Facile Synthesis of Flower-Like TiO 2 -Based Composite for Adsorption–Photocatalytic Degradation of High-Chroma Methylene Blue

: A ﬂower-like TiO 2 -based composite (denoted as Zn-Ti-6) was prepared using a ﬂower-like zinc oxide template for adsorption–photocatalytic degradation of high-chroma methylene blue. The reaction took place in an alkaline environment following hydrochloric acid treatment to remove the template and form TiO 2 -based composite. Sodium hydroxide played both roles of morphology-directing agent and reactive etchant. The possible mechanism for the formation of ﬂower-like Zn-Ti-6 was proposed. The adsorption and photocatalytic degradation behavior of Zn-Ti-6 on methylene blue (MB) removal was also investigated. The results revealed that Zn-Ti-6 showed better adsorption and photocatalytic degradation performance than TiO 2 nanoparticles owing to its much larger speciﬁc surface area, more abundant hydroxyls, and lower photoluminescence intensity. The adsorption and photocatalytic degradation data of Zn-Ti-6 were well ﬁtted to the pseudo-second-order and pseudo-ﬁrst-order kinetics models, respectively. The excellent adsorption performance of Zn-Ti-6 is largely beneﬁcial to the subsequent photocatalytic degradation performance for high-chroma wastewater treatment. Overall, this study contributes a facile fabrication strategy for ﬂower-like TiO 2 -based composite to achieve the adsorption–photocatalytic degradation of high-chroma wastewater.


Introduction
With the rapid development of the textile industry, the resultant organic water pollutions pose a long-term threat to the environment and human health [1,2]. The wastewater from textile dyeing containing high concentrations of dyestuffs, such as methylene blue (MB), is one of the most harmful industrial effluents [3,4]. Currently, a large number of photocatalytic degradation technologies have been developed to remove the dye residues from wastewater. However, the photocatalytic degradation technology performs poorly, especially for high-chroma wastewater. The reason is that the poor light transmittance of high-chroma wastewater suppresses the photocatalyst performances [5][6][7]. In addition, adsorption technology has been widely applied in the field of wastewater treatment for its simple operation and generality [8]. For example, activated carbon is one of the most common adsorbents. However, adsorption alone cannot degrade toxic substances or dyes into non-toxic and pollution-free H 2 O and CO 2 , etc. [9,10]. Recently, an adsorptionphotocatalytic degradation strategy has been proposed by many researchers [11][12][13].
To date, there are two main strategies to achieve adsorption and photocatalytic degradation. A typical method is to stabilize the photocatalyst (titanium dioxide (TiO 2 ), zinc oxide (ZnO), etc.) on the surface of the adsorbent (zeolites, clays, activated carbon, etc.) [14][15][16]. However, the uneven distribution of the photocatalyst exerts a great impact on the photocatalytic degradation efficiency. An alternative method is to improve the adsorption capacity of the photocatalyst itself through structural adjustment [17,18]. For example, TiO 2 is cost-effective, stable, and capable of degrading most pollutants [19]. Kasuga et al. synthesized a new TiO 2 nanotube through a simple wet chemical method with a 5-10 M sodium hydroxide (NaOH) solution [20]. The TiO 2 nanotube, featuring a large specific surface area, displayed not only excellent adsorbing behaviors, but also a certain photocatalytic activity. Since then, a series of relevant work on such material was implemented to further enhance its properties [21][22][23].
