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. 2021 Sep 28;6(41):27121–27128. doi: 10.1021/acsomega.1c03762

Facile Vacuum Annealing-Induced Modification of TiO2 with an Enhanced Photocatalytic Performance

Zhenpeng Cui †,‡, Min Zhao ‡, Xueyan Que §, Jingjing Wang ‡, Yang Xu ‡, Mohamed Nawfal Ghazzal ∥, Christophe Colbeau-Justin ∥, Duoqiang Pan †,‡,*, Wangsuo Wu †,‡
PMCID: PMC8529652  PMID: 34693132

Abstract

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In this work, the photocatalytic performance enhancement of hydrothermally prepared TiO2 was achieved by facile vacuum annealing treatment. Calcination of TiO2 powder in air (CA-TiO2) maintained its white color, while gray powder was obtained when the annealing was performed under vacuum (CV-TiO2). Fourier transform infrared, total organic carbon, X-ray photoelectron spectroscopy, and electron paramagnetic resonance analyses proved that vacuum annealing transformed ethanol adsorbed on the surface of TiO2 into carbon-related species accompanied by the formation of surface oxygen vacancies (Vo). The residual carbon-related species on the surface of CV-TiO2 favored its adsorption of organic dyes. Compared with TiO2 and CA-TiO2, CV-TiO2 exhibited an improved charge carrier separation with surface Vo as trapping sites for electrons. Vacuum annealing-induced improvement of crystallinity, enhancement of adsorption capacity, and formation of surface Vo contributed to the excellent photocatalytic activity of CV-TiO2, which was superior to that of commercial TiO2 (P25, Degussa). Obviously, vacuum annealing-triggered decomposition of ethanol played an important role in the modification of TiO2. In the presence of ethanol, vacuum annealing was also suitable for the introduction of Vo into P25. Therefore, the current work offers an easy approach for the modification of TiO2 to enhance its photocatalytic performance by facile vacuum annealing in the presence of ethanol.

Introduction

Since the discovery of the Fujishima–Honda effect in 1972,1 tremendous efforts have been devoted in investigating the potential applications of photocatalytic oxidation and reduction reactions.2,3 Due to its excellent stability, nontoxicity, and cost effectiveness, titanium dioxide (TiO2) has become one of the most studied photocatalysts.4−6 However, the photocatalytic performance of TiO2 is limited by its intrinsic wide band gap and fast recombination of photogenerated charge carriers.7,8 Thus, the enhancement of photocatalytic performance of TiO2 by modification is of great interest.5,6,9 The photocatalytic activity of TiO2 can be improved either by reducing its band gap to expand its photoresponse range from the UV to the visible range, which takes up the main proportion of solar light, or by preventing the recombination of photogenerated charge carriers by trapping or transfer.6,10,11 To promote its charge carrier separation ability, TiO2 is commonly coupled to other semiconductors with a smaller band gap or to metallic nanoparticles.12−15 The commercially available TiO2 (P25, Degussa) possesses high photocatalytic activity due to the mixed anatase and rutile phase, allowing efficient charge carrier transfer.16,17

In order to reduce its band gap, TiO2 is usually doped with metallic or nonmetallic elements to form an intermediate band state into its band gap, which extends the absorption properties of TiO2 to visible light.9,18,19 Asahi et al.20 reported the incorporation of nitrogen (N) into TiO2 for photocatalytic degradation of organic pollutants under visible light. Carbon (C)-doped TiO2 for efficient water splitting under visible light has been reported.21,22 C-doped TiO2 showed high crystallinity and a unique microstructure, contributing to its remarkable photocatalytic degradation ability.23 Self-doped TiO2 with titanium (Ti3+) was proposed and enabled an increase in the activity under visible light for efficient photocatalytic degradation of methylene blue (MB) and rhodamine B (RhB).24 Co-doped TiO2 with Ti3+ and N exhibited synergistic effects for photocatalytic water oxidation.25

