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Preparation of porous nitrogen-doped titanium dioxide microspheres and a study of their photocatalytic, antibacterial and electrochemical activities
Materials Research Bulletin 47 (2012) 4514–4521
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Short communication
Preparation of porous nitrogen-doped titanium dioxide microspheres and a study of their photocatalytic, antibacterial and electrochemical activities S. Chen a, W. Chu a,*, Y.Y. Huang b, X. Liu a, D.G. Tong a,b,** a b
College of Chemical Engineering, Sichuan University, Chengdu 610065, China College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 October 2011 Received in revised form 7 August 2012 Accepted 5 September 2012 Available online 13 September 2012
Nitrogen-doped titanium dioxide (N-doped TiO2) microspheres with porous structure were prepared via the nitrogen-assisted glow discharge plasma technique at room temperature for the first time. The samples were characterized by X-ray diffraction, scanning electron microscopy, nitrogen adsorption– desorption measurement, UV–Vis diffuse reflectance spectra, photoluminescence spectroscopy and Xray photoelectron spectroscopy. The results indicated that the plasma treatment did not affect the porous structure of the TiO2 microspheres. With the plasma treatment, the N contents in the samples increased. During the photocatalytic degradation of methylene blue under simulative sunlight irradiation, the sample after plasma treatment for 60 min (N-TiO2-60) exhibited higher photocatalytic activity than those of the TiO2 microspheres, P25 and other N-doped TiO2 microspheres. Furthermore, the N-TiO2-60 showed excellent antibacterial activities towards Escherichia coli under visible irradiation. These should be attributed to the enhancement of the visible light region absorption for TiO2 after Ndoping. Electrochemical data demonstrated that the N-doping not only enhanced the electrochemical activity of TiO2, but also improved the reversibility of Li insertion/extraction reactions and the rate behavior of TiO2 during charge–discharge cycles. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds D. Catalytic properties D. Electrochemical properties
1. Introduction Titanium dioxide (TiO2) is an important material that has been recognized for its use in preparing photo-induction antibacterial materials, catalysts in many photocatalytic reactions, and anode materials in lithium-ion batteries. Therefore, the synthesis and the application of TiO2 have both theoretical and practical importance. However, its band gap (3.2 eV for anatase) is wide. In other words, the ion and electron conductivity of anatase TiO2 is poor. To reduce the band gap of TiO2, one of the most common strategies reported is substitution or interstitial doping with other elements, such as Mg, Fe, I, [1–7]. Among them, N substitution is one of the most effective ways. Thus, various strategies have been applied to prepare N-doped anatase TiO2. Bin and Asahi et al. prepared Ndoped TiO2 with good performance via mechanical milling and magnetron sputtering [8,9], respectively. But, these physical methods usually need expensive apparatus. Chemical methods have also been developed to prepare N-doped anatase TiO2. Gohin
* Corresponding author. Tel.: +86 28 8540 3836; fax: +86 28 8407 9074. ** Corresponding author at: College of Chemical Engineering, Sichuan University, Chengdu 610065, China. E-mail addresses: [email protected] (W. Chu), [email protected] (D.G. Tong). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.09.031
et al. [3] prepared N-doped TiO2 thin films via the sol–gel method. In addition, Wu et al. [4] prepared an N-doped TiO2 photocatalyst via the hydrothermal method. In these methods, however, large amounts of toxic chemicals are used. To avoid the use of such chemicals, it is necessary to develop an environmentally friendly and effective method. In recent years, the plasma technique has gained attention as a powerful and environmentally friendly approach for the preparation of functional materials [10–17]. Recently, we successfully prepared N-doped TiO2 nanotube arrays with good photocatalytic activity [17]. However, the N-doped TiO2 nanotube arrays need calcination to improve their performance. During calcination, the atmosphere would affect the N-doping concentration in the TiO2 microspheres. In 2012, Troitskaia et al. prepared nanoporous TiO2 microspheres via a water-solution way without additive thermal treatment, which is interesting in various catalysis and photocatalysis applications [18]. Inspired by these results, we attempted to prepare N-doped TiO2 microspheres via plasma without calcination. In this work, the TiO2 microspheres prepared via the hydrothermal method were used as the precursor of N-doped samples for the first time. The photocatalytic property, antibacterial performance and lithium ion extraction/insertion behaviors of the N-doped TiO2 microspheres were investigated via comparison with those of the TiO2 microspheres and P25.
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2. Experimental 2.1. Preparation of porous N-doped TiO2 microspheres All chemicals were of analytical grade and were used without further purification. Porous TiO2 microspheres were synthesized via a simple hydrothermal method. Typically, 0.1 M Ti(SO4)2 solution (10 mL) was added into hexamethylenetetramine (HMT, C6H12N4) solution (0.3 M, 50 mL) in a 100 mL beaker and stirred for 10 min. Then the mixture was sealed into a Teflon-lined stainless steel
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autoclave, and maintained at 160 8C for 8 h. The autoclave was allowed to cool to room temperature naturally. The collected white precipitates were washed with distilled water, followed by three washings with absolute alcohol. Finally, the obtained product was dried at 80 8C for 12 h in a vacuum oven. For N-doping, the obtained TiO2 microspheres were treated into a discharging tube in a nitrogen-assisted glow discharge plasma system for 10, 30 and 60 min, respectively. The plasma parameters were as follows: discharge voltage 100 V, anodic current 50 mA, and frequency 13.56 MHz. The samples of N-doped TiO2 microspheres prepared by
Fig. 1. SEM images of (a) TiO2; (b) N-TiO2-10; (c) N-TiO2-30 and (d) N-TiO2-60 microspheres; enlarged SEM images of (e) TiO2 and (f) N-TiO2-60 microspheres.
