Unexpected Link between the Template Purification Solvent and the Structure of Titanium Dioxide Hollow Spheres

Abstract

Carbon spheres were applied as templates to synthesize titanium dioxide hollow spheres. The templates were purified with either ethanol or acetone, and the effects of this treatment on the properties of the resulting titania were investigated. The photocatalytic activity of the catalysts was measured via the decomposition of phenol model pollutant under visible light irradiation. It was found that the solvent used for the purification of the carbon spheres had a surprisingly large impact on the crystal phase composition, morphology, and photocatalytic activity. Using ethanol resulted in a predominantly rutile phase titanium dioxide with regular morphology and higher photocatalytic activity (r_0,phenol = 3.9 × 10⁻⁹ M∙s⁻¹) than that containing mainly anatase phase prepared using acetone (r_0,phenol = 1.2 × 10⁻⁹ M∙s⁻¹), surpassing the photocatalytic activity of all investigated references. Based on infrared spectroscopy measurements, it was found that the carbon sphere templates had different surface properties that could result in the appearance of carbonate species in the titania lattice. The presence or absence of these species was found to be the determining factor in the development of the titania’s properties.

1. Introduction

When it comes to photocatalytic water treatment, titanium dioxide (TiO₂) remains the staple choice with good reason. The applicability of TiO₂ in wastewater treatment has been extensively investigated with various pollutants, such as phenol [1,2,3], organic dyes [4], pesticides [5], and pharmaceuticals [6], or by inactivating bacteria in disinfection processes [3,7]. During these investigations, TiO₂ photocatalysts were utilized either in suspension or in an immobilized state. When considering the latter, there are several promising publications about their practical application in pilot plants [7,8]. Despite a tremendous effort to identify a more efficient semiconductor, the good overall characteristics of TiO₂ have not been overcome in practical applications. However, it has its limitations. The most important limitation is that without modifications, its efficiency often proves to be insufficient to replace existing technologies. In order to tackle this problem, numerous attempts have been made to increase photocatalytic activity. This includes (i) doping with various elements, (ii) sensitizing with dyes, (iii) modifying with noble metals, (iv) preparing composites/coupled semiconductors, and (v) synthesizing TiO₂ with various morphologies [9,10,11,12,13,14]. A relatively novel approach to achieve this goal is to synthesize TiO₂-based hollow spheres (HSs) since photocatalysts with such a morphology possess increased light-harvesting capabilities compared to solid spheres [15,16]. There are various methods to prepare TiO₂ HSs (e.g., hydrothermal, sonochemical, and template-based) [17]. The most commonly applied method is the template removal method [18]. Among the most convenient template materials are carbon spheres (CSs) due to their environmentally friendly nature and high hydrophilicity, conveyed by polar –OH and –C=O groups on their surface [18]. However, due to their synthesis method, it is usually necessary to purify them from the various contaminants (for example, aromatic compounds and oligosaccharides [19]) that form during the synthesis. It is known that applying solvents of different polarity can result in the formation of titanium dioxides with different crystal phase compositions and characteristics [20]. However, the effects of the solvents used for the purification of the templates on the properties of the as-prepared titania have been poorly investigated.

In our previous publication, we demonstrated that not only the synthesis conditions of TiO₂ coating but also that the parameters applied during the preparation of CSs can significantly affect the properties of the resulting titania [21]. In this paper, we present the importance of the purification solvent by using either acetone or ethanol to eliminate the residual organic contaminants formed during the synthesis of CSs, since it was observed that this parameter might be the most crucial of all in our system.

