Facile formation of Anatase Nanoparticles on H-Titanate Nanotubes at Low Temperature for Efficient Visible Light-Driven Degradation of Organic Pollutants

: Anatase nanoparticles (5–10 nm) generated on H-titanate nanotube surface (H-titanate/anatase) were prepared by an ingenious and simple method. H-titanate tubes were prepared by a hydrothermal reaction of Ti powder in concentrated NaOH solution and an ion exchange process with HNO 3 solution. After that, at a relatively low drying temperature (100 °C), a small quantity of anatase nanoparticles were in-situ formed on the H-titanate tubes surface by a surface dehydration reaction. In-situ transformation can form a strong interface coupling between H-titanate and anatase, which is conducive to accelerating charge transfer and improving its photocatalytic activity. In addition, the smaller average crystal size, the large specific surface areas (BET), the nanotubed and layered structure and the synergistic effect of dual phases would be beneficial to improving the photocatalytic efficiency.


Introduction
Water contamination resulting from the rapid development of industrialization has attracted worldwide attention. Photocatalytic degradation is the most promising strategy to completely solve the organic pollutants problem [1]. Among all kinds of conversion systems, TiO2-based materials for water pollution are considered to be an environmentally friendly and promising way to efficiently utilize solar energy [2,3]. Under UV irradiation, anatase can degrade a broad range of tenacious and toxic organic contaminants in water, and it is nontoxic, relatively cheap and chemically stable. However, its photocatalytic application is limited owing to the rapid recombination of the excited electron-hole pairs, the low visible light activity and low surface area [4,5]. Coupling anatase with another semiconductor favors a narrow band gap and the electron-hole separation [6][7][8][9], so as to improve the quantum efficiency. Moreover, the synergistic effect between two different phases can also enhance the photocatalytic activity [6,10].
It was reported that H-titanate was formed from TiO2 reacting with a concentrated NaOH solution to form titanate and then the ion exchange reacted with a dilute acid solution [11][12][13]. After calcination at a high temperature, H-titanate can transform into anatase TiO2 [13]. Based on the above formation mechanism of TiO2-H-titanate-TiO2, it is estimated that, at low calcination temperature, a small amount of anatase TiO2 will be formed in situ on the H-titanate nanotube surface. Several papers have reported the synthesis of titanate/anatase composites; however, most preparation processes were carried out under high pressure and high temperature conditions, which consumed more energy [14][15][16]. For example, Xiong et al. reported a nitrogen-doped titanate-anatase core-shell nanobelts. In this paper, the titanate-anatase was obtained by calcination at 500 °C [14]. Yan et al. demonstrated that the titanate nanotube/anatase nanoparticle composites could be prepared by the hydrothermal method using as-obtained titanate tube dispersed into a HNO3 solution [15]. Herein, we report that a dual-phase photocatalyst (anatase nanoparticles (5-10 nm) was generated in situ on a H-titanate nanotube surface) was obtained via a controllable surface dehydration reaction at low temperature (100 °C) and atmospheric pressure, which exhibited a higher visible light photocatalytic activity than P25; pure H-titanate nanotubes and pure anatase. The efficient visible light photocatalytic activity can be attributed to: (1) In situ transformation can form strong interfacial coupling between H-titanate and anatase, which is favorable to accelerating charge transfer [17]; (2) H-titanate has a layered structure, which is composed of TiO6 octahedra sheets sharing four edges, similar to that of anatase crystals, which is easy to form a heterostructure between anatase and H-titanate [14,18,19]. In the meantime, at a low drying temperature, dual-phase catalyst retained the nanotubed and layered structures, which was beneficial for the high BET surfaces to adsorb organic pollutants and promote the diffusion of organic molecules inside the pores. The smaller average crystal size of anatase nanoparticles means a stronger redox ability in the photocatalytic process. Therefore, in the presence of the dual-phase catalyst, rhodamine B (RhB) and methylene blue (MB) can be completely decomposed in a very short time under visible light irradiation. In addition, the synergistic effect would be beneficial to improve the photocatalytic efficiency. Figure 1 shows the X-ray diffraction (XRD) patterns of the as-synthesized material. As shown in Figure 1a, there are several diffraction peaks located at 2θ = 9.8°, 24.4°, 28.4°, 48.4° suggesting that the as-prepared sample is layered titanate with a component of H2Ti2O4(OH)2 [20] and a 0.9 nm interlayer distance, which is further confirmed by TEM observation. After drying at 100 °C, the crystal structure of H-titanate was well maintained; the diffraction peaks of anatase-type TiO2 (JCPDS no. 21-1272) [21] are clearly observed (Figure 1b), showing that dual-phase H-titanate/anatase was obtained at a low temperature, which may be due to the dehydration of the H-titanate. The broad peaks indicate its low crystallinity and nanosized crystallites. With increasing calcination temperature, the peaks of anatase become narrower and sharper (Figure 1c,d). After calcination at 500 °C for 5 h, all diffraction peaks of H-titanate disappeared (Figure 1e), which suggested that the layered H-titanate was completely transformed into anatase. The apparent sharpening of peaks suggests its high crystallinity. To further confirm the coexistence of these two TiO2 phases in the as-synthesized products, Raman spectroscopy was tested for the H-titanate and dual-phase catalysts ( Figure 2). In Figure 2b, the peaks that center at 143 (Eg), 514 (A1g) and 636 cm -1 (Eg) belong to the anatase phase [22], while others match well with the hydrogen titanate phase (shown in Figure 2a). The Raman spectrum in Figure 2b is composed of the characteristic peaks of the H-titanate and anatase phase, which are in good agreement with the XRD analysis.  The detailed characterization and crystal structure of the H-titanate/anatase composite were investigated via transmission electron microscopy (TEM), as shown in Figure 3. H-titanate was formed via the hydrothermal reaction of titanium powder with a concentrated NaOH solution to form sodium titanate and then a subsequent ion exchange reaction with HNO3 solution at room temperature. The dual-phase H-titanate/anatase catalysts were prepared by in situ generation of anatase nanoparticles on the H-titanate tube surfaces by a controllable surface dehydration reaction at a low drying temperature (100 °C). Figure S1a shows images of only the nanotubes, whereas Figure 3b shows that some nanoparticles adhered to the surface of nanotubes, which indicates that a new phase was obtained after drying at 100 °C. The average diameter of the as-synthesized nanoparticles is about 5-10 nm. Figure 3b1 (high-resolution transmission electron microscopy (HRTEM) images) distinctly reveals a lattice fringe spacing of 0.35 nm, which corresponds well with the (101) anatase. Figure 3b2 shows that the nanotubes are crystallized of layered H-titanate. It also indicates that the H-titanate retains its morphology of nanotubes, and the anatase reveals the morphology of nanoparticles. There are only particles in Figure S1b, which indicates that when the calcination temperature increased to 500 °C, H-titanate nanotubes completely transformed into anatase nanoparticles. The above results coincide with the XRD analysis. At a low drying temperature (100 °C), the photocatalyst retained its original nanotube structure and a small amount of anatase nanoparticles were formed on the H-titanate nanotube surfaces. Specific surface area (BET) is a key factor in photodegradation. High specific surface area offers more reaction sites for dye molecules and hydroxyl groups. Therefore, the porosities of the samples were determined by N2 sorption. Figure 3c exhibits the N2 sorption isotherms of all as-synthesized products and the DFT (Density Functional Theory) pore size distributions of the dual-phase photocatalyst. The obtained isotherms of the samples are the typical IUPAC type-IV isotherm. We can see that the isotherms decreased with the decrease in the amount of H-titanate nanotubes. The corresponding DFT pore size distribution curve (inset of Figure 3c) of the dual-phase catalyst shows two pore sizes centered at ~1.0 nm and ~12 nm, respectively, which directly proves the layered and nanotubed structure of the dual phase catalyst. BET measurements show that the as-synthesized H-titanate tubes have a surface area of 245 m 2 g −1 , and dual-phase photocatalyst has a surface area of 174 m 2 g −1 . As the calcination temperature raised to 500 °C, the surface area of the product decreased to 95 m 2 g −1 ( Table  S1). The decrease in surface area may be due to the decreased proportion of layered H-titanate nanotubes. In other words, the specific surface area of the catalysts is enormously increased by the H-titanate nanotubes, which is hoping to enhance the photocatalytic property of the nanomaterials. UV-Vis diffuse reflectance absorption spectra of as-synthesized H-titanate nanotubes (TiO2-60), dual-phase H-titanate/anatase (TiO2-100) and anatase nanoparticles (TiO2-500) are shown in the Figure 4a. As indicated in Figure 4a, in the range of 200-800 nm, the H-titanate nanotubes, dual-phase H-titanate/anatase and anatase samples demonstrate similar absorption. Compared with that of pure H-titanate and anatase, the absorption edge of dual-phase H-titanate/anatase moved toward a longer wavelength. It may be owing to the synergistic effect of H-titanate and anatase, resulting in a narrow band gap, which is a crucial role to the realization of solar energy conversion.

