Fabrication and Photocatalytic Property of One-Dimensional SrTiO₃/TiO_2−𝑥N_𝑥 Nanostructures

Abstract

One-dimensional SrTiO₃/TiO_2−xN_x nanostructures were prepared by the hydrothermal method and investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) measurements. The photocatalytic activities of the prepared samples were evaluated by photodegrading the methylene blue (MB) solution. According to the characterizations, the intermediate product of SrTiO₃/titanate nanotubes was presented after hydrothermal processing of the TiO_2−xN_x nanoparticles with the mixed solution of NaOH and Sr(NO₃)₂. The final product of SrTiO₃/TiO_2−xN_x nanorods was obtained after calcining the intermediate. As compared to the TiO_2−xN_x nanoparticles, the absorption performance of SrTiO₃/titanate nanotubes or SrTiO₃/TiO_2−xN_x nanorods was depressed, instead of improving it. The mechanisms of the absorption property changes were discussed. The SrTiO₃/TiO_2−xN_x nanorods presented better photocatalytic activity than the TiO_2−xN_x nanoparticles or nanorods. However, due to overmuch adsorption, the SrTiO₃/titanate nanotubes gave ordinary photocatalytic performances.

1. Introduction

TiO₂ has been extensively investigated for several decades in photocatalysis and photovoltaic areas. Anatase-type TiO₂ has attracted great interest as one of the most promising photocatalysts due to its impressed advantages such as high degradation capability, chemical stability, and low cost. However, the application potential of TiO₂ photocatalyst is restricted for its low quantum efficiency which is mainly caused by its low light absorption efficiency and high photogenerated charge carrier recombination rate. One-dimensional (1D) nanostructures can facilitate transport of charge carriers and minimize the loss of charge carriers at grain boundaries [1–3]. Hence, the charge carrier recombination of TiO₂ is expected to be held back by fabricating 1D nanostructure. Another efficient method to increase the charge separation efficiency of TiO₂ is modifying the photocatalyst with some other semiconductors because of the charge carrier transfer between valence or conduction band of two different semiconductors [4–6].

Recently, TiO₂ photocatalyst was reported to be modified by various materials such as graphene [7], Fe₃O₄ [8], C₆₀ [9], CdS, and Bi₂S₃ [10], by which the photocatalytic activity of TiO₂ was enhanced. Moreover, to further improve the photogenerated charge carrier seperation rate, some semiconductors were coupled with prepared 1D nanostructures (such as nanotube, nanofiber, and nanorod). Zhai et al. reported high photoactivities in the visible light responsive graphene/titanate nanotubes photocatalysts fabricated by hydrothermal method [11]. Yi et al. fabricated the silver nanoparticles, decorated nanobranched TiO₂ nanofibers, and presented that the photocatalytic degradation rates of silver loaded on nanobranched TiO₂ nanofibers were 1.6 and 1.7 times as those of pure TiO₂ nanofibers in the presence of methylene blue and methyl orange, respectively [12].

Cubic-perovskite-type SrTiO₃, a multimetallic oxide semiconductor with a band gap of 3.4 eV comparable to TiO₂, has drawn considerable interest because of its applications in storage batteries, thermoelectric property, solar cell, and photocatalysis [13–16]. SrTiO₃ is a good candidate for coupling TiO₂ and improving the photocatalytic performance of the photocatalyst because its conduction band edge is more negative than TiO₂. Due to the potential differences between the band edges of these two semiconductors, the photogenerated electrons transfer from the conduction band of SrTiO₃ to that of TiO₂. Conversely, the photogenerated electrons transfer from the conduction band of SrTiO₃ to that of TiO₂. Then the photogenerated charge carriers can be efficiently separated by these processes, resulting in the enhancement of the photocatalytic property in the photocatalyst. Recently, some research work about SrTiO₃/TiO₂ photocatalysts has been reported [17–19]. On the other hand, it has been extensively reported that N-doping could obviously improve the photocatalytic activity of TiO₂ [20, 21]. In this work, we fabricated one-dimensional SrTiO₃/TiO_2−𝑥N_𝑥 nanostructures by the hydrothermal method. The photocatalyic activity for the degradation of methylene blue (MB) solution in the N-doped TiO₂ was greatly enhanced after coupling with SrTiO₃.

