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. 2023 Dec 28;9(1):1695–1713. doi: 10.1021/acsomega.3c08300

Structural Diversity Design, Four Nucleation Methods Growth and Mechanism of 3D Hollow Box TiO2 Nanocrystals with a Temperature-Controlled High (001) Crystal Facets Exposure Ratio

Shengqiang Liao †, Huan Liu †, Yanfei Lu †, Chenglong Tang †, Benjun Xi ‡, Lianqing Chen †,‡,§,*
PMCID: PMC10785669  PMID: 38222646

Abstract

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Three-dimensional (3D) hollow box TiO2 nanocrystals with structural diversity have been designed and grown by four nucleation methods, including the acid dissolution denucleation method with Fe2O3 as heterogeneous nucleation, the topological phase transition method, the sonic solvothermal method, and the air atmosphere sintering method with TiOF2 as homogeneous nucleation. Through full morphology analysis and structural characterization, reasonable growth mechanisms of 3D hollow box TiO2 nanocrystals were proposed, including nucleation dissolution, Oswald ripening, and hydrolysis reactions. It was found that the high energy (001) crystal facets exposure ratio was closely correlated with reaction temperature of four nucleation-methods, which even reached 92% for the first time. Under simulated sunlight irradiation, their hydrogen production performance and photocatalytic degradation efficiency on model dye molecules rhodamine B (RhB) and methylene blue (MB) were evaluated, and as-prepared hollow box TiO2 nanocrystals prepared by the sonic solvothermal method exhibited the best photocatalytic performance, with a hydrogen production rate of 93.88 μmol/g/h. Within 70 min, the photocatalytic degradation rates of RhB and MB reached 96.59 and 75.25%, respectively, which were 5.74 and 5.54 times that of P25. Their properties are closely connected with the orderly cubic and hierarchy configuration structure of hollow box TiO2 nanocrystals, which have a high exposure ratio of (001) facet controlled by reaction temperatures, thereby greatly improving the photocatalytic activity. This study provides a classic reference for improving the properties of hollow box TiO2 nanocrystals through structural diversity design and various methods of nanocrystal growth.

1. Introduction

In recent years, semiconductor nanocrystals have attracted increasingly more interest among researchers.1 Semiconductor nanocrystals exhibit unique optical properties due to their small size, making them widely applicable in optical research.2,3 Tuning the growth and shape evolution of specific crystal facets of semiconductor nanocrystals can enhance their optical characteristics.4,5 Compared to bulk semiconductors, atoms, and molecules, the ultrasmall size of semiconductor nanocrystals alters the energy levels of the conduction and valence bands.6,7 The degeneracy of the band edge states also increases, leading to quantization of nearby states. Additionally, due to their high surface area-to-volume ratio and surface-active sites, semiconductor nanocrystals often exhibit outstanding performance in catalysis and chemical reactions.7,8 Therefore, designing semiconductor nanocrystals with appropriate dimensions and specific facet growth is of paramount importance.

Photocatalysis is a commonly used method to address environmental pollution and energy scarcity issues. TiO2 is the most widely used photocatalytic material due to its high chemical stability,9,10 high degree of commercialization,11,12 and excellent catalytic performance.13,14 However, TiO2 has low solar energy utilization and a high electron–hole recombination rate, indicating that the catalytic performance of TiO2 still needs to be improved. Generally, anatase TiO2 has better photocatalytic activity.15−17 Zhu et al. used TiCl4 and benzyl alcohol to carry out a nonhydrolytic sol–gel reaction at low temperature to prepare anatase TiO2 nanoparticles, which have a much higher degradation effect on phenol than P25.18 Changing the morphology of TiO2 can improve its catalytic performance. Many special nanostructured TiO2 materials have been developed, such as nanosheets,19 nanowire,20 nanotubes,21 nanorods,22 hollow nanoboxes,23 and so on. Wang et al. prepared TiO2 nanosheets with surfaces containing fluoride ions and oxygen vacancies, which showed excellent performance in the photocatalytic degradation of volatile organic compounds.24 Horváth et al. prepared TiO2 nanowire/nanotube composites, which can effectively remove various organic compounds and infectious microorganisms under sunlight.25 So far, experimental and theoretical analyses have shown that the high-energy (001) facet of anatase TiO2 can effectively separate photogenerated electron–hole pairs and improve photocatalytic ability. The research of Yang et al. shows that the average facet energy of anatase TiO2 dioxide follows the order (001) with 0.90 J/m2 > (100) with 0.53 J/m2 > (101) with 0.44 J/m2. By constructing a layered, three-dimensional hollow nanobox of TiO2 composed of six highly exposed (001) facets, the light response can be enhanced.26

Hollow nanobox TiO2 possesses characteristics such as low density,27 good surface permeability,28 and a large surface area,29 which contribute to their strong adsorption capacity.30 Additionally, the hierarchical structure of hollow nanobox TiO2 can generate multilevel scattering of light, where multiple scattering particles exist within the scattering body and each particle’s scattered light can be rescattered by other particles. This leads to an increasing capture of light, effectively enhancing the utilization of light and thus enhancing the photocatalytic activity of TiO2.31 Liu et al. used a simple self-template solvothermal annealing method to prepare novel double-shell ZnFe2O4 hollow microspheres, and the kinetic constants of double-shell ZnFe2O4 hollow spheres were 1.46 and 1.82 times those of the yolk–shell spheres and solid spheres, respectively. This indicates that the multishell hollow structure can adjust the refractive index, which is beneficial for enhancing light scattering.32 Methods for preparing hollow nanobox TiO2 include hydrothermal synthesis, cation-assisted methods, and so on. Zhang et al. used a one-pot method to prepare a Ti3+-doped three-dimensional TiO2 hollow nanobox, which enhanced visible light response.33 Li et al. prepared a hollow TiO2 nanobox with enhanced electrorheological activity using a hydrothermal method and exhibited excellent electrorheological effects in electrorheological suspensions, contributing to improved dielectric performance.34 However, there is limited research on hollow box TiO2 nanocrystals. In order to obtain the optimal method for growing hollow box TiO2 nanocrystals, four nucleation methods have been applied, including the acid dissolution denucleation method with Fe2O3 as heterogeneous nucleation, the topological phase transition method, the sonic solvothermal method, and the air atmosphere sintering method with TiOF2 as homogeneous nucleation.