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this method were designated as N-TiO2-10, N-TiO2-30, and N-TiO260, respectively. The plasma time >60 min was not used because of the limitation of our plasma equipment. 2.2. Characterizations The structural properties of the samples were analyzed with a Philips XPERT-PRO X-ray diffractometer using Cu Ka irradiation at a step size of 0.038. The morphology of the samples was examined using a field-emission scanning electron microscope (Inspect, FEI Corporation, USA). The Brunauer–Emmett–Teller (BET) specific surface area and pore volume analysis of the samples were determined using a N2 adsorption–desorption technique, the sample was degassed at 300 8C for 180 min before the actual measurement. Diffuse-reflectance spectra were recorded on a Shimadzu 3100 UV–Vis–NIR spectrometer equipped with an integrating sphere ISR-3100. BaSO4 powder was used as the reference. Photoluminescence (PL) spectra were obtained by the 340 nm line of a nanosecond Nd:YAG laser (NL303G) as the excitation source. The experimental setup consists of a spectrometer (Spex 1702), a photomultiplier tube (PMT, Hamamatsu R943), a lock-in amplifier, and a computer for data processing. Surface chemical analysis of the samples was conducted on an X-ray photoelectron spectroscope (XPS, XSAM 800) using Al Ka (hy = 1486.6 eV) and an analyzer pass energy of 40 eV. All binding energies were referenced to the C 1s peak with a binding energy of 284.8 eV of the surface adventitious carbon. The elemental analysis was carried out on an Elementar Vario EL III. Photocatalytic experiments were carried out in a homemade photoreactor, containing the required quantity of samples and methylene blue (MB) aqueous solution. The suspension was irradiated with a 300 W Xenon-lamp. The reaction suspension was prepared by adding the sample (0.01 g L 1) into 200 mL of MB solution (10 mg L 1). The suspension was stored in the dark and stirred for 120 min to ensure adsorption/desorption equilibrium prior to reaction. The concentration of MB in the solution was determined using a V-5000 spectrophotometer by collecting the absorbance of MB at 664 nm. The antibacterial activity of the samples against Escherichia coli (E. coli) was investigated based on an antibacterial drop-test according to the literature procedure [19,20]. For the antibacterial test, all samples were coated on glass substrates with size of 12 mm 12 mm. Before the experiment, all glassware and samples were sterilized by heat-treating at 115 8C for 30 min. The microorganisms were cultured on a nutrient agar plate at 37 8C for 24 h. Then all samples were placed into a sterilized Petri dish and 100 mL of a diluted saline solution containing E. coli was spread on the surface of the samples. For photo-induced antibacterial experiments, the samples were irradiated using a 100 mW cm 2 mercury lamp at room temperature (wavelengths below 400 nm were removed by using a cutoff filter). During the irradiation, the substrate of the samples coated by the bacterial suspension was contacted to a reservoir of cool water to control the temperature of the substrate. After 1 h, the bacteria were washed from the surface of the samples using 5 mL phosphate buffer solution. Then 10 mL of each bacteria suspension was spread on a nutrient agar plate and then incubated at 37 8C for 24 h for the purpose of counting the surviving bacterial colonies by using optical microscopy. The reported data were the average value of five separate similar runs. The electrochemical tests were performed using three-electrode cells with lithium serving as both the counter and reference electrodes under ambient temperature. The working electrode was composed of 75 wt.% active material, 17 wt.% carbon black, and 8 wt.% of polyvinylidene difluoride (PVDF, Aldrich). The electrolyte
was 1 M LiPF6 in a 40:60 (w/w) mixture of ethylene carbonate and diethyl carbonate. Cell assembly was carried out in an argon-filled glovebox with both moisture and oxygen contents below 1 ppm. Cyclic voltammetry (CV) was performed using an electrochemical workstation (CHI 660B) with a scanning rate of 0.1 mV s 1. Charge/ discharge cycling was conducted with a battery tester (NEWAER) between 1 V and 3 V at different current rates of 0.5 C, 1 C, 4 C, 10 C and 20 C, where 1 C = 160 mA g 1. 3. Results and discussion The morphologies of the TiO2 microspheres and N-doped TiO2 microspheres (Fig. 1a–f) were studied by SEM, respectively. Obviously, the N-doped TiO2 microspheres maintained the microstructure and size of their precursor after plasma treatment. From the magnified SEM images (Fig. 1e and f), we can see that the uniform nanoparticles were connected to each other to form a porous structure. The isotherms (Fig. 2a) of the TiO2 and N-TiO2-60 microspheres showed typical IUPAC type patterns with an inflection at P/P0 of about 0.50 corresponding to adsorbed N2, indicating the presence of mesopores. The pore size distribution (Fig. 2b) showed that both samples possessed a large quantity of mesopores and a small quantity of macropores (>50 nm) with almost the same average pore diameter of 33.7 nm. The specific surface areas of the TiO2 and N-doped TiO2 microspheres were almost the same (Table 1). These
Fig. 2. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution of TiO2 and N-TiO2-60 microspheres.
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Table 1 Specific surface areas, N contents, average particle diameters, lattice distortions, binding energy of oxygen species, binding energy of Ti 2p3/2, band gap energy of N-doped TiO2 microspheres, TiO2 microspheres and P25. Samples
Specific surface area (m2 g 1) N content (at.%) Average particle diameter (nm) Lattice distortion Binding energy of Ti 2p3/2 (eV) Binding energy of lattice oxygen (Ti–O) (eV) Binding energy of hydroxyl oxygen (eV) Binding energy of physically adsorbed oxygen (eV) Band gap energy (eV)
TiO2
N-TiO2-10
N-TiO2-30
N-TiO2-60
P25
188.0 0.05 8.30 0.018 458.3 529.7 531.3 533.1 3.04
186.3 0.8 8.31 0.022 458.4 529.7 531.3 533.0 2.97
187.8 1.7 8.29 0.027 458.4 529.8 531.3 532.9 2.94
185.7 2.5 8.30 0.034 458.5 529.8 531.3 532.8 2.90
63.0 0 – – – – – – 3.00
results further confirmed that the plasma treatment did not affect the porous structure of the TiO2 microspheres. Meanwhile, the specific surface area of TiO2 microspheres is much larger than that of P25 (63.0 m2 g 1) [21]. Fig. 3 shows the XRD patterns of the TiO2 and N-doped TiO2 microspheres. All the diffraction peaks can be clearly indexed as the anatase phase of TiO2 (JCPDS 21-1272). No impurity peaks are observed for the N-doped TiO2 microspheres, indicating the doping of the nitrogen in the TiO2 lattice sites. The nitrogen contents of TiO2 and N-doped TiO2 microspheres determined by elemental analysis are listed in Table 1. With the plasma treatment, the N contents in the samples increased. The small amount of N detected in the TiO2 precursor (Table 1) should be attributed to the hydrolysis of HMT in the hydrothermal preparation, which introduced a quantity of N onto the surface of TiO2 microspheres. It was also confirmed by the following XPS analysis. According to Scherrer’s equation [22], an average particle diameter of about 8.3 nm can be calculated based on the full width at half maximum (FWHM) of the (1 0 1) peak for the N-doped TiO2 microspheres, which is the same as that of TiO2 (Table 1). In addition, the lattice distortions of TiO2 and N-doped TiO2 microspheres were calculated according to the equation of e = nh k l/(4 tg u), where (nh k l is the FWHM of the diffraction line, u is the diffraction angle) [22], respectively. The slightly increased lattice distortion of TiO2 with plasma treatment further confirmed the nitrogen doping in the TiO2 lattice sites (Table 1). The N 1s XPS spectra of the samples are shown in Fig. 4A. The distinct N 1s peaks for the N-doped TiO2 microspheres
Fig. 3. XRD patterns of (a) TiO2; (b) N-TiO2-10; (c) N-TiO2-30 and (d) N-TiO2-60 microspheres.
