Ammonothermal Synthesis and Photocatalytic Activity of Lower Valence Cation-Doped LaNbON<sub>2</sub>

Izawa, Chihiro; Kobayashi, Takeshi; Kishida, Kazuhisa; Watanabe, Tomoaki

doi:https://doi.org/10.1155/2014/465720

Advances in Materials Science and Engineering

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Research Article | Open Access

Volume 2014 | Article ID 465720 | https://doi.org/10.1155/2014/465720

Ammonothermal Synthesis and Photocatalytic Activity of Lower Valence Cation-Doped LaNbON₂

Chihiro Izawa,^1,2Takeshi Kobayashi,¹Kazuhisa Kishida,¹and Tomoaki Watanabe¹

Academic Editor: Markku Leskela

Received24 Sept 2014

Revised12 Dec 2014

Accepted12 Dec 2014

Published25 Dec 2014

Abstract

Highly crystalline pure perovskite-type LaNbON₂ powders were synthesized in supercritical ammonia using sodium hydroxide as an oxygen source. Additionally, doping LaNbON₂ with cations of lower valence than that of the parent cation was performed to inhibit reduction of Nb⁵⁺. Various characterization methods indicated that crystallinity, particle morphology, and absorption edge of the product, that is, the factors possibly affecting photocatalytic activity, were not significantly changed by the doping of a lower-valence cation. Nevertheless, the doped LaNbON₂ synthesized using the ammonothermal method evolved hydrogen, suggesting that this type of doping decreases the formation of reduced niobium species and consequently enhances the photocatalytic activity of LaNbON₂. In case of doped LaNbON₂ synthesized using conventional method, no hydrogen evolution was observed. This difference is probably due to the higher crystallinity of ammonothermally synthesized LaNbON₂. Therefore, we successfully produced LaNbON₂ with improved potential for photocatalytic activity for hydrogen evolution under visible light irradiation using ammonothermal synthesis and lower-valence cation doping.

1. Introduction

The conversion of solar energy into chemical energy is an attractive technology for sustainable energy systems. An ideal system for this conversion is photocatalytic water splitting using sunlight as a light source to directly produce hydrogen and oxygen from water. For visible light, active water-splitting photocatalysts are desirable for the efficient conversion of solar energy into hydrogen [1]. Recently, (oxy)nitrides containing transition metal cations with empty d orbitals, such as Ti⁴⁺, Zr⁴⁺, Nb⁵⁺, or Ta⁵⁺, have been reported as potentially visible-light-driven photocatalysts [2, 3]. The potential energies of the valence band edges composed of N2p orbitals are higher than those of O2p orbitals, resulting in narrower band gap energies of (oxy)nitrides. Additionally, niobium-based oxynitrides have a wide visible light absorption band [3, 4]. Therefore, the target of this research is LaNbON₂—a niobium based oxynitride with a perovskite structure and a band gap energy of 1.6 eV—which is among the smallest band gap energy values reported for oxynitrides [5]. Perovskite-type material LaTaON₂, with a similar crystal structure to LaNbON₂ and a band gap energy of 1.9 eV, demonstrates photocatalytic activity for hydrogen evolution under visible light irradiation [6, 7]. Therefore, LaNbON₂ is expected to exhibit similar photocatalytic activity. However, in previous studies of this material, no water reduction or oxidation activity under visible light irradiation has been observed [4]. One plausible explanation for this report is that the small band gap of LaNbON₂ decreases the driving force for redox reactions. Additionally, LaNbON₂ contains nitrogen vacancies due to calcination at long durations and high heating temperatures relative to the decomposition temperature of oxynitride [4, 8]. Nb⁵⁺ is reduced to lower oxidation state species for charge compensation of the nitrogen vacancies. These reduction species form a donor level just below the bottom of the conduction band, which may act as a recombination center of excited electrons and electron holes, leading to negligible activity [9]. There are two key requirements for the enhanced photocatalytic activity of LaNbON₂: the inhibition of LaNbON₂ decomposition resulting from calcination and the inhibition of reduced niobium species formation.