The crystallization process is divided into two stages: nucleation and growth. Nucleation is the first step in crystal growth, where small crystals form in a solution. Once nuclei are formed, crystals grow by absorbing material from the solution, gas, or melt.35 During the crystal growth process, heterogeneous nucleation or homogeneous nucleation is crucial.36 In heterogeneous nucleation, the formation of crystals does not begin with free molecules in the solution but rather with heterogeneous nuclei.37 In homogeneous nucleation, crystal formation originates from free ions, atoms, or molecules in the solution, and these molecules aggregate to form small nuclei.38

The acid-dissolution denucleation method is a typical heterogeneous nucleation approach. This method can replicate the micronanostructure of nucleation materials, construct the morphology of synthetic materials,39,40 and precisely control the size and arrangement of nanocrystals,41,42 which has advantages such as simple operation, mild reaction conditions, and high efficiency in preparing zero-dimensional and one-dimensional nanomaterials. Yang et al. used wood as heterogeneous nucleation to prepare TiO2 with a multilayered porous structure, achieving a degradation efficiency of 97.9% for methylene blue within 60 min.43

The topological phase transition method, the sonic solvothermal method, and the air atmosphere sintering method are three important homogeneous nucleation methods. The topological phase transition method and sonic solvothermal method can control the formation of phases, particle size, and shape, resulting in products with good dispersibility.44 The topological phase transition process is typically conducted in a closed system. During the reaction process, the reactants dissolve and disperse in the solution, facilitating the progress of the reaction.45 Jia et al. prepared layered TiO2-x hollow nanoboxes through the in situ topological phase transition solvent thermal method using peroxide.46 The air atmosphere sintering method is primarily used for the manufacturing of ceramics, crystal conversion, separation, and enrichment. Sintering can alleviate the inherent tension in the molecular structure of substances, allowing them to adapt to increasing forces during the shaping process and become more robust upon completion.47,48 When the sintering temperature reaches 500 °C, the anatase phase gradually transforms into the rutile phase, which exhibits lower catalytic activity compared to the anatase phase.49−51 Compared to hollow box TiO2 prepared by traditional methods, the hollow box TiO2 nanocrystals prepared in this study have a higher (001) facet exposure ratio.

Herein, 3D hollow box TiO2 nanocrystals were attempted to grow using one heterogeneous nucleation method and three homogeneous nucleation methods. Comprehensive morphological analysis, structural characterization, and growth mechanisms of hollow box TiO2 nanocrystals by four methods were conducted and proposed. Furthermore, the evolution of the formation process and exposure rate of (001) crystal facets of hollow box TiO2 nanocrystals was studied, and the impact of temperature on the (001) facet exposure rate was investigated. Finally, the photocatalytic performance of the hollow box TiO2 nanocrystals was compared through photocatalytic hydrogen production and the degradation of RhB and MB. This research provides hollow box TiO2 nanocrystals through structural diversity design and various methods of nanocrystal growth.

2. Experimental Section

2.1. Materials

Ferric chloride hexahydrate (FeCl3·6H2O), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), titanium tetra-n-butylate [Ti(OC4H9)4], anhydrous ethanol (CH3CH2OH), hydrochloric acid (HCl), acetic acid (CH3COOH), hydrofluoric acid (HF), coumarin (C9H6O2), 4-chloro-7-nitro-2,1,3-benzoxadiazole (C6H2ClN3O3, NBD-Cl), silver nitrate (AgNO3), isopropanol (C3H8O), benzoquinone (C6H4O2), rhodamine B (C28H31ClN2O3, RhB), and methylene blue (C16H18N3ClS, MB) sourced from China National Pharmaceutical Group Chemical Reagent Co., Ltd. All materials are analytical grade and can be used directly without the need for further purification. Deionized water is used in all experiments.

2.2. Hollow Box TiO2 Nanocrystals Grown by the Heterogeneous Nucleation Method

2.2.1. Acid Dissolution Denucleation Method

Under stirring, 25.0 g of FeCl3·6H2O was added to 50.0 mL of deionized water until fully dissolved. The solution was heated to 80 °C, then 50.0 mL of NaOH solution (5 M) was slowly added while stirring and reacted for 48 h. The precipitate was centrifuged to separate, washed with water and ethanol (3 times), and dried overnight at 60 °C. In 100.0 mL of ethanol, 0.4 mL of sodium carbonate solution (35 wt %) and the prepared nano Fe2O3 (0.2 g) were added. The mixture was ultrasonicated for 15 min. After adding 1.0 mL of tetrabutyl titanate (TBOT), the mixture was stirred at 60 °C for 24 h. To separate the product, it was centrifuged, washed with ethanol (3 times), and dried overnight at 60 °C. The prepared Fe2O3/TiO2 core/shell nanocubes (0.2 g) were dissolved in 10 mL of hydrochloric acid solution (0.5 M). The mixture was ultrasonicated at room temperature for 10 min; then, the resulting suspension was transferred to a high-pressure reaction vessel and placed at 100 °C for 24 h. The resulting mixture was centrifuged and washed several times with deionized water and absolute ethanol (until the pH approached 7) and finally dried overnight in an oven at 60 °C. The obtained product is labeled as TC-1 with a yield of 52.3%.

2.3. Hollow Box TiO2 Nanocrystals Grown by Homogeneous Nucleation Methods

2.3.1. Topological Phase Change Method

The homogeneous nucleation of TiOF2 was prepared according to ref (52). Under magnetic stirring, 15.0 mL of TBOT and 30.0 mL of glacial acetic acid (CH3COOH) were poured into a 100 mL beaker. Then, 5.0 mL of hydrofluoric acid was slowly added to the evenly mixed solution. After 10 min of sonication and 30 min of magnetic stirring, both acid mixtures were transferred to a high-pressure reaction vessel and reacted at 200 °C for 12 h. The resulting supernatant was poured off, leaving a grayish-white precipitate. The precipitate was transferred to a clean plastic bottle containing deionized water, vigorously shaken, and allowed to settle. After complete settling of the precipitate, the supernatant was poured off, and this process was repeated three times. The product was then washed and filtered with anhydrous ethanol. After filtration, the product was dried overnight in a vacuum drying oven at 60 °C and ground in an agate mortar. The obtained white powdered product is TiOF2.