The performance of titanate can be improved by engineering the micro/nanoarchitecture morphology, such as three-dimensional (3D) hierarchical architectures [24]. The 3D hierarchical architectures can avoid the aggregation of low-dimensional nanomaterials, increase the reaction contact surface of the materials, and facilitate the adsorption and photocatalytic performances of the materials. Kuang et al. reported a facile synthesis of protonated flower-like titanate through one-step solvothermal method in the presence of hexamethylenetetramine [25]. The flower-like titanate possessed an extremely large surface area and displayed excellent adsorption performance. The formation of the flower-like titanate is attributed to the intercalation of amino-derivatives or NH 4 + of hexamethylenetetramine. Wang et al. investigated a facile strategy to prepare hollow 3D hierarchical titanate microspheres through the NaOH hydrothermal process. The manipulation of titanate morphology by changing the NaOH concentrations led to the evolution of different morphologies based on approach [26]. Zhang et al. synthesized a 3D hierarchical hollow sodium titanate microsphere via a silica microsphere template-assisted method. The silica microspheres were finally removed by the NaOH hydrothermal process [27]. The samples obtained were with a high surface area and the typical MB adsorption capacity. Although there are many studies that demonstrate the enhanced adsorption and photocatalytic capacities of 3D hierarchical titanate, few reports as yet reflect the synthesis of 3D hierarchical TiO 2 -based composite [28,29]. Significantly, it is still a great challenge to synthesize 3D hierarchical TiO 2 -based composite and study their formation mechanisms and performances.
Herein, we developed a facile alkaline hydrothermal procedure to prepare flowerlike TiO 2 -based composite for adsorption-photocatalytic degradation, and the proposed synthesis mechanism is shown in Figure 1. NaOH was a morphology-directing agent and reactive etchant. The flower-like ZnO was synthesized and used as a sacrificial template. The proper NaOH concentration contributed to form a flower-like ZnO template and further formed the sodium titanate, which was produced by the reaction of TiO 2 and NaOH. Sodium titanate self-assembled onto the ZnO template to form flower-like morphology. After hydrochloric acid (HCl) treatment, the ZnO template was completely removed, and sodium titanate was converted into TiO 2 -based composite. Furthermore, the formation mechanism of TiO 2 -based composite with different morphologies obtained under different NaOH concentrations was studied and proposed. Moreover, the adsorption and photocatalytic performances of the flower-like TiO 2 -based composite obtained toward MB solution were investigated and compared with those of TiO 2 nanoparticles.

Morphologies
To understand the formation mechanism of the flower-like TiO2-based co the morphology depending on NaOH concentration was investigated from field scanning electron microscopy (FESEM) and transmission electron microscopy (T shown in Figure 2. The prepared samples were denoted as Zn-Ti-X, and samples P25 or Zn(NO3)2·6H2O were also prepared following the same process, named Zn X for comparison, where X = 4, 5, 6, 7, and 8 M represented NaOH concentratio precursor solution. From Figure 2a, it can be seen that TiO2 nanoparticles (P2 spherical particles. In addition, the TEM of P25 presented a granular structure w ameter of about 20-30 nm in Figure 2b. In Figure 2c,d, flower-like structures were by a hydrothermal route in the presence of zinc nitrate hexahydrate (Zn(NO3)2·6H NaOH, and the petals were typical prismatic shape with a pencil-like end top. Th like structure and petals were destroyed in Figure 2d, compared with the intac and petals in Figure 2c. This suggests that the flower-like structure was dissol destroyed when the concentration of NaOH solution was 5 M. In addition, the co sample was white in Figure 2c, while the sample was light yellow in Figure 2d. A in Figure 2e, Ti-5 exhibited a tubular or rod-like structure when the precursors w and NaOH (5 M). The morphologies of the samples prepared by Zn(NO3)2·6H2O, different concentrations of NaOH solutions were shown in Figure 2f,j, respective be clearly seen that the morphologies of the samples changed greatly with NaOH tration. When NaOH concentration was 5 M, the flower-like structure with flak was as shown in Figure 2f. The diameter of the flower-like structure was aroun When NaOH concentration was increased to 6 M (Figure 2g), the sample was sented as a flower-like structure. However, the petals were significantly thinner t The TEM image in Figure 2h also shows that 6 M NaOH concentration induced mation of very thin petals. Comparing Figure 2b,h, it can be clearly seen that P25 from granular to very thin flakes, and the edges of some flakes had obviously c possible explanation for this phenomenon is that the Ti-O bond of P25 particles ken and stripped into small nanosheets by concentrated NaOH during the proce

