Facile synthesis of TaO_xN_y photocatalysts with enhanced visible photocatalytic activity

Abstract

N-doped TaO_xN_y is prepared using ethylenediamine as the nitrogen source by a sol–gel method. The obtained samples are subsequently calcined at 500–900 °C. XRD analysis and XPS survey indicate the presence of immature TaO₂ (Ta⁴⁺) in the N-doped TaO_xN_y sample sintered at 550 °C (TaO_xN_y-550), and the UV results show TaO_xN_y-550 has a rather strong absorption in the visible light region. The photocatalytic activities of TaO_xN_y are evaluated in hydrogen generation by water splitting and photodegradation of methyl orange. The N-doped TaO_xN_y sample sintered at 550 °C shows the highest H₂ generation rate of 3.12 mmol g⁻¹ h⁻¹, and only 3.1% methyl orange is left after 90 min irradiation. Such photocatalytic activities are comparable to the conventional nitrogen-doped titania.

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1. Introduction

Tantalum pentoxide (Ta₂O₅) has been widely studied for application as a water splitting photocatalyst under ultraviolet irradiation.¹ Metal oxide semiconductors are highly effective in this area. However, these materials have very large bandgaps that preclude effective solar energy harvesting.² It is important to develop photocatalysts for H₂ production by water splitting and for the decomposition of mostly organic compounds under visible light due to the fossil fuel shortage and environmental sustainability.^1,3 There have been persistent efforts to vary the chemical composition of ubiquitous TiO₂ by adding metals or nonmetal impurities that generate donor or acceptor states in the band gap.^4–6 For example, Chen and his co-workers developed a novel approach to improve visible and infrared optical absorption by engineering the disorder of nanophase TiO₂ with simultaneous dopant incorporation.⁷ The photocatalytic activity of commercial TiO₂ was improved by a facile hydrogenation at room temperature in our previous work and the key evidence of the self-doped Ti³⁺ species with hydrogen atoms on its surface was found.⁸

Nitrogen doping of oxide semiconductors has attracted considerable attentions due to efficient visible-light absorption by bandgap modulation of photocatalysts such as TiO₂, ZnO and Ta₂O₅.^6,9 Such strategy is based on the fact that nitrogen has an atomic valence bond-forming orbital (2p) with a higher oxidation potential than oxygen.^10,11 Various nitriding techniques, including ammonolysis, sol–gel, hydrothermal and solvothermal combined post-nitriding, have been developed.^6,9 As a conventional method, the sol–gel method is facile and inexpensive to synthesize N-doped oxide semiconductors.^2,9,12,13 In 2003, Burda et al. used the sol–gel method to synthesize TiO_2−xN_x with a nitrogen doping level as high as 8%.^14,15 TiO_2−xN_x was prepared by employing anatase TiO₂ with triethylamine at room temperature, and the absorbance regions extended into the visible regions (up to 550 nm). Further studies have demonstrated that hydrazine and ethylenediamine are effective organic sources of nitrogen for doping metal oxides.^16,17 Recently, we prepared N-doped TiO₂ using ethylenediamine as the nitrogen source by the sol–gel method and heat treatment. An appropriate high temperature treatment (500 °C) removed the organic residues and kept the N-dopant.⁹ The resulting N-doped titania (TiON-500) performed a high hydrogen production rate of 2.98 mmol g⁻¹ h⁻¹ and an enhanced methyl organic degradation performance.⁹

Ho et al. prepared N-doped Ta₂O₅ (TaO_xN_y) by the sol–gel method for the first time using ethylenediamine as the nitrogen source. The synthesis solution was refluxed under stirring for 72 h to dope N element, and the resulting TaO_xN_y had a bandgap of 2.3 eV.² However, the structure properties of TaO_xN_y were not discussed in detail. In this work, N-doped TaO_xN_y was prepared by the sol–gel method at room temperature by using ethylenediamine, followed by the heat treatment to remove the organic residues. The combination of Ta⁴⁺ and N dopants makes the N-doped TaO_xN_y photocatalysts exhibit higher activity under visible light, and the photocatalytic performances were evaluated by the degradation of methyl orange and H₂ evolution.