Under ultrasonic conditions, 0.5 g of TiOF2 was mixed with 40.0 mL of absolute ethanol in the inner chamber of a reaction vessel. After complete mixing, the mixture was reacted in an oven at 200 °C for 12 h. After the reaction, the precipitate was washed with deionized water and absolute ethanol, suction-filtered, and dried overnight in a vacuum drying oven at 60 °C, resulting in TiO2. Labeled as TC-2, the yield was 95.1%.

2.3.2. Sonic Solvothermal Method

0.50 g of TiOF2 was weighed into the inner chamber of a reaction vessel and poured into 40 mL of ethylene glycol. After 10 min of ultrasonic dispersion and 30 min of stirring, the mixture was transferred to a high-pressure reaction vessel and reacted at 220 °C (180, 200, 240 °C) for 8 h. Upon completion of the reaction, the upper clear liquid was decanted; then, the product was washed and suction-filtered with deionized water and absolute ethanol. The product was dried in a vacuum drying oven at 60 °C for 6 h to obtain TiO2, and the sample was labeled as TC-3 with a yield of 97.3%.

2.3.3. Air Atmosphere Sintering Method

0.5 g of TiOF2 (0.5 g) was added to a 50.0 mL crucible. The crucible was heated in a muffle furnace at a heating rate of 5 °C/min until it reached 350 °C. The temperature was maintained at 350 °C for 4 h, and then the crucible was cooled back to room temperature at a cooling rate of 5 °C/min to obtain TiO2. It was labeled as TC-4, with a yield of 89.2%. The schematic diagram for the growth of hollow box TiO2 nanocrystals using four methods is shown in Figure 1.

Figure 1.

Figure 1

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Growth of hollow box TiO2 nanocrystals by four nucleation methods.

2.4. Characterization Instruments

Under an accelerating voltage of 200 kV, the morphology and structure of the samples were examined using a scanning electron microscope (SEM) and transmission electron microscope (TEM) (Tecnai G 20, USA). X-ray diffraction (XRD) analysis of the samples was conducted using a D8-advance X-ray diffractometer (Bruker, Germany) with Cu Kα radiation and a scanning rate of 0.02°/0.1 s. Infrared spectra and X-ray photoelectron spectroscopy (XPS) were, respectively, obtained using a Fourier transform infrared spectrometer (Nexus 470, Shimadzu, Japan) and a Kratos XSAM800 XPS system with Mg Kα radiation. The specific surface area and porosity of the samples were determined using a nitrogen adsorption instrument (3 H-2000PS2, Beijing). Ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) and photoluminescence (PL) spectra of the samples were measured by using a UV2600 spectrophotometer (Shimadzu, Japan) and a fluorescence spectrophotometer (F-7000, Hitachi, Japan), respectively. Nitrogen adsorption–desorption isotherms and the BET specific surface area (SBET) of the samples were detected through physical adsorption using a Micromeritics instrument (ASAP 2020, Micromeritics, USA). The photocurrent of the samples was measured using a CHI 660D electrochemical workstation (CH Instruments, Shanghai, China) with nanostructured materials on an FTO working electrode, a saturated Ag/AgCl reference electrode, and a platinum counter electrode. Photocatalytic hydrogen production experiments were conducted using a tandem automatic sampling system (Labsolar 6A, Perfect Light) and a gas chromatograph (GC-9790II, Zhejiang Fulite).

2.5. Photocatalytic Performance Research

The photocatalytic hydrogen production experiment was conducted under simulated solar light by using a xenon lamp. In this experiment, a solution containing 0.04 g of catalyst, 0.025 mol of Na2SO3, and 0.035 mol of Na2S sacrificial agent in 100 mL was added to the reactor. The system was evacuated for 30 min. The solution was continuously stirred, and water circulation cooling was provided in the interlayer of the reactor. During the experiment, every 30 min, the generated gases were automatically collected by using an online gas automatic sampling system. The hydrogen content was analyzed by using gas chromatography.

Catalytic degradation experiments of the model dye molecules RhB and MB were conducted under simulated solar light using a xenon lamp. In a cylindrical Pyrex flask, 50.0 mL of RhB or MB solution (1.0 × 10–5 mol/L) and 40.0 mg of photocatalyst were poured. After ultrasonic treatment, the solution was stirred in the dark for 30 min to establish the adsorption–desorption equilibrium. Then, the solution was irradiated with a xenon lamp (350 W), and 3.0 mL suspension samples were collected every 10 min. The collected solution was centrifuged to remove the photocatalyst particles. The concentration of RhB and MB in the solution was monitored by UV–vis spectroscopy at 555 nm.

We employed 4-chloro-7-nitrobenzofurazan (NBD-Cl) and coumarin probe molecules along with photopolymerization techniques to detect the active species, hydroxyl radicals (•OH) and superoxide radicals (•O2–), of the photocatalyst. A mixture of 0.4 mmol/L NBD-Cl or coumarin with the photocatalyst (1.0 g/L) was prepared, and the suspension was stirred overnight. Subsequently, this suspension was placed under a 210 W xenon lamp. Samples were collected every 2 min and filtered to obtain the filtrate. The fluorescence intensity of the filtrate was measured using a fluorescence spectrophotometer at excitation wavelengths of 454 and 550 nm, respectively.

3. Results and Discussion

3.1. Morphological Characterization

The morphological characteristics of TiOF2 and hollow box TiO2 nanocrystals were observed using a scanning electron microscope (SEM), as depicted in Figure 2. From Figure 2a, it can be observed that the edge length of the prepared cubic TiOF2 crystals was approximately 290 nm. Some of the TiOF2 crystals exhibited a nanorod-like shape, while the majority had a cubic crystal structure with a smooth surface. These cubes were solid cubes. The size of TiO2 grown by the four methods was approximately 285 nm. Figure 2b illustrates that TC-1 has a distinct, hollow cubic structure. Due to the incomplete dissolution of some Fe2O3, the structure of some hollow box TiO2 nanocrystals was not entirely intact. Some TiO2 shells were unstable and were removed as the core dissolved. As shown in Figure 2c, TC-2 exhibited a box-like shape with a rough surface, and some TiO2 nanosheets were attached. These hollow box TiO2 nanocrystals had a complete structure composed of six nanofaces. This structure provided a larger contact area for TiO2 and dyes. From Figure 2d, TC-3 displayed a three-dimensional multilevel hollow box structure composed of numerous TiO2 nanosheets, with many nanosheets embedded on the surface of the hollow nanoboxes. Figure 2e provides a larger-sized image, further demonstrating the multilevel hollow box structure of TC-3. As shown in Figure 2f, TC-4 had six smooth surfaces and exhibited a hollow box structure. However, due to the high reaction temperature in the air atmosphere sintering method, some hollow boxes may become fractured. The surface energy of the system tends to remain at a minimum, leading to structural stacking and a reduction of the surface energy.

