Photocatalytic Oxidation of Acetone Over High Thermally Stable TiO2 Nanosheets With Exposed (001) Facets

Anatase TiO₂ (A-TiO₂) usually exhibits superior photocatalytic activity than rutile TiO₂ (R-TiO₂). However, the phase transformation from A-TiO₂ to R-TiO₂ will inevitably happens when the calcination temperature is up to 600°C, which hampers the practical applications of TiO₂ photocatalysis in hyperthermal situations. In this paper, high energy faceted TiO₂ nanosheets (TiO₂-NSs) with super thermal stability was prepared by calcination of TiOF₂ cubes. With increase in the calcination temperature from 300 to 600°C, TiOF₂ transforms into TiO₂ hollow nanoboxes (TiO₂-HNBs) assembly from TiO₂-NSs via Ostwald Rippening process. Almost all of the TiO₂-HNBs are disassembled into discrete TiO₂-NSs when calcination temperature is higher than 700°C. Phase transformation from A-TiO₂ to R-TiO₂ begins at 1000°C. Only when the calcination temperature is higher than 1200°C can all the TiO₂-NSs transforms into R-TiO₂. The 500°C-calcined sample (T500) exhibits the highest photoreactivity toward acetone oxidation possibly because of the production of high energy TiO₂-NSs with exposed high energy (001) facets and the surface adsorbed fluorine. Surface oxygen vacancy, due to the heat-induced removal of surface adsorbed fluoride ions, is responsible for the high thermal stability of TiO₂-NSs which are prepared by calcination of TiOF₂ cubes.

Introduction

Semiconductor photocatalysis has attracted much attention due to its potential applications such as water (Regmi et al., 2018; Xu et al., 2018) and air purification (Wen et al., 2015; Cui et al., 2017; Li et al., 2017a; Qi et al., 2017) and water splitting for clean H₂ energy (Cheng et al., 2018) due to its peculiar chemical and physical properties. As a typical semiconductor photocatalyst, anatase TiO₂ (A-TiO₂) usually shows excellent photocatalytic activity. However, it usually transforms into poor photoreactive rutile TiO₂ (R-TiO₂) when calcination temperature is higher than about 600°C, which hampers the practical applications of A-TiO₂ in hyperthermal situations (Lv et al., 2011; Liang et al., 2017). For example, the temperature of some industrial waste gases after burning can be as high as 900–1000°C, which makes the photocatalytic oxidation technology lose its power due to the phase transformation of A-TiO₂ at such high temperature. There are many photocatalytically active stable TiO₂-coated ceramic materials which are used for the control of organic contaminants, including sanitary wares, bathroom tiles and self-cleaning glass. They require high processing temperatures and therefore need excellent stability at high temperature (Periyat et al., 2009). Thus, exploration of thermally stable TiO₂ photocatalyst is very important but this question remains unsolved.

As one of volatile organic compounds (VOCs), acetone is a widely used solvent especially in chemical plant. The emission of acetone can not only cause the problems to global environment, but also bring harms to human health (Zhu et al., 2016). When the concentration of acetone vapor is higher than 500 ppm, it can irritate eyes and discomfort respiratory system (Li et al., 2018). So, it is of great importance to explore an efficient method to decompose of acetone. Because semiconductor photocatalysis is considered as an environmentally benign way, it is no doubt very promising for VOCs removal under normal conditions.

It is generally accepted that the quantum efficiency of photocatalysis is highly related to the physical properties of TiO₂ such as crystal structure, relative crystallinity, size of the particle, and specific surface area (Xu et al., 2007; Lan et al., 2015; Wang et al., 2015, 2016; Wen et al., 2015; Li et al., 2016; Sajan et al., 2016; Liang et al., 2017; Xia et al., 2017; Lin et al., 2018a,b). Recently, growing interesting has been devoted to the synthesis of TiO₂ nanocrystals with exposed high energy {001} facets by surface fluorination of TiO₂ to reduce the surface energy (Yang et al., 2008, 2009; Chen et al., 2010; Yu et al., 2014; Sajan et al., 2016; Liang et al., 2017). On considering that TiOF₂ cubes are fluorine-containing materials, they prefer to transform into high energy TiO₂ nanocrystals during calcination. After the removal of surface adsorbed fluorine ions at high temperature, the formed oxygen vacancy (OV) on the surface of the photocatalyst will prevent the fusion of neighboring A-TiO₂ nanocrystals, which is believed to improve the thermal stability of TiO₂ nanocrystals. Herein, we systematically studied the dependence of the structure and photocatalytic activity of TiOF₂ cubes on the calcination temperature. Acetone was selected as the targeted VOCs to evaluate the photoreactivity of the as-prepared photocatalyst.