appearing at 399.3 eV are attributed to the nitrogen doping [1–4,14]. With the plasma treatment, the intensity of N 1s peak for the N-doped TiO2 microspheres increased. It further confirmed the N-doping. The faintness of the N 1s peak for TiO2 further confirmed that the amount of N introduced to the TiO2 microspheres during preparation is very small (Table 1). The higher binding energies of the Ti 2p3/2 peaks for N-doped TiO2 (Table 1) in comparison with that of TiO2 precursor also confirms the N doping [1–4,14]. Fig. 4B compares the Ti 2p3/2 XPS spectra of TiO2 and N-TiO2-60. The slight increase of binding energy of Ti 2p3/2 with the N-doping is due to the enhanced electronic interaction between Ti and N [3,4]. The O 1s bands can be separated into three Lorentzian-type components as shown in Fig. 4C for the TiO2. The peaks at 529.7, 531.3 and 533.1 eV were attributed to the lattice oxygen (Ti–O), hydroxyl oxygen and physically adsorbed oxygen, respectively. Comparatively, three peaks appeared at different binding energies for the N-doped TiO2, respectively, which are listed in Table 1. Fig. 4D shows the O 1s XPS spectra of N-TiO2-60 as the example. In earlier observation of XPS in application to TiO2, the binding energy difference between the O 1s and Ti 2p3/2 was proposed to observe the chemical binding energy shift effects [23]. It is found that this BE difference parameter is insensitive to energy scale calibration methods and the binding energy difference between the O 1s and Ti 2p3/2 is in the range from 71.3 to 71.4 eV [23]. In our case, the binding energy differences between the O 1s and Ti 2p3/2 are 71.4, 71.3, 71.4 and 71.3 eV for pure TiO2, N-TiO2-60, N-TiO2-60 and N-TiO2-60, respectively (Table 1). In other words, the effect of N-doping on the chemical state of Ti–O bonds is not notable in our case. Therefore, the slight shifts of the lattice oxygen (Ti–O) and physically adsorbed oxygen peaks for the Ndoped TiO2 microspheres with plasma time (Table 1) are primarily attributed to the enhanced electronic interaction between Ti and N [1–4,14]. But, the exact mechanism for such interaction between Ti and N requires further study, which is currently under way. Based on experimental observation and analysis, the formation mechanism of the N-doped TiO2 microspheres in our case can be described as follows. First, the porous TiO2 microspheres were formed by the hydrothermal method using HMT as a soft template. Second, the porous TiO2 microspheres were attacked by the N2 plasma. Because of dissociation of N2 molecules during plasma discharge processes, electrons, UV and reactive species such as N, N2+, N2+ and N22+, may be generated [24]. So, the N2-plasma not only produces N radicals but also leads to the reaction between the N radicals and TiO2 microspheres. As a result, the N introduced in the TiO2 lattice sites and the N-doped TiO2 microspheres were formed. The photocatalytic activity of the N-doped TiO2 microspheres during decolorization of the MB solution under visible light
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Fig. 4. (A) N 1s XPS spectra of (a) TiO2; (b) N-TiO2-10; (c) N-TiO2-30 and (d) N-TiO2-60 microspheres; (B) Ti 2p3/2 XPS spectra of (a) TiO2 and (b) N-TiO2-60 microspheres; (C) and (D) O 1s XPS spectra of TiO2 and N-TiO2-60 microspheres.
irradiation was studied. After 30 min irradiation, the MB molecules were degraded to levels of 77%, 81%, and 88%, for N-TiO2-10, NTiO2-30, and N-TiO2-60, respectively, while without the addition of N-doped TiO2, the MB remained largely unchanged (Fig. 5). We also used the precursor TiO2 microspheres and P25 as references to evaluate the photocatalytic performance of the N-doped TiO2 microspheres. The degradation of MB over the pure TiO2 microspheres and P25 are 73% and 65%, respectively. It is well-known that the photocatalysis involves a competitive formation of
Fig. 5. A plot of the photodegradation extent of MB molecules as a function of irradiation time for (§) P25; (*) TiO2 microspheres; (&) N-TiO2-10, (~) N-TiO2-30, (*) N-TiO2-60 and (~) without catalysts.
electron–hole pairs and their recombination [25]. In other words, the band gap and recombination rate of photoexcited electrons and holes in the photocatalysts play an important role in their catalytic performance. Therefore, the enhanced photocatalytic activity of the N-doped TiO2 microspheres should be attributed to the N incorporation into the crystalline lattice of TiO2, which modifies the electronic band structure of TiO2. The N 2p band formed above O 2p valance band narrows the band gap of TiO2 and slows the recombination rate of photoexcited species. They are confirmed by the UV–Vis diffuse reflectance and PL measurements as follows. Fig. 6 shows the UV–Vis diffuse reflectance spectra of the TiO2 and N-doped TiO2 microspheres as examples, and Table 1 lists the estimated band gaps of all samples. From Fig. 6 and Table 1, it can be seen that the band gap of the TiO2 microspheres is decreased with N-doping, which indicates the enhancement of the visible light region absorption. The band gap energy of P25 is 3.00 eV [21]. Thus, the N-doping decreased the band gap energy of the TiO2 microspheres. The PL emission spectra of pure TiO2 microspheres, N-doped TiO2 microspheres, and P25 are presented in Fig. 7. Two peaks around 480 and 525 nm are observed for the TiO2 microspheres, which is attributed respectively to the transition from oxygen vacancies with two-trapped electrons and one-trapped electron to the valence band of TiO2 [26]. The energy levels of these two types of oxygen vacancies are located at 0.51 eV and 0.82 eV below the conduction band of TiO2, respectively [27,28]. Thus, during the PL measurements, the photogenerated electrons in the conduction band can fall into the oxygen vacancies through a non-irradiative process, and then recombine with the photogenerated holes in the valence band, accompanied by the fluorescence emission [26–30].
