Facile synthesis and enhanced visible light photocatalytic activity of N and Zr co-doped TiO2 nanostructures from nanotubular titanic acid precursors
© Zhang et al.; licensee Springer. 2013
Received: 18 November 2013
Accepted: 19 December 2013
Published: 26 December 2013
Zr/N co-doped TiO2 nanostructures were successfully synthesized using nanotubular titanic acid (NTA) as precursors by a facile wet chemical route and subsequent calcination. These Zr/N-doped TiO2 nanostructures made by NTA precursors show significantly enhanced visible light absorption and much higher photocatalytic performance than the Zr/N-doped P25 TiO2 nanoparticles. Impacts of Zr/N co-doping on the morphologies, optical properties, and photocatalytic activities of the NTA precursor-based TiO2 were thoroughly investigated. The origin of the enhanced visible light photocatalytic activity is discussed in detail.
Recently, nanoscale TiO2 materials have attracted extensive interest as promising materials for its applications in environmental pollution control and energy storage . However, TiO2 is only responsive to UV light (λ < 380 nm, 3% to 5% solar energy) due to its large bandgap energy (typically 3.2 eV for anatase). It hinders the practical application of TiO2 for efficient utilization of solar energy . Many studies have been performed to extend the spectral response of TiO2 to visible light and improve visible light photocatalytic activity by doping and co-doping with metals of V, Fe, Cu, and Mo or non-metals of N, B, S, and C [3, 4]. Among the efforts of mono-doping, nitrogen-doped TiO2 was considered to be a promising visible light active photocatalyst. Asahi et al. reported that the effect of N doping into TiO2 achieved enhanced photocatalytic activity in visible region than 400 nm . Theoretical works revealed that the result of the narrowed bandgap is due to N doping-induced localized 2p states above the valence band . However, these states also act as traps for photogenerated carriers and, thus, reduce the photogenerated current and limit the photocatalytic efficiency.
In order to reduce the recombination rate of photogenerated carriers in the nitrogen-doped TiO2, co-doping transition metal and N have been explored . Recently, theoretical calculations have reported that visible light activity of TiO2 can be even further enhanced by a suitable combination of Zr and N co-doping . The Zr/N co-doping of anatase TiO2 could narrow bandgap by about 0.28 eV and enhance the lifetimes of photoexcited carriers. Previously, we had fabricated N-doped TiO2 with visible light absorption and photocatalytic activity using precursor of nanotubular titanic acid (NTA, H2Ti2O4 (OH)2) . The visible light sensitization of N-doped NTA sample was due to the formation of single-electron-trapped oxygen vacancies (SETOV) and N doping-induced bandgap narrowing. It was also found that the N-doped TiO2 prepared by NTA showed the highest visible light photocatalytic activity compared with the TiO2 prepared by different other precursors such as P25 . To obtain further enhanced photocatalytic performance, in this work, we prepared Zr and N co-doped TiO2 nanostructures using nanotubular titanic acid (NTA) and P25 as precursors by a facile wet chemical route and subsequent calcination. A systemic investigation was employed to reveal the effects of Zr and N doping/codoping in the enhancement of visible light absorption and photoactivity of the co-doped TiO2 made by NTA and P25. The results showed that Zr/N-doped TiO2 nanostructures made by nanotubular NTA precursors show significantly enhanced visible light absorption and much higher photocatalytic performance than the Zr/N-doped P25 TiO2 nanoparticles. This work provided a strategy for the further enhancement of visible light photoactivity for the TiO2 photocatalysts in practical applications.
Synthesis of NTA precursors
The precursor of nanotubular titanic acid was prepared and used as a co-doped precursor according to the procedures described in our previous reports [11–13]. Briefly, the Degussa P25 TiO2, a commercial standard TiO2 photocatalyst, reacted with concentrated NaOH solution to obtain Na2Ti2O5 · H2O nanotubes, and then, NTA was synthesized by an ion exchange reaction of Na2Ti2O5 · H2O nanotubes with an aqueous solution of HCl.
Preparation of N and Zr co-doped TiO2
The as-prepared NTA was mixed with urea (mass ratio of 1:2) and dissolved in a 2% aqueous solution of hydrogen peroxide, followed by the addition of pre-calculated amount of Zr(NO3)4 · 5H2O (Zr/Ti atomic ratio, 0%, 0.1%, 0.3%, 0.6%, 1.0%, 5.0%, and 10%). The resultant mixed solution was refluxed for 4 h at 40°C and followed by a vacuum distillation at 50°C to obtain the product of x% Zr/N-NTA. Final Zr/N co-doped TiO2 were prepared by the calcination of x% Zr/N-NTA at a temperature range of 300°C to 600°C for 4 h. The target nanosized TiO2 powder was obtained, denoted as x% Zr/N-TiO2(temperature), for example 0.6% Zr/N-TiO2(500). For reference, Degussa P25 TiO2 powders were used as precursor under the same conditions to prepare Zr/N co-doped TiO2 (denoted as Zr/N-TiO2(P25)).