Morphologies
To understand the formation mechanism of the flower-like TiO 2 -based composite, the morphology depending on NaOH concentration was investigated from field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), as shown in Figure 2. The prepared samples were denoted as Zn-Ti-X, and samples without P25 or Zn(NO 3 ) 2 ·6H 2 O were also prepared following the same process, named Zn-X or Ti-X for comparison, where X = 4, 5, 6, 7, and 8 M represented NaOH concentration in the precursor solution. From Figure 2a, it can be seen that TiO 2 nanoparticles (P25) were spherical particles. In addition, the TEM of P25 presented a granular structure with a diameter of about 20-30 nm in Figure 2b. In Figure 2c,d, flower-like structures were formed by a hydrothermal route in the presence of zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O) and NaOH, and the petals were typical prismatic shape with a pencil-like end top. The flower-like structure and petals were destroyed in Figure 2d, compared with the intact flower and petals in Figure 2c. This suggests that the flower-like structure was dissolved and destroyed when the concentration of NaOH solution was 5 M. In addition, the color of the sample was white in Figure 2c, while the sample was light yellow in Figure 2d. As shown in Figure 2e, Ti-5 exhibited a tubular or rod-like structure when the precursors were P25 and NaOH (5 M). The morphologies of the samples prepared by Zn(NO 3 ) 2 ·6H 2 O, P25, and different concentrations of NaOH solutions were shown in Figure 2f,j, respectively. It can be clearly seen that the morphologies of the samples changed greatly with NaOH concentration. When NaOH concentration was 5 M, the flower-like structure with flake petals was as shown in Figure 2f. The diameter of the flower-like structure was around 3 µm. When NaOH concentration was increased to 6 M (Figure 2g), the sample was still presented as a flower-like structure. However, the petals were significantly thinner than 5 M. The TEM image in Figure 2h also shows that 6 M NaOH concentration induced the formation of very thin petals. Comparing Figure 2b,h, it can be clearly seen that P25 changed from granular to very thin flakes, and the edges of some flakes had obviously curled. A possible explanation for this phenomenon is that the Ti-O bond of P25 particles was broken and stripped into small nanosheets by concentrated NaOH during the process of hydrothermal reaction. Then, these small nanosheets were interconnected to form a large nanosheet structure due to the oxidation of hydroxyls at the original nanosheet edges [30]. With an increase in NaOH concentration (7 M and 8 M), the sample separately presented thinner flakes and tubular (or rod-like) structure (Figure 2i,j). centration of NaOH was appropriate, flower-like sodium titanate was formed on the flower-like ZnO template. When the concentration of NaOH was too high, the flower-like ZnO formed was dissolved and destroyed by the excessively high concentration of NaOH, and the flower-like sodium titanate was not formed. In addition, it should be noted that too high a concentration of NaOH solution caused P25 nanoparticles to be peeled off so thinly that the edges of the sheets were curled and formed tubular or rod shape.

Spectral Properties
X-ray diffraction (XRD) patterns of P25 and Zn-Ti-6 synthesized through hydrothermal method were studied to probe the crystal structure in Figure 3a. All the diffraction peaks of P25 were in a good agreement with anatase and rutile TiO2 (JCPDS No. 89-4921 and JCPDS No. 89-0552). It was observed that P25 was a mixed crystal structure of anatase and rutile, and the sharp diffraction peaks indicated good crystallinity. The diffraction peak positions of the prepared Zn-Ti-6 were not changed, but the diffraction peaks were significantly weakened or disappeared, indicating that its crystallinity was weakened [36]. In addition, the characteristic diffraction peaks of ZnO were not found in the XRD pattern of Zn-Ti-6, which suggested that the ZnO template had been removed [37].
