Anatase Mg_0.05Ta_0.95O_1.15N_0.85: a novel photocatalyst for solar hydrogen production

Abstract

Anatase Mg_0.05Ta_0.95O_1.15N_0.85, exhibiting a narrow band gap for solar hydrogen, is a promising visible-light-response photocatalyst for photocatalytic or photoelectrochemical water splitting. The excellent photocatalytic activity can be attributed to the satisfying light absorption and electron–hole separation rate.

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Photocatalytic splitting of water into H₂ using visible-light driven photocatalysts has received a great deal of attention to overcome energy shortages and serious environmental pollution.¹ To use this technique for large scale applications, highly efficient photocatalysts are necessary. Transition metal oxynitrides are an important emerging class of materials that may combine both the advantages of oxides and nitrides.² As one of the simplest members of the oxynitrides family, TaON is an active visible-light photocatalyst for oxygen evolution. However, the low performance in the reduction of water to hydrogen restricts the application of TaON in the fields of energy and the environment.

The materials with anatase-type crystal structure, for example of famous naturally occurring compound TiO₂, have been used as photocatalyst,³ ferromagnetic semiconductor,⁴ transparent conductor,⁵ and resistive random-access memory.⁶ Recently, transition metal oxynitrides with anatase structure have attracted attention for applications of photocatalysis.⁷ Suzuki et al. had epitaxially grown anatase TaON thin films on lattice-matched (LaAlO₃)_0.3·(SrAl_0.5Ta_0.5O₃)_0.7 single crystals by using nitrogen plasma-assisted pulsed laser deposition method.⁸ The anatase TaON thin film exhibited good electrical conductivity and high Hall mobility. Lüdtke et al. prepared anatase-type TaON nanoparticles using ammonolysis of the amorphous oxide precursor and predicted it can be a promising photocatalyst for redox reactions.⁹ Unfortunately, the reported anatase TaON is metastable and not easy to obtain single-phase product. Recently, a few reports have demonstrated that the pure anantas-type TaON can be obtained by incorporating lower-valent cations such as Mg²⁺ or Sc³⁺ into crystal lattice of TaON.^10,11

In the present work, anatase phase Mg_0.05Ta_0.95O_1.15N_0.85, with an absorption edge longer than that of TaON, was prepared by nitriding amorphous Mg_0.05Ta_0.95O_2.425 and applied in the photocatalytic H₂ evolution under visible light illumination (see ESI†). In general, magnesium does not directly contribute to the formation of band but simply constructs the crystal structure of anatase TaON.¹² To investigate the influence of magnesium on the crystal structure of TaON, XRD patterns of Mg_0.05Ta_0.95O_1.15N_0.85 and TaON were represented in Fig. S1 in ESI.† The XRD pattern for TaON belongs to the baddeleyite phase with a 7-fold tantalum coordination and an ordered anionic sublattice as same as the reported β-TaON.¹³ However, the XRD pattern of Mg-doped TaON revealed a new phase structure that can be assigned to the anatase phase, implying that Mg was incorporated into the crystal structure of TaON. According to the previous report, the chemical compositions of Mg_0.05Ta_0.95O_1.15N_0.85 may be a solid solution of MgTa₂O₆ and TaON with the ideal stoichiometry of 1 : 17.¹⁰ The Rietveld refinement of the diffraction pattern using Panalytical's Highscore plusversion 4.1 was carried out to further confirm the crystal structure of Mg_0.05Ta_0.95O_1.15N_0.85. Fig. 1 presents the experimental powder XRD pattern with the results of the Rietveld refinement. X-ray diffraction analysis of Mg_0.05Ta_0.95O_1.15N_0.85 together with the calculated pattern obtained from the Rietveld refinement accompanied by the difference profile show that the anatase phase has been formed. For TaON material, to obtain the anatase phase is difficult, since it is a metastable polymorph. Otherwise, the structure analysis results indicated that doping of Mg into TaON can stabilize the anatase-type phase. The increased stabilization of the anatase phase structure of Mg_0.05Ta_0.95O_1.15N_0.85 may partially originate from magnesium ions owing to the smaller electronegativity of magnesium atoms compared to that of tantalum atoms.¹⁴

Fig. 1 Powder XRD data and Rietveld refinement plots of the Mg_0.05Ta_0.95O_1.15N_0.85 in space-group I4₁/amd. The experimental data is shown in red dot, the calculated patterns in black, and the difference curves in blue. The short vertical green bars in line represent the positions of the Bragg reflections of Mg_0.05Ta_0.95O_1.15N_0.85.

