. Ismaeel et al [39] determined the optimal composition of reactive magnetron-sputtered combinatorial mixed layers of Titanium oxide and Tin oxide (TiO2-SnO2) for electrochromic purposes. The maximum enhancement in light absorption was found at (30%) TiO2– (70%) SnO2 composition.
In other experiments, also combinatorial material synthesis approach has been applied for the binary MoO3/WO3 system. By using organic propylene carbonate electrolyte cells in a conventional three-electrode configuration, electrochromic redox reactions have been made. Coloration efficiency data has been evaluated from the primary data plotted against the composition displayed a characteristic maximum at around 60% MoO3. The localization of the maximum at 5% accuracy has been allowed in that combinatorial approach [40].
We found only a few publications about the EC behavior of Ti-Mo mixed oxide. Mahajan et al [41] reported the positive effect of doping of Ti (0, 3, 6, 9 at%) in MoO3 thin films prepared by spray pyrolysis technique. Shrestha et al [42] successfully fabricated self-organized TiO2–MoO3 composite oxide nano-tubes with tunable dimensions by anodization. These nano-tube layers exhibited a significantly enhanced electrochromic color contrast compared with plain TiO2 nano-tubes. Haiyan Yu et al [43] prepared MoO3-TiO2 composite core/shell nanorod films by the combination of hydrothermal and electrodeposition method. They attributed the improved electrochromic properties mainly to the porous space among the nanorods array, which makes the ion diffusion easier.
The present study is considered new since its objective was to investigate the electrochromic effectiveness of TiO2-MoO3 mixed layers in a wide compositional range by determination of CE as a function of composition. This paper also aims to assess the results of investigations of such materials showing enhanced electrochromic behavior compared to the pure materials. One can expect that using metal atoms with different diameters in the layers can enhance the CE.

2. Results

The main criterion of the EC device performance is CE. Transmittance changes were directly measured during the coloration process, while the charge was calculated from the integral of the current vs. time data, and the electrolyte wetted area of the sample.
Figure 1 shows the calculated CE data as a function of MoO3 fraction of the layer (individual color-coded curves represent different wavelengths), while Figure 2 is a 3D diagram of the data. Individual points were calculated from the average of three independent measurements. Error is estimated as 3 %, calculated based on the accuracy of sample positioning in the measuring cell and the spot size of the optical beam. The calculated data are given in Table 1 according to the equation (1).
The Coloration Efficiency η is given by following equation:
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where Qi is the electronic charge inserted into the electrochromic material per unit area, ΔOD is the change of optical density, Tb is the transmittance in the bleached state, and Tc is the transmittance in the colored state. The unit of a Coloration Efficiency is cm2 C-1.
Figure 3 and Figure 4. Showed SEM-photos from the Ti-rich side and Mo-rich side.
The precision of the Ti/Mo ratio is 2 %, while the precision of the position is 1 mm. see Figure 5.

3. Discussion

To check the position dependent composition, Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDS) has been used, see Figure 5. To explain the two maximums in CE (Figure 1 and Figure 2) we show here SEM-photos from the Ti-rich side and Mo-rich side, see Figure 3 and Figure 4. These are the proof that the material at the Ti-side is polycrystalline (several hundred nm) and the Mo-side is amorphous or nanocrystalline. We think that the Ti-rich side was at significantly (several hundred C degree) higher temperature (plasma power 4.2 kW) during the deposition process, so the Ti-rich oxide is polycrystalline (several hundred nm grainsize) compared to the Mo-rich side where the oxide remains amorphous or nanocrystalline. This microstructure difference can be the reason for the two peaks in Figure 1 and Figure 2 of the CE curves.
CE values were determined at every composition from the measured relative transmission vs. input charge curves. We used eq. 1 to determine CE at 5 wavelengths (400-800 nm visible range) for every compositional position using the relative transmission curves and fitted exponential curves. The determined results, CE vs. composition and vs. wavelength are shown in Figure 1 and Figure 2.

