富山大学研究推進機構水素同位体科学研究センター研究報告 39：1－8，2019.

論文
窒素プラズマ処理した LaTa2O7 ナノシート光触媒によるトリエタノールアミン水溶液からの水素生成反応
萩原英久、早川克明、野澤一徳

富山大学 研究推進機構 水素同位体科学研究センター〒930-8555 富山市五福 3190

Photocatalytic hydrogen production from triethanolamine aqueous solution by
LaTa2O7 nanosheets treated with nitrogen plasma
Hidehisa Hagiwara, Katsuaki Hayakawa, Ittoku Nozawa
Hydrogen Isotope Research Center, Organization for Promotion of Research,
University of Toyama, Gofuku 3190, Toyama, 930-8555, Japan(Received March 31, 2020; accepted August 7, 2020)
Abstract
N-doped LaTa2O7 nanosheets (LTON-BP) were prepared by polygonal barrel-plasma surface modification
method using N2 plasma. X-ray photoelectron spectroscopy measurements suggest that nitrogen atoms
were added at the interstitial crystal lattice sites of the N2 plasma-treated photocatalyst. The photocatalytic
activity of LTON-BP for H2 production from triethanolamine aqueous solution was found to be 3.3 times
higher than that of N-doped LaTa2O7 nanosheets prepared by a conventional method, which is attributed tothe high oxidation ability of LTON-BP stemming from its band structure.

1. Introduction
Photocatalytic water splitting is a promising process for hydrogen production
because hydrogen can be generated from sunlight and water without emission of carbon
dioxide. Since the report by Jansen and Letschert in 2000 on the photocatalytic activity of amixed anion compound [1], oxynitrides have been actively studied as visible light-responsive1

萩原 英久・早川 克明・野澤 一徳

photocatalysts [2-6].

It has been reported that the solid solution of LaTaON2 andLaMg1/3Ta2/3O3 is suitable for water splitting under visible light with a wavelength shorter
than 600 nm [7]. However, the resulting solar-to-hydrogen conversion efficiency is as low as
0.2% [8], and further research is required. Meanwhile, metal oxide nanosheets prepared fromthe exfoliation of layered metal oxide have recently emerged as promising photocatalystmaterials. Oxide nanosheets are two-dimensional anisotropic semiconductors having several
nanometers of thickness and several micrometers of lateral dimension, and are characterizedby high crystallinity and large surface area. These features provide advantages such as
suppression of recombination of photoexcited charges and reaction site separation, whichrenders the oxide nanosheets suitable for photocatalysis. This has prompted the interest in
nitrogen-doped oxide nanosheets, and several studies on their synthesis have been reported[9,10]. In the conventional preparation method of N-doped oxide nanosheets, a layered metaloxide is heated under NH3 gas flow at high temperature (>1073 K), and is then exfoliated byacid treatment and amine intercalation. This conventional process has some drawbacks,
particularly regarding the generation of impurities such as metal nitrides and coarsening ofparticles by high-temperature heat treatment, which hinders the exfoliation step. To avoid
these problems, we employ in this study a polygonal barrel-sputtering method using N2
plasma to synthesize N-doped oxide nanosheets by direct nitridation of LaTa2O7 nanosheets.
The photocatalytic activity of the as-prepared N-doped LaTa2O7 nanosheets is evaluated andcompared with that of N-doped LaTa2O7 prepared by a conventional NH3 heating process.

2. Experimental
2.1. Photocatalyst preparation and characterization
All reagents were used without further purification. The layered oxide RbLaTa2O7
was synthesized by heating a mixture of Rb2CO3 (97.0%, Wako Pure Chemical Industries,Ltd., Japan), La2O3 (98.0%, Wako Pure Chemical Industries, Ltd., Japan), and Ta2O5 (99.9%,Kojundo Chemical Lab. Co., Ltd., Japan) at 1373 K for 12 h in air. The molar ratio of themixture was Rb:La:Ta = 1.5:1:2. An excess amount (50 mol%) of RbCO3 was added tocompensate for volatilization loss. The as-synthesized RbLaTa2O7 (2.0 g) was added to an
Erlenmeyer flask (300 mL) along with 1 M HCl (150 mL), and the mixture was shaken for aweek to exchange rubidium to proton. The protonated sample was collected by centrifugationand dried in vacuum. The obtained HLaTa2O7 (1.0 g) was stirred in 1 M ethylamine (EA)2