2. Materials and Methods

2.1. Sample Preparation

Anatase nanocrystalline TiO_2−𝑥N_𝑥 was synthesized by a simple sol-gel method as our previous work described [22]. Different amounts of Sr(NO₃)₂ were dissolved in 40 mL of 10 mol/L NaOH solution. Then 0.78 g N-TiO₂ powder was dispersed into the mixture solution to form a suspension which was heated at 180°C for 24 h under hydrothermal condition in a PTFE-lined stainless steel vessel. The observed sediment at the bottom of the vessel was washed with 0.1 mol/L HCl solutions until pH is less than 7, followed by washing with DI water and drying at 80°C in air overnight. The dried powder was milled and calcined at 400°C under air for 2 h. SrTiO₃/N-TiO₂ samples with different ratios of SrTiO₃ were then prepared.

2.2. Characterization and Measurements

X-ray diffraction (XRD) patterns were recorded on a DX-2500 diffractometer (Fangyuan, Dandong) with Cu K𝛼 radiation with 𝜆 = 0.154145 nm. Transmission electron microscopy (TEM) measurements were carried out on a JEOL-2010 TEM at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were recorded on an Axis Ultra system with monochromatic Al K𝛼 X-rays (1486.6 ev) operated at 45 W and 15 kV with a background pressure of approximately 5.0×10⁻⁹ Torr. A survey spot size and 40 eV pass energy were used for the analysis. Emitted photoelectrons perpendicular to the sample surface were collected. The binding energy of all investigated elements was referenced by the C 1s peak at 284.8 eV of the surface adventitious carbon, respectively. The diffuse reflection spectra were obtained on an UV-vis spectrophotometer (Varian Cary 5000) using BaSO₄ as the reference standard and were converted to the absorbance data through the Kubelka-Munk method.

Photocatalytic experiments were carried out in an inner-irradiation-type reactor. A cylindrical reaction cell was used to contain the reaction solution, and a 500 W long-arc xenon lamp surrounded with a water cooling system was fixed in the center of the reaction cell. 30 mg of photocatalyst was suspended in 200 mL of a methylene blue (MB) solution (10 mg/L) under stirring magnetically. The mixture was kept in the dark for 30 minutes to establish an adsorption-desorption equilibrium before the light radiation. The amount of MB in the solution was determined on the basis of its characteristic optical absorption at 665 nm using a visible light spectrophotometer (Model: 723C) based on Lambert-Beer’s law.

3. Results and Discussion

3.1. Crystal Structure and Morphology

The XRD patterns of the uncalcined and calcined samples are presented in Figure 1. The main diffraction peaks at about 9.1°, 24.3°, 28.0°, and 48.3° in the patterns of the uncalcined samples (Figures 1(a), 1(b), and 1(c)) are designated to the protonic titanate phase [23]. Additionally, with the increase of the Sr/Ti ratio, the intensity of the diffraction peaks contributed by SrTiO₃ at about 32.3°, 46.4°, 57.5°, and 67.6° gradually increases. From Figures 1(d), 1(e), and 1(f), it can be found that main crystalline phases in the calcined samples involve the anatase and SrTiO₃ phase. Figure 2 shows the morphologies of the uncalcined and calcined samples. From the TEM results as shown in Figures 2(a), 2(b), and 2(c), it is found that the uncalcined samples are all composed of nanotubes with lengths about 200–300 nm and diameters about 10 nm. After the calcination, the nanotubes in the uncalcined samples morph into nanorods with diameters of 5 10 nm and different lengths as Figures 2(d), 2(e), and 2(f) show.

Figure 1 

XRD patterns for the uncalcined ((a): 1%; (b): 2%; (c): 5%) and calcined ((d): 1%; (e): 2%; (f): 5%) samples with different Sr/Ti ratios.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2 

TEM images of the uncalcined ((a): 1%; (b): 2%; (c): 5%) and calcined ((d): 1%; (e): 2%; (f): 5%) samples with different Sr/Ti ratios.