Figure 2.

Figure 2

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SEM characterization of TiOF2 (a), TC-1 (b), TC-2 (c), and TC-3 (d) with a larger enlarged size of TC-3 (e) and TC-4 (f).

Transmission electron microscopy (TEM) was employed to investigate the microstructures of TiOF2 crystals and hollow box TiO2 nanocrystals. Figure 3a shows that TiOF2 exhibited a standard solid cubic crystal structure. From Figure 3b, it can be observed that only some of the TC-1 had a hollow box structure, while others had a solid cubic structure. This suggests that the yield of hollow box TiO2 nanocrystals prepared by the acid dissolution denucleation method is low because some Fe2O3 may not fully dissolve. All TC-2 was primarily a hollow box structure (Figure 3c), indicating a higher yield of hollow box TiO2 nanocrystals prepared by the topological phase transformation method. Figure 3d displays the three-dimensional multilevel hollow box structure of TC-3, with various sizes of linear structures (nanosheets) surrounding the boxes. Figure 3e clearly shows that TC-4 had a hollow box-like structure with a noticeable stacking arrangement. High temperatures can prevent some TiO2 from forming a hollow box structure, leading to a lower yield of hollow box TiO2 nanocrystals prepared by the air atmosphere sintering method. These TEM results are consistent with the SEM morphological characterizations. Figure 3f is an HR-TEM image of TC-3, with a lattice spacing of 0.235 nm corresponding to the (001) facet of anatase TiO2, indicating that the exposed surface of TC-3 is mainly the (001) facet.

Figure 3.

Figure 3

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TEM characterization of TiOF2 (a), TC-1 (b), TC-2 (c), TC-3 (d), and TC-4 (e) and HR-TEM characterization of TC-3 (f).

Figure 4 shows the energy-dispersive spectroscopy (EDS) of TC-3. From the figure, it can be observed that TC-3 contained two elements, O and Ti. The O elements (red) and Ti elements (cyan) were distributed on the surface of the hollow box and the nanosheets on the surface, further confirming the multilevel hollow box structure of TC-3.

Figure 4.

Figure 4

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EDS of TC-3.

3.2. XRD, FT-IR, and Raman Characterization

Figure 5a shows the X-ray diffraction patterns (XRD) of the samples. The diffraction peaks of TiOF2 matched the standard card (PDF#77-0132), with 2θ values of 23.45, 33.46, 48.04, and 54.14° corresponding to the (100), (110), (200), and (210) crystal facets of TiOF2, respectively. By comparison of the XRD pattern of the TiO2 samples with that of TiOF2, no characteristic diffraction peak of TiOF2 was observed in TiO2, indicating the complete conversion of TiOF2 to TiO2. This is consistent with the conclusions drawn from SEM and TEM. When compared to the standard card (PDF#73-1764), TiO2 prepared by the four methods is in the standard anatase phase, with 2θ values of 25.21, 37.83, 47.95, 54.94, and 62.62° corresponding to the (101), (004), (200), (211), and (204) crystal facets of anatase TiO2. This is attributed to the role of F as structure-directing agents.53 The high-resolution diffraction pattern of the TiO2 (101) facet is shown in Figure 5b. It can be observed that the diffraction peaks of CT-1 to CT-4 become sharper, indicating that the crystallinity of the prepared TiO2 increases.54

Figure 5.

Figure 5

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X-ray diffraction patterns of the samples (a) and high-resolution diffraction patterns of the samples (b).

As shown in Table 1, the crystal size of TiO2 was determined based on the (101) diffraction peak using the Scherrer equation: D = Kλ/β cos θ, where λ and θ represent the X-ray wavelength and X-ray diffraction angle, respectively. In the calculations, the full width at half-maximum (β) of the diffraction peak was chosen, and the shape factor (K) was determined to be 0.89. From the calculated results, it can be observed that the crystal sizes of TiO2 prepared by the four methods are quite similar. The lattice parameters of TiO2 were calculated using Bragg’s law (2d sin θ = λ) and the formula ((l/d2) = (h2 + k2/a2) + (l2/c2)). The lattice constants of TiO2 prepared by the four methods also show little variation, indicating excellent control over the size of TiO2.

Table 1. Crystal Size and Crystal Lattice Parameters of TiO2.

samples XRD(101) peak (deg) XRD(101) relative intensity (%) crystal sizea (nm) crystal lattice parameters
        a/nm b/nm c/nm
TC-1 25.14 31 282 0.3787 0.3787 0.9682
TC-2 25.25 36 282 0.3794 0.3794 0.9503
TC-3 25.25 33 288 0.3797 0.3797 0.9418
TC-4 25.25 38 285 0.3791 0.3791 0.9344
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a

Determined by the Scherrer equation based on the diffraction peak of the (101) facet.

Figure 6a shows the FT-IR spectra for TiOF2, TC-1, and TC-3. The peak at 3438 cm–1 corresponds to the stretching vibration of the surface –OH groups and adsorbed water on the sample. At 1626 cm–1, there is an absorption peak associated with the stretching and bending vibrations of surface water. The characteristic absorption peak at 1079 cm–1 is attributed to the Ti–O–Ti bonds in TiO2.55 The absorption peak at a central wavenumber of 968 cm–1 is assigned to the stretching vibration of Ti–F bonds in TiOF2. The stretching vibrational absorption peak of Ti–O in the sample is around 547 cm–1. All of the characteristic peaks of both TiOF2 and TiO2 are included in the graph, indicating the successful preparation of TiOF2 and TiO2. In order to further investigate the structure of TiO2, Raman spectroscopy was conducted on TiO2. Figure 6b shows the characteristic Raman spectra of TC-1 and TC-3. Peaks centered at 134, 381, 502, and 624 cm–1 correspond to the Eg(1), B1g(1), A1g(1)/B1g(2), and Eg(2) Raman modes of anatase TiO2, respectively.56 These characteristic spectra are located in the low-frequency region (100–800 cm–1 Raman shifts) of the prepared TiO2, further confirming them as anatase TiO2, consistent with XRD results. In comparison to TC-1, the peak positions in TC-3 had shifted by approximately 10 cm–1, which may be related to differences in the concentration of structure-directing agent F. At the same time, the intensity of the peaks has significantly increased, indicating the crystalline enhancement of TiO2.

