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Abstract
Black TiO2 formed by introducing lattice disorder into pristine TiO2 has a narrowed band gap and suppresses the recombination of charge carriers. This provides a potential strategy for visible light photocatalysis. However, the microstructural design of black TiO2 for a higher optimization of visible light is still in high demand. In this work, we proposed the preparation of black TiO2 hollow shells with controllable cavity diameters using silica spheres as templates for the cavities and the NaBH4 reduction method. The decreased cavity size resulted in a hollow shell with an enhanced visible–light absorption and improved photocatalytic performance. Moreover, we demonstrated that this cavity can be combined with gold nanoparticles (AuNPs) to form AuNPs@black TiO2 yolk–shells. The AuNPs provided additional visible light absorption and promoted the separation of photogenerated carriers in the yolk–shell structures. This further improved the photocatalysis, the degradation rate of Cr(VI) can reach 0.066 min-1. Our work evaluated the effect of the cavity size on the photocatalytic performance of hollow and yolk–shell structures and provided concepts for the further enhancement of visible–light photocatalysis.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The shortage of fresh–water resources and contamination of rivers and oceans have become urgent global issues [1–5]. Photocatalysis, particularly visible light catalysis, is a green and sustainable technology that utilizes the abundant and clean solar energy to degrade pollutants. This makes it a preferable technology in the context of water contamination problems. Photocatalysis is based on the redox reaction of photocatalysts under sunlight excitation, which catalyzes the decomposition of water, organic matter, and heavy metal ions [1,2,6,7]. Photocatalysts are at the core of photocatalysis technology. Researchers have conducted substantial work on the development of efficient photocatalysts. Since the identification of the capability of titanium dioxide (TiO2) to photolyze water by Fujishima and Hondain et al. [8] in 1972, TiO2 has shown significant potential as a preferable high–performance photocatalyst for various photocatalytic reactions because of its chemical stability, nontoxicity, and high reactivity [9–11]. However, excessive bandwidth and fast photogenerated carrier complexation [12–14] have limited the application prospects of TiO2. Many strategies have been proposed to solve this problem through energy band engineering using metal or non–metal doping [15–17] and noble metal composites [18]. These strategies can extend the light absorption range to a certain extent. However, none can cover the visible light range. In 2011, Chen et al. reported the identification of black TiO2 [12].
Black TiO2 is a special core–shell structure material with a crystalline interior and an amorphous exterior that forms many oxygen vacancies on the surface of pristine TiO2. It has a lower band–gap and significantly wider light absorption band across the entire visible and near–infrared spectra [12,19,20]. Since then, the preparation and modification of black TiO2 has received significant attention. Several preparation methods have been developed, such as hydrogen thermal treatment [12], hydrogen plasma treatment [21], aluminum [22] and zinc reduction [23], and hydrothermal treatment [24]. These can be classified into reduction, oxidation, and three other major categories. Morphological design has been demonstrated to be an effective strategy to further improve the catalytic performance [25,26]. For example, TiO2 nanorods [23,27], nanosheets [28], nanowires [29], and nano–hollow shells [30–33] have been prepared and demonstrated to enhance the photocatalytic performance. Among these, porous hollow shells with abundant specific surface areas provide a large number of catalytically active sites. This increases the adsorption of substrates [34–36], enables light to enter the interior and be reflected several times inside the cavity, increases light trapping [37–40], and results in the most significant improvement in photocatalytic performance. The thickness of the hollow shell was observed to influence the photocatalytic performance [35]. Furthermore, compounding with other materials is an effective strategy [41–44]. For example, Fu et al. compounded AuNPs on the outer surface of TiO2 hollow spheres to enhance the photocatalytic performance and investigated the effect of loading amount [45]. Singh et al. prepared Ag–modified TiO2 nanorod arrays with remarkable photocatalytic activity and recyclability [46]. The synergistic effect of multiple components can achieve improved photocatalytic properties compared with single–component materials. Most previous studies have focused more on the effect of the composite ratio on the performance. Pristine TiO2 encapsulated with other materials to form core–shell or yolk–shell structures has shown substantial potential for improving the stability of core materials, reducing the agglomeration of nanoparticles, and designing multifunctional photocatalysts [47,48]. Similarly, a few studies have reported that structural parameters influence the final photocatalytic performance of yolk–shell structures [35,49]. However, few studies have systematically investigated hollow shells and yolk–shells, both of which have a cavity structure, for the effect of the cavity size on the photocatalytic and light absorption properties.