窒素プラズマ処理した LaTa2O7 ナノシート光触媒によるトリエタノールアミン水溶液からの水素生成反応

aqueous solution (100 mL) for three days to intercalate the EA into the interlayer. TheEA-treated sample (0.5 g) was stirred for three days in 100 mL of a 0.025 M
tetrabutylammonium hydroxide (TBAOH) aqueous solution to exfoliate into LaTa2O7nanosheets (LTO). The N-doped LaTa2O7 nanosheets (LTON-BP) were synthesized from
LTO using a polygonal barrel-plasma surface modification method (RF output: 200 W, N2
pressure: 7 Pa, time: 1 h) as reported elsewhere [11]. For comparison, a different sample of
N-doped LaTa2O7 nanosheets (LTON-NH) was prepared by heating RbLaTa2O7 at 1073 K
under NH3 flow (200 mL/min) for 5 h, followed by exfoliation as described above.
X-ray diffraction (XRD) patterns of the samples were obtained with an X-ray
diffractometer (D8 DISCOVER, Bruker AXS) equipped with Cu Kα radiation (tube voltage:45 V, current: 40 mA). Surface morphology observation and composition analysis wereperformed with a scanning electron microscope equipped with an energy dispersive X-rayspectrometer (SEM-EDX: JSM-6701F, JEOL). X-ray photoelectron spectroscopy (XPS) wasconducted using an X-ray photoelectron analyzer (ESCALAB 250Xi, Thermo Fisher
Scientific). Ultraviolet–visible (UV–vis) diffuse reflectance spectra were measured using aUV–vis spectrophotometer (V-560, JASCO Corporation) equipped with an integrating sphere
unit. The optical band gap was determined using Tauc’s equation (1) [12](αhν)1/n = A(hν − Eg),

(1)where A is a constant, hν is the photon energy, Eg is the energy gap, n = 1/2 for allowed directtransition, and n = 2 for allowed indirect transition. Since the RbLaTa2O7 compounds used inthis study are indirect transition semiconductors [10], n = 2 was used to prepare the Tauc plot.
2.2. Photocatalytic reaction
To evaluate the photocatalytic activity, a closed-circulation reactor with a quartz
reaction cell was used. The photocatalyst powder (10 mg) and Pt(NH3)4(NO3)2, which wasused as Pt cocatalyst precursor for H2 production, were added to 0.75 M of a triethanolamine(TEOA) aqueous solution (25 mL) in the quartz reaction cell. Then, the reaction cell wasirradiated with an Xe lamp (2.0 W/cm) under magnetic stirring. The evolved gases weredetected using a gas chromatograph (GC-8A, Shimadzu Corp., Japan) with a thermal
conductivity detector.

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萩原 英久・早川 克明・野澤 一徳

3. Results and Discussion
Figure 1 shows the XRD patternof the prepared LaTa2O7 nanosheets (LTO),
which is in accordance with that previously
reported [10]. The sheet-like structure ofLTO, which is composed of La and TaO6
octahedral units, is shown in Figure 2.
Figure 1 also displays the diffraction
patternofLTON-NH

obtained

byexfoliation of heat-treated RbLaTa2O7 in
NH3 atmosphere, and that of LTON-BP,
whichwastreatmentpreparedbyusing

abarrel-sputtering

method.

N2

plasmaFigure 1 XRD patterns of LTO, LTON-NH, andLTON-BP.

polygonal
Since

thediffraction patterns of LTON-NH andLTON-BP were similar to that of LTO, it
can be assumed that all samples have
similar crystal structures.
Figure 3 shows SEM images of

Figure 2 Crystal structure of LTO.

the prepared samples. Each sample normally exhibits a lateral dimension of the order ofmicrons and a thickness of up to several dozen nanometers. In the case of LaTa2O7
nanosheets, the thickness of a single layer nanosheet was reported to be approximately of
1.11 nm [13]. Therefore, it can be concluded that the samples used in this study have a plate
shape with dozens of stacked layers. The compositions of the samples were(a)(b)(c)Figure 3 SEM images of (a) LTO, (b) LTON-NH, and (c) LTON-BP.

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窒素プラズマ処理した LaTa2O7 ナノシート光触媒によるトリエタノールアミン水溶液からの水素生成反応

determined by EDX analysis to be La0.93Ta2.47O6.59, La0.92Ta2.62O6.56N0.06, and La1.12Ta2.75O6.15
for LTO, LTON-NH, and LTON-BP, respectively.
Since nitrogen was not detected inLTON-BP by

EDX

analysis,anXPSmeasurement was performed. Figure 4(a)shows the XPS spectra of Ta 4p and N 1s forLTO, LTON-NH, and LTON-BP. The binding
energy was normalized by adjusting the C 1s
peak to 284.6 eV [14]. In all samples, the Ta
4p peak was observed at 404 eV. Figure 4(b)shows the differential spectra between each
LTON nanosheet and LTO. Broad peaks wereobserved at 397 eV for LTON-NH and at 402
eV for LTON-BP. It was previously reported
that nitrogen atoms were doped into theoxygen sites of the metal oxynitride obtainedby heating a metal oxide with NH3, and N 1s
peaks were observed at 395.8 to 397.5 eV[15]. In the present study, the N 1s peak ofLTON-BP was observed at higher binding
energy than that of LTON-NH. Meanwhile,
previous studies on N-doped TiO2 showedthat the N 1s peak of nitrogen added at theinterstitial crystal lattice was observed at 400eV [16]. These results suggest that the small