The two techniques, an ammonothermal method and cation-doping using lower valence than that of the parent cation, were employed to develop photocatalytic activity in LaNbON₂. The ammonothermal method is a process for synthesizing (oxy)nitride materials in supercritical ammonia. This method has been successfully used to synthesize GaN [10], CaAlSiN₃ [11, 12], and LaTaON₂ [13]. The important advantages of the ammonothermal method over conventional methods are the high crystallinity of (oxy)nitrides at low temperatures and the suppression of defect formation. Doping with cations of lower valence than that of the parent cation intentionally introduces anion vacancies that scavenge reduced transition metal cation species. It has been reported that the lower-valence cation doping in perovskite-type SrTiO₃, a photocatalyst for overall water splitting under UV irradiation, effectively enhances the photocatalytic activity for water splitting [14]. Therefore, a similar effect is expected to exist in LaNbON₂.

Using these techniques, we studied their effects on the crystallinity, morphology, optical properties, and photocatalytic activity of the products.

2. Materials and Methods

2.1. Synthesis of LaNbON₂ by Ammonothermal Method

The starting alloy (La : Nb = 2 : 1) was prepared by the arc discharge melting of La (99.9%, Kojundo Chemical Lab., Japan) and Nb (99.9%, Kojundo Chemical Lab., Japan) powders. For the synthesis of lower valence cation-doped LaNbON₂, the starting alloy (La : Sr : Nb = 1.9 : 0.1 : 1 or La : Nb : Ti = 2 : 0.95 : 0.05) was prepared through a similar process using Sr (99.9%, Kojundo Chemical Lab., Japan) and Ti (99.9%, Kojundo Chemical Lab., Japan) powders. The prepared alloy was ground with a sample mill in an argon atmosphere. The ground alloys NaNH₂ (95%, Aldrich, U.S.A.) and NaOH (97%, Junsei Chemical, Japan) were mixed in molar ratios of Nb : NaNH₂ : NaOH = 1 : 5 : 2 and loaded in a bottom-sealed Ni tube. The nickel tube was transferred to a vertically positioned high-pressure vessel, which was then filled completely with anhydrous liquid ammonia (99.999%, Taiyo Nippon Sanso, Japan) via a cooled condenser. The pressure vessel was heated to 573 K at a rate of 5 K min⁻¹ for 20 h and then heated to 1073 K at a rate of 1 K min⁻¹ for 5 h with an ammonia pressure of 100 MPa. The obtained sample was exposed to air and washed several times with water, ethanol, and diluted chloric acid to remove sodium amide and La(OH)₃. The sample was then dried at 343 K for 6 h in air.

2.2. Synthesis of LaNbON₂ by Conventional Method

A reference sample of LaNbON₂ was synthesized using the conventional method of nitridation of a LaNbO₄ oxide precursor, which was prepared by solid-state synthesis [15]. La₂O₃ (>99.5%, Wako Pure Chemical, Japan) and Nb₂O₅ (>99.5%, Kojundo Chemical Lab., Japan) were mixed in a stoichiometric ratio by wet milling. For the synthesis of lower valence cation-doped LaNbON₂, oxide precursor (La : Sr : Nb = 0.95 : 0.05 : 1 or La : Nb : Ti = 1 : 0.95 : 0.05) was prepared through a similar process using SrCO₃ (99.9%, Kojundo Chemical Lab., Japan) and TiO₂ (99.9%, Kojundo Chemical Lab., Japan) powders. To this mixture, NaCl (>99.5%, Wako Pure Chemical, Japan) and KCl (>99.5%, Wako Pure Chemical, Japan) were added in equimolar quantities as mineralizers, and the sample was calcined in air at 1473 K for 12 h. The product was washed with deionized water in an ultrasonic bath to remove the halides and then dried at 343 K for 6 h in air. LaNbON₂ powder was obtained by nitridation of the oxide precursor at 1223 K for 15 h under flowing ammonia (99.999%, Showa Denko, Japan).