Results and Discussion
The Kubelka-Munk method is often used to estimate the band gap energy (Eg) of as-prepared products [23]. Figure 4b reveals the Tauc plots of (αhν) 2 vs. photon energy (hν) of H-titanate tubes, dual-phase H-titanate/anatase and anatase photocatalyst. The band gap (Eg) can be acquired by extending the vertical segment to the hν axis. As shown in Figure 4b, the Eg of the H-titanate, H-titanate/anatase, and anatase is 3.48, 3.30, and 3.36 eV, respectively, which reveals the same results with the ultraviolet-visible diffffuse reflectance spectra (UV-Vis DRS) analysis. The photoluminescence (PL) technique is usually used to investigate the charge carrier separation and transfer processes because PL emission results from the free photogenerated carriers recombination [24]. Figure 4c shows the PL spectra of dual-phase H-titanate/anatase, H-titanate and anatase excited at 315 nm. The emission intensity of dual-phase H-titanate/anatase is much weaker than that of pure H-titanate and anatase, which can be attributed to the formation of H-titanate/anatase heterojunction in two semiconductor interfaces. Different band edge positions of H-titanate and anatase can reduce the recombination of the carriers [19,25]. That enables more free electrons and holes to participate in the photocatalytic reactions and accelerates the photocatalytic process. The photocatalytic activities of as-prepared samples were studied by degradation of methyl orange (MO), rhodamine B (RhB) and methylene blue (MB) in aqueous solution under visible light at room temperature. In order to evaluate the photocatalytic efficiency, P25 was chosen as the photocatalytic reference material. In general, high specific surface area can improve the adsorption performance of the materials. Therefore, prior to the photocatalytic degradation studies, the adsorption properties of the materials were investigated. In general, a high degree of surface adsorption was observed within the first 30 min of stirring time in dark before attaining the saturation level. In Figure 5a and Figure S2A, P25 shows no obvious adsorption capacity, whereas the as-received dual-phase H-titanate/anatase catalyst shows 90% MB, 25% RhB, 8.9% MO dye adsorption, the H-titanate shows 92% MB, 59% RhB, 9.1% MO adsorption due to their nanotubed and layered structures, larger specific surface areas, which are beneficial for enhancing the photocatalytic activity. In addition, the electrostatic attraction between the catalysts and the dye molecules plays an important role in the different adsorption behavior of several dyes [26]. MB, RhB is cationic and MO is anionic in the aqueous solution, while the nanotubes possess negative surface charge, which is more favorable to absorb MB and RhB. Although both RhB and MB are cationic dyes, the adsorption capacity of nanotubes for MB is much higher than that of RhB, which is due to the different molecular structures of dyes. The more linear shape and smaller size of MB molecule mean a weaker steric hindrance during the adsorption process [26]. In order to verify the adsorption of dye on H-titanate/anatase, FTIR analysis was carried out after a MB adsorption and degradation test ( Figure S3). According to the previous report [27], in adsorption test, the peaks at 2926 cm −1 and 666 cm -1 indicated that the MB was adsorbed onto the H-titanate/anatase nanotubes surfaces. After the degradation process, the peaks disappeared, suggesting that MB degraded completely.