Experimental Section

Synthesis

Precursor TiOF₂ was synthesized through the solvothermal reaction of Tetrabutyl titanium (TBT), acetic acid (HAc) and hydrofluoric acid (HF) (Huang et al., 2013). Typically, 20.0 g of TBT was dropwise added into the mixed solution containing 6.4 ml of HF (47 wt%) and 40.0 ml of HAc under magnetic stirring. The resulted white suspensions were transferred to an autoclave with volume of 100 ml, which was then kept at 200°C for 2 h. The white deposition was collected and washed with ethanol and distilled water until the filtrate is neutral (pH7). After oven dry at 80°C, we obtained the precursor TiOF₂.

Precursor TiOF₂ was then calcined at certain temperature (300-1200°C) for 2 h by keeping the same heating rate (5°C min⁻¹). The obtained photocatalyst is denoted as Tx (Table 1), where x is the calcination temperature. For example, T500 is that the photocatalyst which was prepared by calcination of the precursor TiOF₂ at 500°C for 2 h.

TABLE 1

Table 1. Physical property of the photocatalyst.

Characterization

XRD patterns of the photocatalysts were performed on a X-ray diffractometer (D8- advance, Bruker Co., German), and the scan rate of Cu Ka radiation keeps 0.02° 2θ s⁻¹, using an accelerated voltage and applied current of 15 kV and 20 mA, respectively. We observe the morphology of the prepared photocatalyst by an FESEM with an acceleration voltage of 20 kV (Hitach, Japan) and a TEM (Tecnai G20, USA) using an acceleration voltage of 200 kV, respectively. The optical property of the photocatalyst was measured by a spectrophotometer (UV-2550, Shimadzu, Japan) from 200 to 800 nm using BaSO₄ as background. FTIR was obtained on a infrared spectrometer (NeXUS 470) using the KBr pellet technique. XPS was recorded using monochromatic Al-Ka radiation under vacuum at 2 × 10⁻⁶ Pa on a photoelectron spectrometer (VG Multilab 2000). The C1s peak at 284.8 eV originated from the surface adventitious carbon is used to reference all the binding energies. EPR signal of the photocatalyst was recorded in an EPR spectrometer (JES-FA 200, JEOL) at room temperature (frequency of 100 kHz and microwave power of 0.99 mW). Nitrogen sorption isotherm was measured on an ASAP 2020 nitrogen adsorption equipment (Micromeritics, USA). Before investigating the surface areas of the photocatalysts, all samples were degassed firstly at 200°C.

Photoelectrochemical Measurements

We use CHI760e as electrochemical workstation (Shanghai, China) to measure the transient photocurrent, EIS Nyquist plots and Mott-Schottky plots in a standard there-electrode system, where Pt wire was used as the counter electrode, and the prepared samples and Ag/AgCl in saturated KCl were used as the working and reference electrode, respectively. During the measurement, 0.4 M Na₂SO₄ was used as electrolyte solution. In the Mott-Schottky measurement, direct current potential polarization was kept at a fixed frequency, and the working electrode was prepared on a glassy carbon electrode. Before the test of photocurrent, 50 mg photocatalysts and 30 μL Nafion were dispersed into 1 mL water/absolute ethanol mixed solvent (v/v = 1/1), and then the mixed aqueous solution was dispersed uniformly through ultrasound to form a homogeneous catalyst colloid. The ITO/TiO₂ electrode was prepared using the as-prepared photocatalyst colloid as precursor by a drip coating method. The light source is a 3W LED lamp (Shenzhen LAMPLIC, China) emitted mainly at 365 nm. The intensity for the lamp at working distance is measured to be 0.41 W/cm².

