Preparation of porous titania particles by partial dissolution and hot-water or hydrothermal treatment of hydrous titania

Abstract

Uniform-sized amorphous hydrous titania particles (HTPs) with nonporous or porous surface were prepared and subjected to hot-water or hydrothermal treatment to induce crystallisation of the titania phase for preparing porous titania particles with high specific surface area (SSA) and outer diameter in a micron order. Porous HTPs were crystallised under milder treatment conditions than those required to crystallise nonporous spherical HTPs. This difference was attributed to the difference in the polycondensation states of hydrous titania. The SSA of the porous titania particles crystallised by hot-water treatment at 80°C for 24 h was 216 m² g⁻¹, despite their micron-order particle size. This simple method can synthesise highly functional titania particles at low temperatures and is a general-purpose method for preparing porous particles for use as adsorbents, catalysts and catalyst supports.

Keywords

Titania porous particles hydrothermal

Introduction

Numerous types of nanosized titania particles have been synthesised to increase their specific surface area (SSA), which is one of the important factors affecting their performance as adsorbents, catalysts, catalyst supports and electrode materials [1,2]. Meanwhile, porous titania particles with submicron or micron sizes and relatively high SSAs are attracting attention [3 -5] because of the tendency of nanoparticles to aggregate [3] and concerns over toxicity [6]. The application of porous titania as a carrier for drug release has also been investigated in recent years [7,8].

Various excellent preparation methods for porous titania with well-defined porous structures were reported, including burning out or etching of organic or inorganic templates [5,9], spray-drying [10], hydrothermal or solvothermal treatment [5,11 -13] and nanoparticle assembly [14,15]. We also reported on a method for preparing porous titania particles via the partial dissolution of particle surfaces [16]. In this method, hydrous titania (TiO₂·nH₂O) particles (HTPs) were first prepared via a metal alkoxide method. By selecting a mixed solvent of n-butanol and acetonitrile and performing hydrolysis using ammonia water, HTPs with a uniform size and spherical shape can be obtained [17,18]. When these HTPs prepared with low concentrations of ammonia and water were washed using ethanol, the weakly polycondensed parts of the particle surface were redissolved into ethanol, and a macroporous structure was formed. This process is among the simplest methods for fabricating a porous structure because the porous structure is formed during the same process used to wash the particles prepared via the alkoxide method [19,20].

The porous particles obtained using the aforementioned method were a water-containing amorphous phase. Thus, in our previous study, they were crystallised to anatase via heat treatment at 400°C [16]. After this heat treatment, the Brunauer–Emmett–Teller (BET) SSA (S_BET) of the as-prepared porous HTPs was substantially lowered due to the disappearance of microspores in the as-prepared hydrous titania. Therefore, in the present study, we attempted to crystallise porous HTPs by hydrothermal treatment to attain a higher SSA while avoiding high-temperature heat treatment. For comparison, nonporous spherical particles with a smooth surface were also prepared and subjected to the same crystallisation procedures as the porous particles.

Hydrous titania is well known to crystallise in the anatase structure under hydrothermal treatment [13] and reported that although the primary crystals of titania were grown, the outer shape of the original HTPs was almost retained [20]. Additionally, the presence of water accelerated the crystallisation of the amorphous phase [21,22]. We also previously reported that these porous HTPs could be crystallised as strontium titanate while maintaining the original porous appearance of the particles when heated in Sr(OH)₂ aq. [23]. Thus, hydrothermal treatment appears suitable for crystallising porous HTPs while maintaining their porous structure.

In the present study, in addition to treating particles under hydrothermal conditions (≥100°C), we treated the particles in water at temperatures less than 100°C using an autoclave. The only difference between hydrothermal treatment and treatment less than 100°C was the slight temperature variation from 80°C to 100°C. The operation was exactly the same. Although the water vapour pressure changes, there must be almost no effect in the temperature range used this time. We refer to this treatment, which cannot appropriately be described as a ‘hydrothermal treatment,’ as the ‘hot-water treatment.’ We speculated that crystallising porous HTPs as porous titania via hot-water or hydrothermal treatment at a temperature lower than that used in furnace-based heat treatments would lead the porous titania particles with a higher SSA to acquire a porous structure and a uniform micron-order particle size originating from the original porous hydrous titania.