Besides, the introduction of surface oxygen vacancy (Vo) into TiO2 was proposed as a credible alternative to improve the photocatalytic activity of TiO2.8,26−31 This straightforward method has been widely achieved by annealing treatment under a reducing atmosphere, and it can realize not only doping TiO2 but also creating defects, which improves the surface adsorption of the molecule and prolong the charge carrier lifetime.32 The resultant TiO2 photocatalyst, so-called “black titania”, was obtained upon a thermal treatment under high pressure of H2 and exhibited obvious visible light absorption.33 Due to the H doping of TiO2, the optical band gap energy (Eg) value of black titania was reduced to 1.54 eV by introducing electronic states forming between the valence band and conduction band. Other emerging methods, such as the annealing of TiO2 in anaerobic medium (without oxygen) or under vacuum, led to the formation of Ti3+ and Vo, the amounts of which determined the color of TiO2 (from gray to black).32,34,35 Since vacuum annealing avoids the use of highly flammable H2 gas, it offers safer working conditions and needs more investigations.

In this work, we performed vacuum annealing by sealing hydrothermally prepared TiO2 in a vacuumed glass tube. Meanwhile, calcination of TiO2 in air was also conducted for comparison. The variations of TiO2 before and after heat treatment were characterized in detail. Photocatalytic activities of TiO2 photocatalysts were checked by decomposing organic pollutants, and the differences of their photocatalytic activities were discussed.

Results and Discussion

Characterizations of TiO2 Photocatalysts

X-ray diffraction (XRD) and diffuse reflectance spectroscopy (DRS) measurements were performed to investigate the influences of heat treatment on the crystalline structure and optical band gap of TiO2, CA-TiO2, and CV-TiO2. All TiO2 photocatalysts show similar diffraction patterns (Figure 1a), attributed to the anatase phase of TiO2 (JCPDS-21-1272).36 Therefore, hydrothermally prepared TiO2 maintains the anatase phase after calcination in air and annealing under vacuum. The broad peak (indicated by the arrow, Figure 1a) in the diffraction pattern of CV-TiO2 is due to the formation of amorphous carbon.37 The sharper and narrower (101) peak of CA-TiO2 and CV-TiO2 indicates the enhancement of crystallization of TiO2 after heat treatment.38 Based on the full width at half maximum of (101) plan and Debye–Scherrer formula (eq S1),34 the average crystallite sizes of TiO2, CA-TiO2, and CV-TiO2 are estimated to be 9.9, 16.0, and 21.7 nm, respectively. DRS spectra (Figure 1b) show that all TiO2 photocatalysts absorb in the UV range (200–400 nm), and no visible light absorption can be observed. Compared with TiO2 and CA-TiO2, CV-TiO2 shows decreased absorbance, which is also observed in Chen and co-workers’ work.39 It is clear that both TiO2 and CA-TiO2 appear to be white (the common color of TiO2, Figure 1b, inset), while CV-TiO2 turns out to be gray, similar to carbon-doped TiO2.35 This obvious color difference indicates that annealing of TiO2 under vacuum produces gray calcinate.22,35 Vacuum annealing-induced formation of gray calcinate darkens the color of CV-TiO2. The resultant gray calcinate is composed of carbon-related species resulting from ethanol decomposition on the surface of TiO2, which will be proved by Fourier transform infrared (FT-IR), total organic carbon (TOC), and X-ray photoelectron spectroscopy (XPS) analyses later. The gray calcinate cannot be washed away. It is reported that C-doped TiO2 showed obvious absorption of visible light due to the C doping into the crystal structure of TiO2.23,35 Since CV-TiO2 shows no absorption of visible light, there is no doubt that vacuum annealing of TiO2 results in the formation of gray calcinate without C doping of CV-TiO2. When existing on the surface of CV-TiO2, the gray calcinate may impair the absorption of UV light by CV-TiO2. The Eg values were estimated from the intercepts of their corresponding Kubelka–Munk (K–M) plots (Figure 1c) to be 3.17, 3.18, and 3.20 eV for TiO2, CA-TiO2, and CV-TiO2, respectively.40 All Eg values are around 3.2 eV, which is in agreement with the intrinsic Eg of anatase.19 All TiO2 photocatalysts only absorb UV light due to the wide band gap, as they are composed of TiO2 with a crystal phase of anatase, which agrees with the XRD analysis (Figure 1a). Therefore, XRD and DRS analyses proved that calcination has no influence on the crystal phase and Eg values of all TiO2 photocatalysts but increases the crystallinity and sizes of CA-TiO2 and CV-TiO2.