In this study, we designed and prepared black TiO2 hollow shells with an equal shell thickness (∼50 nm) but with controllable cavity diameters ranging from ∼60 to ∼380 nm. The fabrication process was controlled meticulously to obtain hollow shells with a uniform shell thickness while avoiding the hollow structures collapsed and cracked under high–temperature conditions. This indicates that the black TiO2 hollow shells with decreased cavity size resulted in improved visible–light absorption and correspondingly enhanced the photocatalytic degradation performance. Furthermore, this phenomenon was validated by modulating the cavity size of the yolk–shell structure, and black TiO2 hollow shells were combined with AuNPs to form AuNPs@black TiO2 yolk–shells. AuNPs provide additional visible light absorption because of their surface plasmon resonance (SPR). Moreover, these can facilitate the separation of photogenerated carriers in the yolk–shell structure, which can further enhance the photocatalytic effect. Our work provided significant insights into the improvement of the light utilization and photocatalytic efficiency of black TiO2. It would promote the application of black TiO2 in wastewater purification and water recycling.
2. Experimental
2.1 Preparation of black TiO2 nanoparticles
TiO2 particles were obtained by hydrolysis. Briefly, 1.7 ml of TBOT was mixed with 0.4 ml of KCl solution (0.1 mol/L−1) and 100 ml of alcohol under vigorous magnetic stirring for 30 min (700 rpm). Then, it was reacted at 90°C in an oil bath for 2 h, centrifuged (7500 rpm), washed, dried (60°C, 2 h), and crystallized at 500°C to obtain TiO2 particles. Black TiO2 particles were prepared using the NaBH4 reduction method [50]. A 1:1 sample was mixed with NaBH4 and ground thoroughly for 30 min. The mixture was transferred to a porcelain boat and compacted. Then, it was heated to 350 °C at 10 °C/ min under N2 gas protection in a tube furnace (CHY–1200) and held for 40 min. After being cooled, the product was washed with deionized water and freeze–dried (FD–1A–50).
2.2 Preparation of microporous black TiO2 hollow shells with controllable cavity size
Silica spheres were prepared using the Stöber method [51], with marginal modifications. Ninety milliliters of alcohol, 17.5 ml of deionized water, and 3.5 ml of TEOS were mixed by magnetic stirring. Then, 2.5 ml of ammonia was added dropwise at 25 °C and stirred for 4 h (700 rpm). The size of the cavity was controlled by regulating the size of the silica balls. The size was regulated by controlling the amount of ammonia added while the other parameters remained constant (the raw materials and dosages of different diameters of SiO2 are shown in Table S1). The silica particles were centrifuged (7500 rpm), cleaned, collected, and redispersed in 80 ml of ethanol using ultrasonication (500 W). The EISA coating process was conducted according to a previous report [30] with marginal variations. Twenty milliliters of redispersed SiO2 was removed and added to 80 ml of ethanol, and 0.4 g of HPC was added gradually. Then, 0.4 ml of water was added and stirred vigorously for 30 min. Five milliliters of alcohol and 1 ml of TBOT were mixed (alcohol:TBOT = 5:1) and dropped into the solution with a peristaltic pump for 40 min. This was followed by condensation reflux at 90 °C for 120 min. After centrifugation and cleaning, the TiO2–coated SiO2 (SiO2@TiO2) particles were obtained. The TiO2 shell thickness was controlled by varying the amount of TBOT added (the dosages are listed in Table S2). The solid was ultrasonically dispersed in 50 ml of water, and 0.8 g of PVP was added gradually and stirred for 12 h (700 rpm). The first step was repeated after centrifugal cleaning to enclose another layer of SiO2, and SiO2@TiO2@SiO2 particles were obtained after centrifugal cleaning and drying. It was calcined at 800 °C for 8 h to fully crystallize the amorphous TiO2 into anatase TiO2. The crystallized SiO2@TiO2@SiO2 samples were etched off the surface and internal SiO2 in a strong alkali solution (NaOH), centrifuged, cleaned, and dried to obtain TiO2 hollow shells. The black TiO2 hollow shells were prepared using the NaBH4 reduction method. The entire process is illustrated in Fig. 2(a).
