Ammonothermal Synthesis and Photocatalytic Activity of Lower Valence Cation-Doped LaNbON 2

Highly crystalline pure perovskite-type LaNbON 2 powders were synthesized in supercritical ammonia using sodium hydroxide as an oxygen source. Additionally, doping LaNbON 2 with cations of lower valence than that of the parent cation was performed to inhibit reduction of Nb. 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 lowervalence cation. Nevertheless, the doped LaNbON 2 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 LaNbON 2 . In case of doped LaNbON 2 synthesized using conventional method, no hydrogen evolution was observed. This difference is probably due to the higher crystallinity of ammonothermally synthesized LaNbON 2 . Therefore, we successfully produced LaNbON 2 with improved potential for photocatalytic activity for hydrogen evolution under visible light irradiation using ammonothermal synthesis and lower-valence cation doping.


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 Ti 4+ , Zr 4+ , Nb 5+ , or Ta 5+ , 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, niobiumbased oxynitrides have a wide visible light absorption band [3,4].Therefore, the target of this research is LaNbON 2 -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].Perovskitetype material LaTaON 2 , with a similar crystal structure to LaNbON 2 and a band gap energy of 1.9 eV, demonstrates photocatalytic activity for hydrogen evolution under visible light irradiation [6,7].Therefore, LaNbON 2 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 LaNbON 2 decreases the driving force for redox reactions.Additionally, LaNbON 2 contains nitrogen vacancies due to calcination at long durations and high heating temperatures relative to the decomposition temperature of oxynitride [4,8].Nb 5+ 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 2 Advances in Materials Science and Engineering enhanced photocatalytic activity of LaNbON 2 : the inhibition of LaNbON 2 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 LaNbON 2 .The ammonothermal method is a process for synthesizing (oxy)nitride materials in supercritical ammonia.This method has been successfully used to synthesize GaN [10], CaAlSiN 3 [11,12], and LaTaON 2 [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 SrTiO 3 , 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 LaNbON 2 .
Using these techniques, we studied their effects on the crystallinity, morphology, optical properties, and photocatalytic activity of the products.

Synthesis of LaNbON
2 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 LaNbON 2 , 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 NaNH 2 (95%, Aldrich, U.S.A.) and NaOH (97%, Junsei Chemical, Japan) were mixed in molar ratios of Nb : NaNH 2 : 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.

Synthesis of LaNbON 2 by Conventional Method.
A reference sample of LaNbON 2 was synthesized using the conventional method of nitridation of a LaNbO 4 oxide precursor, which was prepared by solid-state synthesis [15].La 2 O 3 (>99.5%,Wako Pure Chemical, Japan) and Nb 2 O 5 (>99.5%,Kojundo Chemical Lab., Japan) were mixed in a stoichiometric ratio by wet milling.For the synthesis of lower valence cation-doped LaNbON 2 , 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 SrCO 3 (99.9%,Kojundo Chemical Lab., Japan) and TiO 2 (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.LaNbON 2 powder was obtained by nitridation of the oxide precursor at 1223 K for 15 h under flowing ammonia (99.999%,Showa Denko, Japan).

Characterizations.
The crystal structure of the product was determined using powder X-ray diffraction (XRD, Rint-2200, Rigaku, Japan) with Cu K radiation.Particle sizes and morphologies were determined using field emission scanning electron microscopy (FE-SEM, S-5200, Hitachi, Japan).Optical properties were determined using ultravioletvisible 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 H 2 PtCl 6 ⋅6H 2 O. Pt was photodeposited on the LaNbON 2 in situ using H 2 PtCl 6 ⋅6H 2 O as the precursor and the suspension was irradiated by a 70 W ceramic metal halide lamp ( > 380 nm).The evolved gas was analyzed by gas chromatography (GC-8A, TCD, Ar carrier gas, Shimadzu, Japan).