For anatase structure tantalum oxynitrides, possible anion arrangements have already been investigated theoretically and experimentally.^9–11,15 In this investigation, the structure parameters calculated from the Rietveld refinement are presented in Tables 1 and S1 in ESI.† The N and O atoms distributions on the 8e Wyckoff position, while the Ta and Mg atoms distributions on the 4b Wyckoff position. In order to investigate the distribution of Mg atoms we calculated the super cell of Mg_0.05Ta_0.95O_1.15N_0.85 with the most stable O/N distribution. It was found that space group I4₁md (no. 109) is the most preferred which agree with our experimental result. Since the radius of Mg²⁺ (72 pm) comparable to that of Ta⁵⁺ (64 pm), Mg are suitable foreign ions for forming TaON variants. Owing to the difference in valence between the foreign and parent cations, as well as the formation of single Mg–O bond, the substitution of Mg²⁺ for Ta⁵⁺ concurrently causes N³⁻/O²⁻ replacement to maintain the charge balance (Fig. 2). Thus, the stability of anatase phase perhaps originated from adequate intake of magnesium.

Table 1 Structural data and refinement parameters for the Mg_0.05Ta_0.95O_1.15N_0.85 calculated by Rietveld refinement of the experimental XRD powder pattern

Space group	I41/amd
Crystal system	Tetragonal
Lattice parameters	a = 3.90662(6) (Å)
	c = 10.0755(2) (Å)
	c/a = 2.58
Volume of unit cell	V = 155.37 × 10^6 (pm^3)
Formula units	Z = 4
Diffractometer	Bruker D8 advance
2θ range	15–82°
Wavelength	Cu Ka1, λ = 154.053 pm
	Cu Ka2, λ = 154.439 pm
Rwp	11.8%
Rexp	5.1%
RBragg	9.9%
Goodness of fitting	2.29
RIR	14.75

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Fig. 2 Possible atomic distribution of the prepared anatase Mg_0.05Ta_0.95O_1.15N_0.85.

The scanning electron microscopy (SEM) images of Mg_0.05Ta_0.95O_1.15N_0.85 and TaON are shown in Fig. S2.† It can be seen that all the two samples exhibited the aggregation of nanoparticles. The particle size was estimated to be several tens of nanometers, and the doping of magnesium showed little effect on the morphology. We further investigated the microsturcture of the prepared Mg_0.05Ta_0.95O_1.15N_0.85 using TEM, and the result is shown in Fig. 3. Selected area electron diffraction analysis (Fig. 3A) reveals that the Mg_0.05Ta_0.95O_1.15N_0.85 is well crystallized for the anatase phase. The high-resolution TEM image (HRTEM) and related fast Fourier transformation (FFT) pattern (Fig. 3B) shows a lattice plane separation of 0.37 nm that corresponds to the (011) plane of anatase-phase Mg_0.05Ta_0.95O_1.15N_0.85 which is consistent with the XRD result.

Fig. 3 SAED pattern (A) and high-resolution transmission electron microscopy (HRTEM) image (B) of the as-prepared anatase phase Mg_0.05Ta_0.95O_1.15N_0.85. The inset of (D) is the related FFT patterns from the direction along the [101] zone axis. The high-resolution X-ray photoelectron spectra of superposed Ta 4f (C) and N 1s (D) for the anatase Mg_0.05Ta_0.95O_1.15N_0.85.

XPS analyses were carried out to determine the surface composition and the elemental oxidation state in the Mg_0.05Ta_0.95O_1.15N_0.85. The high-resolution spectra for Ta 4f, Ta 4p and N 1s were shown in Fig. 3C and D. Ta 4f peak can be deconvoluted into two peaks: Ta 4f_7/2 at 24.5 eV and Ta 4f_5/2 26.4 eV. The Ta 4f_7/2–Ta 4f_5/2 spin orbit separation was 1.9 eV which demonstrated that the chemical state of Ta was Ta⁵⁺. These binding energy values are lower to Ta⁵⁺ in Ta₂O₅ (26.6 eV and 28.5 eV) or TaON (25 eV and 26.9 eV).¹⁶ The peaks for Ta 4f_7/2 and Ta 4f_5/2 of anatase phase Mg_0.05Ta_0.95O_1.15N_0.85 are shifted to lower binding energies by ca. 2.1 and 1.3 eV compared to those of Ta₂O₅ and TaON.¹⁶ Furthermore, the binding energy for Ta 4p_2/3 and N 1s respectively located at 403.1 eV and 395.1 eV, which lower than those of TaON (Ta 4p_2/3: 404, N 1s: 396 eV). This indicates that the outer electron densities of Ta and N atoms are increased when Mg was doped into TaON.¹⁷ No Mg can be detected in Mg_0.05Ta_0.95O_1.15N_0.85, indicating the amount of the Mg is too low in the surface. Since the electron density around Ta and N atoms could affect the conduction and valence band (CB and VB) positions,^16a this different might change the band gap of the materials.