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Fig. 8. Inactivation percentage of E. coli bacteria on the sample surface under dark condition and under visible light irradiation for 1 h.
Fig. 6. (A) UV–Vis diffuse reflectance spectra and (B) plot of transformed Kubelka– Munk function versus energy of the light absorbed for (a) TiO2 and (b) N-TiO2-60 microspheres.
Compared with pure TiO2, the emission intensity is weakened after N-doping. This implies that the recombination of charge carriers is effectively suppressed upon doping of N. As the PL emission is the result of the recombination of excited electrons and holes, the lower PL intensity indicates the decrease in recombination rate and the enhancement in photocatalytic performance.
Fig. 7. Photoluminescence spectra of (a) TiO2 microspheres; (b) P25; (c) N-TiO2-10; (d) N-TiO2-30 and (e) N-TiO2-60 microspheres.
Although the band gap and PL intensity of the undoped TiO2 microspheres is slightly higher than those of the P25, the photocatalytic activity of the undoped TiO2 is higher than that of P25. It should be due to the large surface area and porous structure of TiO2 microspheres, which can facilitate mass transfer and increase the accessibility of the active sites on the surface of the TiO2 microspheres to the MB molecules [31– 33]. The similar phenomenon has also been confirmed by Wu et al. [4]. The antibacterial activity of the N-TiO2-60 microspheres against E. coli bacteria was also investigated under dark and visible light irradiation (Fig. 8). It can be seen that all samples in the dark condition were not effective for inactivation of the E. coli. But with the visible light irradiation, the samples exhibited effective antibacterial activity. The N-TiO2-60 microspheres were significantly better than those of the TiO2 microspheres and P25. The NTiO2-60 microspheres were found to completely inactivate the bacteria in 1 h, while the TiO2 microspheres and P25 could only inactivate 73% and 60% of the bacteria in the similar conditions, respectively. Although the real mechanism of the antibacterial effect is still uncertain, there is no doubt that this effective antibacterial ability of the N-TiO2-60 microspheres would arise from the enhancement of the visible light region absorption resulted from the N-doping. The better antimicrobial performance of the TiO2 microsphere precursor compared to that of P25 arises from their special structure, in which their interior is expected to inhibit the growth of bacteria and the high specific surface area is expected to increase the efficiency of the antimicrobial performance [34,35]. Fig. 9 shows cyclic voltammograms (CV) of the samples at a scanning rate of 0.1 mV s 1. For the N-TiO2-60 microspheres, there is a pair of cathodic/anodic peaks centered at 1.80 and 1.94 V, corresponding to the lithium insertion/extraction reactions in typical TiO2 samples [5–7], respectively. Furthermore, the N-TiO260 microspheres show a better reversibility of insertion/extraction reactions, compared to the TiO2 microsphere precursor and P25, evidenced by the smallest potential difference between the anodic and cathodic peaks [36–38]. The initial charge–discharge curves of the samples are presented in Fig. 10. As can be seen, the N-TiO2-60 microspheres showed a discharge capacity of 262 mAh g 1, which is superior to that observed in the TiO2 microspheres and P25, respectively. It indicated that N doping improved the electrochemical activity of TiO2. The cycling performance (Fig. 11) at different rates of 0.5 C, 1 C (160 mA g 1), 4 C, 10 C and 20 C clearly suggests a higher efficiency for the solid state diffusion of Li+ ions in
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Fig. 10. The initial charge–discharge curves of (a) P25; (b) TiO2 and (c) N-TiO2-60 microspheres at 0.1 C.
Fig. 11. Cycle performance of (§) P25, (*) TiO2 and (&) N-TiO2-60 microspheres at different rates.
4. Conclusions
Fig. 9. CV curves of (a) P25; (b) TiO2 and (c) N-TiO2-60 microspheres at a scanning rate of 0.1 mV s 1.
the N-TiO2-60 microspheres, leading to an excellent rate behavior compared to the TiO2 microsphere precursor and P25, respectively. Even at 20 C, the N-TiO2-60 still maintained a capacity of 120 mAh g 1. Meanwhile, the better rate behavior of the TiO2 microsphere precursor compared to that of P25 should be attributed to the porous structure, which reduces the diffusion lengths for the electrolyte ions and ensures that sufficient electrochemical reactions can take place at high current rates [39–45]. Furthermore, as the rate is reduced from 20 C to 1 C, the difference in capacity vanishes (Fig. 11). The result suggests the good structural stability of the samples during the Li insertion/ extraction electrochemical reaction.