The phase composition of various Zr/N co-doped TiO2 samples were analyzed by X-ray diffraction (XRD, Philips X’Pert Pro X-ray diffractometer; Cu-Kα radiation, λ = 0.15418 nm). The morphologies of samples were observed using a transmission electron microscopy (TEM, JEOL JEM-2100, accelerating voltage 200 kV). Nitrogen adsorption-desorption isotherms were measured at 77 K on a Quantachrome SI automated surface area and pore size analyzer. The Brunauer-Emmett-Teller (BET) approach was used to evaluate specific surface area from nitrogen adsorption data. The UV-visible diffuse reflectance spectra (DRS) of the samples were obtained on a UV–vis spectrophotometer (Shimadzu U-3010, Kyoto, Japan) using BaSO4 as the reference. The surface composition of the nanocatalysts was analyzed by X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra System with monochromatic Al Ka X-rays (1486.6 eV). An Axis Ultra X-ray photoelectron spectroscope (Quantera) was used for the chemical characterization of photocatalyst samples. The binding energies (BE) were normalized to the signal for adventitious carbon at 284.8 eV. The photoluminescence (PL) spectra were recorded on a fluorescence spectrometer (fluoroSE).
Visible light photocatalytic activity
The photocatalytic activities of various Zr/N co-doped TiO2 samples were evaluated by monitoring the oxidation process of propylene under visible light irradiation. About 25 mg of each photocatalyst sample was spread on one side of a roughened glass plate (ca. 8.4 cm2 active area) and kept in a flat quartz tube reactor. A 300-W xenon lamp (PLS-SXE300/300UV, Beijing Trusttech Co. Ltd., China) was used as the visible light source. A cut filter (λ ≥ 420 nm) was placed between the xenon lamp and reactor. The intensity of visible light irradiated on to be tested samples was ca. 17.6 mW · cm−2. Pure C3H6 (99.99%) stored in a high-pressure cylinder was used as the feed gas, and the flow rate of the feed gas was adjusted to 150 mL/h. The concentration of C3H6, C was determined at a sensitivity of 1 ppm (v/v over volume) using a chromatograph (Shimadzu GC-9A) equipped with a flame ionization detector, a GDX-502 column, and a reactor loaded with Ni catalyst for the methanization of CO2. The photocatalytic activity of visible light photocatalytic oxidation of C3H6 was calculated as (C0 − C)/C0 × 100%, where C0 refers to the concentration of feed gas C3H6 feed gas.
Results and discussion
BET surface areas of the x%-Zr-N-TiO 2 -500 samples with different Zr doping concentration calcined at 500°C
Samples ( x%-Zr-N-TiO2-500)
Surface areas (m2g−1)
BET surface areas of the 0.6%-Zr-N-TiO 2 samples calcined at different temperatures
Calcination temperature (°C)
Surface area (m2g−1)
Figure 6b shows the visible light photocatalytic activities of 0.6% Zr/N-TiO2 samples calcined at different temperatures. The 0.6%Zr/N-TiO2 (400) sample calcined at 400°C shows a lower removal rate of ca. 12%. This lower photocatalytic activity is due to its poor anatase crystallinity as shown in XRD results. Compared with the 0.6% Zr/N-TiO2 (600) sample, 0.6% Zr/N-TiO2(500) sample shows the highest removal rate of ca. 65%. We considered the best photocatalytic performance of Zr/N-TiO2(500) that is due to its higher crystallinity and high surface area according to the above XRD and TEM analysis.
The results showed that the Zr/N codoping significantly enhanced the visible light photocatalytic activities of TiO2 made by NTA precursor. It proves that NTA is a good candidate as a precursor for the preparation of promising visible light TiO2 photocatalyst. As a special structural precursor, the process of loss of water and crystal structural transition during the calcination of NTA is expected to be beneficial for Zr and N doping into the lattice of TiO2. Previously, the visible light absorption and photocatalytic activity of N-doped TiO2 sample N-NTA was found to co-determine by the formation of SETOV and N doping induced bandgap narrowing . Zr doping did not change the bandgap of TiO2 and exhibit no effect on the visible light absorption in our experiments. However, theoretical calculation showed Zr doping brought the N 2p gap states closer to valence band, enhancing the lifetimes of photogenerated carriers . Moreover, Zr doping effectively suppressed the crystallite growth of nano-TiO2 and anatase to rutile phase transformation according to XRD and TEM analysis. Compared with Zr/N-P25, Zr/N-NTA(500) has the advantage of smaller crystallite size, larger surface area, and higher concentration of Zr and N dopant. It is shown that enhanced photocatalytic activity of Zr/N-NTA is achieved in the visible light region as a result of synergy effect of N/Zr codoping and use of nanotubular NTA as precursor.
In summary, Zr/N co-doped TiO2 nanostructures were successfully synthesized using nanotubular titanic acid (NTA) as precursors by a facile wet chemical route. The Zr/N-doped TiO2 nanostructures made by NTA precursors show significantly enhanced visible light photocatalytic activities for propylene degradation compared with that of the Zr/N co-doped commercial P25 powders. Impacts of Zr/N co-doping on the morphologies, optical properties, and photocatalytic activities of the NTA-based TiO2 were thoroughly investigated to find the origin of the enhanced visible light active photocatalytic performance. It is proposed that the visible light response is attributed to the intra-band by the nitrogen doping and calcination-induced single electron-trapped oxygen vacancies (SETOV). Crystallization and growth of Zr/N-doped TiO2 were also impacted by the addition of zirconium. The best visible light photocatalytic activity of Zr/N co-doped NTA was achieved by co-doping with optimal dopant amount and calcination temperature. This work also provided a new strategy for the design of visible light active TiO2 photocatalysts in more practical applications.
The authors thank the National Natural Science Foundation of China (no.21203054) and Program for Changjiang Scholars and Innovation Research Team in University (no. PCS IRT1126).
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