2.3. Characterizations

The crystal structure of the product was determined using powder X-ray diffraction (XRD, Rint-2200, Rigaku, Japan) with Cu Kα radiation. Particle sizes and morphologies were determined using field emission scanning electron microscopy (FE-SEM, S-5200, Hitachi, Japan). Optical properties were determined using ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS, JascoV-550 spectrometer, Japan). Photocatalytic activities for hydrogen evolution from an aqueous methanol solution under visible light irradiation were measured using a closed gas circulation system. This system consisted of a Pyrex top-irradiation type cell in which methanol was used as a sacrificial electron donor. 0.1 g of the sample was dispersed in 10 vol.% methanol solution (100 mL) containing H₂PtCl₆·6H₂O. Pt was photodeposited on the LaNbON₂ in situ using H₂PtCl₆·6H₂O as the precursor and the suspension was irradiated by a 70 W ceramic metal halide lamp (λ > 380 nm). The evolved gas was analyzed by gas chromatography (GC-8A, TCD, Ar carrier gas, Shimadzu, Japan).

3. Results and Discussion

Figure 1 shows XRD patterns of the products synthesized by the ammonothermal and the conventional methods. All diffraction peaks are attributed to perovskite-type LaNbON₂. Ammonothermally synthesized LaNbON₂ has higher crystallinity than that synthesized by the conventional method because LaNbON₂ particles grow by dissolution-recrystallization processes in ammonia under supercritical conditions. The crystallite sizes, calculated by Scherrer’s equation, are 33.9 nm (ammonothermal method) and 24.7 nm (conventional method). The crystal structure characterization therefore indicates that highly crystalline LaNbON₂ was synthesized in supercritical ammonia at low temperatures. In ammonothermal synthesis of LaTaON₂, addition of oxygen species is unnecessary because the moisture occurring as impurities in air or reagents supplies a sufficient amount of oxygen [13]. In this study, water in the ammonia was removed via a cooled condenser, and sodium hydroxide was instead used as an oxygen source to quantify the amount of added oxygen. Sodium hydroxide was chosen as an oxygen source because NaNH₂ reacts with water to form NaOH. Furthermore, XRD patterns indicated that when an insufficient amount of NaOH was added, the resulting products were nitride and hydride of niobium. Therefore, it was determined that the addition of NaOH induces the formation of phase-pure LaNbON₂ via the ammonothermal method.

LaNbON₂ doped with Sr²⁺ (5%) and Ti⁴⁺ (5%) was synthesized using the ammonothermal method to produce lower valence cation-doped LaNbON₂. In the XRD patterns for these materials (Figure 2), all diffraction peaks are attributed to perovskite-type LaNbON₂, with no additional diffraction peaks from impurities. The crystallinity and the crystallite size were not significantly changed by doping.

In the XRD patterns for lower valence cation-doped and undoped LaNbON₂ synthesized by conventional method, all diffraction peaks are attributed to perovskite-type LaNbON₂, with no additional diffraction peaks from impurities. The crystallinity and the crystallite size were not significantly changed by doping.

SEM images of LaNbON₂ synthesized using the ammonothermal and the conventional methods are shown in Figure 3. The ammonothermally synthesized LaNbON₂ powder consists of dispersed cubic particles, which further indicates high crystallinity [6], supporting the interpretation of the XRD analysis. The particle morphology of the product was not significantly affected by doping. The particle size of ammonothermally synthesized LaNbON₂ was 1-2 μm, while the powder synthesized by nitridation of the oxide precursor consists of agglomerated particles with a particle size of 0.2–1 μm. This result suggests that the surface area does not significantly impact the photocatalytic activity.