2. Experimental section

2.1. Preparation of N-doped TaO_xN_y

A series of N-doped tantalum oxynitride (TaO_xN_y) were prepared by a sol–gel method. The samples were synthesized using an alcoholysis-based sol–gel method similar to the method reported in our previous work.^9,18 In a typical sol–gel synthesis, 5 g of TaCl₅ was dissolved in 27.5 mL of anhydrous ethanol, followed by the addition of 2.5 mL of acetylacetone as a stabilizer under continuous stirring for 4 h. 30 mL of ethylenediamine was added into the sol solution and stirred till they gelled. After the sol being aged for 6 d, the yellowish sol was dried at 60 °C for 5 d. The gel was then ground and calcined at 500–900 °C for 1 h with a heating ramp of 2 °C min⁻¹. The final products were named TaO_xN_y-500, TaO_xN_y-550, TaO_xN_y-600, TaO_xN_y-700, TaO_xN_y-800 and TaO_xN_y-900, respectively, based on the calcination temperature.

2.2. Characterization of photocatalysts

The phase structure was examined by X-ray diffraction (XRD) using Rigaku MiniFlex II with Cu Kα radiation (λ = 0.1542 nm) at 40 kV. The UV-vis absorption spectra were recorded on a Perkin-Elmer Lambda 750 UV/vis/NIR spectrometer. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) was used to examine the valence states of the N doped sample, coupled with a spherical capacitor analyzer and Al Kα (hν = 1486.6 eV) as the radiation source. To compensate for a systematic error in XPS measurement, the energy positions were first adjusted by comparing the surface energy of absorbed C on the surface of the specimen with that of the standard binding energy (284.6 eV, C 1s). The specific surface areas were measured by the Brunauer–Emmett–Teller (BET) method employing nitrogen adsorption at 77 K after treating the samples at 100 °C and ∼10⁻⁴ Pa for 2 h using a Tristar-3000 apparatus. FT-IR spectra were obtained on a Nicolet 500 spectrometer. TEM image was obtained on a JEM-2010 electron microscope operated at 200 kV. The morphology and elemental analysis was conducted by SEM/EDAX (Hitachi-S-4800II, scanning electronic microscope with an energy dispersive analysis of X-ray, Japan). Room temperature photoluminescence (PL) spectra were recorded on a spectrofluorometer (FluoroMax-4, HORIBA Jobin Yvon) using a Xe lamp as the excitation source.

2.3. Photocatalysis tests for H₂ generation and photodegradation of methyl orange

Photocatalytic experiments were performed with the same methods described in our recent work.⁹ Typically, 0.5 g of photocatalyst, chloroplatinic acid (1% Pt) were added into 500 mL of 20 vol% aqueous methanol solution under visible light irradiation (>400 nm) with a 300 W high-pressure xenon arc lamp. The amount of H₂ evolved was determined by using a gas chromatograph (GC). The photocatalytic activity was also evaluated by the degradation of methyl orange under visible light irradiation (>400 nm) with a 300 W high-pressure xenon arc lamp. In each experiment, the recyclable photocatalyst from photocatalytic water-splitting (0.5 g) was added into 500 mL of 20 mg L⁻¹ methyl orange aqueous solution (pH: 5). Oxygen was bubbled into the suspension, and the residue of methyl orange was determined by a UV-vis spectrometer.

3. Results and discussion

A series of nitrogen-doped products were sintered at 500–900 °C, and Fig. 1 shows the corresponding XRD patterns. For the samples sintered at 500–600 °C, there are no obvious characteristic peaks appeared (Fig. 1a–c). For the samples calcined at 700 and 800 °C, XRD results show they are composed of δ-Ta₂O₅ phase (JCPDS: 18-1304) with a small amount of β-Ta₂O₅ (JCPDS: 25-0922) (Fig. 1d and e).¹⁹ At the calcination temperature of 900 °C, XRD pattern in Fig. 1f shows TaO_xN_y-900 exhibits a pure β-Ta₂O₅ phase, which is the same as the bulk Ta₂O₅.³

Fig. 1 XRD patterns of different N-doped TaO_xN_y prepared at 500 (a), 550 (b), 600 (c), 700 (d), 800 (e), 900 °C (f) and bulk Ta₂O₅.

Close examinations of XRD patterns of N-doped samples calcined at 500–600 °C indicate there are phase structure changes from “amorphous” to δ-Ta₂O₅ phase with the increase of temperature (Fig. 2). However, the “amorphous” structure of TaO_xN_y-500 and TaO_xN_y-550 includes some immature TaO₂ phase (JCPDS: 19-1297). Such phase would probably give special photocatalytic performances for N-doped TaO_xN_y. The results matched well with Ho's,² if high calcination temperature was carried out. It is fully indicated that calcinations temperature of N-doped metal oxide play an important role on both properties of material and its photocatalytic performances.

Fig. 2 XRD patterns of different N-doped TaO_xN_y sintered at 500 (a), 550 (b) and 600 °C (c).