2.3 Preparation of AuNPs@black TiO2 yolk–shells
To place the AuNPs inside the hollow shells, the SiO2 spheres were replaced with SiO2–coated AuNP (AuNPs@SiO2) spheres. The AuNPs were prepared via citric acid reduction. Briefly, 1.8 ml of HAuCl4 solution was mixed with 30 ml of water and stirred vigorously in an oil bath at 100 °C. After boiling, 1 ml of freshly prepared aqueous sodium citrate solution with a mass fraction of 3% was added rapidly. The heating continued, and it was stirred for 30 min. After cooling to room temperature, 0.235 ml of an aqueous PVP K30 solution with a concentration of 12.8 mg/ml was added and stirred for 24 h. The resulting solution was centrifuged (11000 rpm, 15 min) and redispersed in 2 ml of deionized water. One milliliter of this solution was added to a mixture of 23 ml of alcohol and 3.3 ml of deionized water under vigorous stirring. Then, 0.62 ml of ammonia were added and stirred for 30 min, and 0.86 ml of TEOS was added dropwise. The resulting solution was stirred for 12 h at room temperature, centrifuged (11000 rpm, 5 min), and ultrasonically dispersed in 20 ml of ethanol to obtain an ethanol solution of AuNPs@SiO2. Similarly, different sizes of AuNPs@SiO2 were obtained by controlling the amount of TEOS (Table S3). The subsequent preparation process was identical to that used to prepare the hollow shells.
2.4 Photocatalytic degradation of TC–HCl and reduction of Cr(VI), and active species trapping experiments
To evaluate the visible photocatalytic activity of the catalyst, TC–HCl and Cr(VI) were selected as the targets for catalytic degradation and reduction under irradiation by a xenon lamp with a 400 nm cut–off filter. Specifically, 30 mg of the catalyst was added to 100 ml of a TC–HCl solution (40 mg/L) under light–proof conditions and dispersed completely in the solution by sonication. The suspension was stirred for 30 min to attain adsorption equilibrium. Subsequently, the light source was switched on for photocatalysis. Moreover, 4 ml of the suspension was removed, filtered, and centrifuged every 10 min to obtain the supernatant. The absorption spectra were measured using a UV–Vis spectrophotometer. The absorbance of the characteristic absorption peak (λ = 355 nm) was used to represent the substrate concentration.
Cycling experiments are effective for studying the stability of photocatalysts. The photocatalysts after photocatalytic degradation experiments were separated by centrifugation (7500 rpm), washed, and dried (60°C, 2 h) for the next catalytic experiment.
In addition, 30 ml of Cr(VI) solution (20 mg/L of K2Cr2O7 solution as the source of Cr(VI)) was prepared as the substrate. The pH of the solution was adjusted to 2.0 with dilute sulfuric acid. In a dark room, 30 mg of the catalyst was added to the solution and sonicated. After stirring for 30 min in the dark, visible–light catalytic experiments were performed. One milliliter of the solution was removed from the reaction vessel every 10 min and diluted to 4 ml. The absorbance of the supernatant at λ = 350 nm was tested after filtration and centrifugation to characterize the residual amount of the substrate.
IPA, BQ, AO, and KIO3 were configured as 0.01 M and added to the substrate solution as scavengers for the active species trapping experiments. Then, photocatalysis was performed according to the procedure described above.