Figure 1.

Figure 1

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XRD diffraction patterns (a), DRS spectra (b), K–M plots, and Eg values of (c) of TiO2, CA-TiO2, and CV-TiO2. The inset shows the digital photograph of the as-prepared TiO2, CA-TiO2, and CV-TiO2.

The microscopic structures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the images are shown in Figure 2. SEM images (Figure 2a–c) show that TiO2, CA-TiO2, and CV-TiO2 exhibit granular structures constructed by compactly aggregated nanoparticles. According to TEM images (Figure 2d–f), the average sizes of all TiO2 photocatalysts are in the order of TiO2 < CA-TiO2 < CV-TiO2 and estimated to be around 10, 15, and 20 nm, respectively. This result is in agreement with the estimated crystallite sizes by XRD analysis. The dimensions of CA-TiO2 and CV-TiO2 increase because heat treatment enables TiO2 nanoparticles to merge into bigger ones. Compared with CA-TiO2, CV-TiO2 shows larger particles in size, which indicates that vacuum annealing favors further fusion of TiO2 nanoparticles. Thus, it is obvious that heat treatment enlarges the average crystallite sizes of TiO2, especially vacuum annealing.

Figure 2.

Figure 2

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SEM and TEM images of TiO2 (a,d), CA-TiO2 (b,e), and CV-TiO2 (c,f).

The N2 adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution curves are employed to investigate the surface area and pore diameter of TiO2, CA-TiO2, and CV-TiO2. Figure 3 shows that the isotherms of all samples are of type II according to the IUPAC classification.32 Absorption of N2 by TiO2, CA-TiO2, and CV-TiO2 can be ascribed to the interaggregated pores, which are structured by the neighboring nanoparticles, as shown in the SEM images (Figure 2a–c).32,41 The surface areas and average pore diameters of TiO2, CA-TiO2, and CV-TiO2 are determined to be 128.30, 58.80, and 34.30 m2/g and 133.41, 272.35, and 478.60 Å, respectively. It is reasonable that the surface areas of all TiO2 photocatalysts decrease successively as their average crystallite sizes increase, which is verified by XRD (Figure 1a) and TEM (Figure 2d–f). The average pore diameters of all TiO2 photocatalysts gradually enlarge as heat treatment induces the fusion of TiO2 nanoparticles. Among all TiO2 photocatalysts, CV-TiO2 shows the smallest surface area and biggest pore diameter, corresponding to its largest average crystallite size, implying that vacuum annealing favors the fusion of TiO2 nanoparticles.

Figure 3.

Figure 3

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N2 adsorption–desorption isotherms of TiO2 (a), CA-TiO2 (b), CV-TiO2 (c), and the corresponding BJH pore size distribution curves (inset).