When visible light was turned on, as shown in Figure 5a, RhB was almost decomposed within only 30 min (Figure 5a2 and Figure S4), MO was removed by about 60% (Figure 5a3) within 30 min illumination, and the MB completely decomposed in just 5 min (Figure 5a1) in the test of the dual-phase H-titanate/anatase photocatalyst, whereas P25 showed no significant degradation for the dyes in 30 min. Additionally, without any photocatalyst, RhB, MB and MO were hardly degraded by visible light [28][29][30], which demonstrated the high reactivity of as-synthesized dual-phase nanotubes catalysts. Compared with many reported TiO2-based materials, it also shows higher photocatalytic activity for the degradation of organic dyes under visible light. Xiong et al. reported that under the visible light irradiation, MB degraded completely in about 175 min in the presence of the nitrogen-doped titanate-anatase core-shell nanobelts catalyst [14]. S-doped Na2Ti6O13@TiO2 core-shell nanorods can completely decompose MB in 100 min under visible light [31]. Li et al. demonstrated that within 60 min of UV light irradiation, RhB was completely decomposed in the presence of the double-shell anatase-rutile TiO2 spheres [32]. The report of Pan et al. showed that within 40 min visible light irradiation, MO can be degraded by about 50% in the presence of GQD-TiO2 heterojunctions [33]. Figure S2A shows the photocatalytic activity of all as-synthesized catalysts for MB, RhB and MO under visible light irradiation. Compared with pure H-titanate and anatase catalysts, the dual-phase catalyst exhibited the best photoactivity. Figure S2B displays the degradation rate of the materials (dual-phase H-titanate/anatase and P25) for MO, RhB under visible light irradiation. It is noted that the dual-phase catalyst displays better degradation efficiency than P25. H-titanate/anatase photocatalyst shows 15 times higher efficiency for the photodegradation of RhB and 17 times higher efficiency for the photodegradation of MO compared to P25 under visible light radiation, which demonstrated the high photocatalytic activity of the dual-phase H-titanate/anatase. In order to evaluate the stability of the dual-phase photocatalyst, several photodegradation tests of RhB under visible light were carried out (Figure 5b). After four cycles of photocatalytic degradation, within about 30 min, the RhB could be decomposed completely, which indicates that the dual-phase H-titanate/anatase has excellent photocatalytic stability. Based on above results, the dual-phase (H-titanate/anatase) catalyst reveals a high visible light photo-degradation ability. It can be expounded by the following involved reasons: (1) In-situ generation can form a strong interfacial coupling between H-titanate and anatase, which is useful for accelerating charge transfer and improving the photocatalytic activity [17]; (2) The dual-phase catalyst retains the nanotubed and layered structures, and possesses a high BET surface area. Large specific surface area and pore structure can provide more active sites to adsorb organic pollutants and promote the diffusion of organic molecules inside the pores. The layered titanate product has been considered as an excellent adsorbent [34,35]. It would be beneficial to improve the photocatalytic activity. In addition, the meso-nanotubes structure is also conducive to the rapid diffusion of quantum, which is formed in the photocatalytic process, further promoting photogenerated charge transport to improve the separation rate [36,37]; (3) The synergetic effect between H-titanate and anatase is one of the major ingredients for its enhanced visible light photocatalytic activity. When two phases combined, a staggered band gap was formed, which lead to the efficient charge separation of the cross phase junction [38]. As shown in Figure 6, under visible light irradiation, anatase in dual-phase can be excited to generate electron-hole pairs. Electrons are excited from the valence band (VB) to the conduction band (CB). According to the energy band data, the CB and the VB energy potentials in anatase are −0.26 and 2.94 eV [39], while those of titanate are −0.50 and 3.03 eV [31]. The photogenerated electrons accumulated on anatase will migrate from the CB of anatase to that of titanate due to the potential difference. In this way, titanate can effectively collect photogenerated electrons and anatase collect holes. These electrons react with the surface adsorbed O2 to form O2 -·, because the CB edge potential of titanate (−0.50 eV) is more negative than the standard redox potential of O2/O2 -· (−0.33 eV) [40]. In the mean time, the VB potential of anatase (2.94 eV) is more positive than the standard redox potential of·OH/OH -(1.99 eV) [41,42], the holes react with OHto generate ·OH radicals. Then the organic pollutants could be mineralized by the produced ·OH and O2 -· radicals. Hence, the above efficient separation of photogenerated electron-hole pairs process improves the photodegradation rate of dyes; (4) The new anatase phase with a small average crystal size means a stronger redox ability because of the quantum size effect [43]. Combining all above factors, the dual-phase H-titanate/anatase photocatalysts displayed high visible light activity.