Fig. 3 shows UV-vis spectra of TaO_xN_y prepared at 500–800 °C. The samples sintered at 500 and 550 °C obviously absorb visible light. The absorption in visible light becomes weaker with the increase of sintering temperature from 500 to 800 °C, which is in accordance with the change in color (inset, Fig. 3). The absorption edge shifts to the shorter wavelength with the sintering temperature increase from 500 to 600 °C. The samples sintered over 600 °C become white and most of the organic matters including N element are removed.

Fig. 3 UV-visible spectra of TaO_xN_y samples sintered at 500–800 °C, and the corresponding digital images.

The sintering temperature plays an important role in the photocatalytic activity of N-doped TaO_xN_y. Photocatalytic H₂ generation by water splitting and photocatalytic degradation of methyl orange were conducted. Fig. 4a shows that TaO_xN_y-550 has the highest H₂ generation rate as high as 3.12 mmol g⁻¹ h⁻¹, which is higher than that of N-doped TiO₂ (2.98 mmol g⁻¹ h⁻¹ for TiON-500) reported in our recent work.⁹ N-doped TaO_xN_y sintered at 500 °C (TaO_xN_y-500) has a low H₂ generation rate of 0.61 mmol g⁻¹ h⁻¹. The main reason is that the TaO_xN_y-500 surface (dark in colour) is fully coated by organic residues, which also causes high visible absorption as shown in Fig. 3. TaO_xN_y sintered over 600 °C display low H₂ generation rates due to the removal of N element. At the calcination temperature of 900 °C, the hydrogen generation rate of TaO_xN_y-900 is the same as the bulk Ta₂O₅, indicating N element is fully removed. For the photocatalytic degradation of methyl orange (Fig. 4b), TaO_xN_y-550 shows the high and efficient degradation rate, and only 3.1% methyl orange is left in 90 min irradiation. Such degradation rate is comparable to that of N-doped TiO₂ (TiON-500).⁹ All the samples show the same trend in methyl orange degradation as that of hydrogen generation as shown in Fig. 4a. PL spectra would show the features of excited states and related defects based on the electronic structure and optical characteristics.²⁰ PL spectra of the TaO_xN_y samples were measured with an excitation wavelength of 325 nm at room temperature (see Fig. S1, ESI†). PL peaks between 425 and 625 nm are observed for all samples, which indicate the recombination of photogenerated electrons and holes. TaO_xN_y samples sintered at 500–600 °C have very low photoluminescence intensities, and TaO_xN_y-550 exhibits the lowest photoluminescence signal. This indicates the recombination rate of photogenerated charge carriers is the lowest on the surface of TaO_xN_y-550. The PL results thus confirm that the sample with a lower PL displays a higher photocatalytic activity. TEM image of TaO_xN_y-550 reveals it is composed of irregular nanoparticles (Fig. S2†).

Fig. 4 H₂ generation (a) and photocatalytic degradation of methyl orange (b) on TaO_xN_y sintered at 500–900 °C.

XPS technique was employed to probe and confirm the N-doped content of TaO_xN_y samples sintered at 500–800 °C. Global XPS survey indicates the samples contains Ta, O, N and C. The binding energies for Ta 4f, Ta 4d, C 1s, N 1s (396.4 eV), Ta 4p and O 1s^11,21 are marked in Fig. 5a. The core level binding energy of Ta 4f gives rise to two peaks at 26.9 and 28.1 eV of Ta 4f_7/2 (Ta₂O₅) and Ta 4f_5/2 (Ta₂O₅), respectively, which indicate the presence of Ta in oxidation state of Ta⁵⁺ (Fig. S3†).²¹ The inset in Fig. 5a shows the XPS spectra of TaO_xN_y samples in the N 1s region, where very tiny N 1s peak at 396.4 eV is present for TaO_xN_y sintered at 500–550 °C. The organic residuals and N-doping element are prone to be oxidized at a high temperature. FT-IR spectra of TaO_xN_y samples sintered at 500–800 °C are shown in Fig. S4.† No characteristic peaks are attributed to the vibrations of the N–Ta–O bond. It shows clearly that most of N element has been oxidized over 500 °C.

Fig. 5 XPS survey of TaO_xN_y samples sintered at 500–800 °C, the N 1s peak around the 396.4 eV regions (inset) (a), and the Ta 4f peak of TaO_xN_y-550 around 26 eV region (b).