Evaluation of the Photocatalytic Activity

Photocatalytic oxidation of gasous acetone was used to evaluate the photocatalytic activity of the photocatalyst, which was performed in a 15 L reactor at ambient temperature under UV light irradiation. 0.3 g of the powder was firstly dispersed in 30 mL of double distilled water by sonicating treatment for 5 min. The obtained suspensions were then evenly transferred into three glass dishes in diameters of about 7.0 cm. After drying at 80°C for about 2 h, the dishes that have been coated with a layer of the photocatalyst were placed in the reactor, and then 10 uL of acetone was injected into the reactor by a microsyringe. The vaporated acetone then begins to adsorb on the surface of the photocatalyst. Thirty minutes later, the adsorption–desorption equilibrium of acetone can be established. The photocatalytic oxidation of acetone begins after turning on the UV light, which is 5 cm above the dishes. The concentrations of acetone and the produced carbon dioxide in the reactor were determined online with a Photoacoustic IR Multigas Monitor (Model 1412, INNOVA). Before irradiation, the initial concentration of acetone after the adsorption equilibrium was about 300 ppm, which almost keeps unchanged for about 5 min before lighting a UV lamp (15W@365 nm).

Results and Discussion

Effect of Calcination Temperature on Phase Structure

Phase structure of the photocatalyst is important to the photoreactivity. Therefore, we used XRD to study the phase evolution of the photocatalyst during calcination. From Figure 1, a broad peak centering at 2θ = 23.4°, which corresponds to the (100) plane diffraction of TiOF₂, was observed for the prepared precursor, and no any peak of TiO₂ phases (A-TiO₂ and R-TiO₂) exists, indicating the successful synthesis of TiOF₂(Huang et al., 2013). After calcination at 300°C for 2 h, the phase struccture of the sample (T300) almost keeps unchanged. With increase in the calcination temperature to 400°C, the peak intensity for TiOF₂ decreases. Simultaneously, a small peak at 2θ = 25.3°, which corresponds to the (101) plane diffraction of A-TiO₂, can be observed for T400 sample, indicating that phase transformation from TiOF₂ to anatase begins. This phase transformation becomes obvious after calcination of TiOF₂ at 500°C, and the prepared TiOF₂ totally transforms into anatase TiO₂ at 600°C.

FIGURE 1

Figure 1. XRD patterns of the photocatalysts calcined at different temperatures.

It is proposed that the reaction for the heat-induced phase transformation from TiOF₂ to TiO₂ is as follows (Equation 1) (Zhao et al., 2018).

$\begin{array}{cc}{2\text{TiOF}}_2+\text{heat}\to {\text{TiO}}_2+{\text{TiF}}_4\uparrow & (1)\end{array}$

Further increase in the calcination from 600 to 900°C, the peak intensity of A-TiO₂ increases, indicating the enhanced crystallization. Meanwhile, the narrowing of the width for the (101) plane diffraction peak indicates the increase of A-TiO₂ crystallite size. The peak intensity for the (101) peak of anatase TiO₂ begins to decrease at calcination temperature of 1000°C, which is a sign of anatase-to-rutile phase transfermation. The formation of rutile phase is confirmed from the XRD pattern of the T1100 sample due to the formation of a small peak at 2θ = 27.3°, which corresponds to the (110) plane diffraction of R-TiO₂. Only when the calcination temperature reaches 1200°C can all of the A-TiO₂ transform into R-TiO₂.

Usually, A-TiO₂ nanocrystals begin transform into R-TiO₂ at about 600°C. However, in the present study, the temperature for anatase-to-rutile transformation is as high as 1100°C, which indicates that these samples are promising to be used in hyperthermal situations.

Evolution of the Morphology

The SEM images of the precursor TiOF₂ is shown in Figure 2. It can be clearly observed that these TiOF₂ nanoparticles are in cubic shape and relatively monodispersed (Figure 2a), which is consistent with the reported literatures (Chen et al., 2012; Wang et al., 2012). From the high resolution SEM image shown in Figure 2b, we can estimate that the sidelength of the TiOF₂ cube is about 250 nm.

FIGURE 2

Figure 2. SEM images of the precursor TiOF₂ at low (a) and high magnification (b).

After calcination of the precursor TiOF₂ at 300°C, the morphology of the resulted sample (T300) almost keeps unchanged (SEM image not shown here). However, some nanoboxes with hollow interiors can be clearly seen from the SEM image of T400 sample (Figure 3a). These anatase TiO₂ hollow nanoboxes (TiO₂-HNBs) assemble from TiO₂ nanosheets (TiO₂-NSs) with exposed high energy (001) facets (Wen et al., 2011). The thickness of the TiO₂-NSs is about 30 nm. It has been proven that high energy TiO₂ nanosheets with exposed (001) facets can be prepared by using fluoride ions as shape-directing reagent (Yang et al., 2008; Lv et al., 2012). Then, it is not hard to understand the formation of high energy TiO₂-NSs during calcination of TiOF₂, a kind of fluorine-containing materials.