2.5 Characterizations
X–ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance XRD (2θ range of 20°–80°). X–ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific K–Alpha) was used to analyze the chemical compositions of the samples. An electron paramagnetic resonance (EPR) spectrometer (Bruker E500) was used to characterize the oxygen vacancies. Transmission electron microscopy (TEM) was used to observe the hollow shell structures. High–resolution transmission electron microscopy (HRTEM) was used to observe the amorphous surface layers. The UV–Vis absorption spectra were measured using a UV–Vis spectrophotometer (Shimadzu, (UV–3600). Nitrogen adsorption–desorption isotherms were collected at 77 K using an Autosorb–iQ (Quantachrome Instruments) nitrogen adsorption apparatus. The surface areas were calculated based on the Brunauer–Emmett–Teller (BET) equation. The pore size distributions were obtained by the Barrett–Joyner–Halenda (BJH) method using the adsorption branches of the isotherms.
3. Results and discussions
The pristine TiO2 nanoparticles were processed using the NaBH4 reduction method to form black TiO2. As shown in Fig. 1(a), the HRTEM morphology clearly shows a disordered surface layer on the particles after reduction. It corresponds to a macroscopic color variation from white to dark blue. The disordered layer can be ascribed to the increased number of oxygen vacancies, which result in amorphous characteristics. This phenomenon was verified by the XRD patterns and EPR spectra. Figure 1(b) shows that the TiO2 before and after reduction present nearly identical characteristic peaks that are similar to those of anatase TiO2 (JCPDS No. 21–1272). However, the diffraction peaks of black TiO2 appear significantly broader, which indicates a decrease in the crystallinity. Figure 1(c) presents the EPR spectra of the two samples. There is almost no peak in the EPR spectrum of pristine TiO2, whereas two signals are observed for black TiO2. The two EPR signals at g ≈ 1.956 and g ≈ 2.001 are attributed to Ti3+ and oxygen vacancies [52,53], respectively. This indicates that the reduction process introduced many point defects, which caused the formation of amorphous layers. Furthermore, the UV–Vis absorption spectra shown in Fig. 1(d) indicates that black TiO2 exhibited a remarkable light absorption and wide absorption range (200 nm–1800nm).
Fig. 1. Structural characterization of black TiO2 particles. (a) HRTEM images and optical photographs of white TiO2 and black TiO2 particles. (b) XRD patterns. (c) EPR spectra. (d) UV–Vis absorption spectra.
Black TiO2 nanoparticles with a hollow structure were prepared by the template method. Here, silica was used as a template, and a protective layer was etched off after TiO2 crystallized. The dimensional parameters of the hollow shell, such as the diameter of the cavity and thickness of the shell layer, have been demonstrated to play important roles in determining its optical properties [35,49]. In this study, the preparation method enabled an effective regulation of the above parameters. By controlling the amounts of TEOS and TBOT, a series of samples with equal shell thickness (∼50 nm) and various cavity diameters (∼60, ∼150, and ∼380 nm) were obtained. These were denoted by 60@50 (Fig. 2(b)), 150@50 (Fig. 2(c)), and 380@50 (Fig. 2(d)), respectively. Furthermore, transmission electron microscopy (TEM) images showed that all the three groups of hollow shells had good integrity and dispersion.
Fig. 2. Preparation of black TiO2 hollow shells. (a) Schematic diagram of the fabrication process of black TiO2 hollow shells by the template method. TEM images of black TiO2 hollow shells with equal shell thickness (∼50 nm) but different cavity sizes: (b) ∼60 nm, (c) ∼150 nm, and (d) ∼380 nm.
The specific surface area and pore size affect the photocatalytic performance. Therefore, we measured the N2 adsorption and desorption isotherm curves and the pore size distribution curves of all the samples. The hysteresis loops in the curves (Figure S1 a–c) indicate that all the samples had a good pore structure. The pore size distribution curves (Figure S1 d–f) show that the pore sizes of the three samples were distributed in the range 2–5 nm. Notably, the specific surface areas of the three groups of the samples 60@50, 150@50, and 380@50 obtained by the BET method were 267 m2/g, 200 m2/g, and 104 m2/g, respectively. These decreased significantly with an increase in the cavity diameter.
