Effect of the heating rate on crystallization and densification of TiO₂ films prepared from alkoxide solutions containing various organic additives

ABSTRACT

The effect of the heating rate on crystallization and densification of sol–gel-derived TiO₂ films has been investigated. The TiO₂ gel films were prepared from Ti(OC₃H₇ⁱ)₄ solutions containing alcohols (2-butanol and 2-methoxyethanol), glycols (propylene glycol, diethylene glycol and dipropylene glycol) and chelating agents (acetylacetone and benzoylacetone) as organic additives, followed by heat treatment to 800°C at various heating rates. In the cases without organic additives and with alcohols, anatase films were obtained irrespective of the heating rate, where the refractive indexes of the films were higher for slower heating rate. Conversely, for the films prepared with glycols and chelating agents, rapid heating activated crystallization and densification. In particular, addition of chelating agents led to the formation of rutile films with much higher refractive indexes than those of the anatase films.

KEYWORDS:

• TiO₂ films
• sol-gel coating
• heating rate
• refractive index

1. Introduction

Sol–gel-derived titania (TiO₂) film materials are useful for both academic research and practical applications as optical coatings [1–3], photocatalysts [4–6] and electrodes for solar cells [7–9]. In the sol–gel coating process, precursor gel films are first fabricated from titanium alkoxide solutions containing water, acidic or basic catalysts and alcoholic solvents, and they are then thermally converted to polycrystalline TiO₂ films. The crystallinity and density of the resultant TiO₂ coating layers are influenced by the composition of the coating solution and the heat-treatment conditions, which significantly affect the device properties, such as the refractive index, optical transparency and photocatalytic activity.

Organic molecules are often added to precursor solutions as chelating [10,11] or pore-forming agents [12–17] to control the crystallinity and density of the sol–gel-derived film materials. Organic chelators, such as acetylacetone (ACAC), can coordinate to metal alkoxides and thus suppress the hydrolysis and condensation reactions in the coating solutions, which affects the subsequent crystallization and densification of the coating layer during heating. Moreover, irrespective of the presence or absence of coordinating ability, thermal decomposition of large amounts of organic additives in the coating layers can result in formation of numerous nanopores, providing porous films with high surface areas. The effect of organic additives on the crystallization and densification behavior during heating has been studied by many researchers to control the microstructures of sol–gel-derived TiO₂ films. We previously investigated crystallization and densification of TiO₂ films prepared from Ti(OC₃H₇ⁱ)₄ solutions containing a chelating agent (ACAC) by heating at 800°C [18]. Addition of ACAC led to the formation of a highly dense structure consisting of large grains with sizes of 100‒300 nm and a change of the crystal phase from anatase to rutile. Such dense rutile films were only obtained when the precursor gel films containing ACAC were rapidly heated by direct insertion into a furnace held at 800°C, while a slow heating rate of 5°C min⁻¹ resulted in the formation of anatase films with a porous structure consisting of fine grains with sizes below 50 nm. This suggests that the effect of thermal decomposition of organic species on crystallization and densification drastically varies depending on the heating rate.

A few studies on the effect of the heating rate on sol–gel-derived TiO₂ materials have been reported [19–25]. Keddie et al. investigated the effect of the heating rate on sintering of TiO₂ thin films prepared from Ti(OC₂H₅)₄–H₂O–t-C₄H₉OH solutions [19]. They suggested that a faster heating rate decreases the number of crystallites, which decreases the porosity of the coating layers, resulting in the formation of dense TiO₂ films. Pap et al. prepared N-doped TiO₂ photocatalysts from TiCl₄–HNO₃–HNO₃–CH₃COOH solutions using the sol–gel method [20]. The fast-heat-treated samples had bridged-nitro groups on the surface and exhibited high photocatalytic activity. Dikici et al. investigated the effect of the heating rate on the structure, morphology and photocatalytic properties of TiO₂ particles prepared from Ti(OC₃H₇ⁱ)₄–CH₃COOH–C₂H₅OH solutions [21]. The heating rate did not affect the surface morphology of the TiO₂ particles, but it decreased the band gap energy and improved the photocatalytic properties. However, these studies did not focus on thermal decomposition of the organic additives at different heating rates, and thus the effect of the heating rate on the microstructures of TiO₂ materials has not been fully investigated. A detailed understanding of the influence of the heating rate would allow formation of highly dense, crystalline TiO₂ films.