Experimental details

Materials and methods

The reagents used as the starting materials were titanium n-butoxide (Kishida Chemical Co. Ltd., Japan), n-butanol, acetonitrile and ammonia water (28 wt-%) (Wako Pure Chemical Industries Ltd., Japan). Pure water was prepared through reverse osmosis and electrodeionisation (Elix System; Millipore Corp., USA). The spherical and porous HTPs were prepared following our previously reported procedure [16]. First, two batches of an acetonitrile/n-butanol (acetonitrile: n-butanol = 1:1 by volume) mixed-solvent system were prepared. Titanium n-butoxide was dissolved in one of them, and water and ammonia were added to the other. Both resultant solutions were preheated at 80°C. To prepare the nonporous spherical HTPs, we adjusted the concentrations of the solutes in the combined solutions to 0.05 M titanium butoxide, 0.10 M ammonia and 0.50 M water. We refer to the particles prepared under these conditions as ‘spherical particles.’ Meanwhile, the concentrations of ammonia and water were lowered to 0.03 and 0.33 M, respectively, to prepare the porous HTPs. We refer to these particles as ‘porous particles.’ The hydrolysis of titanium butoxide was performed by mixing equal volumes of the aforementioned solutions. The mixed solution was maintained at 80°C for 30 min under continuous magnetic stirring. The samples were then centrifuged, and the supernatant solutions were discarded. The remaining precipitates were washed thrice with ethanol and water through centrifugation. After the washing process, the obtained HTPs were dried at 75°C. The prepared HTPs were treated with hot water or hydrothermally in a polytetrafluoroethylene-lined autoclave with pure water to induce crystallisation. The weight ratio of the HTPs to pure water in the autoclave was 1/100. For comparison, HTPs were also crystallised by heat treatment using an electric furnace at 400°C for 15 min.

Characterisation

The phases existing in the samples were identified through X-ray diffraction (XRD; MiniFlex, Rigaku Co.) and Raman spectroscopy (NRS-4500, JASCO Co.). The sample morphologies were observed by scanning electron microscopy (SEM; JSM-6330F, JEOL Ltd.) and transmission electron microscopy (TEM; H-7650, Hitachi High-Technologies Co.). The water content of the HTPs was measured by thermogravimetry–differential thermal analysis (TG–DTA; Thermo Plus Evo II TG8120, Rigaku Co.). The N₂ adsorption–desorption isotherms of the prepared particles were measured using an automatic SSA analyzer (BELSORP-max, MicrotracBEL Corp.). The SSA values were determined via the BET method. The Supplementary file provides details of each measurement.

