Photon Factory Activity Report 2014 #32 (2015) B

9C, 12C, NW10A/2014G631

Characterization of Plasmonic Nanoparticles and Dyes Assembled with Layered
Double Hydroxides to Promote the Photoreduction of Carbon Dioxide into Fuels
Shogo Kawamura and Yasuo Izumi*
Graduate School of Science, Chiba University, Yayoi, 1-33, Inage-ku, Chiba 263-8522
1 Introduction
The photoreduction of CO2 to fuels using natural light
can contribute simultaneously to reduction of the major
greenhouse gas and the development of sustainableenergy [1]. Efficient photoreduction of CO2 into methanol(or CO) was reported using layered double oxides(LDHs) [2–4]. Furthermore, the combination of WO3 andLDH photocatalysts as a photoanode to oxidize water anda photocathode to reduce CO2, respectively, wasdemonstrated [5].
The wide band gap of LDHs is advantageous to reducehigh-potential compound, e.g. CO2. Conversely, wideband gap is also drawback because UV light is required toexcite LDHs. In this study, plasmonic nanoparticles of Au
and Ag and Cu phthalocyanine were assembled withLDHs to enable CO2 photoreduction utilizing visible light.
2 Experimental Section
Ag/ZnGa LDH was prepared by ion exchange at 373 K
for 15–180 min [6]. The samples were denoted as
Ag/ZnGa LDH-15 or Ag/ZnGa LDH-180. Au/ZnGa LDH
was prepared by ion exchange and liquid phase reduction
using NaBH4 [6].
CuPcTs/ZnGa LDH was prepared by ion exchange ofsodium salt of Cu phthalocyanine tetrasulfonate hydrate(Na+4CuPcTs4−) with ZnGa LDH [7]. Ag K, Au L3, andCu K-edge XAFS spectra were measured in transmission
mode at 290 K.
3 Results and Discussion
Ag K-edge XANES spectra for Ag/ZnGa LDH-15 andAg/ZnGa LDH-180 were depicted in Figure 1A-b and d,respectively. The XANES patterns for these samples werequite similar to that for Ag metal (a) rather than Ag2O (e).
Thus, Ag0 nanoparticles were combined with LDH. The
metallic Ag0 state of Ag/ZnGa LDH-15 negligiblychanged after the photocatalytic test in CO2 and H2(Figure 1A-c) in comparison to spectrum b for the assynthesized sample. Similarly, metallic Au0 state of fresh
Au/ZnGa LDH unchanged after the photocatalytic test in
CO2 and H2 (Figure 1B-b, c).
The k3-weighted EXAFS function was depicted in
Figure 2a for Ag/ZnGa LDH-15 (row 1) and Ag/ZnGa
LDH-180 (row 2). The peaks at 0.27 nm (phase shiftuncorrected) in the associated Fourier transform (b) werenicely curve-fit by the parameters of Ag–Ag interatomicpair (Figure 2c, d). The N value for Ag/ZnGa LDH-15
was 7.6 (particle size ~1.6 nm), significantly smaller thanN of 10.1 (particle size ~3.7 nm) for Ag/ZnGa LDH-180,demonstrating the gradual growth of Ag nanoparticles
during the synthesis at 373 K.
The N(Au–Au) value of 8.3 (particle size ~1.9 nm) for
fresh Au/ZnGa LDH (Figure 2, row 3) was consistent
with mean particle size of 3.2 nm obtained based on thehigh-resolution transmission electron microscopy.

Figure 1. (A) Normalized Ag K-edge XANES spectra for Ag
metal (a), Ag/ZnGa LDH-15 (b), sample b after photocatalytic
test in CO2 and H2 for 5 h (c; dotted line), Ag/ZnGa LDH-180(d), and Ag2O (e). (B) Normalized Au L3-edge XANES spectra
for Au metal (a), Au/ZnGa LDH (b), sample b after
photocatalytic test in CO2 and H2 for 5 h (c; dotted line), andAu2O3 (d).

Figure 2. Ag K-edge EXAFS for Ag/ZnGa LDH-15 (1) andAg/ZnGa LDH-180 (2) and Au L3-edge EXAFS for Au/ZnGa
LDH (3). k3-weighted EXAFS oscillation (a), its associatedFourier transform (b), and best-fit results in k-space (c) and Rspace (d).

The XANES spectra for Na+4CuPcTs4− andCuPcTs/ZnGa LDH are shown in Figure 3. The spectrum
pattern changed negligibly by the dispersion of CuPcTs4−

Photon Factory Activity Report 2014 #32 (2015) B

over the ZnGa LDH surface as compared withNa+4CuPcTs4− crystallines, suggesting that the framework
structure of CuPcTs4− was retained upon the dispersion
over the LDH surface. In contrast, a sharp shoulder peakat 8988 eV became relatively weaker upon the dispersion
over the LDH surface, suggesting the perturbation ofLUMO of CuPcTs by the interaction with the LDH.
The EXAFS χ oscillation changed negligibly whenNa+4CuPcTs4− was dispersed over the LDH surface(Figure 4A). This fact supported the retention of theCuPcTs4− framework over the LDH surface suggested byXANES. The curve-fit results for the Cu–N interatomic
pair provided a distance of 0.198 nm (fit error ±0.011 nm)with a N value of 3.3 (fit error ±1.3) (Figure 4D, E).
The correlation between characterization by XAFS andthe photocatalytic performance of Ag/ZnGa LDH,Au/ZnGa LDH, and CuPcTs/ZnGa LDH was discussed
elsewhere [6, 7].

22550117) from Japan Society for the Promotion ofScience.
References[1] Y. Izumi, Coordination Chemistry Reviews 257, 171–186 (2013).
[2] M. Morikawa, N. Ahmed, Y. Yoshida, and Y. Izumi,
Applied Catalysis B 144, 561–569 (2014).
[3] N. Ahmed, M. Morikawa, and Y. Izumi, Catalysis
Today 185, 263–269 (2012).
[4] N. Ahmed, Y. Shibata, T. Taniguchi, and Y. Izumi,
Journal of Catalysis 279, 123–135 (2011).
[5] M. Morikawa, Y. Ogura, N. Ahmed, S. Kawamura, G.
Mikami, S. Okamoto, and Y. Izumi, Catalysis Science& Technology 4, 1644–1651 (2014).
[6] S. Kawamura, M. C. Pascasu, Y. Yoshida, Y. Izumi,and G. Carja, Applied Catalysis A, DOI:
10.1016/j.apcata.2014.12.042.
[7] S. Kawamura, N. Ahmed, G. Carja, and Y. Izumi, Oil&Gas
ScienceandTechnology,
DOI:
10.2516/ogst/2015020.
* yizumi@faculty.chiba-u.jp

Figure 3.
Normalized Cu K-edge XANES spectra for
Na+4CuPcTs4− (a) and CuPcTs/ZnGa LDH (b).

Figure 4. (A) Cu K-edge EXAFS χ oscillation and (B) k3weighted EXAFS χ oscillation for Na+4CuPcTs4− (a) andCuPcTs/ZnGa LDH (b), (C) its associated Fourier transform,and (D, E) best-fit results in k-space (D) and R-space (E) for
CuPcTs/ZnGa LDH. Solid line: magnitude and dotted line:
imaginary part (C, E). Red (thick) line: experimental and blue(thin) line: fit (D, E).

Acknowledgements
The authors are grateful for the financial supports fromthe Grant-in-Aid for Scientific Research C (26410204,