In this work, we prepared sol–gel-derived TiO₂ films from Ti(OC₃H₇ⁱ)₄–HNO₃–H₂O–C₂H₅OH solutions containing various organic species, and we investigated the effect of the heating rate on crystallization and densification of the films. First, precursor gel films containing ACAC were prepared and then heated to 800°C at various rates, and the effect of the heating rate on the crystallinity, crystal phase, microstructure and refractive index of the resultant TiO₂ layers was investigated in detail. Next, other organic species, such as alcohols, glycols and chelating agents, were added to the precursor solutions, and the influence of these additives on the structural and optical properties of the TiO₂ layers was compared with that of ACAC. On the basis of these results, the crystallization and densification behavior of sol–gel-derived TiO₂ films containing organic species is systematically discussed.

2. Experimental

2.1. Preparation of the TiO₂ films

Ti(OC₃H₇ⁱ)₄, 69 mass% nitric acid, C₂H₅OH, and the organic additives [ACAC, benzoylacetone (BZAC), propylene glycol (PG), diethylene glycol (DEG), dipropylene glycol (DPG), 2-butanol (2-Bu) and 2-methoxyethanol (2-Me)], which were used as the starting materials, were purchased from Wako Pure Chemical Industries, Osaka, Japan. The molecular weights and boiling points of the organic additives are given in Table 1.

Table 1. Molecular weight and boiling point of organic additives.

Organic additives		Molecular weight/g mol^−1	Boiling point/ºC
Acetylacetone	(ACAC)	100.13	140.0
Benzoylacetone	(BZAC)	162.19	260.0
Propylene glycol	(PG)	76.09	188.2
Diethylene glycol	(DEG)	106.12	244.3
Dipropylene glycol	(DPG)	134.17	230.5
2-Butanol	(2-Bu)	74.12	99.0
2-Methoxyethanol	(2-Me)	76.09	124.0

The Ti(OC₃H₇ⁱ)₄ solutions containing organic additives were prepared by the following procedure, and the molar compositions of the Ti(OC₃H₇ⁱ)₄ solutions with various organic additives are given in Table 2. First, the organic additives and Ti(OC₃H₇ⁱ)₄ were added in this order to half of the prescribed amount of C₂H₅OH. Purified water was added to the remaining C₂H₅OH, followed by nitric acid. The solution containing C₂H₅OH, purified water and nitric acid was added dropwise to the Ti(OC₃H₇ⁱ)₄ solution under stirring in an ice-water bath. The solutions were stirred in the ice-water bath for 30 min, and then at room temperature for 30 min. The resultant transparent solutions were used as coating solutions. In this work, the molar ratios of the solutions were mainly Ti(OC₃H₇ⁱ)₄:H₂O:HNO₃:C₂H₅OH:organic additives = 1:1:0.2:10:2 (Table 2). However, the solutions containing BZAC and PG became cloudy under these conditions, and thus the compositions of the BZAC and PG solutions were adjusted to suppress precipitation (Table 2).

Table 2. Molar compositions of Ti(OC₃H₇ⁱ)₄ solutions prepared with various organic additives.

	Mole ratio
Organic additives	Ti(OC3H7^i)4	H2O	HNO3	C2H5OH	Additives
No additives	1	1	0.2	10	-
- Chelating agent
ACAC	1	1	0.2	10	2
BZAC	1	1	0.2	20*	0.5*
- Glycols
PG	1	1	0.2	20*	2
DEG	1	1	0.2	10	2
DPG	1	1	0.2	10	2
- Alcohols
2-Bu	1	1	0.2	10	2
2-Me	1	1	0.2	10	2

*The mole ratio was adjusted to suppress the precipitation.

The gel films were deposited on silica glass (20 mm × 40 mm × 0.85 mm) or Si(100) substrates (20 mm × 40 mm × 0.5 mm) by dip-coating, where the substrates were withdrawn at 3 cm min⁻¹. The gel films were immediately transferred to an electric furnace held at 800°C or heated to 800°C at a constant rate of 5–260°C min⁻¹ in an electric furnace, followed by heating at 800°C for 10 min and removal from the furnace to the ambient atmosphere. The heat treatments of the gel films were performed in air.