The nitrogen-assisted glow discharge plasma technique was applied to prepare porous N-doped TiO2 microspheres for the first time. The results indicated that plasma treatment did not affect the porous structure of the TiO2 microspheres. With the plasma treatment, the N contents in the samples increased. During the photo-catalytic degradation of methylene blue under simulative sunlight irradiation, the N-TiO2-60 exhibited higher photocatalytic activity than those of the TiO2 microspheres, P25 and other N-doped TiO2 microspheres. Furthermore, the N-TiO2-60 also showed excellent antibacterial activities towards E. coli under visible irradiation. These should be attributed to the enhancement of the visible light region absorption for the TiO2 microspheres after N-doping. Electrochemical data demonstrated that N-doping not only enhanced the reversibility of the Li insertion/extraction reactions, but also improved the rate behavior of the TiO2 microspheres during charge–discharge cycles. References [1] [2] [3] [4] [5]
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Short communication
Preparation of porous nitrogen-doped titanium dioxide microspheres and a study of their photocatalytic, antibacterial and electrochemical activities S. Chen a, W. Chu a,*, Y.Y. Huang b, X. Liu a, D.G. Tong a,b,** a b
College of Chemical Engineering, Sichuan University, Chengdu 610065, China College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 October 2011 Received in revised form 7 August 2012 Accepted 5 September 2012 Available online 13 September 2012
Nitrogen-doped titanium dioxide (N-doped TiO2) microspheres with porous structure were prepared via the nitrogen-assisted glow discharge plasma technique at room temperature for the first time. The samples were characterized by X-ray diffraction, scanning electron microscopy, nitrogen adsorption– desorption measurement, UV–Vis diffuse reflectance spectra, photoluminescence spectroscopy and Xray photoelectron spectroscopy. The results indicated that the plasma treatment did not affect the porous structure of the TiO2 microspheres. With the plasma treatment, the N contents in the samples increased. During the photocatalytic degradation of methylene blue under simulative sunlight irradiation, the sample after plasma treatment for 60 min (N-TiO2-60) exhibited higher photocatalytic activity than those of the TiO2 microspheres, P25 and other N-doped TiO2 microspheres. Furthermore, the N-TiO2-60 showed excellent antibacterial activities towards Escherichia coli under visible irradiation. These should be attributed to the enhancement of the visible light region absorption for TiO2 after Ndoping. Electrochemical data demonstrated that the N-doping not only enhanced the electrochemical activity of TiO2, but also improved the reversibility of Li insertion/extraction reactions and the rate behavior of TiO2 during charge–discharge cycles. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds D. Catalytic properties D. Electrochemical properties
1. Introduction Titanium dioxide (TiO2) is an important material that has been recognized for its use in preparing photo-induction antibacterial materials, catalysts in many photocatalytic reactions, and anode materials in lithium-ion batteries. Therefore, the synthesis and the application of TiO2 have both theoretical and practical importance. However, its band gap (3.2 eV for anatase) is wide. In other words, the ion and electron conductivity of anatase TiO2 is poor. To reduce the band gap of TiO2, one of the most common strategies reported is substitution or interstitial doping with other elements, such as Mg, Fe, I, [1–7]. Among them, N substitution is one of the most effective ways. Thus, various strategies have been applied to prepare N-doped anatase TiO2. Bin and Asahi et al. prepared Ndoped TiO2 with good performance via mechanical milling and magnetron sputtering [8,9], respectively. But, these physical methods usually need expensive apparatus. Chemical methods have also been developed to prepare N-doped anatase TiO2. Gohin
* Corresponding author. Tel.: +86 28 8540 3836; fax: +86 28 8407 9074. ** Corresponding author at: College of Chemical Engineering, Sichuan University, Chengdu 610065, China. E-mail addresses: [email protected] (W. Chu), [email protected] (D.G. Tong). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.09.031
et al. [3] prepared N-doped TiO2 thin films via the sol–gel method. In addition, Wu et al. [4] prepared an N-doped TiO2 photocatalyst via the hydrothermal method. In these methods, however, large amounts of toxic chemicals are used. To avoid the use of such chemicals, it is necessary to develop an environmentally friendly and effective method. In recent years, the plasma technique has gained attention as a powerful and environmentally friendly approach for the preparation of functional materials [10–17]. Recently, we successfully prepared N-doped TiO2 nanotube arrays with good photocatalytic activity [17]. However, the N-doped TiO2 nanotube arrays need calcination to improve their performance. During calcination, the atmosphere would affect the N-doping concentration in the TiO2 microspheres. In 2012, Troitskaia et al. prepared nanoporous TiO2 microspheres via a water-solution way without additive thermal treatment, which is interesting in various catalysis and photocatalysis applications [18]. Inspired by these results, we attempted to prepare N-doped TiO2 microspheres via plasma without calcination. In this work, the TiO2 microspheres prepared via the hydrothermal method were used as the precursor of N-doped samples for the first time. The photocatalytic property, antibacterial performance and lithium ion extraction/insertion behaviors of the N-doped TiO2 microspheres were investigated via comparison with those of the TiO2 microspheres and P25.
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2. Experimental 2.1. Preparation of porous N-doped TiO2 microspheres All chemicals were of analytical grade and were used without further purification. Porous TiO2 microspheres were synthesized via a simple hydrothermal method. Typically, 0.1 M Ti(SO4)2 solution (10 mL) was added into hexamethylenetetramine (HMT, C6H12N4) solution (0.3 M, 50 mL) in a 100 mL beaker and stirred for 10 min. Then the mixture was sealed into a Teflon-lined stainless steel
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autoclave, and maintained at 160 8C for 8 h. The autoclave was allowed to cool to room temperature naturally. The collected white precipitates were washed with distilled water, followed by three washings with absolute alcohol. Finally, the obtained product was dried at 80 8C for 12 h in a vacuum oven. For N-doping, the obtained TiO2 microspheres were treated into a discharging tube in a nitrogen-assisted glow discharge plasma system for 10, 30 and 60 min, respectively. The plasma parameters were as follows: discharge voltage 100 V, anodic current 50 mA, and frequency 13.56 MHz. The samples of N-doped TiO2 microspheres prepared by
Fig. 1. SEM images of (a) TiO2; (b) N-TiO2-10; (c) N-TiO2-30 and (d) N-TiO2-60 microspheres; enlarged SEM images of (e) TiO2 and (f) N-TiO2-60 microspheres.