2. Results and Discussion

2.1. Textural Characterization

X-ray diffraction patterns of the home-made (TiO₂_HS_Ac and TiO₂_HS_EtOH) and reference (Rutile-H2, Aeroxide P25, and Aldrich rutile which is a rutile phase TiO₂ purchased from Sigma-Aldrich, Schnelldorf, Germany) titania are shown in Figure 1. For the sake of completeness, the X-ray diffraction patterns of the CSs purified either by acetone (CS_Ac) or ethanol (CS_EtOH), and the TiO₂-CS composites (i.e., CSs coated with TiO₂; TiO₂_CS_Ac and TiO₂_CS_EtOH, respectively) were also recorded and shown in Supplementary Materials Figure S1. Crystal phase distributions, average primary crystallite sizes, and specific surface areas (SSA) are presented in

Table 1. Phase composition, average crystallite size, and specific surface area of the investigated titanium dioxides.

Titanium-Dioxide	Anatase		Rutile
	wt%	Particle Size (nm)	wt%	Particle Size (nm)	Specific Surface Area (m2·g−1)	r0,phenol (10−9 M·s−1)
TiO2_HS_Ac	79	12.3	21	11.7	51	1.2
TiO2_HS_EtOH	14	10.4	86	13.1	30	3.9
Rutile-H2	<1	-	>99	7.0	237	1.9
Aeroxide P25	90	25.4	10	40.0	49	1.6
Aldrich rutile	4	315.0	96	315.0	3	3.7

. Reference photocatalysts were characterized in detail in our previous publication [22].

As a function of CS template purification solvent, the crystal phases of our home-made titania were remarkably different: TiO₂_HS_Ac contained predominantly anatase phase (79 wt%), whereas TiO₂_HS_EtOH contained predominantly rutile phase (86 wt%). The increased primary crystallite sizes and decreased specific surface areas (TiO₂_HS_Ac: D_anatase = 12.3 nm, SSA = 51 m²∙g⁻¹; TiO₂_HS_EtOH: D_rutile = 13.1 nm, SSA = 30 m²∙g⁻¹), compared to the reference starting material Rutile-H2 (D = 7.0 nm; SSA = 237 m²∙g⁻¹), can be explained by the result of calcination (T = 400 °C, t = 4 h, heating rate = 5 °C∙min⁻¹) that was used for the elimination of the CSs. The SSA of CS_Ac, CS_EtOH, TiO₂_CS_Ac, and TiO₂_CS_EtOH were also measured; their SSA values were 11, 16, 34, and 27 m²∙g⁻¹, respectively.

The significant effect of purification solvent on the properties of home-made titania was also observed during scanning electron microscopy (SEM; Figure 2a–d) and transmission electron microscopy (TEM; Figure 2e,f) measurements.

Following the coating process, the TiO₂ layer evenly surrounded the CS templates, thus retaining their morphology (Figure 2a,c). However, after calcination, the structure of sample TiO₂_HS_Ac was damaged (Figure 2b), while in the case of sample TiO₂_HS_EtOH, the spherical morphology remained mostly intact (Figure 2d). Consequently, TEM measurements were carried out on the latter sample (Figure 2e), and it was observed that it contained mainly hollow spheres, as intended. In Figure 2f, the presence of lattice fringes can be observed, and 3.25 Å was measured predominantly for the d-spacing values, which corresponded to the TiO₂ (110) crystallographic planes (rutile phase). This demonstrates good agreement with the X-ray diffractometry (XRD) results. The predominantly rutile phase and hollow structure of the TiO₂_HS_EtOH sample is presumed to contribute to the photocatalytic activity of this sample under visible light irradiation [15,23].

2.2. Photocatalytic Activity

Photocatalytic degradation curves of phenol are shown in Figure 3.