FIGURE 3

Figure 3. Comparison of the SEM images for T400 (a,b), T500 (c,d), T600 (e,f), T700 (g,h), T1100 (i,j), and T1200 (k,l) samples.

Since the sidelength of the obtained TiO₂-HNBs is similar as these precursor TiOF₂ cube, it is proposed that the formation of TiO₂-HNBs assembly from TiO₂-NSs is through a Ostwald Rippening process (Lou et al., 2008; Huang et al., 2013).

After calcination at 500°C, most of the TiOF₂ cubes transform into TiO₂-HNBs (Figure 3c). Figure 3d shows an enlarged SEM image of a broken TiO₂ hollow nanobox, from which we can see that the thickness of the TiO₂ nanosheet increases to about 50 nm, much thicker than that of T400 sample.

The structure of TiO₂-HNBs was further confirmed by the corresponding TEM image of T500 sample (Figure 4a), from which we can observe the presence of some erected TiO₂-NSs and a TiOF₂ cube that has not totally transformed into anatase TiO₂. This is consistent with the corresponding XRD characterization result (Figure 1d). From the side view HRTEM image of a discrete TiO₂-NS (Figure 4b), we can clearly see the lattice spacing of 0.235 nm that parallels to the top and bottom facets, which corresponds to the (001) planes of A-TiO₂ (Han et al., 2009). This confirms that the obtained TiO₂-HNBs are assembly from high energy TiO₂-NSs with exposed (001) facets (Wang et al., 2010).

FIGURE 4

Figure 4. TEM image (a) and high-resolution TEM image (b) of T500 sample, arrows in (a) indicating the presence of TiOF₂ cube and TiO₂ nanosheet (TiO₂-NS).

When the calcination temperature increases to 600°C, we can see that almost all of the TiO₂-HNBs decompose into discrete TiO₂-NSs (Figure 3e). From the truncated biypramidal shape of a TiO₂ nanocrystals (Figure 3f), we can estimate that the thickness of the TiO₂-NSs is about 100 nm (T600 sample), which increases to ca. 150 nm for T700 sample (Figures 3g,h). Even calcined at 1100°C, some TiO₂ nanosheets still keep bipyramidal shape (decahedron) with exposed (001) and (101) facets (Figures 3i,j), further indicating the thermal stability of high energy TiO₂-NSs.

The bipyramidal shapes of TiO₂ nanostructures disappear (Figures 3k,l) when calcination of TiOF₂ cubes at 1200°C. This can be explained by the sintering of the sample due to phase transformation from A-TiO₂ to R-TiO₂ (Figure 1).

By comparing the morphologies of TiO₂-NSs from Figure 3, we can clearly see that heat treatment of TiOF₂ cubes only results in the growth of TiO₂ nanosheet along (001) direction. Scheme 1 illustrates the morphology evolution and phase transformation of TiOF₂ cube during calcination.

SCHEME 1

Scheme 1. Morphology evolution and phase transformation of TiOF₂ cube during calcination at different temperatures: (a) 300°C, (b) 400°C, (c) 500°C, (d) 600°C, (e) 700°C, (f) 1100°C, and (g) 1200°C.

Uv-vis Absorption and FTIR Spectra

Light-harvesting ability plays a very important role on the photoreactivity of the photocatalyst (Li et al., 2017b). Therefore, we compared the UV-vis absorption spectra of the samples. It can be seen that from Figure 5 that, when calcination temperature is below 1100°C, all samples possess similar absorption spectra (Figure 5). The onset of the UV-vis absorption spectrum for T400 sample is at 389 nm, corresponding to a bangap of 3.19 eV. However, the absorption edge was obviously red-shifted for T1200 sample. The onset of the spectrum for T1200 sample begins at 424 nm, corresponding to a bandgap of 2.92 eV, which can be ascribed to the phase transformation (Figure 1).

FIGURE 5

Figure 5. UV-vis absorption spectra of the photocatalysts.