The optical properties of the undoped and Mg-doped TaON samples were investigated by ultraviolet-visible diffuse reflectance spectroscopy, and the results are shown in Fig. 4A. The absorption edges of the magnesium doped sample shifted significantly to longer wavelengths compared to β-TaON. The Mg_0.05Ta_0.95O_1.15N_0.85 can absorb photoenergy with a wavelength shorter than 600 nm, indicating that incorporating Mg into lattice of TaON can stabilize the anatase structure, thus improving the light absorption of TaON. For a crystalline semiconductor, the optical bandgap (E_g) can be determined from the (αhν)^n/2 vs. hν plot, where α is the absorption coefficient, ν is the frequency of the light (s⁻¹), h is Plank's constant (=6.626 × 10⁻³⁴ J s), and n is decided by the characteristics of transition in the semiconductor (indirect absorption, n = 1; direct absorption, n = 4). In this case, the Mg_0.05Ta_0.95O_1.15N_0.85 belonged to indirect bandgap semiconductor like anatase TiO₂ (ref. 18) and β-TaON¹³ and the E_g is 2.08 eV (see inset in Fig. 4A) which is 0.34 eV smaller than 2.42 eV for TaON. According to the previous investigations, differences in the E_g values for the oxynitride might originate from the difference in anion compositions or in lattice parameters.^8,19 Thus, the replacement of Ta by Mg can possibly alter lattice of Mg–Ta–O–N for decreasing the optical bang gaps.

Fig. 4 (A) Ultraviolet-visible diffusive reflectance spectra of the Mg_0.05Ta_0.95O_1.15N_0.85 (a) and TaON (b) samples, inset shows the corresponding bandgap determined by Tauc plot method; (B) H₂ evolution from methanol solution under visible light irradiation (λ ≥ 420 nm) and room temperature photoluminescence spectra (C and D).

Valence band X-ray photoelectron spectroscopy (VB XPS) spectrum of Mg_0.05Ta_0.95O_1.15N_0.85 (Fig. S3 in ESI†) was measured using PHI5000 Veras Probe instrument. The result revealed that the position of valence band edge located at about 0.42 eV is slightly higher than 0.36 eV for valence band edge of TaON (Fig. S3†). According to E_g = |CBM − VBM|, where CBM is conduction band minimum and VBM is valence band maximum, the conduction band edge of Mg_0.05Ta_0.95O_1.15N_0.85 was determined to be at about −1.66 eV, which is enough to reduce H⁺ to H₂. This indicated that the band structure satisfies the energy criterion of water splitting thermodynamically. Photocatalytic activities of Mg_0.05Ta_0.95O_1.15N_0.85 and TaON were measured by monitoring the variation in hydrogen evolution rate under visible light illumination (λ > 420 nm),²⁰ and the results are shown in Fig. 4B and S4.† When the Pt was used as a cocatalyst, the H₂ evolution rate over anatase type Mg_0.05Ta_0.95O_1.15N_0.85 is 3.5 μmol h⁻¹, which is almost 4 times higher than that of TaON (0.96 μmol h⁻¹). When Ru was used as the cocatalyst, both the Mg_0.05Ta_0.95O_1.15N_0.85 (127 μmol h⁻¹) and TaON (61 μmol h⁻¹) show significant increase in H₂ evolution if compared to the case of Pt co-catalyst, meaning that the Ru is more efficient co-catalyst for oxynitride photocatalysts, as demonstrated in previous report.^13a The photocatalytic hydrogen evolution rate decreased with increasing the cutoff wavelength of incident light, indicating that the H₂ evolution reaction is driven light. Continuous H₂ evolution over Mg_0.05Ta_0.95O_1.15N_0.85 with no obvious degradation was clearly observed during five reaction cycles (Fig. S5†), confirming that the catalyst is stable under 30 h light irradiation. Usually, photocatalytic reactions are dominated by three processes: light absorption and formation of holes and electrons, charge migration to the surface, and surface redox to occur.^1a The incorporation of Mg significantly shifted the light absorption towards longer wavelengths to absorb more visible light. Therefore, the improved hydrogen production over Mg_0.05Ta_0.95O_1.15N_0.85 can be attributed to the more light absorption.