The reasons for the observed photocatalytic activity order between the peroxo group-containing home-made Rutile-H2 and commercial Aldrich rutile, and the well-known commercial Aeroxide P25—which were used as references in this work—have already been described in detail in our previous publication [22]. As expected, sample TiO₂_HS_Ac (containing predominantly anatase phase with no well-defined morphology) proved to be the least efficient under visible light irradiation. In contrast, the photocatalytic activity of sample TiO₂_HS_EtOH (containing predominantly rutile phase with well-defined hollow spherical morphology) surpassed the photocatalytic activity of not only TiO₂_HS_Ac and Aeroxide P25 (as expected), but also the photocatalytic activity of home-made Rutile-H2 and commercial Aldrich rutile references. Sample TiO₂_HS_EtOH achieved such outstanding photocatalytic activity (r_0,phenol = 3.9 × 10⁻⁹ M·s⁻¹) despite its lower specific surface area when compared to TiO₂_HS_Ac (r_0,phenol = 1.2 × 10⁻⁹ M·s⁻¹). The difference is especially prominent when compared to Rutile-H2 (Table 1). It is worth highlighting that, under our applied experimental conditions, no pure rutile phase TiO₂ has ever surpassed the photocatalytic activity of Aldrich rutile before our TiO₂_HS_EtOH sample. The vastly superior photocatalytic activity of TiO₂_HS_EtOH could be attributed partly to the dominance of the rutile phase and its enhanced light-harvesting capability, conveyed by the unique morphology [15]. However, considering the similar synthesis pathway of TiO₂_HS_EtOH compared to TiO₂_HS_Ac (the only difference being the CS purification solvent), vastly different characteristics (crystal phase composition, morphology, and photocatalytic activity) were observed, emphasizing the importance of the CS purification solvent on the properties of the resulting photocatalysts. Thus, to elucidate the effect caused by the application of either acetone or ethanol, Fourier transform infrared (FT-IR) spectroscopy measurements were carried out on the CSs, the TiO₂-CS composites, and on the TiO₂-HSs to seek surface-related differences (Figure 4).

In the FT-IR spectra, a broad band centered at 3400 cm⁻¹ was observed together with a sharp band at 1630 cm⁻¹, which can be associated with adsorbed water and stretching/bending vibrations of surface OH groups, respectively [24,25]. In the titania-containing samples, the bands around 540 cm⁻¹ correspond to Ti–O bonds [26]. Bands at 2854, 2871, 2928, and 2963 cm⁻¹ were more profound in TiO₂_CS_Ac compared with TiO₂_CS_EtOH, which were attributed to CH₂ symmetric stretching, CH₃ symmetric stretching, C–H stretching, and CH₃ asymmetric stretching vibrations, respectively [27]. Most importantly, in TiO₂_HS_Ac, an additional band (i.e., a shoulder, partly covered by the Ti–O fingerprint region) was observed at 802 cm⁻¹, compared with TiO₂_HS_EtOH. This can be attributed to the presence of carbonate species in the TiO₂ lattice, which is characteristic of carbon-modified crystalline titania [28,29,30]. Based on these measurements, a plausible explanation was proposed for the differences that were observed between the TiO₂-HS samples.