Figure 6 compares the FTIR spectra of the photocatalysts treated at different calcination temperature. From which, it can be seen that all samples exhibit strong absorption peaks cenerting at about 3,427, 1,628, 1,403, and 554 cm⁻¹. The peaks of 3,427 and 1,628 cm⁻¹ originate from the -OH groups/H₂O due to the adsorption of moisture from the air, while the peaks centering at about 1403 and 554 cm⁻¹ originate from the vibration of Ti-O and O-Ti-O. Carefully view shows that there is a strong absorption peak at 978 cm⁻¹, which decreases with increasing in the calcination temperature. This peak was attributed to the vibration of Ti-F bond of TiOF₂. When calcination temperature increases to 600°C, the vibration of Ti-F disappears because of the complete phase transformation of TiOF₂-to-anatase TiO₂ (Figure 1; Zhao et al., 2018).

FIGURE 6

Figure 6. FTIR spectra of the photocatalysts.

Analysis of XPS Spectrum

Figure 7A compares the XPS survey spectra of photocatalysts for T400, T500, and T600 photocatalysts. From which, it can be observed that the XPS spectra are similar. All these samples contain titanium (Ti), oxygen (O), fluorine (F), and carbon (C) elements. The C element originates from the adventitious hydrocarbon from the XPS instrument itself. We can also be clearly see that the peak intensity of F element tends to decrease with increasing the calcination temperature from 400 to 600°C. The atomic ratios of F:Ti were determined to be 1.19 for T400 sample, 0.50 for T500 sample, and 0.17 for T600 sample, respectively. The steady decrease in F content with increase in the calcination is because of the TiOF₂ to A-TiO₂ phase transformation (Figure 1 and Equation 1), and the removal of adsorbed fluoride ions on the surface of A-TiO₂.

FIGURE 7

Figure 7. XPS survey spectra of the photocatalysts (A), and the corresponding high resolution XPS spectra in Ti 2p (B), O 1s (C), and F 1s (D) regions, respectively.

Figures 7B,C show the high resolution XPS spectra in Ti 2p and O 1s regions, respectively. It can be seen that, both the binding energies for Ti 2p and O 1s of the samples steady decrease with increasing the calcination temperature. This is attributed to the TiOF₂-to-TiO₂ phase transformation (Figure 1 and Equation 1), and the formation of oxygen vacancy due to the removal of surface adsorbed fluoride ions over TiO₂ upon calcination (Lv et al., 2011). It has been accepted that almost all of the fluoride ions adsorbed on the surface of high energy TiO₂ nanosheets can be removed after calcination at 500°C.

The F 1s binding energy peak centering at about 684 eV (Figure 7D) is designated to the surface fluoride (Ti-F). No peak with binding energy of about 688.5 eV, corresponding to the lattice F⁻ of TiO₂, is found in all the photocatalysts, indicating that calcination of TiOF₂ cannot results in the doping of fluorine into anatase TiO₂ (Yu et al., 2002). We can also clearly see that the peak intensity of F 1s steady decreases with increasing the calcination temperature, this can be attributed to the heat-induced phase transformation from TiOF₂ to A-TiO₂ and the desorption of surface adsorbed fluoride ions (Yang et al., 2008).

Photocatalytic Oxidation of Acetone

Photocatalytic oxidation of acetone was used to monitor the photoreactivity of the prepared photocatalyst. The reaction for the photocatalytic oxidation of acetone is as follows (Equation 2).

$\begin{array}{cc}{({\text{CH}}_3)}_2\text{CO}+{\text{4O}}_2={\text{3CO}}_2+{\text{3H}}_2\text{O}& (2)\end{array}$

Figure 8A compares the relative photoreactivity of the photocatalyst by calculation the decomposed acetone within 2 h. It was found that the photocatalytic activity of the precursor TiOF₂ can be neglected (only 4.0 ppm acetone was decomposed). The poor photocatalytic activity of TiOF₂ is possibly due to its easy recombination of photo-generated carriers.

FIGURE 8

Figure 8. Comparison of the photoreactivity of the photocatalyst (A), and the recycling use of T500 sample (B) in photocatalytic oxidation of acetone.