By controlling the cavity size, hollow shells (150@50) with a size nearly equal to that of the particles (∼250 nm) were obtained. The comparison revealed that the light absorption capacity of the hollow shells was enhanced further (Fig. 3(a)). This was caused by the capability of the incident light to enter the interior of the hollow shell and the multiple reflections at the inner wall of the cavity [38,39]. The light absorption capabilities of the samples with different cavity diameters were tested (Fig. 3(c)). The black TiO2 hollow shells exhibited absorption in the visible range (380 nm–780 nm), thereby indicating a band gap reduction. Evidently, the light absorption of the sample was enhanced as the cavity diameter decreased. We analyzed two main aspects to determine the effect of cavity size on light absorption. Because the amorphous layer was present on the surface of the sample, a larger specific surface area indicated more amorphous layers. The EPR spectra (Figure S2) of several samples show that the smaller the cavity, the higher was the oxygen vacancy concentration. This agreed with our analysis. However, the samples with small cavities displayed more reflections of light in the same period or optical range. This caused an increase in light absorption. The combination of these factors resulted in a negative correlation between the light absorption performance of the sample and the cavity size.
Fig. 3. Light absorption properties of black TiO2 hollow shells (a) Comparison of the light absorption capacity of the broad spectrum of black TiO2 hollow shells, black TiO2 particle, and TiO2 of equal size. (b) Schematic diagram of the principle of light absorption enhancement. The porous hollow shell structure enables light to enter the interior to produce multiple reflections compared with solid particles. (c) The light absorption spectra of black TiO2 hollow shells with different cavity diameters in the visible range reveal improved light absorption performance as the cavity diameter decreases.
TC–HCl was selected as the target degradant to compare the visible photocatalytic properties of all the samples. Because the substrate concentration was linearly correlated with the absorbance of the characteristic peak, the residual amount of the substrate (Cx / C0) was obtained from the ratio of the absorbance (Cx) at each set time point to the absorbance (C0) at the starting point. Figure 4(a) shows the catalytic curves of the samples and control group. The self–degradation of TC–HCl under visible light was negligible. Under identical light conditions, the color of the solution containing the added catalyst faded significantly. This indicated that the concentration of TC–HCl decreased. In contrast, the sample with the smallest cavity diameter (60@50) showed the strongest catalytic activity, with a 79.5% reduction in TC–HCl concentration after 40 min of visible light irradiation. The concentrations of TC–HCl in the other two groups (150@50 and 380@50) were reduced by 68.7% and 62.5%, respectively. To demonstrate the degradation rate more visually, the degradation data were transformed and fitted using the pseudo–first–order kinetic equation ln(C / C0) = kt. The reaction rate constant (k) versus time is shown in Fig. 4(b). The degradation reaction rate constants for the three groups of samples (60@50, 150@50, and 380@50) were 0.045, 0.031, and 0.026 min−1, respectively. Cycling experiments were performed to evaluate the chemical stabilities of the catalysts. The results are shown in Fig. S3. It can be observed that 60@50 ensured a degradation efficiency of nearly 75% after five cycles. This demonstrates that the catalyst has good stability and reuse value.
Fig. 4. Photocatalytic performance of black TiO2 hollow shells with different cavity sizes for the degradation of TC–HCl and Cr(VI). (a) Degradation curves of TC–HCl under visible light. (b) Variation in –ln(C / C0) versus time for the degradation process of TC–HCl. (c) Degradation curve of Cr(VI) under visible light. (d) Variation in –ln(C / C0) versus time for the degradation process of Cr(VI).
To further evaluate the photocatalysis, we used another target, Cr(VI), to study the degradation capability of the samples (Fig. 4(c),(d)). Similarly, Cr(VI) did not self–degrade in the control group without catalyst addition. With the addition of the catalyst, the Cr(VI) concentration in darkness reduced, which could be attributed to adsorption [54–56]. Moreover, Cr(VI) reduced under visible light and the catalytic capacity increased as the diameter of the catalyst cavity decreased. The 60 min reduction rates of 60@50, 150@50, and 380@50 were 90.0%, 63.9%, and 51.4%. The calculated k values were 0.0283, 0.0125, and 0.0093 min−1, respectively.