2.2. Characterization

The thickness and refractive index of the films were measured with an ellipsometer (ESM-1T, ULVAC, Chigasaki, Japan). The ellipsometric measurement was performed at 10 different points on the film sample with a He–Ne laser at an incident angle of 70°. The crystalline phases were identified by X-ray diffraction (XRD) measurement in ordinary 2θ/θ mode using an X-ray diffractometer (Model Rint 2550 V, Rigaku, Tokyo, Japan) with Cu Kα radiation operated at 40 kV and 300 mA. The XRD patterns for the identification of crystal phases were measured between 20–70º at a scanning rate of 4º min⁻¹, and the peak area of the (101) diffraction peaks of anatase phase was calculated from the diffraction patterns measured at 24.5–26º at a scanning rate of 0.5º min⁻¹. The microstructures of the thin-film samples were observed by field-emission scanning electron microscopy (FE-SEM) (JSM-6500F, JEOL, Tokyo, Japan). The XRD measurements and FE-SEM observations were performed for the films deposited on silica glass substrates, and the ellipsometric measurements were performed for the films deposited on Si(100) substrates. The thermogravimetric and differential thermal analysis (TG-DTA) curves were obtained at a heating rate of 10°C min⁻¹ in flowing air using a thermal analyzer (ThermoPlus 2, Rigaku, Tokyo, Japan). The test samples were the gel specimens prepared by drying the coating solutions at room temperature for 24 h.

3. Results and discussion

3.1. Preparation of TiO₂ films from the coating solutions containing ACAC

First, we compared the effect of the heating rate on crystallization and densification of the TiO₂ films obtained without organic additives and with ACAC. The precursor gel films on silica substrates were heated at 800°C for 10 min in air, where the temperature was increased to 800°C at a constant rate of 5–260°C min⁻¹ in an electric furnace, or by direct insertion into an electric furnace held at 800°C. The actual temperature change of the film samples in the electric furnace is shown in Figure S1 (Supplementary Material), where the temperature was measured with a thermocouple attached to the substrates. The temperature of the films increased to 800°C within 1 min for direct insertion into the electric furnace, which means that direct insertion into the electric furnace provided the fastest temperature increase in this work.

Transparent, crack-free films were obtained at all the conditions. The dependence of the film thickness on the heating rate for the TiO₂ films prepared without organic additives and with ACAC is shown in Figure 1. The film thickness was 60‒65 nm irrespective of the heating rate and organic additives, and the films prepared with ACAC were slightly thicker than those prepared without organic additives. The XRD patterns of the films prepared without organic additives and with ACAC heated to 800°C at a constant rate of 5–260°C min⁻¹ and heated by direct insertion into an electric furnace held at 800°C are shown in Figure 2. The diffraction peaks attributed to the anatase phase were detected for all of the films (Figure 2(a,b)). The rutile phase only formed for the films prepared with ACAC by direct insertion into the electric furnace, where the diffraction peaks of anatase drastically weakened (Figure 2(b)). The peak areas calculated from the (101) diffraction peaks of anatase are shown in Figure 3. For the films prepared without organic additives, the peak area (i.e. the degree of crystallization) decreased with increasing heating rate. Conversely, in the case of addition of ACAC, the peak area of anatase tended to slightly increase with increasing heating rate up to 260°C min⁻¹. Moreover, the films prepared with ACAC by direct insertion, which contained a rutile phase, showed the lower peak area and the variation in the peak intensity (shown as the error bar in Figure 3), which could be attributed to the consumption of anatase phase by the phase transformation to rutile. The films prepared without organic additives heated to 800°C at 5–150°C min⁻¹ showed larger peak areas than those prepared with ACAC, while a significant difference between the films prepared with ACAC and without organic additives was not observed at a heating rate of 260°C min⁻¹.

Figure 1. Dependence of the film thickness on the heating rates for the TiO₂ films prepared without organic additives and with ACAC.

Figure 2. XRD patterns of the TiO₂ films prepared without organic additives (a) and with ACAC (b).

Figure 3. Dependence of the peak area of the (101) diffraction peaks of anatase on the heating rates for the TiO₂ films prepared without organic additives and with ACAC.