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this method were designated as N-TiO2-10, N-TiO2-30, and N-TiO260, respectively. The plasma time >60 min was not used because of the limitation of our plasma equipment. 2.2. Characterizations The structural properties of the samples were analyzed with a Philips XPERT-PRO X-ray diffractometer using Cu Ka irradiation at a step size of 0.038. The morphology of the samples was examined using a field-emission scanning electron microscope (Inspect, FEI Corporation, USA). The Brunauer–Emmett–Teller (BET) specific surface area and pore volume analysis of the samples were determined using a N2 adsorption–desorption technique, the sample was degassed at 300 8C for 180 min before the actual measurement. Diffuse-reflectance spectra were recorded on a Shimadzu 3100 UV–Vis–NIR spectrometer equipped with an integrating sphere ISR-3100. BaSO4 powder was used as the reference. Photoluminescence (PL) spectra were obtained by the 340 nm line of a nanosecond Nd:YAG laser (NL303G) as the excitation source. The experimental setup consists of a spectrometer (Spex 1702), a photomultiplier tube (PMT, Hamamatsu R943), a lock-in amplifier, and a computer for data processing. Surface chemical analysis of the samples was conducted on an X-ray photoelectron spectroscope (XPS, XSAM 800) using Al Ka (hy = 1486.6 eV) and an analyzer pass energy of 40 eV. All binding energies were referenced to the C 1s peak with a binding energy of 284.8 eV of the surface adventitious carbon. The elemental analysis was carried out on an Elementar Vario EL III. Photocatalytic experiments were carried out in a homemade photoreactor, containing the required quantity of samples and methylene blue (MB) aqueous solution. The suspension was irradiated with a 300 W Xenon-lamp. The reaction suspension was prepared by adding the sample (0.01 g L 1) into 200 mL of MB solution (10 mg L 1). The suspension was stored in the dark and stirred for 120 min to ensure adsorption/desorption equilibrium prior to reaction. The concentration of MB in the solution was determined using a V-5000 spectrophotometer by collecting the absorbance of MB at 664 nm. The antibacterial activity of the samples against Escherichia coli (E. coli) was investigated based on an antibacterial drop-test according to the literature procedure [19,20]. For the antibacterial test, all samples were coated on glass substrates with size of 12 mm 12 mm. Before the experiment, all glassware and samples were sterilized by heat-treating at 115 8C for 30 min. The microorganisms were cultured on a nutrient agar plate at 37 8C for 24 h. Then all samples were placed into a sterilized Petri dish and 100 mL of a diluted saline solution containing E. coli was spread on the surface of the samples. For photo-induced antibacterial experiments, the samples were irradiated using a 100 mW cm 2 mercury lamp at room temperature (wavelengths below 400 nm were removed by using a cutoff filter). During the irradiation, the substrate of the samples coated by the bacterial suspension was contacted to a reservoir of cool water to control the temperature of the substrate. After 1 h, the bacteria were washed from the surface of the samples using 5 mL phosphate buffer solution. Then 10 mL of each bacteria suspension was spread on a nutrient agar plate and then incubated at 37 8C for 24 h for the purpose of counting the surviving bacterial colonies by using optical microscopy. The reported data were the average value of five separate similar runs. The electrochemical tests were performed using three-electrode cells with lithium serving as both the counter and reference electrodes under ambient temperature. The working electrode was composed of 75 wt.% active material, 17 wt.% carbon black, and 8 wt.% of polyvinylidene difluoride (PVDF, Aldrich). The electrolyte
was 1 M LiPF6 in a 40:60 (w/w) mixture of ethylene carbonate and diethyl carbonate. Cell assembly was carried out in an argon-filled glovebox with both moisture and oxygen contents below 1 ppm. Cyclic voltammetry (CV) was performed using an electrochemical workstation (CHI 660B) with a scanning rate of 0.1 mV s 1. Charge/ discharge cycling was conducted with a battery tester (NEWAER) between 1 V and 3 V at different current rates of 0.5 C, 1 C, 4 C, 10 C and 20 C, where 1 C = 160 mA g 1. 3. Results and discussion The morphologies of the TiO2 microspheres and N-doped TiO2 microspheres (Fig. 1a–f) were studied by SEM, respectively. Obviously, the N-doped TiO2 microspheres maintained the microstructure and size of their precursor after plasma treatment. From the magnified SEM images (Fig. 1e and f), we can see that the uniform nanoparticles were connected to each other to form a porous structure. The isotherms (Fig. 2a) of the TiO2 and N-TiO2-60 microspheres showed typical IUPAC type patterns with an inflection at P/P0 of about 0.50 corresponding to adsorbed N2, indicating the presence of mesopores. The pore size distribution (Fig. 2b) showed that both samples possessed a large quantity of mesopores and a small quantity of macropores (>50 nm) with almost the same average pore diameter of 33.7 nm. The specific surface areas of the TiO2 and N-doped TiO2 microspheres were almost the same (Table 1). These
Fig. 2. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution of TiO2 and N-TiO2-60 microspheres.
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Table 1 Specific surface areas, N contents, average particle diameters, lattice distortions, binding energy of oxygen species, binding energy of Ti 2p3/2, band gap energy of N-doped TiO2 microspheres, TiO2 microspheres and P25. Samples
Specific surface area (m2 g 1) N content (at.%) Average particle diameter (nm) Lattice distortion Binding energy of Ti 2p3/2 (eV) Binding energy of lattice oxygen (Ti–O) (eV) Binding energy of hydroxyl oxygen (eV) Binding energy of physically adsorbed oxygen (eV) Band gap energy (eV)
TiO2
N-TiO2-10
N-TiO2-30
N-TiO2-60
P25
188.0 0.05 8.30 0.018 458.3 529.7 531.3 533.1 3.04
186.3 0.8 8.31 0.022 458.4 529.7 531.3 533.0 2.97
187.8 1.7 8.29 0.027 458.4 529.8 531.3 532.9 2.94
185.7 2.5 8.30 0.034 458.5 529.8 531.3 532.8 2.90
63.0 0 – – – – – – 3.00
results further confirmed that the plasma treatment did not affect the porous structure of the TiO2 microspheres. Meanwhile, the specific surface area of TiO2 microspheres is much larger than that of P25 (63.0 m2 g 1) [21]. Fig. 3 shows the XRD patterns of the TiO2 and N-doped TiO2 microspheres. All the diffraction peaks can be clearly indexed as the anatase phase of TiO2 (JCPDS 21-1272). No impurity peaks are observed for the N-doped TiO2 microspheres, indicating the doping of the nitrogen in the TiO2 lattice sites. The nitrogen contents of TiO2 and N-doped TiO2 microspheres determined by elemental analysis are listed in Table 1. With the plasma treatment, the N contents in the samples increased. The small amount of N detected in the TiO2 precursor (Table 1) should be attributed to the hydrolysis of HMT in the hydrothermal preparation, which introduced a quantity of N onto the surface of TiO2 microspheres. It was also confirmed by the following XPS analysis. According to Scherrer’s equation [22], an average particle diameter of about 8.3 nm can be calculated based on the full width at half maximum (FWHM) of the (1 0 1) peak for the N-doped TiO2 microspheres, which is the same as that of TiO2 (Table 1). In addition, the lattice distortions of TiO2 and N-doped TiO2 microspheres were calculated according to the equation of e = nh k l/(4 tg u), where (nh k l is the FWHM of the diffraction line, u is the diffraction angle) [22], respectively. The slightly increased lattice distortion of TiO2 with plasma treatment further confirmed the nitrogen doping in the TiO2 lattice sites (Table 1). The N 1s XPS spectra of the samples are shown in Fig. 4A. The distinct N 1s peaks for the N-doped TiO2 microspheres
Fig. 3. XRD patterns of (a) TiO2; (b) N-TiO2-10; (c) N-TiO2-30 and (d) N-TiO2-60 microspheres.