Energy dispersive X-ray spectroscopy (EDS) was conducted to determine the elemental composition of all TiO2 photocatalysts. As the EDS analysis was performed by depositing all TiO2 photocatalysts on aluminum foil, the influence of conductive paste, which contained C, was excluded. EDS spectra (Figure S2) of all samples clearly show the presence of O and Ti, further confirming that they are all composed of TiO2. FT-IR analysis was conducted to further analyze the chemical composition of all samples. FT-IR spectra (Figure 4a) of TiO2, CA-TiO2, and CV-TiO2 show bands at 3423 and 1635 cm–1, which are assigned to the stretching and bending vibrations of the O–H bond from adsorbed water or ethanol.42,43 The vibration of the O–Ti–O bond is observed in the range of 500–900 nm.42 The bands at 2972, 1430, and 1505 cm–1 in the FT-IR spectrum of TiO2 are attributed to C–H and C–O vibrations, which may originate from the residual ethanol.22,42 Ethanol can be stabilized on the surface TiO2 through hydrogen bonding interactions between the O–H and Ti–O bonds.43 The absence of C–H and C–O vibration bands in the FT-IR spectra of CA-TiO2 and CV-TiO2 is due to the evaporation and decomposition of ethanol during heat treatment. The total amount of organic carbon in each sample was detected by TOC. The TOC values of TiO2, CA-TiO2, and CV-TiO2 are measured to be 0.68, 0.00, and 0.26% (wt %), respectively. The presence of organic carbon (0.68%) undoubtedly demonstrates the adsorption of ethanol by TiO2. The absence of organic carbon (0.00%) in CA-TiO2 indicates the complete disappearance of ethanol. It is reported that the decomposition of ethanol occurs on the surface of TiO2 in the presence of oxygen (O2), forming carbon dioxide (CO2) and H2O.44,45 As a result, CA-TiO2 maintains the white color of TiO2 due to the complete removal of ethanol. The amount of organic carbon in CV-TiO2 (0.26%) decreases compared with that of TiO2 (0.68%). Under anaerobic conditions, the formation of gray calcinate (Figure 1b, inset) may result from the carbonization of ethanol by dehydration, which extracts lattice O from TiO2, leaving carbon-related species on its surface.44 Hence, heat treatment of TiO2 with adsorbed ethanol on its surface results in quite different appearances of CA-TiO2 and CV-TiO2.

Figure 4.

Figure 4

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FT-IR spectra (a), XPS spectra (b), fine XPS spectra of C (c), Ti (d), and O (e), and EPR spectra (f) of TiO2, CA-TiO2, and CV-TiO2.

To characterize the chemical structure and binding energy of all samples, XPS analysis was performed. XPS spectra (Figure 4b) of all TiO2 photocatalysts show characteristic peaks of C (1s), Ti (2s, 2p), and O (1s). The C 1s XPS spectra (Figures 4c and S3) show that the peaks of C–C (284.6 eV) and C–O (286.1 eV) decrease after heat treatment. As mentioned above, there is no carbon in CA-TiO2, and thus, its C (1s) peak must come from the adventitious carbon species.23 The decrease in the C–C peak (284.6 eV) proves that heat treatment induces evaporation and decomposition of ethanol adsorbed on the surface of TiO2 in air but leads to the carbonation of ethanol under vacuum, producing carbon-related species, as the C 1s peak height of CV-TiO2 is higher than that of CA-TiO2.23,46 There is no C–Ti peak (280.4 eV) in C 1s spectra (Figure S3), which suggests that no C is doped into the lattice of TiO2, which is in accordance with DRS analysis.22,23 Ti 2p XPS spectra (Figure 4d) exhibit almost the same Ti 2p3/2 (458.9 eV) and Ti 2p1/2 (464.6 eV), implying the unchanged binding state of Ti4+ after heat treatment.43 According to the O 1s XPS spectra (Figure 4e), the Ti–O–Ti peak (530.1 eV) is almost the same for TiO2, CA-TiO2, and CV-TiO2, while CV-TiO2 presents a new peak (532.1 eV) (Figure S3), indicating the formation of oxygen vacancy (Vo).23,47,48 Electron paramagnetic resonance (EPR) spectra (Figure 4f) show that CV-TiO2 exhibits a clear signal characterized by the magnetic field strength centering at 3366 G and the electron’s so-called g-factor of 2.004 originating from the presence of unpaired electrons trapped on Vo.14,49−51 There is no Ti3+ signal since its detection at room temperature is not possible.52 As TiO2 and CA-TiO2 have no signal, EPR analysis further proves that vacuum annealing induces the formation of Vo in CV-TiO2. This easy approach for the creation of Vo is also suitable for P25. In the presence of ethanol, vacuum annealing induced the formation of Vo in P25, while no Vo is identified for vacuum annealing of P25 without ethanol (Figure S4). Consequently, vacuum annealing results in the evaporation and decomposition of ethanol, forming carbon-related species on the surface of TiO2 and the formation of Vo on its surface by the extraction of O from the Ti–O–Ti bond.44,45

