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. 2010 Nov 17;11(5):055001. doi: 10.1088/1468-6996/11/5/055001

Preparation of fine, uniform nitrogen- and sulfur-modified TiO2 nanoparticles from titania nanotubes

Mathieu Grandcolas 1,✉, Jinhua Ye 2
PMCID: PMC5090625  PMID: 27877366

Abstract

TiO2 nanoparticles modified with nitrogen and sulfur were prepared from titania nanotubes by a facile wet chemistry method. The samples synthesized with different thiourea/TiO2 ratios showed a uniform nanoparticle size distribution centred at approximately 10 nm with a developed specific surface area of 246 m2 g-1. These modified nanosized photocatalysts exhibited higher photocatalytic activity for the degradation of gaseous isopropanol than unmodified titania nanotubes under visible illumination. This could be attributed to the synergistic effects of a large specific surface area, strong absorption in the visible region, a redshift in the adsorption edge, and surface adsorption modification induced by nitrogen and sulfur compounds.

Keywords: titania nanotube modification, photocatalysis, TiO2 doping

Introduction

Among the various oxide semiconductors, titanium dioxide is recognized as the most suitable for a wide range of applications such as photocatalysis, gas sensing, photovoltaic cells, superhydrophilic coatings and photoelectrolysis [1]. In particular, environmental remediation using TiO2 is widely studied owing to the compound's biological and chemical inertness, strong oxidizing power, non-toxicity, and long-term stability against chemical and photochemical corrosion. With potential applications in air purification, wastewater treatment, as well as an antibacterial, deodorant and self-cleaning material, TiO2 powder has been widely studied for the advantages it offers.

However, titania absorbs only 5% of incoming sunlight in the visible region (≥380 nm), which greatly limits its efficient application. In the late 1980s, researchers began to develop the next generation of titanium dioxide that could absorb both UV and visible light to enhance process efficiencies. Sato reported NOx-doped TiO2 with visible light absorption, produced by annealing titania with NH4Cl or NH4OH [2]. This opened the way to doping TiO2 with non-metallic elements. Asahi et al [3] and Irie et al [4] reported in 2001 and 2003, respectively, that doping TiO2 with nitrogen in an NH3 atmosphere extends its light absorption to the visible region, but some authors indicated that this technique is very reductive and generates many defects in the crystals [5]. Titania photocatalysts were later doped with N [6], C [7], S [8] or F [9] anions or Pt [10], Fe [11], Co [12] or Cu [13] cations or co-doped with several ions [14]. They exhibited an absorption edge that is red-shifted to lower energies as well as enhanced photocatalytic performance in the visible range. Tokudome and Miyauchi reported the synthesis of N-doped TiO2 nanotubes by surface protonation followed by immersion in an NH3 solution and annealing in air [15].

Modification and doping of titania-based nanotubes is now attracting considerable interest. However, little work has been reported on preparing doped or modified TiO2 nanoparticles from titania nanotubes. Titania nanotubes have a high specific surface area and unique properties that could lead to increased surface absorption and thus enhanced doping levels. Moreover, photocatalytic reactions are surface-related, so a large specific surface area usually increases the reaction rate because it can supply more adsorption sites and photocatalytic reaction centres. Note that doping under calcination often leads to a low specific surface area because the porous structure collapses. In addition, the average transfer time of photogenerated electrons and holes is reported to be shorter for small and uniform TiO2 particles than for large particles [16]. The reduced recombination rate is advantageous for photocatalytic activity.

In this work, a facile method is presented for preparing fine TiO2 nanoparticles, which are modified with nitrogen and sulfur and have a high specific surface area, by coupling hydrothermal reaction and wet impregnation methods. Thiourea was used as the source of nitrogen and sulfur. The photocatalytic activity was evaluated by photodegradation of gaseous isopropanol under visible light (≥420 nm).

Experimental details

Synthesis

In a typical synthesis, titania nanotubes were produced by mixing a commercial TiO2 powder (P25 Degussa-Evonik™) with 10 M sodium hydroxide for 24 h in a Teflon autoclave at 130 °C. After rinsing with a large amount of 1 M HCl and distilled water until the pH reached at least 6, the powder was dried at 110 °C for 24 h.

TiO2 was modified with nitrogen and sulfur via thiourea decomposition. Titania nanotubes were soaked for 15 h in 50/50 deionized water to pure ethanol solutions having a thiourea/titania molar ratio of 0.5, 1, 2 or 5; then the solvent was evaporated at 90 °C until dryness. The obtained powder was then calcined at 450 °C for 2 h (heating rate 5 °C min-1) to produce materials with a slight to strong yellow colour (TiO2-NS-x, with 0.5 <x< 5).