Figure 6.

Figure 6

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FT-IR characterization of TiOF2, TC-1, and TC-3 (a). The Raman spectra of TC-1 and TC-3 (b).

3.3. UV–Visible DRS Characterization

Figure 7a shows the UV–visible diffuse reflectance spectra (DRS) of the samples. From the graph, it can be observed that TC-1 exhibits minimal absorption in the visible light range (400–800 nm) but significant absorption in the ultraviolet region (200–400 nm). However, TC-2 and TC-4 showed some absorption in the visible-light region. This suggests that the hollow structure itself enhances visible light absorption to some extent. Compared to TiO2 prepared by other methods, TC-3 exhibited a more substantial absorption in the visible light region. This is likely due to the multilayered hollow box structure of TC-3, which increased the number of active reaction sites, thereby expanding the range of responses to visible light and improving photocatalytic performance. Figure 7b shows the Kubelka–Munk transformed reflectance spectra of the samples. Bandgap values were calculated using the tangent method, which is the point where the tangent to the curve intersects the x-axis.57 The bandgap values for TC-1, TC-2, TC-3, and TC-4 are 3.07, 2.95, 2.90, and 2.99 eV, respectively. The results indicated that TC-3 with the multilayered hollow box structure has the narrowest bandgap.

Figure 7.

Figure 7

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UV–vis diffuse reflectance spectra of the samples (a) and the curve of (ahv)2 versus hv for the samples (b).

3.4. XPS Characterization

Figure 8a displays the surface composition of TiOF2 and TiO2 as revealed by X-ray photoelectron spectroscopy (XPS). The TiOF2 sample contained C, Ti, O, and F elements, while the TiO2 samples contained C, Ti, and O elements, with the presence of C likely originating from the measurement instrument. Figure 8b–d shows high-resolution XPS images for Ti 2p, O 1s, and F 1s, respectively. From Figure 8b, it can be observed that the Ti 2p3/2 and Ti 2p1/2 peaks of TiOF2 appear at 459.78 and 465.48 eV, respectively. For TC-1, the Ti 2p3/2 and Ti 2p1/2 peaks are at 459.31 and 465.01 eV, while for TC-3, they are at 459.19 and 464.89 eV, respectively. The difference between the two peaks is 5.7 eV in all cases, indicating that Ti in both TiOF2 and TiO2 is in the +4 state. From Figure 8c, the Ti–O–Ti peaks for TiOF2, TC-1, and TC-3 occur at 530.50, 530.32, and 530.27 eV, respectively. Additionally, the Ti–O–H peaks for TiOF2, TC-1, and TC-3 appear at 532.19, 531.60, and 531.58 eV, respectively. Compared to TC-1, TC-3 showed a negative shift in both Ti 2p and O 1s peaks, suggesting a higher electron cloud density around TC-3. Figure 8d reveals that the F 1s peak for TiOF2 appears at 685.01 eV. These results confirm the successful preparation of TiOF2 and TiO2.

Figure 8.

Figure 8

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XPS spectra of full spectra (a), Ti 2p (b), O 1s (c), and F 1s (d).

3.5. Comparison of Growth Conditions for Hollow Box TiO2 Nanocrystals by the Acid Dissolution Denucleation Method, the Topological Phase Transition Method, the Sonic Solvothermal Method, and the Air Atmosphere Sintering Method

The reaction conditions for the four methods are compared, as shown in Table 2. The acid dissolution denucleation method for growing hollow box TiO2 nanocrystals is time-consuming with mild reaction conditions. TiO2 was obtained after 24 h of reaction at 45 °C, with a yield of only 52.3%. The topological phase transition method takes more time and has a lower reaction temperature. The reaction is carried out at 200 °C with a yield of approximately 95.1%. The sonic solvothermal method also takes some time and has a higher reaction temperature of 220 °C. The yield is as high as 97.3%. The air atmosphere sintering method has a short reaction time but a high temperature, and the conditions are demanding. The reaction temperature is 350 °C, with a yield of about 89.2%.

Table 2. Comparison of Conditions for the Growth of Hollow Box TiO2 Nanocrystals by Four Methods.

methods acid dissolution denucleation method (TC-1) topological phase transition method (TC-2) sonic solvothermal method (TC-3) air atmosphere sintering method (TC-4)
nuclei Fe2O3 TiOF2 TiOF2 TiOF2
temperature 60°C 200°C 220°C 350°C
reaction time 24 h 12 h 8 h 4 h
yield about 52.3% about 95.1% about 97.3% about 89.2%
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3.6. Growth Mechanism of TiOF2 and Hollow Box TiO2 Nanocrystals

3.6.1. Growth Mechanism of TiOF2

TiOF2 was grown using a solvothermal method with TBOT as the titanium source and HF as the fluorine source and exhibited a regular shape, high purity, and good crystallinity. The possible growth mechanism is depicted in Figure 9. TBOT undergoes a transesterification reaction with acetic acid, where one of TBOT’s butoxy groups is replaced by an acetate group. It then reacts with HF, forming TBOT substituted with one F atom. After a repeat of this cycle, TBOT substituted with two F atoms is obtained. It reacts with acetic acid in an esterification reaction to form Ti(OH)2F2, which finally undergoes a dehydration condensation to produce TiOF2.

Figure 9.

Figure 9

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Growth mechanism of precursor TiOF2 by tetrabutyl titanate.

3.6.2. Growth Mechanism of Hollow Box TiO2 Nanocrystals by the Acid Dissolution Denucleation Method

First, Fe2O3/TiO2 core/shell nanocubes were grown by using a kinetic control encapsulation method. Then, hollow box TiO2 nanocrystals were grown by weak acid corrosion of the hematite (Fe2O3) core. The reaction equations are shown in eqs 1–6, and the mechanism is shown in Figure 10. Under ethanol alkaline conditions, titanium tetrabutoxide was hydrolyzed and aged on the surface of the Fe2O3 nanocubes, forming Fe2O3/TiO2 core/shell nanocubes. Hydrochloric acid (HCl) entered the pores of TiO2 and reacted with Fe2O3, resulting in the formation of an FeCl3 solution. This FeCl3 solution flowed out of the pores, resulting in the formation of hollow box TiO2 nanocrystals.