appearing at 399.3 eV are attributed to the nitrogen doping [1–4,14]. With the plasma treatment, the intensity of N 1s peak for the N-doped TiO2 microspheres increased. It further confirmed the N-doping. The faintness of the N 1s peak for TiO2 further confirmed that the amount of N introduced to the TiO2 microspheres during preparation is very small (Table 1). The higher binding energies of the Ti 2p3/2 peaks for N-doped TiO2 (Table 1) in comparison with that of TiO2 precursor also confirms the N doping [1–4,14]. Fig. 4B compares the Ti 2p3/2 XPS spectra of TiO2 and N-TiO2-60. The slight increase of binding energy of Ti 2p3/2 with the N-doping is due to the enhanced electronic interaction between Ti and N [3,4]. The O 1s bands can be separated into three Lorentzian-type components as shown in Fig. 4C for the TiO2. The peaks at 529.7, 531.3 and 533.1 eV were attributed to the lattice oxygen (Ti–O), hydroxyl oxygen and physically adsorbed oxygen, respectively. Comparatively, three peaks appeared at different binding energies for the N-doped TiO2, respectively, which are listed in Table 1. Fig. 4D shows the O 1s XPS spectra of N-TiO2-60 as the example. In earlier observation of XPS in application to TiO2, the binding energy difference between the O 1s and Ti 2p3/2 was proposed to observe the chemical binding energy shift effects [23]. It is found that this BE difference parameter is insensitive to energy scale calibration methods and the binding energy difference between the O 1s and Ti 2p3/2 is in the range from 71.3 to 71.4 eV [23]. In our case, the binding energy differences between the O 1s and Ti 2p3/2 are 71.4, 71.3, 71.4 and 71.3 eV for pure TiO2, N-TiO2-60, N-TiO2-60 and N-TiO2-60, respectively (Table 1). In other words, the effect of N-doping on the chemical state of Ti–O bonds is not notable in our case. Therefore, the slight shifts of the lattice oxygen (Ti–O) and physically adsorbed oxygen peaks for the Ndoped TiO2 microspheres with plasma time (Table 1) are primarily attributed to the enhanced electronic interaction between Ti and N [1–4,14]. But, the exact mechanism for such interaction between Ti and N requires further study, which is currently under way. Based on experimental observation and analysis, the formation mechanism of the N-doped TiO2 microspheres in our case can be described as follows. First, the porous TiO2 microspheres were formed by the hydrothermal method using HMT as a soft template. Second, the porous TiO2 microspheres were attacked by the N2 plasma. Because of dissociation of N2 molecules during plasma discharge processes, electrons, UV and reactive species such as N, N2+, N2+ and N22+, may be generated [24]. So, the N2-plasma not only produces N radicals but also leads to the reaction between the N radicals and TiO2 microspheres. As a result, the N introduced in the TiO2 lattice sites and the N-doped TiO2 microspheres were formed. The photocatalytic activity of the N-doped TiO2 microspheres during decolorization of the MB solution under visible light
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Fig. 4. (A) N 1s XPS spectra of (a) TiO2; (b) N-TiO2-10; (c) N-TiO2-30 and (d) N-TiO2-60 microspheres; (B) Ti 2p3/2 XPS spectra of (a) TiO2 and (b) N-TiO2-60 microspheres; (C) and (D) O 1s XPS spectra of TiO2 and N-TiO2-60 microspheres.
irradiation was studied. After 30 min irradiation, the MB molecules were degraded to levels of 77%, 81%, and 88%, for N-TiO2-10, NTiO2-30, and N-TiO2-60, respectively, while without the addition of N-doped TiO2, the MB remained largely unchanged (Fig. 5). We also used the precursor TiO2 microspheres and P25 as references to evaluate the photocatalytic performance of the N-doped TiO2 microspheres. The degradation of MB over the pure TiO2 microspheres and P25 are 73% and 65%, respectively. It is well-known that the photocatalysis involves a competitive formation of
Fig. 5. A plot of the photodegradation extent of MB molecules as a function of irradiation time for (§) P25; (*) TiO2 microspheres; (&) N-TiO2-10, (~) N-TiO2-30, (*) N-TiO2-60 and (~) without catalysts.
electron–hole pairs and their recombination [25]. In other words, the band gap and recombination rate of photoexcited electrons and holes in the photocatalysts play an important role in their catalytic performance. Therefore, the enhanced photocatalytic activity of the N-doped TiO2 microspheres should be attributed to the N incorporation into the crystalline lattice of TiO2, which modifies the electronic band structure of TiO2. The N 2p band formed above O 2p valance band narrows the band gap of TiO2 and slows the recombination rate of photoexcited species. They are confirmed by the UV–Vis diffuse reflectance and PL measurements as follows. Fig. 6 shows the UV–Vis diffuse reflectance spectra of the TiO2 and N-doped TiO2 microspheres as examples, and Table 1 lists the estimated band gaps of all samples. From Fig. 6 and Table 1, it can be seen that the band gap of the TiO2 microspheres is decreased with N-doping, which indicates the enhancement of the visible light region absorption. The band gap energy of P25 is 3.00 eV [21]. Thus, the N-doping decreased the band gap energy of the TiO2 microspheres. The PL emission spectra of pure TiO2 microspheres, N-doped TiO2 microspheres, and P25 are presented in Fig. 7. Two peaks around 480 and 525 nm are observed for the TiO2 microspheres, which is attributed respectively to the transition from oxygen vacancies with two-trapped electrons and one-trapped electron to the valence band of TiO2 [26]. The energy levels of these two types of oxygen vacancies are located at 0.51 eV and 0.82 eV below the conduction band of TiO2, respectively [27,28]. Thus, during the PL measurements, the photogenerated electrons in the conduction band can fall into the oxygen vacancies through a non-irradiative process, and then recombine with the photogenerated holes in the valence band, accompanied by the fluorescence emission [26–30].