Results and discussion

The as-prepared particles obtained only by hydrolysis of titanium alkoxide were amorphous hydrous titania, irrespective of whether the hydrolysis conditions for the spherical or porous HTPs were used (Figure 1(a) and Figure 2(a)) [16]. Under hot-water treatment at 80°C, no obvious change was observed in the XRD patterns of either the spherical or porous particles when compared with those of the as-prepared particles (Figure 1(b) and Figure 2(b)). When the treatment temperature was increased to 90°C, only the XRD pattern of the porous particles (Figure 2(c)) showed diffraction peaks assignable to anatase and a small peak assigned to brookite. The anatase and brookite formations were also confirmed by Raman spectroscopy (Figure S1). Brookite is one of the TiO₂ phase polymorphs present in nature. It is more active than anatase and rutile in some photocatalytic reactions [24,25]. This phase requires ingenuity to synthesise as a single phase and can appear as an anatase byproduct during the hydrothermal crystallisation of titania [22,24]. Although the presence of brookite promotes the phase transformation of anatase to rutile [25], the amount in the sample was small, and the rutile phase was not observed in the temperature range examined herein. Thus, the brookite phase existing in the particles must have little effect on their properties. Meanwhile, no obvious diffraction peak was observed in the pattern of the spherical particles subjected to the hot-water treatment at 90°C. The onset of crystallisation was observed when the spherical particles were treated at 100°C. The intensity of the crystalline peaks of the porous particles also increased. These results demonstrate that the porous particles tended to crystallise at lower temperatures than the spherical particles. This difference in the crystallisation temperature was attributed to the difference in stability (degree of polycondensation) between the hydrous titania forming the particles. The porous particles were prepared with lower concentrations of ammonia (0.03 M) and water (0.33 M). Their porous structure was formed by the dissolution of the weakly polycondensed parts of the particles [16]. Although the remaining part of the porous particles after the dissolution process must be more stable than the dissolved part, the degree of polycondensation might be lower than that of the spherical particles prepared with higher concentrations of ammonia (0.10 M) and water (0.50 M). This low degree of polycondensation should facilitate the easy rearrangement of the Ti⁴⁺ and O²⁻ ions for crystallisation. Although the difference in the water content of the spherical and porous particles was not confirmed due to the values estimated by the TG measurement being approximately the same (TiO₂·nH₂O, n ≈ 1.3), subtle differences in the degree of polycondensation may affect the crystallisation temperature.

The SEM observations proved that the as-prepared spherical particles (Figure 3(a)) exhibited a smooth surface and a uniform size (∼0.5 µm). Moreover, meso- and macropores were formed on the surface of the porous particles (Figure 4(a)). The porous particles exhibited a spherical outer shape and a uniform size of ∼1.5 µm. After the hot-water or hydrothermal treatment, both the spherical and porous particles retained the outer shape of the as-prepared particles. Meanwhile, fine protrusions began to appear on the particle surface after the treatment at 90°C, which became larger with the increasing treatment temperature. Although newly formed fine protrusions were observed on the surface of the spherical particles treated at 90°C (Figure 3(c)), their amount appears to be insufficient to be detected by the XRD analysis (Figure 1(c)). When the treatment temperature was increased to 100°C, the protrusions grew and the formation of anatase was confirmed by XRD (Figure 3(d)). Therefore, XRD confirmed that these fine protrusions were crystallites of anatase. Meanwhile, the growth of fine protrusions also grew on the surface of the porous particles. As the protrusions grew, accompanied by the progression of crystallisation, the opening of the meso- and macropores became slightly narrower (Figures 2 and 4).

With the increasing treatment time, the porous HTPs were crystallised into titania, even at 80°C (Figure 5). When the treatment time was prolonged to 24 h (Figure 5(d)), the particles achieved a similar degree of crystallisation as the particles treated at 100°C (Figure 2(d)). The porous particles treated at 80°C for a prolonged time exhibited a similar appearance as those treated at higher temperatures for 6 h (Figures 4 and 6). Figure 7 shows the TEM images of the surface of the as-prepared porous HTPs, porous HTPs subjected to the hot-water treatment at 80°C for 24 h, and porous HTPs heat-treated at 400°C for 15 min using an electric furnace, which crystallised into anatase [16]. The as-prepared HTPs were composed of fine primary particles [26], and their surface was relatively smooth (Figure 7(a)). Primary crystallites with nano-order sizes were observed in a densely packed state on the surface of the heat-treated particles (Figure 7(c)). Meanwhile, newly grown protrusions of crystallites were observed on the surface of the hot-water-treated particles. Voids were also observed between the anatase protrusions that may have formed via the local dissolution of part of the surface of the hydrous titania and its subsequent reprecipitation as anatase (Figure 7(b)).