3.6.2. 1
3.6.2. 2
3.6.2. 3
3.6.2. 4
3.6.2. 5
3.6.2. 6
Figure 10.

Figure 10

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Growth mechanism of hollow box TiO2 nanocrystals by the acid dissolution denucleation method.

3.6.3. Growth Mechanism of Hollow Box TiO2 Nanocrystals by the Topological Phase Transition Method or the Sonic Solvothermal Method

The principle behind the growth of hollow box TiO2 nanocrystals using the topological phase transition method or the sonic solvothermal method is Oswald ripening. This means that relatively smaller solute particles and crystals within the solution dissolve first and then accumulate on the surfaces of relatively larger solute particles and crystals.58 This theory is derived through reverse deduction, meaning that the molecular energy on the surface of particles is higher than the molecular energy inside the particles, thus making them unstable. The reaction equations are shown in eqs 7–13, and the mechanism is shown in Figure 11. The surface of TiOF2 undergoes hydrolysis with hydroxide ions in solution, resulting in the formation of titanium hydroxide. Titanium hydroxide then dehydrates to form TiO2, creating larger crystal facets on the surface of the original crystal. TiOF2 cubic crystals exhibit a porous structure, and OH– ions attack TiOF2 from various directions, forming TiO(OH)2 on both the internal and external surfaces. Dissolution of TiO2 particles generated by dehydration, which then deposit onto the nanoscale surface composed of TiO2. These six surfaces come together to form hollow box TiO2 nanocrystals.

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Figure 11.

Figure 11

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Growth mechanism of hollow box TiO2 nanocrystals by the topological phase change method or the sonic solvothermal method.

3.6.4. Growth Mechanism of Hollow Box TiO2 Nanocrystals by the Air Atmosphere Sintering Method

As shown in Figure 12, the growth of hollow box TiO2 nanocrystals using the air atmosphere sintering method involves a hydrolysis reaction. TiOF2 reacts with water vapor in air within a muffle furnace. Specifically, the cubic surface of TiOF2 starts to undergo hydrolysis, resulting in the formation of tiny TiO2 nanosheets. The reaction primarily involves two steps. TiOF2 reacts with water vapor, losing a molecule of HF, resulting in the substitution of one F on TiOF2 with a hydroxyl group. Then, after this cycle is repeated once, TiO(OH)2 is formed. This compound loses a molecule of H2O to form TiO2. Further hydrolysis of the TiOF2 cube results in the formation of hollow box TiO2 nanocrystals composed of six nanosheets.

Figure 12.

Figure 12

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Growth mechanism of hollow box TiO2 nanocrystals by the air atmosphere sintering method.

The morphologies of 3D hollow box TiO2 nanocrystals grown using the four methods are slightly different but all are composed of anatase-phase nanocrystals. Figure 13 is a schematic representation of the synthesis of hollow box TiO2 nanocrystals using these four methods.

Figure 13.

Figure 13

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Hollow box TiO2 nanocrystals grown by four nucleation methods.

3.7. Evolution Process of Hollow Box TiO2 Nanocrystals (001) Facets and Calculation of the (001) Facet Exposure Ratio

The formation process of anatase TiO2 nanocrystals is depicted in Figure 14. Octahedral anatase TiO2 single crystals gradually undergo compression, leading to the emergence of (001) facets with the area progressively increasing. The lateral area of the octahedron decreases, transforming the anatase TiO2 single crystal into extremely thin octahedral nanosheets. Six such anatase TiO2 nanosheets come together to form hollow box TiO2 nanocrystals.

Figure 14.

Figure 14

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Evolution process of hollow box TiO2 nanocrystal crystal facets.

The formula for calculating the (001) facet exposure rate is as follow59

3.7.

In the formula, θ is the theoretical value of the surface angle of (001) and (101) crystal facets, and b and a are the side length of the bipyramid and the side length of the square at the top of the apical octahedron, respectively, and 0 ≤ a/b ≤ 1. The (001) facets exposure ratio of hollow box TiO2 nanocrystals refers to the research of Huang et al.60 The result is shown in Table 3 below. The (001) facet exposure ratio of TC-3 is the largest. The (001) facets exposure ratio of TC-1 is the smallest. Their (001) facet exposure rates are different, indicating that the amount of water in reaction conditions affects the (001) facet exposure rate of hollow box TiO2 nanocrystals. The more water there is, the lower the (001) facet exposure rate of hollow box TiO2 nanocrystals.

Table 3. (001) Crystal Facet Exposure Ratios of the Samples.

samples θ (deg) a (nm) b (nm) (001) facets exposure ratio (%)
TC-1 63.76 267.6 282 80
TC-2 55.60 270.8 282 87
TC-3 46.16 279.7 288 92
TC-4 58.79 272.8 285 85
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3.8. Nitrogen Adsorption and Photovoltaic Research

In order to obtain more detailed structural information about the prepared hollow box TiO2 nanocrystals, nitrogen adsorption was used to measure the Brunauer–Emmett–Teller (BET) surface area and pore structure of the hollow box TiO2 nanocrystals. Figure 15a shows the nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves for TC-1 and TC-3. Both TC-1 and TC-3 exhibited the shape of the H3 hysteresis loop and type IV isotherm, indicating a slit-like pore shape consistent with the hollow nanobox shape formed by the assembly of six nanosheets of TiO2. Table 4 lists the pore volume, average pore size, and BET specific surface area of TC-1 and TC-3. The results indicated that TC-3 had the largest pore volume (0.099 cm3 g–1), average pore size (23.1 nm), and BET specific surface area (17.1 m2 g–1). This may be related to the multilevel hollow box structure of TC-3. Furthermore, a larger specific surface area can provide more active sites to participate in photocatalytic reactions, thus contributing to the improvement of adsorption and photocatalytic performance. Figure 15b shows the photoluminescence spectrum for TC-1 and TC-3. TiO2 exhibited a pronounced emission peak near 400 nm. Generally, a stronger emission spectrum indicates a higher rate of e––h+ pair recombination. Compared with TC-1, TC-3 had a weaker photoluminescence (PL) intensity, suggesting a lower rate of e––h+ pair recombination. This may be attributed to the multilevel structure of TC-3. As shown in Figure 15c, TC-3 exhibited a transient photocurrent response higher than that of TC-1, indicating a higher efficiency of separating photogenerated e––h+ pairs. The Nyquist plots (EIS) for TC-1 and TC-3 are shown in Figure 15d. Equivalent circuit simulations were performed by using Zview software. CPE1 and CPE2 are constant phase angle elements, R1 and R3 are electrolyte resistances, and R2 is the charge transfer resistance.61 Compared to TC-1, TC-3 had a smaller radius, indicating a lower electron transfer resistance in TC-3. The results indicated that TC-3 had a faster electron transfer rate and a lower e––h+ pair recombination rate.