Materials
The titanium powder was purchased from Aladdin, Tianjin, China. The NaOH and the HNO3 were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.

Catalyst Preparation
The photocatalysts were prepared by the following synthetic route. Firstly, 1.5 g titanium powder was mixed with 35 mL NaOH solution (10 M) and stirred at room temperature for 5 h. The mixture was transferred into a Teflon-lined autoclave and kept at 150 °C for 72 h. The obtained precipitates were washed with deionized water until neutral and dried in an oven (60 °C) overnight. Afterwards, the ion-exchange reaction was followed with 0.5 M HNO3 solution for 3 h at room temperature. There were three times ion-exchange reactions for the product. The final product was dried at 60 °C for 24 h to produce the hydrogen titanate (H-titanate) tubes and then dried at 100 °C for 24 h to yield hydrogen titanate/anatase nanotubes. The as-prepared H-titanate was calcined in air at 200, 300 and 500 °C for 5 h to get TiO2 products. Henceforth, these samples are referred to as TiO2-60, TiO2-100, TiO2-200, TiO2-300 and TiO2-500, respectively.

Catalyst Characterization
X-ray powder diffraction (XRD) analysis was carried out using a D/Max-2550 X-ray powder diffractometer (Tokyo, Japan) with Cu Kα radiation. The ultraviolet-visible diffuse reflectance spectra of the samples were measured on a UV-Vis-NIR spectrophotometer (Shimadzu U-4100, Shanghai, China) detecting absorption over the range of 200-800 nm. The morphologies of samples were measured on a Tecnai G2 S-Twin F20 transmission electron microscopy (TEM, FEI, Hillsboro, FL, USA). N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2020 sorptometer (Norcross, GA, USA). Raman spectra were recorded using a Renishaw InVia Raman spectrometer (London, UK) with a wavelength of 532 nm. Room temperature photoluminescence (PL) spectra with an excitation wavelength of 315 nm were measured on a FLUOROMAX-4 (Beijing, China).

Photocatalytic Activity Test
The photocatalytic activities of the photocatalysts were performed at room temperature in a glass reactor fitted with a Xe lamp (300 W). A 420 nm cut-on filter was used to ensure that only visible light illuminated the photocatalyst. The reaction liquid was prepared by mixing 0.25 g photocatalysts and 100 mL rhodamine B (or 10 mg/L MB; 10 mg/L MO) aqueous solution (10 mg/L). Then, the suspension was stirred in dark for 30 min to reach adsorption-desorption equilibrium before irradiation. Then it was irradiated under visible light. The suspension (8 mL) was withdrawn from the irradiated solution at preset time intervals and centrifuged to separate the photocatalyst particles, and then the supernatants were analyzed by UV-Vis spectrophotometer (UV-2450, Shanghai, China).

Conclusions
In conclusion, anatase nanoparticles generated on H-titanate nanotubes surfaces were successfully synthesized by an ingenious method. H-titanate tubes were prepared by a hydrothermal synthesis of Ti powder in concentrated NaOH solution and an ion exchange process with HNO3 solution. After that, at a relatively low drying temperature, a small amount of anatase nanoparticles were in-situ formed on the surface of the H-titanate tubes by the surface dehydration reaction. It showed higher photocatalytic activity than pure H-titanate nanotube, anatase, and P25 under visible light (100% rhodamine B (RhB) and methylene blue (MB), 60% methyl orange (MO) degraded in 30 min under visible light irradiation) due to the in-situ transformation, the smaller average crystal size, the nanotubed and layered structure, the large BET surface areas and the synergistic effect of the H-titanate/anatase dual phases, which can accelerate the transfer of electron-hole pairs and inhibit their recombination. This work provides an ingenious and simple method to prepare an efficient visible-light-responsive TiO2-based photocatalyst for solving environment problems.