S_BET of the particles crystallised by the hot-water treatment at 80°C for 24 h was 216 m² g⁻¹, which was much higher than that of the famous commercial titania nanoparticles, Evonik AEROXIDE^® TiO₂ P 25 (50 m² g⁻¹ [1]), despite having an outer diameter of micron order. Figure 8 shows the N₂ adsorption and desorption isotherms for the porous particles crystallised via the hot-water treatment at 80°C for 24 h. The isotherms previously reported for the as-prepared porous HTPs (S_BET = 354 m² g⁻¹) and the porous titania particles crystallised by heat treatment at 400°C for 15 min (S_BET = 121 m² g⁻¹) are shown for comparison [16]. The isotherm of the as-prepared porous HTPs shows a rapid increase in the adsorption amount in the low-relative pressure region, indicating the existence of micropores [27]. The microporosity of the particles was substantially decreased by heat treatment, resulting in a low S_BET. The corresponding TEM image (Figure 7(c)) indicated that this decrease in S_BET was attributable to the growth of the primary crystallites as anatase and the accompanying disappearance of microbumps on the particle surface. Although the SSA decreased to some extent with the decreasing microporosity, the particles crystallised by the hot-water treatment retained a larger SSA than the heat-treated particles. In addition, a hysteresis loop indicating the formation of mesopores [27] was also observed in the isotherms of the particles subjected to the hot-water treatment. These mesopores were likely the voids between the protrusions observed in the TEM images. The rapid increase in the adsorption amount in the low-relative pressure region was still observed in the isotherms of the particles subjected to the hot-water treatment. Although this result indicates that some of the amorphous phase remained in the centre of the particles, where water could not penetrate, larger molecules also cannot penetrate this amorphous area. Therefore, the presence of these amorphous regions should not strongly affect the particle performance as adsorbents or catalysts.

Conclusions

Spherical and porous HTPs were prepared via the hydrolysis of titanium butoxide. The porous particles prepared with lower concentrations of ammonia and water were crystallised by hot-water treatment at a temperature lower than that needed to crystallise the spherical particles prepared with higher concentrations of ammonia and water. The crystallised particles maintained the spherical shape of the raw material particles and the pore structure of the porous particles to some extent. Microcrystals of anatase grew on the HTPs as the crystallisation progressed. S_BET of the particles crystallised at 80°C for 24 h was 216 m² g⁻¹. Although this value was lower than that of HTPs, it was much larger than that of the particles crystallised by heat treatment using an electric furnace. In addition, the mesopores originating from the voids between the newly grown nanocrystallites were formed on the particles crystallised using the hot-water treatment. These particles showed relatively large pores at the raw material stage and permitted the utilisation of their inner side. The simple method proposed herein is suitable for obtaining porous particles with a high SSA at a low synthesis temperature. This procedure could potentially be used to prepare various easy-to-handle porous metal oxide particles with a micron-order outer diameter for use in a wide range of applications, such as adsorbents, catalysts and drug carriers.

Footnotes

Acknowledgements

This work was supported by the JSPS KAKENHI Grant Number 21K05223. The authors would like to thank Enago () for the English language review.

Disclosure statement

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

References

Chen

, Mao

SS.

Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chem Rev. 2007;107(7):2891–2959.

Cargnello

, Gordon

, Murray

CB.

Solution-phase synthesis of titanium dioxide nanoparticles and nanocrystals. Chem Rev. 2014;114(19):9319–9345.

Pan

, Dou

, Xiong

, Porous photocatalysts for advanced water purifications. J Mater Chem. 2010;20(22):4512–4528.

Fattakhova-Rohlfing

, Zaleska

, Bein

Three-dimensional titanium dioxide nanomaterials. Chem Rev. 2014;114(19):9487–9558.

Ding

, Yang

, Li

, Nanoporous TiO₂ spheres with tailored textural properties: controllable synthesis, formation mechanism, and photochemical applications. Prog Mater Sci. 2020;109:100620.

Shi

, Magaye

, Castranova

, Titanium dioxide nanoparticles: a review of current toxicological data. Part Fibre Toxicol. 2013;10:15.

Jiang

, Wang

, Development of an amorphous mesoporous TiO₂ nanosphere as a novel carrier for poorly water-soluble drugs: effect of different crystal forms of TiO₂ carriers on drug loading and release behaviors. Micropor Mesopor Mater. 2012;153:124–130.

Wang

, Jiang

, Wan

, Potential application of functional porous TiO₂ nanoparticles in light-controlled drug release and targeted drug delivery. Acta Biomater. 2015;13:354–363.