Fourier transform infrared (FTIR) spectra of P25 and Zn-Ti-6 wereshown in Figure  3b. Two absorption peaks were observed clearly in the FTIR of P25 and Zn-Ti-6. One was the broader absorption peak near 3300 cm −1 , which corresponded to adsorbed water. The other was a narrower absorption peak near 1630 cm −1 , which corresponded to the hydroxyl on the sample surface. However, the intensity of the absorption peaks of Zn-Ti-6 at these two places was significantly stronger than that of P25. This indicates that there were abundant hydroxyl groups on the surface of Zn-Ti-6 [38,39]. These abundant hydroxyl groups not only adsorbed pollutants but also trapped holes to generate Ti-OH + radicals, which benefited the adsorption and photocatalytic performance of Zn-Ti-6 [40]. In order to further characterize the structural evolution from P25 to Zn-Ti-6, Raman spectra were shown in Figure 3c. The Raman spectrum of P25 exhibited strong bands at 142, 396, 443, 516, and 636 cm −1 . In these Raman bands, 142, 396, 516, and 636 cm −1 all corresponded to the vibration modes of the anatase phase. Among them, 516 cm −1 corresponded to the B1g/A1g vibration mode of the anatase phase, and the others corresponded The possible mechanisms for these phenomena were considered to be as follows.
On the one hand, Zn(NO 3 ) 2 ·6H 2 O reacted to produce flower-like ZnO at an appropriate NaOH concentration [31,32]. However, as the concentration of NaOH increased, the resulting flower-like ZnO was dissolved and destroyed [33]. On the other hand, P25 could form white precipitated sodium titanate under the condition of concentrated NaOH and form a TiO 2 -based composite after washing with HCl [34]. When Zn(NO 3 ) 2 ·6H 2 O, P25, and NaOH were present at the same time, the possible formation mechanism is described as follows: first, Zn(NO 3 ) 2 ·6H 2 O and NaOH formed flower-like ZnO; second, the P25 particles were peeled off and adhered to form flower-like sodium titanate on the ZnO template [35]. In other words, both Zn(NO 3 ) 2 ·6H 2 O and P25 were consuming NaOH. When the concentration of NaOH was appropriate, flower-like sodium titanate was formed on the flower-like ZnO template. When the concentration of NaOH was too high, the flower-like ZnO formed was dissolved and destroyed by the excessively high concentration of NaOH, and the flower-like sodium titanate was not formed. In addition, it should be noted that too high a concentration of NaOH solution caused P25 nanoparticles to be peeled off so thinly that the edges of the sheets were curled and formed tubular or rod shape.

Spectral Properties
X-ray diffraction (XRD) patterns of P25 and Zn-Ti-6 synthesized through hydrothermal method were studied to probe the crystal structure in Figure 3a. All the diffraction peaks of P25 were in a good agreement with anatase and rutile TiO 2 (JCPDS No. 89-4921 and JCPDS No. 89-0552). It was observed that P25 was a mixed crystal structure of anatase and rutile, and the sharp diffraction peaks indicated good crystallinity. The diffraction peak positions of the prepared Zn-Ti-6 were not changed, but the diffraction peaks were significantly weakened or disappeared, indicating that its crystallinity was weakened [36]. In addition, the characteristic diffraction peaks of ZnO were not found in the XRD pattern of Zn-Ti-6, which suggested that the ZnO template had been removed [37].
Fourier transform infrared (FTIR) spectra of P25 and Zn-Ti-6 wereshown in Figure 3b. Two absorption peaks were observed clearly in the FTIR of P25 and Zn-Ti-6. One was the broader absorption peak near 3300 cm −1 , which corresponded to adsorbed water. The other was a narrower absorption peak near 1630 cm −1 , which corresponded to the hydroxyl on the sample surface. However, the intensity of the absorption peaks of Zn-Ti-6 at these two places was significantly stronger than that of P25. This indicates that there were abundant hydroxyl groups on the surface of Zn-Ti-6 [38,39]. These abundant hydroxyl groups not only adsorbed pollutants but also trapped holes to generate Ti-OH + radicals, which benefited the adsorption and photocatalytic performance of Zn-Ti-6 [40]. In order to further characterize the structural evolution from P25 to Zn-Ti-6, Raman spectra were shown in Figure 3c. The Raman spectrum of P25 exhibited strong bands at 142, 396, 443, 516, and 636 cm −1 . In these Raman bands, 142, 396, 516, and 636 cm −1 all corresponded to the vibration modes of the anatase phase. Among them, 516 cm −1 corresponded to the B 1g /A 1g vibration mode of the anatase phase, and the others corresponded to the E g vibration mode of the anatase phase. The E g vibration mode of rutile was 443 cm −1 . In the Raman spectrum of Zn-Ti-6, the Raman bands at 184, 272, 450, and 701 cm −1 were clearly shown, which corresponded to the vibration mode of monoclinic TiO 2 -based composite [41]. Among them, the Raman shift at 272 cm −1 was related to the breaking of symmetry caused by the bending of the TiO 6 layered structure during the lamellar formation; the Raman shift at 450 cm −1 represented the 2D lepidocrocite-type TiO 6 octahedron with the existence of the layer; the Raman shift at 701 cm −1 was due to the bending of TiO 6 [42]. In short, the two Raman spectrum lines were significantly different, reflecting the structure transition of the sample from P25 to Zn-Ti-6. In addition, the characteristic Raman bands of ZnO were not found in the Raman spectrum, indicating that ZnO had been removed [43].