The properties (i.e., structure, morphology, and surface) of CS_Ac and CS_EtOH were very similar. After coating them with TiO₂, a broad absorption band between 400–600 cm⁻¹ appeared, as expected, which can be attributed to the transverse optical vibrations of Ti–O bonds [26]. The slight difference between the intensity of the bands in the 2854–2963 cm⁻¹ region could not be attributed to the differences that were observed between the TiO₂-HS samples. However, from the slight difference between their SSAs (34 m²·g⁻¹ for TiO₂_CS_Ac and 27 m²·g⁻¹ for TiO₂_CS_EtOH), it can be assumed that the TiO₂ layer for TiO₂_CS_EtOH was more compact. Moreover, based on their X-ray diffractograms, it was observed that these samples contained both amorphous and crystalline titania, with mixed crystal phases and small crystallite sizes. It was expected that these differences would influence the properties of the resulting titania after calcination. Accordingly, differences were observed between TiO₂_HS_Ac and TiO₂_HS_EtOH in their morphology (TiO₂_HS_EtOH was more regular), crystal phase composition (TiO₂_HS_EtOH contained predominantly rutile, while TiO₂_HS_Ac contained predominantly anatase), and photocatalytic activity (TiO₂_HS_EtOH had much higher efficiency). Regarding the morphology, it was deduced that during calcination, CO₂ could be released from the less compact TiO₂_CS_Ac more rapidly, resulting in a higher number of damaged spheres for TiO₂_HS_Ac (Figure 2b). At the same time, due to the lower SSA of TiO₂_CS_EtOH, the elimination of CSs from the sample was slower, resulting in TiO₂_HS_EtOH with a more regular morphology (Figure 2d). The difference in their crystal phase composition can be explained by the presence of carbon in the titania lattice. Based on the publication of A. Matthews, the presence of carbonate ions facilitates the formation of the anatase crystal phase [31]. As CO₂ was released from TiO₂_CS_Ac more rapidly, it could therefore be incorporated into the crystal lattice of TiO₂_HS_Ac. This was reinforced by the IR measurements, as the band at 802 cm⁻¹ was more profound for TiO₂_HS_Ac (Figure 4), indicating the presence of carbonate species in the titania lattice [28,29,30]. The crystal phase of the carbonate-containing titania samples in these publications was also predominantly anatase. Finally, the difference between the photocatalytic activity of TiO₂_HS_Ac and TiO₂_HS_EtOH (Figure 3) could also be attributed to the presence of carbonate species in TiO₂_HS_Ac. It is known that carbonate ions act as a scavenger for ·OH radicals [32,33], which are the most important reactive oxygen species for the degradation of phenol. The higher photocatalytic activity of TiO₂_HS_EtOH under visible light irradiation can also be attributed to the higher rutile content [15,23] and more regular hollow spherical morphology since it facilitates the utilization of light [15], as previously mentioned. In summary, it was demonstrated that a seemingly insignificant parameter (such as the purification solvent) can have a major influence on the properties of the resulting titania.

3. Materials and Methods

3.1. Materials

CS templates were prepared using ordinary table sugar (sucrose, Magyar Cukor Zrt., Koronás^TM, Kaposvár, Hungary), NaOH (Molar Chemicals, Halásztelek, Hungary; analytical reagent grade), acetone (Molar Chemicals, Halásztelek, Hungary; 99.96%), and ethanol (VWR Chemicals, Debrecen, Hungary; >96%). For the TiO₂ coating Ti(O-nBu)₄ (Sigma-Aldrich, Schnelldorf, Germany; reagent grade; 97%), HCl (VWR Chemicals, Debrecen, Hungary; 37%), H₂O₂ (Sigma-Aldrich, Schnelldorf, Germany; 30%), and ultrapure water (Millipore Milli-Q, Budapest, Hungary) were used. For the evaluation of the photocatalytic activity, phenol (Spektrum 3D, Debrecen, Hungary; analytical grade) was used.

3.2. Synthesis

CS templates were synthesized in accordance with our previous publications [21,34]. In a Teflon-lined stainless-steel autoclave (V_tot = 623 mL), an alkaline (pH = 12), 0.15 M sucrose solution (V_fill = 180.7 mL) was prepared and subjected to hydrothermal treatment (12 h, 180 °C; V_fill/V_tot = 29%). The as-prepared CSs were collected from the suspension and purified via centrifugation in 3 cycles with either acetone or ethanol to remove residual organic contaminants. For the purification of 1 g CS 80 mL solvent was used. Finally, the CSs were dried in air at 40 °C and ground in an agate mortar, prior to the coating process.

The synthesis method of the TiO₂ coating was based on our previous paper [21]. In total, 3.5 mL Milli-Q water, 1.3 mL H₂O₂, and 1.58 mL HCl were added to a beaker followed by the dropwise addition of Ti(O-nBu)₄ precursor at a constant rate of 1 mL·min⁻¹ under vigorous magnetic stirring. The bottom inorganic phase was separated via a separatory funnel after 60 min of phase separation; then, the previously prepared CS templates were added to it, which was then sonicated for 10 min to provide sufficient time for the coating process to take place. Lastly, the templates were eliminated from the as-prepared TiO₂-CS composites via calcination using a tube furnace (Thermolyne 21100; Thermo Fischer Scientific, Prague, Czech Republic) at 400 °C for 4 h. During this process, no additional air supply was provided. The solid white powder was then collected and ground in agate mortar prior to the textural characterization and photocatalytic activity experiments. Herein the samples are referred to as “TiO₂_HS_X”, where X is the applied solvent (“Ac” for acetone and “EtOH” for ethanol) and ‘HS’ stands for ‘hollow spheres’.