Characterization

The crystal structure of the as-prepared photocatalysts was determined using an x-ray diffractometer (XRD, JEOL JDX-3500, Cu-Kα radiation) operated at 35 kV and 100 mA. The UV-visible diffuse reflectance spectra were recorded at room temperature with a UV-2500 spectrophotometer (Shimadzu Corp.) and converted to absorption spectra. Transmission electron microscopy (TEM) micrographs were taken on a JEOL-2100F microscope operated at 200 kV. Selected-area electron diffraction (SAED) patterns were recorded using a 10 nm beam and combined with energy-dispersive X-ray spectroscopy (EDS). Surface area was measured by the conventional Brunauer–Emmett–Teller (BET) method with a Gemini 2360 setup (Shimadzu). Raman spectra were acquired using a laser Raman spectrophotometer (NRS-1000, Jasco) at room temperature. The power of the incident 532 nm laser beam was 2 mW. The beam was focused to a 10 μm spot using an optical microscope with a 20× objective lens. An x-ray photoelectron spectrometer (XPS, PHI Quantera SXM, ULVAC-PHI), which uses monochromatic Al-Kα irradiation (power 100 W, beam diameter 100 μm, incident angle 45°), was used to determine valence states on the surface of powder samples.

Photocatalytic activity

The photocatalytic activity was evaluated via decomposition of gaseous isopropyl alcohol (IPA) over the photocatalyst irradiated with visible light. Typically, 0.1 g of the photocatalyst was spread uniformly in a dish with an irradiation area of 7.8 cm2, and the dish was placed on the base of a 500 mL reactor. After the reactor was sealed with a quartz cover and the inside atmosphere was replaced with synthetic air, gaseous IPA was introduced into the reactor. The reactor was stored in the dark until the concentration stopped changing and the system reached adsorption equilibrium. The reactor was then irradiated with visible light (λ>420 nm) obtained by passing the output of a 300 W Xe lamp through a low-pass filter (L-42, Hoya Corp.) and a water filter. The distance between the sample and the lamp was approximately 15 cm and the light intensity was approximately 30 mW cm-2. The concentrations of IPA, acetone and CO2 were monitored using a gas chromatograph (GC-14B, Shimadzu) equipped with a flame ionization detector and a methanizer.

Results

Sample characterization

Figure 1(a) shows a TEM image of the nanotube sample before calcination. It was taken in conjunction with a previous study, which used the same synthesis method yielding many multiwalled nanotubes with a diameter of 5–15 nm and a length of several hundred nanometres. The same sample after thermal treatment at 400 °C is shown in figure 1(b), which reveals partial degradation of the tubular shape and the formation of grains resulting from the dislocation of titania nanotubes after annealing. Figure 1(c) shows the morphology of the sample doped with 1% thiourea and calcined at 450 °C. TEM observations confirm the porous structure without long-range order, indicating that the mesoporosity mainly originates from interparticle porosity, with a developed specific surface area of 234 m2 g-1. High-resolution TEM of individual nanocrystals revealed well-resolved crystalline planes with a spacing of 0.34 nm, which is similar to that of the (101) planes in anatase [17]. The particle size distribution was rather homogeneous with an average diameter of 10 nm (inset, figure 1(c)).

Figure 1.

Figure 1.

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TEM images of titania nanotube sample (a) before and (b) after calcination at 400 °C, and (c) of TiO2-NS-1 sample calcined at 450 °C with its representative SAED pattern (d). Inset in (c) shows the particle size distribution.

Figure 1(d) shows the SAED pattern of the TiO2-NS-1 sample. Homogeneous and relatively broad rings reveal a poorly crystallized structure, with rings assigned to the (101), (004), (200) and (005) reflections of the anatase phase. EDS results revealed the homogeneous distribution of all constituent elements. These results contrast with the observations of Dong et al wherein a tubular structure was retained for samples calcined at the same temperature of 450 °C [18]. The dissimilarities may arise from different preparation conditions: Dong et al carried out the hydrothermal reaction at a higher temperature (150 °C instead of 130 °C) and for a longer duration (48 h versus 24 h).

Figure 2 shows the UV-visible diffuse reflectance spectra of the TiO2-NS-x samples and titania nanotubes (TiNT). The TiNT sample shows only a single sharp edge, whereas the doped powders exhibit two absorption edges. The edge at 380 nm is attributed to titania absorption, whereas the shoulder between 400 and 500 nm is assigned to N or S compounds interacting with TiO2. The optical adsorption spectra show that the band gaps of the samples prepared with the thiourea/TiO2 ratios of 0.5, 1, 2 and 5 are 2.92, 2.87, 2.79 and 2.73 eV, respectively, and visual inspection confirmed that the colour of the samples changed from yellowish-white to intense yellow with increasing thiourea content. The visible light absorption by TiO2-NS-x samples may be caused by the excitation of electrons from a localized doping level in the band gap. It was previously reported that nitrogen doping can lead to a mixing of N 2p orbitals with O 2p orbitals, creating midgap levels and shifting the absorption edge towards the visible light region [3].