SEM images of the films prepared without organic additives and with ACAC heated to 800°C at a constant rate of 5°C min⁻¹ and heated by direct insertion into an electric furnace held at 800°C are shown in Figure 4. Cracking was not observed for all the films, and crystal grains were found on the surface. The films prepared without organic additives had a porous structure consisting of fine grains below 50 nm in size (Figure 4(a,b)). Conversely, in the case of addition of ACAC, the grain size increased with increasing heating rate (Figure 4(c,d)), where a relatively dense surface consisting of large grains of approximately 200 nm in size was observed for the films heated by direct insertion into the electric furnace (Figure 4(d)).

Figure 4. SEM patterns of the TiO₂ films prepared without organic additives (a,b) and with ACAC (c,d) heated at a constant rate of 5°C min⁻¹ (a,c) and heated by direct insertion into an electric furnace (b,d).

The refractive index of the TiO₂ films was measured by a spectroscopic ellipsometry, where the measurements were performed for the films deposited on Si(100) substrates for avoiding the influence of the back reflection of transparent substrates. In the previous works, we have evaluated the refractive index of sol-gel-derived TiO₂ films on Si(100) substrates and reported the tendency for the variation in the refractive index on the Si(100) substrates agreed with those in the degree of crystallization on the silica substrates [26,27]. The dependence of the refractive index on the heating rate for the films prepared without organic additives and with ACAC heated to 800°C at a constant rate of 5–260°C min⁻¹ and heated by direct insertion into an electric furnace held at 800°C is shown in Figure 5. The refractive index of the films prepared without organic additives decreased with increasing heating rate, while the films prepared with ACAC exhibited higher refractive index for higher heating rate. For the films heated at 5–150°C min⁻¹, addition of ACAC resulted in lower refractive index than for no additives, which agreed well with the variation in the peak area of the XRD pattern (Figure 3). In general, TiO₂ films with high crystallinity and high density have high refractive indexes. Thus, in the present case, the variation in the refractive index can be attributed to the degree of crystallization and densification of the TiO₂ phase. Moreover, the refractive index of highly crystalline rutile TiO₂ (2.65–2.72) is higher than that of anatase (2.48–2.57). The rutile film obtained with ACAC by direct insertion into the electric furnace showed a high refractive index (Figure 5), which was superior to that of the anatase film obtained without organic additives.

Figure 5. Dependence of the refractive index on the heating rates for the TiO₂ films prepared without organic additives and with ACAC.

As described above, the heating rate affected crystallization and densification of the sol–gel-derived TiO₂ films, and the influence changed depending on the organic species added. In the case without organic additives, the peak area of the XRD pattern (i.e. the degree of crystallization) (Figure 3) and refractive index (Figure 5) decreased with increasing heating rate. In addition, rapid heating of the film with ACAC resulted in generation of the rutile phase (Figure 2), a dense surface (Figure 4(d)) and a high refractive index (Figure 5).

3.2. Preparation of TiO₂ films from coating solutions containing alcohols, glycols and chelating agents

Next, we prepared TiO₂ films from coating solutions containing organic species other than ACAC. Here, alcohols (2-Bu and 2-Me), glycols (PG, DEG and DPG) and a different chelating agent (BZAC) were used as organic additives, and the crystallization and densification behaviors of the TiO₂ films were compared with those of the films prepared with ACAC. The precursor gel films were heated at 800°C for 10 min in air, where the temperature was increased to 800°C at a constant rate of 5°C min⁻¹ in an electric furnace, or by direct insertion into an electric furnace held at 800°C.