The photocatalytic activity of the photocatalyst increases with increase in the calcination temperature from 300°C (12.8 ppm) to 500°C (102.5 ppm), which is due to the production of A-TiO₂. When further increase in the heat temperature from 600°C (94.6 ppm) to 1100°C (25.4 ppm), the photocatalytic activity of the photocatalyst steady decreases, possibly due to the decreased specific surface area (Table 1). Only 5.9 ppm of acetone was found to decompose when T1200 sample is used as photocatalyst. This is because of the complete A-TiO₂ to R-TiO₂ phase transformation (Figure 1) and smallest BET surface area (0.4 m²g⁻¹).

Reusability of the photocatalyst is also important from the viewpoint of the practical applications (Liu et al., 2010). Therefore, we monitored the recycling use of T500 sample for 5 times in acetone oxidation (Figure 8B). It can be seen that no obvious reactivity decrease was observed for T500 sample even continual use for 5 times, indicating that T500 sample is potential to be used in practical applications.

Mechanism

The value of photocurrent is usually used to evaluate the ability to generate and transfer of charge carriers for illuminated semiconductor photocatalyst (Cheng et al., 2018; Huang et al., 2018). The photocurrent of the prepared photocatalyst was therefore tested for several on-off cycles (Figure 9). It can be seen that, when irradiation of the TiO₂ film electrodes, obvious prompt photocurrent signals are produced, which exhibit good reproducibility. When the lamp is turned off, the photocurrent value for all the TiO₂ film electrodes are instantaneously close to zero. We can also see, with increasing the calcination temperature from 400 to 600°C, the photocurrent of the photocatalyst increases first and then decreases. The photocurrent of T500 sample is the as high as 2.8 uAcm⁻², which is much higher than that of T400 (1.1 uAcm⁻²) and T600 (2.3 uAcm⁻²) samples. The photoreactivity of semiconductor photocatalyst is closely related to the efficiency of the separation of the photo-generated electron-hole pairs (Fu et al., 2008). So it is safe to predict that the photoreactivity of T500 is higher than that of T400 and T600 samples, which keeps in line with the experimental results (Figure 8).

FIGURE 9

Figure 9. Photocurrents of the photocatalysts.

It has been accepted that fluoride ions show strong affinity to titanium (Equation 3), and the photoreactivity of TiO₂ nanocrystals can be obviously improved after the introduction of surface fluorination (Minero et al., 2000; Xu et al., 2007; Cheng et al., 2008). According to the study of Yu et al., the difference in surface energy makes the photo-generated electrons and holes migrate to (101) and (001) facets, respectively (Yu et al., 2014). The CB electrons aggregated in (101) facets are captured by surface adsorbed oxygen to produce super oxygen radicals (${\text{O}}_2^{·-}$), while the VB holes aggregated on the (001)facets are transfer into hydroxyl radicals (·OH). Both ${\text{O}}_2^{·-}$ and ·OH are important reactive oxygen species (ROSs) for the degradation of organics. When compared with pristine TiO₂, surface fluorination of TiO₂ changes the state of the formed hydroxyl radicals from surface bounded ·OH radicals (Equation 6) to mobile ·OH radicals (·OH_free). This is because the displacement of fluoride to surface OH⁻ groups (Equation 3) induces the direct oxidization solvent water by holes (Equation 7).

$\begin{array}{cc}\equiv \text{Ti}-\text{OH}+{\text{F}}^{-}\to \equiv \text{Ti}-{\text{F+OH}}^{-}& (3)\end{array}$

$\begin{array}{cc}{\text{TiO}}_2+\text{hv}\to {\text{TiO}}_2({\text{h}}^{+}···{\text{e}}^{-})& (4)\end{array}$

$\begin{array}{cc}{\text{O}}_2+{\text{e}}^{-}\to {{\text{O}}_2}^{·-}& (5)\end{array}$

$\begin{array}{cc}\equiv \text{Ti}-\text{OH}+{\text{h}}^{+}\to \equiv \text{Ti}···\text{OH}& (6)\end{array}$

$\begin{array}{cc}\equiv \text{Ti}-\text{F}+{\text{H}}_2\text{O}+{\text{h}}^{+}\to \equiv \text{Ti}-\text{F}+·{\text{OH}}_{\text{free}}& (7)\end{array}$

As free ·OH radical are more active than surface bounded ·OH radicals, the oxidation of acetone into CO₂ and H₂O is greatly enhanced due to the attacks of super oxygen radicals and free ·OH radicals (Equation 8). The proposed mechanism for the enhanced photoreactivity of surface fluorinated TiO₂ nanosheet toward acetone oxidation is shown in Scheme 2.