Figure 2.

Figure 2.

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UV-visible diffuse reflectance spectra of TiO2-NS-x samples prepared with different amounts of thiourea.

Figure 3 shows the XRD patterns of the product after the hydrothermal reaction (TiNT) and the TiO2-NS-x samples after calcination at 450 °C with different thiourea/TiO2 ratios. The XRD profile of pure TiNT is essentially the same as that previously reported for titanate nanotubes, with relatively low crystallinity and characteristic 2θ peaks at 24.7 and 48.7° [19]. The XRD patterns for TiO2-NS-x are close to the reported values of the anatase TiO2 phase with peaks at 25.3, 37.7, 48.1, 54.2 and 62.4° (Joint Committee on Powder Diffraction Standards, card No. 21-1272). No notable changes were observed in the lattice parameters, but the diffraction intensity decreased when more thiourea was used.

Figure 3.

Figure 3.

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XRD patterns of TiO2-NS prepared with different thiourea/TiO2 ratios and calcined at 450 °C and of pristine titania nanotubes (TiNT) obtained via hydrothermal synthesis at 110 °C.

We determined the size of the coherent diffraction domains D using Scherrer's equation: D=0.8λ/β cos θ, where λ is the x-ray wavelength (0.1542 nm for Cu-Kα radiation), β is the full width at half maximum, and θ is the diffraction angle. The particle sizes vary from 7 to 12 nm, which agree well with the results of TEM observations. As the amount of thiourea increases, the catalyst particle size decreases, but the unit cell parameters do not change.

The Raman scattering spectrum of TiNT in figure 4(a) features bands at 185, 270, 388, 448, 669, 835 and 928 cm−1, which are ascribed to titanate nanotubes [20, 21]. Figure 4(b) shows Raman spectra for TiO2-NS-x with different thiourea/titania ratios. The peaks at 635, 510, 390 and 140 cm−1 in doped samples originate from the anatase TiO2 phase [22]; they weaken with increasing thiourea content—a trend also seen in the XRD patterns (figure 3). Okata et al reported the role of nitrogen in epitaxial films of anatase and had the same observations [23]. A small concentration of N can promote the growth of anatase; however, the effect is controversial at high dopant levels.

Figure 4.

Figure 4.

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Raman spectra of (a) TiNT and (b) TiO2 nanoparticles modified with nitrogen and sulfur at different thiourea/TiO2 ratios.

XPS spectra were examined to identify the surface properties of the samples with the addition of nitrogen and sulfur. Figure 5(a) shows the wide-range spectrum revealing the presence of Ti, O, N, S and C atoms. The doping level of this sample is approximately 0.6 at.% for N atoms, as estimated from the XPS spectra, and 1.6 at.% for S atoms. The C 1 s peaks at 284.6 eV represent organic carbon pollutants and were used for calibration.

Figure 5.

Figure 5.

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XPS spectra of TiO2-NS-1: (a) wide-scale spectrum, (b) N 1 s and (c) S 2p regions. Underlying curves in (b) and (c) represent fittings.

The Ti 2p XPS spectrum (not shown here) shows the spin-orbit split peaks of Ti 2p3/2 and Ti 2p1/2 at 458.6 and 464.7 eV, respectively, which is characteristic of the Ti (IV) oxidation state. No reduction in the Ti(III) signal was observed. The O 1 s XPS spectrum (not shown) exhibits a strong peak at 529.9 eV, which is characteristic of the oxides of transition metals. The peak at 530.7 eV is attributed to hydroxyl groups on the surface of the photocatalyst, and peaks at higher energies correspond to contamination with carbon compounds.

Figure 5(b) shows the N 1 s XPS spectrum; a clear asymmetric broad peak centred at approximately 400 eV reveals that more than one chemical state of nitrogen exists in the sample. Fitting of the experimental line shape was achieved with three peaks centred at 402.1, 400.1 and 397.8 eV. The attribution of each peak to a specific compound or chemical bond has been debated recently and depends on the synthesis method. The peak at 397.8 eV is assigned here to anionic N-doping, O-Ti-N, where oxygen atoms are replaced by nitrogen atoms involving substitutional N doping [24]. The peaks at 400.1 and 402.1 eV are attributed to molecular nitrogen compounds adsorbed on the surface of TiO2 (NOx, NH3 or NH4+). Therefore, it could be concluded that the lattice oxygen atoms of TiO2 were replaced by N, altering the energy band structure.

