**1. Introduction**

Functional materials are used in wide-ranging industrial fields including, but not limited to solar cells, energy storage devices, displays, smart windows, catalysts for chemical reactions and sensors [1]. Among them the tungsten oxide (WO3) represents an inorganic transition metal oxide with chromogenic and semiconductor properties, which enable its applicability in numerous applications mentioned above [2–4]. The WO3 layers can be produced with complex and expensive techniques (physical vapor deposition—PVD, chemical vapor deposition—CVD and electrodeposition) or by less expensive application from a solution phase (sol-gel, mixture of powder and different solvents) [5]. A variety of the sol-gel chromogenic devices enabling optical modulation of the interior light in the buildings [6,7], such as electrochromic [7,8], photoelectrochromic [9,10], photochromic [11–13] and gasochromic [14] have been prepared with the dip-coated WO3 layers from the corresponding sols. This publication focuses on the in-depth study of the rheological characteristics of the sol-gel derived WO3 inks suitable for inkjet printing [15].

The sol-gel process enables the fabrication of the variety of the functional materials [16]. The process involves the conversion of small molecules (monomers) into a colloidal solution (sol) that transforms to an integrated network (gel) in which the solvent is trapped. Further drying of the gel leads to the formation of a solvent free xerogel while annealing of the xerogel results in the powder. On the other hand, the printing is becoming more and

**Citation:** Opara Krašovec, U.; Vidmar, T.; Klanjšek Gunde, M.; Cerc Korošec, R.; Slemenik Perše, L. In-Depth Rheological Characterization of Tungsten Sol-Gel Inks for Inkjet Printing. *Coatings* **2022**, *12*, 112. https://doi.org/10.3390/ coatings12020112

Academic Editor: Michelina Catauro

Received: 30 December 2021 Accepted: 17 January 2022 Published: 19 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

more linked with the applications such as printed boards, displays, smart labels, smart packaging, and various printed electronic such as different sensors, solar cells, etc., which all require the availability of the functional materials in ink or paste form required for the planned printing technique [17–20]. Therefore, nowadays, functional sol-gel materials are entering the printing production, but the area is still new and underexplored. Every ink or paste should exhibit proper rheological properties, which are essential for every specific deposition process. The inkjet printing requires low viscosity of the inks (1–30 mPa.s) and surface tension around 30 mN/m, the ink typically contains a complex mixture of many solvents with high and low boiling points [18,20]. This allows proper drop generation, avoids nozzle clogging and provides optimal printing results. However, due to irreversible behavior of sol-gel material, inkjet printing of functional sol-gel materials is even more complex and requires a careful gelation control of inks, with necessary fine tuning of their rheological properties [15].

In 1997, Atkinson and coworkers reported one of the first examples of inkjet printouts using sol-gel inks [21]. Since then, the research interest in using sol-gel materials for inkjet printing production of metal oxides has increased, which is demonstrated by numerous scientific publications over the past decade [19,22–25]. Furthermore, the printability of the tungsten sol using inkjet printing has been successfully demonstrated [15]. The tungsten oxide—WO3 printouts with the thickness of around 300 nm were realized of very good optical quality and enabled the realization of the electrochromic devices [15]. The tungsten sol used as functional ink has been modified by using 2-propoxy propanol to match the inkjet printing requirements for proper drop generation (jetting characteristics), smooth ink transfer through printer and uniformity of deposited films [15].

Moreover, inkjet printing has complex drying behavior, therefore an appropriative solvent system, temperature modulation of printer vacuum plates (up to 60 ◦C) and also cartridge temperature (up to 70 ◦C) should be carefully chosen for each individual system and substrate in order to form uniform deposited films [18,20]. From this perspective, it is of paramount importance to perform a rheological study of inks by simulating the conditions in printing process. An in-depth study of the sol rheology could enable better insight of the sol-gel inks limitations as well as the control of the sol-gel material stability (sol-gel transition) which is required for continuous inkjet printing without clogging. The inks should have Newtonian behavior, which means that viscosity of fluid or ink is constant with applied shear rates. In the case of the sol-gel inks the transition of the sol to gel occurs therefore the ink changes from Newtonian to non-Newtonian fluid. In this regard printing of functional sol-gel materials is even more complex and requires a careful gelation control of the ink, with necessary understanding of its rheological properties.

To our knowledge, we are the first to reveal in-depth rheological characterization of the WO3 sols for inkjet printing in which we simulate the conditions of the printing process. Publication by Karimi-Nazarabad et al. [26] describes the rheological properties of nanofluids of tungsten oxide nanoparticles in ethylene glycol and glycerol. The authors don't report on practical usage of studied nanofluids, neither on the rheological characteristics of the gelation process of the samples. Moreover, D. Tripkovic et al. [27] demonstrated tailoring of BaTiO4 sol-gel inks for inkjet printing. The most relevant publication [28], describes the study of rheological properties of TiO2 sol for direct write assembly in planar and 3D configuration.

The aim of this study is to characterize the sol to gel transition of the WO3 inks at various temperatures that the ink could be exposed to during the inkjet printing. The results were obtained by coupling two measurement techniques, IR spectroscopy and rheological characterization. Rheological study enabled the insight into the gelation process, while the IR spectroscopy shed the light on the changes of the chemical structure of the WO3 sols taking place during the transformation of the sol to the gel.

#### **2. Materials and Methods**