The thicknesses of the TiO₂ films prepared with various organic species heated to 800°C at a constant rate of 5°C min⁻¹ and directly inserted into an electric furnace held at 800°C are given in Table 3. The thickness slightly decreased from approximately 70 nm to 40‒50 nm by addition of 2-Bu, 2-Me, PG, ACAC and BZAC, and the heating rate did not affect the thickness irrespective of the organic additive. The XRD patterns of the TiO₂ films prepared with alcohols (2-Bu and 2-Me), glycols (PG, DEG and DPG) and chelating agents (ACAC and BZAC) are shown in Figure 6. Only the anatase phase was detected in the XRD patterns of the films prepared with alcohols (Figure 6(a)) and glycols (Figure 6(b)), irrespective of the heating rate. In contrast, addition of BZAC, as well as the addition of ACAC with direct insertion into an 800°C electric furnace, resulted in the formation of the rutile phase (Figure 6(c)). For the films prepared with BZAC, the rutile phase formed irrespective of the heating rate and the peak intensity became stronger with increasing heating rate (Figure 6(c)), where only the (110) diffraction peak of rutile was detected, which might indicate the oriented growth rutile crystallites. SEM images of the films prepared with alcohols (2-Bu and 2-Me), glycols (PG, DEG and DPG) and chelating agents (ACAC and BZAC) by heating to 800°C at constant rate of 5°C min⁻¹ and by direct insertion into an 800°C electric furnace are shown in Figure S2 (Supplementary Material). The anatase films prepared with alcohols (2-Bu and 2-Me) and glycols (PG, DEG and DPG) consisted of fine grains below 50 nm in size, while the rutile films prepared with chelating agents (ACAC and BZAC) contained large grains above 200 nm in size.

Figure 6. XRD patterns of the TiO₂ films prepared with alcohols (2-Bu, 2-Me) (a), glycols (PG, DEG, DPG) (b) and chelating agents (ACAC, BZAC) (c) heated at constant rate of 5°C min⁻¹ and by the direct insertion.

Table 3. Thickness of TiO₂ films prepared with various organic additives.

Organic additives	Heating rate	Film thickness/nm
No additives	5°C min^−1	67.6
	Direct insertion	70.2
ACAC	5°C min^−1	62.7
	Direct insertion	56.5
BZAC	5°C min^−1	43.8
	Direct insertion	39.6
PG	5°C min^−1	44.9
	Direct insertion	43.4
DEG	5°C min^−1	72.5
	Direct insertion	68.6
DPG	5°C min^−1	68.9
	Direct insertion	69.7
2-Bu	5°C min^−1	50.8
	Direct insertion	55.3
2-Me	5°C min^−1	52.9
	Direct insertion	51.5

The dependence of the refractive index on the heating rate for the TiO₂ films prepared with various organic additives by heating to 800°C at constant rate of 5°C min⁻¹ and by direct insertion into an 800°C electric furnace is shown in Figure 7. The refractive indexes of the films prepared with alcohols were not influenced by the heating rate, and they were lower than those of the films prepared without organic additives (Figure 7(a)). Conversely, in the case of addition of glycols (PG, DEG and DPG) and chelating agents (BZAC and ACAC), the refractive indexes increased with increasing heating rate (Figure 7(b)). The films prepared with BZAC, containing the rutile phase, showed higher refractive indexes than those prepared with the other organic species (Figure 7(b)), where the films heated by direct insertion especially exhibited the highest refractive index of ca. 2.7. The ordinary (n_o) and extraordinary (n_e) refractive indexes of rutile TiO₂ have been reported to be 2.584 and 2.872, respectively, at 632.8 nm [28], and thus the average values can be calculated to be 2.680 and 2.677 with the two types of equations, (2n_o + n_e)/3 and (n_o²n_e)^1/3 [29], respectively. The refractive index of the films prepared with BZAC by direct insertion was higher than the calculated average values, which was thought to be not reasonable. On the other hand, if the rutile crystallites expose the (110) planes to the surface of coating layers, the average refractive index could be estimated to be 2.728 and 2.724 with the equations, (n_o + n_e)/2 and (n_on_e)^1/2, respectively. In fact, such high refractive index is reported to be found for the oriented rutile films [30]. The high refractive index and the XRD patterns (Figure 6(c)) of the films prepared with BZAC might indicate the possibility of the oriented growth of rutile crystallites.

Figure 7. Refractive index of the TiO₂ films prepared with alcohols (2-Bu, 2-Me) (a), glycols (PG, DEG, DPG) and chelating agents (ACAC, BZAC) (b) heated at constant rate of 5°C min⁻¹ and by the direct insertion.