violet light absorption characteristics near 400 nm, which was similar to the results of other studies [44][45][46]. As a non-destructive technique technology, photoluminescence spectroscopy was used to explore the separation efficiency of electron-hole pairs in semiconductor materials. Usually, lower fluorescence intensity represented a lower photogenerated carrier recombination rate [47]. Figure 3e showed the photoluminescence spectra of P25 and Zn-Ti-6 under the excitation wavelength at 300 nm. It can be observed that P25 and Zn-Ti-6 both exhibited photoluminescence peaks at 398, 411, 419, 436, 450, 467, 481, and 492 nm. The difference was that the intensities of all the fluorescence peaks of Zn-Ti-6 were significantly lower than that of P25, which implies that Zn-Ti-6 inhibited the recombination of photogenerated electron-hole pairs effectively, benefiting the photocatalytic performance of the material. Among these, the peaks at 450, 467, 481, and 492 nm were all defect peaks generated by fluorescence excitation of surface defects. The peaks at 450 and 467 nm were the defect peaks due to the excitation of band edge electrons, and the peaks at 481 and 492 nm were the defect peaks due to the fluorescence excitation of band-bound electrons [48,49].

Adsorption and Photocatalytic Activity Evaluation
The N2 adsorption-desorption isotherms were shown in Figure 4. From the adsorption-desorption isotherms, it can be seen that the specific surface area from Brunauer-Emmett-Teller (BET) analysis of Zn-Ti-6 was 130.0 m 2 /g, which was much larger than P25 To investigate the light response ability of P25 and Zn-Ti-6, the UV-Vis diffuse reflectance spectra were depicted in Figure 3d. Clearly, P25 and Zn-Ti-6 both had strong ultraviolet light absorption characteristics near 400 nm, which was similar to the results of other studies [44][45][46]. As a non-destructive technique technology, photoluminescence spectroscopy was used to explore the separation efficiency of electron-hole pairs in semiconductor materials. Usually, lower fluorescence intensity represented a lower photogenerated carrier recombination rate [47]. Figure 3e showed the photoluminescence spectra of P25 and Zn-Ti-6 under the excitation wavelength at 300 nm. It can be observed that P25 and Zn-Ti-6 both exhibited photoluminescence peaks at 398, 411, 419, 436, 450, 467, 481, and 492 nm. The difference was that the intensities of all the fluorescence peaks of Zn-Ti-6 were significantly lower than that of P25, which implies that Zn-Ti-6 inhibited the recombination of photogenerated electron-hole pairs effectively, benefiting the photocatalytic performance of the material. Among these, the peaks at 450, 467, 481, and 492 nm were all defect peaks generated by fluorescence excitation of surface defects. The peaks at 450 and Catalysts 2021, 11, 515 6 of 12 467 nm were the defect peaks due to the excitation of band edge electrons, and the peaks at 481 and 492 nm were the defect peaks due to the fluorescence excitation of band-bound electrons [48,49].