3.3. Characterization Methods and Instrumentation

Crystalline composition of titanium dioxide samples was investigated by XRD using a Rigaku Miniflex II diffractometer (Rigaku, Neu-Isenburg, Germany) with the following parameters: λ_{Cu Kα} = 0.15406 nm, 40 kV, and 30 mA, 20–40 (2θ°) region. Average primary crystal sizes were calculated using the Scherrer equation. Anatase–rutile weight fractions were estimated from their corresponding peak areas at 25.3 (2θ°) and 27.5 (2θ°), respectively. Morphology of the samples was observed via scanning and transmission electron microscopy (SEM and TEM) using a Hitachi S-4700 Type II (Hitachi, Tokyo, Japan) and a FEI TECNAI G2 20 X-Twin device (FEI, Hillsboro, OR, USA), respectively. The specific surface area was calculated following N₂ adsorption measurements, carried out with a BELCAT-A device (Microtrac MRB, Osaka, Japan) using the Brunauer–Emmett–Teller (BET) method. Surface properties were investigated with a Jasco 6000 FT-IR (Jasco, Tokyo, Japan) spectrometer. The spectra were recorded applying 4 cm⁻¹ spectral resolution in the 400–4000 cm⁻¹ range.

3.4. Photocatalytic Activity Experiments

Photoactivity of the titania samples was investigated via the photocatalytic decomposition of phenol model pollutant (c₀ = 0.1 mM) under visible light irradiation. The experiments were carried out in a double-walled glass vessel that was surrounded by four energy-saving, compact fluorescence lamps (Düwi 25920/R7S, 24W; Düwi Kft., Budapest, Hungary). The light intensity, which was measured by iron(III) oxalate actinometry, was I_Vis = 1.07 ± 0.03 × 10⁻⁵ einstein∙L⁻¹∙s⁻¹. The spectrum of the lamps was slightly modified by circulating 1 M NaNO₂ aqueous solution in the thermostat jacket (T = 25.0 °C). NaNO₂ solution absorbs ultraviolet (UV) photons; therefore, solely visible light irradiation was provided (Figure S2). The photocatalyst suspensions were stirred in the dark for 30 min to reach adsorption/desorption equilibrium. Constant magnetic stirring and air supply were provided during the tests. The concentration of phenol was measured via a high-performance liquid chromatograph (HPLC) equipped with a Lichrospher RP 18 column (Merck, Darmstadt, Germany) using 50–50% methanol/water mixture as eluent (λ_detection = 210 nm). As references, our homemade “Rutile-H2” sample (a peroxo group-containing rutile TiO₂ with high visible light excitability [35]) and commercial Sigma Aldrich (Schnelldorf, Germany) rutile and Aeroxide P25 were used.

4. Conclusions

The solvents used for the purification of the templates had a large influence on the properties of the resulting titania. Using acetone as solvent resulted in anatase phase TiO₂ with no well-defined morphology, whereas the application of ethanol resulted in rutile phase TiO₂ with well-defined hollow spherical morphology. The photocatalytic activity of the latter surpassed the efficiency of each investigated titania, both home-made and commercial references. The enhanced photocatalytic activity was attributed to the enhanced light-harvesting capability conveyed by the unique morphology to the crystal phase composition and the differences in the surface properties of the titania. Based on the infrared spectroscopy results, the purification solvent was identified to be a determining factor. It influenced the presence/absence of carbonate ions on the titania surface, which was deduced to be a significant parameter in shaping the characteristics of the resulting photocatalysts.