3.3. Effect of organic additives on crystallization and densification of TiO₂ films heated at various rates

For the TiO₂ films prepared without organic additives, the degree of crystallization and the refractive index increased with decreasing heating rate (Figure 5). This indicates that a gradual increase of the temperature and an increase in the total heating time improved the crystallinity and density of the TiO₂ film, resulting in higher refractive index. However, as shown in Section 3.2, crystallization and densification of the TiO₂ films prepared with glycols and chelating agents were activated by rapid heating, which was not observed when alcohols were used as additives. To discuss the influence of the organic additives during heat treatment, the thermal decomposition behavior of the organic additives was investigated by TG-DTA analysis. The TG-DTA curves of the precursor gels prepared with alcohols (2-Bu and 2-Me), glycols (PG, DEG and DPG) and chelating agents (ACAC and BZAC) are shown in Figure 8, respectively. In the case without organic additives, gradual weight loss was observed below 200°C (Figure 8(a-c)). No exothermic peaks attributed to burning of the organic species were observed in the DTA curves (Figure 8(a-c)), which indicate that the weight loss was mainly caused by evaporation of C₂H₅OH as the main solvent and H₂O. For the gels prepared with glycols (Figure 8(b)) and chelating agents (Figure 8(c)), additional weight loss accompanied by exothermic peaks occurred at 200–300 and 400–500°C. The exothermic reactions at 200–300°C can be attributed to burning of organic species, such as the remaining organic additives and unhydrolyzed i-propoxide ligands [10,31]. The weight loss above 400°C is thought to be caused by combustion of residual carbon owing to incomplete burning of organic species [31]. As mentioned above, rapid heating to 800°C activated crystallization and densification of the TiO₂ films prepared with glycols and chelating agents. Generally, sol–gel-derived TiO₂ films crystallize in the anatase phase above 400°C, and the anatase phase transforms to the rutile phase above 800°C. When the gel films containing glycols and chelating agents were rapidly heated, nucleation of the anatase and rutile phases and burning of the organic species simultaneously progressed, and the heat of burning of the organic species could activate crystallization and densification. In particular, in the present case, addition of chelating agents led to generation of the rutile phase (Figure 6(c)), providing TiO₂ films with high refractive indexes (Figure 7(b)). Chelating agents are thought to inhibit development of the Ti–O–Ti network in the gel film before heat treatment, which more easily allows rearrangement of the Ti–O–Ti bonds in the initial stage of heat treatment, promoting transformation to the rutile phase.

Figure 8. TG and DTA curves of the TiO₂ gels prepared with alcohols (2-Bu, 2-Me) (a), glycols (PG, DEG, DPG) (b) and chelating agents (ACAC, BZAC) (c).

For addition of alcohols (2-Bu and 2-Me), rapid heating did not affect the refractive indexes of the TiO₂ films (Figure 7(a)), which indicates that crystallization and densification were not promoted. For the gels with 2-Bu and 2-Me, several exothermic peaks were detected at 100–300°C (Figure 8(c)), which could be caused by burning of the remaining alcohols with relatively high boiling points. However, the gels with 2-Bu and 2-Me showed almost the same weight loss as the gel without organic additives, and no other weight loss and exothermic reactions were observed above 300°C (Figure 8(a)). Thus, 2-Bu and 2-Me were inferred to burn before nucleation of TiO₂ even for direct insertion into a 800°C furnace, and thus they did not activate crystallization and densification.

4. Conclusions

The effect of the heating rate on crystallization and densification of TiO₂ film materials has been investigated for alkoxide-derived films containing various organic species (alcohols, glycols and chelating agents). For the TiO₂ films prepared without organic additives and with alcohols (2-Bu and 2-Me), a slow heating rate resulted in relatively high refractive index, where a gradual increase of the temperature and an increase in the total heating time increased the crystallinity and density of the film. For the films prepared with glycols (PG, DEG and DPG) and chelating agents (ACAC and BZAC), rapid heating activated crystallization and densification, leading to the formation of TiO₂ films with relatively high refractive indexes. In particular, the rutile phase is formed by the addition of chelating agents. The heat of burning of the glycols and chelating agents activated crystallization and densification, resulting in higher refractive index than without these additives. These crystallization and densification behaviors of sol–gel-derived TiO₂ films containing organic species during heating at various rates are essential for forming highly dense, crystalline thin-film materials.

Acknowledgments

We thank Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplemental data

Supplemental data for this article can be accessed online at https://doi.org/10.1080/21870764.2023.2252230

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by Nippon Sheet Glass Foundation for Materials Science and Engineering, 2022 Research Grant (No. 22-002).