Photocatalytic Activity

The photocatalytic activities of all TiO2 photocatalysts were evaluated by the photocatalytic degradation of organic dyes under UV light. In the absence of TiO2 photocatalysts, the photolysis of RhB, methyl orange (MO), and MB is negligible (Figure 5a,c). The photocatalytic activities of TiO2, CA-TiO2, and CV-TiO2 are compared to the photocatalytic performance of P25 (as a reference photocatalyst). As shown in Figure 5a, compared with TiO2 and CA-TiO2 photocatalysts (∼10%), increased adsorption of RhB by CV-TiO2 (∼30%) was observed, although it possesses the smallest surface area (Figure 3). This high adsorption capacity of RhB may result from the presence of carbon-related species on the surface of CV-TiO2, which favors the adsorption of organic dyes. Under UV light irradiation, CA-TiO2 and CV-TiO2 show enhanced photocatalytic activity relative to TiO2. The photocatalytic performance of CV-TiO2 is drastically improved, and it is even higher than that of P25. Cycling tests are performed to check the stabilities of TiO2, CA-TiO2, and CV-TiO2. Figure 5b shows the repeated four cycling tests of photocatalytic degradation of RhB by recovering all TiO2 photocatalysts after each run. It is obvious that CV-TiO2 shows the best photocatalytic activity, and the photocatalytic degradation rate becomes slow as the adsorption of RhB gradually decreases. The photocatalytic activity of CV-TiO2 is also investigated by decomposing MO and MB. Figure 5c shows that CV-TiO2 can efficiently decompose MO and MB, which is similar to the degradation of RhB. The excellent photocatalytic performance of CV-TiO2 may originate from its high adsorption capacity and the presence of Vo, which contributes to the separation of photogenerated electrons and holes.

Figure 5.

Figure 5

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Photocatalytic degradation curves of RhB by TiO2, CA-TiO2, CV-TiO2, and P25 as a function of irradiation time with photolysis of RhB without a photocatalyst as the control (a), cycling tests (b), and photocatalytic degradation curves of MO and MB by CV-TiO2 with photolysis of MO and MB without a photocatalyst as the control (c).

Proposed Photocatalytic Mechanism

Figure 6a shows the transient photocurrent response of TiO2, CA-TiO2, and CV-TiO2. Compared with TiO2, CA-TiO2 and CV-TiO2 present clear enhancement of transient photocurrent, indicating the improved charge carrier separation properties, which originate from their improved crystallinity.17,34 The relative lower photocurrent of CV-TiO2 than that of CA-TiO2 may be caused by the trapping of electrons by Vo.53,54 Trapping electrons by Vo on the surface of CV-TiO2 can prevent the recombination of electrons and holes, further elongating the lifetime of holes. Thus, more holes contribute to the formation of more hydroxyl radicals (OH•), which play a key role in the photocatalytic degradation of RhB by oxidation.55 Photoluminescence (PL) spectra further support this assumption. Figure 6b shows that the fluorescence intensity of CV-TiO2 is lower than that of TiO2 and CA-TiO2. The emission signals in the PL spectrum are from the recombination of photogenerated electrons and holes.35 The lower fluorescence intensity implies less recombination of electrons and holes as a result of the trapping of electrons by Vo. The reduced recombination of electrons and holes favors the formation of more OH• for photocatalytic oxidation. Therefore, the enhanced photocatalytic activity of CV-TiO2 is due to the improved crystallinity, increased adsorption capacity, and formation of Vo.

