A novel oxynitride photocatalyst system for water splitting : La₂Zn_1-xTi_1+x(O,N)₆

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A novel oxynitride photocatalyst system for water

splitting : La2Zn1-xTi1+x(O,N)6

Vincent Michel, François Cheviré, Franck Tessier, Tilak Das, Xavier

Rocquefelte, Stéphane Jobic, Kazuhiko Maeda, Kazunari Domen

To cite this version:

Vincent Michel, François Cheviré, Franck Tessier, Tilak Das, Xavier Rocquefelte, et al.. A novel

oxyni-tride photocatalyst system for water splitting : La2Zn1-xTi1+x(O,N)6. ISNT 2012 - 7th International

Symposium on Nitrides, Jun 2012, Saint Malo, France. �hal-01261270�

10,00 20,00 30,00 40,00 50,00 60,00 70,00 2θ (°) In te n s it y ( U .A .) 0 20 40 60 80 100 120 250 350 450 550 650 750 Wavelength (nm) % R -10 -8 -6 -4 -2 0 2 4 6 8 10 0 20 40 Total-DOS G G A : B u lk -D O S o f L a2 Z n T iO 6 O-Total Synthesis Citrate route 1000°C crystallized powders

La2ZnTiO6 NH3- 900°C - 10h La2ZnTiO6-3xN2x with 0 < x < 0.129 “La2ZnTiO6” - type structure - double perovskite

Citrate route 600°C “poorly crystallized” powders

“La2ZnTiO6” NH3- 900°C - 10h La2Zn1-xTi1+x(O,N)6+ La2O3+ “ZnO”

“LaTiO2N” - type structure - simple perovskite

Note: La2O3easily removed by washing with dilute HCl (0.1M)

XRD patterns

Air 1000°C 2h / NH3900°C 10h

Air 600°c 2h / NH3900°C 10h

Air 1000°C 2h

Air 600°C 2h

A novel oxynitride photocatalyst system for water splitting :

La

₂

Zn

_1-x

Ti

_1+x

(O,N)

₆

**_{Department of Chemical System Engineering - The University of Tokyo - JAPAN}

Conclusion

The development of alternative routes for visible-light-photocatalysis represents a promising challenge where nitride-type compounds can play a major role. Within few years, several advances have been made in the number of phases tested for overall water splitting. With the example of La-Zn-Ti-O-N type phases, we have demonstrated that oxynitrides containing lanthanides, d0_{-type, and d}10_{-type metal ions} exhibit interesting photocatalytic behavior. The effect of the Zn/Ti ratio on the performance of the oxynitride will be investigated.

Contact informations: vincent.michel@univ-rennes1.fr

The photocatalytic splitting of water under solar radiations is a challenging and promising domain of research to provide clean and

renewable H₂ as an alternative energy resource. Current oxide photocatalysts (TiO₂, ZnO, SrTiO₃, NaTaO₃…) are wide-gap

semiconductors for which UV-light is necessary to produce electron-hole pairs by photoexcitation. However, the UV light represents only a small fraction (5%) of the solar spectrum. So far the (oxy)nitride approach become attractive for the efficient utilization of solar energy. A consequence of the anionic N3-_/O2-_{substitution results in an increase of the covalent character which is illustrated by a shift} of the absorption edge towards higher wavelength values (band gap < 3 eV). However other requirements have to be considered to synthesize an efficient visible-light-driven photocatalyst such as the mobility of the photogenerated charge carriers, the crystal structure or the chemical stability. Here, we have investigated the monoclinic perovskite-type oxynitride system La₂Zn_1-xTi_1+x(O,N)₆.

The performance of this new photocatalyst is compared to the reference material LaTiO2N.

Samples Temperature %O %N Sg(m²/g)

Optical band gap (eV) Color La2Zn1-xTi1+x(O,N)6 Calcination 600°C 2h NH3900°C 10h 17.6 5.0 32 ±1 2.35 orange La2ZnTiO6-3xN2x Calcination 1000°C 2h NH3900°C 10h 18.7 0.5 7 ±1 3.18 yellow

Photocatalytic water splitting UV-Vis diffuse reflectance spectra

Activity Time

La2Zn1-xTi1+x(O,N)6 La2ZnTiO6-3xN2x LaTiO2N

a_{H2 (µmol.h}-1₎b_{O2 (µmol.h}-1₎a_{H2 (µmol.h}-1₎b_{O2 (µmol.h}-1₎a_{H2 (µmol.h}-1₎b_{O2 (µmol.h}-1₎

1 h 1.76 4.03 0 3.88 0.52 5.88 2 h 3.56 6.24 0.10 6.81 1.16 9.03 4 h 6.63 8.40 0.53 9.65 2.27 14.69

a conditions:catalyst (0.2g); 200ml 20% MeOH; 300 W Xe lamp (>420nm); 0.2 g La2O3

b conditions:catalyst (0.1g); 200mL 0.02 M AgNO3; 300 W Xe lamp (>420nm); 0.2 g La2O3 ; 2% Co3O4

Photocatalytic activity

It is expected that the expanded nature of the Zn 4s band improves the mobility of the photogenerated carriers thus enhancing the photocatalytic activity of the material.

Efficient charge separation and fast transport of the carriers must be achieved in order to avoid recombination between electrons and holes.

H2evolution reaction O2evolution reaction

V (vs. NHE) (pH 0) 0 + 1.0 + 2.0 + 3.0 H+_{/ H} 2 O2/ H2O e -O2 H2O H+ H2 h+ Conduction Band (C.B.) Valence Band (V.B.) Water oxidation Water reduction Visible light + 1.23 Charge migration Charge migration N 2p O 2p Zn 3d Zn 4s Ti 3d Diffuse reflectance (%R)

Citrate route / air 600°C Citrate route / air 1000°C

Density of States (DOS) from GGA calculations for La2ZnTiO6

DFT: Egap= 3.00 eV DR US-VIS: Egap≈4.00 eV

Energy (eV) a b c La+3 O-2 Ti+4 Zn+2 La2ZnTiO6 Structure used for calculation

Unit Cell parameters:

a = 7.895 Å b = 5.5964 Å c = 5.5809 Å

β= 90.03°

Iwakura et al. , J. Nov. Carb. Res. Sciences, 3 (2011) 1-5

Note: complete ordering of Zn/Ti on B-site used for calculation calculation in progress on the oxynitride composition La2Zn1-xTi1+x(O,N)6 air 600°C / NH3900°C La2ZnTiO6-3xN2x air 1000°C / NH3900°C 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 time / h H 2 / u m o l La - Total O - Total Ti - Total Zn - Total 0 2 4 6 8 10 12 14 16 18 20 0 1 2 3 4 5 time / h O 2 / u m o l

Absorption of solar energy, excitation, and creation of an electron / hole pair.

*_{Institut des Matériaux Jean Rouxel (UMR CNRS 6502) – Université de Nantes - FRANCE}

La2ZnTiO6

La2ZnTiO6

Space Group:

P1 21/n 1 (No: 14)

Monoclinic

Vincent Michel, François Cheviré, Franck Tessier,

Tilak Das*_{, Xavier Rocquefelte}*_{, Stéphane Jobic}*_{, Kazuhiko Maeda}**_{, Kazunari Domen}**