Ismail

, Bahnemann

DW.

Mesoporous titania photocatalysts: preparation, characterization and reaction mechanisms. J Mater Chem. 2011;21(32):11686–11707.

10.

Nandiyanto

ABD

, Ogi

, Wang

, Template-assisted spray-drying method for the fabrication of porous particles with tunable structures. Adv Powder Technol. 2019;30(12):2908–2924.

11.

Wang

, Xu

, Wang

, A general approach to porous crystalline TiO₂, SrTiO₃, and BaTiO₃ spheres. J Phys Chem B. 2006;110(28):13835–13840.

12.

Tong

, Zhang

, Tian

, Preparation and characterization of anatase TiO₂ microspheres with porous frameworks via controlled hydrolysis of titanium alkoxide followed by hydrothermal treatment. Mater Lett. 2008;62(17–18):2970–2972.

13.

Mamaghani

, Haghighat

, Lee

CS.

Hydrothermal/solvothermal synthesis and treatment of TiO₂ for photocatalytic degradation of air pollutants: preparation, characterization, properties, and performance. Chemosphere. 2019;219:804–825.

14.

, Ye

, Li

, Self-assembled TiO₂ nanocrystal clusters for selective enrichment of intact phosphorylated proteins. Angew Chem Int Ed. 2010;49(10):1862–1866.

15.

Hwang

, Lee

, Jang

, Electrospray preparation of hierarchically-structured mesoporous TiO₂ spheres for use in highly efficient dye-sensitized solar cells. ACS Appl Mater Interfaces. 2011;3(7):2719–2725.

16.

Kojima

, Baba

, Ota

, Preparation of porous titania particles by partial dissolution and heat treatment of hydrous titania. J Ceram Soc Japan. 2016;124(12):1226–1228.

17.

Sugimoto

, Kojima

Formation mechanism of amorphous TiO₂ spheres in organic solvents. 1. Roles of ammonia. J Phys Chem C. 2008;112(48):18760–18771.

18.

Kojima

, Sugimoto

Formation mechanism of amorphous TiO₂ spheres in organic solvents 3. Effects of water, temperature, and solvent composition. J Phys Chem C. 2008;112(47):18445–18454.

19.

Barringer

, Bowen

HK.

High-purity, monodisperse TiO₂ powders by hydrolysis of titanium tetraethoxide. 1. Synthesis and physical properties. Langmuir. 1985;1(4):414–420.

20.

Oguri

, Riman

, Bowen

HK.

Processing of anatase prepared from hydrothermally treated alkoxy-derived hydrous titania. J Mater Sci. 1988;23(8):2897–2904.

21.

Yanagisawa

, Yamamoto

, Feng

, Formation mechanism of fine anatase crystals from amorphous titania under hydrothermal conditions. J Mater Res. 1998;13(4):825–829.

22.

Yanagisawa

, Ovenstone

Crystallization of anatase from amorphous titania using the hydrothermal technique: effects of starting material and temperature. J Phys Chem B. 1999;103(37):7781–7787.

23.

Ujiie

, Kojima

, Ota

, Preparation of spherical and porous strontium titanate particles by hot water and hydrothermal conversion of hydrous titania. Ceram Int. 2020;46(5):6146–6153.

24.

Di Paola

, Bellardita

, Palmisano

Brookite, the least known TiO₂ photocatalyst. catalysts. 2013;3(1):36–73.

25.

, Tsai

H-L

, Huang

C-L.

Effect of brookite phase on the anatase–rutile transition in titania nanoparticles. J Eur Ceram Soc. 2003;23(5):691–696.

26.

Eiden-Assmann

, Widoniak

, Maret

Synthesis and characterization of porous and nonporous monodisperse colloidal TiO₂ particles. Chem Mater. 2004;16(1):6–11.

27.

Sing

KSW

, Everett

, Haul

RAW

, Reporting physisorption data for gas solid systems with special reference to the determination of surface-area and porosity. Pure Appl Chem. 1985;57(4):603–619.