3.2. XPS Measurements

Figure 3 shows the XPS spectra of O 1s, N 1s, Ti 2p, and Sr 3d for the SrTiO₃/TiO_2−𝑥N_𝑥 with the Sr/Ti ratio of 5%. After fitting the result of O 1s XPS, it can be found that two peaks centered at 529.8 eV and 531.2 are observed in the O 1s XPS peak, attributing to bulk oxygen bonded to titanium and surface hydroxyl oxygen in TiO₂, respectively [22, 24]. From Figure 3, a similar broad peak from 397 eV to 403 eV is displayed in the XPS pattern of N 1s state with our previous work where the broad N 1s peak in the XPS pattern of the (Y, N)-codoped TiO₂ was attributed to the formation of the O–Ti–N (substitutional N) and Ti–O–N (interstitial N) structures in the lattice of TiO₂ during the nitridation process [22]. The XPS result of N 1s also indicates that N still exists in the lattice of TiO₂ after the hydrothermal process. As shown in Figure 3, two peaks at 458.6 and 464.6 eV are observed in the spectrum of Ti 2p, which corresponds to the 2p_3/2 and Ti 2p_1/2 states of Ti⁴⁺, respectively [22]. By fitting the XPS result of Sr 3d, two peaks at about 133.0 and 134.7 eV are revealed, assigning to the Sr 3d electronic states in the SrTiO₃ perovskite material and SrO complexes, respectively [24–26].

Figure 3 

XPS spectra of O 1s, N 1s, Ti 2p, and Sr 3d for the SrTiO3/TiO2−𝑥N𝑥 with the Sr/Ti ratio of 5%.

3.3. UV-Vis Spectroscopy

Figure 4 shows the UV-vis spectra of the uncalcined and calcined samples. It can be observed that all the absorption spectra in Figure 4 exhibit typical absorption behavior of the wide band gap oxide semiconductor, having an intense absorption band with a steep edge. From the spectra, the absorption edges of the uncalcined (SrTiO₃/titanate) samples with the Sr/Ti ratio of 0%, 1%, 2%, and 5%, respectively, lie at 390.9, 392.5, 408.4, and 396.7 nm as shown in Figure 4 (curves (a)–(d)). The absorption edges of the calcined (SrTiO₃/TiO_2−𝑥N_𝑥) samples with the Sr/Ti ratio of 0%, 1%, 2%, and 5%, respectively, lie at 424.5, 408.9, 418.8, and 402.1 nm (see curves (e)–(h) in Figure 4). Compared with the absorption edges of uncalcined samples, the absorption edges of the calcined samples shift slightly toward lower energy, respectively, which can be attributed to the differences of band structure and morphologies between titanate nanotubes and anatase nanorods. On the other hand, slight red shifts can be observed in the SrTiO₃/titanate samples after decorating titanate with SrTiO₃. However, different absorption edge blue shifts occur in the SrTiO₃/TiO_2−𝑥N_𝑥 samples as compared to the undecorated TiO_2−𝑥N_𝑥. The origins of absorption edge shifts after the decoration of SrTiO₃ are still under discussion. It is known that the N-doping in TiO₂ nanoparticles can result in obvious visible photocatalytic activity because the band narrowing and oxygen vacancies caused by the N-doping can greatly extend the optical absorption range to visible light region. The inset of Figure 4 shows the absorption spectrum of the TiO_2−𝑥N_𝑥 nanoparticles which reveals that the absorption edge lies at 426.0 nm, and a broad absorption shoulder is observed from 380 to 550 nm. According to the calculational results by Dong et al. [27], the highest localized state for the substitutional N state is 0.14 eV above the top of the valence band and 0.73 eV for the interstitial N state. N-doping simultaneously favored the formation of oxygen vacancy whose electronic states are located 0.8 eV below the conduction band minimum. Herein, in our opinion, the absorption edge at 426.0 nm can be attributed to the electron transition from the substitutional N states to the conduction band, and the absorption shoulder is contributed by the electron transition between the valence (or conduction) band and the oxygen vacancy (or interstitial N) states. It should be noted that the absorption shoulder is not presented in the spectra of the uncalcined or calcined samples, indicating the decrease or vanishing of oxygen vacancies and interstitial N after the hydrothermal process.

Figure 4 

UV-vis absorption spectra of the uncalcined ((a): 0%; (b): 1%; (c): 2%; (d): 5%) and calcined ((e): 0%; (f): 1%; (g): 2%; (h): 5%) samples with different Sr/Ti ratios.