The XPS survey indicates TaO_xN_y-550 is composed of Ta₂O₅, TaO₂ and Ta₃N₅ (Fig. 5b).²¹ The Ta 4f peak around the 26 eV regions shows Ta 4f_7/2 (Ta₂O₅) and Ta 4f_5/2 (Ta₂O₅) peaks at 26.3 and 28.1 eV, respectively. Two peaks of Ta 4f_7/2 (25.3 eV) and Ta 4f_5/2 (27.2 eV) indicate the presence of TaO₂. A small peak at 24.7 eV is ascribed to Ta 4f_7/2 (Ta₃N₅). The Ta⁴⁺ and Ta₃N₅ in TaO_xN_y-550 is beneficial for the high photocatalytic activities under visible light. EDAX was performed for the element analysis of TaO_xN_y-550. Only 3.5 wt% N is detected in the sample (Fig. S5†).

As the porous structure is one of the important aspects for catalyst, the surface area and the pore size distribution of TaO_xN_y-550 was investigated by nitrogen sorption test. Fig. 6a shows the nitrogen adsorption–desorption isotherm that is a typical type IV curve, indicating the presence of meso- and macroporous structure. The BET surface area of TaO_xN_y-550 is about 72 m² g⁻¹ and the peak pore size is centered at around 3.6 nm (Fig. 6b). As a comparison, the sample sintered at 800 °C (TaO_xN_y-800) was also conducted by nitrogen sorption test (Fig. 6a). The nitrogen sorption value is greatly lower than that of TaO_xN_y-550, which confirms high crystallinity of TaO_xN_y-800. Its BET surface area decreases to about 7 m² g⁻¹, and the mean pore size increases to around 26 nm. Based on the nitrogen adsorption–desorption isotherm (Fig. 6a), the large pore size should arise from the intra-crystalline pore.

Fig. 6 N₂ adsorption–desorption isotherms (a) and pore size distributions (b) of TaO_xN_y-550, TaO_xN_y-800.

N-doped Ta₂O₅ was prepared using ethylenediamine as the nitrogen source by sol–gel method and heat treatment. The organic residues that negatively affect the photocatalytic activity on Ta₂O₅ surface would be removed after appropriate high temperature (550 °C) treatment. Apart from this, some factors, including crystallinity, N doping level and Ta⁴⁺ in TaO_xN_y, will affect the visible photocatalytic activity of the samples. Obviously, a low crystallinity of TaO_xN_y samples sintered at less than 500 °C is a reason for their low photocatalytic activity, but it is not the most important reason. Similar to N-doped TiO₂, the Ta⁴⁺ can produce isolated states in the forbidden gap as well. These Ta⁴⁺ defects are occupied states and usually act as donors. The electrons in these sites are excited to the conduction band by a thermal or photoexcitation process to form the unoccupied states. This process corresponds to the visible light absorption.²² Meanwhile, the substituted N atoms (appropriate N doping) will introduce localized N 2p states above the valence band that should be responsible for the red shift of the absorption and narrow the band gap due to the mixing of N 2p states with O 2p states, and thus lower the photon transition energy from the valence band to the conduction band.²³ The visible photocatalytic activity of TaO_xN_y-550 is the highest due to the above reasons.

4. Conclusions

N-doped TaO_xN_y was prepared using ethylenediamine as the nitrogen source by the sol–gel method at room temperature and heat treatment. XRD analysis and XPS survey indicate the presence of immature TaO₂ in the N-doped TaO_xN_y sample sintered at 550 °C (TaO_xN_y-550), and the UV results show TaO_xN_y-550 has a rather strong absorption in the visible light regions. Photocatalytic hydrogen generation and photodegradation of methyl orange also reveal TaO_xN_y-550 has the highest photocatalytic activity, which is comparable to the conventional N-doped TiO₂. The H₂ generation rate is as high as 3.12 mmol g⁻¹ h⁻¹, and only 3.1% methyl orange is left in 90 min irradiation for TaO_xN_y-550. High temperature calcination (>600 °C) of N-doped TaO_xN_y removes organic residues and N element, which results in higher crystallinity of Ta₂O₅ and low photocatalytic activities. TaO_xN_y-900 has no photocatalytic performance under visible light that is the same as bulk Ta₂O₅.

Acknowledgements

This research was financially supported by National Natural Science Foundation of China (Project U1332107) and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (2013137). J.Y. thanks the financial support from Natural Science Key Project of the Jiangsu Higher Education Institutions (15KJA220001) and the Priority Academic Program Development of Jiangsu Higher Education.

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Footnote

† Electronic supplementary information (ESI) available: Photoluminescence spectra, TEM image, XPS spectra, FT-IR spectra and energy dispersive analysis of X-ray. See DOI: 10.1039/c5ra23087j

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