Figure 5(c) shows the S 2p XPS spectrum, which exhibits a similar asymmetry. After fitting the experimental spectrum profile, two different peaks were identified as the S6+ oxidation states of S 2p1/2 and S 2p3/2; the intensity ratio is about 1/2. The S4+ peak is expected at around 166 eV and is not present in the sample. This sulfur signal may be assigned to the SOx ions adsorbed on the surface of TiO2 nanoparticles. No signal has been detected that can be assigned to S atoms replacing O atoms in the TiO2 lattice, which would produce a peak at about 160–161 eV.

BET experiments were conducted to determine the specific surface area and pore size distribution. Unmodified material, i.e. TiNT, had a large specific surface area of 390 m2 g-1. It decreased to around 246 m2/g after calcinations at 450 °C, which is still about five times higher than that of the commercial TiO2 powder used in the hydrothermal synthesis. The nitrogen sorption (adsorption and desorption) isotherm and the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution of TiO2-NS-1 material are shown in figure 6. The N,S-modified TiO2 isotherm corresponds to type IV in the IUPAC classification, which is associated with mesopores and shows a specific surface area of 234 m2 g-1; this value is very close to that of undoped material. The specific surface area was almost independent of the thiourea/TiNT ratio indicating that the doping procedure did not affect the sample's porosity. Furthermore, the measurement reveals a monomodal pore size distribution centred between 4 and 6 nm. According to a t plot, no micropores were detected in any sample. The mesopores allow rapid diffusion of gas reactants and products during photocatalytic reactions and enhance the reaction rate.

Figure 6.

Figure 6.

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Isotherm of low-temperature nitrogen adsorption and desorption, and pore size distribution for TiO2-NS-1.

Photocatalytic activity

The photocatalytic activity of mesoporous TiO2 modified with nitrogen and sulfur was examined using the decomposition of IPA under visible light irradiation. In a dark test, the IPA concentration went to zero, and no CO2 formation was detected. No catalytic process was observed, and a rapid decrease in the concentration occurred with IPA adsorption due to the high specific surface area of the powder. Under visible light irradiation, gaseous IPA is gradually oxidized through an acetone intermediate to CO2. However, the concentration of acetone in the system was very low, only a few ppm in the first hour, probably owing to the high specific surface area of the powder and its high adsorption capability. Therefore, we focused on the process of complete conversion into CO2. The changes in IPA and CO2 concentration versus time are shown in figure 7. TiO2 nanoparticles prepared without thiourea (TiO2-NS-0) show no photocatalytic activity under the present conditions even if IPA was completely adsorbed on the photocatalyst during the experiments, as only a few ppm of CO2 was detected. The photocatalytic activity under visible irradiation increased when the thiourea/titania mass ratio was 1. Thus, we can conclude that the TiO2-NS-x samples exhibit enhanced activity relative to the pristine TiNT material. The best results are obtained for the powder with a thiourea/titania ratio of 1, which showed a complete conversion of IPA to CO2 within the first 12 h under visible light.

Figure 7.

Figure 7.

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Time evolution of concentrations of IPA (open symbols) and CO2 (closed symbols) in the presence of TiO2-NS-x nanoparticles irradiated with visible light (catalyst: 0.1 g, 300 W Xe lamp, 420 nm cut-off filter).

The photocatalytic activity of a nanopowder is typically associated with different parameters such as surface area, surface-reactive species density, phase structure and crystallinity [25]. The enhanced photocatalytic activity at a thiourea/titania mass ratio of 1 can be ascribed to (i) a clear improvement in the crystallinity of anatase, as shown by XRD and Raman spectroscopy, (ii) the high specific surface area of the powder, which provides not only more surface area for visible light absorption but also more catalytic sites for reactions, (iii) reduction of the bandgap by N doping via the introduction of midgap levels and (iv) surface modification (mainly with NH3 and SOx species) and interaction with reactants and products. Although doping of TiO2 with N and/or S atoms is discussed in numerous publications, point (iv) has been insufficiently discussed in the past. The presence of sulfate species on the surface of titania has been found to enhance its catalytic activity towards oxidation processes owing to an increase in surface acidity and synergy with the support [26]. It is also well known that surface groups play an important role in the photocatalytic degradation of organic contaminants, particularly hydroxyl and acidic groups.

Conclusions

TiO2 nanoparticles modified with nitrogen and sulfur were synthesized by an impregnation method for titania nanotubes and thiourea as a nitrogen and sulfur source. Their photocatalytic activities were evaluated via degradation of gaseous IPA under visible light irradiation. The modified nanoparticles exhibited a high specific surface area and their bandgap decreased with increasing thiourea content. TiO2-NS with 1 wt% thiourea showed the highest photocatalytic activity. Such modification could enhance the photocatalytic activity of titania nanotubes, which are a promising starting material for doped TiO2 nanoparticles. This strategy could also be applied to other anions or metal oxides.

Acknowledgments

This work was supported by the International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), Tsukuba, Japan. The authors are grateful to Dr H Iwai for the XPS analysis.

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