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Fig. 8. Inactivation percentage of E. coli bacteria on the sample surface under dark condition and under visible light irradiation for 1 h.
Fig. 6. (A) UV–Vis diffuse reflectance spectra and (B) plot of transformed Kubelka– Munk function versus energy of the light absorbed for (a) TiO2 and (b) N-TiO2-60 microspheres.
Compared with pure TiO2, the emission intensity is weakened after N-doping. This implies that the recombination of charge carriers is effectively suppressed upon doping of N. As the PL emission is the result of the recombination of excited electrons and holes, the lower PL intensity indicates the decrease in recombination rate and the enhancement in photocatalytic performance.
Fig. 7. Photoluminescence spectra of (a) TiO2 microspheres; (b) P25; (c) N-TiO2-10; (d) N-TiO2-30 and (e) N-TiO2-60 microspheres.
Although the band gap and PL intensity of the undoped TiO2 microspheres is slightly higher than those of the P25, the photocatalytic activity of the undoped TiO2 is higher than that of P25. It should be due to the large surface area and porous structure of TiO2 microspheres, which can facilitate mass transfer and increase the accessibility of the active sites on the surface of the TiO2 microspheres to the MB molecules [31– 33]. The similar phenomenon has also been confirmed by Wu et al. [4]. The antibacterial activity of the N-TiO2-60 microspheres against E. coli bacteria was also investigated under dark and visible light irradiation (Fig. 8). It can be seen that all samples in the dark condition were not effective for inactivation of the E. coli. But with the visible light irradiation, the samples exhibited effective antibacterial activity. The N-TiO2-60 microspheres were significantly better than those of the TiO2 microspheres and P25. The NTiO2-60 microspheres were found to completely inactivate the bacteria in 1 h, while the TiO2 microspheres and P25 could only inactivate 73% and 60% of the bacteria in the similar conditions, respectively. Although the real mechanism of the antibacterial effect is still uncertain, there is no doubt that this effective antibacterial ability of the N-TiO2-60 microspheres would arise from the enhancement of the visible light region absorption resulted from the N-doping. The better antimicrobial performance of the TiO2 microsphere precursor compared to that of P25 arises from their special structure, in which their interior is expected to inhibit the growth of bacteria and the high specific surface area is expected to increase the efficiency of the antimicrobial performance [34,35]. Fig. 9 shows cyclic voltammograms (CV) of the samples at a scanning rate of 0.1 mV s 1. For the N-TiO2-60 microspheres, there is a pair of cathodic/anodic peaks centered at 1.80 and 1.94 V, corresponding to the lithium insertion/extraction reactions in typical TiO2 samples [5–7], respectively. Furthermore, the N-TiO260 microspheres show a better reversibility of insertion/extraction reactions, compared to the TiO2 microsphere precursor and P25, evidenced by the smallest potential difference between the anodic and cathodic peaks [36–38]. The initial charge–discharge curves of the samples are presented in Fig. 10. As can be seen, the N-TiO2-60 microspheres showed a discharge capacity of 262 mAh g 1, which is superior to that observed in the TiO2 microspheres and P25, respectively. It indicated that N doping improved the electrochemical activity of TiO2. The cycling performance (Fig. 11) at different rates of 0.5 C, 1 C (160 mA g 1), 4 C, 10 C and 20 C clearly suggests a higher efficiency for the solid state diffusion of Li+ ions in
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Fig. 10. The initial charge–discharge curves of (a) P25; (b) TiO2 and (c) N-TiO2-60 microspheres at 0.1 C.
Fig. 11. Cycle performance of (§) P25, (*) TiO2 and (&) N-TiO2-60 microspheres at different rates.
4. Conclusions
Fig. 9. CV curves of (a) P25; (b) TiO2 and (c) N-TiO2-60 microspheres at a scanning rate of 0.1 mV s 1.
the N-TiO2-60 microspheres, leading to an excellent rate behavior compared to the TiO2 microsphere precursor and P25, respectively. Even at 20 C, the N-TiO2-60 still maintained a capacity of 120 mAh g 1. Meanwhile, the better rate behavior of the TiO2 microsphere precursor compared to that of P25 should be attributed to the porous structure, which reduces the diffusion lengths for the electrolyte ions and ensures that sufficient electrochemical reactions can take place at high current rates [39–45]. Furthermore, as the rate is reduced from 20 C to 1 C, the difference in capacity vanishes (Fig. 11). The result suggests the good structural stability of the samples during the Li insertion/ extraction electrochemical reaction.
The nitrogen-assisted glow discharge plasma technique was applied to prepare porous N-doped TiO2 microspheres for the first time. The results indicated that plasma treatment did not affect the porous structure of the TiO2 microspheres. With the plasma treatment, the N contents in the samples increased. During the photo-catalytic degradation of methylene blue under simulative sunlight irradiation, the N-TiO2-60 exhibited higher photocatalytic activity than those of the TiO2 microspheres, P25 and other N-doped TiO2 microspheres. Furthermore, the N-TiO2-60 also showed excellent antibacterial activities towards E. coli under visible irradiation. These should be attributed to the enhancement of the visible light region absorption for the TiO2 microspheres after N-doping. Electrochemical data demonstrated that N-doping not only enhanced the reversibility of the Li insertion/extraction reactions, but also improved the rate behavior of the TiO2 microspheres during charge–discharge cycles. References [1] [2] [3] [4] [5]
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