4. Materials and Methods

Optimal composition of reactive magnetron-sputtered have determined the combinatorial mixed layers of Titanium oxide and Molybdenum oxide (MoxTi1-x) oxide thin film system, where 0 < x < 1) for electrochromic purposes. Ti and Mo targets were put separately from each other, and the ITO-covered glass and Si-probes on a glass substrate (30 cm × 30 cm) were moved under the two separated targets (Ti and Mo) in a reactive Argon-Oxygen (Ar-O2) gas mixture, see Figure 6a. Titanium-Molybdenum-Oxide layers were deposited onto Indium-Tin-Oxide (ITO) covered 100x25 mm size glass surfaces. Layer depositions were made by reactive sputtering in an (Ar + O2) gas mixture at ~2 × 10−4 Pa base pressure and at ~10−1 Pa process pressure. The target - substrate working distance was 6 cm. 30 sccm/s Ar and 70 sccm/s O2 volumetric flow rates were applied in the magnetron sputtering chamber. The plasma powers of the Ti and Mo metal targets were selected as (4200 and 1500) W respectively. Samples were moved back and forth at 25 cm/s of walking speed between the Mo and Ti targets and a mixed oxide film was deposited onto the ITO surface, see Figure 6b. 5 min cooling interrupt was applied after every 50 walking cycles.
Spectroscopic Ellipsometry (SE) is an optical characterization technique with high-accuracy [44]. Several researchers have used SE for combinatorial or pure materials investigation [45,46,47,48,49]. The combinatorial approach used to investigate mixed metal oxides has several advantages, Fried et al. [49] have used SE (which is a fast, cost-effective, and non-destructive method) for the investigation and mapping of WO3-MoO3 mixed layers after sputtering. Different optical models, such as EMA and 2T–L, have been used to achieve the composition map and thickness map of the sample layers. We used SE similar manner to determine the composition map and thickness map of our Ti-Mo combinatorial layers.
The best determination of the optimal composition as a part of electrochromic (EC) properties, the layers were deposited onto ITO-covered glass. The composition map and thickness map were measured on the Si-probes, see Figure 6a,b. We checked the resulted compositional map on the Si-probes Figure 6b,c by using Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDS), see Figure 5.
The Coloration Efficiency (CE) has been determined in a transmission electrochemical cell, see Figure 7. The cell was filled with 1M lithium perchlorate (LiClO4) / propylene carbonate electrolyte. A 5 mm width masked (Ti-Mo oxide-free) stripe of the slides remained above the liquid level allowing direct electric contact onto the ITO layer. A Pt wire counter electrode was placed into the electrolyte alongside with a reference electrode. This arrangement was a fully functional electrochromic cell. The applied current was controlled through the cell using a Farnell U2722 Source Measurement Unit (SMU). Constant current was registered through coloration and bleaching cycles of the electrochromic layer. Simultaneous spectral transmission measurements were performed by using the Woollam M2000 spectroscopic ellipsometer into transmission mode.

5. Conclusions

We could enhance the coloration efficiency of mixed Titanium oxide and Molybdenum oxide (TiO2-MoO3) layer deposited by reactive magnetron sputtering. We prepared combinatorial samples by moving the samples under the Ti and Mo sputtering targets in a reactive Argon-Oxygen (Ar-O2) gas mixture.
By using this combinatorial process, all the compositions (from 0 to 100%) were achieved in the same sputtering chamber after one sputtering. The mixed metal oxides showed better CE as a part of EC properties than the pure oxides, because the electrochromic effectiveness can be higher in mixed metal-oxide layers and mixing metal atoms with different diameters in the layers can enhance the CE.
The Ti-rich side was at significantly higher temperature during the deposition process, so the Ti-rich oxide is polycrystalline compared to the Mo-rich side where the oxide remains amorphous or nanocrystalline.
Coloration Efficiency has been considered an important parameter in this study. The maximum value of the CE is 22.2 cm2 C−1 at the wavelength 600 nm at ~ 60% - 40 % Ti-Mo ratio on the Ti-rich polycrystalline material, while CE is 19.8 cm2 C−1 at the wavelength 600 nm at ~ 20% - 80 % Ti-Mo ratio on the Mo-rich amorphous (or nanocrystalline) material.

Author Contributions

Z.L. and M.F.: conceptualization; methodology; Z.L.: investigation (sputtering); N. T. I., Z.L. and M.F.: investigation (CE and SE); N. T. I.: computing; M.F.: resources, funding acquisition; N. T. I., Z.L. and M.F.: writing—original draft preparation. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by NKFIH OTKA K 143216 and 146181 projects and Project TKP2021-EGA-04 has been implemented with the support provided by the Ministry of Innovation and Technology of Hungary from the National Research, Development, and Innovation Fund, financed under the TKP2021 funding scheme.