The room temperature photoluminescence (PL) spectra of the nanocrystalline Mg_0.05Ta_0.95O_1.15N_0.85 and TaON were recorded using the excitation wavelength of 380 nm and 250 nm respectively, as shown in Fig. 4C and D. A broadened and intensive emission of photoluminescence signal at wavelengths of 530–850 nm was observed in TaON under excitation wavelength of 380 nm (Fig. 4C).²¹ This broad emission band can be resolved into two peaks of 629 and 704 nm (Fig. S6 in ESI†). The PL peak at ca. 629 nm is closed to the band gap of 600 nm for Mg_0.05Ta_0.95O_1.15N_0.85, can be assigned to band edge recombination of excitons.²² However, the PL peak at ca. 704 nm can be attributed to the excitonic emission from the irradiate recombination of surface defects on TaON.^22c Such a photoluminescence process was not observed in Mg_0.05Ta_0.95O_1.15N_0.85, indicating that the electron–hole recombination was suppressed for Mg doping into TaON (Fig. 4C), and alluding the improving photocatalytic activity for Mg_0.05Ta_0.95O_1.15N_0.85. It is worth noting that broad photoluminescence spectrum from 700 to 830 nm was observed in both of the Mg_0.05Ta_0.95O_1.15N_0.85 and TaON under excitation wavelength of 250 nm (Fig. 4D), which can also be resolved into five peaks 704, 732, 760, 799 and 827 nm, respectively (Fig. S7 in ESI†). The origins of these emissions are not yet well understood, but the most reasonable explanation is possibly due to the surface structure defects. Stronger emission was observed in the Mg_0.05Ta_0.95O_1.15N_0.85 if compared to TaON, indicating that the Mg doping into TaON might induce the more surface structure defects. An efficient method to remove the surface structure defects of Mg_0.05Ta_0.95O_1.15N_0.85 is needed for improving its photocatalytic activity.

In summary, the anatase-type Mg_0.05Ta_0.95O_1.15N_0.85 with a narrow band gap of 2.08 eV was synthesized and exhibited the high performance in photocatalytic hydrogen production. Mg doping can stabilize the crystal structure of anatase-type TaON and narrows the band gap by altering the crystal lattice. Our results indicated that the anatase-type Mg_0.05Ta_0.95O_1.15N_0.85 is a promising photocatalyst for water splitting due to its narrow band gap for harvesting more sunlight.

Acknowledgements

The authors wish to thank Dr Yuan Lu of Worcester Polytechnic Institute for the help on Rietveld refinement. This research is financially supported by the National Basic Research Program of China (973 Program, 2013CB632404) and the National Natural Science Foundation of China (51572121 and 51262030), Natural Science Fund of Yunnan province (2016FB084), Key Project of the Department of Education of Yunnan Province (2015Z003), PhD Research Start up Foundation of Yunnan University (XT412004).