(a)

(b)

UV-Vis DRS of LaNbON₂ synthesized by the ammonothermal method and the conventional method are shown in Figure 4. The absorption edge of ammonothermally synthesized LaNbON₂ is at ca. 710 nm, which was not significantly changed by doping. This result suggests that doping does not significantly affect the band gap of LaNbON₂.

The absorption edge of LaNbON₂ synthesized by the conventional method is slightly shorter wavelength compared to ammonothermally synthesized LaNbON₂. In case of oxynitride, it has been reported that absorption edge of oxynitride is red-shift with increasing of nitrogen content in the oxynitride [16]. It is considered that ammonothermal synthesis led to higher nitrogen content in LaNbON₂ compared to conventional method. However, it has been reported that crystallinity and particle morphology affect the optical properties of oxide [17, 18]. Because of differences in crystallinity and particle morphology of products synthesized by ammonothermal and conventional method, it is difficult to consider chemical composition of product from optical properties.

Figure 5 shows the time course of hydrogen evolution from an aqueous methanol solution over undoped, Sr-doped, and Ti-doped LaNbON₂ synthesized using the ammonothermal method with a Pt cocatalyst under visible light ( nm). In the case of undoped LaNbON₂, only nitrogen evolution was observed, which is presumably due to the oxidation of N³⁻ species near the N₂ surface. This self-decomposition has been observed for other (oxy)nitride photocatalysts during water splitting reactions [4, 19–21]. However, no hydrogen evolution was observed in our study. In the case of Sr-doped and Ti-doped LaNbON₂, 0.03 μmol/h of hydrogen evolution was observed after 3 h.

In case of doped LaNbON₂ synthesized using conventional method, only evolution of nitrogen and oxygen was observed, which is presumably due to the self-decomposition of the photocatalyst. However, no hydrogen evolution was observed. This difference is probably due to the higher crystallinity of ammonothermally synthesized LaNbON₂. Thus, we successfully produced photocatalytic activity in LaNbON₂ for hydrogen evolution under visible light as a result of combination with the ammonothermal synthesis and doping with lower-valence cations. The crystallinity, morphology, and absorption edge of LaNbON₂ synthesized by ammonothermal method, that is, the factors possibly affecting photocatalysis, were not significantly changed by doping with a lower-valence cation. Nevertheless, the doped LaNbON₂ synthesized using the ammonothermal method evolved hydrogen, suggesting that this type of doping decreases the formation of reduced niobium species and consequently enhances the photocatalytic activity of LaNbON₂. Hydrogen evolution was observed in both cases after an induction period in which Pt was deposited by photogenerated electrons that would otherwise have been consumed by the reduction of water. However, two of the three products ammonothermally synthesized under the same conditions showed photocatalytic activity for hydrogen evolution.

4. Conclusion

We report the successful synthesis of highly crystalline perovskite-type LaNbON₂ in supercritical ammonia using sodium hydroxide as an oxygen source. The LaNbON₂ powder synthesized by the ammonothermal reaction consists of dispersed cubic particles that reflect the high crystallinity. The crystallinity, morphology, and absorption edge of LaNbON₂, the factors possibly affecting photocatalysis, were not significantly changed by doping with lower-valence cations. Nevertheless, the doped LaNbON₂ synthesized using the ammonothermal method evolved hydrogen, suggesting that this type of doping decreases the formation of reduced niobium species and consequently enhances the photocatalytic activity of LaNbON₂. In case of doped LaNbON₂ synthesized using conventional method, no hydrogen evolution was observed. Therefore, we successfully produced LaNbON₂ with improved potential for photocatalytic activity for hydrogen evolution under visible light using ammonothermal synthesis and doping with cations of lower valence than that of the parent cation. To the best of our knowledge, there are no reports as yet for hydrogen evolution under visible light over LaNbON₂. Verifying the reproducibility of the synthesis requires further investigation.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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Copyright © 2014 Chihiro Izawa et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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