Acknowledgments

Authors are grateful to Noemi Szasz for the SEM–EDS measurements. This work was supported and funded by NKFIH OTKA NN 131269 (VOC-DETECT M-ERA.NET Transnational Call 2018) and NKFIH OTKA K 143216 and 146181 projects. Project TKP2021-EGA-04 has been implemented with the support provided by the Ministry of Innovation and Technology of Hungary from the National Research, Development, and Innovation Fund, financed under the TKP2021 funding scheme. The work in frame of the 20FUN02 ‘‘POLight’’ project has received funding from the EMPIR programme, co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. Noor Taha Ismaeel is grateful for the Stipendium Hungaricum scholarship.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Coloration Efficiency of TiO2-MoO3. vs. Mo % for wavelengths from (400-800) nm by home-made software version 1.0 coded in Python version 3.11 language. (Individual color-coded curves represent different wavelengths: 1 – 400 nm, 2 – 500 nm, 3 – 600 nm, 4 – 700 nm, 5 – 800 nm).
Figure 1. Coloration Efficiency of TiO2-MoO3. vs. Mo % for wavelengths from (400-800) nm by home-made software version 1.0 coded in Python version 3.11 language. (Individual color-coded curves represent different wavelengths: 1 – 400 nm, 2 – 500 nm, 3 – 600 nm, 4 – 700 nm, 5 – 800 nm).
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Figure 2. diagram of the Coloration Efficiency data of TiO2-MoO3. vs. Mo % for wavelengths from (400-800) nm visible spectral range.
Figure 2. diagram of the Coloration Efficiency data of TiO2-MoO3. vs. Mo % for wavelengths from (400-800) nm visible spectral range.
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Figure 3. SEM micrograph from the TiO2-MoO3 surface Ti-rich side.
Figure 3. SEM micrograph from the TiO2-MoO3 surface Ti-rich side.
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Figure 4. SEM micrograph from the TiO2-MoO3 surface Mo-rich side.
Figure 4. SEM micrograph from the TiO2-MoO3 surface Mo-rich side.
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Figure 5. Ti/Mo ratio measured on the Si-probes by SEM-EDS.
Figure 5. Ti/Mo ratio measured on the Si-probes by SEM-EDS.
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Figure 6. TiO2-MoO3 (a) schematic of the two targets arrangements in a closer position (35 cm from each other) and the chamber for the DC magnetron sputtering device after being air vacuumed, blue light is from the Ar-O2 plasma. (b) ITO-covered glass and Si-probes on a glass substrate, before-electrochromic-experiments, the Ti in the left and the Mo in the right. (c) after-electrochromic-experiments.
Figure 6. TiO2-MoO3 (a) schematic of the two targets arrangements in a closer position (35 cm from each other) and the chamber for the DC magnetron sputtering device after being air vacuumed, blue light is from the Ar-O2 plasma. (b) ITO-covered glass and Si-probes on a glass substrate, before-electrochromic-experiments, the Ti in the left and the Mo in the right. (c) after-electrochromic-experiments.
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Figure 7. TiO2-MoO3-during-electrochromic-experiments by SE.
Figure 7. TiO2-MoO3-during-electrochromic-experiments by SE.
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Table 1. calculated data for the CE according to the wavelengths (400-800) nm.
Table 1. calculated data for the CE according to the wavelengths (400-800) nm.
X(cm) Mo (%) 400 nm 500 nm 600 nm 700 nm 800 nm
2 14.5 2.4 3.04 3.8 3.6 3.5
2.5 15 4.5 4.5 5.1 4.5 4.3
3 16.6 3.4 5.6 5.9 5.3 4.8
3.5 19.1 3.2 4.4 5.2 4.7 4.3
4 21.9 10.2 12.2 12.1 9.8 8.9
4.5 25.1 12.7 18.6 17.7 15.1 12.8
5 27.8 11.1 17.2 16.9 14.4 11.8
5.5 31.8 11.9 17.6 17.7 15.3 13.1
6 35.6 14.2 21.5 21.5 18.8 15.5
6.5 40.5 13.6 21.1 22.2 19.5 15.8
6.9 44.1 7.7 14.1 15.9 15.3 13.4
7.4 49.1 8.5 15.05 16.8 15.9 13.4
7.9 53.55 8.4 12.6 13.9 13.3 11.7
8.4 58.15 5.1 9 9.7 8.8 7.7
8.9 62.55 4.3 6.6 7.03 6.1 5.4
9.3 65.2 2.3 4.4 5.5 5.6 5.1
9.8 69.3 2.08 2.2 2.4 2.1 2.6
10.3 73.3 1.9 2.9 3.2 3.05 3.4
10.8 76.7 6.5 9.9 10.8 9.7 10.04
11.3 79.7 11.3 17.7 19.3 18.7 17.7
11.8 83.2 7.6 10.2 11.2 14.3 11.01
12.3 85.6 6.6 7.5 8.2 8.4 7.5
12.8 87.7 4.2 5.2 5.9 6.4 5.8
13.3 89.5 5.6 6.8 7.9 8.3 7.7
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