The cavity structure of the black TiO2 proposed in this study provides a platform for combining with other nanoparticles. Here the AuNPs were encapsulated in the cavities of black TiO2 (AuNPs@B–TiO2) to form a yolk–shell structure. The main optical properties of AuNPs are closely related to their SPR, which is the absorption and radiation of electromagnetic waves by free electrons in metals driven away from the nucleus by electromagnetic fields in a collective excitation mode [57]. For spherical AuNPs, SPR mainly manifests as an absorption band in the visible spectral region. This combination further enhances the light absorption of the samples. A schematic diagram of the preparation process is shown in Figure S4a. AuNPs were prepared using the sodium citrate reduction method. PVP was added to modify the surface. Then, the surface of the AuNPs was enclosed by a layer of SiO2. The subsequent preparation process was identical to that used to prepare the hollow shells. Similarly, the structural parameters of the yolk–shell and cavity diameters were modulated by controlling the amount of the precursors. Black TiO2 yolk–shells with AuNP cores (∼20 nm); black TiO2 shells (∼50 nm); and cavity diameters of ∼60, ∼150, and ∼250 nm were prepared. These were named as Au20@60@50, Au20@150@50, and Au20@250@50, respectively. One of the main features of black TiO2 is the introduction of self–doped defects on its surface. This forms a crystalline layer, as observed in the HRTEM images. Figure S4d shows a thin amorphous layer on the surface of the hollow shell. Clear lattice stripes can be observed in the crystalline part with a measured crystal plane spacing of approximately 0.35 nm, which is the lattice stripe of the anatase TiO2 (101) surface. It was verified that the internal TiO2 exhibited good crystallinity.
The XRD pattern (Figure S5) also verifies this result, with TiO2 crystallizing as the anatase phase in the yolk–shell and AuNPs displaying good crystallinity. After the reduction treatment, TiO2 decreased in crystallinity owing to the introduction of lattice defects. Therefore, although the intensity of the characteristic peaks decreased, it did not significantly affect the crystallinity of the AuNPs. Meanwhile, the effect of the cavity diameter on the crystallinity of the core and shell materials was not significant.
Furthermore, the elemental distribution of AuNPs@B–TiO2 was characterized by high–angle annular dark–field scanning transmission electron microscopy (HAADF–STEM) and an energy spectrometry face–scan elemental analysis. As shown in Figure S4 f–i, the Ti and O elemental distributions demonstrate that the shell layer consisted of TiO2, whereas the inner particles were AuNPs. Significant gaps existed between the core and shell, which is a typical yolk–shell structure with core–shell separation.
XPS was used to analyze the valence states of the elements on the surfaces of the samples. The Ti 2p spectrum of the AuNPs@B–TiO2 yolk–shell sample (Figure S6a) contains two peaks of Ti 2p3/2 and Ti 2p1/2 that can be divided into four peak positions. The two peaks located at 458.79 eV and 464.62 eV correspond to Ti4+, whereas those at 458.47 eV and 463.97 eV correspond to Ti3+ [58] . This further demonstrates that the surface of the external TiO2 shell was partially reduced by the reduction treatment. Theoretically, the thermal decomposition of NaBH4 produces reactive hydrogen [59], which removes lattice oxygen from the surface by forming H2O while leaving oxygen vacancies and reducing Ti3+ ions [60]. In contrast, the two peaks in the Ti 2p spectrum of the yolk–shell before reduction treatment in Figure S6c could not be separated. Moreover, the two peaks at 458.49 eV and 464.23 eV (both of which are Ti4+ peaks) demonstrate that Ti3+ was almost absent from the samples.
The EPR spectra of the yolk–shell structures before and after reduction are shown in Figure S7 for detecting point defects in the samples. Almost no oxygen vacancies were observed in the AuNPs@W–TiO2 yolk–shell structures. In contrast, a Ti3+ signal located at g = 1.952 and an oxygen vacancy signal at g = 2.003 appeared in the spectrum of AuNPs@B–TiO2 yolk–shells. This was consistent with previous studies [52,53]. Combined with the above characterization methods, it was clearly demonstrated that the surface shell layer was black TiO2. Moreover, this preparation method did not affect the state of the internal AuNPs.
