Photon Factory Activity Report 2018 #36 (2019)

BL-9C, BL-12C, AR-NW10A /2016G577

Monitoring of Heat via Localized Surface Plasmon Resonance to Convert CO2 over
Silver–Zirconium Oxide
1Hongwei Zhang,1 Takehisa Konishi, 1 and Yasuo Izumi1,*
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku,Chiba 263-8522, Japan

1 Introduction
Photocatalytic conversion of CO2 into fuels has been
widely investigated to enable carbon-neutral cycle
including solar fuel in contrast to irreversible
consumption of fossil fuel. Noble metal nanoparticles
supported on semiconductor were reported to be efficientfor photocatalytic conversion of CO2 [1], however, themechanism of enhancement photoreaction over the metal
nanoparticles is still unclear. Based on the mechanistic
investigation, dual roles of UV–visible light wereclarified for the first time using Ag–ZrO2: (i) charge
separation at the band gap and (ii) the heat transformed
via localized surface plasmon resonance (LSPR) by using
extended X-ray absorption fine structure (EXAFS). TheAg site temperature was monitored via Debye–Waller
factor obtained by EXAFS [2].

much as by 31% during the light irradiation of 100 min(Figure 1C). On the contrary, the peak intensity quickly
recovered when the light was turned off.

2 Experimental Section
Silver K-edge EXAFS spectra were measured at 290 K
in the transmission mode in the Photon Factory on
beamline NW10A, 9C, and 12C. At NW10A, Si(3 1 1)
double-crystal monochromator and a Pt-coated focusing
cylindrical mirror were inserted into the X-ray beam path.
Piezo transducer was used to detune the X-ray intensity totwo thirds of the maximum to suppress higher harmonics.
The Ag K-edge absorption energy was calibrated to25516.5 eV using the X-ray spectrum of a Ag metal foil.
A disk (Φ = 10 mm) of Ag (3.0 wt %)–ZrO2
photocatalyst (125 mg) was set in a Pyrex glass reactor
equipped with a Kapton film for X-ray transmission and apolyethylene terephthalate film for both UV–visible lightand X-ray transmission filled with 2.3 kPa of CO2 and21.7 kPa of H2 and were measured in thepresence/absence of UV–visible irradiation provided by a500-W xenon arc lamp (Ushio) at the beamline. Ascontrol experiments, only visible or infrared light, orunder Ar atmosphere were also performed. XAFS data
were analyed using the XDAP software package.

3 Results and Discussion
Ag K-edge EXAFS spectra were monitored for Ag (3.0wt %)–ZrO2 under CO2 (2.3 kPa) and H2 (21.7 kPa)
irradiated by UV–visible light at beamline. In the Fouriertransform (FT; Figure 1C), a peak due to Ag–Ag
interatomic scattering of photoelectrons was predominant
at 0.28 nm (phase shift uncorrected), demonstrating that
the Ag nanoparticles were exclusively metallic. The FT
peak intensity of EXAFS was significantly suppressed asFigure 1. The time-course changes of (A) N and (B) σ valuesand (C) FT obtained from k3-weighted Ag K-edge EXAFS χfunction for Ag (3.0 wt %)–ZrO2 under CO2 (2.3 kPa) andH2 (21.7 kPa) irradiated by UV–visible light for 100 min

Photon Factory Activity Report 2018 #36 (2019)

followed by under dark for 20 min. (D) The change of FT ofAg K-edge EXAFS for fresh Ag–ZrO2: 3.0, 5.0, and 10 wt %of Ag. (E) The correlation between σ value and temperaturefor bulk sites (circle, ○) and surface sites (vertical motion;square, ◻︎) in/on Ag metal generated by the correlated Debye
model using a code FEFF8.

Such light-induced change in the FT was quantitativelyevaluated by the curve-fit analysis based on plane-wave
approximation for amplitude Ai(k), coordination number
Ni, interatomic distance Ri, Debye–Waller factor σi,backscattering amplitude fi, and mean free path ofphotoelectrons λ for shell i using a code XDAP:Ai (k) =⎡ ⎛NiR ⎞⎤fi (k) exp ⎢−2 ⎜σ i2 k 2 + i ⎟⎥, i = Ag2kRiλ ⎠⎦⎣ ⎝(1).