References

1. (a) F. E. Osterloh, Chem. Soc. Rev., 2013, 42, 2294; (b) Y. Tachibana, L. Vayssieres and J. R. Durrant, Nat. Photonics, 2012, 6, 511.
2. (a) M. Yang, J. Oró-Solé, J. A. Rodgers, A. B. Jorge, A. Fuertes and J. P. Attfield, Nat. Chem., 2011, 3, 47; (b) S. Ida, Y. Okamoto, M. Matsuka, H. Hagiwara and T. Ishihara, J. Am. Chem. Soc., 2012, 134, 15773.
3. H. Yang, C. Sun, S. Qiao, J. Zou, G. Liu, S. C. Smith, H. Cheng and G. Lu, Nature, 2008, 453, 638.
4. Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara and H. Koinuma, Science, 2001, 291, 854.
5. Y. Furubayashi, T. Hitosugi, Y. Yamamoto, K. Inaba, G. Kinoda, Y. Hirose, T. Shimada and T. Hasegawa, Appl. Phys. Lett., 2005, 86, 252101.
6. M. Fujimoto, H. Koyama, M. Konagai, Y. Hosoi, K. Ishihara, S. Ohnishi and N. Awaya, Appl. Phys. Lett., 2006, 89, 223509.
7. (a) M. Lerch, J. Janek, K. D. Becker, S. Berendts, H. Boysen, T. Bredow, R. Dronskowski, S. G. Ebbinghaus, M. Kilo, M. W. Lumey, M. Martin, C. Reimann, E. Schweda, I. Valov and H. D. Wiemhöfer, Prog. Solid State Chem., 2009, 37, 81; (b) Y. Liu, Y. Zhou, G. Chen, T. Guo, L. Wang, X. Huang and W. Zeng, Mater. Lett., 2015, 148, 155.
8. A. Suzuki, Y. Hirose, D. Oka, S. Nakao, T. Fukumura, S. Ishii, K. Sasa, H. Matsuzaki and T. Hasegawa, Chem. Mater., 2014, 26, 976.
9. T. Lüdtke, A. Schmidt, C. Göbel, A. Fischer, N. Becker, C. Reimann, T. Bredow, R. Dronskowski and M. Lerch, Inorg. Chem., 2014, 53, 11691.
10. (a) H. Schilling, M. Lerch, A. Börger, K. D. Becker, H. Wolff, R. Dronskowski, T. Bredow, M. Tova and C. Baehtz, J. Solid State Chem., 2006, 179, 2416; (b) H. Wolff, M. Lerch, H. Schilling, C. Bähtz and R. Dronskowski, J. Solid State Chem., 2008, 181, 2684.
11. A. Stork, H. Schilling, C. Wessel, H. Wolff, A. Börger, C. Baehtz, K. D. Becker, R. Dronskowski and M. Lerch, J. Solid State Chem., 2010, 183, 2051.
12. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253.
13. (a) M. Hara, T. Takat, J. N. Kondo and K. Domen, Catal. Today, 2004, 90, 313; (b) C. M. Fang, E. Orhan, G. A. de Wijs, H. T. Hintzen, R. A. de Groot, R. Marchand, J. Y. Saillardd and G. de With, J. Mater. Chem., 2001, 11, 1248.
14. S. Chen, S. Shen, G. Liu, Y. Qi, F. Zhang and C. Li, Angew. Chem., Int. Ed., 2015, 54, 3047.
15. (a) T. Bredow, M. W. Lumey, R. Dronskowski, H. Schilling, J. Pickardt and M. Lerch, Z. Anorg. Allg. Chem., 2006, 632, 1157; (b) H. Schilling, A. Stork, E. Irran, H. Wolff, T. Bredow, R. Dronskowski and M. Lerch, Angew. Chem., Int. Ed., 2007, 46, 2931.
16. (a) W. J. Chun, A. Ishikawa, H. Fujisawa, T. Takata, J. N. Kondo, M. Hara, M. Kawai, Y. Matsumoto and K. Domen, J. Phys. Chem. B, 2003, 107, 1798; (b) Y. Chen, S. Liang, L. Wen, W. Wu, R. Yuan, X. Wang and L. Wu, Phys. Chem. Chem. Phys., 2013, 15, 12742.
17. (a) S. Chen, J. Yang, C. Ding, R. Li, S. Jin, D. Wang, H. Han, F. Zhang and C. Li, J. Mater. Chem. A, 2013, 1, 5651; (b) S. Chen, Y. Qi, G. Liu, J. Yang, F. Zhang and C. Li, Chem. Commun., 2014, 50, 14415.
18. H. Tang, K. Prasad, R. Sanjines, P. E. Schmid and F. Lévy, J. Appl. Phys., 1994, 75, 2042.
19. R. Aguiar, D. Logvinovich, A. Weidenkaff, A. Rachel, A. Reller and S. G. Ebbinghaus, Dyes Pigm., 2008, 76, 70.
20. H. L. Gao, S. C. Yan, J. J. Wang and Z. G. Zou, Dalton Trans., 2014, 43, 8178.
21. (a) J. Das and D. Khushalani, J. Phys. Chem. C, 2010, 114, 2544; (b) Y. Chen, S. Liang, L. Wen, W. Wu, R. Yuan, X. Wang and L. Wu, Phys. Chem. Chem. Phys., 2013, 15, 12742; (c) Y. Du, L. Zhao and Y. Zhang, J. Hazard. Mater., 2014, 267, 55.
22. (a) Y. Zhang, Q. Pan, G. Chai, M. Liang, G. Dong, Q. Zhang and J. Qiu, Sci. Rep., 2013, 3, 1943; (b) D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent and O. M. Bakr, Science, 2015, 347, 519; (c) D. Zhang, S. W. Eaton, Y. Yu, L. Dou and P. Yang, J. Am. Chem. Soc., 2015, 137, 9230.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, characterization date, XRD and SEM. See DOI: 10.1039/c6ra17152d

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