1. Highly efficient H2 generation by oxide nanostructures
M. Miki-Yoshida*, P. Amézaga-Madrid, P. Pizá-Ruiz, W. Antúnez-Flores, J. Salinas-Gutiérrez, M. Meléndez-Zaragoza and V. Collins-Martínez.
Centro de Investigación en Materiales Avanzados, S.C., Miguel de Cervantes No. 120, CP 31136, Chihuahua, Chihuahua, Mexico.
Corresponding author:
ABSTRACT
Oxide based nanostructures were grown by aerosol assisted chemical vapour deposition (AACVD) onto borosilicate glass substrates covered by a TiO2 thin film as a buffer. Details of the experimental setup and general synthesis conditions were reported elsewhere1,2. Nanostructures were a multilayered coating composed by: oxides of Ti, Ti-Fe, Ti-Ni, and TiOx-FeOx. All the nanostructures were covered by a continuous layer of Pt nanoparticles. Microstructure of the samples were analysed by electron microscopy and x-ray diffraction. Optical properties were also determined in the UV-visible-near IR interval. Finally photocatalytic generation of H2 was evaluated in a batch reactor under visible light irradiation, using a filtered low vapour pressure Hg lamp of 250 W. H2 evolution was tested every hour by gas chromatography.
Crystalline structure
Synthesis of nanostructured oxides
Fig. 1. Schematic representation of the aerosol assisted CVD system used for the synthesis of the nanostructured materials [1]. Substrates were borosilicate glass of 12 x 4 cm2.
Table 1. Layer configuration of the different samples
Ultrasonic nebulizer
Nozzle
Substrate and substrate holder
Heating plate
Nozzle displacement device
(3)
(5)
(2)
(4)
(1)
AACVD system
Fig. 3 X-ray diffraction pattern of representative sample. Only diffraction lines of TiO2 (anatase) can be noticed. Indicating that the other oxide layers are amorphous.
Optical Properties
Sample A
Pt/TiO2 B
Pt/TiO2-NiOx/TiO2 C
Pt/TiO2-FeOx D
Pt/TiO2/FeOx/TiO2
1st layer
TiO2
TiOx-FeOx
2nd layer
Nano Pt
TiOx-NiOx
FeOx
3rd layer
4rd layer
Table 2. Precursor, deposition temperature and deposition time of the different layers
Fig. 4. a) Absorptance spectra of the nanostructured oxide sample in the UV-VIS-NIR interval. b) Semilog plot of the absorption coefficient as a function of the photon energy. c) Tauc’s plot for the determination of the optical band gap energy. A permitted direct transition was considered. From the figure it can be deduced that the material absorbs most of the incoming light in the UV-VIS region and that there is a sharp edge related to the optical band gap in the limit of UV-VIS interval. Modulation of absorptance spectra indicate smooth interfaces. Obtained optical band gap energy were in agreement to values reported elsewhere [3]
Layer
TiO2
Ti-Ni oxide
Ti-Fe oxide
FeOx Pt
Precursor
Titanium (IV) Oxiacetylacetonat
Nickel (II) acetylacetonate
Iron (III) acetilacetonate
Platinum Acetylacetonate
Temperature [K]
673
773
573
Deposition time [min]  
100
 110 33
Photocatalytic generation of H2
(a)
(b)
Microstructural Characterization Pt
TiO2 buffer
BSG
TiO2
FeOx
(d)
(a)
(c)
(b)
Fig. 5. a) Home made batch reactor for the evaluation of the photocatalytic generation of H2 under visible light irradiation, using a filtered low vapor pressure Hg lamp of 250 W. H2 evolution was tested every hour by gas chromatography. b) H2 generation as a function of time.
Table 3. Correlation between H2 generation and optical properties. It is shown an increase of the H2 generation with the diminution of the optical band gap energy of the nanostructured oxide. There is also a direct correlation between absorption coefficient and H2 generation.
Sample A
Pt/TiO2 B
Pt/TiO2-NiOx/TiO2 C
Pt/TiO2-FeOx D
Pt/TiO2/FeOx/TiO2
H2 generation [µmol h-1 g-1] 14 29 97
302
Eg [eV]
3.75
3.70
3.62
3.57
(Eg) [cm-1]  
Fig. 2. SEM micrographs of representative oxide nanostructures. a) Secondary electron SEM micrographs of sample A showing its surface morphology. b) Backscattered electron SEM micrograph of samples B. c) Secondary electron SEM micrographs of sample D, it is shown the size and distribution of Pt nanoparticles on the surface. d) Backscattered electron images of the cross section of sample D, showing the different layers (Pt/TiO2/FeOx/TiO2/BSG).
CONCLUSION
AACVD is a versatile technique capable of synthesize nanostructured oxides for the efficient photocatalytic generation of H2. Most efficient coating was a multilayered Pt/TiO2/FeOx/TiO2 with up to 302 µmol h-1 g-1 of generation. A clear inverse correlation between H2 generation and optical band gap energy was obtained. Also as expected, generation increased with the increase of the coating´s absorption coefficient.
Composition (atomic % and atomic ratio)
Sample
Ti [at.%]
Ni or (Fe) [at.%]
Pt [at.%]
Pt / [Ti + Ni (Fe)] [%]
[Ni (Fe)] / Ti [%]
A – Pt/TiO2 23 - 2 9
B – Pt/TiO2-NiOx/TiO2 22 12 3 55
D – Pt/TiOx/FeOx/TiOx 20
(8)
(11)
(40)
Authors thanks to CONACyT and to the Laboratorio Nacional de Nanotecnología for the support provided during the realization of this work. Also to the Red Temática del Hidrógeno for partial economical support.
F. Paraguay-Delgado, W. Estrada-Lopez, E. Andrade, M. Miki-Yoshida, Thin Solid Films, 1999, 350, 192.
P. Amézaga-Madrid et al., Thin Solid Films, 2008, 516, 8282.
W. Kang and M. S. Hybertsen, Phys. Rev. B 82, (2010).
ACKNOWLEDGEMENTS
REFERENCES