The visible light absorption spectra reveal an absorption peak for the AuNPs at 525 nm (Fig. 5(c)). Comparing the hollow and yolk shells with identical size–parameters, the absorption spectrum of the yolk–shell has an absorption peak at 525 nm, and the visible light absorption capacity was also enhanced. To reveal the mechanism of the AuNPs@B-TiO2 yolk–shell nanostructures to improve the light absorption performance, the electric field distribution of the SPR of AuNPs has been simulated with CST simulation software, as presented in Fig. 5(d). The corresponding model structure is established, according to TEM images shown in Fig. 5(b) (diameter of Au core, inner diameter and outer diameter of black TiO2 hollow shells are 20, 60, and 160 nm, respectively). An electromagnetic pulse ranging in 420–800 nm for the incident light was launched to the targeted Au20@60@50 to simulate the interaction of visible light with nanostructures. In the calculations, the electromagnetic wave is vertically incident on the model from the z direction, the x and y directions are open (add space) boundary condition, and the optical parameters of black TiO2 and AuNPs were set with reference to previous works [42,61]. The simulation results showed that an obvious electric field was generated around the AuNPs in the yolk-shell under visible light irradiation. This is consistent with the observed enhancement of visible light absorption and the appearance of AuNPs absorption peak in the spectra. Furthermore, the visible absorption spectra of yolk–shell structures with a range of cavity diameters were compared (Fig. 6(a)). The light absorption capacity of the samples increased to different degrees in the visible region as the cavity size decreased: the sample with the smallest cavity size, Au20@60@50, displayed the best light absorption properties. In addition, when the cavity size increased, the AuNP peak became less pronounced. We used the electromagnetic field simulation software COMSOL Multiphysics to compute the absorption features of the black TiO2 hollow shells with Au nanoparticles. The refractive index of the dielectric materials was set to 1.33 for simplicity. The refractive index and extinction coefficient of the black TiO2 material as functions of wavelength were set to be identical to those in previous works [61]. Figure 6(b) shows the absorption properties of different types of black TiO2 yolk–shell structures. These agree with the measured results. The sample with a small cavity diameter caused the AuNPs to occupy a larger relative volume, and the light was more likely to reach the surface of the AuNPs. This resulted in more apparent AuNP absorption peaks for the samples with small cavities.
Fig. 5. Characterization of AuNPs@B–TiO2 yolk–shell. (a) Schematic diagram of the structure of the AuNPs@B–TiO2 yolk–shell. (b) TEM images of the AuNPs@B–TiO2 yolk–shell. (c) Visible light absorption spectra of hollow shell, AuNPs, and AuNPs@B–TiO2 yolk–shell. (d) The distribution of the electric field for the SPR of the AuNPs in Au20@60@50 simulated by CST simulation software.
Fig. 6. Light absorption properties of AuNPs@B–TiO2 yolk–shell. (a) Light absorption spectra of AuNPs@B–TiO2 yolk–shell with different cavity diameters. It shows that visible light absorption increases with a decrease in the cavity size. (b) The absorption properties of different types of AuNPs@B–TiO2 yolk–shell simulated by COMSOL Multiphysics simulation software. (c) Schematic diagram of multiple reflections of light inside the yolk–shell. The more the AuNPs account for the total cavity volume in samples with a small cavity diameter, the more the light that can be irradiated on the AuNPs.
Similarly, the photocatalytic performance of various AuNPs@B–TiO2 yolk–shell samples was also evaluated (Figure S8). For TC–HCl, it degraded by 79.27% within 30 min at a rate of 0.059 min−1 (Figure S8b) and reduced Cr(VI) at a rate of 0.066 min−1 (Figure S8d). The rates of degradation of both the targets were enhanced compared with those of hollow shells of an equal size (Fig. 7). The experimental results illustrate that the yolk–shell structure improved further in terms of the light absorption capacity and photocatalytic capability compared with the hollow shells. Table 1 presents the previous work on the reduction of Cr(VI) by TiO2–based photocatalysts, with Au20@60@50 showing good reduction rates under visible–light irradiation.
Fig. 7. Comparison of photocatalytic performance of AuNPs@B–TiO2 yolk–shells and hollow shells for the degradation of Cr(VI). (a) Degradation curves of two photocatalysts for the degradation of Cr(VI) under visible light. (b) Variation in –ln(C/C0) versus time for the degradation of Cr(VI) process.