Figure 6.

Figure 6

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Transient photocurrent curves (a) and PL spectra (b) of TiO2, CA-TiO2, and CV-TiO2.

Conclusions

In summary, the effects of heat treatment on the crystal phase and structure, band gap, morphology, composition, and photocatalytic properties of TiO2 photocatalysts were investigated in detail. It is proved that all TiO2 photocatalysts are composed of the anatase phase, and calcination improves the crystallinity and increases the average crystallite size and pore diameter but decreases the surface area of CA-TiO2 and CV-TiO2 with quite different appearances. CA-TiO2 maintains the white color of TiO2 because calcination in air can completely evaporate and decompose the ethanol adsorbed by as-synthesized TiO2. However, vacuum annealing leads to the carbonization of ethanol by dehydration and the extraction of lattice O from TiO2, leaving gray calcinate and Vo on the surface of CV-TiO2. The gray calcinate is composed of carbon-related species, which darken the color of CV-TiO2 and favor the adsorption of organic dyes. Meanwhile, the formation of Vo on the surface of CV-TiO2 traps photogenerated electrons, which promotes the charge carrier separation and enables more holes to participate in the formation of oxidizing species involved in the photocatalytic degradation. The improved crystallinity, enhanced adsorption capacity, and formation of Vo contribute to the superior photocatalytic performance of CV-TiO2 relative to P25 in the degradation of organic pollutants. Thus, the decomposition of ethanol plays a vital role in vacuum annealing-induced modification of TiO2. This work offers an easy approach for the modification of TiO2 to enhance its photocatalytic performance by facile vacuum annealing in the presence of ethanol. Vacuum annealing with ethanol-induced formation of Vo is also suitable for commercial TiO2 (P25). Therefore, this easy method may be universal for the modification of semiconductor photocatalysts to improve the photocatalytic activity. The quantitative creation of Vo is of great importance, as too much Vo is detrimental to the photocatalytic performance. Further work concerning the controllable formation of Vo via vacuum annealing by controlling the amount of adsorbed ethanol is underway.

Experimental Section

Materials

Titanium tetraisopropoxide (TTIP, 95%, Macklin) was used as the precursor for the hydrothermal synthesis of TiO2. Deionized water (DI, 18.2 MΩ·cm, Millipore system), ethanol, isopropanol, and acetone (AR grade, Rionlon) were used as solvents. A custom-built high-borosilicate glass tube was applied for vacuum annealing. Nafion D-521 dispersion (5% w/w in water and 1-propanol, Alfa Aesar) and fluorine-doped tin oxide substrates (FTO, Youxuan Tech) were used to prepare the working electrode for transient photocurrent measurements. RhB (AR grade, Shanghai Zhongqin), MO, and MB (AR grade, Beijing Chemical Reagent) were selected as model organic pollutants for the evaluation of the photocatalytic activity.

Synthesis of TiO2 Photocatalysts

The synthesis of TiO2 was conducted according to a reported hydrothermal method.56 In a typical synthesis, TTIP (10 mL) was placed in a Teflon liner, and DI water (3 mL) was added dropwise while stirring at room temperature. Then, the mixture was stirred for 30 min, transferred to a Teflon-lined autoclave, and kept at 180 °C for 24 h in a high-temperature furnace. After cooling down to room temperature, white precipitates appeared at the bottom of the Teflon liner and were separated by centrifugation. The as-synthesized TiO2 powders were washed three times with ethanol and dried by lyophilization overnight. The white TiO2 powders were placed in a porcelain crucible or sealed in a high-borosilicate glass tube for calcination in air or annealing under vacuum at 500 °C for 2 h at a heating rate of 2 °C/min. The treated TiO2 powders in air and under vacuum were labeled as CA-TiO2 and CV-TiO2, respectively. All the as-prepared and sintered TiO2 powders were collected for further characterizations and tests.