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 LaNbON 2 .Ammonothermally synthesized LaNbON 2 has higher crystallinity than that synthesized by the conventional method because LaNbON 2 particles grow by dissolutionrecrystallization 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 LaNbON 2 was synthesized in supercritical ammonia at low temperatures.In ammonothermal synthesis of LaTaON 2 , 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 NaNH 2 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 Advances in Materials Science and Engineering that the addition of NaOH induces the formation of phasepure LaNbON 2 via the ammonothermal method.LaNbON 2 doped with Sr 2+ (5%) and Ti 4+ (5%) was synthesized using the ammonothermal method to produce lower valence cation-doped LaNbON 2 .In the XRD patterns for these materials (Figure 2), all diffraction peaks are attributed to perovskite-type LaNbON 2 , 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 LaNbON 2 synthesized by conventional method, all diffraction peaks are attributed to perovskite-type LaNbON 2 , with no additional diffraction peaks from impurities.The crystallinity and the crystallite size were not significantly changed by doping.
SEM images of LaNbON 2 synthesized using the ammonothermal and the conventional methods are shown in Figure 3.The ammonothermally synthesized LaNbON 2 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 LaNbON 2 was 1-2 m, while the powder synthesized by nitridation of the oxide precursor consists of agglomerated particles with a particle size of 0.2-1 m.This result suggests that the surface area does not significantly impact the photocatalytic activity.
UV-Vis DRS of LaNbON 2 synthesized by the ammonothermal method and the conventional method are shown in Figure 4.The absorption edge of ammonothermally synthesized LaNbON 2 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 LaNbON 2 .
The absorption edge of LaNbON 2 synthesized by the conventional method is slightly shorter wavelength compared to ammonothermally synthesized LaNbON 2 .In case of oxynitride, it has been reported that absorption edge of oxynitride is red-shift with increasing nitrogen content in the oxynitride [16].It is considered that ammonothermal synthesis led to higher nitrogen content in LaNbON 2 compared to conventional method.it has been reported that crystallinity morphology affect the optical properties of oxide [17,18].Because of in crystallinity and particle morphology of products synthesized by ammonothermal conventional method, difficult to consider chemical composition of product optical properties.
Figure 5 shows the of hydrogen evolution from an aqueous solution over undoped, Sr-doped, and Ti-doped 2 synthesized using the ammonothermal method with a Pt cocatalyst under light ( > 380 nm).In the case of undoped 2 , only nitrogen evolution was which is presumably due to the oxidation N 3− near the N 2 surface.This selfdecomposition has been observed for other photocatalysts during water splitting [4,[19][20][21].However, no hydrogen evolution was observed in our study.In the case of Sr-doped and Ti-doped LaNbON 2 , 0.03 of hydrogen evolution was observed after 3 h.
In case of doped LaNbON 2 synthesized using conventional method, only evolution of nitrogen was observed, which is presumably due to the self-decomposition of the photocatalyst.However, no hydrogen evolution was difference is probably due to the higher crystallinity of ammonothermally synthesized LaNbON 2 .Thus, we successfully produced activity in LaNbON 2 for 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 LaNbON 2 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 LaNbON 2 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 LaNbON 2 .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.

Conclusion
We report the successful synthesis of highly crystalline perovskite-type LaNbON 2 in supercritical ammonia using hydroxide as source.The 2 powder synthesized by reaction consists of dispersed cubic particles that reflect the crystallinity.The crystallinity, morphology, and absorption edge of 2 , the possibly affecting photocatalysis, were not sigchanged by doping with lower-valence cations.Nevertheless, the doped LaNbON synthesized using the ammonothermal method evolved hydrogen, suggesting this type doping decreases the formation reduced niobium species and consequently enhances the photocatalytic activity of LaNbON 2 .In doped LaNbON 2 synthesized using conventional method, no hydrogen evolution was observed.Therefore, we successfully produced LaNbON 2 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 LaNbON 2 .Verifying the reproducibility of the synthesis requires further investigation.

Figure 1 :
Figure 1: XRD patterns of products synthesized by the ammonothermal method and by the conventional method.

Figure 3 :Figure 4 :
Figure 3: SEM images of LaNbON 2 synthesized by the ammonothermal method (a) and by nitridation of an oxide precursor (b).