Figure 15.

Figure 15

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Nitrogen adsorption isotherms and corresponding pore size distribution curves of TC-1 and TC-3 (a). PL spectra of TC-1 and TC-3 (b). Photocurrent response diagram of TC-1 and TC-3 (c). EIS of TC-1 and TC-3 (d).

Table 4. Volume, Mean Pore Size, and Specific Surface Area for TC-1 and TC-3.

samples V (cm3 g–1) pore size (nm) SBET (m2 g–1)
TC-1 0.054 18.1 12.0
TC-2 0.086 21.4 15.9
TC-3 0.099 23.1 17.1
TC-4 0.067 20.5 14.7
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3.9. Photocatalytic Properties of Hollow Box TiO2 Nanocrystals

The hydrogen production performance of test samples was evaluated through photocatalytic experiments under simulated sunlight. As shown in Figure 16a, the hydrogen production amounts for TC-1, TC-2, TC-3, and TC-4 within 4 h are 10.00, 12.97, 15.02, and 11.07 μmol, respectively. TC-3 had the highest hydrogen production, which can be attributed to the multi-hollow box structure of TC-3. From Figure 16b, it can be observed that the hydrogen production rates for TC-1, TC-2, TC-3, and TC-4 are 62.49, 81.08, 93.88, and 69.19 μmol g–1 h–1, respectively. The results indicated that the high (001) facet exposure rate and unique hollow box structure of TiO2 contribute to enhancing the photocatalytic hydrogen production performance.

Figure 16.

Figure 16

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Photocatalytic hydrogen production amount over time for the samples (a) and the hydrogen production rate graph of the samples (b).

By conducting photocatalytic degradation of RhB and MB dyes under Xe lamp irradiation, we explored the differences in photocatalytic activity of hollow box TiO2 nanocrystals prepared by four methods. Figure 17a shows the change in the C/C0 ratio of the samples over time t, where C0 is the initial concentration of RhB in the aqueous solution, and C is the remaining concentration of RhB in the solution after degradation. RhB without a catalyst and RhB with were P25 hardly degraded. TC-3 exhibited the best catalytic activity, degrading 96.59% of RhB within 70 min. This is because TC-3 has a larger (001) facet exposure and a multilevel hollow box structure. TC-1 and TC-4 showed relatively poor degradation performance due to their relatively minimal (001) facet exposure and incomplete hollow box structure. Figure 17b represents the degradation rate of the samples with respect to RhB, and the reaction follows the Langmuir–Hinshelwood first-order reaction kinetics. The photocatalytic degradation rate constant (k) for RhB can be calculated by using the formula k = −1/t ln(C/C0), where C0 and C (mg/L–1) are the concentrations of RhB at time 0 and t (min–1), respectively. The degradation rate constants for TC-1, TC-2, TC-3, and TC-4 were 0.00725, 0.03111, 0.04556, and 0.00987, respectively. Figure 17c shows the degradation of the samples with respect to MB. The degradation efficiency of the samples toward MB was weaker than that for RhB. The results for the degradation of MB by hollow box TiO2 nanocrystals prepared by the four methods were consistent with those for RhB. TC-3 exhibited the best degradation performance, degrading 75.25% of MB within 70 min. From Figure 17d, the degradation rate constants for TC-1, TC-2, TC-3, and TC-4 were 0.00837, 0.01496, 0.01959, and 0.00932, respectively. Figure 17e,f represents the five-cycle degradation of RhB and MB by TC-3. The results indicate that TC-3 possesses good photocatalytic stability. After five cycles, the degradation rates only decreased by 2.93 and 4.24%.

Figure 17.

Figure 17

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RhB photocatalytic degradation plots of the samples (a,b). Plot of ln(C0/C) vs time for the samples (c,d). Cyclic degradation of RhB and MB over the samples (e,f).

3.10. Detection and Capture of Active Species

Figure 18a,b depicts the detection of active species hydroxyl radicals (•OH) and superoxide radicals (•O2–) respectively. With increasing irradiation time, the peak intensities at 454 and 550 nm continuously increase. The results indicate that TC-3 can generate •OH and •O2– under light conditions, and their concentrations increase as time goes on. The capture of active species on TC-3 is shown in Figure 18c. The external conditions for active species capture experiments are consistent with the conditions for RhB photocatalytic degradation. In addition, 0.05 mmol of ethylenediaminetetraacetic acid (EDTA), isopropanol (IPA), benzoquinone (BZQ), and AgNO3 were added. EDTA is used to capture h+, IPA captures •OH, BZQ captures •O2–, and AgNO3 captures e–. Compared to TC-3, the degradation efficiency significantly decreased when EDTA and BZQ scavengers were added, indicating that •O2– and h+ are the main active species in the degradation reaction. When IPA and AgNO3 were used as scavengers, the degradation efficiency also decreased, suggesting that •OH and e– had some influence on the reaction. Figure 18d displays the proportion of free radicals generated during the degradation of RhB in TC-3. The ratio of •OH, e–, h+, and •O2– generated by TC-3 within 70 min was approximately 1:2.7:5.5:8.7.

Figure 18.

Figure 18

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Fluorescence detection of •OH (a) and •O2– (b) for TC-3. The influence of active species scavengers on the degradation of RhB by TC-3 (c). Proportion of free radicals generated in the degradation of RhB by TC-3 (d).