Table 1. Comparison of different photocatalysts for Cr(VI) reduction under visible light.
To study the dominant active species in the photocatalytic process and clarify the mechanism, active–species trapping experiments were conducted using different scavengers in the photocatalytic process. IPA, BQ, AO, and KIO3 were added as ⋅OH, superoxide radical (⋅O2−), hole (h+), and electron (e−) scavengers [35,56,62–64], respectively. The experimental results (see Figure S9) show that the main active species for degrading TC–HCL are h+ and ⋅O2−, and that for reducing Cr(VI) is e−. Meanwhile, the other scavengers do not have a significant effect on the reduction rate. AO can quench h+ and improve the separation efficiency of charge carriers. This can marginally promote reduction. BQ is photolyzed during the production of hydroquinone and semiquinone. This may also contribute to the reduction of Cr(VI) [56].
Based on the experimental results, the photocatalytic reaction mechanism of the AuNPs@B–TiO2 yolk–shell is proposed in Fig. 8. For the structure with AuNPs encapsulated inside the black TiO2 and separated core–shell, the black TiO2 shell layer absorbed the incident light and excited photogenerated carriers. These were captured by AuNPs through the heterogeneous interface between the AuNPs and shell contact. This improved the separation efficiency of the photogenerated carriers in the shell layer. In addition, the incident light passed through the shell on the inner wall to undergo multiple reflections, and part of it hit the shell again and was absorbed. Meanwhile, the other part excited the SPR of the AuNPs to be absorbed by the AuNPs. Meanwhile, a few high–energy hot electrons were excited and transferred to the TiO2 surface for chemical reactions. The photogenerated electron–hole pairs generated by the above process and the reactive groups obtained from these can participate in photocatalytic redox reactions. Therein, the reactive groups (⋅O2−) and holes can degrade pollutants such as TC–HCl, whereas the electrons can reduce Cr(VI). In addition, the effects of the cavity size and AuNPs on the photocatalytic performance are summarized. The surface defects in black TiO2 introduce defect energy levels in the energy band (thereby expanding the range of light harvesting to generate more photogenerated carriers). At the appropriate concentration, these function as traps facilitating electron–hole separation and transportation, and contribute to the adsorption of the substrate. Thus, variations in the specific surface area resulting from the cavity–size control cause variations in the surface defect content. This, in turn, affects the photocatalytic performance. The photogenerated carriers generated by black TiO2 are transferred to the AuNPs owing to the Fermi energy level difference, which also promotes electron–hole separation. Simultaneously, the hot electrons generated by the AuNPs owing to the SPR effect would also be involved in the photocatalytic reaction, and the two work in combination to promote the photocatalytic efficiency.
4. Conclusions
To conclude, a series of black TiO2 hollow shells with equal shell thickness and controllable cavity size were prepared using the template method and NaBH4 reduction. Additionally, AuNPs were encapsulated to form AuNPs@B–TiO2 yolk–shell structures. The light absorption performance of the yolk–shell was enhanced by the multiple internal reflections and SPR of the AuNPs. The light absorption performance improved further as the cavity diameter decreased. This was consistent with the simulated calculation results. Meanwhile, the increase in the specific surface area provided more active sites and AuNPs to promote the separation of photogenerated carriers. This resulted in a remarkable photocatalytic performance. A direction for future research is to obtain the structural parameters with the best performance by adjusting other parameters such as the thickness of the hollow shell and size of the AuNPs. Our work enhanced the light absorption and photocatalytic performance through a fine modulation of the nanostructure of black TiO2 hollow shells and AuNPs@B–TiO2 yolk–shells. This has made these potential candidates for catalysts for wastewater degradation and heavy metal reduction, and provides new concepts for improving the performance of other nanostructured catalysts.
Funding
National Natural Science Foundation of China (U20A20212); Sichuan Province Science and Technology Support Program (2022NSFSC0498); Fundamental Research Funds for the Central Universities.
Disclosures
The authors declare no conflicts of interest related to this study.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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