Characterizations

The diffraction patterns of TiO2, CA-TiO2, and CV-TiO2 were recorded using XRD (X’Pert PRO) with a Cu Kα radiation source (λ = 1.5406 Å), and their corresponding crystallite sizes were estimated using the Debye–Scherrer formula (eq S1).34 The absorption properties of all TiO2 photocatalysts were studied by UV–vis DRS (UV-2600), and their Eg values were estimated from the intercepts of their corresponding K–M plots (eqs S2–S4).40,57 The morphologies of all samples were observed by SEM (Apreo S) and TEM (JEM-2100F). Brunauer–Emmett–Teller surface areas and the nitrogen adsorption and desorption isotherms of all TiO2 photocatalysts were measured using an accelerated surface area and porosity analyzer (ASAP 2460). The pore size distribution was collected by the BJH method. The elemental composition was determined by EDS (X1 Analyzer, AMETEK). To do so, the powders of each TiO2 photocatalyst were dispersed in DI water, deposited on aluminum foil, and dried naturally in air. Further chemical structure analyses were performed by FT-IR spectra (Bruker Alpha) and XPS (Escalab Xi+) with Al Kα radiation. The binding energy of the C 1s peak (adventitious carbon species) was fixed at 284.6 eV to set the binding energy scale.23 The contents of organic carbon in all TiO2 photocatalysts were analyzed on a TOC analyzer (TOC-L) at 900 °C. EPR (Bruker ER200DSRC) spectra of all samples were taken by applying an X-band (9.44 GHz, 2.47 mW) microwave and sweeping magnetic center field (3362 G) at room temperature. The transient photocurrent curve of each sample was measured using an electrochemical workstation (CHI 660D) in a typical three-electrode potentiostat system with a xenon lamp (300 W) as the light source. Alcohol suspensions of TiO2 photocatalysts (20 μL, 10 mg/mL) were added to FTO substrates precoated with the Nafion D-521 dispersion. The surface area of the FTO substrate is 1 × 1 cm2. Steady-state PL spectra were recorded on a spectrofluorometer (FLS920) under 280 nm laser excitation wavelength.

Photocatalytic Degradation Tests

Photocatalytic degradation tests of organic pollutants were carried out under UV light irradiation (mercury lamp, 500 W) in a photochemical reaction apparatus (Beijing Princess Technology) equipped with short-wavelength pass filters (<400 nm) to filter the visible light. TiO2, CA-TiO2, and CV-TiO2 (1 mg/mL) were added into aqueous solutions of RhB (10 ppm, 25 mL) and stirred in the dark for 120 min before photocatalytic tests. For comparison, the photocatalytic performance of commercial TiO2 (P25, Degussa) was also evaluated. The photocatalytic activity of CV-TiO2 was further assessed through the degradation of MO (25 ppm) and MB (10 ppm) organic dyes under the same conditions. The photolysis of organic dyes under the same conditions without TiO2 photocatalysts was performed as the control. The concentrations of organic dyes were monitored by following the maximum absorbance of RhB at 554 nm, MO at 464 nm, and MB at 664 nm. The ratios of residual organic dyes versus irradiation time were calculated by the expression C/C0, where C and C0 were the recorded concentration at different time intervals of irradiation and the initial concentration, respectively. The stability and reusability of all TiO2 photocatalysts were checked by performing cycling tests of photocatalytic degradation of RhB four times.

Acknowledgments

This work has been supported by the National Natural Science Funds of China (12005086) and the Fundamental Research Funds for the Central Universities (lzujbky-2020-kb06 and lzujbky-2021-sp29).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03762.

  • Digital photograph of vacuum annealing of TiO2; vacuum annealing of P25; EDS and XPS of TiO2 photocatalysts; EPR spectra of P25; UV–vis spectra of organic dyes; and corresponding digital photographs during the photocatalytic degradation (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c03762_si_001.pdf (1.1MB, pdf)

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