3.11. Photocatalytic Mechanism

Figure 19a shows the Mott–Schottky (MS) plot for TC-3, obtained using the formula C–2 = 2(V – VFB – kBT/e)/εε0eND.62 Here, C, V, VFB, kB, T, e, ε, ε0, and ND represent the space charge capacitance, electrode potential, flat-band potential, Boltzmann constant, temperature, elementary charge, dielectric constant, vacuum permittivity, and charge carrier density, respectively. V is the applied potential correction to AgCl of 0.197 eV.63 A positive slope indicates that TC-3 was an n-type semiconductor, consistent with TiO2. Furthermore, the VFB potential of TC-3 was obtained by the intersection of the curve’s tangent with the x-axis, which was −0.68 eV compared to Ag/AgCl and −0.48 eV compared to NHE. The voltage difference between the ECB and VFB for n-type semiconductors is typically between 0 and 0.2 V. Here, the voltage difference between ECB and VFB is 0.1 V.64 According to calculations, the ECB of TC-3 was −0.58 V. Based on the formula EVB = ECB + Eg, where Eg is seen in Figure 7b, the EVB of TC-3 was +2.32 eV. The energy band of TC-3 is shown in Figure 19b.

Figure 19.

Figure 19

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Mott–Schottky plot (a) and energy band schematic (b) of TC-3.

In n-type semiconductors, there are two types of charge carriers: h+ in the valence band and e– in the conduction band. Hollow box TiO2 nanocrystals generate e– and h+ when exposed to sunlight. e– and h+ can react with H2O, O2, and other substances. As shown in eqs 14–17, •OH and •O2– are formed on the surface of TiO2. Additionally, generated •O2– can further react to produce •OH, as shown in eqs 18–20. These active groups can reduce water to hydrogen gas and participate in redox reactions with organic pollutants on the surface of the catalyst.65Figure 20 is a schematic diagram illustrating the mechanism of photocatalytic reactions using hollow box TiO2 nanocrystals. The multiscattering effect of the hollow box structure enhances the utilization of light by the catalyst. Compared to other crystal facets, the (001) crystal facet of TiO2 has the highest surface energy (0.90 J/m2), which enhances the photocatalytic capability of TiO2.

3.11. 14
3.11. 15
3.11. 16
3.11. 17
3.11. 18
3.11. 19
3.11. 20

Figure 20.

Figure 20

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Photocatalytic mechanism of hollow box TiO2 nanocrystals.

3.12. Analysis of (001) Facet Formation under Temperature Control

Table 5 presents an analysis of the (001) facet exposure rate of TC-3 under temperature control. With increasing temperature, the (001) facet exposure rate of TC-3 initially rises and then declines. The highest (001) facet exposure rate of TC-3 was observed at a reaction temperature of 220 °C. This is because, as the temperature increases, the reaction rate accelerates. The stronger the influence of the structure-directing agent F within the molecule, the higher the (001) facet exposure rate rises. When the temperature exceeds 220 °C, the multilevel hollow box structure of TC-3 breaks down, resulting in a decrease in the (001) facet exposure rate. At the same time, a higher (001) facet exposure rate of TiO2 corresponds to increased activity and improved photocatalytic performance. As shown in Figure 21a, during the preparation of hollow box TiO2 nanocrystals, F acted as a structural orientation. The dihedral angle between the facets of the octahedron and the plane became smaller, and the exposure rate of the (001) crystal facet increased. There are two critical situations. When the dihedral angles are 90 and 0°, the exposure rates of the (001) crystal plane are 0 and 100%, respectively.

Table 5. (001) Facet Exposure Ratios of TC-3 under Controlled Temperatures.

sample temperature (°C) θ (deg) a (nm) b (nm) (001) facets exposure ratio (%)
TC-3 180 71.57 279.7 288 84
  200 63.79 279.7 288 88
  220 46.16 279.7 288 92
  240 68.29 279.7 288 86
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Figure 21.

Figure 21

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Relationship between the exposure rate of the (001) facet and the angle between the (101) facet and the reference plane.

Figure 22a,b depicts Hirshfeld surface maps and fingerprint plots generated using Crystal Explorer 21.5. These were employed to visualize intermolecular interactions and calculate interatomic contacts within the crystal framework.66 The blue and red regions represent low and high electron densities, respectively, due to short and long contacts between atoms. The white region indicates intermediate electron density, corresponding to contact points located at the van der Waals radii.67 The contributions of Ti···F, F···Ti, Ti···O, and O···Ti (TiOF) were 7.6, 10.8, 19.9, and 14.6%, respectively, and the contributions of Ti···O and O···Ti (TiO2) were 28.0 and 28.9%, respectively. This indicated that the presence of F in the molecule had an impact on the formation of the (001) crystal facet of TiO2.

Figure 22.

Figure 22

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Dnorm and fingerprint plots of TiOF (a) and TiO2 (b).

Hollow box TiO2 nanocrystals were grown using four nucleation methods: the acid dissolution denucleation method, the topological phase transition method, the sonic solvothermal method, and the air atmosphere sintering method. Possible growth mechanisms were investigated, including the nucleation dissolution reaction, Oswald ripening, and hydrolysis. All four methods can produce complete hollow box structures, primarily composed of nanosheets with high exposed (001) crystal facets, enhancing the photocatalytic performance of TiO2. Calculations revealed that the (001) crystal facet exposed rates for TC-1, TC-2, TC-3, and TC-4 were 80, 87, 92, and 85%, respectively. It was found that the higher amount of water in the preparation conditions led to a lower exposed (001) crystal facet, which was related to the structural orientation of F. Photocatalytic performance studies were conducted under simulated sunlight irradiation, and TC-3 exhibited the best photocatalytic activity. TC-3 showed a hydrogen production rate of 93.88 μmol/(g h) and photocatalytic degradation rates of 96.59% for RhB and 75.25% for MB within 70 min. This was primarily attributed to the highly exposed (001) crystal facet and multilevel hollow box structure of TC-3. This study provides some references for growing highly active TiO2 nanocrystals.

Acknowledgments

This project was supported by the National Natural Science Foundation of China (21177161 and 52272102), the Hubei Province Excellent Talents Training Plan (RCJH15001), the Natural Science Foundation of Hubei Province for Distinguished Yong Scholars (2013CFA034), the Fundamental Research Funds for the Central University, South-Central Minzu University (CXY22006 and CZY23013), and the Open and Innovation Fund of Hubei Three Gorges Laboratory (SC232015).

The authors declare no competing financial interest.

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