Adsorption and Photocatalytic Activity Evaluation
The N 2 adsorption-desorption isotherms were shown in Figure 4. From the adsorptiondesorption isotherms, it can be seen that the specific surface area from Brunauer-Emmett-Teller (BET) analysis of Zn-Ti-6 was 130.0 m 2 /g, which was much larger than P25 (48.0 m 2 /g). Additionally, the typical isotherms of P25 and Zn-Ti-6 were both type IV (Brunauer-Deming-Deming-Teller classification), with a hysteresis loop of type H3 at high relative pressure. This suggests the existence of slit-like mesopores in the samples. The difference was that the pores of P25 were formed by the disorderly accumulation of small particles, while the pores of Zn-Ti-6 were formed by the stacking of petals [50]. These were consistent with the results observed by FESEM and TEM. The high BET surface area and open mesoporous morphology were vital in increasing the number of sites for the adsorption and redox reaction [51]. These were beneficial to the adsorption and photocatalytic performance of Zn-Ti-6.
Catalysts 2021, 11, x FOR PEER REVIEW (48.0 m 2 /g). Additionally, the typical isotherms of P25 and Zn-Ti-6 were both (Brunauer-Deming-Deming-Teller classification), with a hysteresis loop of typ high relative pressure. This suggests the existence of slit-like mesopores in the s The difference was that the pores of P25 were formed by the disorderly accumul small particles, while the pores of Zn-Ti-6 were formed by the stacking of peta These were consistent with the results observed by FESEM and TEM. The high B face area and open mesoporous morphology were vital in increasing the number for the adsorption and redox reaction [51]. These were beneficial to the adsorpt photocatalytic performance of Zn-Ti-6. In order to easily describe the process of sample adsorption and photocataly radation of MB solution, the decontamination efficiency can be expressed as where C0 and Ct (mg/L) represent the initial concentration and the concentration a of the MB solution, respectively. Adsorption and photocatalytic degradation of MB solution (60 mg/L, 50 mL) and Zn-Ti-6 were shown in Figure 5a, which was calculated and plotted accor Equation (1). The dark adsorption time ranged from −240 min to 0 min, and the catalytic degradation time ranged from 0 min to 240 min. It was clear that P25 h adsorption capacity for MB, while Zn-Ti-6 had a strong adsorption capacity for M tion. For Zn-Ti-6, the dark adsorption was very fast in the first 30 min, and decon tion efficiency reached 62.83%. This was because of the strong electrostatic interac tween negatively charged Zn-Ti-6 and positively charged MB. Subsequently, the d In order to easily describe the process of sample adsorption and photocatalytic degradation of MB solution, the decontamination efficiency can be expressed as where C 0 and C t (mg/L) represent the initial concentration and the concentration at time t of the MB solution, respectively. Adsorption and photocatalytic degradation of MB solution (60 mg/L, 50 mL) by P25 and Zn-Ti-6 were shown in Figure 5a, which was calculated and plotted according to Equation (1). The dark adsorption time ranged from −240 min to 0 min, and the photocatalytic degradation time ranged from 0 min to 240 min. It was clear that P25 had little adsorption capacity for MB, while Zn-Ti-6 had a strong adsorption capacity for MB solution. For Zn-Ti-6, the dark adsorption was very fast in the first 30 min, and decontamination efficiency reached 62.83%. This was because of the strong electrostatic interaction between negatively charged Zn-Ti-6 and positively charged MB. Subsequently, the dark adsorption capacity slowed down. This could be explained by the fact that the adsorption vacancies on the surface of Zn-Ti-6 decreased, and the repulsion between MB molecules caused the adsorption rate to decrease with the extension of the adsorption time [48]. At 180 min, the dark adsorption reached equilibrium, and the decontamination efficiency was 73.30%, indicating that Zn-Ti-6 had a good adsorption capability. Under visible light irradiation, P25 showed weak photodegradation ability for MB, while Zn-Ti-6 exhibited better photodegradation ability for MB. In general, under dark adsorption and visible light irradiation, the decontamination efficiency of P25 for MB solution was only 2.70%, and the decontamination efficiency of Zn-Ti-6 for MB solution was 85.76%, which was about 32 times stronger than that of P25. This suggested that Zn-Ti-6 had a strong ability to remove the pollution of high-concentration MB solution through the adsorptionphotocatalytic degradation method. The reasons for the above phenomena were as follows: there were a few adsorption sites on the surface of P25, and its specific surface area was small (BET was 48.0 m 2 /g), which led to the poor adsorption capacity of P25 for MB. Notably, the adsorption capacity of P25 was too poor to be displayed when the pollutant was a high-chroma MB solution. In addition, although P25 had a good photocatalytic ability in the ultraviolet region, the photocatalytic ability in the visible region was very weak. A high chroma of the MB solution resulted in poor light transmission, which further led to the weak photodegradation ability of P25. In contrast, Zn-Ti-6 possessed the flowerlike structure, the large specific surface area (BET was 130.0 m 2 /g), and the rich surface hydroxyl groups, which helped to provide a large number of adsorption sites for MB, leading to the efficient adsorption ability of Zn-Ti-6. In addition, the flower-like structure, the surface abundant hydroxyl groups, and the strong adsorption capacity for MB were conducive to take full advantage of light, which was favorable for the photodegradation of MB by Zn-Ti-6. Zn-Ti-6 was larger, at 0.0031 min −1 . This showed that Zn-Ti-6 had a positive photodegradation effect on MB compared with P25.