3.4. Photocatalytic Activity

The photocatalytic activities of the prepared samples were evaluated by monitoring the degradation of MB solution, where the concentration of MB was obtained from the linear relation between the absorbance and the concentration of MB solution. Figure 5 displays the degradation rate of MB as a function of light irradiation time without any photocatalyst (the blank test) and over the photocatalysts of TiO_2−𝑥N_𝑥 nanoparticles, TiO_2−𝑥N_𝑥 nanorods, and SrTiO₃/TiO_2−𝑥N_𝑥 nanorods with different amounts of SrTiO₃ and SrTiO₃/titanate nanotube with 5% Sr/Ti ratio. Firstly, the TiO_2−𝑥N_𝑥 nanorods sample presents a similar photocatalytic performance with the sample of TiO_2−𝑥N_𝑥 nanoparticles even though the photoabsorption efficiency of the former is obviously lower than that of the latter. According to the C/C₀ values of these two samples after 30 of minutes dark reaction, the adsorption capability of the TiO_2−𝑥N_𝑥 nanorods is better than that of the nanoparticles. Moreover, as described above, the charge carrier recombination of TiO₂ is expected to be held back by fabricating 1D nanostructure. The above two factors are both positive to the photocatalytic activity of TiO_2−𝑥N_𝑥 nanorods. Secondly, the photocatalytic property of TiO_2−𝑥N_𝑥 nanorods was greatly enhanced after the decoration of SrTiO₃ as shown in Figure 5. With the increase of the amount of SrTiO₃, the photocatalytic properties of the SrTiO₃/TiO_2−𝑥N_𝑥 samples are improved gradually. Here, the visible light photodegradation experiments using a light source with a UV cut were also performed which revealed that there was almost no visible light photocatalytic activity in the samples of SrTiO₃/TiO_2−𝑥N_𝑥. This result is expected; after all, the decoration of SrTiO₃ cannot enhance the visible light absorption in the TiO_2−𝑥N_𝑥 photocatalyst. As described above, SrTiO₃ is a good candidate for coupling TiO₂ to improve the photocatalytic performance of the photocatalyst because the photogenerated charge carriers can be efficiently separated after the modification of SrTiO₃. As a result, better photocatalytic properties were presented in the composite samples than the sample of TiO_2−𝑥N_𝑥. Lastly, as Figure 5 shows, the photocatalytic activity of the SrTiO₃/titanate nanotubes is obviously inferior to the other samples even though its adsorption capability is best among all the samples. From the C/C₀ values of all the samples after the dark reaction, the adsorption capability of the nanotubes is obviously more prominent than the other samples. Generally, the better adsorption capability will indicate a better photocatalytic performance in a type of photocatalyst. However, overmuch adsorbed degrading dye will cover the photocatalytic active sites existing on the photocatalyst surface which is detrimental to the photocatalytic activity of photocatalyst. Herein, in our opinion, overmuch adsorption on the nanotubes surface causes the decrease of photocatalytic property in the SrTiO₃/titanate sample.

Figure 5 

Photodegradation of MB under light irradiation without any photocatalyst (blank) and over the prepared photocatalysts.

4. Conclusions

In summary, SrTiO₃/TiO_2−𝑥N_𝑥 nanostructures were fabricated by the hydrothermal method and characterized by XRD, TEM, XPS, and UV-vis DRS measurements. After hydrothermal processing of the TiO_2−𝑥N_𝑥 nanoparticles with the mixed solution of NaOH and Sr(NO₃)₂, SrTiO₃/titanate nanotubes were obtained and presented ordinary photocatalytic performances even though their adsorption properties were greatly improved, indicating that overmuch adsorption is detrimental to the photocatalytic activity of photocatalysts. After calcining the SrTiO₃/titanate nanotubes, SrTiO₃/TiO_2−𝑥N_𝑥 nanorods were produced and showed greatly enhanced photocatalytic activities even though their optical absorption performances were depressed as compared to the TiO_2−𝑥N_𝑥. The mechanisms about the absorption and photocatalytic properties in the prepared samples were discussed in detail.

Acknowledgments

This work is financially supported by the Scientific and Technological Research Project of Henan Province (Grant no. 122102210229) and the Research Project of Province-ministry Coconstructing Henan University (SBGJ090505).