$\begin{array}{cc}{\text{C}}_3{\text{H}}_6\text{O}+{{\text{O}}_2}^{·-}/·{\text{OH}}_{\text{free}}\to {\text{CO}}_2+{\text{H}}_2\text{O}& (8)\end{array}$

SCHEME 2

Scheme 2. Proposed mechanism for the enhanced photocatalytic activity of surface fluorinated TiO2 nanosheets with exposed high energy (001) facets toward acetone oxidation.

Recently, significantly growing interest has been devoted to studying of hierarchical nanostructures due to their unique properties and widespread potential applications (Lou et al., 2008). For example, when compared with solid spheres, TiO₂ hollow spheres usually shows much higher photocatalytic activity mainly because they possess better light-harvesting ability (Li et al., 2009). In the present study, the hierarchically structured T500 sample, that is TiO₂ hollow nanobox assembly from TiO₂ nanosheets should also benefit the use of light (Figure 5), enhancing the photoreactivity.

However, the photoreactivity of TiO₂-NSs steady decrease with increase in the calcination temperature from 500 to 1100°C due to the collapse of the hierarchical TiO₂ hollow nanobox and the removal of surface adsorbed fluorine, leaving surface oxygen vacancy (Figure 8; Cheng et al., 2018).

$\begin{array}{cc}\equiv \text{Ti}-\text{F}+\text{heat}\to \equiv \text{Ti}-\square & (9)\end{array}$

The formation of surface oxygen vacancy was proved by electron paramagnetic resonance (EPR) technique. It was found that the signal intensity for oxygen vacancy of the photocatalyst increases with increase in the calcination temperature from 400 to 600°C (Figure 10) due to the heat-induced desorption of surface adsorbed fluoride ions (Equation 9).

$\begin{array}{cc}\equiv \text{Ti}-\text{OH}+\text{HO}-\text{Ti}\equiv \to \equiv \text{Ti}-\text{O}-\text{Ti}\equiv +{\text{H}}_2\text{O}& (10)\end{array}$

$\begin{array}{cc}\equiv \text{Ti}-\square +\square -\text{Ti}\equiv \to \text{no}\text{reaction}& (11)\end{array}$

It was believed that only when the crystalline size of the nanocrystal is larger than a critical size can phase transformation begin (Padmanabhan et al., 2007; Periyat et al., 2008, 2009). Therefore, the growth of TiO₂ nanocrystal is a prerequisite before the phase transformation of A-TiO₂ to R-TiO₂. The growth of pristine TiO₂ nanocrystal is relatively easy by formation of =Ti-O-Ti= chain between two neighboring TiO₂ nanoparticles (Equation 10). However, the growth of the =Ti-O-Ti= chain is prevented due to the formation of surface oxygen vacancy (Equation 11). Only when the lattice oxygen is diffused from the bulk to the surface of TiO₂ nanosheet with oxygen vacancy at high temperaure can the fusion of neighboring TiO₂-NSs become possible (Lv et al., 2011). Therefore, it is not hard to understand the super thermal stability of TiO₂-NSs prepared by calcination of TiOF₂ cubes.

FIGURE 10

Figure 10. EPR spectra of the photocatalysts.

After the removal of surface adsorbed fluoride ions by calcination, these TiO₂-NSs prefer to aggregate and grow along (001) direction to reduce the high surface energy (Lv et al., 2011). Then we can clearly observe the steady increase in the thickness of TiO₂-NSs (Figure 3).

Conclusions

TiO₂ nanosheets with high thermal stability were prepared by calcination of TiOF₂ cubes. The anatase-to-rutile phase transformation temperature reaches as high as 1100°C. 500°C-calcined sample shows the highest photoreactivity toward acetone oxidation due to the surface fluorination. The high thermal stability of TiO₂ nanosheets is ascribed to the introduction of surface oxygen vacancy after removal of the surface adsorbed fluoride ions, which prevents the growth of TiO₂ nanosheets. The present study provide a novel way in design of thermally stable materials.

Author Contributions

TS and YD performed the experiments. KL planned the project, designed the experiments, and wrote the manuscript. ZH, QL and XL assisted in the analysis and interpretation of the data. ML revised the manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51672312, 21571192 and 21373275), the Science and Technology Program of Wuhan (2016010101010018) and Fundamental Research Funds for the Central Universities, South-Central University for Nationalities (CZT18016).

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