Figure 4 (a) XPS spectra of LTO, LTON-NH,and LTON-BP, (b) Differential XPS spectra
between LTON samples and LTO.

amount of nitrogen atoms in LTON-BP wasdoped not in the oxygen sites but in the interstitial crystal lattice sites. In addition, the factthat the nitrogen atoms added to LTON-BP were detected only in the XPS measurement
indicates that LTON-BP contained a high nitrogen concentration near the surface.
Figure 5(a) shows the UV–vis diffuse reflection spectra of the photocatalysts. The
absorption edge of LTO, LTON-NH, and LTON-BP were observed at around 300, 600, and350 nm, respectively. According to previous reports, the valence band maximum ofLaTa2O7-xNx is more negative than that of LaTa2O7 because the potential energy of N 2p is5

萩原 英久・早川 克明・野澤 一徳

higher than that of O 2p [10]. Therefore, theshift of valence band maximum between
LTON-NH and LTON-BP suggests that theirphotoabsorption

properties

changedbydoping nitrogen. Figure 5(b) shows the Taucplot of each sample. The band gaps of LTO,
LTON-NH, and LTON-BP were estimated tobe 3.99, 1.95, and 2.96 eV, respectively,
being the narrowing of the band gap of both
LTON samples attributable to the effect ofthe doped nitrogen.
Figure 6 depicts the time course ofH2 production on LTON-BP and LTON-NH
from triethanolamine aqueous solution. Bothcatalysts showed an induction period at thebeginning of the reaction, followed by alinear H2 production phase. The induction
period most likely involves the reduction ofPt(NH3)4(NO3)2 and loading of the resulting

Figure 5 (a) UV–vis diffuse reflectance spectraand (b) Tauc plots of LTO, LTON-NH, andLTON-BP.

Pt particles on the photocatalyst. Oxygen
formation was not observed on both catalysts,
since triethanolamine works as a sacrificialreagent.

Under

thiscondition,thephotocatalytic reaction can be assumed toproceed as follows [17]:Photocatalyst + hυ → e−(CB) + h+(VB)(2)

2H+ + 2e−(CB) → H2(3)N(CH2CH2OH)3 + H2O + 2h+(VB) →HN(CH2CH2OH)2 + HOCH2CHO + 2H+ (4)Figure 6 Time course of H2 formation from0.75 M triethanolamine aqueous solution underXe lamp irradiation by Pt/LTON-NH andPt/LTON-BP.

The formation of H2 occurred with the concomitant oxidation of triethanolamine. The H2
production rates were 6.2 mol/h and 1.9 mol/h for LTON-BP and LTON-NH, respectively,

6窒素プラズマ処理した LaTa2O7 ナノシート光触媒によるトリエタノールアミン水溶液からの水素生成反応

indicating that the activity of LTON-BP was about 3.3 times higher than that of LTON-NH.
The difference in the oxidizing power of the photogenerated holes can be invoked to explain
the higher activity of LTON-BP. As described above, the valence band maximum ofLTON-BP was more positive than that of LTON-NH, being the higher oxidation ability of thephotogenerated holes in LTON-BP more suitable for the oxidation reaction of triethanolamine.
Other possible reasons for the higher activity of LTON-BP are the difference in charge
separation lifetime due to the different surface states of both catalysts and the difference inthe loading state of the Pt cocatalyst. These possibilities will be explored in future studies. At
this stage, the higher photocatalytic activity of LTON-BP is attributed to the high oxidation
ability that stems from its band structure. Although this study only addresses the absorptionof UV light by LTON-BP, the light absorption of LTON-BP catalysts can be adjusted bychanging the N2 plasma treatment conditions. Therefore, highly active visible
light-responsive photocatalysts can be expected to be developed by optimizing the plasma
treatment conditions.

4. Summary
N-doped LaTa2O7 nanosheets (LTON-BP) were prepared by N2 plasma treatment,and their photocatalytic activity was compared with that of N-doped LaTa2O7 nanosheets(LTON-NH) prepared by conventional NH3 heat treatment and exfoliation. XPS
measurements suggest that nitrogen atoms were added at the interstitial crystal lattice sites ofLTON-BP. The difference in the band structure of LTON-BP and LTON-NH were also
supported by UV–vis diffuse reflectance spectroscopy measurement. Comparison of thephotocatalytic activities of LTON-BP and LTON-NH in the H2 production reaction fromaqueous triethanolamine showed that LTON-BP was 3.3 times more active than LTON-NH.
This is most likely due to the high oxidation ability of LTON-BP arising from its bandstructure. The development of highly active visible light-responsive photocatalysts byoptimizing N2 plasma treatment conditions will be the subject of further studies.

Acknowledgment
This research was partially supported by JSPS KAKENHI Grant-in-Aid forSpecially Promoted Research (JP16H06293), and a Grant-in-Aid for Scientific Research (C)(JP18K05292).

7萩原 英久・早川 克明・野澤 一徳

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