Materials and Methods
All of the chemicals were analytical grade and used without any further purification. P25 was purchased from Degussa Co. Ltd., China. NaOH, HCl, Zn(NO3)2·6H2O, ethyl alcohol, and MB were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, The characteristics in the adsorption process can be further studied by the adsorption kinetics models. There are two commonly used adsorption kinetic models, the pseudofirst-order model (Equation (2)) and the pseudo-second-order model (Equation (3)): where k 1 and k 2 (min −1 ) represent the rate constants of the pseudo-first-order model and the pseudo-second-order model, respectively; Q t and Q e (mg/g) represent the adsorption capacity of MB at time t and at adsorption equilibrium, respectively. The calculation equations of Q t and Q e are defined below: where C 0 and C t (mg/L) are defined above; C e represents the concentration of the MB solution at equilibrium; V is the initial volume of the MB solution (L); and W is the mass of the adsorbent used (g). Figure 5b,c are the pseudo-first-order model and the pseudo-second-order model diagrams based on Equations (2) and (3) for the adsorption of MB onto Zn-Ti-6, respectively. It can be seen that the adsorption of MB onto Zn-Ti-6 conformed to the pseudo-secondorder model (correlation coefficient R 2 = 0.9999, the rate constant k 2 = 0.0218 min −1 ). The equilibrium adsorption capacity calculated by the pseudo-second-order model was 45.8716 mg/g, which was in good agreement with the equilibrium adsorption capacity obtained by experiments of 43.9790 mg/g. This indicated that the adsorption process could be well described by the pseudo-second-order kinetics model.
The pseudo-first-order kinetics model is still used to describe the rate constant of dye photodegradation with respect to the degradation time, and the relationship is described in Equation (6): where C 0 and C t are defined above, and k is the pseudo-first-order kinetics model rate constant (min −1 ). According to Equation (6), the first-order kinetics model of P25 and Zn-Ti-6 was as shown in Figure 5d for the stage of photodegradation of MB. The good linear correlation confirmed that both of the photodegradation processes followed the pseudo-first-order kinetics. In contrast, the rate constant k of P25 was very small, while the rate constant k of Zn-Ti-6 was larger, at 0.0031 min −1 . This showed that Zn-Ti-6 had a positive photodegradation effect on MB compared with P25.

Materials and Methods
All of the chemicals were analytical grade and used without any further purification. P25 was purchased from Degussa Co. Ltd., China. NaOH, HCl, Zn(NO 3 ) 2 ·6H 2 O, ethyl alcohol, and MB were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. Deionized water was used throughout the experiments.
In a typical synthesis procedure, 0.8 g of P25 was dispersed in 50 mL of different molar concentrations of NaOH solution. The solution was kept under magnetic stirring for 30 min. Subsequently, a certain amount of Zn(NO 3 ) 2 ·6H 2 O solution was dropped slowly into the above suspensions. Then, the mixture was treated at 180 • C for 6 h in a Teflon-lined autoclave of 100 mL. After the reaction was complete, the autoclave was cooled naturally to room temperature. The white precipitate was washed with deionized water until the pH reached 7. Subsequently, the white precipitate was dispersed in 500 mL of diluted HCl solution overnight. Finally, the white precipitate was washed with deionized water and absolute ethanol, in order, and then dried at 60 • C for 12 h.