The obtained N value was 9.0 ± 0.7 before lightirradiation and did not change significantly during lightirradiation (8.3–10.0) and after the light was turned off(10.1±0.7; Figure 1A). Obtained N values of 9–10
correspond to particle size of 2.5–3.7 nm assuming
spherical face-centered cubic (fcc) nanoparticle modeland the surface dispersion is 0.54–0.36 (mean value 0.45).
The σ value was calculated to 0.009 95 nm for Ag metal
at 290 K by the correlated Debye model using ab initio
multiple-scattering calculation code FEFF8 and theDebye temperature for Ag [θD(Bulk) 225 K] [3]. The code
XDAP provides the experimental difference of σ2 value
from that for Ag metal (model) based on equation 1.
Thus-obtained initial σ value 0.010 1 nm for Ag (3.0wt %)–ZrO2 before light irradiation quickly increased to0.010 7 nm at 10 min of irradiation and progressively
increased to 0.011 7 nm at 90 min of light irradiation(Figure 1B). Then, the value quickly decreased to 0.010 1nm after the light was turned off at 112 min.
Furthermore, the Ag site temperature was evaluatedbased on the σ values. The temperature dependence of σvalue is supplied using FEFF8 by the correlated Debye
model both for bulk and surface Ag sites using bulk andsurface Debye temperature (Figure 1E-a and b,respectively). Preferable exposure of thermodynamicallystable fcc(1 1 1) face was assumed for latter value [θD(Surf,⊥) 155 K]. We also approximated the mean temperature
for Ag nanoparticles as the arithmetic mean temperatureof that based on θD(Surf, ⊥ ) weighted by 1/2·1/3D [D:dispersion of nanoparticles (0.45), for effective verticaldegree of freedom at free hemisphere surface] and thatbased on θD(Bulk) weighted by (1 – D) + 1/2D + 1/2·2/3D(bulk site, non-free hemisphere in contact with ZrO2, andtwo lateral degrees of freedom at free hemispheresurface).
As the result, the initial 286 K before irradiation rose to325 K at 10 min of irradiation and progressively
increased to 392 K until 90 min of light irradiation(Figure 1B). The temperature quickly dropped to 290 K
after the light was turned off. Such temperature rise/dropshould be due to the heat transformed from LSPR. Quite
similarly, the initial temperature increased to 363 K until

90 min of UV–visible light irradiation and quickly
dropped to 397 K once the light was off using Ag (5.0wt %)–ZrO2 sample. The method based on σ value foundin this study by EXAFS is very common and directlyprobe for all nanoparticles to exhibit LSPR, e.g. Au, Ag.
Furthermore, temperature rise/drop was also observed
under argon and UV–visible light (367 K at 50 min oflight, 302 K at 10 min of light off) [2]. The σvalue/temperature negligibly changed up to 329 K when
the wavelength of incident light was filtered to be morethan 580 nm whereas essentially identical rise/drop to fulllight (365 K at 90 min, 298 K when light was off) of σvalue/temperature was observed when λ was more than
320 nm. The reaction heat did not affect in this study
because the reaction to reduce CO2 into CO isendothermic.
In summary, the contribution of mean 2.5–3.7 nm ofAg nanoparticles was investigated using in-situ EXAFS.
Temperature rise from 286 K to 392 K under theirradiation of UV–visible light and quick drop to 290 K
under dark were directly monitored based on Debye–
Waller factor change for Ag–Ag interatomic pairinterference. The H2 should be oxidized associated withthe reduction of bicarbonate species over ZrO2, but any
evidences of available O on Ag were found in this study.
Thus, H2 activation is simply over heated Ag surface andthe H atoms spill over to bicarbonate species over ZrO2
rather than hot electron injection or plasmonic resonant
energy transfer to/with ZrO2.

Acknowledgements
The authors are grateful for the financial supports fromthe Grant-in-Aid for Scientific Research C (17K05961)
from Japan Society for the Promotion of Science, Grants
for Academic Research from the Japan Gas Association(2018), and Leading Research Promotion Program (2015–2019) from the Institute for Global Prominent Research,Chiba University.

References[1] Y. Izumi, Coord. Chem. Rev. 257,171–186 (2013).
[2] H. Zhang, T. Itoi, T. Konishi, and Y. Izumi, J. Am.
Chem. Soc., 141, 6292–6301 (2019).
[3] American Institute of Physics Handbook, Third
Edition, Gray, D. E. Ed., p.4-116, 1972, McGraw-Hill,
New York.
* yizumi@faculty.chiba-u.jp