The morphological features of the samples were examined by FESEM (FEI Quanta-450, operated at 20 kV in high vacuum) (FEI, Hillsboro, OR, USA) and TEM (JEOL JEM-3010, operated at 100 kV)(JEOL, Tokyo, Japan). The crystalline phase data of the samples were collected by XRD (Rikagu MiniFlex600, Rikagu, Tokyo, Japan) with Cu Kα radiation (wavelength = 1.54 Å). The diffractograms of samples were recorded in 2 from 10 • to 70 • at a step of 0.02 • . The specific surface area and pore size distribution were measured by N 2 adsorption-desorption isotherm using the equipment (GW-BK100B, Beijing, China). FTIR spectra were determined by PerkinElmer Spotlight 400 & Frontier spectrometer (PerkinElmer, Waltham, MA, USA) to detect bonding structures and functional groups. Raman spectra were collected by Renishaw inVia Reflex Raman spectrometer (Renishaw, England) with an excitation wavelength of 514 nm. UV-Vis diffuse reflectance spectra of the samples were recorded on PerkinElmer Lambda 950 spectrometer (PerkinElmer, Waltham, MA, USA), and BaSO 4 was used as reflectance standard. The fluorescence spectra were analyzed by HORIBA FluoroMax+ fluorescence spectrometer (HORIBA, Irvine, CA, USA) with a Xe lamp as an excitation source to investigate the recombination efficiency of photogenerated electron-hole pairs.
The adsorption and photocatalytic activities of samples were evaluated by the degradation of MB solution. The instrument is a photochemical reaction system (Zhong Jiao Jin Yuan, Beijing, China). The light source was a 300 W Xe lamp, and a cut-off filter (420 nm) was used to simulate visible light. The Xe lamp was surrounded by water to keep a constant temperature. In the adsorption and photocatalytic reaction, the suspensions used were obtained by adding 0.05 g of samples into 50 mL of 60 mg/L MB solution, and the suspensions were under constant magnetic stirring. In order to investigate the regular pattern of adsorption with time, the suspensions were in the dark, and some MB solutions were removed at different time points. The Xe lamp was turned on to conduct the photocatalytic experiments after the adsorption-desorption equilibrium was established. Like dark adsorption, some MB solutions were removed at different photocatalytic reaction time points to explore the variation of photodegradation over time. Specifically, 1.6 mL of solutions were collected at the specified time points and centrifuged to separate the samples at a speed of 1500 r/min for 5 min. Following this, 0.8 mL of supernatant was removed and mixed with 2.4 mL of deionized water as the final test solutions. The concentrations of MB solution were gauged by iDH2000-BSC optical fiber spectrometer (ideaoptics, Shanghai, China).

Conclusions
In summary, TiO 2 -based composite with flower-like morphology (denoted as Zn-Ti-6) was successfully fabricated by a facile alkaline hydrothermal procedure. NaOH was a morphology-directing agent and reactive etchant. The flower-like ZnO was synthesized and used as a sacrificial template. A possible formation mechanism was proposed. Zn-Ti-6 exhibited better adsorption and photocatalytic degradation capacity toward MB solution than P25, which was attributed to Zn-Ti-6 having a larger specific surface area, more abundant hydroxyls, and lower photoluminescence intensity than P25. For high-chroma wastewater, excellent adsorption performance is largely beneficial to the subsequent photocatalytic degradation performance. The adsorption data of Zn-Ti-6 fitted well with the pseudo-second-order kinetics model, and the photocatalytic degradation fitted with the pseudo-first-order kinetics model. Relying on the adsorption-photocatalytic degradation, MB was almost completely removed under visible light. Zn-Ti-6 synthesized by the alkaline hydrothermal process has the potential to combine with other photocatalytic material to further expand its application in adsorption-photocatalytic degradation of high-chroma wastewater.

Conflicts of Interest:
The authors declare no conflict of interest.